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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Exponential expansion in IndVarSimplify/ScalarEvolutionExpander"
href="https://llvm.org/bugs/show_bug.cgi?id=31543">31543</a>
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<th>Summary</th>
<td>Exponential expansion in IndVarSimplify/ScalarEvolutionExpander
</td>
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<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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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>opt
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>jesper.antonsson@ericsson.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17805" name="attach_17805" title="reduced ll file that gives exponential expansion with opt">attachment 17805</a> <a href="attachment.cgi?id=17805&action=edit" title="reduced ll file that gives exponential expansion with opt">[details]</a></span>
reduced ll file that gives exponential expansion with opt
File size of output file and time taken by opt is an exponential function of a
loop iteration count for the attached file:
opt -S -O1 -o foo.opt.ll foo.ll
alternatively, if -O1 is expanded and reduced:
opt -S -instcombine -loop-rotate -licm -instcombine -indvars -o foo.opt.ll
foo.ll
It's IndVarSimplify::rewriteLoopExitValues() that calls ScalarEvolutionExpander
to calculate loop exit value, but the expansion becomes exponential.
If I change the loop iteration count on this line:
%_tmp12 = icmp slt i16 %_tmp11, 6
The output ll file grows and compilation time lengthens exponentially. Some
sample values:
bound file size of foo.opt.ll
6 4912
7 8944
8 17088
..
15 2065266</pre>
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