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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Regression in loop vectorizer (trunk vs. llvm 9)"
href="https://bugs.llvm.org/show_bug.cgi?id=43656">43656</a>
</td>
</tr>
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<th>Summary</th>
<td>Regression in loop vectorizer (trunk vs. llvm 9)
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>size_t h_dist (const uint8_t *restrict a, const uint8_t *restrict b, int n) {
size_t r = 0;
// #pragma clang loop vectorize(enable)
for (int i = 0; i < n; i++)
r += __builtin_popcount (a[i] ^ b[i]);
return r;
}
This loop is no longer vectorized with Clang trunk -O3 and we have a 20-30%
perf drop here.
The loop is vectorized with Clang <= 9.
remark: vectorized loop (vectorization width: 2, interleaved count: 2)
[-Rpass=loop-vectorize]
for (int i = 0; i < n; i++)
<a href="https://godbolt.org/z/Td8Ibo">https://godbolt.org/z/Td8Ibo</a></pre>
</div>
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