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

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

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?

> 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
>