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

[Aurelien Jarno <aurelien@aurel32.net>]

On 2022-10-01 15:06, Noah Goldstein wrote:
> 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)
> >        && 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)),
> 
> Also needs BMI2 for the `shlx`. Likewise for avx2 versions.

Good catch, I haven't look for that one, so I haven't encountered the
issue. Similarly there is 'shrx'.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net