[MemorySSA] Add API to update MemoryPhis, following CFG changes.

Summary:
When splitting predecessors in BasicBlockUtils, we create a new block as an immediate predecessor of the original BB, then we connect a given set of predecessors to the new block.
The API in this patch will be used to update MemoryPhis for this CFG change.
If all predecessors are being moved, we move the MemoryPhi directly. Otherwise we create a new MemoryPhi in the NewBB and populate its incoming values, while deleting them from BB's Phi.
[Split from D45299 for easier review]

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D49156

llvm-svn: 337581

llvm/include/llvm/Analysis/MemorySSA.h

@@ -578,10 +578,11 @@ class MemoryPhi final : public MemoryAccess {
     setNumHungOffUseOperands(getNumOperands() - 1);
   }
 
-  // After deleting incoming block BB, the incoming blocks order may be changed.
-  void unorderedDeleteIncomingBlock(const BasicBlock *BB) {
+  // After deleting entries that satisfy Pred, remaining entries may have
+  // changed order.
+  template <typename Fn> void unorderedDeleteIncomingIf(Fn &&Pred) {
     for (unsigned I = 0, E = getNumOperands(); I != E; ++I)
-      if (block_begin()[I] == BB) {
+      if (Pred(getIncomingValue(I), getIncomingBlock(I))) {
         unorderedDeleteIncoming(I);
         E = getNumOperands();
         --I;
@@ -590,17 +591,17 @@ class MemoryPhi final : public MemoryAccess {
            "Cannot remove all incoming blocks in a MemoryPhi.");
   }
 
+  // After deleting incoming block BB, the incoming blocks order may be changed.
+  void unorderedDeleteIncomingBlock(const BasicBlock *BB) {
+    unorderedDeleteIncomingIf(
+        [&](const MemoryAccess *, const BasicBlock *B) { return BB == B; });
+  }
+
   // After deleting incoming memory access MA, the incoming accesses order may
   // be changed.
   void unorderedDeleteIncomingValue(const MemoryAccess *MA) {
-    for (unsigned I = 0, E = getNumOperands(); I != E; ++I)
-      if (getIncomingValue(I) == MA) {
-        unorderedDeleteIncoming(I);
-        E = getNumOperands();
-        --I;
-      }
-    assert(getNumOperands() >= 1 &&
-           "Cannot remove all incoming values in a MemoryPhi.");
+    unorderedDeleteIncomingIf(
+        [&](const MemoryAccess *M, const BasicBlock *) { return MA == M; });
   }
 
   static bool classof(const Value *V) {

llvm/include/llvm/Analysis/MemorySSAUpdater.h

@@ -124,6 +124,14 @@ class MemorySSAUpdater {
   /// |------|        |------|
   void moveAllAfterMergeBlocks(BasicBlock *From, BasicBlock *To,
                                Instruction *Start);
+  /// BasicBlock Old had New, an empty BasicBlock, added directly before it,
+  /// and the predecessors in Preds that used to point to Old, now point to
+  /// New. If New is the only predecessor, move Old's Phi, if present, to New.
+  /// Otherwise, add a new Phi in New with appropriate incoming values, and
+  /// update the incoming values in Old's Phi node too, if present.
+  void
+  wireOldPredecessorsToNewImmediatePredecessor(BasicBlock *Old, BasicBlock *New,
+                                               ArrayRef<BasicBlock *> Preds);
 
   // The below are utility functions. Other than creation of accesses to pass
   // to insertDef, and removeAccess to remove accesses, you should generally

llvm/lib/Analysis/MemorySSAUpdater.cpp

@@ -499,6 +499,35 @@ static MemoryAccess *onlySingleValue(MemoryPhi *MP) {
   return MA;
 }
 
+void MemorySSAUpdater::wireOldPredecessorsToNewImmediatePredecessor(
+    BasicBlock *Old, BasicBlock *New, ArrayRef<BasicBlock *> Preds) {
+  assert(!MSSA->getWritableBlockAccesses(New) &&
+         "Access list should be null for a new block.");
+  MemoryPhi *Phi = MSSA->getMemoryAccess(Old);
+  if (!Phi)
+    return;
+  if (pred_size(Old) == 1) {
+    assert(pred_size(New) == Preds.size() &&
+           "Should have moved all predecessors.");
+    MSSA->moveTo(Phi, New, MemorySSA::Beginning);
+  } else {
+    assert(!Preds.empty() && "Must be moving at least one predecessor to the "
+                             "new immediate predecessor.");
+    MemoryPhi *NewPhi = MSSA->createMemoryPhi(New);
+    SmallPtrSet<BasicBlock *, 16> PredsSet(Preds.begin(), Preds.end());
+    Phi->unorderedDeleteIncomingIf([&](MemoryAccess *MA, BasicBlock *B) {
+      if (PredsSet.count(B)) {
+        NewPhi->addIncoming(MA, B);
+        return true;
+      }
+      return false;
+    });
+    Phi->addIncoming(NewPhi, New);
+    if (onlySingleValue(NewPhi))
+      removeMemoryAccess(NewPhi);
+  }
+}
+
 void MemorySSAUpdater::removeMemoryAccess(MemoryAccess *MA) {
   assert(!MSSA->isLiveOnEntryDef(MA) &&
          "Trying to remove the live on entry def");