Multithread and automatic variable

[Parth]

Hi , 

For below code , for case-1 and case-2 output is different ,

 

module my_module; 
    int a=0; 
    initial 
    begin 
      for(int i=0;i<5;i++) 
      begin     
         fork 
                // Check differance here , between 2 statements 
                // ##case 1 (declaration and assignment). automatic int j= i;
                // and 
                automatic int j;  // ##case 2 declare 
                j=i;                  //  then assign
                begin
                    $display("Chk auto %d",j);
                end    
         join_none  
      end
    end 
endmodule : my_module

 

For case-1 output is  0 1 2 3 4 

 

For case-2 output is 5 5 5 5 5 

 

Can anyone explain me how??

Thanks , 
Parth

[bhunter1972]

Put the assignment of j=i into the begin/end (for that matter, put the declaration there, too, to be safe).

 

In case 2, you have forked off two separate processes:  one assigns j to i and the other prints j. All of these take zero time, and the order in which they occur is indeterminate.

[dave_59]

bhunter, that is incorrect; only case 1 will work. The initialization of automatic variables happens as the scope they declared in come into existance(activated).

 

Each iteration of the for loop creates a new fork/join_none block with a unique value of i.  That unique value of i is used to initialize the local copy of j created for each fork/join_none block. If you put the variable assigment into procedural code of the child threads, the ordering of the execution of those child threads are indetermiate, and at best, deferred until the parent thread blocks or terminates.

[uwes]

hi,

 

what you essentially wrote in case#2 is 

 

module my_module; 
    int a=0; 
    initial 
    begin 
      for(int i=0;i<5;i++) 
      begin     
         fork 
                // Check differance here , between 2 statements 
                // ##case 1 (declaration and assignment). automatic int j= i;
                // and 
                automatic int j;  // ##case 2 declare 
begin
                 j=i;                  //  then assign
end
                begin
                    $display("Chk auto %d",j);
                end    
         join_none  
      end
    end 
endmodule : my_module

note that the $display and the "j=i" are actually parallel threads (a race). if you switch the order of the two  blocks your result might be different (if your simulator chooses this)

 

                begin
                    $display("Chk auto %d",j);
              end    

begin
                 j=i;                  //  then assign
end

 

so for what you want to accomplish case#1 is the right path.

 

/uwe

[Parth]

Thanks bhunter , dave and uwe . 

 

What I conclude is , automatic int j= i; does not create process in fork_join but always executed before $display , so case-1 works and not case-2. In case-2 also if we remove automatic type then result would be same as case-1.