https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115029
Bug ID: 115029
Summary: FFT computation performance regression, x86, between
gcc-14 and gcc-13 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 58172
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58172&action=edit
reproducer source code
I'm seeing a ~0.8-1.4% performance regression in gcc-14 compared to gcc-13,
using gcc on Ubuntu 24.04 computing Fast Fourier Transforms on 4096 values.
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 reproducer-fft.c -lm -O2
cking@skylake:~$ ./a.out
1927.23 fft ops per sec
CFLAGS="" gcc-14 reproducer-fft.c -lm -O2
cking@skylake:~$ ./a.out
1906.73 fft ops per sec
I did some analysis on 20 runs of each gcc-13 and gcc-14 runs I noted a ~0.44
percentage std.deviation jitter in my results, but it's clear that there gcc-14
build is always 0.8%-1.4% slower on my i7-6700 test machine, so I think this is
a significant regression in performance to be reported.
Attached is the reproducer.
The original issue appeared when regression testing stress-ng cpu fft number
stressor [1]. I've managed to extract the attached reproducer from the original
code (see attached).
Attached are the reproducer C source and disassembled object code.
References: [1]
https://github.com/ColinIanKing/stress-ng/blob/master/stress-cpu.c
