[Bug target/121994] New: [16 Regression] 15% slowdown of 538.imagick_r on AMD Zen2 since r16-3396-g9823624395a946

[pheeck at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121994

            Bug ID: 121994
           Summary: [16 Regression] 15% slowdown of 538.imagick_r on AMD
                    Zen2 since r16-3396-g9823624395a946
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pheeck at gcc dot gnu.org
                CC: liuhongt at gcc dot gnu.org
            Blocks: 26163
  Target Milestone: ---
              Host: x86_64-linux
            Target: x86_64-linux

As seen here

https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=287.507.0

there was a 15% exec time slowdown of the 538.imagick_r SPEC 2017
benchmark when compiled with -Ofast -march=native -flto on an AMD Zen2
machine. I bisected it to r16-3396-g9823624395a946.

commit 9823624395a946bb08a74e5aa4fb5d8bcebacfdf
Author: liuhongt <hongtao....@intel.com>
Date:   Mon Jul 28 18:06:06 2025 -0700

    Enable unroll in the vectorizer when there's reduction for
FMA/DOT_PROD_EXPR/SAD_EXPR


Referenced Bugs:

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