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From: "nathanael.schaeffer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114107] New: poor vectorization at -O3 when
 dealing with arrays of different multiplicity, good with -O2
Date: Sun, 25 Feb 2024 23:40:30 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114107

            Bug ID: 114107
           Summary: poor vectorization at -O3 when dealing with arrays of
                    different multiplicity, good with -O2
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathanael.schaeffer at gmail dot com
  Target Milestone: ---

A simple loop multiplying two arrays, with different multiplicity fails to
vectorize efficiently with -O3.
Target is AVX x86_64.
The loop is the following, where 4 consecutive values in data are multiplie=
d by
the same factor :

    for (int i=3D0; i<n; i++) {
     for (int k=3D0; k<4; k++) data[4*i+k] *=3D factor[i];
    }

See the very poor generated assembly with -O3 on godbolt, while=20
the correct solution of a simple vbroadcastsd is generated by gcc 12.1+ with
-O2=20

https://godbolt.org/z/fWj34bbhq=