GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same
options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options"
allow you to stop this process at an intermediate stage. For example, the -c option says not to run the
linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the
compiler itself. Yet other options control the assembler and linker; most of these are not documented here,
since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only
useful with another language (usually C++), the explanation says so explicitly. If the description for a
particular option does not mention a source language, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore
multiple single-letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does
matter when you use several options of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and
so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This
manual documents only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -no-canonical-prefixes -pipe -pass-exit-codes -x language -v -###
--help[=class[,...]] --target-help --version -wrapper @file -fplugin=file -fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fallow-parameterless-variadic-functions -fno-asm
-fno-builtin -fno-builtin-function -fhosted -ffreestanding -fopenmp -fms-extensions -fplan9-extensions
-trigraphs -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch
-flax-vector-conversions -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconstexpr-depth=n -ffriend-injection
-fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fdiagnostics-color=[auto|never|always]
-fno-diagnostics-show-option -fno-diagnostics-show-caret
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Waddress
-Waggregate-return -Waggressive-loop-optimizations -Warray-bounds -Wno-attributes
-Wno-builtin-macro-redefined -Wc++-compat -Wc++11-compat -Wcast-align -Wcast-qual -Wchar-subscripts
-Wclobbered -Wcomment -Wconversion -Wcoverage-mismatch -Wno-cpp -Wno-deprecated
-Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wdouble-promotion -Wempty-body
-Wenum-compare -Wno-endif-labels -Werror -Werror=* -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k
-Wframe-larger-than=len -Wno-free-nonheap-object -Wjump-misses-init -Wignored-qualifiers -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline -Wmaybe-uninitialized
-Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len
-Wunsafe-loop-optimizations -Wlogical-op -Wlong-long -Wmain -Wmaybe-uninitialized -Wmissing-braces
-Wmissing-field-initializers -Wmissing-include-dirs -Wno-mudflap -Wno-multichar -Wnonnull -Wno-overflow
-Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded -Wparentheses -Wpedantic-ms-format
-Wno-pedantic-ms-format -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls
-Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wsign-conversion
-Wsizeof-pointer-memaccess -Wstack-protector -Wstack-usage=len -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wsuggest-attribute=[pure|const|noreturn|format]
-Wmissing-format-attribute -Wswitch -Wswitch-default -Wswitch-enum -Wsync-nand -Wsystem-headers
-Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas
-Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wunused-but-set-parameter
-Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla
-Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type -Wmissing-prototypes
-Wnested-externs -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fsanitize=style -fdbg-cnt-list -fdbg-cnt=counter-value-
list -fdisable-ipa-pass_name -fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-noaddr -fdump-unnumbered
-fdump-unnumbered-links -fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes -fdump-statistics -fdump-tree-all
-fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-cfg -fdump-tree-alias -fdump-tree-ch
-fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw]
-fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiprop[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv
-fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-forwprop[-n] -fdump-tree-fre[-n]
-fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n] -fdump-final-insns=file
-fcompare-debug[=opts] -fcompare-debug-second -feliminate-dwarf2-dups -fno-eliminate-unused-debug-types
-feliminate-unused-debug-symbols -femit-class-debug-always -fenable-kind-pass -fenable-kind-pass=range-
list -fdebug-types-section -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report
-fprofile-arcs -fopt-info -fopt-info-options[=file] -frandom-seed=string -fsched-verbose=n
-falign-loops[=n] -fassociative-math -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize
-fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcheck-data-deps
-fcombine-stack-adjustments -fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce
-fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize -fdse -fearly-inlining -fipa-sra
-fexpensive-optimizations -ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -fforward-propagate -ffp-contract=style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads
-fif-conversion -fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once
-finline-limit=n -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-pta -fipa-profile -fipa-pure-const
-fipa-reference -fira-algorithm=algorithm -fira-region=region -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots -fira-verbose=n -fivopts -fkeep-inline-functions
-fkeep-static-consts -floop-block -floop-interchange -floop-strip-mine -floop-nest-optimize
-floop-parallelize-all -flto -flto-compression-level -flto-partition=alg -flto-report
-fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants fmudflap -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline
-fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole
-fno-peephole2 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder
-fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays
-fprofile-report -fprofile-correction -fprofile-dir=path -fprofile-generate -fprofile-generate=path
-fprofile-use -fprofile-use=path -fprofile-values -freciprocal-math -free -fregmove -frename-registers
-freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math -fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining
-fsel-sched-pipelining-outer-loops -fshrink-wrap -fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector -fstack-protector-all
-fstack-protector-strong -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-inline-vars -ftree-coalesce-vars
-ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-loop-if-convert -ftree-loop-if-convert-stores -ftree-loop-im -ftree-phiprop
-ftree-loop-distribution -ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear
-ftree-loop-optimize -ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc
-ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vect-loop-version -ftree-vectorize -ftree-vrp -funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops -fvariable-expansion-in-unroller
-fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-ld=linker -fuse-linker-plugin --param name=value
-O -O0 -O1 -O2 -O3 -Os -Ofast -Og
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir
-include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir
-imultilib dir -isysroot dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fdebug-cpp
-ftrack-macro-expansion -fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor
option -no-integrated-cpp
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static
Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num
-msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest -mlong-calls
-mshort-calls -msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check
-mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian -mwords-little-endian -mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking -mtp=name -mtls-dialect=dialect
-mword-relocations -mfix-cortex-m3-ldrd -munaligned-access
AVR Options -mmcu=mcu -maccumulate-args -mbranch-cost=cost -mcall-prologues -mint8 -mno-interrupts -mrelax
-mstrict-X -mtiny-stack -Waddr-space-convert
Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k
-mstack-check-l1 -mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data -mno-sep-data -mlong-calls
-mno-long-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4
-metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit
-m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops -mdata-model=model
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist
-flat_namespace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -iframework
-image_base -init -install_name -keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle
-read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols
-sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library
-sub_umbrella -twolevel_namespace -umbrella -undefined -unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=version -mkernel
-mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic -minline-plt
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0
-mportable-runtime -mschedule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static
-threads
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=opt -mvzeroupper
-mprefer-avx128 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -maes -mpclmul
-mfsgsbase -mrdrnd -mf16c -mfma -msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt -mbmi2
-mrtm -mlwp -mpku -mthreads -mno-align-stringops -minline-all-stringops -minline-stringops-dynamically
-mstringop-strategy=alg -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double
-mlong-double-64 -mlong-double-80 -mregparm=num -msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32
-mpc64 -mpc80 -mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -mabi=name -maddress-mode=mode -m32 -m64 -mx32 -mlarge-data-threshold=num -msse2avx
-mfentry -m8bit-idiv -mavx256-split-unaligned-load -mavx256-split-unaligned-store
i386 and x86-64 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread
-municode -mwin32 -mwindows -fno-set-stack-executable
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop
-mregister-names -msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency -minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec -msched-ar-in-data-spec
-msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled
-muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name
-mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60 -m68030
-m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float
-m68881 -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot -mno-xgot
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates
-mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n -mclip -mconfig=name -mcop -mcop32
-mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms
-mdmx -mno-mdmx -mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32
-mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt
-membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata
-mcode-readable=setting -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-24k
-mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r10000 -mno-fix-r10000
-mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func
-mno-flush-func -mbranch-cost=num -mbranch-likely -mno-branch-likely -mfp-exceptions -mno-fp-exceptions
-mvr4130-align -mno-vr4130-align -msynci -mno-synci -mrelax-pic-calls -mno-relax-pic-calls
-mmcount-ra-address
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend
-mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type
-mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb
Moxie Options -meb -mel -mno-crt0
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32
-mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap -munix-asm -mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RL78 Options -msim -mmul=none -mmul=g13 -mmul=rl78
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mpowerpc64 -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf
-mpopcntb -mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb -mmfpgpr -mno-mfpgpr
-mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32
-mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple
-mno-multiple -msingle-float -mdouble-float -msimple-fpu -mstring -mno-string -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd -mbit-align
-mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic
-maltivec -mswdiv -msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no
-mpaired -mgen-cell-microcode -mwarn-cell-microcode -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb
-mno-dlmzb -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -G num
-pthread -mrecip -mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type -mfriz
-mno-friz -mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector -mcrypto
-mno-crypto -mdirect-move -mno-direct-move -mquad-memory -mno-quad-memory -mquad-memory-atomic
-mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data -mlittle-endian-data
-m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media
-m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml -mdalign
-mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range -mindexed-addressing -mgettrcost=number -mpt-fixed
-maccumulate-outgoing-args -minvalid-symbols -matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mno-zdcbranch -mcbranchdi -mcmpeqdi -mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options -mimpure-text -mno-impure-text -pthreads -pthread
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32 -m64
-mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu
-mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis
-mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mpopc -mno-popc
-mfix-at697f -mfix-ut699
SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem -mlarge-mem
-mstdmain -mfixed-range=register-range -mea32 -mea64 -maddress-space-conversion
-mno-address-space-conversion -mcache-size=cache-size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=cpu -m32 -m64 -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e2v3
-mv850e2 -mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float -mhard-float
-mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mtarget-align
-mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions
-fdelete-dead-exceptions -funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -finstrument-functions -finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,... -fno-common -fno-ident -fpcc-struct-return -fpic
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program
work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++
program work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C,
C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally
x. Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.go
Go source code.
file.ads
Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or
generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or
subprogram renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are also
called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated this
way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a
default based on the file name suffix). This option applies to all following input files until the next
-x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for
example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The
ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with
.o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler
code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with
.s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of
preprocessed source code, which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies to whatever sort of output is being produced, whether it be an
executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for
source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch,
and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the
version number of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and arguments are quoted unless they contain only
alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated
command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This
fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
--help
Print (on the standard output) a description of the command-line options understood by gcc. If the -v
option is also specified then --help is also passed on to the various processes invoked by gcc, so that
they can display the command-line options they accept. If the -Wextra option has also been specified
(prior to the --help option), then command-line options that have no documentation associated with them
are also displayed.
--target-help
Print (on the standard output) a description of target-specific command-line options for each tool. For
target
Display target-specific options. Unlike the --target-help option however, target-specific options of
the linker and assembler are not displayed. This is because those tools do not currently support the
extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is the name of one of the languages
supported in this version of GCC.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal sign in the same continuous piece of
text, such as: --help=target.
separate
Display options taking an argument that appears as a separate word following the original option, such
as: -o output-file.
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display
all binary warning options (i.e., ones that are either on or off and that do not take an argument) that
have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is
nothing to display. One case where it does work, however, is when one of the classes is target. For
example, to display all the target-specific optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use displays its requested class
of options, skipping those that have already been displayed.
If the -Q option appears on the command line before the --help= option, then the descriptive text
displayed by --help= is changed. Instead of describing the displayed options, an indication is given as
to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows
this at the point where the --help= option is used).
Alternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when
generating a relative prefix.
--version
Display the version number and copyrights of the invoked GCC.
-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are
passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 ....
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The
base name of the shared object file is used to identify the plugin for the purposes of argument parsing
(See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in
the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in file. This generates Go
"const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface
to code written in some other language.
@file
Read command-line options from file. The options read are inserted in place of the original @file option.
If file does not exist, or cannot be read, then the option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by
surrounding the entire option in either single or double quotes. Any character (including a backslash)
may be included by prefixing the character to be included with a backslash. The file may itself contain
additional @file options; any such options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header
files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and
Objective-C++) that the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of
standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros
such as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable
and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style //
comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite
-ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in
header files that might be included in compilations done with -ansi. Alternate predefined macros such as
"__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is
required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may notice
this macro and refrain from declaring certain functions or defining certain macros that the ISO standard
doesn't call for; this is to avoid interfering with any programs that might use these names for other
things.
Functions that are normally built in but do not have semantics defined by ISO C (such as "alloca" and
"ffs") are not built-in functions when -ansi is used.
-std=
Determine the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards,
such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following
that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off
certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but
not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:"
expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by
the compiler are enabled, even when those features change the meaning of the base standard. As a result,
some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to
identify which features are GNU extensions given that version of the standard. For example -std=gnu90
-Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as
-ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features). This is the default for C code.
gnu99
gnu9x
GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will become the default. The
name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. Support is incomplete and experimental. The name gnu1x is deprecated.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same
as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98. This is the default for C++ code.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. Support for C++11 is still experimental, and may change in
incompatible ways in future releases. The name c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. Support for C++11 is still experimental, and may change in incompatible
ways in future releases. The name gnu++0x is deprecated.
c++1y
The next revision of the ISO C++ standard, tentatively planned for 2017. Support is highly
experimental, and will almost certainly change in incompatible ways in future releases.
gnu++1y
GNU dialect of -std=c++1y. Support is highly experimental, and will almost certainly change in
incompatible ways in future releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in
C99 mode.
This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions
4.3 and later it changes the behavior of GCC in C99 mode. Using this option is roughly equivalent to
adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or
gnu99 mode (i.e., it specifies the default behavior). This option was first supported in GCC 4.3. This
option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which
semantics are in effect for "inline" functions.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to read
the arguments. This is only supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords.
You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99
or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard
keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance,
calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy"
may become inline copy loops. The resulting code is often both smaller and faster, but since the function
calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the
behavior of the functions by linking with a different library. In addition, when a function is recognized
as a built-in function, GCC may use information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the resulting code still contains calls to that
function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built
in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not
begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is
ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is
one in which the entire standard library is available, and in which "main" has a return type of "int".
Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding
environment is one in which the standard library may not exist, and program startup may not necessarily be
at "main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is
specified, the compiler generates parallel code according to the OpenMP Application Program Interface v3.0
<http://www.openmp.org/>. This option implies -pthread, and thus is only supported on targets that have
support for -pthread.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with this option.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions
that expect pointers to elements of the type of the field, and permits referring to anonymous fields
declared using a typedef. This is only supported for C, not C++.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies
-trigraphs.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only
supported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP
manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such
an expression is void. This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element
types. This option should not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by
default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the
signedness of an object. But many programs have been written to use plain "char" and expect it to be
signed, or expect it to be unsigned, depending on the machines they were written for. This option, and
its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its
behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
This section describes the command-line options that are only meaningful for C++ programs. You can also use
most of the GNU compiler options regardless of what language your program is in. For example, you might
compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any
language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 2.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI
obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function
pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another
parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the
mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used
as a template argument.
See also -Wabi.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control
code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage
allocated. This check is normally unnecessary because the C++ standard specifies that "operator new" only
returns 0 if it is declared throw(), in which case the compiler always checks the return value even
without this option. In all other cases, when "operator new" has a non-empty exception specification,
memory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect
endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
-fdeduce-init-list
Enable deduction of a template type parameter as "std::initializer_list" from a brace-enclosed initializer
list, i.e.
deprecated, and may be removed in a future version of G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the
class in which they are declared. Friend functions were documented to work this way in the old Annotated
C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend
function that is not declared in an enclosing scope can only be found using argument dependent lookup.
This option causes friends to be injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future release of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary that is only used to initialize
another object of the same type. Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications at run time. This option violates
the C++ standard, but may be useful for reducing code size in production builds, much like defining
NDEBUG. This does not give user code permission to throw exceptions in violation of the exception
specifications; the compiler still optimizes based on the specifications, so throwing an unexpected
exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime)
initialization. To support this, any use of such a variable goes through a wrapper function that performs
any necessary initialization. When the use and definition of the variable are in the same translation
unit, this overhead can be optimized away, but when the use is in a different translation unit there is
significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer
can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization
(either because the variable is statically initialized, or a use of the variable in the defining TU will
be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init
option.
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support
symbol aliases, the default is -fno-extern-tls-init.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for
loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables
declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old
versions of G++, and other (traditional) implementations of C++.
If neither flag is given, the default is to follow the standard, but to allow and give a warning for old-
style code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the
keyword "__typeof__" instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code
member function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs",
"alloca", "_exit", "index", "bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a "noexcept" specification to reduce or eliminate
the text size overhead relative to a function with no exception specification. If the function has local
variables of types with non-trivial destructors, the exception specification actually makes the function
smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an
exception thrown out of a function with such an exception specification results in a call to "terminate"
rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as
synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such
diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive
allows some nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function template, the compiler normally prints the
signature of the template followed by the template arguments and any typedefs or typenames in the
signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it's clear which template is
involved. When an error message refers to a specialization of a class template, the compiler omits any
template arguments that match the default template arguments for that template. If either of these
behaviors make it harder to understand the error message rather than easier, you can use
-fno-pretty-templates to disable them.
-frepo
Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ run-time
type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you
can save some space by using this flag. Note that exception handling uses the same information, but G++
generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time
type information, i.e. casts to "void *" or to unambiguous base classes.
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is generally
only useful to the G++ development team.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the
values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in
the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if
the program uses a cast to convert an arbitrary integer value to the enumerated type.
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of
local statics. You can use this option to reduce code size slightly in code that doesn't need to be
thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than
the "atexit" function. This option is required for fully standards-compliant handling of static
destructors, but only works if your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be
incorrect, but is necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to inline functions or methods
where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility
("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection
when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO
as it massively reduces the size of the dynamic export table when the library makes heavy use of
templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it
does not affect static variables local to the function or cause the compiler to deduce that the function
is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that
method. For example, if you do want to compare pointers to a particular inline method, you might mark it
as having default visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise
cross a shared library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of
Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are
defined in more than one shared object: those declarations are permitted if they are permitted when
this option is not used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be
externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual
Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name
but defined in different shared objects are different, so changing one does not change the other; and that
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions have linkage like inline functions; they just aren't inlined by default.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an
effort has been made to warn about all such cases, there are probably some cases that are not warned
about, even though G++ is generating incompatible code. There may also be cases where warnings are
emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code
generated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities in -fabi-version=2 (the default) include:
· A template with a non-type template parameter of reference type is mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This is fixed in -fabi-version=3.
· SIMD vector types declared using "__attribute ((vector_size))" are mangled in a non-standard way that
does not allow for overloading of functions taking vectors of different sizes.
The mangling is changed in -fabi-version=4.
The known incompatibilities in -fabi-version=1 include:
· Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as
a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ places "B::f2" into the same byte as "A::f1"; other compilers do not. You can avoid
this problem by explicitly padding "A" so that its size is a multiple of the byte size on your
platform; that causes G++ and other compilers to lay out "B" identically.
· Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out
virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ does not place "B" into the tail-padding for "A"; other compilers do. You can avoid
this problem by explicitly padding "A" so that its size is a multiple of its alignment (ignoring
virtual base classes); that causes G++ and other compilers to lay out "C" identically.
· Incorrect handling of bit-fields with declared widths greater than that of their underlying types,
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ places the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++
mistakenly believes that the "A" data member of "B" is already at offset zero.
· Names of template functions whose types involve "typename" or template template parameters can be
mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
It also warns about psABI-related changes. The known psABI changes at this point include:
· For SysV/x86-64, unions with "long double" members are passed in memory as specified in psABI. For
example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private,
and it has neither friends nor public static member functions. Also warn if there are no non-private
methods, and there's at least one private member function that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual
destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the
base class does not have a virtual destructor. This warning is enabled by -Wall.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-suffix which does not begin with an
underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order
to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>". For
example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing suppresses the diagnostic required by the standard. Note that this does
not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in
SFINAE context.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a
non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never
throw an exception.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-virtual destructor, in which case it is
possible but unsafe to delete an instance of a derived class through a pointer to the base class. This
warning is also enabled if -Weffc++ is specified.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does not match the order in which they must
be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for i and j to match the declaration order of the members,
emitting a warning to that effect. This warning is enabled by -Wall.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this
option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is
on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11,
-std=gnu++1y. This option is off by default for ISO C++11 onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++, Second Edition
book:
· Item 11: Define a copy constructor and an assignment operator for classes with dynamically-allocated
memory.
· Item 12: Prefer initialization to assignment in constructors.
· Item 14: Make destructors virtual in base classes.
· Item 15: Have "operator=" return a reference to *this.
· Item 23: Don't try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the advent
of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e.,
friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary,
nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids
could be interpreted as a particular specialization of a templatized function. Because this non-
conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler
to check existing code for potential trouble spots and is on by default. This new compiler behavior can
be turned off with -Wno-non-template-friend, which keeps the conformant compiler code but disables the
helpful warning.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts
(dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects
and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over
a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++
programs. You can also use most of the language-independent GNU compiler options. For example, you might
compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use
the other options with any language supported by GCC.
constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most
types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including
Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit ensure
that the receiver is not "nil". This allows for more efficient entry points in the runtime to be used.
