Gotcha: SystemVerilog’s post_randomize() is Called Top-Down Not Bottom-Up

SystemVerilog classes contain the pre-defined function post_randomize(), which is automatically called at the end of the randomization. One can override this function to do extra processing after randomization.

In SystemVerilog post_randomize() is called top-down and not bottom-up!

The top-down call of post_randomize() is counter-intuitive, especially for those of you in love with e-Language, and it can cause problems that require extra debug effort.

Let’s consider for the sake of the example that we have a simple packet made of a header, 8 bytes of data and a CRC. The header has its own CRC.

Both Header’s and Packet’s CRCs are computed once randomization is finished:

class header;
  //random info
  rand byte header_info[2];
   
  //crc calculated based on the generated data
  byte header_crc;
  ...
  function void post_randomize();
    header_crc = compute_header_crc();
    $display("header: calling post_randomize() - CRC is: %X", header_crc);
  endfunction
endclass

class packet;
  rand header header_i;
	
  rand byte data[8];
   
  // crc calculated based on the generated header
  byte packet_crc;
  ...
  function void post_randomize();
    // At this point the header is randomized, but its CRC ia not computed yet
    packet_crc = compute_packet_crc();
    $display("packet: calling post_randomize() - header_i.CRC is: %X", header_i.header_crc);
  endfunction
endclass

The issue in the code snippet above is: header_crc isn’t available at the time packet_crc is computed since packet.post_randomize() is called before header.post_randomize(). packet_crc will be computed using the default value of header_crc which is 8’h00.
Here is the output:

OUTPUT:
packet: calling post_randomize() - header_i.CRC is: 00
header: calling post_randomize() - CRC is: 8e

Take care e-Language speakers!!!