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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Failure to reduce loop w/known exit conditions (pass ordering?)"
href="http://llvm.org/bugs/show_bug.cgi?id=20812">20812</a>
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<th>Summary</th>
<td>Failure to reduce loop w/known exit conditions (pass ordering?)
</td>
</tr>
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<th>Product</th>
<td>libraries
</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>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>listmail@philipreames.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12961" name="attach_12961" title="C++ source code for example">attachment 12961</a> <a href="attachment.cgi?id=12961&action=edit" title="C++ source code for example">[details]</a></span>
C++ source code for example
LLVM O3 will currently convert this loop into a single increment of "counter":
void test_return_high_gt() {
for(int i = 0; i < 20; i++) {
if( i > 10 ) return;
counter++;
}
}
But will not perform the same optimization on this loop:
int test_return_value_high_gt() {
for(int i = 0; i < 20; i++) {
if( i > 10 ) return 0;
counter++;
}
return 1;
}
I have not analyzed this extensively, but I suspect this is a pass ordering
problem. If I run the IR generated by Clang for the second example through
"opt -O3" twice, I get the exact same IR as in the first example.
I've attached both the cpp example and the LL file generated by clang 3.4 w/O3.
I used a recent TOT opt version for the pass ordering investigation.</pre>
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