RE: [sv-ec] Mantis 2149 - Covergroups sample method with arguments

Scott,

 

My comments inlined below.

 

            Arturo

 

________________________________

From: David Scott [mailto:david_scott@mentor.com] 
Sent: Friday, November 09, 2007 2:27 PM
To: Arturo Salz
Cc: SV-EC; sv-ac@eda-stds.org
Subject: Re: [sv-ec] Mantis 2149 - Covergroups sample method with
arguments

 

Arturo:

Some reaction from the Mentor covergroup team and Gordon (Dave R and Ray
not represented):

#1.  Syntax.  There was a preference to drop the @, to allow a
declaration like so:

covergroup ct function sample(int x);

The issue is that "@ function" seems like a cue to wait on an event
related to the function, or that it seems somehow related to event-based
sampling.



We're open to syntactical changes.

The reason for the "@ function" syntax was to use an operator to
separate the coverage declaration from the sampling method declaration;
it seems more readable this way. We'd like to hear from other people.


#2.  We'd prefer some re-wording around the first sentence of the last
paragraph.  Maybe something like this:

Within coverpoint and guard expressions, the formal arguments of an
overridden sample method shall be searched after other covergroup items
but before the scope enclosing the covergroup.

("Other covergroup items" might be the option or type_option members,
though not coverpoints and crosses according to Mantis 1279.)



I'm not sure this is better. The more complex verbiage does not appear
to solve any issue.

How does the limitation to coverpoint and guard expressions needed? 


#3.  "error to use a sample argument" should read "error to use a sample
formal".



OK.


#4.  The last example needs a "ref" added in the covergroup arguments,
like so: "covergroup C1 (ref int v)".



No. That's the whole point of the example.

Users may think that it's OK to use the "v" covergroup argument by the
constructor (to set the weight option) and the "v" sample formal by the
definition of the coverpoint., The rule is intended to avoid this
ambiguity.


#5.  Just for people to consider, we are expecting the following to be
legal (with our preferred syntax)?

int a;
covergroup ct (ref int b) function sample(int c);
    coverpoint a + b + c;
endgroup

 

Yes. We agree. Awkward, but legal nonetheless.

 

#6.  A host of issues around sample arguments.  One thing that occurred
to us -- without, I'll note, Gordon's blessing! -- is that we could
possibly allow this:

int a;
covergroup ct function sample(ref int x = a);
    coverpoint a;

This would allow event-based sampling to occur as well as procedural
sampling, let's say with this syntax (which I know would be a change
from the proposal):

covergroup ct function sample(ref int x = a) @(e);

With event-based sampling, the default arguments would be used, which as
you see cause "a" from the enclosing scope to be sampled.  It wasn't
clear from Mike's original e-mail whether the override was expected to
be optional (used occasionally) or required (used always).



I am very uncomfortable with this change. The current proposal makes it
clear that there are two mutually exclusive ways to trigger a
covergroup: via an events (or block event) or procedurally through the
sample method. Otherwise, there are other issues like "strobe" sampling
that rear their ugly head.


If default arguments are used, what restrictions are there?  Can they be
covergroup formals, like this?

covergroup ct (ref int a) function sample(ref int x = a);

 

Good point. I would say no. Covergroup formals should not be visible in
the scope of default arguments.

 

Can they be non-static class properties for an embedded covergroup?



Yes. Any reason to disallow them?


What about "output" arguments?  While this sounds nonsensical, someone
might find a use for something like this:

function int f(output o, input i);
    o = 2*i;
    return o;
endfunction

covergroup ct function sample(output o, input i);
    coverpoint f(o,i);
endgroup

 

As you say, "output" arguments are nonsensical. Do we need an explicit
error rule? Is there any plausible expectation on the part of users?

 

Finally, what about a null argument list to sample?  That sounds to me
like it should be disallowed.



The current BNF allows it and I see no problem for an overridden method
to have the same signature as the default method. In a way the proposal
is saying that omitting the coverage_event is the same as an explicit
"covergroup <name> @ function sample();"



-- David Scott, Mentor Graphics


Arturo Salz wrote: 

I've uploaded a proposal for Mantis 2149. Please review.
 
        Arturo
 
-----Original Message-----
From: owner-sv-ec@eda.org On Behalf Of Michael Burns
Sent: Wednesday, October 24, 2007 4:38 PM
To: SV_EC List
Cc: John Havlicek
Subject: [sv-ec] Mantis 2149 - Covergroups sample method with arguments
 
 
Mantis 2149 - Covergroups sample method with arguments has been
submitted 
describing what we'd like to see in a solution for the covergroup sample
issue I 
raised a week or two ago.
 
--Mike Burns
 
 
  

 


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