Re: [PATCH] x86/build/64: Prevent native builds from generating APX instructions

[David Laight]

On Sat, 1 Aug 2026 17:00:36 -0700
"Chang S. Bae" <chang.seok.bae@xxxxxxxxx> wrote:

> On 8/1/2026 4:44 AM, David Laight wrote:
> > 
> > Much like when AVX support was added, -march-native needs to check that the
> > operating system is saving the registers not that the cpu supports them.  
> 
> This GCC bug looks relevant:
> 
>    "__builtin_cpu_supports avx does not verify OS supports it"
>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85100

Yes, except I thought it was much older...

> The resulting commit 059cc8aca774 ("i386: Enable AVX/AVX512 features 
> only if supported by OSXSAVE") switched the compiler to read XCR0 before 
> set_feature().
> 
> Similarly, the APX enablement commit e686416b6217 ("[APX_EGPR] Initial 
> support for APX_F") extends the XCR0 check to determine whether APX is 
> usable. So, the compiler seemingly won't emit APX instructions without 
> OS setting XCR0.APX.

Does that mean that this patch is completely unnecessary?

	David

> 
> Thanks,
> Chang