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

[Sander de Smalen 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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sdesmalen-arm wrote:

This is only free iff the target has an instruction that implements the partial reduction in a way that it folds in the extend. I think this warrants another `CastContextHint` enum value for the case where the extend is used in a partial reduction context. Then the `TTI.getCastInstrCost` can determine whether this is folded into the operation, and is therefore `TCC_Free`.

This does mean widening the meaning of `CastContextHint` beyond loads, but I can't see any fundamental reason why that hint should only be limited to loads.

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