[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:

For our initial motivating use case (the AArch64 dot product instructions), there would be an extra recipe (a multiply) between the extends and the partial reduction. So we would need to look for uses of uses. I'm not sure if anyone has a case for more than one operation between the extends and the reduction, but I decided to keep it simple for now instead of walking the vplan.

If it's preferable to do the walk, then sure, I can convert it.

(I guess we could also make the multiply free, but so far that hasn't been an issue in using wider VFs)

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