Re: [PATCH] libatomic: Handle AVX+CX16 AMD like Intel for 16b atomics [PR104688]

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Nov 14, 2022 at 04:19:48PM +0800, Xi Ruoyao wrote:
> > > --- libatomic/config/x86/init.c.jj      2022-03-17
> > > 18:48:56.708723194 +0100
> > > +++ libatomic/config/x86/init.c 2022-11-13 18:23:26.315440071 -1200
> > > @@ -34,18 +34,6 @@ __libat_feat1_init (void)
> > >    unsigned int eax, ebx, ecx, edx;
> > >    FEAT1_REGISTER = 0;
> > >    __get_cpuid (1, &eax, &ebx, &ecx, &edx);
> > > -#ifdef __x86_64__
> > > -  if ((FEAT1_REGISTER & (bit_AVX | bit_CMPXCHG16B))
> > > -      == (bit_AVX | bit_CMPXCHG16B))
> > > -    {
> > > -      /* Intel SDM guarantees that 16-byte VMOVDQA on 16-byte
> > > aligned address
> > > -        is atomic, but so far we don't have this guarantee from
> > > AMD.  */
> > > -      unsigned int ecx2 = 0;
> > > -      __get_cpuid (0, &eax, &ebx, &ecx2, &edx);
> > > -      if (ecx2 != signature_INTEL_ecx)
> > > -       FEAT1_REGISTER &= ~bit_AVX;
> > 
> > We still need this, but also bypass it for AMD signature. There are
> > other vendors than Intel and AMD.
> 
> Mayshao: how about the status of this feature on Zhaoxin product lines?
> IIRC they support AVX (but disabled by default in GCC for Lujiazui), but
> we don't know if they make the guarantee about atomicity of 16B aligned
> access.

I did the change on the assumption that only Intel and AMD implement AVX.
Looking around, I'm afraid Zhaoxin Zhangjiang/Wudaokou/Lujiazui
and VIA Eden C and VIA Nano C CPUs do support AVX too, the question is
if they implement CMPXCHG16B too.
From what is in i386-common.cc, none of non-Intel CPUs in there have
PTA_AVX and only Lujiazui has CX16.  But that doesn't need to match what
the HW actually does and one can just compile with -mcx16 -mavx -m64
rather than using some -march=whatever.

Sure, can change the check so that it checks for AMD too for now and
therefore discard the sync.md patch, the question is whom do we talk at
Zhaoxin and VIA and if there are any further other CX16+AVX CPUs

	Jakub