This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only
for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for
properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support
for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is
specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial
default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method which runs non-
trivial default constructors on any such instance variables, in order, and then return "self". Similarly,
check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a
special "- (void) .cxx_destruct" method which runs all such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly generated only operate on instance
variables declared in the current Objective-C class, and not those inherited from superclasses. It is the
responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy.
The "- (id) .cxx_construct" methods are invoked by the runtime immediately after a new object instance is
allocated; the "- (void) .cxx_destruct" methods are invoked immediately before the runtime deallocates an
object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "- (id)
.cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by
C++ and Java. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and
@synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available
in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available
with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not
require special compiler flags.
-fobjc-nilcheck
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow
dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course
of program execution, without the need to restart the program itself. Currently, Fix-and-Continue
functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")"
(when the name of the class is known at compile time) with static class references that get initialized at
load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior
and causes calls to "objc_getClass("...")" to be retained. This is useful in Zero-Link debugging mode,
since it allows for individual class implementations to be modified during program execution. The GNU
runtime currently always retains calls to "objc_get_class("...")" regardless of command-line options.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued for every method in the protocol that
is not implemented by the class. The default behavior is to issue a warning for every method not
explicitly implemented in the class, even if a method implementation is inherited from the superclass. If
you use the -Wno-protocol option, then methods inherited from the superclass are considered to be
implemented, and no warning is issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector are found during compilation. The check
is performed on the list of methods in the final stage of compilation. Additionally, a check is performed
for each selector appearing in a "@selector(...)" expression, and a corresponding method for that
selector has been found during compilation. Because these checks scan the method table only at the end of
compilation, these warnings are not produced if the final stage of compilation is not reached, for example
because an error is found during compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return types are found for a given selector when
attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is
off (which is the default behavior), the compiler omits such warnings if any differences found are
confined to types that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is
considered undeclared if no method with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol declaration, or implicitly in an
@implementation section. This option always performs its checks as soon as a "@selector(...)" expression
is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces
the coding style convention that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location
information once; that is, in case the message is too long to fit on a single physical line and has to be
wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation
lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source
location information (as prefix) for physical lines that result from the process of breaking a message
which is too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The default is auto. auto means to use color
only when the standard error is a terminal. The forms -fdiagnostics-color and -fno-diagnostics-color are
aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of
capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal
or terminal emulator. (See the section in the documentation of your text terminal for permitted values
and their meanings as character attributes.) These substring values are integers in decimal
representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold,
4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground
colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes
foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for
16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background
colors.
The default GCC_COLORS is error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01 where 01;31
is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 01;32 is bold green and 01 is bold. Setting
GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"caret="
SGR substring for caret line.
"locus="
SGR substring for location information, file:line or file:line:column etc.
"quote="
SGR substring for information printed within quotes.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly
controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
Check the code for syntax errors, but don't do anything beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to
continue processing the source code. If n is 0 (the default), there is no limit on the number of error
messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this
option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a warning is appended; for example
-Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form,
to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings
not be errors, even when -Werror is in effect.
The warning message for each controllable warning includes the option that controls the warning. That
option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the
warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply
anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to
keep going and printing further error messages.
You can request many specific warnings with options beginning with -W, for example -Wimplicit to request
warnings on implicit declarations. Each of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms,
whichever is not the default. For further language-specific options also refer to C++ Dialect Options and
Objective-C and Objective-C++ Dialect Options.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating
that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no
diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the
use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an
unrecognized option is present.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden
extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version
of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few
require -ansi or a -std option specifying the required version of ISO C). However, without this option,
certain GNU extensions and traditional C and C++ features are supported as well. With this option, they
are rejected.
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end
there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.
Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense
for such warnings to be given only for features not in the specified GNU C dialect, since by definition
the GNU dialects of C include all features the compiler supports with the given option, and there would be
nothing to warn about.)
-pedantic-errors
Like -Wpedantic, except that errors are produced rather than warnings.
-Wall
This enables all the warnings about constructions that some users consider questionable, and that are easy
to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some
language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect
Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds (only with -O2) -Wc++11-compat -Wchar-subscripts -Wenum-compare (in C/ObjC; this
is on by default in C++) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and
Objective-C only) -Wcomment -Wformat -Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized -Wmissing-braces (only for C/ObjC) -Wnonnull -Wparentheses -Wpointer-sign -Wreorder
-Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch
-Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users
generally do not consider questionable, but which occasionally you might wish to check for; others warn
about constructions that are necessary or hard to avoid in some cases, and there is no simple way to
modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be
enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W.
The older name is still supported, but the newer name is more descriptive.)
-Wclobbered -Wempty-body -Wignored-qualifiers -Wmissing-field-initializers -Wmissing-parameter-type (C
only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare -Wtype-limits -Wuninitialized
-Wunused-parameter (only with -Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following cases:
· A pointer is compared against integer zero with <, <=, >, or >=.
· (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
· (C++ only) Ambiguous virtual bases.
· (C++ only) Subscripting an array that has been declared register.
· (C++ only) Taking the address of a variable that has been declared register.
· (C++ only) A base class is not initialized in a derived class's copy constructor.
-Wchar-subscripts
-Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can
result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to
an existing code-base. Completely disabling the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32-bit
"single-precision" floating-point unit implement "float" in hardware, but emulate "double" in software.
On such a machine, doing computations using "double" values is much more expensive because of the overhead
required for software emulation.
It is easy to accidentally do computations with "double" because floating-point literals are implicitly of
type "double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because the floating-point literal is a
"double".
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate
to the format string specified, and that the conversions specified in the format string make sense. This
includes standard functions, and others specified by format attributes, in the "printf", "scanf",
"strftime" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific
families). Which functions are checked without format attributes having been specified depends on the
standard version selected, and such checks of functions without the attribute specified are disabled by
-ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all
ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these features; GCC does not support
warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used
with -Wformat, warnings are given about format features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since
-Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.
Some aspects of this level of format checking can be disabled by the options:
-Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by
-Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that contain NUL bytes.
length formats are allowed.
-Wformat=2
Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be
checked, unless the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security
problems. At present, this warns about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments, as in "printf (foo);". This may be
a security hole if the format string came from untrusted input and contains %n. (This is currently a
subset of what -Wformat-nonliteral warns about, but in future warnings may be added to
-Wformat-security that are not included in -Wformat-nonliteral.)
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull"
function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with themselves. Note this option can only be
used with the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the following snippet only when -Winit-self has
been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or -std=gnu99),
this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also
enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wignored-qualifiers (C and C++ only)
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the
initializer for a is not fully bracketed, but that for b is fully bracketed. This warning is enabled by
-Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context
where a truth value is expected, or when operators are nested whose precedence people often get confused
about.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a
different interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which "if" statement an "else" branch
belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible "if" statement, which in this example is
"if (b)". This is often not what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this confusion, GCC issues a warning
when this flag is specified. To eliminate the warning, add explicit braces around the innermost "if"
statement so there is no way the "else" can belong to the enclosing "if". The resulting code looks like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Also warn for dangerous uses of the GNU extension to "?:" with omitted middle operand. When the condition
in the "?": operator is a boolean expression, the omitted value is always 1. Often programmers expect it
to be a value computed inside the conditional expression instead.
This warning is enabled by -Wall.
not specified. All these rules describe only a partial order rather than a total order, since, for
example, if two functions are called within one expression with no sequence point between them, the order
in which the functions are called is not specified. However, the standards committee have ruled that
function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect.
Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that
"Between the previous and next sequence point an object shall have its stored value modified at most once
by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the
value to be stored.". If a program breaks these rules, the results on any particular implementation are
entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more
complicated cases are not diagnosed by this option, and it may give an occasional false positive result,
but in general it has been found fairly effective at detecting this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the
sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after
the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that defaults to "int". Also warn about any
"return" statement with no return value in a function whose return type is not "void" (falling off the end
of the function body is considered returning without a value), and about a "return" statement with an
expression in a function whose return type is "void".
For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type
is specified. The only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case"
labels outside the enumeration range also provoke warnings when this option is used (even if there is a
"default" label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of
the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings
when this option is used. The only difference between -Wswitch and this option is that this option gives
a warning about an omitted enumeration code even if there is a "default" label.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This
warning is enabled by -Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused.
This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return
value. The default is -Wunused-result.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This
warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the
unused expression to void. This includes an expression-statement or the left-hand side of a comma
expression that contains no side effects. For example, an expression such as x[i,j] causes a warning,
while x[(void)i,j] does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused
(note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by
a "setjmp" call. In C++, warn if a non-static reference or non-static const member appears in a class
without constructors.
printed.
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from the function entry to a use of the variable that is
initialized, but there exist some other paths for which the variable is not initialized, the compiler
emits a warning if it cannot prove the uninitialized paths are not executed at run time. These warnings
are made optional because GCC is not smart enough to see all the reasons why the code might be correct in
spite of appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. To
suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp".
These warnings as well are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a
signal handler could call it at any point in the code. As a result, you may get a warning even when there
is in fact no problem because "longjmp" cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as
"noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option
is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the
warnings are only enabled by the -Wall command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between
pragmas. See also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code that might break the
strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases,
but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to
-Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about code that might break the
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives.
Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference
pattern in the front end: "*(int*)&some_float". If optimization is enabled, it also runs in the back end,
where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when
the converted pointer is dereferenced. Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It warns about cases where the compiler
optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about
all cases where the code might overflow: it only warns about cases where the compiler implements some
optimization. Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the
variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily
give a false positive: a warning about code that is not actually a problem. To help focus on important
issues, several warning levels are defined. No warnings are issued for the use of undefined signed
overflow when estimating how many iterations a loop requires, in particular when determining whether a
loop will be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid. For example, with -fstrict-overflow,
the compiler simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow is enabled by -Wall; higher
levels are not, and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >=
0". This can only be simplified when -fstrict-overflow is in effect, because "abs (INT_MIN)"
overflows to "INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified
to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is
simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison.
For example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at the highest warning
level because this simplification applies to many comparisons, so this warning level gives a very
large number of false positives.
-Wsuggest-attribute=[pure|const|noreturn|format]
Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed
below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes "pure", "const" or "noreturn". The
initialization, the type of the parameter variable, or the return type of the containing function
respectively should also have a "format" attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for "format" attributes. Again,
these are only possible candidates. GCC guesses that "format" attributes might be appropriate for any
function that calls a function like "vprintf" or "vscanf", but this might not always be the case, and
some functions for which "format" attributes are appropriate may not be detected.
-Warray-bounds
This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about
subscripts to arrays that are always out of bounds. This warning is enabled by -Wall.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned
about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are
normally suppressed, on the assumption that they usually do not indicate real problems and would only make
the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system
headers as if they occurred in user code. However, note that using -Wall in conjunction with this option
does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point
values as approximations to infinitely precise real numbers. If you are doing this, then you need to
compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the
computation introduces, and allow for it when performing comparisons (and when producing output, but
that's a different problem). In particular, instead of testing for equality, you should check to see
whether the two values have ranges that overlap; and this is done with the relational operators, so
equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C
constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
· Macro parameters that appear within string literals in the macro body. In traditional C macro
replacement takes place within string literals, but in ISO C it does not.
· In traditional C, some preprocessor directives did not exist. Traditional preprocessors only
considered a line to be a directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C understands but ignores because the # does not
enough context to avoid warning in these cases.
· A function declared external in one block and then used after the end of the block.
· A "switch" statement has an operand of type "long".
· A non-"static" function declaration follows a "static" one. This construct is not accepted by some
traditional C compilers.
· The ISO type of an integer constant has a different width or signedness from its traditional type.
This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values,
which typically represent bit patterns, are not warned about.
· Usage of ISO string concatenation is detected.
· Initialization of automatic aggregates.
· Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
· Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the
assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid
missing initializer warnings and relies on default initialization to zero in the traditional C case.
· Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these
prototypes when compiling with traditional C causes serious problems. This is a subset of the
possible conversion warnings; for the full set use -Wtraditional-conversion.
· Use of ISO C style function definitions. This warning intentionally is not issued for prototype
declarations or variadic functions because these ISO C features appear in your code when using
libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed
for nested functions because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would happen to the same argument
in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed-point argument except when the same as the default
promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was
introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not
supported by GCC versions before GCC 3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable or type declaration shadows another variable, parameter, type, or class
member (in C++), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a
local variable shadows an explicit typedef, but not if it shadows a struct/class/enum.
Do not warn when attempting to free an object that was not allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len bytes. The computation done to determine
the stack usage is conservative. Any space allocated via "alloca", variable-length arrays, or related
constructs is included by the compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
· If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
· If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
· If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop
indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the
warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows
targets, which depend on the MS runtime.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types
a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++,
warn also when an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic.
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data type, but do not
warn for constant expressions. For example, warn if an unsigned variable is compared against zero with <
or >=. This warning is also enabled by -Wextra.
-Wbad-function-cast (C and Objective-C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast
to "anything *".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for
implicit conversion from "void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers
in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by
-Wall.
-Wcast-qual
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example,
warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-
byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of one
into a non-"const" "char *" pointer produces a warning. These warnings help you find at compile time code
that can try to write into a string constant, but only if you have been very careful about using "const"
in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall
request these warnings.
When compiling C++, warn about the deprecated conversion from string literals to "char *". This warning
is enabled by default for C++ programs.
-Wclobbered
Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra.
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between real and integer,
like "abs (x)" when "x" is "double"; conversions between signed and unsigned, like "unsigned ui = -1"; and
conversions to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int) x)" and
"ui = (unsigned) -1", or if the value is not changed by the conversion like in "abs (2.0)". Warnings
about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that
never use a type conversion operator: conversions to "void", the same type, a base class or a reference to
them. Warnings about conversions between signed and unsigned integers are disabled by default in C++
unless -Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal '0' is used as null pointer constant. This can be useful to facilitate the conversion
to "nullptr" in C++11.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by
-Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++ enumeral mismatches in
conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is
enabled by -Wall.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a variable,
or jumps backward to a label after the variable has been initialized. This only warns about variables
that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++
option is enabled also by -Wconversion.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses
"sizeof". This warning warns e.g. about "memset (ptr, 0, sizeof (ptr));" if "ptr" is not an array, but a
pointer, and suggests a possible fix, or about "memcpy (&foo, ptr, sizeof (&foo));". This warning is
enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include using the address of a function in a
conditional expression, such as "void func(void); if (func)", and comparisons against the memory address
of a string literal, such as "if (x == "abc")". Such uses typically indicate a programmer error: the
address of a function always evaluates to true, so their use in a conditional usually indicate that the
programmer forgot the parentheses in a function call; and comparisons against string literals result in
unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to
use "strcmp". This warning is enabled by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using logical operators in
contexts where a bit-wise operator is likely to be expected.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where you can
return an array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some
statement during one or more of the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes
applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of
"__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style function
definition is permitted without a warning if preceded by a declaration that specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-
class specifiers like "static" are not the first things in a declaration. This warning is also enabled by
-Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous
prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
Warn if a global function is defined without a previous declaration. Do so even if the definition itself
provides a prototype. Use this option to detect global functions that are not declared in header files.
In C, no warnings are issued for functions with previous non-prototype declarations; use
-Wmissing-prototype to detect missing prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the following code causes such a
warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification does not trigger a
warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's
code, as they have implementation-defined values, and should not be used in portable code.
-Wnormalized=<none|id|nfc|nfkc>
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters.
However, sometimes when characters outside the basic ASCII character set are used, you can have two
different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out
some normalization rules which when applied ensure that two sequences that look the same are turned into
the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this
option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any
identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most
uses.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned
into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C
or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters.
It is hoped that future versions of the standards involved will correct this, which is why this option is
not the default.
You can switch the warning off for all characters by writing -Wnormalized=none. You should only do this
if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs
that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display
methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN
SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO 10646
defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if
your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every
identifier that contains the letter O because it might be confused with the digit 0, and so is not the
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-override-init.
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or
size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this
code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" does not itself have the
packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This has
been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you
when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding
between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align the
whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is
valid and changes nothing.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C
variadic constructor; the warning is on by default because the ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does
who are aware that they are writing nonportable code and who have deliberately chosen to ignore the
warning about it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a
pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't be used.
-Wlong-long
Warn if long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and
C++98 modes. To inhibit the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic
ISO C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to handle variable arguments like va_start. This is
default. To inhibit the warning messages, use -Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for
the performance tuning. Vector operation can be implemented "piecewise", which means that the scalar
operation is performed on every vector element; "in parallel", which means that the vector operation is
implemented using scalars of wider type, which normally is more performance efficient; and "as a single
scalar", which means that vector fits into a scalar type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator.
This is dangerous because if the virtual base is reachable along more than one path, it will be moved
multiple times, which can mean both objects end up in the moved-from state. If the move assignment
operator is written to avoid moving from a moved-from object, this warning can be disabled.
-Wvla
Warn if variable length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the
variable length array.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all
optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by
-Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is
anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code
effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize
programs when the optimization itself is likely to take inordinate amounts of time.
-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length specified in the C standard.
Modern compilers generally allow string constants that are much longer than the standard's minimum limit,
but very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the
limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum
maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a suffix. When used together with
-Wsystem-headers it warns about such constants in system header files. This can be useful when preparing
code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2).
GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can
use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or
refuse to read the program. If you want to control for certain whether to generate the extra information,
use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce
surprising results: some variables you declared may not exist at all; flow of control may briefly move
where you did not expect it; some statements may not be executed because they compute constant results or
their values are already at hand; some statements may execute in different places because they have been
moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer
for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one debugging
format.
-gsplit-dwarf
Separate as much dwarf debugging information as possible into a separate output file with the extension
.dwo. This option allows the build system to avoid linking files with debug information. To be useful,
this option requires a debugger capable of reading .dwo files.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available
(DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at
all possible.
-gpubnames
Generate dwarf .debug_pubnames and .debug_pubtypes sections.
-gstabs
GCC normally emits debugging information for classes because using this option increases the size of
debugging information by as much as a factor of two.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead
of making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat
sections since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types"
sections yet and on some objects ".debug_types" produces larger instead of smaller debugging information.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions understood only
by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse
to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported). This is the format used by SDB on
most System V systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX
debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only
by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse
to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). The value of version may be either
2, 3 or 4; the default version for most targets is 4.
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions
in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.
-grecord-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect code generation to
be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated
with spaces separating them from each other and from the compiler version. See also -frecord-gcc-switches
for another way of storing compiler options into the object file. This is the default.
-gno-record-gcc-switches
Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most
targets using non-conflicting DWARF extensions from later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't
plan to debug. This includes descriptions of functions and external variables, but no information about
local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some
debuggers support macro expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that
meant to generate debug information in version 1 of the DWARF format (which is very different from version
2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be
changed now. Instead use an additional -glevel option to change the debug level for DWARF.
-gtoggle
Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2
otherwise. The position of this argument in the command line does not matter; it takes effect after all
other options are processed, and it does so only once, no matter how many times it is given. This is
mainly intended to be used with -fcompare-debug.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access instructions will be instrumented to
detect out-of-bounds and use-after-free bugs. See <http://code.google.com/p/address-sanitizer/> for more
details.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be instrumented to
detect data race bugs. See <http://code.google.com/p/data-race-test/wiki/ThreadSanitizer> for more
details.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is
"."), the name of the dump file is determined by appending ".gkd" to the compilation output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and
-fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables
-fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for
opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which
disables the dumping of the final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say
-fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather
than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to
-w%n-fcompare-debug not overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug,
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation source file
matches the base name of file in which the struct is defined.
This option substantially reduces the size of debugging information, but at significant potential loss in
type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation source file
matches the base name of file in which the type is defined, unless the struct is a template or defined in
a system header.
This option significantly reduces the size of debugging information, with some potential loss in type
information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates debug information. The intent is to reduce
duplicate struct debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which
serves for most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are used directly (dir:) or used
indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect
uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is
indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:).
Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of
template classes, or non-template classes within the above. Other programming languages have generics,
but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug
information. The values none and any have the normal meaning. The value base means that the base of name
of the file in which the type declaration appears must match the base of the name of the main compilation
file. In practice, this means that when compiling foo.c, debug information is generated for types
declared in that file and foo.h, but not other header files. The value sys means those types satisfying
base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF 2.
-fno-merge-debug-strings
-p Generate extra code to write profile information suitable for the analysis program prof. You must use
this option when compiling the source files you want data about, and you must also use it when linking.
-pg Generate extra code to write profile information suitable for the analysis program gprof. You must use
this option when compiling the source files you want data about, and you must also use it when linking.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about each
pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after interprocedural
optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the (estimated) profile and effect of
individual passes.
