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

[Graham Hunter 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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huntergr-arm wrote:

There's a bit of a conflict there, though – the current code looks up the operand and calculates a CCH based on that, whereas this focuses on users. This would override the existing CCH... I guess we could do that if the current CCH is Normal or None, otherwise leave it as is?

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