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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 removal of cross-iteration dep in Loop Vectorizer"
href="https://bugs.llvm.org/show_bug.cgi?id=52462">52462</a>
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</tr>
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<th>Summary</th>
<td>Missed removal of cross-iteration dep in Loop Vectorizer
</td>
</tr>
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<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>
<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>
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<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>#include<stdint.h>
void
foo (uint64_t* __restrict pdst, uint64_t* psrc, uint64_t shift)
{
for (int64_t i = 0; i != 64; i++)
{
uint64_t shift_i = shift >> i;
pdst[i] = psrc[i] + shift_i;
}
}
void
bar (uint64_t* __restrict pdst, uint64_t* psrc, uint64_t shift)
{
for (int64_t i = 0; i != 64; i++)
{
pdst[i] = psrc[i] + shift;
shift >>= 1;
}
}
LLVM/GCC can vectorize foo but not bar since there's cross-iteration dep for
shift >>= 1, but shift >>= 1 is just shift >> i in loop.
<a href="https://godbolt.org/z/GfccMhszc">https://godbolt.org/z/GfccMhszc</a></pre>
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</p>
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