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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 - new PM/SimpleLoopUnswitch 11% regression from old PM/LoopUnswitch"
href="https://bugs.llvm.org/show_bug.cgi?id=37420">37420</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>new PM/SimpleLoopUnswitch 11% regression from old PM/LoopUnswitch
</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>
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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>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>brzycki@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=20293" name="attach_20293" title="reduced C++ test-case">attachment 20293</a> <a href="attachment.cgi?id=20293&action=edit" title="reduced C++ test-case">[details]</a></span>
reduced C++ test-case
The new pass manager (-fexperimental-new-pass-manager) is showing an 11%
regression on a proprietary benchmark on Aarch64 (around 6% on x86_64). Upon
further analysis the core of the problem is the SimpleLoopUnswitch code does
not catch a case the original LoopUnswitch code does. This check is very hot
and without this optiomization another ~40M instructions are executed inside
the hot region.
Attached is the reduced test-case (reduced.cpp). To compile simply use:
good:
clang++ -O3 -emit-llvm -S reduced.cpp -o reduced_good.ll
bad:
clang++ -O3 -fexperimental-new-pass-manager -emit-llvm -S reduced.cpp -o
reduced_bad.ll
The successful case will have:
for.body.i.i.i.us:
...
br i1 %cmp.i.i.i68.us, label %for.body.i.i.i.us, label
%_ZNK2JJ2paERK2EE.exit.loopexit.i
for.body.i.i.i:
...
br i1 %cmp.i.i.i68, label %for.body.i.i.i, label
%_ZNK2JJ2paERK2EE.exit.loopexit.i</pre>
</div>
</p>
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