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Explain why DenseMap is still used here instead of MapVector.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166454 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Julien Lerouge 2012-10-23 00:23:46 +00:00
parent 2128aaebd8
commit 6ecdd0e824
1 changed files with 9 additions and 1 deletions
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10
lib/Transforms/Utils/PromoteMemoryToRegister.cpp
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@ -212,7 +212,11 @@ namespace {
/// ///
DenseMap<AllocaInst*, unsigned> AllocaLookup; DenseMap<AllocaInst*, unsigned> AllocaLookup;
/// NewPhiNodes - The PhiNodes we're adding. /// NewPhiNodes - The PhiNodes we're adding. That map is used to simplify
/// some Phi nodes as we iterate over it, so it should have deterministic
/// iterators. We could use a MapVector, but since we already maintain a
/// map from BasicBlock* to a stable numbering (BBNumbers), the DenseMap is
/// more efficient (also supports removal).
/// ///
DenseMap<std::pair<unsigned, unsigned>, PHINode*> NewPhiNodes; DenseMap<std::pair<unsigned, unsigned>, PHINode*> NewPhiNodes;
@ -588,6 +592,10 @@ void PromoteMem2Reg::run() {
while (EliminatedAPHI) { while (EliminatedAPHI) {
EliminatedAPHI = false; EliminatedAPHI = false;
// Iterating over NewPhiNodes is deterministic, so it is safe to try to
// simplify and RUAW them as we go. If it was not, we could add uses to
// the values we replace with in a non deterministic order, thus creating
// non deterministic def->use chains.
for (DenseMap<std::pair<unsigned, unsigned>, PHINode*>::iterator I = for (DenseMap<std::pair<unsigned, unsigned>, PHINode*>::iterator I =
NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E;) { NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E;) {
PHINode *PN = I->second; PHINode *PN = I->second;