Re: sc_event naming: Cadence position on proposed extension

All,

Here is an updated summary of the event naming proposal now on the table:

* Events are not sc_objects

* All static events are named (the default constructor generates a name)
* Dynamic events can be named or unnamed
* Kernel events can be named or unnamed, and the names are
implementation-defined

* Named events have a parent in the object hierarchy (which can only be a
module or a process)
* Add a method sc_event::get_parent_object() to return this parent (null
for unnamed events)
* The prefix to each event's name is the hierarchical name of the event's
parent
* An event name shall not clash with an object name (sc_gen_unique_name is
called iteratively)

* Add methods get_child_events, get_top_level_events, and sc_find_event
that mirror the methods of the object hierarchy

* Add a method sc_name_exists(const char*) that returns true if the name
is in either the object or the event namespace

Can everyone now live with that?

I have one question regarding the constraints on implementation-defined
kernel names. Do they have to be legal names? If so, there might be a risk
of a clash with a subsequent user-defined name, which would then get given
a suffix from sc_gen_unique_name. If not, the only way to create an
illegal name would be to include a space character (the AZaz09_ character
set for string names is recommended, but not obligatory).

I would vote for implementation-defined kernel names being legal names.
The kernel could always include separators such as -> or / that are
unlikely to be used by an application. Moreover, as I read the LRM, I
think we could allow implementation-defined hierarchical event names to
include extra '.' characters since these would not technically be embedded
in string names (arguments passed to ctors) but would be part of the
hierarchical name of a new kind of event object.

To clarify, when you construct an sc_object or an sc_event you pass in a
string name that must not contain dots. But an implementation-defined
kernel event name could include extra dot separators, I think.

Thanks,

John A

From:
Martin Janssen <Martin.Janssen@synopsys.com>
To:
"john.aynsley@doulos.com" <john.aynsley@doulos.com>
Cc:
"systemc-p1666-technical@eda.org" <systemc-p1666-technical@eda.org>
Date:
23/11/2010 17:27
Subject:
Re: sc_event naming: Cadence position on proposed extension

Hi John,

To avoid further delays in the process, I'll change my vote to option 2
provided it states that kernel event naming is implementation-defined,
as Bishnupriya proposed in an earlier email.

One more question: How is a vendor supposed to experiment with
developing a sane object/event naming and parent/child relationship
hierarchy while remaining compliant with the 2010 standard if the '.'
character in the name is an error? (not only for kernel event naming)

-Martin

On 2010-11-23 17:35, Bishnupriya Bhattacharya wrote:
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
 
----------------------------------------------------------------------------------------------------------------------------------------------------

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

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