[PATCH] D101722: [SCEV] Don't require ControlsExit for gt/lt NoWrap

[Nikita Popov via Phabricator via llvm-commits]

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When determining the exit count for gt/lt predicates for an AddRec with non-unit stride, we need to check whether the AddRec may overflow. NoWrap flags are used for this purpose, but currently only if `ControlsExit` is true. Among other things, this means that it must be a single-exit loop.

I believe the `ControlsExit` requirement is not necessary. To explain why, it's useful to consider the case where it **is** needed, which is for equality exit counts (https://github.com/llvm/llvm-project/blob/ec2e3e331e6d60dee77e4da83bbb192597069f15/llvm/lib/Analysis/ScalarEvolution.cpp#L9261). If we have something like `{0,+,3}<nw> == 4`, then it would be not correct to just return `4 u/ 3` as the exit count: It could be that a prior normal or abnormal exit exits before the UB branch on poison from the `<nw>` flag violation is encountered. It would be wrong to upper-bound the trip count using this exit.

The same issue doesn't apply to gt/lt predicates. If we have something like `{0,+,2}<nuw> < Len` then the loop will exit after `((1 + Len) /u 2)` iterations, if it exits through that exit. It could also exit through an earlier (normal or abnormal) exit that might prevent a poison value from reaching the branch, but that ultimately doesn't matter.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D101722

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/trip-count-non-unit-stride.ll

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