[sv-bc] RE: [sv-ac] checker: Clarification on functions & side effects

Hi Srini,

We are talking about functions called in the RHS in the checker variable assignment. Obviously a void function cannot appear there directly, and we can talk only about the case when this void function is called from another function entering the RHS of the checker variable assignment.

The problem here is that though the term "side effect" is used in the LRM several times, there is no exact definition of this term. According to my understanding, calling $display is a side effect, and therefore $display cannot appear in this context in checkers. I agree that conventional usage of $display in checkers would not harm (unless you use auto-increment operations there, etc.), but this needs refinement of the "side effect" term. I believe that this activity belongs to SV-BC.

I definitely agree with the benefit of direct usage of procedural control statements in checkers, and we can elaborate this issue if it becomes part of the next PAR.

Regards,
Dmitry

From: Srinivasan Venkataramanan [mailto:svenka3@gmail.com]
Sent: Thursday, October 01, 2009 2:53 AM
To: Korchemny, Dmitry
Cc: ben@systemverilog.us; sv-ac@server.eda.org; Seligman, Erik
Subject: Re: [sv-ac] checker: Clarification on functions & side effects

Dmitry,
  Can you please explain why you believe a simple VOID function is illegal/meaningless in checker? Especially referring to an earlier issue raised by Ben/Abhishek on $display, I can think of "debug-only" functions being used in checker, a skeleton code below, simply added a "display function" that may potentially be reused for few cases. One may find WA by putting $display inline etc. but my point is why exclude a void function that someone may find it useful later on?

I also have questions on if.else, case usage in checker, will raise it little later - once this LRM work is done from SV-AC side. I understand the rationale of preventing them as of today is from its origin in Formal World, but now that it is becoming lot more widely accessible, it may be good to relook at the target usage.

Thanks
Srini
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On Thu, Oct 1, 2009 at 1:10 AM, Korchemny, Dmitry <dmitry.korchemny@intel.com<mailto:dmitry.korchemny@intel.com>> wrote:
Hi Ben,

Yes, assignment of a variable that does not belong to the function (i.e., neither its local variable, no argument) is illegal. But if a is a local variable in the function then a++ is legal.

It also turns out that void functions are illegal (or at least, meaningless) in checkers.

Regards,
Dmitry

From: owner-sv-ac@server.eda.org<mailto:owner-sv-ac@server.eda.org> [mailto:owner-sv-ac@server.eda.org<mailto:owner-sv-ac@server.eda.org>] On Behalf Of ben cohen
Sent: Wednesday, September 30, 2009 7:27 PM
To: sv-ac@server.eda.org<mailto:sv-ac@server.eda.org>; Seligman, Erik
Subject: [sv-ac] checker: Clarification on functions & side effects

LRM states that
Functions shall be automatic (or preserve no state information) and have no side effects,

1) What is a side effect? To me a side effect is an assignment of a variable (free variable of the checker or a module).
A function can (in a checker) only return a value.   Thus the following is illegal:
checker t_chk(...);
  logic a, x, y;
 function void inc_a;
       a ++; // same as    a=a+1 // illegal blocking assignment?
     // a <= a+1;  // Illegal also, cannot change value of a variable
   endfunction : a_to_1
 ap_test: assert property(@ (posedge clk) (x, inc_a()) |-> y) ;
endchecker : t_chk
   --
 2) Can functions be of type void in a checker?
   Since a void function must have a side effect, such as assigning a value to a variable, I believe that they are illegal.
Function with return type of void cannot be used in/as an expression.





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