[clang-tools-extra] [llvm] [clang] [VPlan] Add new VPUniformPerUFRecipe, use for step truncation. (PR #78113)

[Florian Hahn via cfe-commits]

fhahn wrote:

> The VPUniformPerUFRecipe of the current patch deals only with casts, so best name it as such, at-least for now, and define it comparable to VPWidenCastRecipe. Perhaps VPScalarCastRecipe would work - it effectively serves Invariant values placed outside the loop that are Uniform across all trip-count iterations, not only across the loop's VF and/or UF. Down the road, it should presumably be possible to query whether its VPValue operand is scalar or vector along with its uniformity across VF and/or UF or the entire Trip Count, and use that for cost and code-generation, or for asserting the case of distinct recipes.

Thanks, for now I updated the naming to reflect the cast-only case. Codegen is only implemented for the cases where only the first lane is used (as per `isOnlyFirstLaneUsed` check).


> How to refactor VPReplicateRecipe, relieving it of relying on an underlying IR instruction, and dealing with uniform cases, deserves further thoughts. Renaming may be adequate when recipe(s) generate scalar instruction(s) in general, rather than "replicate" an existing underlying Instruction. I.e., in contrast to VPReplicateCastRecipe.

Sounds good!

> One aspect of VPWidenCastRecipe is its destination type. A vector type, currently set during ::execute, can be set as soon as VF is known. I.e., when VPlan's range of VF's holds a single value - either initially or eventually during optimizeForVFAndUF.

Yes, that may be another option down the line.

https://github.com/llvm/llvm-project/pull/78113