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-function basis. The filename
for the dump is made by appending .su to the auxname. auxname is generated from the name of the output
file, if explicitly specified and it is not an executable, otherwise it is the basename of the source
file. An entry is made up of three fields:
· The name of the function.
· A number of bytes.
· One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack statically: a fixed number of bytes
are allocated for the frame on function entry and released on function exit; no stack adjustments are
otherwise made in the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the
static allocation described above, stack adjustments are made in the body of the function, for example to
push/pop arguments around function calls. If the qualifier "bounded" is also present, the amount of these
adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack
used by the function. If it is not present, the amount of these adjustments is not bounded at compile
time and the second field only represents the bounded part.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many times
each branch and call is executed and how many times it is taken or returns. When the compiled program
exits it saves this data to a file called auxname.gcda for each source file. The data may be used for
coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source
file in a program.
· Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
· Run the program on a representative workload to generate the arc profile information. This may be
repeated any number of times. You can run concurrent instances of your program, and provided that the
file system supports locking, the data files will be correctly updated. Also "fork" calls are
detected and correctly handled (double counting will not happen).
· For profile-directed optimizations, compile the source files again with the same optimization and code
generation options plus -fbranch-probabilities.
· For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda
files. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a
spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the
compiler adds code to count the number of times that these arcs are executed. When an arc is the only
exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new
basic block must be created to hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source
file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of
auxname and instructions on how to generate test coverage data. Coverage data matches the source files
more closely if you do not optimize.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-value-list is a comma-separated list of name:value
pairs which sets the upper bound of each debug counter name to value. All debug counters have the initial
upper bound of "UINT_MAX"; thus "dbg_cnt()" returns true always unless the upper bound is set by this
option. For example, with -fdbg-cnt=dce:10,tail_call:0, "dbg_cnt(dce)" returns true only for first 10
invocations.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable optimization passes. These options
are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling
passes instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler
multiple times, the pass name should be appended with a sequential number starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler
multiple times, the pass name should be appended with a sequential number starting from 1. range-list
is a comma-separated list of function ranges or assembler names. Each range is a number pair
separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified by letters. This is used for debugging
the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass
number and a word to the dumpname, and the files are created in the directory of the output file. In case
of =filename option, the dump is output on the given file instead of the pass numbered dump files. Note
that the pass number is computed statically as passes get registered into the pass manager. Thus the
numbering is not related to the dynamic order of execution of passes. In particular, a pass installed by
a plugin could have a number over 200 even if it executed quite early. dumpname is generated from the
name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename
of the source file. These switches may have different effects when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible
letters for use in pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto
dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion
passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination
passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-regmove
Dump after the register move pass.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
-fdump-rtl-see
Dump after sign extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5
enable dumping after five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging information.
-dD Dump all macro definitions, at the end of preprocessing, in addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which pattern and alternative is used. The
length of each instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp
annotation.
-dx Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging
dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap
/ stack / dso start locations.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible
to use diff on debugging dumps for compiler invocations with different options, in particular with and
without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the
previous and next instructions in a sequence.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file name is
made by appending .tu to the source file name, and the file is created in the same directory as the output
file. If the -options form is used, options controls the details of the dump as described for the
-fdump-tree options.
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
inline
Dump after function inlining.
-fdump-passes
Dump the list of optimization passes that are turned on and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file name is generated by appending
a suffix ending in .statistics to the source file name, and the file is created in the same directory as
the output file. If the -option form is used, -stats causes counters to be summed over the whole
compilation unit while -details dumps every event as the passes generate them. The default with no option
is to sum counters for each function compiled.
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the intermediate language tree to a file. The file
name is generated by appending a switch-specific suffix to the source file name, and the file is created
in the same directory as the output file. In case of =filename option, the dump is output on the given
file instead of the auto named dump files. If the -options form is used, options is a list of - separated
options which control the details of the dump. Not all options are applicable to all dumps; those that
are not meaningful are ignored. The following options are available
address
Print the address of each node. Usually this is not meaningful as it changes according to the
environment and source file. Its primary use is for tying up a dump file with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME".
Its primary use is ease of use working backward from mangled names in the assembly file.
slim
When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of
a function merely because that scope has been reached. Only dump such items when they are directly
reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like
representation.
raw Print a raw representation of the tree. By default, trees are pretty-printed into a C-like
representation.
details
Enable more detailed dumps (not honored by every dump option). Also include information from the
optimization passes.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in certain passes).
missed
Enable showing missed optimization information (only available in certain passes).
notes
Enable other detailed optimization information (only available in certain passes).
=filename
Instead of an auto named dump file, output into the given file name. The file names stdout and stderr
are treated specially and are considered already open standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is output on to stderr. If two conflicting
dump filenames are given for the same pass, then the latter option overrides the earlier one.
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and note.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
gimple
Dump each function before and after the gimplification pass to a file. The file name is made by
appending .gimple to the source file name.
cfg Dump the control flow graph of each function to a file. The file name is made by appending .cfg to
ccp Dump each function after CCP. The file name is made by appending .ccp to the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file
name.
pre Dump trees after partial redundancy elimination. The file name is made by appending .pre to the
source file name.
fre Dump trees after full redundancy elimination. The file name is made by appending .fre to the source
file name.
copyprop
Dump trees after copy propagation. The file name is made by appending .copyprop to the source file
name.
store_copyprop
Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the
source file name.
dce Dump each function after dead code elimination. The file name is made by appending .dce to the source
file name.
mudflap
Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap
to the source file name.
sra Dump each function after performing scalar replacement of aggregates. The file name is made by
appending .sra to the source file name.
sink
Dump each function after performing code sinking. The file name is made by appending .sink to the
source file name.
dom Dump each function after applying dominator tree optimizations. The file name is made by appending
.dom to the source file name.
dse Dump each function after applying dead store elimination. The file name is made by appending .dse to
the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline code. The file name is made by
appending .phiopt to the source file name.
forwprop
Dump each function after forward propagating single use variables. The file name is made by appending
.forwprop to the source file name.
copyrename
Dump each function after applying the copy rename optimization. The file name is made by appending
.copyrename to the source file name.
nrv Dump each function after applying the named return value optimization on generic trees. The file name
is made by appending .nrv to the source file name.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the -options form is used, options is a
list of - separated options to select the dump details and optimizations. If options is not specified, it
defaults to all for details and optall for optimization groups. If the filename is not specified, it
defaults to stderr. Note that the output filename will be overwritten in case of multiple translation
units. If a combined output from multiple translation units is desired, stderr should be used instead.
The options can be divided into two groups, 1) options describing the verbosity of the dump, and 2)
options describing which optimizations should be included. The options from both the groups can be freely
mixed as they are non-overlapping. However, in case of any conflicts, the latter options override the
earlier options on the command line. Though multiple -fopt-info options are accepted, only one of them can
have =filename. If other filenames are provided then all but the first one are ignored.
The dump verbosity has the following options
optimized
Print information when an optimization is successfully applied. It is up to a pass to decide which
information is relevant. For example, the vectorizer passes print the source location of loops which
got successfully vectorized.
missed
Print information about missed optimizations. Individual passes control which information to include
in the output. For example,
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
will print information about missed optimization opportunities from vectorization passes on stderr.
note
Print verbose information about optimizations, such as certain transformations, more detailed messages
about decisions etc.
all Print detailed optimization information. This includes optimized, missed, and note.
The second set of options describes a group of optimizations and may include one or more of the following.
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
vec Enable dumps from all vectorization optimizations.
For example,
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all.
gcc -O3 -fopt-info
Note that -fopt-info-vec-missed behaves the same as -fopt-info-missed-vec.
As another example, consider
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed.
In this case, only the first option takes effect and the subsequent options are ignored. Thus only the
vec.miss is produced which cotaints dumps from the vectorizer about missed opportunities.
-ftree-vectorizer-verbose=n
This option is deprecated and is implemented in terms of -fopt-info. Please use -fopt-info-kind form
instead, where kind is one of the valid opt-info options. It prints additional optimization information.
For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got
vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer reports locations
which could not be vectorized and the reasons for those. For any higher verbosity levels all the analysis
and transformation information from the vectorizer is reported.
Note that the information output by -ftree-vectorizer-verbose option is sent to stderr. If the equivalent
form -fopt-info-options=filename is used then the output is sent into filename instead.
-frandom-seed=string
This option provides a seed that GCC uses in place of random numbers in generating certain symbol names
that have to be different in every compiled file. It is also used to place unique stamps in coverage data
files and the object files that produce them. You can use the -frandom-seed option to produce
reproducibly identical object files.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the
scheduler prints. This information is written to standard error, unless -fdump-rtl-sched1 or
-fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched1 or
.sched2 respectively. However for n greater than nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and
-fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready list
information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and
regions info. And for n over four, -fsched-verbose also includes dependence info.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place them in the current directory and name
them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s,
as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally
uses an integrated preprocessor.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over
writing an input source file with the same extension as an intermediate file. The corresponding
intermediate file may be obtained by renaming the source file before using -save-temps.
are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like
-save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself. The
second number is "system time", time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like
this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program name, and the options passed to the
program are displayed, so that one can later tell what file was being compiled, and with which options.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better
debugging information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g)
and the debug info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the annotations over
throughout the compilation all the way to the end, in an attempt to improve debug information while
optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained,
but discarded at the end.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't do
anything else. With this option, GCC does not compile or link anything; it just prints the file name.
libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS
libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or
if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can
do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc
searches---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or
directory. To resolve this you either need to put cpp0 and the other compiler components where gcc
expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you
installed them. Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during compilation. This is the target sysroot specified
either at configure time or using the --sysroot option, possibly with an extra suffix that depends on
compilation options. If no target sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler
is not configured with such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being
built.)
-fno-eliminate-unused-debug-types
Normally, when producing DWARF 2 output, GCC avoids producing debug symbol output for types that are
nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging
information for all types declared in a compilation unit, regardless of whether or not they are actually
used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is
not actually used in your program (but is declared). More often, however, this results in a significant
amount of wasted space.
once to a single output file mode allows the compiler to use information gained from all of the files when
compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in
this section.
Most optimizations are only enabled if an -O level is set on the command line. Otherwise they are disabled,
even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled
at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact
set of optimizations that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations
that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse
-fguess-branch-probability -fif-conversion2 -fif-conversion -fipa-pure-const -fipa-profile -fipa-reference
-fmerge-constants -fsplit-wide-types -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop
-ftree-slsr -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed
tradeoff. As compared to -O, this option increases both compilation time and the performance of the
generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:
-fthread-jumps -falign-functions -falign-jumps -falign-loops -falign-labels -fcaller-saves
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fexpensive-optimizations -fgcse -fgcse-lm -fhoist-adjacent-loads -finline-small-functions
-findirect-inlining -fipa-sra -foptimize-sibling-calls -fpartial-inlining -fpeephole2 -fregmove
-freorder-blocks -freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2 -fstrict-aliasing -fstrict-overflow -ftree-switch-conversion
-ftree-tail-merge -ftree-pre -ftree-vrp
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the
-finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize,
-fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone options.
-O0 Reduce compilation time and make debugging produce the expected results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also
performs further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops
If you use multiple -O options, with or without level numbers, the last such option is the one that is
effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative
forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one
you typically use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related
to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be
performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the class scope (C++
only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by
default; i.e., you don't need to add inline in front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines that must
pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for
several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the
result can be simplified. If loop unrolling is active, two passes are performed and the second is
scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2, -O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-
point expression contraction such as forming of fused multiply-add operations if the target has native
support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the
language standard. This is currently not implemented and treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't need one. This avoids the
instructions to save, set up and restore frame pointers; it also makes an extra register available in many
functions. It also makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence
automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-
description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag.
Starting with GCC version 4.6, the default setting (when not optimizing for size) for 32-bit GNU/Linux x86
and 32-bit Darwin x86 targets has been changed to -fomit-frame-pointer. The default can be reverted to
-fno-omit-frame-pointer by configuring GCC with the --enable-frame-pointer configure option.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
overall size of program gets smaller). The compiler heuristically decides which functions are simple
enough to be worth integrating in this way. This inlining applies to all functions, even those not
declared inline.
Enabled at level -O2.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining.
This option has any effect only when inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at level -O2.
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler heuristically
decides which functions are worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function
is normally not output as assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not marked
"inline". If a call to a given function is integrated, then the function is not output as assembler code
in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function call
overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes
profiling significantly cheaper and usually inlining faster on programs having large chains of nested
wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of
parameters passed by reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this
limit. n is the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using
--param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are
declared using the "dllexport" attribute or declspec
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function has
been inlined into all of its callers. This switch does not affect functions using the "extern inline"
extension in GNU C90. In C++, emit any and all inline functions into the object file.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables aren't
referenced.
GCC enables this option by default. If you want to force the compiler to check if a variable is
referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts
option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants) across compilation
units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant
initialized arrays or initialized constant variables with integral or floating-point types. Languages
like C or C++ require each variable, including multiple instances of the same variable in recursive calls,
to have distinct locations, so using this option results in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at
innermost loops and reorders their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag
certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-
range analysis. This option is effective only with -fmodulo-sched enabled.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of
instructions that decrement a register, compare it against zero, then branch based upon the result. This
option is only meaningful on architectures that support such instructions, which include x86, PowerPC,
IA-64 and S/390.
The default is -fbranch-count-reg.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function contain
the function's address explicitly.
The default is -fzero-initialized-in-bss.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations,
some standard library string/heap functions, and some other associated constructs with range/validity
tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other
classes of C/C++ programming errors. The instrumentation relies on a separate runtime library
(libmudflap), which is linked into a program if -fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env
MUDFLAP_OPTIONS=-help a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use
-fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads.
This produces less instrumentation (and therefore faster execution) and still provides some protection
against outright memory corrupting writes, but allows erroneously read data to propagate within a program.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a location where another comparison subsumed
by the first is found. If so, the first branch is redirected to either the destination of the second
branch or a point immediately following it, depending on whether the condition is known to be true or
false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the
registers apart and allocate them independently. This normally generates better code for those types, but
may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is
not reached by any other path. For example, when CSE encounters an "if" statement with an "else" clause,
CSE follows the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks.
When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow
the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and copy
propagation.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This
pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a
load/store sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads
that come after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The
purpose of this pass is to clean up redundant spilling.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to derive bounds for the number of
iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations
of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is
enabled by default.
-funsafe-loop-optimizations
This option tells the loop optimizer to assume that loop indices do not overflow, and that loops with
nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the
loop optimizer itself cannot prove that these assumptions are valid. If you use
-Wunsafe-loop-optimizations, the compiler warns you if it finds this kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size.
The resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always skipped on
architectures that do not have instructions to support this. Enabled by default at -O and higher on
architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional
moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use
of conditional execution on chips where it is available is controlled by "if-conversion2".
cannot be null.
Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks
to disable this optimization for programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all levels. Otherwise it is enabled at all
levels: -O0, -O1, -O2, -O3, -Os. Passes that use the information are enabled independently at different
optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and
interprocedurally as part of indirect inlining ("-findirect-inlining") and interprocedural constant
propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is especially helpful for the x86-64
architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.
Enabled for x86 at levels -O2, -O3.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple instructions in
order to maximize the amount of register tying. This is especially helpful on machines with two-operand
instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be
priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring.
Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support
it, it is the default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The region argument should be one of the
following:
all Use all loops as register allocation regions. This can give the best results for machines with a
small and/or irregular register set.
mixed
Use all loops except for loops with small register pressure as the regions. This value usually gives
the best results in most cases and for most architectures, and is enabled by default when compiling
with optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in the smallest code size, and is
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard
register gets a separate stack slot, and as a result function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a
hard register gets a separate stack slot, and as a result function stack frames are larger.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If
the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus
10.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots
available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to
required data being unavailable. This helps machines that have slow floating point or memory load
instructions by allowing other instructions to be issued until the result of the load or floating-point
instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register
allocation has been done. This is especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling
before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is normally enabled by default when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register allocation. This only makes sense when
scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage
of this option can improve the generated code and decrease its size by preventing register pressure
increase above the number of available hard registers and subsequent spills in register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate
for premature removal from the queue of stalled insns. This has an effect only during the second
scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling. This allows motion across basic
block boundaries, resulting in faster schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or
higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a
schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical
path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative
instructions with greater dependency weakness. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic
block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is
less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has
more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later
scheduling passes may change its schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the
first scheduler pass.
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at the top of the
function. This flag is enabled by default at -O and higher.
-fcaller-saves
Enable allocation of values to registers that are clobbered by function calls, by emitting extra
instructions to save and restore the registers around such calls. Such allocation is done only when it
seems to result in better code.
This option is always enabled by default on certain machines, usually those which have no call-preserved
registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to
combine them.
Enabled by default at -O1 and higher.
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the
program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-
frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only
considers expressions that are computed on all paths leading to the redundant computation. This analysis
is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and
higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and
higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations
in the same structure and the target architecture has a conditional move instruction. This flag is
enabled by default at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is
-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold functions are marked as
cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are
identified. Cold functions and loop less parts of functions executed once are then optimized for size.
Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the program to determine when
values passed to functions are constants and then optimizes accordingly. This optimization can
substantially increase performance if the application has constants passed to functions. This flag is
enabled by default at -O2, -Os and -O3.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger. When enabled,
interprocedural constant propagation performs function cloning when externally visible function can be
called with constant arguments. Because this optimization can create multiple copies of functions, it may
significantly increase code size (see --param ipcp-unit-growth=value). This flag is enabled by default at
-O3.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information.
This pass only operates on local scalar variables and is enabled by default at -O and higher. It requires
that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar
variables and is enabled by default at -O and higher.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar array. This
flag is enabled by default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the other. This optimization
is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The
compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-
merge-iterations parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but
are otherwise side-effect free. This flag is enabled by default at -O2 and higher if -Os is not also
specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range
propagation and expression simplification) based on a dominator tree traversal. This also performs jump
threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
-ftree-loop-linear
Perform loop interchange transformations on tree. Same as -floop-interchange. To use this code
transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop
transformation infrastructure.
-floop-interchange
Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner and
outer loops. For example, given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange transforms the loop as if it were written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when "N" is larger than the caches, because in Fortran, the elements of an array
are stored in memory contiguously by column, and the original loop iterates over rows, potentially
creating at each access a cache miss. This optimization applies to all the languages supported by GCC and
is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and
--with-cloog to enable the Graphite loop transformation infrastructure.
-floop-strip-mine
Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested loops.
The outer loop has strides equal to the strip size and the inner loop has strides of the original loop
within a strip. The strip length can be changed using the loop-block-tile-size parameter. For example,
given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining transforms the loop as if it were written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use
this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite
loop transformation infrastructure.
-floop-block
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when "M" is larger than the caches, because the innermost loop iterates over a
smaller amount of data which can be kept in the caches. This optimization applies to all the languages
supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured
with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation
and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the
GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code
generator CLooG, like index splitting and dead code elimination in loops.
-floop-nest-optimize
Enable the ISL based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto
optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism.
This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the
loops that can be analyzed to not contain loop carried dependences without checking that it is profitable
to parallelize the loops.
-fcheck-data-deps
Compare the results of several data dependence analyzers. This option is used for debugging the data
dependence analyzers.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is
to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass
to handle these loops. This is enabled by default if vectorization is enabled.
-ftree-loop-if-convert-stores
Attempt to also if-convert conditional jumps containing memory writes. This transformation can be unsafe
for multi-threaded programs as it transforms conditional memory writes into unconditional memory writes.
For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
is transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library. This flag is
enabled by default at -O3.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL
level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it
also moves operands of conditions that are invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number of iterations
requires complicated analysis. Later optimizations then may determine the number easily. Useful
especially in connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and induction
variable elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops
whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on
multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory
bandwidth. This option implies -pthread, and thus is only supported on targets that have support for
-pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at -O and higher.
but only if they were inlined from other functions. It is a more limited form of -ftree-coalesce-vars.
This may harm debug information of such inlined variables, but it will keep variables of the inlined-into
function apart from each other, such that they are more likely to contain the expected values in a
debugging session. This was the default in GCC versions older than 4.7.
-ftree-coalesce-vars
Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too,
instead of just compiler temporaries. This may severely limit the ability to debug an optimized program
compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of
user variables, including inlined ones. This option is enabled by default.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries
are replaced at their use location with their defining expression. This results in non-GIMPLE code, but
gives the expanders much more complex trees to work on resulting in better RTL generation. This is
enabled by default at -O and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related expressions involving
multiplications and replaces them by less expensive calculations when possible. This is enabled by
default at -O and higher.
