[Bug target/115069] New: 8 bit integer vector performance regression, x86, between gcc-14 and gcc-13 using avx2 target clones on skylake platform

["colin.king at intel dot com" <gcc-bugzilla@gcc.gnu.org>]

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