[VPlan] Implement VPReductionRecipe::computeCost(). NFC (#107790)

Implementation of `computeCost()` function for `VPReductionRecipe`.

Note that `in-loop` and `any-of` reductions are not supported by
VPlan-based cost model currently.

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2476,6 +2476,10 @@ class VPReductionRecipe : public VPSingleDefRecipe {
   /// Generate the reduction in the loop
   void execute(VPTransformState &State) override;
 
+  /// Return the cost of VPReductionRecipe.
+  InstructionCost computeCost(ElementCount VF,
+                              VPCostContext &Ctx) const override;
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
   /// Print the recipe.
   void print(raw_ostream &O, const Twine &Indent,

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -2071,6 +2071,40 @@ void VPReductionEVLRecipe::execute(VPTransformState &State) {
   State.set(this, NewRed, /*IsScalar*/ true);
 }
 
+InstructionCost VPReductionRecipe::computeCost(ElementCount VF,
+                                               VPCostContext &Ctx) const {
+  RecurKind RdxKind = RdxDesc.getRecurrenceKind();
+  Type *ElementTy = Ctx.Types.inferScalarType(this);
+  auto *VectorTy = cast<VectorType>(ToVectorTy(ElementTy, VF));
+  TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
+  unsigned Opcode = RdxDesc.getOpcode();
+
+  // TODO: Support any-of and in-loop reductions.
+  assert(
+      (!RecurrenceDescriptor::isAnyOfRecurrenceKind(RdxKind) ||
+       ForceTargetInstructionCost.getNumOccurrences() > 0) &&
+      "Any-of reduction not implemented in VPlan-based cost model currently.");
+  assert(
+      (!cast<VPReductionPHIRecipe>(getOperand(0))->isInLoop() ||
+       ForceTargetInstructionCost.getNumOccurrences() > 0) &&
+      "In-loop reduction not implemented in VPlan-based cost model currently.");
+
+  assert(ElementTy->getTypeID() == RdxDesc.getRecurrenceType()->getTypeID() &&
+         "Inferred type and recurrence type mismatch.");
+
+  // Cost = Reduction cost + BinOp cost
+  InstructionCost Cost =
+      Ctx.TTI.getArithmeticInstrCost(Opcode, ElementTy, CostKind);
+  if (RecurrenceDescriptor::isMinMaxRecurrenceKind(RdxKind)) {
+    Intrinsic::ID Id = getMinMaxReductionIntrinsicOp(RdxKind);
+    return Cost + Ctx.TTI.getMinMaxReductionCost(
+                      Id, VectorTy, RdxDesc.getFastMathFlags(), CostKind);
+  }
+
+  return Cost + Ctx.TTI.getArithmeticReductionCost(
+                    Opcode, VectorTy, RdxDesc.getFastMathFlags(), CostKind);
+}
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 void VPReductionRecipe::print(raw_ostream &O, const Twine &Indent,
                               VPSlotTracker &SlotTracker) const {