Re: [PATCH] Bug 201689: Belt-and-suspenders detection of FMA.

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Oct 14, 2016 at 11:09 AM, Carlos O'Donell <carlos@redhat.com> wrote:
> In the Intel Architecture Instruction Set Extensions Programming reference
> the recommended way to test for FMA in section '2.2.1 Detection of FMA'
> is:
>
> "Application Software must identify that hardware supports AVX as explained
>  in ... after that it must also detect support for FMA..."
>
> We don't do that in glibc. We use osxsave to detect the use of xgetbv, and
> after that we check for AVX and FMA orthogonally. It is conceivable that
> you could have the AVX bit clear and the FMA bit in an undefined state.
>
> I have never seen a machine with the AVX bit clear and the FMA bit set, but
> we should follow the intel specifications and adjust our check as the following
> patch works.

One can't write a program with FMA nor AVX2 without using AVX
instructions.  AVX2/FMA aren't usable if AVX isn't.   We should do

if (CPU_FEATURES_CPU_P (cpu_features, AVX))
   {
      Set AVX available
      if (CPU_FEATURES_CPU_P (cpu_features, AVX2))
         Set AVX2 available
      if (CPU_FEATURES_CPU_P (cpu_features, FMA))
         Set FMA available
   }

> OK to checkin?
>
> 2016-10-14  Carlos O'Donell  <carlos@redhat.com>
>
>         [BZ #20689]
>         * sysdeps/x86/cpu-features.c: Only enable FMA is AVX is present.
>
> diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
> index 11b9af2..1d52f22 100644
> --- a/sysdeps/x86/cpu-features.c
> +++ b/sysdeps/x86/cpu-features.c
> @@ -83,8 +83,10 @@ get_common_indeces (struct cpu_features *cpu_features,
>                       |= bit_arch_AVX512DQ_Usable;
>                 }
>             }
> -         /* Determine if FMA is usable.  */
> -         if (CPU_FEATURES_CPU_P (cpu_features, FMA))
> +         /* Determine if FMA is usable.  The recommended Intel procedure
> +            is to check for AVX && FMA to decide if FMA is available.  */
> +         if (CPU_FEATURES_CPU_P (cpu_features, AVX)
> +             && CPU_FEATURES_CPU_P (cpu_features, FMA))
>             cpu_features->feature[index_arch_FMA_Usable]
>               |= bit_arch_FMA_Usable;
>         }
> ---
>
>
> --
> Cheers,
> Carlos.



-- 
H.J.