[sv-ec] Clarification of the random stability in object instances

Title:
   Clarification of the random stability in object instances

Description:
   In section 12.13, it is claimed that each object (class instance) has

an independent RNG. This mechanism could cause a misuse of generating
random objects by user.
   It is very commonly seen in may testbenches that a new class instance

is created and used per each test-sequence. In such case, multiple
class instances are created and sent to the testing subsystem. For
example, a class 'packet' can be created and sent to the testing
subsystem as:

   // commonly used OOP coding style
   packet pkt ;
   int i ;

   for( i = 0 ; i < 1000 ; i++ ) begin
     pkt = new ;
     packet_driver.send( pkt ) ;
     ..
   end

This coding style can not be used with randomize() function due to the
random object stability. If the 'packet' class is declared as:

   class packet ;
     rand int id ;
     randc bit [3:0] dest ;
     ..
   endclass ;

And the code above is modified to call 'randomize()' as:

   // code uses random class but not in a single random sequence
   for( i = 0 ; i < 1000 ; i++ ) begin
     pkt = new ;
     success = pkt.randomize() ;
     packet_driver.send( pkt ) ;
     ..
   end

Because a new RNG is created on each packet instance, and the random
stability among instances, this code won't satisfy the non-repeating
nature of 'dest' variable, and the distribution of 'id' is
questionable. (In a cense that the distribution of 'id' purely relies
on the RNG initialization mechanism, and not utilize the
distribution-safeness of the RNG mechanism itself.)

In order to satisfy the randomness that a user may expect, the code
must be written as:

   // the follow coding style is required to get a random sequence
   packet pkt, ppp ;
   int i, success ;

   pkt = new ;
   for( i = 0 ; i < 1000 ; i++ ) begin
     success = pkt.randmize() ;
     ppp = new pkt ; // create a copy for actual testing
     packet_driver.send( ppp ) ;
     ..
   end

To avoid the confusion and possible testing error by users, a detailed
explanation like above should be placed in the specifictation.

Also, it is desirable to have a mechanism to declare a random variable
without the instance level stability in the same class. This allows to
write the code in more intuitive way, and avoid many copy-object
operation which is possibly costly. This can be done with a new
property qualifier. For a possible implementation, the example here
uses 'cross' as an additional qualifier.

   <BNF>
     property_qualifier ::=
       [ 'cross' ] 'rand'
       |
       [ 'cross' ] 'randc'
       |
       class_item_qualifier

   Example:

     class packet ;
       cross rand int id ; // RNG is per class, not per object
       cross randc bit [3:0] dest ; // per class cyclic
       ..
     endclass ;

Note: non-terminal symbol in BNF is encapsulated in ' ' (single quote),

as bold characters can not be used in plain text.
Received on Sun Sep 5 14:25:49 2004