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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Failure to optimize comparison of variable with same variable involved in bitwise or using andnot"
href="https://bugs.llvm.org/show_bug.cgi?id=47305">47305</a>
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<th>Summary</th>
<td>Failure to optimize comparison of variable with same variable involved in bitwise or using andnot
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>bool f(int a, int b)
{
return a == (a | b);
}
With -mbmi, GCC outputs this:
f(int, int):
andn eax, edi, esi
sete al
ret
LLVM outputs this:
f(int, int):
or esi, edi
cmp esi, edi
sete al
ret
I have no idea whether there is an LLVM instruction that fits along with this
pattern (if not, I guess you could make a new one or else just change this to
an x86-specific issue) but this transformation seems useful.</pre>
</div>
</p>
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