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

[Sunil Pandey <skpgkp2@gmail.com>]

On Wed, Oct 5, 2022 at 7:23 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Tue, Oct 4, 2022 at 6:11 PM Sunil Pandey <skpgkp2@gmail.com> wrote:
> >
> > 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.
> >
> > How do you know it is a "by chance" fix, do you have any evidence to back
> > your claim?
>
> There might not be evidence about the intention of the authors but clearly
> the strchr commit message does not clarify that it also fixes memchr/strlen.

ifunc-avx2.h header file is used in many functions, so fix in ifunc-avx2.h fixes
all those functions too. It's not "by chance", I scan all the functions where
ifunc-avx2.h are used before backporting it.

Since this is a backport commit and no extra changes are made, there is no
need to modify the original author commit message.

> >
> > >
> > > NB: at this stage, I haven't verified the consistency of the ifunc
> > > selectors with ifunc-impl-list.c.
> > >
> > > --
> > > Aurelien Jarno                          GPG: 4096R/1DDD8C9B
> > > aurelien@aurel32.net                 http://www.aurel32.net