[llvm] [TTI] Support scalable offsets in getScalingFactorCost (PR #88113)

[Sander de Smalen via llvm-commits]

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@@ -834,7 +834,7 @@ class TargetTransformInfo {
   /// If the AM is not supported, it returns a negative value.
   /// TODO: Handle pre/postinc as well.
   InstructionCost getScalingFactorCost(Type *Ty, GlobalValue *BaseGV,
-                                       int64_t BaseOffset, bool HasBaseReg,
+                                       StackOffset BaseOffset, bool HasBaseReg,
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sdesmalen-arm wrote:

It seems we now have three approaches for passing a 'scalable' and a 'fixed-length' offset to interfaces for this:
1. One using `StackOffset` (this patch for `getScalingFactorCost`)
2. One passing in two separate variables (`isLegalAddressingMode`)
3. Another one with two separate interfaces (`isLegalAddImmediate` vs `isLegalAddScalableImmediate`)

It would be good to settle on a single approach. @paulwalker-arm is there a particular reason you're pushing for (1) for this interface?

If we go for (1), I think we should rename `StackOffset` to `MemOffset` to make it more generic, because in this instance the offset does not necessarily apply only to stack addresses.

https://github.com/llvm/llvm-project/pull/88113