[PATCH] D87344: [IndVars] Remove range checks for which we can predicate ends of the range

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: fhahn, lebedev.ri, sanjoy.google, reames, skatkov.
Herald added subscribers: llvm-commits, javed.absar, hiraditya.
Herald added a project: LLVM.
mkazantsev requested review of this revision.

This patch teaches IndVars to remove the following kind of range checks:

  for (i = X; cond(); i--) {
    if (i predicate Y) deopt();
    ...
  }

basing on the following set of facts:

- Predicate is monotonic;
- Condition is met on the 1st iteration;
- Max possible number of iterations (given that we do not exit by the check being deleted) is limited and does not exceed max unsigned value of `Type(i)`;
- Condition is met on the max possible iteration;
- `X - max possible itereration` does not overflow.

If all conditions above are true, then the range check can be trivially eliminated on every iteration.

The max number of iterations is computed by SCEV basing on all loop exit blocks, with block
of interest excluded.


https://reviews.llvm.org/D87344

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
  llvm/test/Transforms/IndVarSimplify/monotonic_checks.ll

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