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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
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6
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
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@ -8009,7 +8009,11 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
const ConstantInt *CaseVal = I.getCaseValue();
uint32_t Weight = 1;
if (BPI) {
Weight = BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex());
// TODO - BPI used to guarantee non-zero weights, but this produces
// information loss (see PR 22718). Since we can't handle zero weights
// here, use the same flooring mechanism previously used by BPI.
Weight = std::max(
1u, BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex()));
assert(Weight <= UINT32_MAX / SI.getNumSuccessors());
}
Clusters.push_back(CaseCluster::range(CaseVal, CaseVal, Succ, Weight));