[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:

Immediately below my change is the code to look up the CCH based on the operand. I was imagining overriding the CCH after that code and letting the existing `getCastInstrCost` call below make use of the info. I think it'd be more appropriate that way – a normal load of `<vscale x 16 x i8>` can be used directly by an sdot instruction, whereas the result of a gather for the same type couldn't be.

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