[Bug target/115029] New: FFT computation performance regression, x86, between gcc-14 and gcc-13 on skylake platform

[Bug target/115029] New: FFT computation performance regression, x86, between gcc-14 and gcc-13 on skylake platform

[colin.king at intel dot com]

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

[Bug target/115029] FFT computation performance regression, x86, between gcc-14 and gcc-13 on skylake platform

[colin.king at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115029

--- Comment #1 from Colin Ian King <colin.king at intel dot com> ---
Created attachment 58174
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58174&action=edit
gcc-13 disassembly

[Bug target/115029] FFT computation performance regression, x86, between gcc-14 and gcc-13 on skylake platform

[colin.king at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115029

--- Comment #2 from Colin Ian King <colin.king at intel dot com> ---
Created attachment 58175
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58175&action=edit
gcc-14 disassembly

[Bug target/115029] [14/15 regression] FFT computation performance regression, x86, between gcc-14 and gcc-13 on skylake platform

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115029

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.2

[Bug target/115029] [14/15 regression] FFT computation performance regression, x86, between gcc-14 and gcc-13 on skylake platform

[lin1.hu at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115029

Hu Lin <lin1.hu at intel dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lin1.hu at intel dot com

--- Comment #3 from Hu Lin <lin1.hu at intel dot com> ---
According to my investigation, the regression is about 0.9% regression on
cascadelake. And for Sapphire rapids, gcc14 has about a 4% improvement.