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From: "colin.king at intel dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/115025] New: prime computation performance regression,
 x86, between gcc-14 and gcc-13 on skylake platform
Date: Fri, 10 May 2024 08:18:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115025

            Bug ID: 115025
           Summary: prime 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 58163
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D58163&action=3Dedit
reproducer source code

I'm seeing a ~7% performance regression in gcc-14 compared to gcc-13, using=
 gcc
on Ubuntu 24.04 computing prime numbers:

Versions:
gcc version 13.2.0 (Ubuntu 13.2.0-23ubuntu4)=20
gcc version 14.0.1 20240412 (experimental) [master r14-9935-g67e1433a94f]
(Ubuntu 14-20240412-0ubuntu1)=20

cking@skylake:~$ CFLAGS=3D"" gcc-13 -O2 reproducer-prime.c -lm
cking@skylake:~$ ./a.out=20
473.04 prime ops per sec

cking@skylake:~$ CFLAGS=3D"" gcc-14 -O2 reproducer-prime.c -lm
cking@skylake:~$ ./a.out=20
439.86 prime ops per sec

Attached is the reproducer. Note that the use of
__attribute__((optimize("-O3"))) and/or __builtin_expect((x), 0) does not
affect the performance regression.

The original issue appeared when regression testing stress-ng cpu prime num=
ber
stressor [1]. I've managed to extract the attached reproducer from the orig=
inal
code (see attached).

Attached are the reproducer C source and disassembled object code.=20

References: [1]
https://github.com/ColinIanKing/stress-ng/blob/master/stress-cpu.c=