[PATCH v2 0/6] x86: Fix AVX2 string functions requiring BMI1, BMI2 or LZCNT (BZ #29611)

[Aurelien Jarno]

On 2022-10-02 17:11, Noah Goldstein via Libc-alpha wrote:
> On Sun, Oct 2, 2022 at 2:09 PM Aurelien Jarno <aurelien@aurel32.net> wrote:
> >
> > On 2022-10-02 09:21, Noah Goldstein wrote:
> > > On Sun, Oct 2, 2022 at 5:34 AM Aurelien Jarno <aurelien@aurel32.net> wrote:
> > > >
> > > > Some early Intel Haswell CPU have AVX2 instructions, but do not have
> > > > BMI1 and BMI2 instructions. Some AVX2 string functions only check for
> > > > AVX2, but use BMI1, 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
> > >
> > > str(case)cmp as well, correct?
> >
> > Oops, yes forgot to update the cover letter on that aspect.
> >
> > > > 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 BMI1 and BMI2 instructions in the
> > > > source code by the "ud2" instruction and disabling the BMI1, BMI2
> > > > feature detection, and running the testsuite.
> > > >
> > > > Resolves: BZ #29611
> > > >
> > > > Change v1 -> v2:
> > > > - Better scan for BMI2 instructions (shlx and shrx) and BMI1
> > > >   instructions (blsmsk) instructions following the feedback from Noah
> > > >   Goldstein
> > > >
> > > > Aurelien Jarno (6):
> > > >   x86: include BMI1 and BMI2 in x86-64-v3 level
> > > >   x86-64: Require BMI2 for AVX2 str*cmp and wcs(n)cmp implementations
> > > >   x86-64: Require BMI2 for AVX2 (raw|w)memchr implementations
> > > >   x86-64: Require LZCNT for AVX2 memrchr implementation
> > > >   x86-64: Require BMI1/BMI2 for AVX2 strrchr and wcsrchr implementations
> > > >   x86-64: Require BMI2 for AVX2 memrchr implementation
> > > >
> > > >  sysdeps/x86/get-isa-level.h                 |  2 +
> > > >  sysdeps/x86/isa-level.h                     |  2 +
> > > >  sysdeps/x86_64/multiarch/ifunc-avx2.h       |  2 +
> > > >  sysdeps/x86_64/multiarch/ifunc-impl-list.c  | 86 ++++++++++++++++-----
> > > >  sysdeps/x86_64/multiarch/ifunc-strcasecmp.h |  1 +
> > > >  sysdeps/x86_64/multiarch/strcmp.c           |  4 +-
> > > >  sysdeps/x86_64/multiarch/strncmp.c          |  4 +-
> > > >  7 files changed, 76 insertions(+), 25 deletions(-)
> > > >
> > > > --
> > > > 2.35.1
> > > >
> > >
> >
> > --
> > Aurelien Jarno                          GPG: 4096R/1DDD8C9B
> > aurelien@aurel32.net                 http://www.aurel32.net
> 
> Patchset looks good.
> 
> Do you have commit permissions? If not I can push them for you.

Yes, I can commit them, though I'll wait for v3 to be reviewed.

> Thanks for the bugfix!
> 
> NB:
> the str*(case)cmp, wcs(n)cmp bug affects 2.36, 2.35, 2.34, 2.33.

Yep, I have already prepared a backport of the whole patchset to those
branches.

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