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 25BB9386184A; Sun, 20 Sep 2020 20:36:06 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 25BB9386184A
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1600634166;
 bh=gX46Ue4nJFQsuz0+yJOET8BF0V2ItjuruD2U8T34nqg=;
 h=From:To:Subject:Date:From;
 b=ihN0tarQMKmonZmV599n0uqFsUyPu4XWYjpjzOQ/3Mho/74FCiXF98cWGbEWwOj/1
 cVxu3SkHl1EKH26ZJZkTnm+E88UPNM/aX98qX35XNFLT1wyPjB3b0YqDa19pKs2x6C
 vygEkMHHlJpPWNUl4+xzikRdWOUA49WQM8SBMY2Q=
From: "already5chosen at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/97127] New: FMA3 code transformation leads to slowdown
 on Skylake
Date: Sun, 20 Sep 2020 20:36:06 +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: 10.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: already5chosen at yahoo 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
Message-ID: <bug-97127-4@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
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sun, 20 Sep 2020 20:36:06 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97127

            Bug ID: 97127
           Summary: FMA3 code transformation leads to slowdown on Skylake
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: already5chosen at yahoo dot com
  Target Milestone: ---

The following clever gcc transformation leads to generation of slower code =
than
non-transformed original:
 a =3D *mem;
 a =3D a + b * c;
where both b and c are reused further down is transformed to:
 a =3D b
 a =3D *mem + a * c;

Or, expressing the same in asm terms
 vmovuxx      (mem), %ymmA
 vfnmadd231xx %ymmB, %ymmC, %ymmA
transformed to
 vmovaxx      %ymmB, %ymmA
 vfnmadd213xx (mem), %ymmC, %ymmA

You may ask "Why transformed variant is slower?" and I can try my best to
answer (my guess is that performance bottleneck is in rename stage rather t=
han
in the execution stage and transformed code occupies 3 rename slots vs 2 re=
name
slots by original) but it would be mostly pointless. What's matters that on
Skylake the transformed variant is slower and I can prove it with benchmark.
BTW, on Haswell too.

You can see comparison of two variants at
https://github.com/already5chosen/others/tree/master/cholesky_solver/gcc-ba=
dopt-fma3
The interesting spot is starting at line 367 in file chol.cpp.
Or starting two lines below .L21: in asm generated by gcc 10.2.0 (chol_a.s).
Run 's_chol_a 100' vs 's_chol_b 100' and see the difference in favor of the
second (de-transformed) variant.
The difference, in this particular case, is small, order of 2-4 percents, b=
ut
very consistent.
In more tight loops I would expect a bigger difference.=