-ftree-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O3.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when
-ftree-vectorize is enabled.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable
except that data alignment or data dependence cannot be determined at compile time, then vectorized and
non-vectorized versions of the loop are generated along with run-time checks for alignment or dependence
to control which version is executed. This option is enabled by default except at level -Os where it is
disabled.
-fvect-cost-model
Enable cost model for vectorization. This option is enabled by default at -O3.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead
of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks
like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null
pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of
the function allowing other optimizations to do a better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop. This option makes code larger, and may or may not make it
run faster.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables when unrolling a loop,
which can result in superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at level -O2.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores)
performed in previous iterations of loops.
This option is enabled at level -O3.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of
loops that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops
within the source code.
Disabled at level -Os.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and
-fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a
few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are
specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of
the control flow graph, taking the __builtin_expect info into account. The interactions between the
heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the
heuristics so that the effects of __builtin_expect are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code
locality.
Enabled at levels -O2, -O3.
executed functions. Reordering is done by the linker so object file format must support named sections
and linker must place them in a reasonable way.
Also profile feedback must be available to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For
C (and C++), this activates optimizations based on the type of expressions. In particular, an object of
one type is assumed never to reside at the same address as an object of a different type, unless the types
are almost the same. For example, an "unsigned int" can alias an "int", but not a "void*" or a "double".
A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called "type-
punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is
accessed through the union type. So, the code above works as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has
undefined behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fstrict-overflow
Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For
C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means
that the compiler may assume that it does not happen. This permits various optimizations. For example,
the compiler assumes that an expression like "i + 10 > i" is always true for signed "i". This assumption
integers. With -fwrapv certain types of overflow are permitted. For example, if the compiler gets an
overflow when doing arithmetic on constants, the overflowed value can still be used with -fwrapv, but not
otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For
instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24
aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This
option can easily make code slower, because it must insert dummy operations for when the branch target is
reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are
used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no
alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. If the loops are
executed many times, this makes up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached
by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be
executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
Do not reorder top-level functions, variables, and "asm" statements. Output them in the same order that
they appear in the input file. When this option is used, unreferenced static variables are not removed.
This option is intended to support existing code that relies on a particular ordering. For new code, it
is better to use attributes.
Enabled at level -O0. When disabled explicitly, it also implies -fno-section-anchors, which is otherwise
enabled at -O0 on some targets.
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo
register. This allows the register allocation pass to operate on pseudos directly, but also strengthens
several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can,
however, make debugging impossible, since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public
functions and variables with the exception of "main" and those merged by attribute "externally_visible"
become static functions and in effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with "-flto". Instead relying on a linker plugin should
provide safer and more precise information.
-flto[=n]
This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE
(one of GCC's internal representations) and writes it to special ELF sections in the object file. When
the object files are linked together, all the function bodies are read from these ELF sections and
instantiated as if they had been part of the same translation unit.
To use the link-time optimizer, -flto needs to be specified at compile time and during the final link.
For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside
foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two
files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are
merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work
across the two files as if they were a single one. This means, for example, that the inliner is able to
inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation
and optimizes them as usual to produce myprog.
The only important thing to keep in mind is that to enable link-time optimizations the -flto flag needs to
be passed to both the compile and the link commands.
Additionally, the optimization flags used to compile individual files are not necessarily related to those
used at link time. For instance,
gcc -c -O0 -flto foo.c
gcc -c -O0 -flto bar.c
gcc -o myprog -flto -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but the resulting binary myprog is
optimized at -O3. If, instead, the final binary is generated without -flto, then myprog is not optimized.
When producing the final binary with -flto, GCC only applies link-time optimizations to those files that
contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and
final object code. GCC automatically selects which files to optimize in LTO mode and which files to link
without further processing.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used
during the final link stage. Currently, the following options are saved into the GIMPLE bytecode files:
-fPIC, -fcommon and all the -m target flags.
At link time, these options are read in and reapplied. Note that the current implementation makes no
attempt to recognize conflicting values for these options. If different files have conflicting option
values (e.g., one file is compiled with -fPIC and another isn't), the compiler simply uses the last value
read from the bytecode files. It is recommended, then, that you compile all the files participating in
the same link with the same options.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to
be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued.
The behavior is still undefined at run time.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in
different languages. This requires support in the language front end. Currently, the C, C++ and Fortran
front ends are capable of emitting GIMPLE bytecodes, so something like this should work:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to
get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same
link command options as when mixing languages in a regular (non-LTO) compilation; all you need to add is
-flto to all the compile and link commands.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible
to extract and use them in an LTO link if you are using a linker with plugin support. To enable this
feature, use the flag -fuse-linker-plugin at link time:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them
on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects
inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization
Link-time optimization does not work well with generation of debugging information. Combining -flto with
-g is currently experimental and expected to produce wrong results.
If you specify the optional n, the optimization and code generation done at link time is executed in
parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may
be used to override the program used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel
jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a +
to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is
GNU make.
This option is disabled by default.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either "1to1" to specify
a partitioning mirroring the original source files or "balanced" to specify partitioning into equally
sized chunks (whenever possible) or "max" to create new partition for every symbol where possible.
Specifying "none" as an algorithm disables partitioning and streaming completely. The default value is
"balanced". While "1to1" can be used as an workaround for various code ordering issues, the "max"
partitioning is intended for internal testing only.
-flto-compression-level=n
This option specifies the level of compression used for intermediate language written to LTO object files,
and is only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no compression) to 9
(maximum compression). Values outside this range are clamped to either 0 or 9. If the option is not
given, a default balanced compression setting is used.
-flto-report
Prints a report with internal details on the workings of the link-time optimizer. The contents of this
report vary from version to version. It is meant to be useful to GCC developers when processing object
files in LTO mode (via -flto).
Disabled by default.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in
the linker, which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This
improves the quality of optimization by exposing more code to the link-time optimizer. This information
specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking).
Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are
similar to "-fwhole-program". See -flto for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a
linker supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the object code. This
makes them usable for both LTO linking and normal linking. This option is effective only when compiling
with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be
This pass only applies to certain targets that cannot explicitly represent the comparison operation before
register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, perform a copy-propagation
pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to
missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such
inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects only the profile
data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and
-fbranch-probabilities and its related options. Both absolute and relative paths can be used. By
default, GCC uses the current directory as path, thus the profile data file appears in the same directory
as the object file.
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile useful for later
recompilation with profile feedback based optimization. You must use -fprofile-generate both when
compiling and when linking your program.
The following options are enabled: "-fprofile-arcs", "-fprofile-values", "-fvpt".
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
-fprofile-use
-fprofile-use=path
Enable profile feedback directed optimizations, and optimizations generally profitable only with profile
feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops",
"-ftracer", "-ftree-vectorize", "ftree-loop-distribute-patterns"
By default, GCC emits an error message if the feedback profiles do not match the source code. This error
can be turned into a warning by using -Wcoverage-mismatch. Note this may result in poorly optimized code.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
The following options control compiler behavior regarding floating-point arithmetic. These options trade off
-fexcess-precision=style
This option allows further control over excess precision on machines where floating-point registers have
more precision than the IEEE "float" and "double" types and the processor does not support operations
rounding to those types. By default, -fexcess-precision=fast is in effect; this means that operations are
carried out in the precision of the registers and that it is unpredictable when rounding to the types
specified in the source code takes place. When compiling C, if -fexcess-precision=standard is specified
then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments
cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This
option is enabled by default for C if a strict conformance option such as -std=c99 is used.
-fexcess-precision=standard is not implemented for languages other than C, and has no effect if
-funsafe-math-optimizations or -ffast-math is specified. On the x86, it also has no effect if
-mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess
precision, and in the latter, rounding is unpredictable.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math,
-fno-signaling-nans and -fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for
programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed with a single instruction, e.g., "sqrt".
A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while
maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result in incorrect output for programs that
depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to
consider the possibility that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and
(b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files
that change the default FPU control word or other similar optimizations.
This option is not turned on by any -O option since it can result in incorrect output for programs that
depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++
language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero
subexpression elimination. Note that this loses precision and increases the number of flops operating on
the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or
+-Infs.
This option is not turned on by any -O option since it can result in incorrect output for programs that
depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic
specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of
expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a
zero result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps
include division by zero, overflow, underflow, inexact result and invalid operation. This option requires
that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-
stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for
programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default floating-point rounding behavior. This is
round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic
truncations. This option should be specified for programs that change the FP rounding mode dynamically,
or that may be executed with a non-default rounding mode. This option disables constant folding of
floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic
transformations that are unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are
affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's
"FENV_ACCESS" pragma. This command-line option will be used to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point
operations. Setting this option disables optimizations that may change the number of exceptions visible
with signaling NaNs. This option implies -ftrapping-math.
When enabled, this option states that a range reduction step is not needed when performing complex
division. Also, there is no checking whether the result of a complex multiplication or division is "NaN +
I*NaN", with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but
is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the
option applies to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex
division, but there is no checking whether the result of a complex multiplication or division is "NaN +
I*NaN", with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O
options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When
a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called
sourcename.gcda for each source file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code and the same optimization options
for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be
used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing
which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which
path is taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program
is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for
usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the compiler to add code to gather information
about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based
on them. Currently the optimizations include specialization of division operations using the knowledge
about the value of the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register
allocation. This optimization most benefits processors with lots of registers. Depending on the debug
information format adopted by the target, however, it can make debugging impossible, since variables no
longer stay in a "home register".
makes code larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually
makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not roll much (from profile feedback). It
also turns on complete loop peeling (i.e. complete removal of loops with small constant number of
iterations).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches
(modified according to result of the condition).
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports arbitrary
sections. The name of the function or the name of the data item determines the section's name in the
output file.
Use these options on systems where the linker can perform optimizations to improve locality of reference
in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2
have linkers with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so. When you specify these options,
the assembler and linker create larger object and executable files and are also slower. You cannot use
"gprof" on all systems if you specify this option, and you may have problems with debugging if you specify
both this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of target
registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-
block scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't reuse branch target registers within any
basic block.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a
guard variable to functions with vulnerable objects. This includes functions that call "alloca", and
functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and
then checked when the function exits. If a guard check fails, an error message is printed and the program
targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it
accesses the variables from a common anchor point instead. The effect is similar to the following
pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For
example, GCC does not inline functions that contain more than a certain number of instructions. You can
control some of these constants on the command line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the
compiler, and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are:
predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold (in percent), then it
is considered well predictable. The default is 10.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping. The algorithm used by
-fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more
aggressive optimization, making the compilation time increase with probably small improvement in
executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the end of two blocks before cross-jumping
is performed on them. This value is ignored in the case where all instructions in the block being
cross-jumped from are matched. The default value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is
relative to a jump instruction. The default value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid
O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process,
and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more
than max-goto-duplication-insns are unfactored. The default value is 8.
max-gcse-memory
The approximate maximum amount of memory that can be allocated in order to perform the global common
subexpression elimination optimization. If more memory than specified is required, the optimization
is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value for any expression, then
RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the
instruction stream. The default value is 20.
max-pending-list-length
The maximum number of pending dependencies scheduling allows before flushing the current state and
starting over. Large functions with few branches or calls can create excessively large lists which
needlessly consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop.
Larger values can exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This number sets the maximum number of
instructions (counted in GCC's internal representation) in a single function that the tree inliner
considers for inlining. This only affects functions declared inline and methods implemented in a
class declaration (C++). The default value is 400.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be
considered for inlining by the compiler are investigated. To those functions, a different (more
restrictive) limit compared to functions declared inline can be applied. The default value is 40.
inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this threshold (in precent),
the function can be inlined regardless the limit on --param max-inline-insns-single and --param max-
inline-insns-auto.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after inlining,
inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid
extreme compilation time caused by non-linear algorithms used by the back end. The default value is
2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. The default value is 100
which limits large function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units larger than this
limit is limited by --param inline-unit-growth. For small units this might be too tight. For
example, consider a unit consisting of function A that is inline and B that just calls A three times.
If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very
large units consisting of small inlineable functions, however, the overall unit growth limit is needed
to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param
large-unit-insns before applying --param inline-unit-growth. The default is 10000.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents. The default value is
1000 which limits large stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function
can grow into by performing recursive inlining.
For functions declared inline, --param max-inline-insns-recursive is taken into account. For
functions not declared inline, recursive inlining happens only when -finline-functions (included in
-O3) is enabled and --param max-inline-insns-recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.
For functions declared inline, --param max-inline-recursive-depth is taken into account. For
functions not declared inline, recursive inlining happens only when -finline-functions (included in
-O3) is enabled and --param max-inline-recursive-depth-auto is used. The default value is 8.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt for
function having little recursion depth by increasing the prologue size or complexity of function body
to other optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed
from probability that function recurses via a given call expression. This parameter limits inlining
only to call expressions whose probability exceeds the given threshold (in percents). The default
value is 10.
early-inlining-insns
Specify growth that the early inliner can make. In effect it increases the amount of inlining for
code having a large abstraction penalty. The default value is 10.
max-early-inliner-iterations
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls
the early inliner can resolve. Deeper chains are still handled by late inlining.
comdat-sharing-probability
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility are shared across multiple
compilation units. The default value is 20.
min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used.
The number of iterations after vectorization needs to be greater than the value specified by this
option to allow vectorization. The default value is 0.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations.
This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive
code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in
tree tail merging. The default value is 10.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function. This is used to limit compilation
time in tree tail merging. The default value is 2.
max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this
parameter also determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop may have to
be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is
unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this
parameter also determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which all candidates are considered for
each use in induction variable optimizations. If there are more candidates than this, only the most
relevant ones are considered to avoid quadratic time complexity.
Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions
slow the analyzer.
omega-max-vars
The maximum number of variables in an Omega constraint system. The default value is 128.
omega-max-geqs
The maximum number of inequalities in an Omega constraint system. The default value is 256.
omega-max-eqs
The maximum number of equalities in an Omega constraint system. The default value is 128.
omega-max-wild-cards
The maximum number of wildcard variables that the Omega solver is able to insert. The default value
is 18.
omega-hash-table-size
The size of the hash table in the Omega solver. The default value is 550.
omega-max-keys
The maximal number of keys used by the Omega solver. The default value is 500.
omega-eliminate-redundant-constraints
When set to 1, use expensive methods to eliminate all redundant constraints. The default value is 0.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alignment
in the vectorizer. See option -ftree-vect-loop-version for more information.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alias in
the vectorizer. See option -ftree-vect-loop-version for more information.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of
iterations of the loop tries to evaluate.
hot-bb-count-ws-permille
A basic block profile count is considered hot if it contributes to the given permillage (i.e.
0...1000) of the entire profiled execution.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of basic block in function given basic
block needs to have to be considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where a function
contains a single loop with known bound and another loop with unknown bound. The known number of
iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This
means that the loop without bounds appears artificially cold relative to the other one.
align-threshold
Select fraction of the maximal frequency of executions of a basic block in a function to align the
basic block.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is a rather artificial
limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much
higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in
percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge has probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback
and one for compilation without. The value for compilation with profile feedback needs to be more
conservative (higher) in order to make tracer effective.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers. The default is 10.
max-cse-insns
The maximum number of instructions CSE processes before flushing. The default is 1000.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the
minimum percentage by which the garbage collector's heap should be allowed to expand between
collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is
available, the notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC
is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this
parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This
is extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first
collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning
this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or
RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound
is used. Setting this parameter very large effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register. Increasing
values mean more aggressive optimization, making the compilation time increase with probably slightly
better performance. The default value is 100.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account. Increasing values mean more
aggressive optimization, making the compilation time increase with probably slightly better
The maximum number of instructions ready to be issued the scheduler should consider at any given time
during the first scheduling pass. Increasing values mean more thorough searches, making the
compilation time increase with probably little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling. The default
value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the selective scheduler.
The default value is 15.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling. The default value
is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the selective scheduler.
The default value is 200.
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock speculative
scheduling. The default value is 40.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of 0 (the default) disables
region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion. The default value is
3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative insns are scheduled.
The default value is 40.
sched-spec-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state across it. The default
value is 10.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory locations. The default
value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth of search for
available instructions. The default value is 50.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled during selective scheduling. This is the
limit on the number of iterations through which the instruction may be pipelined. The default value
is 2.
selsched-max-insns-to-rename
increasing its speed. This sets the maximum value of a shared integer constant. The default value is
256.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing protection when
-fstack-protection is used.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.
The default is zero for -O0 and -O1, and 100 for -Os, -O2, and -O3.
prefetch-latency
Estimate on average number of instructions that are executed before prefetch finishes. The distance
prefetched ahead is proportional to this constant. Increasing this number may also lead to less
streams being prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches to enable
prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory references to enable
prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system. By default, this should always be 1,
which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However,
if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable
canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-
branch-ratio times the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy elimination
optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the
enhanced partial redundancy elimination optimization can run away, consuming all of the memory
available on the host machine. This parameter sets a limit on the length of the sets that are
function entry. The default maxmimum number of queries is 1000.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops than the number
given by this parameter, only at most the given number of the most frequently-executed loops form
regions for regional register allocation. The default value of the parameter is 100.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still
require excessive amounts of memory for huge functions. If the conflict table for a function could be
more than the size in MB given by this parameter, the register allocator instead uses a faster,
simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table.
The default value of the parameter is 2000.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop
invariants (see -O3). The number of available registers reserved for some other purposes is given by
this parameter. The default value of the parameter is 2, which is the minimal number of registers
needed by typical instructions. This value is the best found from numerous experiments.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-
time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop
invariant motion optimization performed on them. The default value of the parameter is 1000 for -O1
and 10000 for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dapendencies is expensive for very large loops. This parameter limits the number of
data references in loops that are considered for data dependence analysis. These large loops are no
handled by the optimizations using loop data dependencies. The default value is 1000.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any
function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that
function is retried without it, after removing all debug insns from the function. If the limit is
exceeded even without debug insns, var tracking analysis is completely disabled for the function.
Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries
to value expressions. This trades compilation time for more complete debug information. If this is
set too low, value expressions that are available and could be represented in debug information may
end up not being used; setting this higher may enable the compiler to find more complex debug
expressions, but compile time and memory use may grow. The default is 12.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved
exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-
overlapping) uids above it if the reserved range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative
size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
is bounded. The default value is 100 basic blocks.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine
each loop in the loop nest by a given number of iterations. The strip length can be changed using the
loop-block-tile-size parameter. The default value is 51 iterations.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's parameter in order to
propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values
and types it stores per one formal parameter of a function.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The number of partitions
should exceed the number of CPUs used for compilation. The default value is 32.
lto-minpartition
Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting
very small programs into too many partitions.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an
identifier. The default is 1000.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative to a statement's
original block to allow statement sinking of a statement. Larger numbers result in more aggressive
statement sinking. The default value is 75. A small positive adjustment is applied for statements
with memory operands as those are even more profitable so sink.
max-stores-to-sink
The maximum number of conditional stores paires that can be sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. The default is 2.
allow-load-data-races
Allow optimizers to introduce new data races on loads. Set to 1 to allow, otherwise to 0. This
option is enabled by default unless implicitly set by the -fmemory-model= option.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0. This
option is enabled by default unless implicitly set by the -fmemory-model= option.
allow-packed-load-data-races
Allow optimizers to introduce new data races on packed data loads. Set to 1 to allow, otherwise to 0.
This option is enabled by default unless implicitly set by the -fmemory-model= option.
allow-packed-store-data-races
Allow optimizers to introduce new data races on packed data stores. Set to 1 to allow, otherwise to
0. This option is enabled by default unless implicitly set by the -fmemory-model= option.
case-values-threshold
The smallest number of different values for which it is best to use a jump-table instead of a tree of
conditional branches. If the value is 0, use the default for the machine. The default is 0.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that will be considered when seeking a basis for a new
straight-line strength reduction candidate.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only
together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor.
If option contains commas, it is split into multiple options at the commas. However, many options are
modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and
-Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to
change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor
options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the
option and once for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an
integrated part of input tokenization and parsing. If this option is provided, the appropriate language
front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice,
once for preprocessing only and once for actual compilation of the preprocessed input. This option may be
useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform
additional processing of the program source between normal preprocessing and compilation.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase three
in a #define directive. In particular, the definition will be truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's
quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding
parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and
-include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc
has a different interpretation of a second non-option argument, so you must use -o to specify the output
file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment,
-Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on by default and have no options to
control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears
in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form
an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends.
Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs
-Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C
constructs that have no traditional C equivalent, and problematic constructs which should be avoided.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined.
