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From: "jamborm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102178] New: SPECFP 2006 470.lbm regressions
 on AMD Zen CPUs after r12-897-gde56f95afaaa22
Date: Thu, 02 Sep 2021 15:38:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102178

            Bug ID: 102178
           Summary: SPECFP 2006 470.lbm regressions on AMD Zen CPUs after
                    r12-897-gde56f95afaaa22
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jamborm at gcc dot gnu.org
            Blocks: 26163
  Target Milestone: ---
              Host: x86_64-linix
            Target: x86_64-linux

LNT has detected an 18% regression of SPECFP 2006 benchmark 470.lbm
when it is compiled with -Ofast -march=3Dnative on a Zen2 machine:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=3D421.240.0&plot.1=
=3D301.240.0&

...and similarly a 6% regression when it is run on the same machine
with -Ofast:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=3D450.240.0&plot.1=
=3D24.240.0&

I have bisected both on another zen2 machine to commit
r12-897-gde56f95afaaa22 (Run pass_sink_code once more before
store_merging).

Zen1 machine has also seen a similar -march=3Dnative regression in the
same time frame:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=3D450.240.0&plot.1=
=3D24.240.0&

Zen1 -march=3Dgeneric seems to be unaffected, which is also the case for
the Intel machines we track.

Although lbm has been known to have weird regressions caused entirely
by code layout where the compiler was not really at fault, the fact
that both generic code-gen and Zen1 are affected seems to indicate this
is not the case.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D26163
[Bug 26163] [meta-bug] missed optimization in SPEC (2k17, 2k and 2k6 and 95=
)=