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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - clang loop vectorizer doesn't work when reduction variable is a slot in an array"
href="https://llvm.org/bugs/show_bug.cgi?id=27625">27625</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>clang loop vectorizer doesn't work when reduction variable is a slot in an array
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.8
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>albertnetymk@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>```
#define SIZE (1 << 15)
#define XMM_ALIGNMENT_BYTES 16
float *vec_a __attribute__((aligned (XMM_ALIGNMENT_BYTES)));
float *vec_b __attribute__((aligned (XMM_ALIGNMENT_BYTES)));
float *vec_c __attribute__((aligned (XMM_ALIGNMENT_BYTES)));
void matvec_autovec()
{
int i, j;
for (i = 0; i < SIZE; i++) {
for (j = 0; j < SIZE; j++) {
vec_c[i] += vec_a[j] * vec_b[j];
}
}
}
```
Command used: `clang-3.8 -c -S -Ofast -Rpass-analysis=loop-vectorize test.c`
```
test.c:17:22: remark: loop not vectorized: value that could not be identified
as reduction is used outside the loop [-Rpass-analysis=loop-vectorize]
vec_c[i] += vec_a[j] * vec_b[j];```
```
I was expecting that `vec_c[i]` to be treated as inner loop invariant and the
vectorization would work. The auto-vectorization works fine if I use a tmp
variable for accumulation. Could someone explain why the vectorizer bails out
in this case?</pre>
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