RE: [sv-ec] 1900 mantis (checkers): checker variable semantics: interoperability and portability issues, multiple semantics in SV LRM

From: Mirek Forczek <mirekf_at_.....>
Date: Wed Apr 09 2008 - 08:12:48 PDT
Hi Dmitry,
 
I've listen this discussion very carefully.
 
The most important conclusion IMHO was: the SV LRM no longer refer to the
single (simulation) semantic only, rather than it refer to the at least
three different (in nature) semantics now: the (event-based) simulation,
formal checking and symbolic simulation were mentioned.
The simulation semantics seems to keep dominate position in LRM text, but
certain paragraphs start to refer other semantics now ...
 
But the basic problem is: only the simulation semantic's foundation is well
defined in LRM - this is chapter 9 "Scheduling semantics". This is where any
"semantics" shall start at all in the SV LRM ...
 
I think this chapter shall be extended with foundation definitions for the
other two semantics (formal checking and symbolic simulation), the today
contant of the chapter shall become sub-paragraph.
It does not imply whole LRM text shall tripled immediately - initally we can
assume the today content apply to the all semantics unless it is specified
otherwise for particular construct.
Than only specific constructs can be supplemented with definitions for a new
semantics.
 
With such document organization, dealing with things like "free checkvar"
will be much more easy:
 
The LRM text can contain full definition of some construct (i.e.: the "free
checkvar") semantics for one domain (i.e: formal checking), without need to
provide full mapping of that content into the other semantics domain (i.e.:
simulation) if it (obviously) do not fit at all.
For another domain a completely different semantics definition could be made
- in particular: an empty one - the construct shall be ignored by a tools
that implement such semantics.
 
(this is not exactly the case for "free checkvar" probably - "free checkvar"
is expected to act like random-constrained variable in simulation, but still
this demonstrates that different domains may require different semantins
explained for a construct - on a basis of different fundametals of the
domain.)
 
All this is not possible without having good foundation chapter for multiple
semantics ...
 
Regards,
Mirek
 

  _____  

From: Korchemny, Dmitry [mailto:dmitry.korchemny@intel.com] 
Sent: 9 kwietnia 2008 15:39
To: Mirek Forczek; sv-ec@server.eda.org
Cc: sv-ac@eda.org
Subject: RE: [sv-ec] 1900 mantis (checkers): checker variable semantics:
interoperability and portability issues.



Hi Mirek,

 

We started discussing this issue at our yesterday meeting, See some more
comments below.

 

Thanks,

Dmitry

 

  _____  

From: owner-sv-ec@server.eda.org [mailto:owner-sv-ec@server.eda.org] On
Behalf Of Mirek Forczek
Sent: Monday, April 07, 2008 2:37 PM
To: sv-ec@server.eda.org
Subject: [sv-ec] 1900 mantis (checkers): checker variable semantics:
interoperability and portability issues.

 

 

In "16.18.6 Checker variables" section of the proposal there is:

 

"Simulators shall assign arbitrary values to the free checker variables
provided that these values are consistent with the free checker variable
assignments; they also may use the assumptions to constrain these values if
they have the capability to do so. If the values assigned by a simulator
contradict some of the assumptions, the simulator shall report a violation
of the corresponding assumptions. The simulator shall report a violation of
an assertion containing free checker variables if the assertion is violated
for the values of the free checker variables assigned by the simulator.
Optionally the simulator may choose to check the assertions for all possible
values of the free checker variables imposed by the assumptions and
assignments if it supports relevant formal verification techniques, e.g., if
it supports model checking on the fly."

The style of some sentences: may(/may-not ?), if they have capability,
optionally - make a potential threat to interoperability and portability,
IMHO.

With the same input (source code) simulators are allowed to work in a
different manner.

[DK] Yes. the interoperability is important, but let's ask a different
question: given two different formal tools checking the same assertion,
would you expect that both tools would generate the same counterexample? To
achieve it you have to standardize the verification strategy, the model
checking algorithms and its parameters, search heuristics, SAT solvers, etc.
The situation in our case is very similar.

 

The sentence: "If the values assigned by a simulator contradict some of the
assumptions, the simulator shall report a violation of the corresponding
assumptions." rise a questions:

- once the assumptions were specified in a source code, what is the point to
assign by a simulator the values that obviusly will contradict with the
assumptions,  to check the assumptions after that and report violation ?

[DK] There are several reasons for doing that. One is that the assumptions
may be contradictory or contradict the RTL. The other reason is not to
require from a simulator to have an embedded formal verification engine.
Without such an engine you cannot guarantee to satisfy all the assumptions
(and even with it you can have a blow-up).

  Wouldn't it be better to obligate simulator to check the new values
against assumptions before an assignment and not to assign them if they
violate assumptions, just to select another one .. ?

[DK] See my previous comment.

- is it good idea to allow simulators decide whether to ignore the
assumptions or not, without any explicit directive in source code ?

[DK] Same.

It is good idea to classify and organize checker variable semantics
accordingly to the assumed simulator capabilites, but it would be better to
have strictly defined behaviour for particular syntax.

[DK] ?

More advanced semantics could be denoted with additional directives or
additional options to the existing directives/declarations.

An advanced semantics - once denoted in a source code - shall obligate
simulator to follow them.

Users shall be aware of their simulator limits and to achieve code
portabiity an advanced directives could be explicitly placed under
conditional compilation (which is a popular coding style to achieve code
portability).

[DK] See my previous comment.

Regards,

Mirek

 

 

 

 

 

 

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Received on Wed Apr 9 08:19:30 2008

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