Re: [PATCH v5 2/2] x86: Optimize strlen-avx2.S

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Oct 4, 2022 at 2:20 PM Aurelien Jarno <aurelien@aurel32.net> wrote:
>
> On 2022-09-28 06:54, Sunil Pandey via Libc-stable wrote:
> > Attached patch fixes BZ# 29611.
> >
> > I would like to backport it to 2.32,2.31,2.30,2.29 and 2.29. Let me know
> > if there is any objection.
>
> Sorry to be late on this. I have a few comments about that patch:
>
> > From 86e1d88e1a3c126597ef39165275ada7564cfce9 Mon Sep 17 00:00:00 2001
> > From: "H.J. Lu" <hjl.tools@gmail.com>
> > Date: Mon, 19 Apr 2021 10:45:07 -0700
> > Subject: [PATCH] x86-64: Require BMI2 for strchr-avx2.S
> >
> > Since strchr-avx2.S updated by
> >
> > commit 1f745ecc2109890886b161d4791e1406fdfc29b8
> > Author: noah <goldstein.w.n@gmail.com>
> > Date:   Wed Feb 3 00:38:59 2021 -0500
> >
> >     x86-64: Refactor and improve performance of strchr-avx2.S
> >
> > uses sarx:
> >
> > c4 e2 72 f7 c0        sarx   %ecx,%eax,%eax
> >
> > for strchr-avx2 family functions, require BMI2 in ifunc-impl-list.c and
> > ifunc-avx2.h.
> >
> > (cherry picked from commit 83c5b368226c34a2f0a5287df40fc290b2b34359)
> > ---
> >  sysdeps/x86_64/multiarch/ifunc-avx2.h      |  4 ++--
> >  sysdeps/x86_64/multiarch/ifunc-impl-list.c | 12 +++++++++---
> >  2 files changed, 11 insertions(+), 5 deletions(-)
>
> First of all 1f745ecc2109890886b161d4791e1406fdfc29b8 never got
> backported to 2.32 and older branches, and strchr-avx2.S in those
> branches do not use BMI2 instructions. So it doesn't make sense to
> backport it.
>
> That said the change in ifunc-avx2.h fixes:
>
> - memchr and rawmemchr, broken by the backport of acfd088a1963 ("x86:
>   Optimize memchr-avx2.S")
> - strlen and strnlen, broken by the backport of aaa23c350715 ("x86:
>   Optimize strlen-avx2.S")
>
> So the issues are fixed, but mostly by chance.
>
> NB: at this stage, I haven't verified the consistency of the ifunc
> selectors with ifunc-impl-list.c.
>

Changes to ifunc-impl-list.c aren't strictly needed since strchr functions
don't use BMI2.  AVX2 strchr functions are still tested on machines with
AVX2 and BMI2.

-- 
H.J.