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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - *Potential* missing optimization opportunity for bzhi when input is 64bit but output is 32 bit?"
href="https://bugs.llvm.org/show_bug.cgi?id=36419">36419</a>
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<th>Summary</th>
<td>*Potential* missing optimization opportunity for bzhi when input is 64bit but output is 32 bit?
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>It's a very simple function that gets some low bits from the cache.
<a href="https://godbolt.org/g/f9JjP3">https://godbolt.org/g/f9JjP3</a>
The first variant does not work when count == 32, but is optimized down to just
the bzhi.
The second variant *does* work when count == 32, but is *NOT* optimized down to
just the bzhi.
The third variant *does* work when count == 32, and *IS* optimized down to just
the bzhi.
The difference between the first and second/third variant is the 32 vs 64 bit
size of mask.
The difference between the first/second and third variant is the 32 vs 64 bit
return type.
I'm wondering if the second variant can still be optimized to bzhi,
and then still return 32-bit by discarding the upper half?</pre>
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