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From: "aurelien at aurel32 dot net" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug string/29611] New: Optimized AVX2 string functions
 unconditionally use BMI2 instructions
Date: Sun, 25 Sep 2022 08:17:58 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: glibc
X-Bugzilla-Component: string
X-Bugzilla-Version: 2.31
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: critical
X-Bugzilla-Who: aurelien at aurel32 dot net
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: goldstein.w.n at gmail dot com
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29611

            Bug ID: 29611
           Summary: Optimized AVX2 string functions unconditionally use
                    BMI2 instructions
           Product: glibc
           Version: 2.31
            Status: NEW
          Severity: critical
          Priority: P2
         Component: string
          Assignee: goldstein.w.n at gmail dot com
          Reporter: aurelien at aurel32 dot net
  Target Milestone: ---

Starting with commit aaa23c350715 ("x86: Optimize strlen-avx2.S"), the
optimized AVX2 string functions uses the bzhiq and sarxl instructions, caus=
ing
SIGILL on CPU which does have AVX2 instructions, but does not have BMI2
instructions.

For instance this happens on a Intel(R) Core(TM) i3-4000M CPU @ 2.40GHz:

cpu family      : 6
model           : 60
model name      : Intel(R) Core(TM) i3-4000M CPU @ 2.40GHz
stepping        : 3
microcode       : 0x12

It *seems* that upgrading the microcode to level 0x28 fixes the issue. That
said why it is acceptable to have performance issues with an older microcod=
e,
this should not prevent a system to boot.

This has been reported to affected Debian Bullseye (as the optimization have
been backported to the upstream release/2.31/master tree), Debian Sid and
Fedora Rawhide.

I think that the code should probably be reverted, or at least the IFUNC
functions using BMI2 instructions should be probably be gated by the CPU fl=
ag.
It also poses the question of backporting optimizations to stable release
trees.

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