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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [loop vectorize] failure to vectorize after GVN load-pre"
href="https://bugs.llvm.org/show_bug.cgi?id=39235">39235</a>
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<th>Summary</th>
<td>[loop vectorize] failure to vectorize after GVN load-pre
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
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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>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
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<th>Reporter</th>
<td>mpfergu@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=20981" name="attach_20981" title="test showing the problem">attachment 20981</a> <a href="attachment.cgi?id=20981&action=edit" title="test showing the problem">[details]</a></span>
test showing the problem
I'm seeing an issue where adding !noalias metadata seems to prevents loop
vectorization from occurring. The issue seems to be that GVN does a PRE of a
load and this prevents the loop vectorizer from functioning.
See the attached .ll file. Supposing it's named opt-me.ll, to reproduce you can
do this:
opt -O3 -loop-vectorize -force-vector-width=4 -force-vector-interleave=1
opt-me.ll -S
Result:
remark: <unknown>:0:0: loop not vectorized: value that could not be identified
as reduction is used outside the loop
remark: <unknown>:0:0: loop not vectorized: value that could not be identified
as reduction is used outside the loop
-> loop not vectorized
Compare with
opt -O3 -loop-vectorize -force-vector-width=4 -force-vector-interleave=1
-enable-load-pre=false opt-me.ll -S
-> loop is vectorized, I see 4 x i32, extractelements, and no loop not
vectorized remarks.
If you remove the !noalias metadata from opt-me.ll, vectorization occurs
whether or not -enable-load-pre=false is passed. This seems unfortunate, as
front-ends would like to provide !noalias metadata where possible to improve
(and not hinder!) optimization.
I believe this problem is present in LLVM 6 as well as master.</pre>
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