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

[Aurelien Jarno <aurelien@aurel32.net>]

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.

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