Re: [PATCH v3 4/8] x86-64: Require BMI2 for AVX2 strncmp implementation

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Mon, Oct 3, 2022 at 12:59 PM Aurelien Jarno <aurelien@aurel32.net> wrote:
>
> The AVX2 strncmp implementations uses the 'bzhi' instruction, which
> belongs to the BMI2 CPU feature.
>
> NB: It also uses the 'tzcnt' BMI1 instruction, but it is executed as BSF
> as BSF if the CPU doesn't support TZCNT, and produces the same result
> for non-zero input.
>
> Partially fixes: b77b06e0e296 ("x86: Optimize strcmp-avx2.S")
> Partially resolves: BZ #29611
> ---
>  sysdeps/x86_64/multiarch/ifunc-impl-list.c | 7 +++++--
>  sysdeps/x86_64/multiarch/strncmp.c         | 4 ++--
>  2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/ifunc-impl-list.c b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> index a42b0a4620..aebef3daaf 100644
> --- a/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> +++ b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> @@ -1176,13 +1176,16 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>    IFUNC_IMPL (i, name, strncmp,
>               X86_IFUNC_IMPL_ADD_V4 (array, i, strncmp,
>                                      (CPU_FEATURE_USABLE (AVX512VL)
> -                                     && CPU_FEATURE_USABLE (AVX512BW)),
> +                                     && CPU_FEATURE_USABLE (AVX512BW)
> +                                     && CPU_FEATURE_USABLE (BMI2)),
>                                      __strncmp_evex)
>               X86_IFUNC_IMPL_ADD_V3 (array, i, strncmp,
> -                                    CPU_FEATURE_USABLE (AVX2),
> +                                    (CPU_FEATURE_USABLE (AVX2)
> +                                     && CPU_FEATURE_USABLE (BMI2)),
>                                      __strncmp_avx2)
>               X86_IFUNC_IMPL_ADD_V3 (array, i, strncmp,
>                                      (CPU_FEATURE_USABLE (AVX2)
> +                                     && CPU_FEATURE_USABLE (BMI2)
>                                       && CPU_FEATURE_USABLE (RTM)),
>                                      __strncmp_avx2_rtm)
>               X86_IFUNC_IMPL_ADD_V2 (array, i, strncmp,
> diff --git a/sysdeps/x86_64/multiarch/strncmp.c b/sysdeps/x86_64/multiarch/strncmp.c
> index 4ebe4bde30..c4f8b6bbb5 100644
> --- a/sysdeps/x86_64/multiarch/strncmp.c
> +++ b/sysdeps/x86_64/multiarch/strncmp.c
> @@ -41,12 +41,12 @@ IFUNC_SELECTOR (void)
>    const struct cpu_features *cpu_features = __get_cpu_features ();
>
>    if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX2)
> +      && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI2)
>        && X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
>                                       AVX_Fast_Unaligned_Load, ))
>      {
>        if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
> -         && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512BW)
> -         && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI2))
> +         && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
>         return OPTIMIZE (evex);
>
>        if (CPU_FEATURE_USABLE_P (cpu_features, RTM))
> --
> 2.35.1
>
LGTM.
Reviewed-by: Noah Goldstein  <goldstein.w.n@gmail.com>