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

[David Sherwood via llvm-commits]

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@@ -1693,6 +1693,15 @@ InstructionCost VPWidenRecipe::computeCost(ElementCount VF,
     if (RHS->isLiveIn())
       RHSInfo = Ctx.TTI.getOperandInfo(RHS->getLiveInIRValue());
 
+    // The mul is folded into another target instruction when participating
+    // in scaled reductions.
+    if (Opcode == Instruction::Mul && !hasMoreThanOneUniqueUser()) {
+      if (all_of(users(), [](const VPUser *U) {
+            return isa_and_present<VPPartialReductionRecipe>(U);
+          }))
+        return TTI::TCC_Free;
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david-arm wrote:

See my comment on `isScaledReductionExtension` - I have concerns this is essentially AArch64-specific code being used for all targets.

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