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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Improve cost model for lengthens followed by truncates"
href="http://llvm.org/bugs/show_bug.cgi?id=21370">21370</a>
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<th>Summary</th>
<td>Improve cost model for lengthens followed by truncates
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>james.molloy@arm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The loop vectorizer does not have a model for sequences of
lengthen->arithmetic->truncate. These sequences are often generated due to
implicit int promotions when dealing with sub-int types, and the
lengthen/truncate can be elided.
One of the "obvious" solutions to this would be to question why the
lengthen/truncate sequence hadn't been removed in the first place. This has
been discussed on-list, and keeping it is deliberate. If the sequence is not
vectorized, the lengthens and truncates would have to be generated because most
architectures can't do scalar arithmetic on sub-word types. Only if it is
vectorized can we do 8-bit or 16-bit arithmetic. So the model for this needs to
be in the vectorizer.
In fact, if the vectorizer is forced to vectorize, it does actually remove the
lengthen/truncate in its generated code. It just doesn't account for this in
its cost model.
char arr[5000];
char arr2[5000];
void f() {
for (int i = 0; i < 5000; ++i) {
int a = arr[i] + 1;
arr2[i] = a;
}
}
./bin/clang -O3 -o - -S test-vec2.c -target arm64 -Rpass=loop-vectorize
-Rpass-analysis=loop-vectorize
test-vec2.c:5:3: remark: unrolled with interleaving factor 2 (vectorization not
beneficial) [-Rpass=loop-vectorize]
for (int i = 0; i < 5000; ++i) {
^</pre>
</div>
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