[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Sam Tebbs via llvm-commits]

================
@@ -291,6 +291,68 @@ InstructionCost VPRecipeBase::computeCost(ElementCount VF,
   llvm_unreachable("subclasses should implement computeCost");
 }
 
+InstructionCost
+VPPartialReductionRecipe::computeCost(ElementCount VF,
+                                      VPCostContext &Ctx) const {
+  std::optional<unsigned> Opcode = std::nullopt;
+  VPRecipeBase *BinOpR = getOperand(0)->getDefiningRecipe();
+  if (auto *WidenR = dyn_cast<VPWidenRecipe>(BinOpR))
+    Opcode = std::make_optional(WidenR->getOpcode());
+
+  VPRecipeBase *ExtAR = BinOpR->getOperand(0)->getDefiningRecipe();
+  VPRecipeBase *ExtBR = BinOpR->getOperand(1)->getDefiningRecipe();
+
+  auto GetExtendKind = [](VPRecipeBase *R) {
+    auto *WidenCastR = dyn_cast<VPWidenCastRecipe>(R);
+    if (!WidenCastR)
+      return TargetTransformInfo::PR_None;
+    if (WidenCastR->getOpcode() == Instruction::CastOps::ZExt)
+      return TargetTransformInfo::PR_ZeroExtend;
+    if (WidenCastR->getOpcode() == Instruction::CastOps::SExt)
+      return TargetTransformInfo::PR_SignExtend;
+    return TargetTransformInfo::PR_None;
+  };
+
+  auto *PhiType = Ctx.Types.inferScalarType(getOperand(1));
+  auto *ExtTy = Ctx.Types.inferScalarType(ExtAR->getOperand(0));
+
+  return Ctx.TTI.getPartialReductionCost(
+      getUnderlyingInstr()->getOpcode(), ExtTy, PhiType, VF,
+      GetExtendKind(ExtAR), GetExtendKind(ExtBR), Opcode);
+}
+
+void VPPartialReductionRecipe::execute(VPTransformState &State) {
+  State.setDebugLocFrom(getDebugLoc());
+  auto &Builder = State.Builder;
+
+  assert(getUnderlyingInstr()->getOpcode() == Instruction::Add &&
----------------
SamTebbs33 wrote:

There isn't a use of it at the moment but I was asked to set the underlying instruction as part of the review and there was no push-back against it, so I'm hesitant to go round in circles and remove it.

https://github.com/llvm/llvm-project/pull/92418