This happens in testcase gcc.dg/vect/vect-62.c: looks like on the alias branch pre is more powerful, as it moves the load into the latch block; as a result the latch block is not empty, and we fail to vectorize (with -fno-tree-pre vectorization succeeds).
Related non-empty-latch PRs that prevernt vectorization: PR28643, PR33447 -- Summary: [alias-improvements-branch] pre makes latech-block non- empty Product: gcc Version: unknown Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: tree-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: dorit at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37698