[PATCH] D74695: [VPlan] Replace VPWidenRecipe with VPInstruction (WIP).

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added a comment.

Breaking the recipes into VPInstructions is indeed on the roadmap for evolving VPlan to support transformations, cost modeling, and more.

The approach we’ve taken so far is to first migrate the def-use dependencies between recipes, which originally rely on the underlying immutable def-use dependencies among their IR ingredient Instructions and their Operands, to VPlan VPValue-VPUser dependencies. Once two recipes feed each other via a VPValue-VPUser pair, new VPInstructions can be introduced freely between them. The primary motivation for this approach is to facilitate VPlan def-use analysis and transformations, including reordering recipes, replacing them, and introducing new VPInstructions. Once the uses of all recipes migrate to VPUsers, a rather mechanical process following D70865 <https://reviews.llvm.org/D70865>, any single-def recipe can be converted independently into a VPInstruction. VPWidenRecipe is indeed an easy first candidate, which would break up naturally at this time.

An approach that first breaks recipes into VPInstructions, will migrate all def-use dependencies when the *last* recipe is broken. Other recipes are however more challenging to break than VPWidenRecipe, because they have multiple defs, entail SCEV expansions, un/pack predicated scalars along a to-be-unrolled loop of VF*UF iterations, and more. These design challenges require careful attention, which should preferably be devoted in parallel to facilitating VPlan transformation rather than blocking it.

Breaking VPWidenRecipe into a VPInstruction at this time, by letting dependent recipes continue to rely on their underlying IR Operands, via a lookup for potential VPValues (VPInstructions) that cover their corresponding IR Value def, presumably builds a VPlan def-use graph but in effect does not support introducing a new VPInstruction between a VPWidenInstruction and its users. It seems preferable to first focus on migrating away from the original IR def-use relations, gradually and consistently.

Gil&Ayal.


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  https://reviews.llvm.org/D74695/new/

https://reviews.llvm.org/D74695