https://llvm.org/bugs/show_bug.cgi?id=29103
Bug ID: 29103
Summary: Probability of taking branch when checking for NaN is
too high
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: ameh...@ca.ibm.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
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)
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