From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 2B41C381DC0F; Thu, 17 Dec 2020 15:24:41 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 2B41C381DC0F From: "ktkachov at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/98350] New: Reassociation breaks FMA chains Date: Thu, 17 Dec 2020 15:24:41 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: tree-optimization X-Bugzilla-Version: unknown X-Bugzilla-Keywords: missed-optimization X-Bugzilla-Severity: normal X-Bugzilla-Who: ktkachov at gcc dot gnu.org 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 keywords bug_severity priority component assigned_to reporter target_milestone Message-ID: 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 List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 17 Dec 2020 15:24:41 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98350 Bug ID: 98350 Summary: Reassociation breaks FMA chains Product: gcc Version: unknown Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: ktkachov at gcc dot gnu.org Target Milestone: --- Consider the testcase: #define N 1024 double a[N]; double b[N]; double c[N]; double d[N]; double e[N]; double f[N]; double g[N]; double h[N]; double j[N]; double k[N]; double l[N]; double m[N]; double o[N]; double p[N]; void foo (void) { for (int i =3D 0; i < N; i++) { a[i] +=3D b[i]* c[i] + d[i] * e[i] + f[i] * g[i] + h[i] * j[i] + k[i] *= l[i] + m[i]* o[i] + p[i]; } } For -Ofast --param=3Dtree-reassoc-width=3D1 GCC generates the loop: .L2: ldr q1, [x1, x0] ldr q0, [x12, x0] ldr q3, [x14, x0] fadd v0.2d, v0.2d, v1.2d ldr q1, [x13, x0] ldr q2, [x11, x0] fmla v0.2d, v3.2d, v1.2d ldr q1, [x10, x0] ldr q3, [x9, x0] fmla v0.2d, v2.2d, v1.2d ldr q1, [x8, x0] ldr q2, [x7, x0] fmla v0.2d, v3.2d, v1.2d ldr q1, [x6, x0] ldr q3, [x5, x0] fmla v0.2d, v2.2d, v1.2d ldr q1, [x4, x0] ldr q2, [x3, x0] fmla v0.2d, v3.2d, v1.2d ldr q1, [x2, x0] fmla v0.2d, v2.2d, v1.2d str q0, [x1, x0] add x0, x0, 16 cmp x0, 8192 bne .L2 with --param=3Dtree-reassoc-width=3D4 it generates: .L2: ldr q5, [x11, x0] ldr q4, [x7, x0] ldr q0, [x3, x0] ldr q3, [x12, x0] ldr q1, [x8, x0] ldr q2, [x4, x0] fmul v3.2d, v3.2d, v5.2d fmul v1.2d, v1.2d, v4.2d fmul v2.2d, v2.2d, v0.2d ldr q16, [x1, x0] ldr q18, [x14, x0] ldr q17, [x13, x0] ldr q0, [x2, x0] ldr q7, [x10, x0] ldr q6, [x9, x0] ldr q5, [x6, x0] ldr q4, [x5, x0] fmla v3.2d, v18.2d, v17.2d fadd v0.2d, v0.2d, v16.2d fmla v1.2d, v7.2d, v6.2d fmla v2.2d, v5.2d, v4.2d fadd v0.2d, v0.2d, v3.2d fadd v1.2d, v1.2d, v2.2d fadd v0.2d, v0.2d, v1.2d str q0, [x1, x0] add x0, x0, 16 cmp x0, 8192 bne .L2 The reassociation is evident. The problem here is that the fmla chains are something we'd want to preserve. Is there a way we can get the reassoc pass to handle FMAs more intelligentl= y?=