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[NFCI][BasicBlockUtils] MergeBlockIntoPredecessor(): improve Dominator Tree updating

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Same as with TryToSimplifyUncondBranchFromEmptyBlock() patch.
This commit is contained in:
Roman Lebedev
2021-04-11 23:08:19 +03:00
parent 2def9c3d8e
commit e5692a564a
1 changed files with 10 additions and 8 deletions
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18
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
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@@ -228,20 +228,22 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
// These dominator edges will be redirected from Pred.
std::vector<DominatorTree::UpdateType> Updates;
if (DTU) {
SmallPtrSet<BasicBlock *, 2> UniqueSuccessors(succ_begin(BB), succ_end(BB));
Updates.reserve(1 + (2 * UniqueSuccessors.size()));
SmallPtrSet<BasicBlock *, 2> SuccsOfBB(succ_begin(BB), succ_end(BB));
SmallPtrSet<BasicBlock *, 2> SuccsOfPredBB(succ_begin(PredBB),
succ_begin(PredBB));
Updates.reserve(Updates.size() + 2 * SuccsOfBB.size() + 1);
// Add insert edges first. Experimentally, for the particular case of two
// blocks that can be merged, with a single successor and single predecessor
// respectively, it is beneficial to have all insert updates first. Deleting
// edges first may lead to unreachable blocks, followed by inserting edges
// making the blocks reachable again. Such DT updates lead to high compile
// times. We add inserts before deletes here to reduce compile time.
for (BasicBlock *UniqueSuccessor : UniqueSuccessors)
// This successor of BB may already have PredBB as a predecessor.
if (!llvm::is_contained(successors(PredBB), UniqueSuccessor))
Updates.push_back({DominatorTree::Insert, PredBB, UniqueSuccessor});
for (BasicBlock *UniqueSuccessor : UniqueSuccessors)
Updates.push_back({DominatorTree::Delete, BB, UniqueSuccessor});
for (BasicBlock *SuccOfBB : SuccsOfBB)
// This successor of BB may already be a PredBB's successor.
if (!SuccsOfPredBB.contains(SuccOfBB))
Updates.push_back({DominatorTree::Insert, PredBB, SuccOfBB});
for (BasicBlock *SuccOfBB : SuccsOfBB)
Updates.push_back({DominatorTree::Delete, BB, SuccOfBB});
Updates.push_back({DominatorTree::Delete, PredBB, BB});
}