How to set `-march=skylake-avx512` on a file correctly

[John Stiles]

Hi there! I am working on some Skia optimizations (specifically in skcms, the color management package) and I've hit a stumbling block that seems to involve the ChromeOS build system.

Previously, we would put Haswell/AVX512 optimizations into the same cc file as the rest of skcms, and used __attribute__(target…) to enable it. (We ensure that we only call these functions when the CPU supports it, of course.) However, due to a bug/ABI issue in Clang, __attribute__(target…) can generate pretty lousy code in some cases (https://github.com/llvm/llvm-project/issues/64706). To counteract this, I'm splitting the architecture-specific code out into separate cc files with -march= specified directly as a build flag. This is handled here:

https://source.chromium.org/chromium/chromium/src/+/main:skia/BUILD.gn;l=780-807;drc=ff297868ea15ec0944a53388fa98ec1609e11b00

This works fine in most cases. Unfortunately, in Lacros and ChromeOS builds only, I get a build error because -march=x86-64 -msse3 has been tacked onto the command line after my -march flag, overriding it.

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket/8763895282307470081/+/u/compile__with_patch_/stdout#L6811_0

It's not obvious to me how I should work around this issue. I am not sure where the extra -march is being added or how I can neutralize it.

Any help would be most appreciated—thanks in advance! 

[Mike Frysinger]

it sounds like you're compiling code under chromium/src/ which is a browser question, not an OS question, so you want to ask on chromium-dev@ instead

-mike

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