res->back_edge_src = bitset_obstack_alloc(obst, res->n_blocks);
res->back_edge_tgt = bitset_obstack_alloc(obst, res->n_blocks);
res->map = obstack_alloc(obst, res->n_blocks * sizeof(res->map[0]));
res->back_edge_src = bitset_obstack_alloc(obst, res->n_blocks);
res->back_edge_tgt = bitset_obstack_alloc(obst, res->n_blocks);
res->map = obstack_alloc(obst, res->n_blocks * sizeof(res->map[0]));