if (!user_island) continue;
      // We have labeled `user_island` before so it also must have been added to
      // `worklist` before.
      if (pred_map.count(user_island) > 0) continue;

      worklist.push(user_island);
      pred_map[user_island] = curr_op;
    }
  }

  // Construct path by following predecessor pointers.
  IslandOp curr_op = target_op;
  while (curr_op != source_op) {