https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126850
Bug ID: 126850
Summary: [17 Regression] 5% slowdown of milc on Zen2 since
r17-2979-g141edd02af7056
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: jakub at redhat dot com
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=685.70.0
there was a 5% exec time slowdown of the milc SPEC 2006
benchmark when compiled with -O2 -g -flto=128 PGO on an Zen2
machine. I bisected it to r17-2979-g141edd02af7056.
141edd02af705696f89ff9a804879dc51834898d is the first 'bad' commit
commit 141edd02af705696f89ff9a804879dc51834898d
Author: Jakub Jelinek <[email protected]>
Date: Wed Aug 5 12:15:24 2026 +0200
fold-const, match.pd: Improve ptr_difference_const optimizations
This is a regression against GCC 14 and 15, not GCC 16. See the comparison
here:
https://lnt.opensuse.org/db_default/v4/SPEC/graph?plot.0=732.70.0&plot.1=734.70.0&plot.2=689.70.0&plot.3=746.70.0&plot.4=744.70.0&plot.5=762.70.0&plot.6=921.70.0&plot.7=1031.70.0&plot.8=1192.70.0&plot.9=1498.70.0&plot.10=685.70.0&;
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)
