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

[Sam Tebbs 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,
+                                       PartialReductionChain &Chain) const {
+    auto PairIt = PartialReductionChains.find(I);
+    if (PairIt == PartialReductionChains.end())
+      return false;
+    Chain = PairIt->second;
+    return true;
+  }
+
+  void addPartialReductionIfSupported(Instruction *Instr, ElementCount VF) {
+    Value *ExpectedPhi;
+    Value *A, *B;
+
+    using namespace llvm::PatternMatch;
+    auto Pattern =
+        m_BinOp(m_OneUse(m_BinOp(m_OneUse(m_ZExtOrSExt(m_OneUse(m_Value(A)))),
+                                 m_OneUse(m_ZExtOrSExt(m_OneUse(m_Value(B)))))),
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SamTebbs33 wrote:

I've done some work to facilitate removing the one use restriction, let me know what you think of it. I've also added a second match call to check for when the phi is the first operand.

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