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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - BranchProbabilityInfo is not using the right heuristics inside loops"
href="http://llvm.org/bugs/show_bug.cgi?id=18705">18705</a>
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<th>Summary</th>
<td>BranchProbabilityInfo is not using the right heuristics inside loops
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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
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>bob.wilson@apple.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>The BranchProbabilityInfo analysis applies a series of heuristics to determine
branch weights. In cases when multiple heuristics could apply, it uses the
first one. The LoopBranchHeuristics are the 4th of 8 heuristics that it tries.
The calcLoopBranchHeuristics function starts like this:
Loop *L = LI->getLoopFor(BB);
if (!L)
return false;
That is the only "return false" in the function. I.E., any basic block inside a
loop will be handled by this heuristic, and we will never even try using any of
the later heuristics inside a loop. Worse, the "InEdges" (anything that is
neither an exit nor a back edge) are always handled by dividing the loop weight
by the number of InEdge successors, which seems unlikely to be the right thing
to do in many cases.</pre>
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