[llvm] [LSR] Recognize vscale-relative immediates (PR #88124)

[Graham Hunter via llvm-commits]

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@@ -1308,9 +1390,9 @@ void Cost::RateRegister(const Formula &F, const SCEV *Reg,
 
       // If the step size matches the base offset, we could use pre-indexed
       // addressing.
-      if (AMK == TTI::AMK_PreIndexed) {
+      if (AMK == TTI::AMK_PreIndexed && !F.BaseOffset.isScalable()) {
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huntergr-arm wrote:

There's no 'isFixed()' method on FixedOrScalableQuantity, so I just used the inverse of isScalable(). They're logically equivalent, though there's semantic reinforcement from separating them.

But I do wonder if we want an uninitialized state for scalable-ness, and allow that to be freely used with either fixed or scalable offsets. The result would then be 'locked in' to one or the other.

https://github.com/llvm/llvm-project/pull/88124