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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 - missed DCE optimization after loop unrolling"
href="https://bugs.llvm.org/show_bug.cgi?id=43550">43550</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>missed DCE optimization after loop unrolling
</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>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>code-quality
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>trass3r@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#include <cstdint>
#include <utility>
const int N = 4;
void bitreverse(int* data)
{
uint32_t j = 0;
for (uint32_t i = 0; i < N; ++i)
{
if (j > i)
std::swap(data[i], data[j]);
uint32_t k = N/2;
while (k <= j)
{
j -= k;
k /= 2;
}
j += k;
}
}
Even with -O3 after everything is unrolled the while loop unnecessarily
remains: <a href="https://godbolt.org/z/cA-S8J">https://godbolt.org/z/cA-S8J</a>
Of course it's even more severe with N=8: <a href="https://godbolt.org/z/niZPeS">https://godbolt.org/z/niZPeS</a>
Interestingly if you change the swap line slightly it suddenly starts using
pshufd which seems quite clever: <a href="https://godbolt.org/z/5YJnJ1">https://godbolt.org/z/5YJnJ1</a></pre>
</div>
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