[sv-ac] Re: Clock flow

Hi Manisha:

> Is there any specific reason that the instance of a sequence with
> methods ended/matched are getting excluded in the above case ?=20

First, I believe that these restrictions can be relaxed without 
introducing backward compatibility problems.  This is because saying 
that the clock does not flow through ended/matched means the sequences 
instances to which these methods are applied must already have initial 
clocking events defined.  Thus, if the contextual clock flows in, it
will immediately be overridden by the internal clocks of the sequences.

As I recall, the intuition for these restrictions was basically that 
a clock flowing into a sequence instance starts affecting the 
beginning of that sequence, while ended/matched are references
to the endpoints of sequences.  Thus, there is something 
counterintuitive about have the clock flow "backwards" to get to the 
start of the sequence instance to which ended/matched is applied.

Also, "matched" involves synchronization between clocks, so I think the
committee preferred to avoid the clock flow in this case.

On the other hand, especially with "ended", I think the case can be
made that it is a nuisance to forbid the clock to flow in.  

For example, if I want to code my sequences as unclocked and only 
attach my clock at the top level of a property, then I want that clock 
to be able to flow to all parts of the property.  If I need to use "ended",
then I am stuck--I have to declare an explicit clock for each sequence
to which I want to apply ended, and I have to make sure it matches the clock 
in the context where ended appears.  This makes those sequences less
re-usable, or it forces me to a different coding style in which the 
clock gets passed in as an argument.

My current opinion is that it would be a good thing to relax this
restriction for "ended", at least if "ended" is applied to an instance of
an unclocked sequence.

In the case of matched, it is less clear to me that it is a good idea
to relax the restriction.

I am curious to hear other opinions on this topic.

Best regards,

John H.

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> From: "Kulshrestha, Manisha" <Manisha_Kulshrestha@mentor.com>
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> Hi John/All,
> 
> In section 17.12.3 (Clock Flow), P1800 draft, it says that:
> 
> The scope of a clocking event flows into an instance of a named
> property. The scope of a clocking event flows into an instance of a
> named sequence provided neither method ended nor method matched is
> applied to the instance of the sequence.
> 
> Is there any specific reason that the instance of a sequence with
> methods ended/matched are getting excluded in the above case ?=20
> 
> Thanks.
> Manisha
>