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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [Vectorization] Recognize strided access for vectorization"
href="http://llvm.org/bugs/show_bug.cgi?id=17673">17673</a>
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<th>Summary</th>
<td>[Vectorization] Recognize strided access for vectorization
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Loop Optimizer
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>renato.golin@linaro.org
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>Stride Access is not supported in the LLVM vectorizer. In order to recognize
loops like:
for (i..N/3) {
a[3*i] = b[3*i] + I;
a[3*i+1] = b[3*i+1] + J;
a[3*i+2] = b[3*i+2] + K;
}
We can't re-roll the loop, nor we can use sequential access, since I, J and K
might have different values (assuming I, J, K, N constant). Detecting the
stride in this case is essential to be able to use interleaved loads/stores
(such as VLDN/VSTN on ARM).
The case where I==J==K can, sometimes, be rerolled, and on such cases, the
current vectorizer can already deal with the transformed loop, but we might
still want the vectorizer to deal with such cases, when loop re-roller is
disabled or it can't detect specific semantics.
Another case, when "a" is a reduction variable, like:
for (i..N/3) {
a += b[3*i] + I;
a += b[3*i+1] + J;
a += b[3*i+2] + K;
}
Should be dealt with the same way, so we need to make sure that the reduction
logic can also cope with non-unit strides.
Finally, non-unit memory induction should be transformed/mapped into the stride
access above, so that loops such as this:
for (i..N/3) {
a++ = b++ + I;
a++ = b++ + J;
a++ = b++ + K;
}
can also be vectorized.</pre>
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