https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66916
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> --- It's a combination of sign-changed compare and X - Y CMP 0 to X CMP Y. Bad is <bb 4>: _9 = end_8 - start_6; length_10 = (size_t) _9; if (start_6 == end_8) and I guess good was if (length_10 == 0) where the sign-change actually didn't matter. The single-use restriction on the X - Y CMP 0 to X CMP Y doesn't help us because it has just a single use - the conversion. The sign-change and widen comparison optimization has no such restriction though. Adding that restriction avoids the above transform and fixes the testcase.
