https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115069
Bug ID: 115069
Summary: 8 bit integer vector performance regression, x86,
between gcc-14 and gcc-13 using avx2 target clones on
skylake platform
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: colin.king at intel dot com
Target Milestone: ---
Created attachment 58188
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58188&action=edit
reproducer source code
I'm seeing a ~12-14% performance regression in gcc-14 compared to gcc-13, using
gcc on Ubuntu 24.04:
Versions:
gcc version 13.2.0 (Ubuntu 13.2.0-23ubuntu4)
gcc version 14.0.1 20240412 (experimental) [master r14-9935-g67e1433a94f]
(Ubuntu 14-20240412-0ubuntu1)
cking@skylake:~$ gcc-13 reproducer-vecmath.c -O2
cking@skylake:~$ ./a.out
13540.16 vec8 ops per sec, duration = 14.77 secs
cking@skylake:~$ gcc-14 reproducer-vecmath.c -O2
cking@skylake:~$ ./a.out
11720.25 vec8 ops per sec, duration = 17.06 secs
The original issue appeared when regression testing stress-ng vecmath stressor
[1]. I've managed to extract the attached reproducer from the original code
(see attached).
Salient point to focus on:
1. The issue is also dependant on the TARGET_CLONES macro being defined as
__attribute__((target_clones("mmx,avx,avx2,default"))) - the avx2 target
clones seems to be an issue in reproducing this problem, remove it for gcc-14
and the performance regression is reduced.
Attached are the reproducer C source and disassembled object code.
References: [1]
https://github.com/ColinIanKing/stress-ng/blob/master/stress-vecmath.c
