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[SimplifyCFG] MergeBlockIntoPredecessor(): switch to non-permissive DomTree updates 

... which requires not deleting edges that were just deleted already,
    by not processing the same successor more than once.
This commit is contained in:
Roman Lebedev 2021-01-08 00:51:29 +03:00
parent 05adc73db0
commit 66189212bb
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GPG Key ID: 083C3EBB4A1689E0
1 changed files with 9 additions and 7 deletions
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16
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
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@ -230,19 +230,21 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
// These dominator edges will be redirected from Pred.
std::vector<DominatorTree::UpdateType> Updates;
if (DTU) {
Updates.reserve(1 + (2 * succ_size(BB)));
SmallSetVector<BasicBlock *, 2> UniqueSuccessors(succ_begin(BB),
succ_end(BB));
Updates.reserve(1 + (2 * UniqueSuccessors.size()));
// 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 (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
for (BasicBlock *UniqueSuccessor : UniqueSuccessors)
// This successor of BB may already have PredBB as a predecessor.
if (!llvm::is_contained(successors(PredBB), *I))
Updates.push_back({DominatorTree::Insert, PredBB, *I});
for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
Updates.push_back({DominatorTree::Delete, BB, *I});
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});
Updates.push_back({DominatorTree::Delete, PredBB, BB});
}
@ -303,7 +305,7 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
isa<UnreachableInst>(BB->getTerminator()) &&
"The successor list of BB isn't empty before "
"applying corresponding DTU updates.");
DTU->applyUpdatesPermissive(Updates);
DTU->applyUpdates(Updates);
DTU->deleteBB(BB);
} else {
BB->eraseFromParent(); // Nuke BB if DTU is nullptr.