Re: [PATCH 4/4] x86-64: Require LZCNT for AVX2 memrchr implementation

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

On Sat, Oct 1, 2022 at 12:09 PM Aurelien Jarno <aurelien@aurel32.net> wrote:
>
> The AVX2 memrchr implementation uses the lzcntl and lzcntq instructions,
> which belongs to the LZCNT CPU feature.
>
> Fixes: af5306a735eb ("x86: Optimize memrchr-avx2.S")
> Partially resolves: BZ #29611
> ---
>  sysdeps/x86_64/multiarch/ifunc-avx2.h      | 1 +
>  sysdeps/x86_64/multiarch/ifunc-impl-list.c | 7 +++++--
>  2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/ifunc-avx2.h b/sysdeps/x86_64/multiarch/ifunc-avx2.h
> index a57a9952f3..f1741083fd 100644
> --- a/sysdeps/x86_64/multiarch/ifunc-avx2.h
> +++ b/sysdeps/x86_64/multiarch/ifunc-avx2.h
> @@ -37,6 +37,7 @@ IFUNC_SELECTOR (void)
>
>    if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX2)
>        && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI2)
> +      && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, LZCNT)

This causes a build failure. Need a corresponding macro in
sysdeps/x86/isa-level.h

Something like:

#define LZCNT_X86_ISA_LEVEL 3

after the BMI2 one.
>        && X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
>                                       AVX_Fast_Unaligned_Load, ))
>      {
> diff --git a/sysdeps/x86_64/multiarch/ifunc-impl-list.c b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> index c628462d47..db5a2032d6 100644
> --- a/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> +++ b/sysdeps/x86_64/multiarch/ifunc-impl-list.c
> @@ -209,13 +209,16 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
>    IFUNC_IMPL (i, name, memrchr,
>               X86_IFUNC_IMPL_ADD_V4 (array, i, memrchr,
>                                      (CPU_FEATURE_USABLE (AVX512VL)
> -                                     && CPU_FEATURE_USABLE (AVX512BW)),
> +                                     && CPU_FEATURE_USABLE (AVX512BW)
> +                                     && CPU_FEATURE_USABLE (LZCNT)),
>                                      __memrchr_evex)
>               X86_IFUNC_IMPL_ADD_V3 (array, i, memrchr,
> -                                    CPU_FEATURE_USABLE (AVX2),
> +                                    (CPU_FEATURE_USABLE (AVX2)
> +                                     && CPU_FEATURE_USABLE (LZCNT)),
>                                      __memrchr_avx2)
>               X86_IFUNC_IMPL_ADD_V3 (array, i, memrchr,
>                                      (CPU_FEATURE_USABLE (AVX2)
> +                                     && CPU_FEATURE_USABLE (LZCNT)
>                                       && CPU_FEATURE_USABLE (RTM)),
>                                      __memrchr_avx2_rtm)
>               /* ISA V2 wrapper for SSE2 implementation because the SSE2
> --
> 2.35.1
>