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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Inconsistent optimization of bit twiddling function"
href="https://bugs.llvm.org/show_bug.cgi?id=45694">45694</a>
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<th>Summary</th>
<td>Inconsistent optimization of bit twiddling function
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>unsigned r1(unsigned a, unsigned b, unsigned mask)
{
return a ^ ((a ^ b) & mask);
}
unsigned r2(unsigned a, unsigned b, unsigned mask)
{
return (~mask & a) | (b & mask);
}
On x86, if `-mbmi` is on, `r1` is translated to `r2`, and then `andn` is used
to generate faster code. If `-mbmi` is off, this optimization does not occur.
Presumably, this would mean that then `r1` is faster than `r2`. I would thus
suggest that in those cases `r2` be optimized to `r1`.
With `-mbmi` : <a href="https://godbolt.org/z/CrJboc">https://godbolt.org/z/CrJboc</a>
Without `-mbmi` : <a href="https://godbolt.org/z/kg3vH6">https://godbolt.org/z/kg3vH6</a></pre>
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</p>
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