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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Possible O(n^2) slowness in Global Value Numbering"
href="https://bugs.llvm.org/show_bug.cgi?id=51157">51157</a>
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<th>Summary</th>
<td>Possible O(n^2) slowness in Global Value Numbering
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>10.0
</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>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>wsnyder@wsnyder.org
</td>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=25044" name="attach_25044" title="Examples and runtimes">attachment 25044</a> <a href="attachment.cgi?id=25044&action=edit" title="Examples and runtimes">[details]</a></span>
Examples and runtimes
This was noticed due to a much larger gcc performance bug
(<a href="https://github.com/verilator/verilator/issues/3072">https://github.com/verilator/verilator/issues/3072</a>) but looking into this
there seems a possible clang issue also.
On clang version 10.0.0-4ubuntu1 the attached takes 37 seconds total.
clang++ -I. -c -Os -ftime-report -o /dev/null Vara___024root__114_0.cpp
In clang++, 33 of 37 seconds are in the "Global Value Numbering" stage. The
same splitting above in this stage takes 4.6, 2.9, and 3.9 seconds
(totalling 11.3). This thus seems a O(n^2) performance problem suspect.
This issue was originally reported using gcc 8.2.0 by Matheus Cavalcante
at <a href="https://github.com/verilator/verilator/issues/3072">https://github.com/verilator/verilator/issues/3072</a></pre>
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