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

[Graham Hunter via llvm-commits]

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@@ -1556,7 +1556,105 @@ class LoopVectorizationCostModel {
   getReductionPatternCost(Instruction *I, ElementCount VF, Type *VectorTy,
                           TTI::TargetCostKind CostKind) const;
 
+  using PartialReductionList = DenseMap<Instruction *, PartialReductionChain>;
+
+  PartialReductionList getPartialReductionChains() {
+    return PartialReductionChains;
+  }
+
+  bool getInstructionsPartialReduction(Instruction *I,
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huntergr-arm wrote:

This could return a `std::optional<PartialReductionChain *>` to reduce the possibility of misuse.

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