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

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

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.

Finally a corresponding fix is needed for strcmp.c as well (there is
missing BMI2 check in strcmp.c ifunc selection as well as missing
checks in the impl list).

> >  sysdeps/x86/get-isa-level.h                 |  2 +
> >  sysdeps/x86_64/multiarch/ifunc-avx2.h       |  1 +
> >  sysdeps/x86_64/multiarch/ifunc-impl-list.c  | 44 +++++++++++++++------
> >  sysdeps/x86_64/multiarch/ifunc-strcasecmp.h |  1 +
> >  sysdeps/x86_64/multiarch/strncmp.c          |  4 +-
> >  5 files changed, 38 insertions(+), 14 deletions(-)
> >
> > --
> > 2.35.1
> >