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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Loop unrolling breaks vectorization"
href="https://bugs.llvm.org/show_bug.cgi?id=35448">35448</a>
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<th>Summary</th>
<td>Loop unrolling breaks vectorization
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<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>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>sanjoy@playingwithpointers.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Perhaps this is already a known issue, but Clang/LLVM trunk does not vectorize
the inner matmult loop unless the "#pragma unroll" is enabled:
void f(int * __restrict__ a, int * __restrict__ b, int * __restrict__ r) {
for (int m = 0; m < 64; m++) {
int c = 0;
// #pragma unroll
for (int i = 0; i < 32; i++) {
c += a[i] * b[m * 32 + i];
}
r[m] = c;
}
}
It looks like the loop unroller fully unrolls the inner loop and the SLP
vectorizer is unable to vectorize as well as the Loop vectorizer would have
vectorized the not-unrolled loop.</pre>
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</p>
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