[llvm] [VectorCombine] foldShuffleChainsToReduce - add support for partial vector reductions (PR #195119)

[Simon Pilgrim via llvm-commits]

================
@@ -4147,14 +4156,32 @@ bool VectorCombine::foldShuffleChainsToReduce(Instruction &I) {
   if (!ReducedOp)
     return false;
 
-  IntrinsicCostAttributes ICA(ReducedOp, FinalVecVTy, {FinalVecV});
-  InstructionCost NewCost = TTI.getIntrinsicInstrCost(ICA, CostKind);
+  InstructionCost NewCost = 0;
+  FixedVectorType *ReduceVecTy = FinalVecVTy;
+
+  if (IsPartialReduction) {
+    unsigned SubVecSize = ShuffleMaskHalf;
+    ReduceVecTy = FixedVectorType::get(FVT->getElementType(), SubVecSize);
+    NewCost += TTI.getShuffleCost(TargetTransformInfo::SK_ExtractSubvector,
----------------
RKSimon wrote:

Create the ExtractMask earlier and add the mask to getShuffleCost - its always better to provide an explicit mask to getShuffleCost if you can, even for something as mundane as SK_ExtractSubvector

https://github.com/llvm/llvm-project/pull/195119