[PATCH] D39082: [IRCE] Smarter detection of empty ranges using SCEV

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.

For a SCEV range, this patch replaces the naive emptiness check for SCEV ranges
which looks like `Begin == End` with a SCEV check. The range is guaranteed to be
empty of `Begin >= End`. We should filter such ranges out and do not try to perform
IRCE for them.

For example, we can get such range when intersecting range `[A, B)` and `[C, D)`
where `A < B < C < D`. The resulting range is `[max(A, C), min(B, D)) = [C, B)`.
This range is empty, but its `Begin` does not match with `End`.

Making IRCE for an empty range is basically safe, but unprofitable because we
never actually get into the main loop where the range checks are supposed to
be eliminated. This patch uses SCEV mechanisms to treat loops with proved
`Begin >= End` as empty.


https://reviews.llvm.org/D39082

Files:
  lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
  test/Transforms/IRCE/clamp.ll
  test/Transforms/IRCE/empty_ranges.ll

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