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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113632] New: Range info for a^CST could be
 improved
Date: Sun, 28 Jan 2024 06:23:30 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: pinskia at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113632

            Bug ID: 113632
           Summary: Range info for a^CST could be improved
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
void dummy();
_Bool f(unsigned long a)
{
        _Bool cmp =3D a > 8192;
        if (cmp) goto then; else goto e;
then:
        unsigned long t =3D __builtin_clzl(a); // [0,50]=20
        t^=3D63; // [13,63]
        return t >=3D 13;
e:
  dummy();
  return 0;
}
```

Currently after the t^=3D63; we get:
```
  # RANGE [irange] int [1, 63] MASK 0x3f VALUE 0x0
  _7 =3D _1 ^ 63;
```

But this could/should be improved to [13,63].

If we change to using minus instead:
```
t =3D 63 - t;
```

We get the better range and the comparison (t >=3D 13) is optimized away.
```
Folding statement: t_10 =3D 63 - t_9;
Global Exported: t_10 =3D [irange] long unsigned int [13, 63] MASK 0x3f VAL=
UE 0x0
Not folded
```

Yes this should up in real code, see the LLVM issue for more information on
that.=