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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Probability of taking branch when checking for NaN is too high"
href="https://llvm.org/bugs/show_bug.cgi?id=29103">29103</a>
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<th>Summary</th>
<td>Probability of taking branch when checking for NaN is too high
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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
</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
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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>amehsan@ca.ibm.com
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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>Currently when we have a check to find if a FP value is NaN, the probability of
the path taken, when the input is NaN is 3/8.
If we reduce this probability to something smaller (I tried a value slightly
less than 5%), that will allow that basic blocks to be placed differently and
we will get a higher number of fall throughs (assuming NaNs are really rare).
One place that this matters, is in std::sqrt, where the result will be NaN if
the input is negative. I was looking at a FP code with many calls to std::sqrt,
and realized this.
Currently the function below is where these branch probabilities are decided:
bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB)</pre>
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</p>
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