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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/112304] New: cinc is not being used for (small) constant
Date: Tue, 31 Oct 2023 04:48:23 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: pinskia at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: pinskia at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112304

            Bug ID: 112304
           Summary: cinc is not being used for (small) constant
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64-linux-gnu

Take:
```
int f(int a)
{
        return (a!=3D0)+42;
}
int f1(int a, int b)
{
        return (a!=3D0)+b;
}
```
Currently we get:
```
f:
        cmp     w0, 0
        cset    w0, ne
        add     w0, w0, 42
        ret
f1:
        cmp     w0, 0
        cinc    w0, w1, ne
        ret
```

But f should really be:
```
        mov     w1, #42
        cmp     w0, #0
        cinc    w0, w1, ne
        ret
```

Because for many newer aarch64 cores the `mov` in this case is "free" and d=
oes
not take up an issue slot or even the for a dual issue, the mov and cmp cou=
ld
be issued together.  Even though it is the same # of instruction, the second
case is going to be faster due to latencies.=