http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60533
--- Comment #7 from Bill Schmidt <wschmidt at gcc dot gnu.org> --- The problem is actually introduced much earlier, during the cunrolli (complete unroll inner) pass. I'm attaching dumps from 055t.copyrename2 and 056t.cunrolli to show what happens. Prior to unrolling, we have a loop formed by blocks 47,19,20,...,46, with the original call to makeUnion at the end of block 45. The unroller decides that this loop will be executed exactly 3 times and unrolls it completely. (It's not clear to me on what basis this decision is made in the first place; it doesn't seem justified on the surface.) In any case, the third copy of the loop comprises blocks 74 through 101, with the call to makeUnion duplicated at the end of block 100. The unroller then inserts a __builtin_unreachable at the end of block 101 for some reason. This survives until the rewrite into RTL, at which point it is converted to the barrier that causes the bad block reordering.
