[PATCH] D44676: [SCEV] Make exact taken count calculation more optimistic

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: sanjoy, apilipenko, anna, skatkov, reames.

Currently, `getExact` fails if it sees two exit counts in different blocks. There is
no solid reason to do so, given that we only calculate exact non-taken count
for exiting blocks that dominate latch. Using this fact, we can simply take min
out of all exits of all blocks to get the exact taken count.

This patch makes the calculation more optimistic with enforcing our assumption
with asserts. It allows us to calculate exact backedge taken count in trivial loops
like

  for (int i = 0; i < 100; i++) {
    if (i > 50) break;
    . . .
  }


https://reviews.llvm.org/D44676

Files:
  include/llvm/Analysis/ScalarEvolution.h
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/exact_iter_count.ll
  test/Analysis/ScalarEvolution/max-trip-count.ll
  test/Analysis/ScalarEvolution/trip-count14.ll
  test/Transforms/IndVarSimplify/loop_evaluate10.ll
  test/Transforms/LoopSimplify/preserve-scev.ll

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