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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - SCEV hangs when creating SCEVs for a long chain of add/mul instructions"
href="https://llvm.org/bugs/show_bug.cgi?id=28830">28830</a>
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<th>Summary</th>
<td>SCEV hangs when creating SCEVs for a long chain of add/mul instructions
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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>All
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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
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<th>Component</th>
<td>Global Analyses
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>li1.huang@intel.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>The following code causes SCEV to hang due to endless multiplication expanding
in getMulExpr (ScalarEvolution.cpp), when compiled with clang –S –O2.
int get(unsigned n)
{
unsigned i, j, mult = 1;
for (i = 0; i < 1; i++) {
for (j = 0; j < 30; j++) {
mult *= n++;
}
}
return mult;
}
At some point the inner loop is completed unrolled (by 30) to a long chain of
“add” and “mult” instructions, then indvars follows loop-unroll and triggers
SCEV creations for this long instruction chain. If I change the trip count of
inner loop to 15, then it could finish the work after a short delay, but the
time it spends on creating SCEV seems to grow exponentially.</pre>
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