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From: "xxs_chy at outlook dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113487] New: Missed optimization:simplify
 demanded bits on multi-use instructions like select
Date: Thu, 18 Jan 2024 17:34:44 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113487

            Bug ID: 113487
           Summary: Missed optimization:simplify demanded bits on
                    multi-use instructions like select
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xxs_chy at outlook dot com
  Target Milestone: ---

Godbolt example: https://godbolt.org/z/EfKrTbK77

Based on common demanded bits from s&8 and s&16,
```
void src(bool c, unsigned a, unsigned b) {
    unsigned s =3D c ? a & 24 : b & 25;
    use(s & 8);
    use(s & 16);
}
```

can be folded to:

```
void tgt(bool c, unsigned a, unsigned b) {
    unsigned s =3D c ? a : b ;
    use(s & 8);
    use(s & 16);
}
```

Both LLVM and GCC missed this optimization opportunity.=