Such identifiers are replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested
for existence at least once. The preprocessor will also warn if the macro has not been used at the time
it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned
about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as
unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by,
for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with
something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since
they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes
mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the
dependencies of the main source file. The preprocessor outputs one make rule containing the object file
name for that source file, a colon, and the names of all the included files, including those coming from
-include or -imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source
file with any suffix replaced with object file suffix and with any leading directory parts removed. If
there are many included files then the rule is split into several lines using \-newline. The rule has no
commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug
output with the dependency rules you should explicitly specify the dependency output file with -MF, or use
an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output
stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header files that
are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in
itself determine whether that header will appear in -MM dependency output. This is a slight change in
semantics from GCC versions 3.0 and earlier.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header
files are generated files and adds them to the dependency list without raising an error. The dependency
filename is taken directly from the "#include" directive without prepending any path. -MG also suppresses
preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each
to depend on nothing. These dummy rules work around errors make gives if you remove header files without
updating the Makefile to match.
This is typical output:
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise
it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file,
but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the
compilation process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to also list the files
from the precompiled header's dependencies. If not specified only the precompiled header would be listed
and not the files that were used to create it because those files are not consulted when a precompiled
header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma
GCC pch_preprocess "filename"" in the output to mark the place where the precompiled header was found, and
its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input
to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file
is available in a different location. The filename may be absolute or it may be relative to GCC's current
directory.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards
conformance or extensions; it merely selects which base syntax to expect. If you give none of these
options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some
other common extensions for C++ and assembly are also recognized. If cpp does not recognize the
extension, it will treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards
conformance level. This option has been removed, because it conflicts with the -l option.
The -ansi option is equivalent to -std=c90.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
"iso9899:2011"
"c11"
"c1x"
The revised ISO C standard, published in December 2011. Before publication, this was known as C1X.
"gnu90"
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"gnu11"
"gnu1x"
The 2011 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I- Split the include path. Any directories specified with -I options before -I- are searched only for
headers requested with "#include "file""; they are not searched for "#include <file>". If additional
directories are specified with -I options after the -I-, those directories are searched for all #include
directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search
directory for "#include "file"". This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the directories you have specified
with -I options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the other
standard directories. (This option is used when building the C++ library.)
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the
first directory searched for file is the preprocessor's working directory instead of the directory
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard system
directories have been exhausted. dir is treated as a system include directory. If dir begins with "=",
then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory,
you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the
include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where
-idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin targets,
where it applies to both header files and libraries). See the --sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system
directories. Mark it as a system directory, so that it gets the same special treatment as is applied to
the standard system directories. If dir begins with "=", then the "=" will be replaced by the sysroot
prefix; see --sysroot and -isysroot.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not searched for
"#include <file>", before all directories specified by -I and before the standard system directories. If
dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error".
Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In
addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as
"__LINE__", which are contextually dependent, are handled normally. This enables compilation of files
previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full
preprocessing of files previously preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings
or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option
is ignored. The default is 8.
-fdebug-cpp
This option is only useful for debugging GCC. When used with -E, dumps debugging information about
location maps. Every token in the output is preceded by the dump of the map its location belongs to. The
dump of the map holding the location of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the
current macro expansion stack when a compilation error occurs in a macro expansion. Using this option
makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the
level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0
of level de-activates this option just as if no -ftrack-macro-expansion was present on the command line.
Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode
all tokens resulting from the expansion of an argument of a function-like macro have the same location.
Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is
given no argument, the default parameter value is 2.
Note that -ftrack-macro-expansion=2 is activated by default.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8. charset
can be any encoding supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is UTF-32
or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any
encoding supported by the system's "iconv" library routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the source
character set used by GCC. If the locale does not specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by either the locale or this command line option.
Currently the command line option takes precedence if there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the compiler know the current
working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it's present in the preprocessed input, as the directory
emitted as the current working directory in some debugging information formats. This option is implicitly
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space.
Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so
are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define directives for all the macros defined during
the execution of the preprocessor, including predefined macros. This gives you a way of finding out
what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
D Like M except in two respects: it does not include the predefined macros, and it outputs both the
#define directives and the result of preprocessing. Both kinds of output go to the standard output
file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
U Like D except that only macros that are expanded, or whose definedness is tested in preprocessor
directives, are output; the output is delayed until the use or test of the macro; and #undef
directives are also output for macros tested but undefined at the time.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running
the preprocessor on something that is not C code, and will be sent to a program which might be confused by
the linemarkers.
-C Do not discard comments. All comments are passed through to the output file, except for comments in
processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as
tokens in their own right. For example, comments appearing at the start of what would be a directive line
have the effect of turning that line into an ordinary source line, since the first token on the line is no
longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments
contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a
macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently
commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form
of the include path.
-H Print the name of each header file used, in addition to other normal activities. Each name is indented to
show how deep in the #include stack it is. Precompiled header files are also printed, even if they are
found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes,
exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple options
at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler options
that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option
and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are
meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or
library. (Object files are distinguished from libraries by the linker according to the file contents.)
If linking is done, these object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used as
arguments.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a separate
argument is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes
libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
-lobjc
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used
normally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify are passed to the
linker, and options specifying linkage of the system libraries, such as "-static-libgcc" or
"-shared-libgcc", are ignored. The standard startup files are used normally, unless -nostartfiles is
used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through some other mechanism when this
option is specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only the
libraries you specify are passed to the linker, and options specifying linkage of the system libraries,
such as "-static-libgcc" or "-shared-libgcc", are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through some other mechanism when this
option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal
subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some
languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words,
when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that
you have no unresolved references to internal GCC library subroutines. (An example of such an internal
subroutine is __main, used to ensure C++ constructors are called.)
-pie
Produce a position independent executable on targets that support it. For predictable results, you must
also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you
specify this linker option.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to
add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of
"dlopen" or to allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On other
systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all
systems support this option. For predictable results, you must also specify the same set of options used
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or
a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to
do.
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always
linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or
a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into
shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking
with the shared version of libgcc, linking with the static version of libgcc by default. This allows
exceptions to propagate through such shared libraries, without incurring relocation costs at library load
time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using
the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the GCC driver automatically links against
libasan. If libasan is available as a shared library, and the -static option is not used, then this links
against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan
statically, without necessarily linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the GCC driver automatically links against
libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links
against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan
statically, without necessarily linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If
libstdc++ is available as a shared library, and the -static option is not used, then this links against
the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the
version of libstdc++ used by the program without going all the way to a fully static link. The
-static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking
other libraries statically.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved references
(unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this
option.
-T script
Use script as the linker script. This option is supported by most systems using the GNU linker. On some
targets, such as bare-board targets without an operating system, the -T option may be required when
linking to avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options that
GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the
option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker
-assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u
multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This can be
used to override a system header file, substituting your own version, since these directories are searched
before the system header file directories. However, you should not use this option to add directories
that contain vendor-supplied system header files (use -isystem for that). If you use more than one -I
option, the directories are scanned in left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I,
the -I option is ignored. The directory is still searched but as a system directory at its normal
position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers
and the ordering for the "include_next" directive are not inadvertently changed. If you really need to
change the search order for system directories, use the -nostdinc and/or -isystem options.
-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so.
This option is not meant to be used by the user, but only passed by the driver.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header files only for the
case of #include "file"; they are not searched for #include <file>, otherwise just like -I.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the
compiler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a
prefix for each program it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not
found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and
/usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by the -B refers to a directory, and if necessary it adds
a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the
compiler translates these options into -L options for the linker. They also apply to includes files in
the preprocessor, because the compiler translates these options into -isystem options for the
preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not
found there, the two standard prefixes above are tried, and that is all. The file is left out of the link
right.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler normally
searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and
dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but
the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker
does not support this option, the header file aspect of --sysroot still works, but the library aspect does
not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other
options used, so that headers may for example be found in dir/suffix/usr/include instead of
dir/usr/include. This option disables the addition of such a suffix.
-I- This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove
the -I-. Any directories you specify with -I options before the -I- option are searched only for the case
of #include "file"; they are not searched for #include <file>.
If additional directories are specified with -I options after the -I-, these directories are searched for
all #include directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input file came
from) as the first search directory for #include "file". There is no way to override this effect of -I-.
With -I. you can specify searching the directory that is current when the compiler is invoked. That is
not exactly the same as what the preprocessor does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and -nostdinc
are independent.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or
machine-gcc-version to run a version other than the one that was installed last.
Hardware Models and Configurations
Each target machine types can have its own special options, starting with -m, to choose among various hardware
models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed
version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with
other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general registers.
of each other. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions about addresses and sizes of sections.
Pointers are 64 bits. Programs can be statically linked only.
-mstrict-align
Do not assume that unaligned memory references will be handled by the system.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behaviour is the default.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is
the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This will involve
inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate
instructions.
-march=name
Specify the name of the target architecture, optionally suffixed by one or more feature modifiers. This
option has the form -march=arch{+[no]feature}*, where the only value for arch is armv8-a. The possible
values for feature are documented in the sub-section below.
Where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This
option can be used in conjunction with or instead of the -mcpu= option.
-mcpu=name
Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This
option has the form -mcpu=cpu{+[no]feature}*, where the possible values for cpu are generic, large. The
possible values for feature are documented in the sub-section below.
Where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses this name to determine what kind of instructions it can emit when generating assembly code.
-mtune=name
Specify the name of the processor to tune the performance for. The code will be tuned as if the target
processor were of the type specified in this option, but still using instructions compatible with the
target processor specified by a -mcpu= option. This option cannot be suffixed by feature modifiers.
-march and -mcpu feature modifiers
Feature modifiers used with -march and -mcpu can be one the following:
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that
lack these registers.
-mprefer-short-insn-regs
Preferrentially allocate registers that allow short instruction generation. This can result in increased
instruction count, so this may either reduce or increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is
faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two
different small numbers are compared such that their difference is calculated as zero. The default is
-msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the
eight bytes in the range "sp+0...sp+7" can be used by leaf functions without stack allocation. Values
other than 8 or 16 are untested and unlikely to work. Note also that this option changes the ABI;
compiling a program with a different stack offset than the libraries have been compiled with generally
does not work. This option can be useful if you want to evaluate if a different stack offset would give
you better code, but to actually use a different stack offset to build working programs, it is recommended
to configure the toolchain with the appropriate --with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating. The default is
-mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" /
"bl" instructions, and therefore load the function address into a register before performing a (otherwise
direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl"
instructions, so use these instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for
which -mlong-calls semantics are in effect.
-mfp-mode=mode
choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with truncating (i.e. round towards zero)
rounding mode. That includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer
multiply-and-accumulate.
The default is -mfp-mode=caller
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment
addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and
-mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred
SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this
is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD
vector modes in places that affect size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this can give better register
allocation, but so far the reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative constants and certain
bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed
register, and none, which means that no register is used for this purpose. The default is -m1reg-none.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and
iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if
this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with
this option causes the stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations.
softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-
float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific
calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs
are not link-compatible; you must compile your entire program with the same ABI, and link with a
compatible set of libraries.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard
configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a little-
endian processor.
-mwords-little-endian
This option only applies when generating code for big-endian processors. Generate code for a little-
endian word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this
option should only be used if you require compatibility with code for big-endian ARM processors generated
by versions of the compiler prior to 2.8. This option is now deprecated.
-march=name
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. This option can be used in conjunction with or
instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5,
armv5t, armv5e, armv5te, armv6, armv6j, armv6t2, armv6z, armv6zk, armv6-m, armv7, armv7-a, armv7-r,
armv7-m, armv7e-m armv8-a, iwmmxt, iwmmxt2, ep9312.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present, this
feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
-mtune=name
This option specifies the name of the target ARM processor for which GCC should tune the performance of
the code. For some ARM implementations better performance can be obtained by using this option.
Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d,
arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm720, arm7500, arm7500fe,
arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110,
arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore,
mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, cortex-a5, cortex-a7, cortex-a8,
cortex-a9, cortex-a15, cortex-r4, cortex-r4f, cortex-r5, cortex-m4, cortex-m3, cortex-m1, cortex-m0,
cortex-m0plus, marvell-pj4, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626,
fa726te.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within
architecture arch. The aim is to generate code that run well on the current most popular processors,
Permissible names for this option are the same as those for -mtune.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune
for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature
is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware emulation) is available on the target.
Permissible names are: vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon,
neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fp-armv8, neon-fp-armv8, and crypto-neon-fp-armv8.
If -msoft-float is specified this specifies the format of floating-point values.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-
point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is
also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for
floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON
instructions may lead to a loss of precision.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none, ieee,
and alternative; the default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this
option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the
COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI
supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the
program. Different values are potentially incompatible. Code compiled with one value cannot necessarily
expect to work with code or libraries compiled with another value, if they exchange information using
structures or unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It is executed if the
function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register
and then performing a subroutine call on this register. This switch is needed if the target function lies
outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that
static functions, functions that have the short-call attribute, functions that are inside the scope of a
#pragma no_long_calls directive, and functions whose definitions have already been compiled within the
current compilation unit are not turned into long calls. The exceptions to this rule are that weak
function definitions, functions with the long-call attribute or the section attribute, and functions that
are within the scope of a #pragma long_calls directive are always turned into long calls.
suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI
based or stack-checking is enabled, otherwise the default is R10.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The
generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace
function then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a
function name embedded immediately preceding this location and has length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb states. The default for most configurations
is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the
--with-mode=state configure option.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf
functions. (A leaf function is one that does not call any other functions.) The default is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions.
(A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which
switches to Thumb mode before executing the rest of the function. This allows these functions to be
called from non-interworking code. This option is not valid in AAPCS configurations because interworking
is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of
whether the target code has been compiled for interworking or not. There is a small overhead in the cost
of executing a function pointer if this option is enabled. This option is not valid in AAPCS
configurations because interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The valid models are soft, which generates
calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the
arm6k architecture), and auto, which uses the best available method for the selected processor. The
default setting is auto.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and
base registers are used. This option avoids generating these instructions. This option is enabled by
default when -mcpu=cortex-m3 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32-
bit aligned. By default unaligned access is disabled for all pre-ARMv6 and all ARMv6-M architectures, and
enabled for all other architectures. If unaligned access is not enabled then words in packed data
structures will be accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" will be set in the generated object file to either true or
false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor
symbol "__ARM_FEATURE_UNALIGNED" will also be defined.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is@tie{}"avr2".
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "attiny22", "attiny26",
"at90c8534", "at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433",
"at90s4434", "at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8@tie{}KiB of program memory and with the "MOVW" instruction. mcu@tie{}=
"ata5272", "ata6289", "attiny13", "attiny13a", "attiny2313", "attiny2313a", "attiny24", "attiny24a",
"attiny25", "attiny261", "attiny261a", "attiny43u", "attiny4313", "attiny44", "attiny44a", "attiny45",
"attiny461", "attiny461a", "attiny48", "attiny84", "attiny84a", "attiny85", "attiny861", "attiny861a",
"attiny87", "attiny88", "at86rf401".
"avr3"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory. mcu@tie{}= "at43usb355",
"at76c711".
"avr31"
"Classic" devices with 128@tie{}KiB of program memory. mcu@tie{}= "atmega103", "at43usb320".
"avr35"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory and with the "MOVW"
instruction. mcu@tie{}= "ata5505", "atmega16u2", "atmega32u2", "atmega8u2", "attiny1634",
"attiny167", "at90usb162", "at90usb82".
"avr4"
"Enhanced" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "ata6285", "ata6286",
"atmega48", "atmega48a", "atmega48p", "atmega48pa", "atmega8", "atmega8a", "atmega8hva", "atmega8515",
"atmega328p", "atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3290", "atmega3290a",
"atmega3290p", "atmega3290pa", "atmega406", "atmega48hvf", "atmega64", "atmega64a", "atmega64c1",
"atmega64hve", "atmega64m1", "atmega64rfa2", "atmega64rfr2", "atmega640", "atmega644", "atmega644a",
"atmega644p", "atmega644pa", "atmega645", "atmega645a", "atmega645p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega649", "atmega649a", "atmega649p", "atmega6490", "atmega6490a", "atmega6490p",
"at90can32", "at90can64", "at90pwm161", "at90pwm216", "at90pwm316", "at90scr100", "at90usb646",
"at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128@tie{}KiB of program memory. mcu@tie{}= "atmega128", "atmega128a",
"atmega128rfa1", "atmega1280", "atmega1281", "atmega1284", "atmega1284p", "at90can128", "at90usb1286",
"at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128@tie{}KiB of program memory. mcu@tie{}=
"atmega2560", "atmega2561".
"avrxmega2"
"XMEGA" devices with more than 8@tie{}KiB and up to 64@tie{}KiB of program memory. mcu@tie{}=
"atmxt112sl", "atmxt224", "atmxt224e", "atmxt336s", "atxmega16a4", "atxmega16a4u", "atxmega16c4",
"atxmega16d4", "atxmega16x1", "atxmega32a4", "atxmega32a4u", "atxmega32c4", "atxmega32d4",
"atxmega32e5", "atxmega32x1".
"avrxmega4"
"XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory. mcu@tie{}=
"atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3",
"atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory and more than
64@tie{}KiB of RAM. mcu@tie{}= "atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128@tie{}KiB of program memory. mcu@tie{}= "atmxt540s",
"atmxt540sreva", "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3",
"atxmega128d3", "atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3",
"atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3",
"atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM.
mcu@tie{}= "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler only. mcu@tie{}=
"attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function
arguments once in function prologue/epilogue. Without this option, outgoing arguments are pushed before
calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space
might lead to smaller executables because arguments need not to be removed from the stack after such a
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1 byte, an "int" is 1
byte, a "long" is 2 bytes, and "long long" is 4 bytes. Please note that this option does not conform to
the C standards, but it results in smaller code size.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if
applicable. Setting "-mrelax" just adds the "--relax" option to the linker command line when the linker
is called.
Jump relaxing is performed by the linker because jump offsets are not known before code is located.
Therefore, the assembler code generated by the compiler is the same, but the instructions in the
executable may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and linker stubs below.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is
zero. In general, you don't need to set this option by hand.
This option is used internally by the compiler to select and build multilibs for architectures "avr2" and
"avr25". These architectures mix devices with and without "SPH". For any setting other than "-mmcu=avr2"
or "-mmcu=avr25" the compiler driver will add or remove this option from the compiler proper's command
line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no
"SPH" register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that "X" is only used in indirect,
post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z" which then is emulated by
additional instructions. For example, loading a value with "X+const" addressing with a small non-negative
"const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8@tie{}bits of the stack pointer.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the resulting address space is not
contained in the incoming address space.
"EIND" and Devices with more than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The address of a function or label is represented as
word address so that indirect jumps and calls can target any code address in the range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than 128@tie{}Ki bytes of program memory space,
particular, "EIND" is not saved/restored in function or interrupt service routine prologue/epilogue.
· For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads
sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a
direct jump to the desired address.
· Linker relaxation must be turned on so that the linker will generate the stubs correctly an all
situaltion. See the compiler option "-mrelax" and the linler option "--relax". There are corner cases
where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error
message.
· The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a setup
with "EIND != 0", a custom linker script has to be used in order to place the sections whose name start
with ".trampolines" into the segment where "EIND" points to.
· The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from libgcc
and AVR-LibC. For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
· It is legitimate for user-specific startup code to set up "EIND" early, for example by means of
initialization code located in section ".init3". Such code runs prior to general startup code that
initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets "EIND"
to the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
· Stubs are generated automatically by the linker if the following two conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
· The compiler emits such "gs" modifiers for code labels in the following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
Instead, a stub has to be set up, i.e. the function has to be called through a symbol ("func_4" in the
example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with "-Wl,--defsym,func_4=0x4". Alternatively, "func_4" can be defined in
the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64@tie{}KiB range that can be accessed with 16-bit pointers.
To access memory locations outside this 64@tie{}KiB range, the contentent of a "RAMP" register is used as high
part of the address: The "X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ"
special function register, respectively, to get a wide address. Similarly, "RAMPD" is used together with
direct addressing.
· The startup code initializes the "RAMP" special function registers with zero.
· If a AVR Named Address Spaces,named address space other than generic or "__flash" is used, then "RAMPZ" is
set as needed before the operation.
· If the device supports RAM larger than 64@tie{KiB} and the compiler needs to change "RAMPZ" to accomplish
an operation, "RAMPZ" is reset to zero after the operation.
· If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that SFR and
initializes it with zero in case the ISR code might (implicitly) use it.
· RAM larger than 64@tie{KiB} is not supported by GCC for AVR targets. If you use inline assembler to read
from locations outside the 16-bit address range and change one of the "RAMP" registers, you must reset it
to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features.
Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the
"-mmcu=" command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies the architecture and depends on the
"-mmcu=mcu" option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6, 102, 104, 105, 106, 107
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6", "avrxmega2",
"avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case for devices with at least 16@tie{}KiB
of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than
128@tie{}KiB of program memory. This also means that the program counter (PC) is 3@tie{}bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2@tie{}bytes wide. This is the case for devices with up to 128@tie{}KiB of
program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The
definition of these macros is affected by "-mtiny-stack".
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack
pointer, respectively. The definition of these macros is affected by "-mmcu=" and in the cases of
"-mmcu=avr2" and "-mmcu=avr25" also by "-msp8".
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the "-mno-interrupts" command line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516,
bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539,
bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds
available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are
enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The
"__SILICON_REVISION__" macro is defined to two hexadecimal digits representing the major and minor numbers
in the silicon revision. If sirevision is none, the "__SILICON_REVISION__" is not defined. If sirevision
is any, the "__SILICON_REVISION__" is defined to be 0xffff. If this optional sirevision is not used, GCC
assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes
the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator. This causes the simulator BSP provided by
libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options,
such as -mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set
up and restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all functions, which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not contain speculative loads after jump
instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions
too soon after conditional branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a
conditional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into
the low 64k of memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
Generate code that supports shared libraries via the library ID method, but assumes that this library or
executable won't link against any other ID shared libraries. That allows the compiler to use faster code
for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared libraries. Slower code is
generated for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a value of
0 generates more compact code; specifying other values forces the allocation of that number to the current
library but is no more space- or time-efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text
segment. This allows for execute in place in an environment without virtual memory management by
eliminating relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register
and then performing a subroutine call on this register. This switch is needed if the target function lies
outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note
these switches have no effect on how the compiler generates code to handle function calls via function
pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's
rules for checking inputs against Not-a-Number (NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has
no effect without -mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin processors. This option causes proper start files
and link scripts supporting multicore to be used, and defines the macro "__BFIN_MULTICORE". It can only
be used with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming
model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this
model, the main function of Core B should be named as "coreb_main".
If this option is not used, the single-core application programming model is used.
-mcorea
Build a standalone application for Core A of BF561 when using the one-application-per-core programming
model. Proper start files and link scripts are used to support Core A, and the macro "__BFIN_COREA" is
defined. This option can only be used in conjunction with -mmulticore.
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux
targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x,
c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the .neardata section, which is pointed to by register "B14". Put
small uninitialized global and static data in the .bss section, which is adjacent to the .neardata
section. Put small read-only data into the .rodata section. The corresponding sections used for large
pieces of data are .fardata, .far and .const.
-msdata=all
Put all data, not just small objects, into the sections reserved for small data, and use addressing
relative to the "B14" register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access all data. Put
all initialized global and static data in the .fardata section, and all uninitialized data in the .far
section. Put all constant data into the .const section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where
the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the set
of available instructions. The choices for architecture-type are the same as for -march=architecture-
type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
Do not use condition-code results from previous instruction; always emit compare and test instructions
before use of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and
constants to be aligned for the maximum single data access size for the chosen CPU model. The default is
to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack frame, writable
data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are
omitted and no return instructions or return sequences are generated in the code. Use this option only
together with visual inspection of the compiled code: no warnings or errors are generated when call-saved
registers must be saved, or storage for local variables needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions
from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The
default is -mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-elf, arranges to link with input-output functions from a
simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at
0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
Generates "sbit"/"cbit" instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or medium. medium is default. However, far is
not valid with -mcr16c, as the CR16C architecture does not support the far data model.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture
that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are
used; it does so by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the
ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override
this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits
instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit
instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an
error if asked to create a shared library with a less restrictive subtype than its input files (for instance,
trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the
executable the most restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header files.
These directories are interleaved with those specified by -I options and are scanned in a left-to-right
order.
A framework directory is a directory with frameworks in it. A framework is a directory with a Headers
and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework
is the name of this directory excluding the .framework. Headers associated with the framework are found
in one of those two directories, with Headers being searched first. A subframework is a framework
directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear
in a header of a framework that contains the subframework, or in a sibling subframework header. Two
subframeworks are siblings if they occur in the same framework. A subframework should not have the same
name as a framework; a warning is issued if this is violated. Currently a subframework cannot have
subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can
be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like "#include
<Framework/header.h>", where Framework denotes the name of the framework and header.h is found in the
PrivateHeaders or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory. The main difference between this
-iframework and -F is that with -iframework the compiler does not warn about constructs contained within
header files found via dir. This option is valid only for the C family of languages.
-gused
Emit debugging information for symbols that are used. For stabs debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for
Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Using this switch may require recompiling all other modules in a program,
including system libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load ".o"
files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards
compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when
the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will load the build output file being linked. See man ld(1) for
more information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead of an executable when linking, using the
Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march
option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When
-msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless
they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to
call such emulations routines, these routines issue floating-point operations. If you are compiling for
an Alpha without floating-point operations, you must ensure that the library is built so as not to call
them.
Note that Alpha implementations without floating-point operations are required to have floating-point
registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies
-msoft-float. If the floating-point register set is not used, floating-point operands are passed in
compilation. The resulting code is less efficient but is able to correctly support denormalized numbers
and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this
option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this
option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may
execute significantly slower than the code generated by default. Since there is very little code that
depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this
option -fptm trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot be
disabled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like u, but the instructions are marked to be safe for software completion (see Alpha architecture
manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The
rounding-mode can be one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or
towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture
reference manual) controls the rounding mode in effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round
towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means without software assistance it
is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC
can generate code that can assist operating system trap handlers in determining the exact location that
caused a floating-point trap. Depending on the requirements of an application, different levels of
precisions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which
program caused a floating-point exception.
f Function precision. The trap handler can determine the function that caused a floating-point
exception.
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller
constants in two or three instructions. If it cannot, it outputs the constant as a literal and generates
code to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants using code, even if it takes more
instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself a shared library, it must
relocate itself in memory before it can find the variables and constants in its own data segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The
default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of
the CPU on which GCC was built if none is specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and
double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of
these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a
new syntax that allows the compiler to explicitly mark which relocations should apply to which
instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the
assembler when it is built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When
-msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and
".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size
of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that
require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in
the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library)
fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can
assume that all local symbols share the same $gp value, and thus reduce the number of instructions
required for a function call from 4 to 1.
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not
changed.
Native toolchains also support the value native, which selects the best architecture option for the host
processor. -mtune=native has no effect if GCC does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the application.
This number is highly dependent on the memory access patterns used by the application and the size of the
external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the
Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that
L3 is only valid for EV5.
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any
PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are
within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32
bits. With a bfin-elf target, this option implies -msim.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections.
It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table
smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which
may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so
it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and
-fpic without -mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This
option is enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the functions
to be placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only
has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to
existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by
default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500,
fr450, fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this
option causes the preprocessor macro "__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and
-fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or
-ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mexr
Extended registers are stored on stack before execution of function with monitor attribute. Default option
is -mexr. This option is valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before execution of function with monitor attribute. Default
option is -mno-exr. This option is valid only for H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and
H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte
boundaries. This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1
for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine
the proper architecture option for your machine. Code compiled for lower numbered architectures runs on
higher numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
that perform lazy context switching of floating-point registers. If you use this option and attempt to
perform floating-point operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when
compiling MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect
calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs
faster indirect calls.
This option does not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator cannot use. This is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is
equivalent to the +k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are
700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the
proper scheduling option for your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also
triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking
some programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library that
comes with GCC, with -msoft-float in order for this to work.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with
the HP linker. This option does not affect which ld is called; it only changes what parameters are passed
to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search
path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc
-print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker
generated stubs. The default is to generate long calls only when the distance from the call site to the
beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the
branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the
PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when
using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance. However, it may be useful in
large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code
being generated. The impact on systems that support long absolute calls, and long pic symbol-difference
or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems
in pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for
unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and
later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX
10.10 though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for
"XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional
predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also
affects the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the
variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX
10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive
version of libdld.sl. Thus, when the -static option is specified, special link options are needed to
resolve this dependency.
-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes
the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not
run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-
type.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation time by determining the processor type of the
compiling machine. Using -march=native enables all instruction subsets supported by the local machine
(hence the result might not run on different machines). Using -mtune=native produces code optimized
for the local machine under the constraints of the selected instruction set.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
pentiumpro
Intel Pentium Pro CPU.
i686
When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family
chips. When used with -mtune, it has the same meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set
support. Used by Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction
core-avx-i
Intel Core CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AES, PCLMUL,
FSGSBASE, RDRND and F16C instruction set support.
core-avx2
Intel Core CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2,
AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
atom
Intel Atom CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron,
Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and
64-bit instruction set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE,
SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX,
XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C,
FMA, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.)
BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit
instruction set extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented
for this chip.)
geode
AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of
available instructions. While picking a specific cpu-type schedules things appropriately for that
particular chip, the compiler does not generate any code that cannot run on the default machine type
unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then
-mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In addition, -mtune supports an extra choice for
cpu-type:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which
your code will run, then you should use the corresponding -mtune or -march option instead of
-mtune=generic. But, if you do not know exactly what CPU users of your application will have, then
you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change.
Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will
change to reflect the processors that are most common at the time that version of GCC is released.
There is no -march=generic option because -march indicates the instruction set the compiler can use,
and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates
the processor (or, in this case, collection of processors) for which the code is optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The choices for unit are:
387 Use the standard 387 floating-point coprocessor present on the majority of chips and emulated
otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed
in 80-bit precision instead of the precision specified by the type, resulting in slightly different
results compared to most of other chips. See -ffloat-store for more detailed description.
This is the default choice for i386 compiler.
bits.
This is the default choice for the x86-64 compiler.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This effectively doubles the amount of available
registers, and on chips with separate execution units for 387 and SSE the execution resources too.
Use this option with care, as it is still experimental, because the GCC register allocator does not
model separate functional units well, resulting in unstable performance.
-masm=dialect
Output assembly instructions using selected dialect. Supported choices are intel or att (the default).
Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case
where the result of a comparison is unordered.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C
compiler are used, but this can't be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for cross-compilation.
On machines where a function returns floating-point results in the 80387 register stack, some floating-
point opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU register,
even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this
option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD.
This option is overridden when -march indicates that the target CPU always has an FPU and so the
instruction does not need emulation. These instructions are not generated unless you also use the
-funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a
one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat
faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double"
is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a
"long double".
Warning: if you override the default value for your target ABI, this changes the size of structures and
arrays containing "long double" variables, as well as modifying the function calling convention for
functions taking "long double". Hence they are not binary-compatible with code compiled without that
switch.
-mlong-double-64
-mlong-double-80
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type
equivalent to the "double" type. This is the default for Bionic C library.
Warning: if you override the default value for your target ABI, this changes the size of structures and
arrays containing "long double" variables, as well as modifying the function calling convention for
functions taking "long double". Hence they are not binary-compatible with code compiled without that
switch.
-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger than threshold are placed in the large data
section. This value must be the same across all objects linked into the binary, and defaults to 65535.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "ret num" instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function
attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it
if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally,
extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to pass
arguments, and at most 3 registers can be used. You can control this behavior for a specific function by
using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value,
including any libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can control
this behavior for a specific function by using the function attribute sseregparm.
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of
results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the
significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the
significands of results of floating-point operations to 64 bits (extended double precision), which is the
default. When this option is used, floating-point operations in higher precisions are not available to
the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs
by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-
point operations are enabled by default; routines in such libraries could suffer significant loss of
accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the
precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the Intel x86, the -mstackrealign option generates an alternate prologue
and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep
4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See
also the attribute "force_align_arg_pointer", applicable to individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with SSE extensions disabled,
-mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since
x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled
environment where stack space is important limitation. This option will lead to wrong code when functions
compiled with 16 byte stack alignment (such as functions from a standard library) are called with
misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In
addition, variable arguments will be handled incorrectly for 16 byte aligned objects (including x87 long
double and __int128), leading to wrong results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup
modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is
not specified, the one specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values should be aligned to an 8-byte boundary (see
-malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD
Extension (SSE) data type "__m128" may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that
required by any value stored on the stack. Further, every function must be generated such that it keeps
the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a
function compiled with a lower preferred stack boundary most likely misaligns the stack. It is
recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is
sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce
the preferred alignment to -mpreferred-stack-boundary=2.
-msse4.2
-mno-sse4.2
-msse4
-mno-sse4
-mavx
-mno-avx
-mavx2
-mno-avx2
-maes
-mno-aes
-mpclmul
-mno-pclmul
-mfsgsbase
-mno-fsgsbase
-mrdrnd
-mno-rdrnd
-mf16c
-mno-f16c
-mfma
-mno-fma
-msse4a
-mno-sse4a
-mfma4
-mno-fma4
-mxop
-mno-xop
-mlwp
-mno-lwp
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
-mbmi
-mbmi2
-mno-bmi
-mno-bmi2
-mlzcnt
-mno-lzcnt
-mrtm
-mpku
-mno-pku
-mtbm
-mno-tbm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, BMI, BMI2, LZCNT, RTM, PKU or
3DNow! extended instruction sets. These extensions are also available as built-in functions: see X86
Built-in Functions, for details of the functions enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions),
see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX
be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used.
This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld
configure option. Generation of "cld" instructions can be suppressed with the -mno-cld compiler option in
this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of the
function to minimize the AVX to SSE transition penalty as well as remove unnecessary "zeroupper"
intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-
vectorizer.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions. "CMPXCHG16B" allows for atomic operations
on 128-bit double quadword (or oword) data types. This is useful for high-resolution counters that can be
updated by multiple processors (or cores). This instruction is generated as part of atomic built-in
functions: see __sync Builtins or __atomic Builtins for details.
-msahf
This option enables generation of "SAHF" instructions in 64-bit code. Early Intel Pentium 4 CPUs with
Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the "LAHF" and
"SAHF" instructions which were supported by AMD64. These are load and store instructions, respectively,
for certain status flags. In 64-bit mode, the "SAHF" instruction is used to optimize "fmod", "drem", and
"remainder" built-in functions; see Other Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to implement "__builtin_bswap32" and
"__builtin_bswap64".
-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi",
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to generate the "crc32" machine instruction.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS" and
"RSQRTPS") with an additional Newton-Raphson step to increase precision instead of "DIVSS" and "SQRTSS"
(and their vectorized variants) for single-precision floating-point arguments. These instructions are
generated only when -funsafe-math-optimizations is enabled together with -finite-math-only and
-fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the
non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the
inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or "RSQRTPS") already with -ffast-math (or
the above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-
float division and vectorized "sqrtf(x)" already with -ffast-math (or the above option combination), and
doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of
options, which may be preceded by a ! to invert the option:
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for
type are svml for the Intel short vector math library and acml for the AMD math core library. To use this
option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-
compatible library must be specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldLog102", "vmldPow2", "vmldTanh2",
"vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2",
"vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsLog104",
"vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4",
"vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding
function type when -mveclibabi=svml is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log",
"__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f",
"__vrs4_log10f" and "__vrs4_powf" for the corresponding function type when -mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention. Permissible values are sysv for the ABI used on
GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when
targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for a
specific function by using the function attribute ms_abi/sysv_abi.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative
default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be
satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as
method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments is computed in the function
prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and
reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable
increase in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling
must compile and link all code with the -mthreads option. When compiling, -mthreads defines "-D_MT"; when
linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-
handling data.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular algorithm to use for inlining string
operations. The allowed values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set
up, and restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit,
%fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends
on the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on
by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86
architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit
integer divide. This option generates a run-time check. If both dividend and divisor are within range of
0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a
128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt
handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag
-mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of
the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This
model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small
symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large
data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no assumptions about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the
default address mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the
default address mode for 32-bit and x32 environments.
i386 and x86-64 Windows Options
These additional options are available for Microsoft Windows targets:
-mconsole
This option specifies that a console application is to be generated, by instructing the linker to set the
PE header subsystem type required for console applications. This option is available for Cygwin and MinGW
targets and is enabled by default on those targets.
-mdll
This option is available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link
library---is to be generated, enabling the selection of the required runtime startup object and entry
point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the "dllimport" attribute should
be ignored.
-mthread
This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be
predefined, and chooses Unicode-capable runtime startup code.
nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as
there the User32 API, which is used to set executable privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only
section is put into .data section. This is a necessary for older runtimes not supporting modification of
.rdata sections for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file
format that permits the correct alignment of COMMON variables should be used when generating code. It is
enabled by default if GCC detects that the target assembler found during configuration supports the
feature.
See also under i386 and x86-64 Options for standard options.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent code,
and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler
output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working around
optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel
code.
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default
is to use these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF 2 line number debugging info. This may be useful when
not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop
bit. This can improve instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator cannot use. This is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced,
itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX
specific flags.
-mno-sched-br-data-spec
(i.e. before reload). This results in generation of the "ld.s" instructions and the corresponding check
instructions "chk.s". The default is 'disable'.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads
before reload. This is effective only with -msched-br-data-spec enabled. The default is 'enable'.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads
after reload. This is effective only with -msched-ar-data-spec enabled. The default is 'enable'.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative
loads. This is effective only with -msched-control-spec enabled. The default is 'enable'.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule only if there are no other choices at
the moment. This makes the use of the data speculation much more conservative. The default is 'disable'.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule only if there are no other choices at
the moment. This makes the use of the control speculation much more conservative. The default is
'disable'.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during computation of the instructions priorities.
This makes the use of the speculation a bit more conservative. The default is 'disable'.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same
instruction group. This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory
insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank
conflicts. The default value is 1.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the
M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to be
linked in which supports, for example, file I/O. You must not use this option when generating programs
that will run on real hardware; you must provide your own runtime library for whatever I/O functions are
needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-
registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into
available registers, and the performance penalty of using memory instead of registers. Note that all
modules in a program must be compiled with the same value for this option. Because of that, you must not
use this option with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3"
instructions to load their addresses), and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3"
them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to reference
them.
-G num
Put global and static objects less than or equal to num bytes into the small data or BSS sections instead
of the normal data or BSS sections. The default value of num is 8. The -msdata option must be set to one
of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or
may not work; if it doesn't the linker gives an error message---incorrect code is not generated.
-mdebug
Makes the M32R-specific code in the compiler display some statistics that might help in debugging
programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then
the opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and
15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is
_flush_cache, but a function call is only used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which
architecture was selected when the compiler was configured; the defaults for the most common choices are given
below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch
for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures
are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus,
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march
are rejected.
GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines
__mcf_family_family, where the value of family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The
M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire
microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040
targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the
same tuning decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines
mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of
architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every
architecture in the range.
GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one
of the arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.
It is equivalent to -march=68000.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.
It is equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.
It is equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the
68040. Use this option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems.
It is equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on
the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems.
It is equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332,
68333, 68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for
520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent
-mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the
equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent
-mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent
-mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-
point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that
option.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code that can run
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices
that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire
targets.
-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the default for 68000,
68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without
-mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the
default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For
example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
GCC defines the macro __mcfhwdiv__ when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack
are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use
a configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "rtd" instruction, which pops their arguments while returning. This saves one instruction
in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you
need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally,
extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not
by the 68000 or 5200.
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is the default.
present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is
not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text
segment. This allows for execute-in-place in an environment without virtual memory management. This
option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute-in-place
and shared libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a value of
0 generates more compact code; specifying other values forces the allocation of that number to the current
library, but is no more space- or time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the GOT has more than
8192 entries. This code is larger and slower than code generated without this option. On M680x0
processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient,
it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error
such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs.
However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of
a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT
entries. If you have such a linker, you should only need to use -mxgot when compiling a single object
file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker
command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values can increase the speed of
programs that contain functions that need a large amount of stack space, but they can also trigger a
segmentation fault if the stack is extended too much. The default value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute difference between two registers.
-mall-opts
Enables all the optional instructions---average, multiply, divide, bit operations, leading zero, absolute
difference, min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the average of two registers.
-mbased=n
Variables of size n bytes or smaller are placed in the ".based" section by default. Based variables use
the $tp register as a base register, and there is a 128-byte limit to the ".based" section.
-mbitops
Enables the bit operation instructions---bit test ("btstm"), set ("bsetm"), clear ("bclrm"), invert
("bnotm"), and test-and-set ("tas").
-mc=name
Selects which section constant data is placed in. name may be "tiny", "near", or "far".
-mclip
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near" section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io" attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by "-mall-opts".
