[all-commits] [llvm/llvm-project] 73b951: [SCEV] Clarify requirements for zero-stride to be UB

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 73b951a7f7dc07be37bcb3d545d228098c4e19f4
      https://github.com/llvm/llvm-project/commit/73b951a7f7dc07be37bcb3d545d228098c4e19f4
  Author: Philip Reames <listmail at philipreames.com>
  Date:   2021-09-01 (Wed, 01 Sep 2021)

  Changed paths:
    M llvm/lib/Analysis/ScalarEvolution.cpp

  Log Message:
  -----------
  [SCEV] Clarify requirements for zero-stride to be UB

There's a silent bug in our reasoning about zero strides. We assume that having a single static exit implies that if that exit is not taken, then the loop must be infinite. This ignores the potential for abnormal exits via exceptions. Consider the following example:

for (uint_8 i = 0; i < 1; i += 0) {
  throw_on_thousandth_call();
}

Our reasoning is such that we'd conclude this loop can't take the backedge as that would lead to a (presumed) infinite loop.

In practice, this is a silent bug because the loopIsFiniteByAssumption returns false strictly more often than the loopHaNoAbnormalExits property. We could reasonable want to change that in the future, so fixing the codeflow now is worthwhile.

Differential Revision: https://reviews.llvm.org/D109029