[VPlan] Add VPPhiAccessors to provide interface for phi recipes (NFC)

Add a VPPhiAccessors class to provide interfaces to access incoming
values and blocks, with corresponding iterators.

The first user is VPWidenPhiRecipe, with the other phi-like recipes
following soon.

This will also be used to verify def-use chains where users are phi-like
recipes, simplifying llvm#124838.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -3069,11 +3069,9 @@ void InnerLoopVectorizer::fixNonInductionPHIs(VPTransformState &State) {
       PHINode *NewPhi = cast<PHINode>(State.get(VPPhi));
       // Make sure the builder has a valid insert point.
       Builder.SetInsertPoint(NewPhi);
-      for (unsigned Idx = 0; Idx < VPPhi->getNumOperands(); ++Idx) {
-        VPValue *Inc = VPPhi->getIncomingValue(Idx);
-        VPBasicBlock *VPBB = VPPhi->getIncomingBlock(Idx);
+
+      for (const auto &[Inc, VPBB] : VPPhi->incoming_values_and_blocks())
         NewPhi->addIncoming(State.get(Inc), State.CFG.VPBB2IRBB[VPBB]);
-      }
     }
   }
 }

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1088,6 +1088,62 @@ class VPIRInstruction : public VPRecipeBase {
   void extractLastLaneOfOperand(VPBuilder &Builder);
 };
 
+/// Helper type to provide functions to access incoming values and blocks for
+/// phi-like recipes. RecipeTy must be a sub-class of VPRecipeBase.
+template <typename RecipeTy> class VPPhiAccessors {
+  /// Return a VPRecipeBase* to the current object.
+  const VPRecipeBase *getAsRecipe() const {
+    return static_cast<const RecipeTy *>(this);
+  }
+
+public:
+  /// Returns the \p I th incoming VPValue.
+  VPValue *getIncomingValue(unsigned I) const {
+    return getAsRecipe()->getOperand(I);
+  }
+
+  /// Returns an interator range over the incoming values
+  VPUser::const_operand_range incoming_values() const {
+    return getAsRecipe()->operands();
+  }
+
+  /// Returns the \p I th incoming block.
+  const VPBasicBlock *getIncomingBlock(unsigned Idx) const;
+
+  using const_incoming_block_iterator =
+      mapped_iterator<detail::index_iterator,
+                      std::function<const VPBasicBlock *(size_t)>>;
+  using const_incoming_blocks_range =
+      iterator_range<const_incoming_block_iterator>;
+
+  const_incoming_block_iterator incoming_block_begin() const {
+    return const_incoming_block_iterator(
+        detail::index_iterator(0),
+        [this](size_t Idx) { return getIncomingBlock(Idx); });
+  }
+  const_incoming_block_iterator incoming_block_end() const {
+    return const_incoming_block_iterator(
+        detail::index_iterator(getAsRecipe()->getVPDefID() ==
+                                       VPDef::VPWidenIntOrFpInductionSC
+                                   ? 2
+                                   : getAsRecipe()->getNumOperands()),
+        [this](size_t Idx) { return getIncomingBlock(Idx); });
+  }
+
+  /// Returns an iterator range over the incoming blocks.
+  const_incoming_blocks_range incoming_blocks() const {
+    return make_range(incoming_block_begin(), incoming_block_end());
+  }
+
+  /// Returns an iterator range over pairs of incoming values and corrsponding
+  /// incoming blocks.
+  detail::zippy<llvm::detail::zip_shortest, VPUser::const_operand_range,
+                const_incoming_blocks_range>
+  incoming_values_and_blocks() const {
+    return zip(incoming_values(), incoming_blocks());
+  }
+};
+
 /// An overlay for VPIRInstructions wrapping PHI nodes enabling convenient use
 /// cast/dyn_cast/isa and execute() implementation.
 struct VPIRPhi : public VPIRInstruction {
@@ -1968,7 +2024,8 @@ class VPScalarPHIRecipe : public VPHeaderPHIRecipe {
 /// recipe is placed in an entry block to a (non-replicate) region, it must have
 /// exactly 2 incoming values, the first from the predecessor of the region and
 /// the second from the exiting block of the region.
-class VPWidenPHIRecipe : public VPSingleDefRecipe {
+class VPWidenPHIRecipe : public VPSingleDefRecipe,
+                         public VPPhiAccessors<VPWidenPHIRecipe> {
 public:
   /// Create a new VPWidenPHIRecipe for \p Phi with start value \p Start and
   /// debug location \p DL.
@@ -1994,12 +2051,6 @@ class VPWidenPHIRecipe : public VPSingleDefRecipe {
   void print(raw_ostream &O, const Twine &Indent,
              VPSlotTracker &SlotTracker) const override;
 #endif
-
-  /// Returns the \p I th incoming VPBasicBlock.
-  VPBasicBlock *getIncomingBlock(unsigned I);
-
-  /// Returns the \p I th incoming VPValue.
-  VPValue *getIncomingValue(unsigned I) { return getOperand(I); }
 };
 
 /// A recipe for handling first-order recurrence phis. The start value is the

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -1005,6 +1005,29 @@ void VPIRInstruction::print(raw_ostream &O, const Twine &Indent,
 }
 #endif
 
+static const VPBasicBlock *getIncomingBlockForRecipe(const VPRecipeBase *R,
+                                                     unsigned I) {
+  const VPBasicBlock *Parent = R->getParent();
+  const VPBlockBase *Pred = nullptr;
+  if (Parent->getNumPredecessors() > 0) {
+    Pred = Parent->getPredecessors()[I];
+  } else {
+    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 &&
+        (R->getNumOperands() == 2 || isa<VPWidenIntOrFpInductionRecipe>(R)) &&
+        "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;
+  }
+
+  return Pred->getExitingBasicBlock();
+}
+
 void VPIRPhi::execute(VPTransformState &State) {
   PHINode *Phi = &getIRPhi();
   for (const auto &[Idx, Op] : enumerate(operands())) {
@@ -3599,25 +3622,10 @@ void VPReductionPHIRecipe::print(raw_ostream &O, const Twine &Indent,
 }
 #endif
 
-VPBasicBlock *VPWidenPHIRecipe::getIncomingBlock(unsigned I) {
-  VPBasicBlock *Parent = getParent();
-  VPBlockBase *Pred = nullptr;
-  if (Parent->getNumPredecessors() > 0) {
-    Pred = Parent->getPredecessors()[I];
-  } else {
-    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;
-  }
-
-  return Pred->getExitingBasicBlock();
+template <>
+const VPBasicBlock *
+VPPhiAccessors<VPWidenPHIRecipe>::getIncomingBlock(unsigned Idx) const {
+  return getIncomingBlockForRecipe(getAsRecipe(), Idx);
 }
 
 void VPWidenPHIRecipe::execute(VPTransformState &State) {