`uvm_do_with with inline constraints

Question

I was using inline constraints in my sequence with the uvm_do_with macro and I came across something I don't quite understand. Hoping someone can shed some insight into this:

Assume my my_seq_item has 2 rand variables - data and addr

my_sequence.sv

class my_sequence extends uvm_sequence;
// Constructor, .... etc etc

virtual task body();
  my_sequence_item tx;
  bit [31:0] addr = 'h0101_0101;
  bit [31:0] data = 'hDEAD_BEEF;

  `uvm_do_with(tx , { tx.addr == addr;
                      tx.data == data;
                    });
endtask: body

This doesn’t play nice. Instead:

 `uvm_do_with(tx , { tx.addr == local::addr;
                     tx.data == local::data;
                   }

Works fine.

The other solution is naming my local variables something other than addr and data.

The expansion of the uvm_do_with macro says that it calls item.randomize(), so I assume in this case, the call would be

item.randomize(addr, data) with {addr == local::addr;
                                 data == local::data;
                                } 

Does this mean that if the scope of the variables within the constraint block is that of item rather than the current object? If this is the case, then I should be able to call:

 `uvm_do_with(tx , { addr == local::addr;
                     data == local::data;
                   }

If this also true, then why does naming the variables to something else work?

Answer 1

In-line constraints have complex identifier search semantics. It first searches the object that randomize() is called on (item in this case). If no identifiers are found, the search proceeds normally from the point where the call to randomize() occurs (from the body() task).

So when you have the same identifier names in both the item and the body, the constraint finds the item identifiers first. You don't have this problem when the names are not the same. Using local:: is the way to show your intent that you do not want the item to be search. I suggest using local:: whenever you just want the local scope search, regardless of when or not there are identifiers with the same names in the item.