Using functions inside constraints

SystemVerilog
1 
 rand bit tmp1, tmp2;
  function bit my_f(tmp);
   return tmp;
  endfunction
  constraint tmp_cnstr {
    my_f(tmp1 + tmp2) == 2;
  }

It sounds like tmp1 and tmp2 are solved before the constraint is applied and are assigned to 0. How can I use a function inside constraints?

my_f( .tmp( ( tmp1 + tmp2 ) ) ) (U1) SOLVED EARLY 0 (0x0)
tmp1 (U1) SOLVED EARLY 0 (0x0)
tmp2 (U1) SOLVED EARLY 0 (0x0)

2

You are very limited in how you can use a user defined function inside a constraint.

Either the function input arguments cannot reference any random variables, or the function return value cannot have any constraints that restrict the function inputs.

rand bit [7:0] tmp1, tmp2;
  function bit [7:0] my_f(tmp);
   return tmp+1;
  endfunction
  constraint c1 {
    tmp1 < 10;  tmp2 < 10;
  }
  constraint c2 {
    my_f(tmp1)+ tmp2 == 10;
  }

In this example, you could not constrain the sum in c2 to anything other than 10 because my_f(tmp1) could return any value between 1 and 10.

3

Thank you, Dave.
Below is what I am trying to do. But it seems that the array is randomized before the constraint takes affect. Is there a way to make it work?

rand bit arr[5][5];
function int sum_2d_array(int arr[5][5]);
    int sum = 0;
    for (int i = 0; i < 5; i++) {
        for (int j = 0; j < 5; j++) {
            sum += arr[i][j];
        }
    }
    return sum;
endfunction

constraint sum_constraint {
    sum_2d_array(arr) == target_sum;
}
4

If you are looking to constrain the sum of all the elements in a 2D array, you can use the built-in array reduction method, sum().

rand bit arr[5][5];
constraint c_sum { arr.sum(D1) with (D1.sum(D2))== target_sum; }

Unlike user defined function, the built-in reduction methods get expanded into one big constraint equasion. See section 18.5.8.2 Array reduction iterative constraints in the IEEE 1800-2017 LRM.

5

Thank you, Dave

6

I thought that it worked, but it didn’t.
“Identifier ‘D2’ has not been declared yet.” with VCS
“‘with’ clause not specified for the iterator argument.” with Xrun
Any idea why?

7

I switched to 1D array, which solved the constraint issue, but it would be nice to have the constraint with 2D array.

8

D2 is unnessary in theis example, but there appears to be tool issues

See 2D array constraint randomization - #7 by dave_59

9

Thank you, Dave!