[llvm] [VPlan] Replicate VPScalarIVStepsRecipe by VF outside replicate regions. (PR #170053)

[via llvm-commits]

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@@ -150,7 +148,7 @@ void UnrollState::addStartIndexForScalarSteps(VPScalarIVStepsRecipe *Steps,
     StartIndex = Builder.createScalarCast(Instruction::SIToFP, StartIndex,
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ayalz wrote:

Independent thoughts, worth documenting what can and should be said about the StartIndex VPValue operand:

- a compile-time constant value for fixed VF
- a runtime constant value for scalable VF
- has the same type as base IV - the first operand of VPScalarIVStepsRecipe
- computing StartIndex may unsigned wrap, to fit the type of base IV
- can be either fp or unsigned integer

?

https://github.com/llvm/llvm-project/pull/170053