Constraining Dynamic array size

Hi Sundar.

I think the generator is a bit more intelligent than you give it credit for ;-)
It should be able to understand that the array_size and data.size() are linked, and thus generate them together.

By way of demonstrating this point, I expanded on your example, using an in-line constraint on data.size(). If you run this, you should see that array_size is never greater then 4.

module test;
 class dynamic_array;
   rand int array_size;
   rand byte data [] ;
   constraint array_constriant {
     array_size inside { [ 1:10 ] } ;
     data.size() == array_size;
   }
   function void post_randomize();
     $display("Array_size=%0d", array_size);
   endfunction

 endclass

  dynamic_array c = new();

initial begin
  for (int a=0; a<100; a++)
    void'( c.randomize() with { data.size() < 5; } );
end
endmodule

In SystemVerilog, a dynamic array marked with "rand" and its size are considered as two different random variables. In addition, an implicit ordering exists between generation of the size of a dynamic array and generation of that dynamic array, where the size variable is always generated first. In your examples, random variables "array_size" and "data.size()" are solved first and assigned random variables. Then random array "data" is resized to the value assigned to "data.size()" and then its members are randomized. This approach effectively means that size of array "data" in your example will always be the same as the random value generated for "array_size". Note that this implicit ordering is very important to keep in mind since it can lead to generation problems (i.e. not finding a solution where one exists) if a constraint is given between "array_size" and one of the elements of array "data" (e.g. data[3]). See section 12.3 in LRM.

I should also clarify that if the size of a dynamic array is not constrained, then the size of that array remains the same across generation. In this case, only members of the array are randomized. So size of an array is considered as a random variable if and only if a constraint is specified for it. Also, if there is a constraint specified for size of an array, I THINK that the array is resized only if the randomly generated size is bigger than the previous size. For example, if a dynamic array size is 100, and I constrain the size to 50, then the actual array size doesn't change, but randomization only randomizes the first 50 elements. But if I constrain the size to be 200, then the array is resized and the first 200 elements are randomized. Sasan