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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 65120] Missed optimization for bitset based equality
From:       LLVM Bugs via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2023-08-30 19:40:10
Message-ID: 20230830194010.5b5b89ae620f1894 () email ! llvm ! org
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<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/65120>65120</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Missed optimization for bitset based equality
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          bmaurer
      </td>
    </tr>
</table>

<pre>
    https://godbolt.org/z/nTseoW9z1
```
bool test(unsigned long long x) {
    auto mask = x &amp; 0xf;
    return (mask == 10 || mask == 1 || mask == 2);
}
```

clang generates:
```
test(unsigned long long): # @test(unsigned long long)
        and     edi, 15
        cmp rdi, 11
        setb    cl
        mov     eax, 1030
        bt eax, edi
        setb    al
        and     al, cl
 ret
```

the comparison against 11 is unnecessary given the range of the mask operation. GCC \
emits shorter code:

```
test(unsigned long long):
        and     edi, 15
        mov     eax, 1030
        bt rax, rdi
        setc    al
        ret
```
</pre>
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