https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114987
Bug ID: 114987
Summary: floating point vector regression, x86, between gcc 14
and gcc-13 using -O3 and target clones on skylake
platforms
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: colin.king at intel dot com
Target Milestone: ---
Created attachment 58126
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58126&action=edit
reproducer.c source code
I'm seeing a ~10% 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)
king@skylake:~$ CFLAGS="" gcc-13 reproducer.c; ./a.out
4.92 secs duration, 2130.379 Mfp-ops/sec
cking@skylake:~$ CFLAGS="" gcc-14 reproducer.c; ./a.out
5.46 secs duration, 1921.799 Mfp-ops/sec
The original issue appeared when regression testing stress-ng vecfp stressor
[1] using the floating point vector 16 add stressor method. I've managed to
extract the attached reproducer (reproducer.c) from the original code.
Salient points to focus on:
1. The issue is dependant on the OPTIMIZE3 macro in the reproducer being
__attribute__((optimize("-O3")))
2. The issue is also dependant on the TARGET_CLONES macro being defined as
__attribute__((target_clones("mmx,avx,default"))) - the avx target clones
seems to be an issue in reproducing this problem.
Attached are the reproducer.c C source and disassembled object code. The
stress_vecfp_float_add_16.avx from gcc-13 is significantly different from the
gcc-14 code.
References: [1]
https://github.com/ColinIanKing/stress-ng/blob/master/stress-vecfp.c
