[llvm] [VPlan] Explicitly handle scalar pointer inductions. (PR #83068)

[via llvm-commits]

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@@ -547,12 +552,40 @@ static VPValue *createScalarIVSteps(VPlan &Plan, const InductionDescriptor &ID,
 /// provide them by building scalar steps off of the canonical scalar IV and
 /// update the original IV's users. This is an optional optimization to reduce
 /// the needs of vector extracts.
+/// If all users of VPWidenPointerInductionRecipe only use its scalar values,
+/// replace it with a PtrAdd (IndStart, ScalarIVSteps (0, Step)).
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ayalz wrote:

This sounds more like a mandatory, functional legalization of induction recipes, rather than an optional, performance optimization - referring to the `optimizeInductions()` name.

Furthermore, this conceptually introduces two types or stages of the recipes - before and after legalization - which could be represented as two distinct recipes/opcodes, or by recording an indicator whether the recipe was legalized or not. Although these seem unneeded atm.

https://github.com/llvm/llvm-project/pull/83068