[llvm] [LoopVectorizer][AArch64] Add support for partial reduce subtraction (PR #123636)

[Sam Tebbs via llvm-commits]

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@@ -282,7 +283,20 @@ InstructionCost
 VPPartialReductionRecipe::computeCost(ElementCount VF,
                                       VPCostContext &Ctx) const {
   std::optional<unsigned> Opcode = std::nullopt;
-  VPRecipeBase *BinOpR = getOperand(0)->getDefiningRecipe();
+  VPValue *BinOp = getOperand(0);
+  VPRecipeBase *BinOpR = BinOp->getDefiningRecipe();
+
+  using namespace llvm::PatternMatch;
+  if (auto *UnderInst =
+          dyn_cast_if_present<Instruction>(BinOp->getUnderlyingValue())) {
+    if (match(UnderInst, m_Neg(m_BinOp()))) {
+      BinOpR = BinOpR->getOperand(1)->getDefiningRecipe();
+    }
+  }
+  // BinOp is never used again, any further interaction should be via the
+  // defining recipe `BinOpR`
+  BinOp = nullptr;
+
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SamTebbs33 wrote:

It feels a bit strange to set something to null like this, perhaps it should just be left alone. Or we could just not declare the variable in the first place and use `getOperand(0)` instead.

https://github.com/llvm/llvm-project/pull/123636