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-rw-r--r--lib/Transforms/Utils/PromoteMemoryToRegister.cpp10
1 files changed, 9 insertions, 1 deletions
diff --git a/lib/Transforms/Utils/PromoteMemoryToRegister.cpp b/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
index f6ad3b8b9e..fb65a1847c 100644
--- a/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
+++ b/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -212,7 +212,11 @@ namespace {
///
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;
@@ -588,6 +592,10 @@ void PromoteMem2Reg::run() {
while (EliminatedAPHI) {
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 =
NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E;) {
PHINode *PN = I->second;