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note that there is a 65536-byte limit to this
section. Accesses to these variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the compiler does not currently generate these itself, but
this option is included for compatibility with other tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based runtime.
Variables that are n bytes or smaller are allocated to the ".tiny" section. These variables use the $gp
base register. The default for this option is 4, but note that there's a 65536-byte limit to the ".tiny"
section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where
X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a,
v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub.
This uses startup file crt1.o and sets the start address of the program to 0x800.
bootstrap
for applications that are loaded using a bootloader. This model uses startup file crt2.o which does
not contain a processor reset vector handler. This is suitable for transferring control on a processor
reset to the bootloader rather than the application.
novectors
for applications that do not require any of the MicroBlaze vectors. This option may be useful for
applications running within a monitoring application. This model uses crt3.o as a startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
-march=arch
Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular
processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips64 and mips64r2. The
processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1,
24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc,
1004kf2_1, 1004kf1_1, loongson2e, loongson2f, loongson3a, m4k, octeon, octeon+, octeon2, orion, r2000,
r3000, r3900, r4000, r4400, r4600, r4650, r4700, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000,
r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp. The
special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for
32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects the best architecture option
for the host processor. -march=native has no effect if GCC does not recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional,
and vr may be written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the
form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2
refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons,
nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name
of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized
value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro
_MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full
prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved
architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is
given.
-mtune=arch
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use
of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16"
attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed
MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.
-minterlink-mips16
-mno-interlink-mips16
Require (do not require) that non-MIPS16 code be link-compatible with MIPS16 code.
For example, non-MIPS16 code cannot jump directly to MIPS16 code; it must either use a call or an indirect
jump. -minterlink-mips16 therefore disables direct jumps unless GCC knows that the target of the jump is
not MIPS16.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select
a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html>.
default for SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can therefore be linked into
shared libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and
-fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for
locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to
locally-defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU
linker. However, the option does not affect the ABI of the final executable; it only affects the ABI of
relocatable objects. Using -mno-shared generally makes executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option
only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt
otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient,
it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error
such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although
the code is also less efficient, since it takes three instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use
-mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
configurations, where it may select a special set of libraries that lack all floating-point support
(including, for example, the floating-point "printf" formats). If code compiled with "-mno-float"
accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the default.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When
neither option is specified, GCC uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when
compiling for nonstandard ISAs. You can make either option the default by configuring GCC with
--with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the
installation documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev
to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating
64-bit code and requires hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option
is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and
faster references to symbolic addresses.
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num
bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata
is the default for all configurations.
If the linker complains that an application is using too much small data, you might want to try rebuilding
the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries
with -mno-local-sdata, so that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small data section if the size of that data is
within the -G limit. -mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is
no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is
defined by another module, you must either compile that module with a high-enough -G setting or attach a
"section" attribute to Var's definition. If Var is common, you must link the application with a high-
enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G
option. However, you may wish to build a library that supports several different small data limits. You
can do this by compiling the library with the highest supported -G setting and additionally using
-mno-extern-sdata to stop the library from making assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G,
-mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if
the code is part of a library that might be used in a boot monitor, programs that call boot monitor
routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled
with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data section if
-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other instructions must not
do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit
set. It is also useful on processors that can be configured to have a dual instruction/data SRAM
interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option can be useful on targets that are
configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not
automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative,
selected by -mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation
operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break instruction.
Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of
the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use
-mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force
the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using
--with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which
allows GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient
but requires the caller and callee to be in the same 256 megabyte segment.
infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some
circumstances. On other processors the result is numerically identical to the equivalent computation
using separate multiply, add, subtract and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when
assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by
the assembler rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting
an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer
multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a
jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting
an integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on
revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000
is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these
functions are only provided by the "mips64vr*-elf" configurations.
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side-effects of speculation on R10K
processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and
speculatively executes instructions from the "taken" branch. It later aborts these instructions if the
predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a
speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even
if the store itself is later aborted. If a DMA operation writes to the same area of memory before the
"dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a
full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be
speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting
controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the
following regions does not have side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC
honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-
written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed and that might have
side effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that might have side
effects even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If called,
the function must take the same arguments as the common "_flush_func()", that is, the address of the
memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to
selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that
implement those architectures; for those, Branch Likely instructions are not be generated by default
because the MIPS32 and MIPS64 architectures specifically deprecate their use.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some
processors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use
both FP pipes. Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one
is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should
execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the
expense of making it bigger. It is enabled by default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that support it. The "synci"
instructions (if enabled) are generated when "__builtin___clear_cache()" is compiled.
This option defaults to "-mno-synci", but the default can be overridden by configuring with
"--with-synci".
When compiling code for single processor systems, it is generally safe to use "synci". However, on many
multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to
undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only
possible if the linker can resolve the destination at link-time and if the destination is within range for
a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the
".reloc" assembly directive and "-mexplicit-relocs" is in effect. With "-mno-explicit-relocs", this
optimization can be performed by the assembler and the linker alone without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's return address. When
enabled, this option extends the usual "_mcount" interface with a new ra-address parameter, which has type
"intptr_t *" and is passed in register $12. "_mcount" can then modify the return address by doing both of
the following:
· Returning the new address in register $31.
· Storing the new address in "*ra-address", if ra-address is nonnull.
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as
registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load
instructions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX"
assembly directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable
branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically generates a
request (handled by the assembler and the linker) for a constant to be set up in a global register. The
register is used for one or more base address requests within the range 0 to 255 from the value held in
the register. The generally leads to short and fast code, but the number of different data items that can
be addressed is limited. This means that a program that uses lots of static data may require
-mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not
change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both "a0" and "d0". Otherwise,
the pointer is returned only in "a0", and attempts to call such functions without a prototype result in
errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls
and absolute memory addresses. This option only has an effect when used on the command line for the final
link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later.
This is the default. This option defines the preprocessor macro __LIW__.
-mnoliw
Do not allow the compiler to generate Long Instruction Word instructions. This option defines the
preprocessor macro __NO_LIW__.
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is
the default. This option defines the preprocessor macro __SETLB__.
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor
macro __NO_SETLB__.
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-* configurations.
-mel
Generate little-endian code.
-mno-crt0
Do not link in the C run-time initialization object file.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not
supported.)
-msoft-float
Do not use hardware floating point.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than
pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction scheduling parameters for array element type
ae_type. Supported values for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code generated with this option runs on any of the other
AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types
of operation (e.g., multiplication) do not work properly on all types of AE.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These warnings can be generated, for example,
when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no
hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word
load/store operations. This is inefficient and a warning is generated to indicate that you should rewrite
the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This
option disables these warnings.
PowerPC Options
These are listed under
RL78 Options
-msim
Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g13
-mmul=rl78
Specifies the type of hardware multiplication support to be used. The default is "none", which uses
software multiplication functions. The "g13" option is for the hardware multiply/divide peripheral only
on the RL78/G13 targets. The "rl78" option is for the standard hardware multiplication defined in the
RL78 software manual.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on the processor you are using. The
default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than
the options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General
Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the
optional PowerPC architecture instructions in the Graphics group, including floating-point select.
V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions
implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the
full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602,
603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3,
e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6,
power6x, power7, power8, powerpc, powerpc64, powerpc64le, and rs64.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit
big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate,
generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options runs best on that
processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -mpowerpc64 -mpowerpc-gpopt
-mpowerpc-gfxopt -msingle-float -mdouble-float -msimple-fpu -mstring -mmulhw -mdlmzb -mmfpgpr -mvsx
-mcrypto -mdirect-move -mpower8-fusion -mpower8-vector -mquad-memory -mquad-memory-atomic
The particular options set for any particular CPU varies between compiler versions, depending on what
setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's
capabilities. If you wish to set an individual option to a particular value, you may specify it after the
-mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present
because AIX does not have full support for these options. You may still enable or disable them
individually if you're sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type
or register usage, as -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as for -mcpu.
If both are specified, the code generated uses the architecture and registers set by -mcpu, but the
scheduling parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of 4G in
size.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is only
limited by the 64-bit address space.
-maltivec
Generate Altivec instructions using big-endian element order, regardless of whether the target is big- or
little-endian. This is the default when targeting a big-endian platform.
The element order is used to interpret element numbers in Altivec intrinsics such as "vec_splat",
"vec_extract", and "vec_insert". By default, these will match array element order corresponding to the
endianness for the target.
-maltivec=le
Generate Altivec instructions using little-endian element order, regardless of whether the target is big-
or little-endian. This is the default when targeting a little-endian platform. This option is currently
ignored when targeting a big-endian platform.
The element order is used to interpret element numbers in Altivec intrinsics such as "vec_splat",
"vec_extract", and "vec_insert". By default, these will match array element order corresponding to the
endianness for the target.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-mgen-cell-microcode
Generate Cell microcode instructions.
-mwarn-cell-microcode
Warn when a Cell microcode instruction is emitted. An example of a Cell microcode instruction is a
variable shift.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-executable
".plt" and ".got" sections. This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and requires ".plt" and ".got" sections
that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe
-mno-spe
This switch enables or disables the generation of SPE simd instructions.
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mvsx
and the vector/scalar (VSX) registers that were added in version 2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be
fused together on power8 and later processors.
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07
of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to the vector
instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory
option requires use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic
option requires use of 64-bit mode.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating-point operations on the general-purpose
registers for architectures that support it.
The argument yes or single enables the use of single-precision floating-point operations.
The argument double enables the use of single and double-precision floating-point operations.
The argument no disables floating-point operations on the general-purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The
32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC
variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code
for PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The
-mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each
unique non-automatic variable reference in your program. GCC also places floating-point constants in the
TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can
reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure
needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and
implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass
floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in
addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is
properly rounded when comparing values and converting to double. Use XL symbol names for long double
support routines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of
calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL
compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is
compiled without optimization. Because always storing floating-point arguments on the stack is
inefficient and rarely needed, this option is not enabled by default and only is necessary when calling
subroutines compiled by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with
special startup code to enable the application to run. The system must have PE installed in the standard
location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the
appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and
the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The
option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the
standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating-point emulation
is provided if you use the -msoft-float option, and pass the option to GCC when linking.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point operations. -mdouble-float implies
-msingle-float.
-msimple-fpu
Do not generate "sqrt" and "div" instructions for hardware floating-point unit.
-mfpu=name
Specify type of floating-point unit. Valid values for name are sp_lite (equivalent to -msingle-float
-msimple-fpu), dp_lite (equivalent to -mdouble-float -msimple-fpu), sp_full (equivalent to
-msingle-float), and dp_full (equivalent to -mdouble-float).
-mxilinx-fpu
Generate code that uses (does not use) the load string instructions and the store string word instructions
to save multiple registers and do small block moves. These instructions are generated by default on POWER
systems, and not generated on PowerPC systems. Do not use -mstring on little-endian PowerPC systems,
since those instructions do not work when the processor is in little-endian mode. The exceptions are
PPC740 and PPC750 which permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register to the
address of the calculated memory location. These instructions are generated by default. If you use
-mno-update, there is a small window between the time that the stack pointer is updated and the address of
the previous frame is stored, which means code that walks the stack frame across interrupts or signals may
get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These
instructions can incur a performance penalty on Power6 processors in certain situations, such as when
stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting
Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the
IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those
processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and
476 processors. This instruction is generated by default when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields
to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is aligned
to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a
1-byte boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled
by the system.
-mrelocatable
-mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a
global area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The
-mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external
references are relocatable. The resulting code is suitable for applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each
function. The runtime system is responsible for initializing this register with an appropriate value
before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during the
second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or
second-highest (respectively) priority to dispatch-slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during instruction scheduling.
The argument dependence_type takes one of the following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the second scheduling pass. The argument
scheme takes one of the following values:
no Don't insert NOPs.
1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the
default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec,
no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble, elfv1, elfv2.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the
SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Book-E SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is a PowerPC 32-bit SYSV ABI
option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is a PowerPC 32-bit Linux ABI
option.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is likely to fail in spectacular ways.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C
libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries
are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended
relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary
Interface (EABI), which is a set of modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up
the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small
data areas. Selecting -mno-eabi means that the stack is aligned to a 16-byte boundary, no EABI
initialization function is called from "main", and the -msdata option only uses "r13" to point to a single
small data area. The -meabi option is on by default if you configured GCC using one of the
powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the
.sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and
static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global
and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which
is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section,
which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable
option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi,
otherwise compile code the same as -msdata=sysv.
-msdata=data
is target-specific.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small
data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch
is also passed to the linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output
using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more expensive calling sequence is
required. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current
location. A short call is generated if the compiler knows the call cannot be that far away. This setting
can be overridden by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these
systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do
this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for
32-bit PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr callee, L42", plus a branch island (glue code).
The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the
first address and generates a "bl callee" if the PPC "bl" instruction reaches the callee directly;
otherwise, the linker generates "bl L42" to call the branch island. The branch island is appended to the
body of the calling function; it computes the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and
the Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The
relocation allows the linker to reliably associate function call with argument setup instructions for TLS
optimization, which in turn allows GCC to better schedule the sequence.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the
preprocessor and linker.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with
additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide
for floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -finite-math-only, -freciprocal-math and -fno-trapping-math). Note that
while the throughput of the sequence is generally higher than the throughput of the non-reciprocal
instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals
0.99999994) for reciprocal square roots.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the
"FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision reciprocal square
root calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than
is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically
selects -mrecip-precision. The double-precision square root estimate instructions are not generated by
default on low-precision machines, since they do not provide an estimate that converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type
supported at present is "mass", which specifies to use IBM's Mathematical Acceleration Subsystem (MASS)
libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to "acosd2",
"acosf4", "acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2", "atan2f4", "atand2",
"atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4", "erfcd2",
"erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2", "expm1f4", "hypotd2",
"hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4", "log2d2", "log2f4",
"logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2", "sqrtf4", "tand2",
"tanf4", "tanhd2", and "tanhf4" when generating code for power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The MASS libraries must be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations option is used to
optimize rounding of floating-point values to 64-bit integer and back to floating point. The "friz"
instruction does not return the same value if the floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register (r11) when calling through a pointer
on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function
address, TOC value to be loaded in register r2, and static chain value to be loaded in register r11. The
-mpointers-to-nested-functions is on by default. You cannot call through pointers to nested functions or
pointers to functions compiled in other languages that use the static chain if you use the
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in the function
prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not
saved in the prologue, it is saved just before the call through the pointer. The -mno-save-toc-indirect
option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum alignment of 64 bits, for
compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit
boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more
recent versions of GCC. This option may be used to generate code that is compatible with functions
compiled with older versions of GCC.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the
RX600 series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU
hardware is not used for doubles if the -m64bit-doubles option is used.
Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This
is because the RX FPU instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and
RX200 series hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default when
this type is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store
data in the little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be placed into the small data
area. Using the small data area can lead to smaller and faster code, but the size of area is limited and
it is up to the programmer to ensure that the area does not overflow. Also when the small data area is
used one of the RX's registers (usually "r13") is reserved for use pointing to this area, so it is no
longer available for use by the compiler. This could result in slower and/or larger code if variables are
pushed onto the stack instead of being held in this register.
Note, common variables (variables that have not been initialized) and constants are not placed into the
small data area as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default with
higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a
register. It is up to the programmer to experiment and discover whether this feature is of benefit to
their program. See the description of the -mpid option for a description of how the actual register to
hold the small data area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board-specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler. This
syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by
default.
-mmax-constant-size=N
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler functions. The value N can be
between 0 and 4. A value of 1 means that register "r13" is reserved for the exclusive use of fast
interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11",
and a value of 4 reserves "r13" through "r10". A value of 0, the default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register. This is only
necessary if normal code might use the accumulator register, for example because it performs 64-bit
multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access to constant data is done via
an offset from a base address held in a register. This allows the location of constant data to be
determined at run time without requiring the executable to be relocated, which is a benefit to embedded
applications with tight memory constraints. Data that can be modified is not affected by this option.
Note, using this feature reserves a register, usually "r13", for the constant data base address. This can
result in slower and/or larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends upon whether the
-msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register
"r13" and proceeding downwards, registers are allocated first to satisfy the requirements of
-mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area
register to be "r8" if both -mint-register=4 and -mpid are specified on the command line.
By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is
compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the
RX only supports one such interrupt.
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with
the "interrupt" function attribute. This attribute indicates a function intended to process fast interrupts.
GCC ensures that it only uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the normal
use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line
options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point operations.
When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When
-mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations.
A backchain may be needed to allow debugging using tools that do not understand DWARF 2 call frame
information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the
stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160
byte register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain;
however, use of the backchain for debugging purposes usually requires that the whole binary is built with
-mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.
In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all
fields of the 96/160 byte register save area only for their default purpose; unused fields still take up
stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the
register save area; unused space is reused for other purposes, allowing for more efficient use of the
available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register is always saved two words below the
backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible
with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or
zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes.
Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination
of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed 64k. The default is to use the "basr" instruction
instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified,
generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate
64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa
is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible
with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When
generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of instructions available with the transactional
execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in
The -mzvector option enables vector language extensions and builtins using instructions available with the
vector extension facility introduced with the IBM z13 machine generation. This option adds support for
vector to be used as a keyword to define vector type variables and arguments. vector is only available
when GNU extensions are enabled. It will not be expanded when requesting strict standard compliance e.g.
with -std=c99. In addition to the GCC low-level builtins -mzvector enables a set of builtins added for
compatibility with Altivec-style implementations like Power and Cell. In order to make use of these
builtins the header file vecintrin.h needs to be included. -mzvector is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle
is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug
information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a system representing a certain processor type.
Possible values for cpu-type are g5, g6, z900, z990, z9-109, z9-ec, z10, z196, zEC12, and z13. When
generating code using the instructions available on z/Architecture, the default is -march=z900.
Otherwise, the default is -march=g5.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The list of cpu-type values is the same as for -march. The default is the value
used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating
system. This option is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time check
it doesn't need to be a real problem when the program runs. It is intended to identify functions that
most probably cause a stack overflow. It is useful to be used in an environment with limited stack size
e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a bad
idea with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional instructions in the function prologue
that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack
on S/390 grows downward). If the stack-guard option is omitted the smallest power of 2 larger than the
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the "hotpatch" attribute.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generation of "bcnz" instructions.
-muls
Enable generation of unaligned load and store instructions.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is
not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-point unit is not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
-m4a-single-only
-m5-32media
Generate 32-bit code for SHmedia.
-m5-32media-nofpu
Generate 32-bit code for SHmedia in such a way that the floating-point unit is not used.
-m5-64media
Generate 64-bit code for SHmedia.
-m5-64media-nofpu
Generate 64-bit code for SHmedia in such a way that the floating-point unit is not used.
-m5-compact
Generate code for SHcompact.
-m5-compact-nofpu
Generate code for SHcompact in such a way that the floating-point unit is not used.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some
functions from the standard C library do not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available. This
option is the default for all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the
result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is
enabled -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal
comparisons. The implcit settings can be overridden by specifying either -mieee or -mno-ieee.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH
ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters as a comma separated list. For details on
the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
none
Disable compiler generated atomic sequences and emit library calls for atomic operations. This is the
default if the target is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The
generated atomic sequences require additional support from the interrupt/exception handling code of
the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by
default when the target is "sh*-*-linux*" and SH3* or SH4*. When the target is SH4A, this option will
also partially utilize the hardware atomic instructions "movli.l" and "movco.l" to create more
efficient code, unless strict is specified.
soft-tcb
Generate software atomic sequences that use a variable in the thread control block. This is a
variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic
sequences require additional support from the interrupt/exception handling code of the system and are
only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be
specified as well.
soft-imask
Generate software atomic sequences that temporarily disable interrupts by setting "SR.IMASK = 1111".
This model works only when the program runs in privileged mode and is only suitable for single-core
systems. Additional support from the interrupt/exception handling code of the system is not required.
