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

[David Green 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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davemgreen wrote:

This doesn't sound like a job for CCH, as it cannot handle the mul. In the long run I would expect this to work like #113903, where there is one vplan recipe that gets the entire cost of the extending reduction added to it. And we happily leave the legacy cost model behind us.

The mul should also be free, as in https://github.com/llvm/llvm-project/blob/de6c9096ba5d186c0ebe11bae76425af70959232/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp#L6655. I was hoping that #113903 would be done by now and we could have one solution for the two sets of extending reductions, that should ideally work the same. The mul should also be free though, and the entire cost of the dot should come from getPartialReductionCost.

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