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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86, loop vectorizer] cost model causes loop to not vectorize"
href="https://llvm.org/bugs/show_bug.cgi?id=27619">27619</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[x86, loop vectorizer] cost model causes loop to not vectorize
</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>
<tr>
<th>OS</th>
<td>All
</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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Re-filing the example from <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - [ppc] slow data reorganization in VSX register (through memory)"
href="show_bug.cgi?id=27078">bug 27078</a> as a potential x86 cost model bug and/or
loop vectorizer bug. Note that this vectorizes for PPC but not x86 w/ AVX. But
I don't think the vectorization is being done optimally for PPC: no shuffles
should be needed if we treat everything as <12 x float>.
$ ./clang -O2 27078_orig.c -S -o - -emit-llvm -Rpass-analysis=loop-vectorize
-mavx
27078_orig.c:5:30: remark: the cost-model indicates that vectorization is not
beneficial [-Rpass-analysis=loop-vectorize]
float* blurred_pixel = blurred_row[j];
^
27078_orig.c:5:30: remark: the cost-model indicates that interleaving is not
beneficial [-Rpass-analysis=loop-vectorize]
typedef float Vector3_f[3];
void foo(Vector3_f* blurred_row, int width, float* pixel, float pixel_diff_avg)
{
for (int j = 0; j < width; ++j, pixel += 3) {
float* blurred_pixel = blurred_row[j];
float pixel_diff[3];
pixel_diff[0] = blurred_pixel[0] - pixel[0];
pixel_diff[1] = blurred_pixel[1] - pixel[1];
pixel_diff[2] = blurred_pixel[2] - pixel[2];
pixel_diff[0] -= pixel_diff_avg;
pixel_diff[1] -= pixel_diff_avg;
pixel_diff[2] -= pixel_diff_avg;
pixel[0] += pixel_diff[0];
pixel[1] += pixel_diff[1];
pixel[2] += pixel_diff[2];
}
}</pre>
</div>
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