RE: sc_event naming: Cadence position on proposed extension

John, Martin,

Comments below.

-Bishnupriya

________________________________
From: owner-systemc-p1666-technical@eda.org [mailto:owner-systemc-p1666-technical@eda.org] On Behalf Of Martin Janssen
Sent: Tuesday, November 23, 2010 7:31 PM
To: john.aynsley@doulos.com
Cc: systemc-p1666-technical@eda.org
Subject: Re: sc_event naming: Cadence position on proposed extension

John,

A lot of nice words from Bishnupriya, but the only objection against our extension
proposal is hierarchical naming of objects, which is not part of our proposal, and
for good reason.

But does this mean that we should do a lousy job on hierarchical event naming?
Or, put differently, should we treat events as first class citizens or not?

Let's take a look at the current objects types. We have modules and processes,
which are the most important object types when modeling and simulating with
SystemC. Their hierarchical name is just perfect, and also in other regards we
can say that these object types are indeed first class citizens. The other object
types, such as ports, exports, primitive channels, are already in the "poor objects"
(or second class citizens) category.

Enter named events. We already decided not to make them objects, but to
treat them somehow like objects. In which of the two above categories (first
class citizens or second class citizens) do events belong? Bishnupriya is
saying that we should put them with the second class citizens, and do as
poor a job at naming them. But events _are_ first class citizens that belong
with modules and processes. During simulation processes and events are
at center stage.

So my question is: why not treat events as we do modules and processes?

>>> [bpriya: The logic presented above does not make any sense to me. With the SNPS extension proposal, how are events getting treated the same as modules and processes? Today, the only thing that distinguishes modules/processes from other objects is that modules/processes can be parents, owning children. The SNPS extension is *not* proposing that events be able to own children. It is proposing that objects be able to own events, which actually creates inconsistency between events and other objects, including processes - e.g. when an object instantiates an event and a process inside it, the event will get a *nicer* hierarchical name "mymod.myobj.myevent" compared to the process's name "mymod.myproc". How is that treating events like processes? On the contrary, it pegs events at a notch higher than processes (and all other objects for that matter) as "gold class citizens" perhaps.]

An implementation that has been out there for 2 years, on its own, cannot be
enough reason for unamended standardization.

>>> [bpriya: Cadence has never presented this as the rationale either for standardization of the Cadence proposal, or for vetoing the SNPS extension proposal. The Cadence proposal has been presented early in this round, and it has been scrupulously explained technically and thoroughly debated. The fact that an implementation exists and has been tested by users is always considered a positive for anything but non-trivial proposals, and especially so for unchartered territory.

Cadence has pushed back on standardizing the SNPS extension in this round because it creates a clear inconsistency in the hierarchy for events and objects, and at this last minute, there isn't the time for technical discussions in p1666 (and for experimental implementations) in order to explore if this inconsistency can be removed/minimized. Cadence does not support standardizing something half-baked that leaves obvious, unanswered questions in the users' mind. That is not a prudent strategy. Following precedence, this rightfully belongs to the LWG for further exploration, and can be presented in the next round if so deemed.]

-Martin

On 2010-11-22 11:35, Bishnupriya Bhattacharya wrote:
John, All,

Over the past few days, within Cadence, we have carefully reviewed the SNPS extension to the Cadence event naming proposal. Our position is as below. There is strong agreement within Cadence on this position, including from Stuart .

First, about allowing objects to parent other objects, there are certainly issues (as several of you have pointed out) if we try for an ambitious scheme to arbitrarily change the object hierarchy at run-time. However, that is not our intent. We're trying to explore if we can carve out a small subset that will be useful. We're not envisaging a dynamic module hierarchy. Any object hierarchy changes will be restricted to elaboration time only.

So, what is the motivation? One thing that comes to mind is a natural grouping of things to better express the intent of a class, and prevent "leakage" of internal implementation objects to the outside of this class.
Consider an aggregate class like the TLM2 simple_target_socket. It has a number of other objects and events inside it - these are all internal implementation artifacts of this socket. Today all of these internal objects/events end up in the parent module's scope. The output of get_child_objects() and get_child_events() on a target module that owns a simple_target_socket is given below.

top.target.socket
top.target.socket_port_0
top.target.m_peq_0
top.target.b2nb_thread_0
top.target.peq_with_cb_and_phase_0
top.target.fec_0
top.target.event_0
top.target.event_1
top.target.event_2
top.target.event_3
Everything besides the target socket itself, does not really belong to the parent module's scope; the rest is useless clutter that a user never wants to see. It is safe to say that the simple_target_socket class does not intentionally want to expose these internal implementation artifacts, but today it has no choice but to do so. With the SNPS extension proposal, the events will now go out of this list - the poor objects will be left behind

top.target.socket
top.target.socket_port_0
top.target.m_peq_0
top.target.b2nb_thread_0
top.target.peq_with_cb_and_phase_0
top.target.fec_0

This is not consistent. It does not feel right that events will end up with the property that they can be grouped more hierarchically, but objects can't. As things stand today (or rather I should say with the Cadence proposal), things are not perfect, but at least an user can expect consistency when constructing an user-defined object wrt scope of internal objects and events. With the SNPS extension, that is no longer true. It is as if events have become more of a first class citizen than objects.

We all agree that a proper "fix" for this - such that objects can also be children of other objects in some restricted, but useful fashion - is not simple. Many issues need to be considered, especially wrt adjusting existing object's ctor, and other issues that Philip has raised. Realistically speaking there isn't time in this round to consider all the ramifications, and arrive at a solution (or conclude one is not possible).

Given that, we feel it will be inappropriate to only standardize the object-owning-events feature, and leave things in a half-baked state. We do not buy the argument that since this feature pertains to event naming it will make it to this round - we DO need to consider the ramifications of proposed solutions to the rest of the ecosystem. We feel it is more appropriate for the LRM to leave kernel event naming as implementation-defined in this round, and include this feature as an addition in the next round if it is so deemed. We're opposed to rushing something into the LRM that leaves things in an inconsistent state without having the time to consider the complete problem.

Leaving this aspect implementation-defined in p1666-2010 allows each vendor (including the OSCI ref sim) to experiment with developing a sane object/event naming and parent/child relationship hierarchy while remaining compliant with the 2010 standard. In 1666-2012 (or whenever the next round is) we can then propose standardizing that which we have previously implemented and tested.

We would like to hear what others have to say on this topic.

Thanks,
-Bishnupriya

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     Bishnupriya Bhattacharya | Software Architect - Virtual System Platform | Cadence
     P: +91.80.4184.1197 www.cadence.com<http://www.cadence.com/>
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