How to force add compiler options for building the Linux kernel?

[Debian_SuperUser]

This is just an experiment. I want to force add compiler options such as "-Ofast -march=native -mtune=native -m64 -flto -s" to each file that compiles in the build process and also remove certain ones such as -g along the replacement of the ones I mentioned. Just settings your CFLAGS or Kbuild flags or setting in the Makefile don't do anything. The build system of each modules will have their own compiler options.

I made a script to intercept all calls to gcc and add/remove compiler options. That worked whenever I tried to compile anything but not for the kernel. The build fails and using V=1 I can see my options aren't being applied.

Why doesn't the build system even make better use of the compiler options to prioritize optimization?

Does anyone have any better idea?

 

[dos2unix]

Don't use Kbuild for custom builds, it overrides compiler flags. Just run ./configure and make from the command line.

Even if you jhard-code changes in the makefile Kbuild can over-ride them. Do not use Kbuild for custom Kernel compiles.

If you run from the command line, you can pass options like this

make KCFLAGS="-Ofast -march=native -mtune=native -m64 -flto -s -g0"

As a rule, the Linux kernel doesn't want to be hardware optimized. (There are ways around this).
It wants to be portable and run on as many kinds of hardware as possible. I will tell you from experience, even
if you do get it fully optimized, it will save you something like .0001 seconds in execution.

Kbuild is great if you're just doing a standard compile.

 

[Debian_SuperUser]

Just run ./configure

I can't find any file called configure.

Even if you jhard-code changes in the makefile Kbuild can over-ride them. Do not use Kbuild for custom Kernel compiles.

I actually replaced all instances of -O2 with better optimization flags in all sub directories. The kernel actually compiled and I saw those flags being applied in the build process. I am going to run some benchmarks. But I still don't want to do it like this. I didn't have good control over the compiler options and left some on like -g.

As a rule, the Linux kernel doesn't want to be hardware optimized. (There are ways around this).
It wants to be portable and run on as many kinds of hardware as possible. I will tell you from experience, even
if you do get it fully optimized, it will save you something like .0001 seconds in execution.

Of course march=native won't be distributable. But I don't see any -march option for better optimization. Or at least there should be a build profile to enable them easily. That's how I would write a build system.

 

[dos2unix]

I can't find any file called configure.

I can't remember now, I think it's config or xconfig.
We had a thread about this a few months go.

Post in thread 'How To Install A Higher Version of the Kernel, Build/Compile and Make the System Used It'

Jan 10, 2025

This is usually somewhat distro dependent. For example the build tools are usually named differently
for different distro's. Some distro's such as fedora require openssl-devel because parts of the communication
between the kernel and other system processes is encrypted. Also it's slightly different depending on it you
do it from the command line, or if you do it with GUI config tool.

But here is the basic outline. I have done a number of times so I know it works with these instructions for my distro.
Results may vary.

Step-by-Step Guide to Download, Compile, and Install a Newer...​

• dos2unix

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[Debian_SuperUser]

I can't remember now, I think it's config or xconfig.

config and xconfig are both targets of make. The config target makes me decide all the configs and xconfig is just a QT frontend.

 

[sailed]

Modifies KBUILD_CFLAGS in the kernel Makefile, not just CFLAGS.