[PATCH] D95026: [SimplifyCFG] Update FoldBranchToCommonDest to be poison-safe

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

I could reproduce the failures from two-stage build and submitted a bug report: https://bugs.llvm.org/show_bug.cgi?id=49495

Consider this code snippet:

  vector<ty> v; // empty
  ty* b = v.begin(), *e = v.end()
  if (b != e && b < (e - 1))
    // reverse items in [b, e)

After the patch, LLVM was optimizing it into

  vector<ty> v; // empty
  ty* b = v.begin(), *e = v.end()
  if (b < (e - 1))
    // reverse items in [b, e)

But this is wrong. Since v is empty, v.begin() and v.end() can return NULL. `e - 1` raises overflow, hence the branch is taken, which is wrong.

PopulateLoopsDFS<llvm::BasicBlock, llvm::Loop>::insertIntoLoop was containing a code snippet that looked like this.


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