[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,
+                                       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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huntergr-arm wrote:

If it's not too much work then we can support the extra matching, but if it turns out to require a lot more work I think I'd prefer to land it with the current restrictions and implement the extra cases in follow-up patches.

You don't need to add the temporaries (`Pattern` and `Matches`) here, you can just call match directly in an if statement.

That said, it may be best to split the matching up a bit; this matcher assumes the phi is always the second argument, but I'm not sure if that's guaranteed.

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