Writing functions in Covergroup

Coverage
1

All I am working on understanding how to use functions to create coverpoint and cross bins. Consider the following example code:

module top;

logic [5:0] small_bv;

covergroup cg;
  cp : coverpoint small_bv {
    bins b0 = {'d0};
    bins b_interting[] = {['d1:'d62]} with (bins_interesting_func(item));
    bins  b63 = {'d63};
  }
end group

function automatic bins_interesting_func(logic [5:0] val);
  automatic boolean not_prime = 0;
  if (int'(val) > 1) begin
    for(int i=2; i < int'(val)/2; i++) begin
      if ((int'(val) % i) == 0) begin
        not_prime = 1;
        break;
      end
    end
  end
  if (not_prime == 0) return 1 else return 0;
endfunction

initial begin
  for (int sim_loop=0; sim_loop<256; sim_loop++) begin
    small_bv = $urandom;
  end
end

endmodule

With this example I have a small bit_vector in which I am randomizing the values. I am creating a dynamic list of bins based on if the value of the variable for the coverpoint is a PrimeNumber.

Yes I know the PrimeNumber search is brute force, you cannot use the $sqrt operator inside a covergroup function call?

Questions:
Is the word “item” a SV keyword of some sort meaning the variable referenced in the coverpoint?

The return type of the function is not declared, I can see in my analysis of running this code (BTW this was hand transcribed from another system so there may be a typo or two) that I do indeed get a bunch of bins that are only the prime values. I got what I wanted but I need to understand the mechanisms better for my real problem.

I will be posting an update as I try to get a function to build bins for the cross product, see my previous example: Building Sparse Cross Coverage Bins for arrays of enum'ed datatypes
for some ideas of what I am trying to solve.

Thanks in advance to anyone who can answer my couple of questions.
TomT…

2

Did you make any progress on this?
This is something that interests me as well.

As for your question:

Is the word “item” a SV keyword of some sort meaning the variable referenced in the coverpoint?

no, the keyword item is a reference to the item from the array manipulation methods, but I would say you should do something like this:

covergroup cg;
  // not tested!!!
  cp : coverpoint small_bv {
    bins b0 = {'d0};
    bins b_interting[] = {['d1:'d62]}.find() with (bins_interesting_func(item));
    bins  b63 = {'d63};
  }
endgroup

The return of the array find() method is a queue filled by the return value of the bins_interesting_func(item) where item is the one iterated on.
See: 7.12.4 Iterator index querying.

As mentioned in the code, it is not tested, but I don’t see why it shouldn’t work and it would be nice if someone can chime in and comment.