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From: "ktkachov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97984] New: [10/11 Regression] Worse code for
 -O3 than -O2 on aarch64 vector multiply-add
Date: Wed, 25 Nov 2020 11:49:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97984

            Bug ID: 97984
           Summary: [10/11 Regression] Worse code for -O3 than -O2 on
                    aarch64 vector multiply-add
           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: ---
            Target: aarch64

The code:
void x (long * __restrict a, long * __restrict b)
{
  a[0] *=3D b[0];
  a[1] *=3D b[1];
  a[0] +=3D b[0];
  a[1] +=3D b[1];
}

at -O2 generates:
x:
        ldp     x4, x3, [x0]
        ldp     x2, x1, [x1]
        madd    x2, x2, x4, x2
        madd    x1, x1, x3, x1
        stp     x2, x1, [x0]
        ret

whereas at -O3 it does:
x:
        ldp     x2, x3, [x0]
        ldr     x4, [x1]
        ldr     q1, [x1]
        mul     x2, x2, x4
        ldr     x4, [x1, 8]
        fmov    d0, x2
        ins     v0.d[1], x3
        mul     x1, x3, x4
        ins     v0.d[1], x1
        add     v0.2d, v0.2d, v1.2d
        str     q0, [x0]
        ret

which is clearly inferior.
GCC 9 used to generate the good code for both -O2 and -O3=