Re: [PATCH 0/4] x86: Fix AVX2 string functions requiring BMI2 or LZCNT (BZ #29611)

[Aurelien Jarno <aurelien@aurel32.net>]

On 2022-10-01 15:17, Noah Goldstein via Libc-alpha wrote:
> On Sat, Oct 1, 2022 at 3:11 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > On Sat, Oct 1, 2022 at 12:09 PM Aurelien Jarno <aurelien@aurel32.net> wrote:
> > >
> > > Some early Intel Haswell CPU have AVX2 instructions, but do not have
> > > BMI2 instructions. Some AVX2 string functions only check for AVX2, but
> > > use BMI2 or LZCNT instructions. This patchset tries to fix that.
> > >
> > > While most fixes only change ifunc-impl-list.c, and thus only concerns
> > > the testsuite, the strn(case)cmp is a real issue affecting early Intel
> > > Haswell CPU, reported to affect Debian Sid and Fedora Rawhide.
> > >
> > > On the other hand, the check for LZCNT in memrchr is purely for
> > > correctness, I am not aware of a CPU implementing AVX2 without LZCNT.
> > >
> > > This has been tested by remplacing all BMI2 and LZCNT instruction in the
> > > source code by the "ud2" instruction and disabling the BMI1, BMI2
> > > feature detection, and running the testsuite.
> > >
> > > Resolves: BZ #29611
> > >
> > > Aurelien Jarno (4):
> > >   x86: include BMI1 and BMI2 in x86-64-v3 level
> > >   x86-64: Require BMI2 for AVX2 strn(case)cmp and wcsncmp
> > >     implementations
> > >   x86-64: Require BMI2 for AVX2 (raw|w)memchr implementations
> > >   x86-64: Require LZCNT for AVX2 memrchr implementation
> > >
> >
> > We also need BMI2 check in ifunc-impl-list for:
> > strcasecmp
> > strcmp
> > strcasecmp_l
> > strrchr
> > wcsrchr
> > wcscmp
> >
> > If you want you can make patches, otherwise I can.
> 
> This is a duplicate of a comment I left in the strn(case)cmp patchset,
> but leaving here so the information is not scattered:
> 
> The ifunc change in strncmp.c and ifunc-strcasecmp.h need to be backport
> to 2.33, 2.34, 2.35.
> 
> Also separate changes for ifunc need to be backport to strncmp.c:
> 2.32, 2.31, 2.30, 2.29, 2.28 for a `tzcnt` usage that needs
> BMI1.

Is that really correct? According the commit log TZCNT is used in a way
that is compatible with BSF:

commit 1457016337072d1b6739f571846b619596990cb7
Author: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Date:   Thu May 3 11:09:30 2018 -0500

    x86-64: Optimize strcmp/wcscmp and strncmp/wcsncmp with AVX2
    
    Optimize x86-64 strcmp/wcscmp and strncmp/wcsncmp with AVX2. It uses vector
    comparison as much as possible. Peak performance observed on a SkyLake
    machine: 9x, 3x, 2.5x and 5.5x for strcmp, strncmp, wcscmp and wcsncmp,
    respectively. The larger the comparison length, the more benefit using
    avx2 functions, except on the strcmp, where peak is observed at length
    == 32 bytes. Select AVX2 strcmp/wcscmp on AVX2 machines where vzeroupper
    is preferred and AVX unaligned load is fast.
    
    NB: It uses TZCNT instead of BSF since TZCNT produces the same result
    as BSF for non-zero input.  TZCNT is faster than BSF and is executed
    as BSF if machine doesn't support TZCNT.

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