[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:

Yes, the documentation for CastContextHint says:
```
/// Represents a hint about the context in which a cast is used.
///
/// For zext/sext, the context of the cast is the operand, which must be a
/// load of some kind. For trunc, the context is of the cast is the single
/// user of the instruction, which must be a store of some kind.
```
The first sentence suggests adding a new hint would make sense for this case, and the second part suggests that the interpretation is currently specific to loads/stores. For partial reductions, we'd indeed look at the single use.

> This would override the existing CCH

If I read the code correctly for the zext/sext case it can only be set to `TTI::CastContextHint::None` at the point where you want to define it?


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