[Bug tree-optimization/126435] New: [17 Regression] 4-8% slowdown of blender_r on Zen{2,3,4} since r17-2654-gfb0838bc1073aa

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126435

            Bug ID: 126435
           Summary: [17 Regression] 4-8% slowdown of blender_r on
                    Zen{2,3,4} since r17-2654-gfb0838bc1073aa
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jmelcr at gcc dot gnu.org
                CC: rguenth at gcc dot gnu.org
            Blocks: 26163
  Target Milestone: ---
              Host: x86_64-suse-linux
            Target: x86_64-suse-linux
             Build: x86_64-suse-linux

As seen here

https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=286.487.0

there was a 4% exec time slowdown of the blender_r SPEC 2017
benchmark when compiled with  -Ofast -march=native -g -flto=128 PGO on an Zen2
machine. I bisected it to r17-2654-gfb0838bc1073aa.

fb0838bc1073aa5f22ec38e0cca2d70d625c6d71 is the first 'bad' commit
commit fb0838bc1073aa5f22ec38e0cca2d70d625c6d71
Author: Richard Biener <[email protected]>
Date:   Wed Jul 15 10:37:57 2026 +0200

    Support two-lane vector BB reductions without target support

This is a regression against GCC 16. See the comparison
here:

https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=326.487.0&plot.1=313.487.0&plot.2=350.487.0&plot.3=279.487.0&plot.4=402.487.0&plot.5=415.487.0&plot.6=775.487.0&plot.7=929.487.0&plot.8=1077.487.0&plot.9=1207.487.0&plot.10=1520.487.0&plot.11=286.487.0&

There were also these blender_r slowdowns in the same timeframe (so probably
caused by the same commit):

6% Zen3 -Ofast -march=native -g -flto=128 PGO
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=477.487.0

6% Zen3 -Ofast -march=native -g -flto=128
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=475.487.0

8% Zen4 -Ofast -march=x86-64-v3 -g -flto=128 
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=1118.487.0


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26163
[Bug 26163] [meta-bug] missed optimization in SPEC (2026, 2k17, 2k and 2k6 and
95)