[llvm] [AArch64][LV] Reduce cost of scaled reduction extends (PR #134074)

[David Sherwood via llvm-commits]

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@@ -1796,6 +1796,11 @@ InstructionCost VPWidenCastRecipe::computeCost(ElementCount VF,
   // For Z/Sext, get the context from the operand.
   else if (Opcode == Instruction::ZExt || Opcode == Instruction::SExt ||
            Opcode == Instruction::FPExt) {
+    // If the extend is performed as part of another operation, it can be
+    // considered 'free'.
+    const VPlan *Plan = getParent()->getPlan();
+    if (Plan->isScaledReductionExtension(getUnderlyingInstr()))
+      return TargetTransformInfo::TCC_Free;
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david-arm wrote:

Is it possible for the sext/zext to have multiple uses, i.e. one for the partial reduction and one used for something else? If so, then it won't be free. We might need to check that all uses are for partial reductions.

https://github.com/llvm/llvm-project/pull/134074