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From: "christophm30 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/102793] New: AArch64: sequential comparisons with equal
 conditional blocks don't use ccmp
Date: Fri, 15 Oct 2021 21:07:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102793

            Bug ID: 102793
           Summary: AArch64: sequential comparisons with equal conditional
                    blocks don't use ccmp
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: christophm30 at gmail dot com
  Target Milestone: ---

The following code:

int ccmp(uint64_t* s1, uint64_t* s2, int(*foo)(void))
{
    uint64_t d1, d2, bar;

    d1 =3D *s1++;
    d2 =3D *s2++;
    bar =3D (d1 ^ d2) & 0xabcd;
    if (bar =3D=3D 0 || d1 !=3D d2)
      return foo();
    return 0;
}

int noccmp(uint64_t* s1, uint64_t* s2, int(*foo)(void))
{
    uint64_t d1, d2, bar;

    d1 =3D *s1++;
    d2 =3D *s2++;
    bar =3D (d1 ^ d2) & 0xabcd;
    if (bar =3D=3D 0)
      return foo();
    if (d1 !=3D d2)
      return foo();
    return 0;
}

...produces (GCC master or earlier, ARM64, with -O3):

ccmp:
        ldr     x3, [x0]
        mov     x4, 43981
        ldr     x0, [x1]
        eor     x1, x3, x0
        tst     x1, x4
        ccmp    x3, x0, 0, ne
        bne     .L5
        mov     w0, 0
        ret
.L5:
        mov     x16, x2
        br      x16
noccmp:
        ldr     x3, [x0]
        mov     x4, 43981
        ldr     x0, [x1]
        eor     x1, x3, x0
        tst     x1, x4
        beq     .L8
        cmp     x3, x0
        beq     .L9
.L8:
        mov     x16, x2
        br      x16
.L9:
        mov     w0, 0
        ret

Since both conditional blocks do exactly the same (they call foo()), both
functions could use the ccmp instruction.
I just observed this and have not analysed this any further.=