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From: "rguenth at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug string/27457] New: vzeroupper use in AVX2 multiarch string
 functions cause HTM aborts
Date: Mon, 22 Feb 2021 12:40:32 +0000
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X-Bugzilla-Component: string
X-Bugzilla-Version: 2.31
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27457

            Bug ID: 27457
           Summary: vzeroupper use in AVX2 multiarch string functions
                    cause HTM aborts
           Product: glibc
           Version: 2.31
            Status: NEW
          Severity: normal
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

The use of vzeroupper in for example strcmp on a AVX2 capable machine like
Skylake-X causes HTM aborts when used inside transactions.  This causes sev=
ere
performance degradation for some workloads compared to glibc without those
multiarch implementations.

For one specific benchmark the following hack restores performance (as does
removing the VZEROUPPER or replacing it with the way more costly VZEROALL):

diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S
b/sysdeps/x86_64/multiarch/strcmp-avx2.S
index ee82fa3e19..208b396557 100644
--- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
+++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
@@ -127,7 +127,8 @@ L(return):
        movzbl  (%rsi, %rdx), %edx
        subl    %edx, %eax
 # endif
-       VZEROUPPER
+       vpxor   %ymm0, %ymm0, %ymm0
+       vpxor   %ymm1, %ymm1, %ymm1
        ret

        .p2align 4

--=20
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