RE: [sv-ec] New Mantis item on Coverage: bins rhs non-constant expressions

Doug,

I can give the user perspective.  I would like the meaning of the ignore
clause to be identical between when I run the simulation and when I view
the results (which could be an entirely separate run).  It seems
plausible that the restriction on constant expressions is required to
enforce the results being identical.  I *would* like to be able to
specify the value of the constant for a given run (in the code would be
best); I think that is what instance constants would do for me.

If there is a way to enforce the constant behavior of the ignore clause
without the current restriction, then from a user's standpoint, it would
be OK to remove the restriction.

Mark

-----Original Message-----
From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of
Warmke, Doug
Sent: Sunday, May 06, 2007 4:48 PM
To: sv-ec@eda.org
Subject: [sv-ec] New Mantis item on Coverage: bins rhs non-constant
expressions

Hi All,

I've uploaded a new Mantis item
   http://www.eda-stds.org/svdb/bug_view_page.php?bug_id=0001802

Following is the Mantis Description, reproduced for convenience.
Comments, anyone?  Based on feedback, I can either create a proposal for
this issue, or close the Mantis as "Not to be fixed".

Consider this testcase:

class myclass;
     int id;
     int ign_id;
     covergroup cg;
         coverpoint id {
             ignore_bins ign = {ign_id};
         }
     endgroup
endclass

The P1800-2005 LRM currently states, in 18.4:

    The open_range_list used to specify the set of values
    associated with a bin shall be constant expressions,
    instance constants (for classes only), or non-ref
    arguments to the coverage group.

Thus, the above testcase is illegal.
Is this overly restrictive?

The need for requiring constant expressions isn't clear. It would be
possible to accept non-constant expressions, and take a "snapshot" of
the expression value at covergroup construction time. There may be some
subtelties involved, such as function calls with side effects as part of
the expression. This should be considered, but we think it won't be a
showstopper. Non-constant variables could easily be handled, in any
case.

One clear fact is that the expressions used on the rhs of coverage bins
must not be allowed to vary with time after the covergroup is
constructed.

Regards,
Doug

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