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From: "goon.pri.low at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112657] New: missed optimization: cmove not
 used with multiple returns
Date: Tue, 21 Nov 2023 21:42:59 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.2.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112657

            Bug ID: 112657
           Summary: missed optimization: cmove not used with multiple
                    returns
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goon.pri.low at gmail dot com
  Target Milestone: ---

This function

int unopt(int c) {
    if (c =3D=3D 14)
        return -9;
    else
        return c;
}

unopt:
        mov     eax, edi
        cmp     edi, 14
        je      .L4
        ret
.L4:
        mov     eax, -9
        ret

Should probably be optimized to:

int opt(int c) {
    if (c =3D=3D 14)
        c =3D -9;

    return c;
}

opt:
        cmp     edi, 14
        mov     eax, -9
        cmovne  eax, edi
        ret

This seems to only really happen when negative numbers are used,

int positive(int c) {
    if (c =3D=3D 14)
        return 9;
    else
        return c;
}

positive:
        mov     eax, edi
        cmp     edi, 14
        mov     edx, 9
        cmove   eax, edx
        ret

Though use of positive values still isn't completely optimized (possibly sa=
me
as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97968)

Also it seems like if the order is reversed:

int reverse(int c) {
    if (c !=3D 14)
        c =3D 9120;

    return c;
}

reverse:
        cmp     edi, 14
        mov     edx, 14
        mov     eax, 9120
        cmove   eax, edx
        ret

We could use %edi in the cmove and eliminate the 2nd instruction.=