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From: "ktkachov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/98477] New: aarch64: Unnecessary GPR -> FPR moves for
 conditional select
Date: Wed, 30 Dec 2020 09:35:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98477

            Bug ID: 98477
           Summary: aarch64: Unnecessary GPR -> FPR moves for conditional
                    select
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64

Code like
void
foo (int a, double *b)
{
  *b =3D a ? 10000.0 : 200.0;
}

generates:
foo:
        cmp     w0, 0
        mov     x2, 149533581377536
        movk    x2, 0x40c3, lsl 48
        mov     x0, 4641240890982006784
        fmov    d0, x2
        fmov    d1, x0
        fcsel   d0, d0, d1, ne
        str     d0, [x1]
        ret

We don't need to do the FCSEL on the FPR side if we're just storing it to
memory. We can just do a GPR CSEL and avoid the FMOVs.
I've seen this pattern in the disassembly of some math library routines.
Maybe we should add a =3Dw,w,w alternative to the CSEL patterns in the back=
end?=