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[LV] Make sure prefer-predicate-over-epilogue works for short TC loops
Needs ReviewPublic

Authored by ebrevnov on Dec 14 2021, 1:37 AM.

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Reviewers
fhahn
Summary

Currently "short trip count" heuristic overrides user settings of PreferPredicateOverEpilogue and llvm.loop.vectorize.predicate.enable. That doesn't seem right for several reasons. Typically user settings are expected to override compiler chosen heuristics. Another reason is surprising difference in behavior for "known" short trip count loops and all other cases.

PS: This change is exploited in https://reviews.llvm.org/D115713 to be able to keep testing coverage.

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