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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Loop unroll by N iterations takes O(N^2) time in domtree updater"
href="https://bugs.llvm.org/show_bug.cgi?id=43134">43134</a>
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<th>Summary</th>
<td>Loop unroll by N iterations takes O(N^2) time in domtree updater
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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>Windows NT
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</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>efriedma@quicinc.com
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</tr>
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<th>CC</th>
<td>alina.sbirlea@gmail.com, brzycki@gmail.com, kubakuderski@gmail.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Testcase:
unsigned x() {
unsigned z = 3;
unsigned LIMIT = 16000;
_Pragma("unroll(16384)") for (int i = 0; i < LIMIT; ++i)
z *= z;
return z;
}
The compile times look roughly like the following, with "clang -O2 -mllvm
-unroll-threshold=999999":
LIMIT=1000: 0.3s
LIMIT=2000: 1.0s
LIMIT=4000: 3.6s
LIMIT=8000: 14.3s
LIMIT=16000: 57.8s
This is obviously non-linear; I think it's roughly quadratic. -ftime-report
says the time is spent in unrolling. I "profiled" in a debugger, and it looks
like the time is spent updating the domtree.
Without the "-unroll-threshold", LLVM refuses to unroll the loop 16000 times,
but it will unroll 8000 times.</pre>
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