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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - High loop unroll counts can result in long compile times"
href="http://llvm.org/bugs/show_bug.cgi?id=20058">20058</a>
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<th>Summary</th>
<td>High loop unroll counts can result in long compile times
</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
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>meheff@google.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Compilation can take a long time with high loop counts and is looks to be
quadratic with loop count. The addition of loop unroll pragmas
(<a href="http://reviews.llvm.org/D4147">http://reviews.llvm.org/D4147</a>, not yet submitted) will make it more likely
that users will encounter this bad behavior.
Attached is the IR from the following simple C code:
void loop(int *a)
{
for (int i = 0; i < 1000000; i++) {
a[i]++;
}
}
I measured ~2s to unroll this loop 16K times with release build at head, ~10s
to unroll roll it 32K times, and ~50s for 64K, so it appears to be growing (at
least) quadratically. Profiling suggests time is being taken in somewhere
beneath llvm::UnrollLoop -> FoldBlockIntoPredecessor ->
llvm::ScalarEvolution::forgetLoop.
Run:
opt loop.ll -loop-rotate -loop-unroll -unroll-count 16000 -unroll-allow-partial
-unroll-threshold=10000000</pre>
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</p>
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