UVM pack and unpack

[girodias]

The following example of packing and unpacking does not work. Integral fields (length) get packed and unpacked properly but arrays do not get unpacked (unless I'm unpacking to an existing data structure that previously allocated space for the array). Is this a known issue?

program test;

import uvm_pkg::*;
`include "uvm_macros.svh"

class my_item extends uvm_sequence_item;

  rand int length;
  rand bit [7:0]  payload[];

  constraint data_length_cons      { length inside { [1:10] }; }
  constraint data_payload_sizecons { payload.size() == length; }

  `uvm_object_utils_begin(my_item)
    `uvm_field_int(length, UVM_ALL_ON + UVM_DEC)
    `uvm_field_array_int(payload, UVM_ALL_ON)
  `uvm_object_utils_end

endclass

initial begin

  my_item a = new();
  my_item b = new();
  bit bitstream[];
  int num_bits = 0;

  a.randomize();
  `uvm_info("a", {"\n", a.sprint()}, UVM_NONE);
  num_bits = a.pack(bitstream);
  b.unpack(bitstream);
  `uvm_info("b", {"\n", b.sprint()}, UVM_NONE);

end

endprogram : test

Typical output for the unpacked object will look like:

UVM_INFO n.sv(32) @ 0: reporter [b]
--------------------------------------------
Name               Type          Size  Value
--------------------------------------------
uvm_sequence_item  my_item       -     @465
  length           integral      32    'd8
  payload          da(integral)  0     -
--------------------------------------------

[jadec]

Make sure you set the packer to use_metadata = 1 during both the pack and unpack in order for array sizes to be stored and retrieved.

[girodias]

Should that not be handled by the field automation macros?

[jadec]

This is handled by the automation macros, but by default the packer doesn't generate the extra "metadata" needed to do this.

In your case, you have a redundant length field, but there's no automated association between that length and the array size.

[girodias]

I'm not following; the length property is redundant so why does it need to be considered for the unpacking? If I take the length property out and hard-code in the constraint the size of the array, how is the problem going to be different? The array will still be packed the same way.