This model is enabled by default when the target is "sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l" and "movco.l" instructions only. This is only
available on SH4A and is suitable for multi-core systems. Since the hardware instructions support
only 32 bit atomic variables access to 8 or 16 bit variables is emulated with 32 bit accesses. Code
compiled with this option will also be compatible with other software atomic model interrupt/exception
handling systems if executed on an SH4A system. Additional support from the interrupt/exception
handling code of the system is not required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the variable in the thread control block structure
that should be used by the generated atomic sequences when the soft-tcb model has been selected. For
other models this parameter is ignored. The specified value must be an integer multiple of four and
in the range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic models, even though they would be compatible,
and will make the compiler generate atomic sequences of the specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that depending on the particular hardware
and software configuration this can degrade overall performance due to the operand cache line flushes that
user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division operations. For SHmedia strategy can be one of:
fp Performs the operation in floating point. This has a very high latency, but needs only a few
instructions, so it might be a good choice if your code has enough easily-exploitable ILP to allow the
compiler to schedule the floating-point instructions together with other instructions. Division by
zero causes a floating-point exception.
inv Uses integer operations to calculate the inverse of the divisor, and then multiplies the dividend with
the inverse. This strategy allows CSE and hoisting of the inverse calculation. Division by zero
calculates an unspecified result, but does not trap.
inv:minlat
A variant of inv where, if no CSE or hoisting opportunities have been found, or if the entire
operation has been hoisted to the same place, the last stages of the inverse calculation are
intertwined with the final multiply to reduce the overall latency, at the expense of using a few more
instructions, and thus offering fewer scheduling opportunities with other code.
call
Calls a library function that usually implements the inv:minlat strategy. This gives high code
density for "m5-*media-nofpu" compilations.
call2
Uses a different entry point of the same library function, where it assumes that a pointer to a lookup
table has already been set up, which exposes the pointer load to CSE and code hoisting optimizations.
inv:call
inv:call2
inv:fp
Use the inv algorithm for initial code generation, but if the code stays unoptimized, revert to the
call, call2, or fp strategies, respectively. Note that the potentially-trapping side effect of
division by zero is carried by a separate instruction, so it is possible that all the integer
instructions are hoisted out, but the marker for the side effect stays where it is. A recombination
to floating-point operations or a call is not possible in that case.
inv20u
inv20l
Variants of the inv:minlat strategy. In the case that the inverse calculation is not separated from
the multiply, they speed up division where the dividend fits into 20 bits (plus sign where applicable)
by inserting a test to skip a number of operations in this case; this test slows down the case of
larger dividends. inv20u assumes the case of a such a small dividend to be unlikely, and inv20l
assumes it to be likely.
For targets other than SHmedia strategy can be one of:
call-div1
Calls a library function that uses the single-step division instruction "div1" to perform the
operation. Division by zero calculates an unspecified result and does not trap. This is the default
except for SH4, SH2A and SHcompact.
When a division strategy has not been specified the default strategy will be selected based on the current
target. For SH2A the default strategy is to use the "divs" and "divu" instructions instead of library
function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around each call.
Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack
frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only affects the name
used in the call and inv:call division strategies, and the compiler still expects the same sets of
input/output/clobbered registers as if this option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator can not use. This is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware
and/or OS implement 32-bit wrap-around semantics for the indexed addressing mode. The architecture allows
the implementation of processors with 64-bit MMU, which the OS could use to get 32-bit addressing, but
since no current hardware implementation supports this or any other way to make the indexed addressing
mode safe to use in the 32-bit ABI, the default is -mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the "gettr" instruction to number. The default is 2 if -mpt-fixed is in effect,
100 otherwise.
-mpt-fixed
Assume "pt*" instructions won't trap. This generally generates better-scheduled code, but is unsafe on
current hardware. The current architecture definition says that "ptabs" and "ptrel" trap when the target
anded with 3 is 3. This has the unintentional effect of making it unsafe to schedule these instructions
before a branch, or hoist them out of a loop. For example, "__do_global_ctors", a part of libgcc that
runs constructors at program startup, calls functions in a list which is delimited by -1. With the
-mpt-fixed option, the "ptabs" is done before testing against -1. That means that all the constructors
run a bit more quickly, but when the loop comes to the end of the list, the program crashes because
"ptabs" loads -1 into a target register.
Since this option is unsafe for any hardware implementing the current architecture specification, the
default is -mno-pt-fixed. Unless specified explicitly with -mgettrcost, -mno-pt-fixed also implies
-mgettrcost=100; this deters register allocation from using target registers for storing ordinary
integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the compiler are always valid to
load with "movi"/"shori"/"ptabs" or "movi"/"shori"/"ptrel", but with assembler and/or linker tricks it is
possible to generate symbols that cause "ptabs" or "ptrel" to trap. This option is only meaningful when
-mno-pt-fixed is in effect. It prevents cross-basic-block CSE, hoisting and most scheduling of symbol
loads. The default is -mno-invalid-symbols.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more
-mcmpeqdi
Emit the "cmpeqdi_t" instruction pattern even when -mcbranchdi is in effect.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for sine and cosine approximations. The
option "-mfsca" must be used in combination with "-funsafe-math-optimizations". It is enabled by default
when generating code for SH4A. Using "-mno-fsca" disables sine and cosine approximations even if
"-funsafe-math-optimizations" is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal square root approximations.
The option "-mfsrra" must be used in combination with "-funsafe-math-optimizations" and
"-ffinite-math-only". It is enabled by default when generating code for SH4A. Using "-mno-fsrra"
disables reciprocal square root approximations even if "-funsafe-math-optimizations" and
"-ffinite-math-only" are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result
in faster code on the SH4 processor.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when
linking a shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker
error message. However, the necessary relocations trigger copy-on-write, and the shared object is not
actually shared across processes. Instead of using -mimpure-text, you should compile all source code with
-fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
Add support for multithreading using the POSIX threads library. This option sets flags for both the
preprocessor and linker. This option does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it.
-pthread
This is a synonym for -pthreads.
SPARC Options
These -m options are supported on the SPARC:
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register
window model. This model is compatible with the regular register window model. The local registers and
the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as
needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf
functions). This is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not
available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do
provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library that
comes with GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-point instructions. The
functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word
floating-point instructions. They all invoke a trap handler for one of these instructions, and then the
trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much
slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in
another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment.
Specifying this option avoids some rare compatibility problems with code generated by other compilers. It
is not the default because it results in a performance loss, especially for floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant only for the "casa"
instruction emitted for the LEON3 processor. The default is -mno-user-mode.
-mno-faster-structs
-mfaster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables
the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as
Default instruction scheduling parameters are used for values that select an architecture and not an
implementation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture.
With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from
V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in
SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the
SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds
the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not
in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which
is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the
Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the
integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in
SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC
SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer
and floating-point move instructions, 3 additional floating-point condition code registers and conditional
move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC
I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2
chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With
-mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those
that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3,
leon3v7, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3 and
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction
Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as
UltraSPARC-III and later. Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction
Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as
niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of compare-and-branch instructions, as defined in
the Sparc Architecture 2011. The default is -mcbcond when targeting a cpu that supports such
instructions, such as niagara-4 and later.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC population count instruction. The
default is -mpopc when targeting a cpu that supports such instructions, such as Niagara-2 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point
extensions. The default is -mfmaf when targeting a cpu that supports such instructions, such as Niagara-3
and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds
to erratum #13 of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the
UT699 processor.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory.
Programs can be statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory,
supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the Sparc V9 architecture manual, as set in the
processor's "PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047
which must be added back when making stack frame references. This is the default in 64-bit mode.
Otherwise, assume no such offset is present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By default, GCC gives an error when it generates
code that requires a dynamic relocation. -mno-error-reloc disables the error, -mwarn-reloc generates a
warning instead.
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not be reordered with respect to loads and
stores of the memory that is being accessed. With -munsafe-dma you must use the "volatile" keyword to
protect memory accesses, but that can lead to inefficient code in places where the memory is known to not
change. Rather than mark the memory as volatile, you can use -msafe-dma to tell the compiler to treat the
DMA instructions as potentially affecting all memory.
-mbranch-hints
By default, GCC generates a branch hint instruction to avoid pipeline stalls for always-taken or probably-
taken branches. A hint is not generated closer than 8 instructions away from its branch. There is little
reason to disable them, except for debugging purposes, or to make an object a little bit smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are never larger than 18 bits. With -mlarge-mem
code is generated that assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the SPU-style main function interface (which has
object code in an executable must be compiled with the same setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of the generic address space. This enables
explicit type casts between "__ea" and generic pointer as well as implicit conversions of generic pointers
to "__ea" pointers. The default is to allow address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler links to an executable and selects a
software-managed cache for accessing variables in the "__ea" address space with a particular cache size.
Possible options for cache-size are 8, 16, 32, 64 and 128. The default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler links to an executable and selects whether
atomic updates to the software-managed cache of PPU-side variables are used. If you use atomic updates,
changes to a PPU variable from SPU code using the "__ea" named address space qualifier do not interfere
with changes to other PPU variables residing in the same cache line from PPU code. If you do not use
atomic updates, such interference may occur; however, writing back cache lines is more efficient. The
default behavior is to use atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC inserts nops to increase dual issue when it expects it to increase performance. n can be
a value from 0 to 10. A smaller n inserts fewer nops. 10 is the default, 0 is the same as
-mno-dual-nops. Disabled with -Os.
-mhint-max-nops=n
Maximum number of nops to insert for a branch hint. A branch hint must be at least 8 instructions away
from the branch it is affecting. GCC inserts up to n nops to enforce this, otherwise it does not generate
the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be within 256 instructions of the branch it
is affecting. By default, GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC inserts the "hbrp"
instruction to make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those
systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
absolute addresses.
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to
32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only
supported behavior so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads
the function's address into a register, and calls indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer
into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by
default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a
function. The external functions are slower, but use less code space if more than one function saves the
same number of registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function
options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register "ep"
points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register "gp"
points to. The small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
Specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if
this option is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this
option is used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are
defined if this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this
option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a
default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant
is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for the v850e, v850e1, v850e2, v850e2v3 and
v850e3v5 flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default
when the GCC ABI is in use. If "CALLT" instructions are being generated then the C preprocessor symbol
"__V850_CALLT__" will be defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command line option to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point instructions. This option is only
significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions
are being generated then the C preprocessor symbol "__FPU_OK__" will be defined, otherwise the symbol
"__NO_FPU__" will be defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when
the e3v5 architecture is selected because its use is still experimental.
-mrh850-abi
-mghs
option is not supported.
When this version of the ABI is enabled the C preprocessor symbol "__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules
apply:
· Integer sized structures and unions are returned in register "r10".
· Large structures and unions (more than 8 bytes in size) are passed by reference.
· Functions are aligned to 32-bit boundaries, unless optimizing for size.
· The -m8byte-align command line option is not supported.
· The -mdisable-callt command line option is supported but not enabled by default.
When this version of the ABI is enabled the C preprocessor symbol "__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "doubles" and "long long" types to be aligned on 8-byte boundaries. The default is to
restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the C
preprocessor symbol "__V850_8BYTE_ALIGN__" will be defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about
out of range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by the compiler. This setting is the
default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot
handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that the GNU assembler is being used.
-mg Output code for G-format floating-point numbers instead of D-format.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error) codes.
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are
listed with the other options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from
the former to the latter. It also defines the preprocessor macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather than static libraries. The options -static and
-shared can also be used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for
compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for compatibility with
Diab.
x86-64 Options
These are listed under
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in
place of the standard "L32R" instructions. The use of "CONST16" is enabled by default only if the "L32R"
instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point
option. This has no effect if the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the
multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant
results are required: the fused multiply add/subtract instructions do not round the intermediate result,
thereby producing results with more bits of precision than specified by the IEEE standard. Disabling
fused multiply add/subtract instructions also ensures that the program output is not sensitive to the
compiler's ability to combine multiply and add/subtract operations.
Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals
in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM,
and it also allows the linker to combine literal pools from separate object files to remove redundant
literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text
section in order to keep them as close as possible to their references. This may be necessary for large
assembly files.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce
branch penalties at the expense of some code density. The assembler attempts to widen density
instructions to align branch targets and the instructions following call instructions. If there are not
enough preceding safe density instructions to align a target, no widening is performed. The default is
-mtarget-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which
the assembler always aligns, either by widening density instructions or by inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls
unless it can determine that the target of a direct call is in the range allowed by the call instruction.
This translation typically occurs for calls to functions in other source files. Specifically, the
assembler translates a direct "CALL" instruction into an "L32R" followed by a "CALLX" instruction. The
default is -mno-longcalls. This option should be used in programs where the call target can potentially
be out of range. This option is implemented in the assembler, not the compiler, so the assembly code
generated by GCC still shows direct call instructions---look at the disassembled object code to see the
actual instructions. Note that the assembler uses an indirect call for every cross-file call, not just
those that really are out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table
below, only one of the forms is listed---the one that is not the default. You can figure out the other form
by either removing no- or adding it.
-fbounds-check
For front ends that support it, generate additional code to check that indices used to access arrays are
within the declared range. This is currently only supported by the Java and Fortran front ends, where
this option defaults to true and false respectively.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto variables and compiler generated
temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables
and temporaries, named_vars enables the reuse only for user defined local variables with names, and none
disables stack reuse completely. The default value is all. The option is needed when the program extends
the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by
the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing
compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose
live range does not overlap with it. Legacy code extending local lifetime will likely to break with the
stack reuse optimization.
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a
temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its
stack space with other temporaries or scoped local variables whose live range does not overlap with it.
However some of the legacy code relies on the behavior of older compilers in which temporaries' stack
space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control
the temporary stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement representation. This flag enables some optimizations
and disables others. This option is enabled by default for the Java front end, as required by the Java
language specification.
-fexceptions
to be thrown from arbitrary signal handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of
the program can be optimized away. This option is enabled by default for the Ada front end, as permitted
by the Ada language specification. Optimization passes that cause dead exceptions to be removed are
enabled independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed static data, but does not affect the
generated code in any other way. You normally do not need to enable this option; instead, a language
processor that needs this handling enables it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF 2 format, if supported by target machine. The table is exact at each
instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or
garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE" binding to
make sure that definitions of template static data members and static local variables in inline functions
are unique even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library used
by two different "RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing
with the other one about the binding of the symbol. But this causes "dlclose" to be ignored for affected
DSOs; if your program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can use
-fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This
convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled
files and files compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled
with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small
structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention
is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return,
except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we
chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled
with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values.
Specifically, the "enum" type is equivalent to the smallest integer type that has enough room.
This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-default application binary interface.
-fno-common
In C code, controls the placement of uninitialized global variables. Unix C compilers have traditionally
permitted multiple definitions of such variables in different compilation units by placing the variables
in a common block. This is the behavior specified by -fcommon, and is the default for GCC on most
targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed
or code size penalty on variable references. The -fno-common option specifies that the compiler should
place uninitialized global variables in the data section of the object file, rather than generating them
as common blocks. This has the effect that if the same variable is declared (without "extern") in two
different compilations, you get a multiple-definition error when you link them. In this case, you must
compile with -fcommon instead. Compiling with -fno-common is useful on targets for which it provides
better performance, or if you wish to verify that the program will work on other systems that always treat
uninitialized variable declarations this way.
-fno-ident
Ignore the #ident directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is
split in the middle, and the two halves are placed at locations far apart in memory. This option is used
when compiling crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This option is
generally only of use to those who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two
assembler files.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be recorded into the object file that
is being created. This switch is only implemented on some targets and the exact format of the recording
is target and binary file format dependent, but it usually takes the form of a section containing ASCII
text. This switch is related to the -fverbose-asm switch, but that switch only records information in the
assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches
for another way of storing compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target
machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic
loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating that -fpic does not work; in that case,
recompile with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The
386 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the
386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position independent code can be only linked
into executables. Usually these options are used when -pie GCC option is used during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and
2 for -fPIE.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code
generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that
forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump
tables do not require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps
as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in
the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be
allocated for temporaries or variables that do not live across a call. Functions compiled this way do not
save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other
registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even for
temporaries or variables that live across a call. Functions compiled this way save and restore the
register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other
registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a register in which function values may
be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is specified
(which must be a small power of two), pack structure members according to this value, representing the
maximum alignment (that is, objects with default alignment requirements larger than this are output
potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-
The first argument is the address of the start of the current function, which may be looked up exactly in
the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls
indicate where, conceptually, the inline function is entered and exited. This means that addressable
versions of such functions must be available. If all your uses of a function are expanded inline, this
may mean an additional expansion of code size. If you use extern inline in your C code, an addressable
version of such functions must be provided. (This is normally the case anyway, but if you get lucky and
the optimizer always expands the functions inline, you might have gotten away without providing static
copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation is not
done. This can be used, for example, for the profiling functions listed above, high-priority interrupt
routines, and any functions from which the profiling functions cannot safely be called (perhaps signal
handlers, if the profiling routines generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the description of
"-finstrument-functions"). If the file that contains a function definition matches with one of file, then
that function is not instrumented. The match is done on substrings: if the file parameter is a substring
of the file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain "/bits/stl" or "include/sys".
If, for some reason, you want to include letter ',' in one of sym, write ','. For example,
"-finstrument-functions-exclude-file-list=',,tmp'" (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to "-finstrument-functions-exclude-file-list", but this option sets the list of function
names to be excluded from instrumentation. The function name to be matched is its user-visible name, such
as "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g.,
"_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring of the
function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name
must be given in UTF-8, not using universal character names.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag
if you are running in an environment with multiple threads, but you only rarely need to specify it in a
single-threaded environment since stack overflow is automatically detected on nearly all systems if there
is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language
runtime must do that. The switch causes generation of code to ensure that they see the stack being
extended.
You can additionally specify a string parameter: "no" means no checking, "generic" means force the use of
old-style checking, "specific" means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but
been added in the compiler.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a
register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For
most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the
signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the
flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack
limit of 128KB. Note that this may only work with the GNU linker.
-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting program has a
discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is
most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to
use for each thread. This is currently only implemented for the i386 and x86_64 back ends running
GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much
stack space available for the latter code to run. If compiling all code, including library code, with
-fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without
-fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU
binutils release 2.21 and later.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are
represented in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary interface. Not
all targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of "global-dynamic",
"local-dynamic", "initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".
-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified option---all symbols are marked with this
unless overridden within the code. Using this feature can very substantially improve linking and load
times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent
symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.
Despite the nomenclature, "default" always means public; i.e., available to be linked against from outside
the shared object. "protected" and "internal" are pretty useless in real-world usage so the only other
commonly used option is "hidden". The default if -fvisibility isn't specified is "default", i.e., make
every symbol public---this causes the same behavior as previous versions of GCC.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by
"How To Write Shared Libraries" by Ulrich Drepper (which can be found at
readability and self-documentation of the code. Note that due to ISO C++ specification requirements,
"operator new" and "operator delete" must always be of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other
library you use, may not be expecting to be compiled with visibility other than the default. You may need
to explicitly say #pragma GCC visibility push(default) before including any such headers.
extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with
-fvisibility=hidden with no modifications. However, this means that calls to "extern" functions with no
explicit visibility use the PLT, so it is more effective to use "__attribute ((visibility))" and/or
"#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception
class that is be thrown between DSOs must be explicitly marked with default visibility so that the
type_info nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at
<http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields, although the
compiler usually honors those types anyway) should use a single access of the width of the field's type,
aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers
might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-
fields as "unsigned short" (assuming short is 16 bits on these targets) to force GCC to use 16-bit
accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example,
that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion
of the bit-field, or memory-mapped registers unrelated to the one being updated.
If the target requires strict alignment, and honoring the field type would require violating this
alignment, a warning is issued. If the field has "packed" attribute, the access is done without honoring
the field type. If the field doesn't have "packed" attribute, the access is done honoring the field type.
In both cases, GCC assumes that the user knows something about the target hardware that it is unaware of.
The default value of this option is determined by the application binary interface for the target
processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the "__sync" family of functions may be used to
implement the C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the only useful form of the option is
-fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by
specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify
other aspects of the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take precedence
over places specified using environment variables, which in turn take precedence over those specified by the
configuration of GCC.
escape characters that are otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise,
LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these
variables are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold
the output of one stage of compilation which is to be used as input to the next stage: for example, the
output of the preprocessor, which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the
documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the
compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the
pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the
subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed
compiler. In many cases prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header files.
For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely,
with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to
produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the
standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix then the
value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the
directories thus specified when searching for subprograms, if it can't find the subprograms using
GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a
native compiler, GCC tries the directories thus specified when searching for special linker files, if it
can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for
ordinary libraries for the -l option (but directories specified with -L come first).
LANG
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as
defined by the default locale to recognize and translate multibyte characters.
Some additional environment variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which
to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at
GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets
it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given
with -I options on the command line. This environment variable is used regardless of which language is
being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if specified with -isystem, but after any paths given
with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory.
Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system
header files processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that
file, guessing the target name from the source file name. Or the value can have the form file target, in
which case the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an
optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not
ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.
BUGS
For instructions on reporting bugs, see <http://bugzilla.redhat.com/bugzilla>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-
libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply
the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is
innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info
the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.8.5 2015-06-23 GCC(1)
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