!fixup address latest comments, thanks!

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1961,7 +1961,7 @@ class VPScalarPHIRecipe : public VPHeaderPHIRecipe {
 /// A recipe for widened phis. Incoming values are operands of the recipe and
 /// their operand index corresponds to the incoming predeocessor block. If the
 /// recipe is placed in an entry block to a (non-replicate) region, it must have
-/// exactly 2 incoming values, from from the predecessors of the region and one
+/// exactly 2 incoming values, the first from the predecessors of the region and the second
 /// from the exiting block of the region.
 class VPWidenPHIRecipe : public VPSingleDefRecipe {
 public:

llvm/lib/Transforms/Vectorize/VPlanHCFGBuilder.cpp

@@ -145,13 +145,13 @@ void PlainCFGBuilder::fixPhiNodes() {
     }
 
     // Add operands for VPPhi in the order matching its predecessors in VPlan.
-    DenseMap<const VPBasicBlock *, VPValue *> IncomingValues;
+    DenseMap<const VPBasicBlock *, VPValue *> VPPredToIncomingValue;
     for (unsigned I = 0; I != Phi->getNumOperands(); ++I) {
-      IncomingValues[BB2VPBB[Phi->getIncomingBlock(I)]] =
+      VPPredToIncomingValue[BB2VPBB[Phi->getIncomingBlock(I)]] =
           getOrCreateVPOperand(Phi->getIncomingValue(I));
     }
     for (VPBlockBase *Pred : VPPhi->getParent()->getPredecessors())
-      VPPhi->addOperand(IncomingValues.lookup(Pred->getExitingBasicBlock()));
+      VPPhi->addOperand(VPPredToIncomingValue.lookup(Pred->getExitingBasicBlock()));
   }
 }
 

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3613,10 +3613,12 @@ VPBasicBlock *VPWidenPHIRecipe::getIncomingBlock(unsigned I) {
   VPBasicBlock *Parent = getParent();
   VPBlockBase *Pred = nullptr;
   if (Parent->getNumPredecessors() == 0) {
-    auto *R = Parent->getParent();
-    assert(R && R->getEntry() == Parent);
-    assert(I < 2);
-    Pred = I == 0 ? R->getSinglePredecessor() : R;
+    auto *Region = Parent->getParent();
+    assert(Region && !Region->isReplicator() && Region->getEntry() == Parent && "must be in the entry block of a non-replicate region");
+    assert(I < 2 && getNumOperands() == 2 && "when placed in an entry block, only 2 incoming blocks are available");
+
+    // I ==  0 selects the predecessor of the region, I == 1 selects the region itself whose exiting block feeds the phi across the backedge.
+    Pred = I == 0 ? Region->getSinglePredecessor() : Region;
   } else {
     Pred = Parent->getPredecessors()[I];
   }