How many matches in two queues

SystemVerilog
1

I have two queues of sequence items, one from the dut and one from the ref. I’d like to count the amount of transactions that match so I thought about using the sum reduction method:

my_seq_item ref_queue[$];
my_seq_item dut_queue[$];

// find_matches() relies on caller to make sure the queues are not empty
function int find_matches();
  return dut_queue.sum() with (item.compare(ref_queue[item.index]);
endfunction

but apparently it doesn’t behave like I would expect.

The 1800-2017 states (7.12.4):

The following reduction methods are supported:
— sum() returns the sum of all the array elements or, if a with clause is specified, returns the sum of the values yielded by evaluating the expression for each array element.

I assume the the ‘sum of the values yielded’ would be the sum of 1'b1 and 1'b0 reported by the compare operation.

Should I cast the result into a bit datatype for the sum to work? Like this:

function int find_matches();
  return dut_queue.sum() with (item.compare(bit'(ref_queue[item.index]));
endfunction

I’m trying the above and will report, but if in the meantime someone is able to catch another issue that would be great.

2

The result of the compare method is a single bit, so the result of the sum method will also be a single bit without a cast. You want to cast the result to an int, not the inputs to the compare method.

function int find_matches();
  return dut_queue.sum() with (int'(item.compare(ref_queue[item.index]));
endfunction
1 Like
3

Hi Dave, actually what I wrote was not what I intended! What I said I was going to do it was supposed to be:

function int find_matches();
  return dut_queue.sum() with (bit'(item.compare(ref_queue[item.index]));
endfunction

Nonetheless, what you say makes sense, I assumed (wrongly) that summing bits will automatically be cast to an int but I’m not sure why I thought so.

Thanks a lot!