any restrictions on expression in wait(expression)?

[mastrick]

The LRM says that an "event control expression" can include a non-virtual function as long as it has a singular return value. Do those restrictions apply to the expression within wait()? If any function is allowed within wait, should I expect that the wait will unblock within the same time that the function return value changes (to whatever would make the expression true)?

[dave_59]

The issue with functions in wait expressions or event controls is that the LRM only defines a few conditions when the expression must be evaluated to see if the result has changed. And that is when any operand in the expression changes. That includes arguments to any function calls as well as any objects referenced directly in the expression. An object is considered changed when any of it members has changed.

The problem is that a class member can be a reference to another child class object, but when the child object changes, the reference in the parent object does not. It is the same problem as shallow copy versus deep copy, except here it is shallow events versus deep events. If you have a parent-child hierarchy of class objects and you are waiting on a method that looks into the value of child object, it won't unblock until something in the parent changes, or some other operand in the expression changes and causes the whole expression to be re-evaluated. For example

class A;

bit B1;

endclass

class B;

A a = new;

bit B2;

function bit getB1;

return a.B1;

endfunction

function bit getB2;

return B2;

endfunction

endclass

task run;

wait(getB2()); // evaluated when B2 changes

wait(getB1()); // evaluated when B2 changes not when a.B1 changes

endtask

[mastrick]

Thanks for the detailed response, Dave. Can you clarify one point? Your example shows that wait(getB2()) is evaluated when B2 changes, but your text says "when any operand in the expression changes. That includes arguments to any function calls as well as any objects referenced directly in the expression". B2 is not an argument to the function nor referenced directly in the expression. Is there another category of operands that covers B2?

[dave_59]

getB2() is implicitly this.getB2(). Since B2 is a member of 'this', a change event to B2 is also change event to 'this'.

So wait(this.getB2()) will call getB2 for evaluation whenever there is a change event on 'this' (as well as calling getB2() upon first executing the wait statement)

[mastrick]

Thanks again, Dave. That makes perfect sense to me. Perhaps we could get some of your text in the next update to the LRM?

[salmantanvir]

Hi, 

 

Could someone clarify if there is a performance impact when using a function in an event control expression instead of a signal?

 

Br,

Salman

 

[dave_59]

Of course @(func(signal)) is going to cost more in performance than just plain @(signal).