[SystemVerilog] disable label and disable fork

The disable fork statement differs from disable in that disable fork considers the dynamic parent-child relationship of the processes, whereas disable uses the static, syntactical information of the disabled block. Thus, disable shall end all processes executing a particular block, whether the processes were forked by the calling thread or not, while disable fork shall end only the processes that were spawned by the calling thread.
from LRM

---

disable fork会kill disable fork 所在的当前线程以及所有子线程；
disable label 会kill 含有相同label 的进程；

program automatic test;
  task dis_thread(int t);
    fork:fork_la
      repeat(t)begin
        $display("%0d,still alive %t");
        #500;
      end

      begin
        #1000;
        $display("timeout!!");
      end
    join_any
    disable fork_la;   
  endtask

  initial begin
    #2000;
  end

  initial begin
    fork
      dis_thread(1);
      dis_thread(2);
    join
  end
endprogram

result:
1,still alive 0
2,still alive 0
timeout!!

可以看出，当dis_thread(1)被kill后，dis_thread(2)也被kill，因为两个task 进程具有相同的label。

---

program automatic test;
  task dis_thread(int t);
    fork:fork_la
      repeat(t)begin
        $display("%0d,still alive %t");
        #500;
      end

      begin
        #1000;
        $display("timeout!!");
      end
    join_any
    disable fork;   
  endtask

  initial begin
    #2000;
  end

  initial begin
    fork
      dis_thread(1);
      dis_thread(2);
    join
  end
endprogram

result:
1,still alive 0
2,still alive 0
2,still alive 500
timeout!!

可以看出，当dis_thread(1)被kill后，dis_thread(2)没有被影响。

---

program automatic test;
  task dis_thread(int t);
    fork
      #600;
      $display("%0d,fork..join_none %t");
    join_none

    fork:fork_la
      repeat(t)begin
        $display("%0d,still alive %t");
        #500;
      end

      begin
        #1000;
        $display("timeout!!");
      end
    join_any
    disable fork; //两个fork均会被kill  
  endtask

  initial begin
    #2000;
  end

  initial begin
    dis_thread(1);
  end
endprogram

result:
1,still alive 0
timeout!!

disable fork 和两个fork 处于同一个父进程中（由于没有离它最近的fork 父进程，所以会kill 整个父进程），因此两个fork 会被kill。

---

program automatic test;
  task dis_thread(int t);
    fork
      #600;
      $display("fork..join_none %t");
    join_none

    fork  //always use addtional fork to wrap up all the forks that you want to disable
      begin
        fork:fork_la
          repeat(t)begin
            $display("%0d,still alive %t");
            #500;
          end

         begin
           #1000;
           $display("timeout!!");
         end
       join_any
      disable fork; 
     end
     begin//这个进程不会被disable
       #1500;
       $display("extra!!!");
     end
   join
  endtask

  initial begin
    #2000;
  end

  initial begin
    dis_thread(1);
  end
endprogram

result:
1,still alive 0
fork..join_none 600
timeout!!
extra!!

建议把想要disable的fork 用 fork join 包起来，如上面的例子所示。