[Bug tree-optimization/126445] New: [17 Regression] 4-5% slowdown of imagick_r on Zen4 since r17-2654-gfb0838bc1073aa

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126445

            Bug ID: 126445
           Summary: [17 Regression] 4-5% slowdown of imagick_r on Zen4
                    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=1118.507.0

there was a 4% exec time slowdown of the imagick_r SPEC 2017
benchmark when compiled with -Ofast -march=x86-64-v3 -g -flto=128 on an Zen4
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, on par with GCC 15. See the comparison
here:

https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=1117.507.0&plot.1=1135.507.0&plot.2=1126.507.0&plot.3=1138.507.0&plot.4=1174.507.0&plot.5=1589.507.0&plot.6=1118.507.0&


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)