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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112457] Possible better vectorization of
 different reduction min/max reduction
Date: Thu, 09 Nov 2023 12:45:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112457

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |tree-optimization
             Blocks|                            |53947

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, this is because MAX_EXPR detection fails when store motion inserts fl=
ags
(the max =3D max is elided) to avoid store-data races.  Also when using
-Ofast we avoid this but then the next phiopt comes too late to discover
MAX after store motion is applied.

The more practical example is

int foo2 (int max, int n, int * __restrict a)
{
  for (int i =3D 0; i < n; ++i)
    if (max < a[i]) {
        max =3D a[i];
    }
  return max;
}

and that's handled OK.  For your second example, index reduction, there's
already bugreports.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations=