RE: [sv-ec] Transition coverage and covergroup instances

Just to complement this discussion. I fully agree with Scott's write-up.
The per_instance option is intended only as a request to persist the
coverage information of each covergroup object in the database. As Scott
wrote, there is no implication that covergroup instances interact in any
way except for aggregation - or perhaps by sampling the same data.

 

I'm sorry Jonathan, but the answer did seem obvious to me as well.
Perhaps you can point out what part of the LRM may have mislead you.

 

            Arturo

 

From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of
David Scott
Sent: Friday, August 08, 2008 5:21 PM
To: Mark Strickland (mastrick)
Cc: jonathan.bromley@doulos.com; sv-ec@eda.org
Subject: Re: [sv-ec] Transition coverage and covergroup instances

 

Jonathan's question was whether you could sample foo==0 in one
covergroup instance followed by foo==1 in a different covergroup
instance and expect the bin to increment.  Since sampling is
per-instance, I say not.

The solution to your problem is sample the values in the same covergroup
instance.

To use Jonathan's example, if there were a way to make the covergroup cg
static within class Data, that would be an elegant way to do it.
However, SystemVerilog doesn't offer a syntax for that, so the best bet
is to create another class with embedded covergroup, where a variable of
that class is declared static in class Data.  Then the value "B" gets
assigned (and sampled and the transition covered) within the static
class variable.

If you needed both -- sometimes covering transitions within a given
packet, sometimes covering transitions between packets -- then obviously
you'd have to maintain sampled variables and covergroups for both
intra-packet and inter-packet transitions.

David


Mark Strickland (mastrick) wrote: 

David,

 

If the "rise" bin is not incremented in the "not per-instance"
covergroup, how would you describe the coverage such that you could
detect one packet with foo=1 followed a packet with foo=0?  That is a
useful thing to measure.

 

Mark

 

________________________________

From: owner-sv-ec@server.eda.org [mailto:owner-sv-ec@server.eda.org] On
Behalf Of David Scott
Sent: Friday, August 08, 2008 5:56 PM
To: jonathan.bromley@doulos.com
Cc: sv-ec@server.eda.org
Subject: Re: [sv-ec] Transition coverage and covergroup instances

Jonathan,

I may be too close to our implementation, and maybe too close to the
covergroup chapter having studied it for a few years now, but the
question never occurred to me.  And I still don't find anything
ambiguous now that you mention it.

The option.per_instance has been recently clarified as a cue for the
requirement of saving an instance in the coverage database.  To my way
of thinking, the use of multiple instances of a given covergroup has an
implication only for the aggregation of coverage: multiple instances
will be aggregated into the "overall coverage information for the
covergroup type" (LRM's words).  I don't think there is an implication
that covergroup instances interact in any way except with regard to this
"overall coverage", which is some kind of aggregation.  The 2009 LRM did
add type_option.merge_instances to allow a couple of different ways to
aggregate instances into the covergroup type.

Your question seems to imply that covergroup instances might interact
with each other in how bins are incremented.  The bin increment -- not
least because sample() is an instance method -- is entirely local to the
covergroup instance.  Whether there is a bin associated with the type is
really just an artifact of aggregation: with
type_option.merge_instances==1, the type can indeed appear to have a
bin; with type_option.merge_instances==0, the type probably does not
(although I suppose a tool could do report one if it chose.)

Anyway, to answer your last two questions directly, from my perspective:




Given that it is not a per-instance
covergroup, did we increment the "rise" bin?


No.




It may be that the answer to this is so blindingly 
obvious to coverage experts that it isn't worth saying,
but it would be nice if the LRM were explicit about it.
Or have I missed something?


I'm not sure.  The answer was blindingly obvious to me, but sometimes
clarifications don't hurt.  We can see what others have to say ...

Cheers,

David Scott



jonathan.bromley@doulos.com wrote: 

hi EC,
 
I know it's too late to change anything now, but this question arose
in discussion here recently and I can't find clarification of it in any
version of the LRM so it might be worth asking.
 
How are transition coverpoints incremented when multiple
instances of a covergroup exist?  It seems reasonable to
assume that, if the covergroup's per_instance option is true,
each CG instance detects transitions as seen by just that
one instance alone.  But what happens when the CG's
per-instance option is false? 
 
For example:
 
module User;
 
  class Data;
    bit b;
    covergroup cg;
      coverpoint b { bins rise =  (0 => 1); }
    endgroup
    function new(bit B);
      b = B;
      cg = new;
    endfunction
  endclass
 
  initial begin
      Data d;
      d = new(0);
      d.cg.sample();
      d = new(1);
      d.cg.sample();
  end
 
endmodule 
 
At the end of this code we have created one instance of
the covergroup that sampled b==0, and then a second 
instance that sampled b==1.  Given that it is not a per-instance
covergroup, did we increment the "rise" bin?
 
It may be that the answer to this is so blindingly 
obvious to coverage experts that it isn't worth saying,
but it would be nice if the LRM were explicit about it.
Or have I missed something?
 
Thanks
  



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