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From: "jamborm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102058] New: 450.soplex regressed on x86_64
 with -Ofast -march=generic (by 8-15%)
Date: Wed, 25 Aug 2021 08:41:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102058

            Bug ID: 102058
           Summary: 450.soplex regressed on x86_64 with -Ofast
                    -march=3Dgeneric (by 8-15%)
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jamborm at gcc dot gnu.org
                CC: rguenth at gcc dot gnu.org
            Blocks: 26163
  Target Milestone: ---

All three LNT x86_64 testers have experienced a regression when
running SPECFP 2006 benchmark 450.soplex compiled with -Ofast
-march=3Dgeneric (as opposed to -march=3Dnative builds which seem not to
be affected).

A znver2 machine regressed by 15%:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=3D414.150.0&plot.1=
=3D300.150.0&

A znver1 machine regressed by 8%:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=3D451.150.0&plot.1=
=3D27.150.0&

An Intel Kabylake machine regressed by 9%:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=3D429.150.0&plot.1=
=3D25.150.0&

I have bisected the regression (on another znver2 machine) to revision
r12-2733-g31855ba6b16:

31855ba6b16cd138d7484076a08cd40d609654b8 is the first bad commit
commit 31855ba6b16cd138d7484076a08cd40d609654b8
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Jul 29 14:14:48 2021 +0200

    Add emulated gather capability to the vectorizer

    This adds a gather vectorization capability to the vectorizer
    without target support by decomposing the offset vector, doing
    sclar loads and then building a vector from the result.  This
    is aimed mainly at cases where vectorizing the rest of the loop
    offsets the cost of vectorizing the gather.
    [...]


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=
)=