http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52701
--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-03-26 11:44:55 UTC --- During analysis we have {c_50, +, pretmp.51_172}_4 as access function. This is analyzed to be unknown: 9: Unknown def-use cycle pattern. but we still vectorize this loop, as the induction (with unknown step, thus not detected as such) is irrelevant, only the final value after the loop (thus, a missed SCEV "const"-prop). Which means we should simply always set STMT_VINFO_LOOP_PHI_EVOLUTION_PART. I have a patch.