https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115024
Bug ID: 115024
Summary: 128 bit division performance regression, x86, between
gcc-14 and gcc-13 using 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 58158
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58158&action=edit
reproducer source code for __int128_t division regression
I'm seeing a 5% 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:~$ CFLAGS="" gcc-13 -O2 reproducer-div128.c
cking@skylake:~$ ./a.out
1650.83 div128 ops per sec
cking@skylake:~$ CFLAGS="" gcc-14 -O2 reproducer-div128.c
cking@skylake:~$ ./a.out
1567.48 div128 ops per sec
The original issue appeared when regression testing stress-ng cpu div128
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("avx,default"))) - the avx target clones seems to
be an issue in reproducing this problem.
Attached are the reproducer C source and disassembled object code.
References: [1]
https://github.com/ColinIanKing/stress-ng/blob/master/stress-cpu.c
