[all-commits] [llvm/llvm-project] 48d7cc: [SCEV] Fix incorrect treatment of max taken count....

[max-azul via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 48d7cc6ae23b0e5b1922457462d0f6e4582a1ae7
      https://github.com/llvm/llvm-project/commit/48d7cc6ae23b0e5b1922457462d0f6e4582a1ae7
  Author: Max Kazantsev <mkazantsev at azul.com>
  Date:   2020-11-23 (Mon, 23 Nov 2020)

  Changed paths:
    M llvm/lib/Analysis/ScalarEvolution.cpp
    M llvm/test/Analysis/ScalarEvolution/pr48225.ll

  Log Message:
  -----------
  [SCEV] Fix incorrect treatment of max taken count. PR48225

SCEV makes a logical mistake when handling EitherMayExit in
case when both conditions must be met to exit the loop. The
mistake looks like follows: "if condition `A` fails within at most `X` first
iterations, and `B` fails within at most `Y` first iterations, then `A & B`
fails at most within `min (X, Y)` first iterations". This is wrong, because
both of them must fail at the same time.

Simple example illustrating this is following: we have an IV with step 1,
condition `A` = "IV is even", condition `B` = "IV is odd". Both `A` and `B`
will fail within first two iterations. But it doesn't mean that both of them
will fail within first two first iterations at the same time, which would mean
that IV is neither even nor odd at the same time within first 2 iterations.

We can only do so for known exact BE counts, but not for max.

Differential Revision: https://reviews.llvm.org/D91942
Reviewed By: nikic