[MemorySSA] Update MemoryPhi wiring for block splitting to consider if identical edges were merged.

Summary:
Block splitting is done with either identical edges being merged, or not.
Only critical edges can be split without merging identical edges based on an option.
Teach the memoryssa updater to take this into account: for the same edge between two blocks only move one entry from the Phi in Old to the new Phi in New.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D51563

llvm-svn: 341709

1 files changed, 10 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
index abd67cc2e93..a889b607d21 100644
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -498,7 +498,8 @@ static MemoryAccess *onlySingleValue(MemoryPhi *MP) {
 }
 
 void MemorySSAUpdater::wireOldPredecessorsToNewImmediatePredecessor(
-    BasicBlock *Old, BasicBlock *New, ArrayRef<BasicBlock *> Preds) {
+    BasicBlock *Old, BasicBlock *New, ArrayRef<BasicBlock *> Preds,
+    bool IdenticalEdgesWereMerged) {
   assert(!MSSA->getWritableBlockAccesses(New) &&
          "Access list should be null for a new block.");
   MemoryPhi *Phi = MSSA->getMemoryAccess(Old);
@@ -513,9 +514,17 @@ void MemorySSAUpdater::wireOldPredecessorsToNewImmediatePredecessor(
                              "new immediate predecessor.");
     MemoryPhi *NewPhi = MSSA->createMemoryPhi(New);
     SmallPtrSet<BasicBlock *, 16> PredsSet(Preds.begin(), Preds.end());
+    // Currently only support the case of removing a single incoming edge when
+    // identical edges were not merged.
+    if (!IdenticalEdgesWereMerged)
+      assert(PredsSet.size() == Preds.size() &&
+             "If identical edges were not merged, we cannot have duplicate "
+             "blocks in the predecessors");
     Phi->unorderedDeleteIncomingIf([&](MemoryAccess *MA, BasicBlock *B) {
       if (PredsSet.count(B)) {
         NewPhi->addIncoming(MA, B);
+        if (!IdenticalEdgesWereMerged)
+          PredsSet.erase(B);
         return true;
       }
       return false;