res->n_blocks = dfs_get_n_nodes(res->dfs);
res->back_edge_src = bitset_obstack_alloc(obst, res->n_blocks);
res->back_edge_tgt = bitset_obstack_alloc(obst, res->n_blocks);
res->n_blocks = dfs_get_n_nodes(res->dfs);
res->back_edge_src = bitset_obstack_alloc(obst, res->n_blocks);
res->back_edge_tgt = bitset_obstack_alloc(obst, res->n_blocks);