[PATCH] D76838: [LV] WIP: Tail-folding counting down loops

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added a comment.

It would indeed be good to have FoldTail handle loops that currently lack a Primary Induction Variable (0, +1). Such an %iv is needed in order to build the "%iv < %TripCount" comparison for masking.
In order to handle loops with reversed (%TripCount, -1) induction variables %riv instead of a PIV, this comparison should use %iv = %TripCount - %riv, or simply compare "%riv > 0" instead.
Note that LV eventually always builds a new PIV to control the vector loop, so best use it instead of an %riv; but this PIV is not (yet) represented in VPlan directly, i.e., in the absence of an original PIV.
There are however non-primary cases other than reversed whose tail would also be good to fold, e.g., when the start index is non-zero or when SCEV can somehow determine the trip count as in PR40816.

So it would be better to provide a more general solution, such as (1) having LV build this comparison using a new PIV if needed, which requires additional VPlan support; (2) canonicalize loops to have a PIV before they reach LV, based on (Predicated?) SCEV, possibly by extending/rerunning `loop-simplify` or `indvars` as suggested in https://reviews.llvm.org/D68577#1742745.
Note that (1) follows the general **long-term** direction of modelling the entire vector loop in VPlan.
Note that (2) may help LV in additional aspects, e.g., by clearing the discrepancy that last reverted D68577 <https://reviews.llvm.org/D68577>.

Sounds reasonable to go for (2) at this time?


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  https://reviews.llvm.org/D76838/new/

https://reviews.llvm.org/D76838