"A variable index into the for generate block is illegal" error

Question

I created a group tasks using "generate" and was trying to call them using a for loop.

I have used generate block to create multiple instances of module, so I thought I could use it similarly to create multiple tasks. I plan to have no arguments for the tasks, and always call the tasks in a loop. But the number of tasks to be called is dependent on a parameter (instead of "4" in the example below, it will be a parameter).

Here is an example of what I was trying to do:

  task display;
    fork
      other_tasks();
      for(int i=0; i<4; i++) begin
        create_task[i].print_i;
      end
//      create_task[0].print_i;
//      create_task[1].print_i;
//      create_task[2].print_i;
//      create_task[3].print_i;
    join
  endtask

  generate
    for(genvar i=0; i<4; i++) begin : create_task
      task print_i();
        $display("%t, %m, Message #%0d", $time, i);
        #1;
      endtask
    end : create_task
  endgenerate

The code gives me no error if I call the tasks separately (commented lines) instead of using "for". However, I would prefer to do this using a loop because the number of tasks I need to call may vary. What would be the correct way of doing this?

Answer 1

Based on the the limited requirements you provided, the simplest solution is triggering a single event that calls all the tasks.

 event print;
 task display;
    fork
      other_tasks();
      ->print;
    join
  endtask

  generate
    for(genvar i=0; i<4; i++) begin : create_task
      task print_i();
        $display("%t, %m, Message #%0d", $time, i);
        #1;
      endtask
    always @print print_i;
    end : create_task
  endgenerate

If you want to only call some of the tasks, you could create an array of events, and trigger the array indexed by a variable.

 event print[4];
 task display;
    fork
      other_tasks();
      ->print[some_index];
    join
  endtask

  generate
    for(genvar i=0; i<4; i++) begin : create_task
      task print_i();
        $display("%t, %m, Message #%0d", $time, i);
        #1;
      endtask
    always @print[i] print_i;
    end : create_task
  endgenerate

If you ever need to pass arguments to the task, there are ways of using interface classes instead of using events.

interface class abstract_print;
  pure virtual task printer(input int arg);
endclass

abstract_print list[4];

 task display;
    fork
      other_tasks();
      list[some_index].print(some_arg);
    join
  endtask

  for(genvar i=0; i<4; i++) begin : create_task
      task print_i(int arg);
        $display("%t, %m, Message #%0d", $time, i);
        #1;
      endtask
    class concrete_print implements abstract_print;
       function new;
         list[i] = this;
       endfunction
       task print(int arg);
          print_i(arg);
       endtask
    endclass
    concrete_print p = new;
    
  end : create_task