From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 1DF9A3849AEA; Fri, 10 May 2024 13:30:50 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 1DF9A3849AEA
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1715347850;
	bh=utIaOYzr2p5nJFL3QJMdPeCQWYtupMqNd2qCe1CxlW0=;
	h=From:To:Subject:Date:From;
	b=Kg0rzGsA0c0/KU9TKf9wKF2ylPaXoSmurOmhfLNXDmvi10lMg9yKZ3K/7IKxetWcr
	 EG+YQ0EC5BjlmT3oQab5r+XG2XeCoUDVdghpwzyzX9WVTps2O40eIO1i8UCP61gUMe
	 odXQJo03ZW9g8ED35aoF9bC0uQO0nydooATwcMLk=
From: "colin.king at intel dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/115029] New: FFT computation performance regression,
 x86, between gcc-14 and gcc-13 on skylake platform
Date: Fri, 10 May 2024 13:30:49 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: colin.king at intel dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
 attachments.created
Message-ID: <bug-115029-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115029

            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=3D58172&action=3Dedit
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)=20
gcc version 14.0.1 20240412 (experimental) [master r14-9935-g67e1433a94f]
(Ubuntu 14-20240412-0ubuntu1)

cking@skylake:~$ CFLAGS=3D"" gcc-13 reproducer-fft.c -lm -O2
cking@skylake:~$ ./a.out=20
1927.23 fft ops per sec

CFLAGS=3D"" gcc-14 reproducer-fft.c -lm -O2
cking@skylake:~$ ./a.out=20
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 gc=
c-14
build is always 0.8%-1.4% slower on my i7-6700 test machine, so I think thi=
s 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 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=