[sv-bc] What constitutes the actual lval in an output?

The LRM does not address whether "lval" determination is done
in an eager or lazy manner.

Consider the following:

     module top;
         class C;
               int id;
         endclass
         C cid = new();
         C cid_prev;

         task automatic next_val(output int val);
             #10 val = 5;                       // line 1
         endtask

         initial begin
             #10 next_val(cid.id);              // line 2
         end

         initial begin
             #15 cid_prev = cid;                    // line 3
             cid = new();
             #10 $display(cid_prev.id,, cid.id);    // line 4
         end
     endmodule


At time 10, line 2 calls next_val; with "cid" having some
handle value (call it object1).  At time 15, "cid" changes
to associate to a new object (call it object2).  At time 20
the next_val routine terminates and the copy out occurs.  The
question is whether the lval of "cid.id" was determined
at the time of the call or at the time of the copy-out.
In this case, the difference determines whether the
assignment targets object1 or object2.

Either approach has semantic peculiarities.  If "cid"
would become "null" before the copy-out, the "determination
at exit" approach would cause a simulation error.  If
the "determination at entry" is adopted, the call might
be required to keep an object alive if the routine becomes the
last reference.

There are similar issues with respect to array indexing using
an index expression that changes during the task evaluation.

In addition, if the formal in "next_val" was "inout", then
adopting "determination at exit" would mean that the copy-in
and copy-out could actually relate to different locations
(object1's id would be copied-in; object2's id would be
the copy out target).


The LRM doesn't clearly discuss this issue, but I suspect that
"determination at exit" is the expected answer (meaning that
"object2" is the target in my original example).

Does everyone concur with that expectation?

Gord.
-- 
--------------------------------------------------------------------
Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com


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