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Allow 0-weight branches in BranchProbabilityInfo.

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Summary:
When computing branch weights in BPI, we used to disallow branches with
weight 0. This is a minor nuisance, because a branch with weight 0 is
different to "don't have information". In the context of
instrumentation, it may mean "never executed", in the context of
sampling, it means "never or seldom executed".

In allowing 0 weight branches, I ran into issues with the switch
expansion code in selection DAG. It is currently hardwired to not handle
branches with weight 0. To maintain the current behaviour, I changed it
to use 1 when it finds 0, but perhaps the algorithm needs changes to
tolerate branches with weight zero.

Reviewers: hansw

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9533

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236617 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Diego Novillo
2015-05-06 17:55:11 +00:00
parent 5a6ea245b0
commit 26e46f2283
5 changed files with 26 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
lib/Analysis/BranchProbabilityInfo.cpp
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@ -201,8 +201,7 @@ bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
if (!Weight)
return false;
Weights.push_back(
std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
Weights.push_back(Weight->getLimitedValue(WeightLimit));
}
assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
@ -553,7 +552,7 @@ uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
uint32_t PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
assert(Sum >= PrevSum); (void) PrevSum;
}
return Sum;
@ -616,14 +615,17 @@ uint32_t BranchProbabilityInfo::getEdgeWeight(const BasicBlock *Src,
uint32_t BranchProbabilityInfo::
getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
uint32_t Weight = 0;
bool FoundWeight = false;
DenseMap<Edge, uint32_t>::const_iterator MapI;
for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
if (*I == Dst) {
MapI = Weights.find(std::make_pair(Src, I.getSuccessorIndex()));
if (MapI != Weights.end())
if (MapI != Weights.end()) {
FoundWeight = true;
Weight += MapI->second;
}
}
return (Weight == 0) ? DEFAULT_WEIGHT : Weight;
return (!FoundWeight) ? DEFAULT_WEIGHT : Weight;
}
/// Set the edge weight for a given edge specified by PredBlock and an index