Re: [sv-ac] Initial proposal text for messages in sequences.

Subject: Re: [sv-ac] Initial proposal text for messages in sequences.
From: Arturo Salz (Arturo.Salz@synopsys.com)
Date: Wed Nov 26 2003 - 14:30:27 PST

Adam,

I wonder why your proposal is limited only to those error reporting tasks.
Why not allow any arbitrary "pure" function (including void functions) that
will allow users to massage the data before issuing the message, or perhaps
adding it to an error database (which may involve file i/o). What's so
special
about these tasks? I believe the motivation is to provide access to the data
internal to the property but without changing the design state.

    Arturo

----- Original Message -----
From: "Adam Krolnik" <krolnik@lsil.com>
To: <sv-ac@eda.org>
Sent: Wednesday, November 26, 2003 1:08 PM
Subject: [sv-ac] Initial proposal text for messages in sequences.

Hello;

Here is an initial wording on the proposal. I wrote it and then saw that
John's if() proposal only allows for a boolean if expression. I will have
to remove that section or we can discuss alternatives.

     Thanks.

    Adam Krolnik
    Verification Mgr.
    LSI Logic Corp.
    Plano TX. 75074
    Co-author "Assertion Based Design"

---------------------------------------------------------------------------- 

----

Problem:

The problem with error reporting is that the error message does not
directly have access to the signal values and local variables used
in the property to determine a legal sequence of actions or an illegal
sequence.

For example, this property has signals and variables:

property follow_req(a, b, c, address)
  reg [31:0] req_addr;
  @(posedge clk) // Simple example
  (a, (req_addr = address)
     => b ##1 c)
endproperty

assert follow_req(a, b, c, address)
 else
  $error("C (%0d) did not follow b after a with address %0h.\n", c,
req_addr);
  // 'c' and req_addr may show the wrong value.

In addition to the difficulty in creating an error message, it will be
difficult
to track operation of some sequences and the setting of local variables.

Motivation:

1. Provide support for error messages using same context of signal values as
the property.
2. Provide support for error messages using local variables from the
assertion context.
3. Provide debug support for debugging the flow of sequences and local
variable settings.

Solution:

[We currently can not define a solution to provide for one error message for
the failure
of properties. There are too many questions to be resolved to come up with a
full
definition.]

Insert after section 17.8 "Manipulating data in a sequence"

17.9 Messages within sequences.

Messages can be included in sequences to aid in debug of sequences, or to
produce
error messages with contextual information. Messages are produced through
these
system tasks: $display, $fatal $error, $warning, $info.

Messages are added within the sequence, after an expression.

  sequence_expr ::=
     ...
    | expression, blocking_assign_or_message

As an example, after 'a' we expect 'b' and we store the value of 'v' into
the local
variable 'loc_v'. We then report a message using $display() with the shown
string.
We then proceed to expect 'c' one cycle later.

   a ##1 b, (loc_v = v, $display("loc_v is %0h at time %d.", loc_v, $stime))
     ##1 c

These system tasks do not produce a value (a function) they simply execute
in the
order they are listed. These messages can be for debug or for error
reporting.

[ Not yet. if (b |-> pass_p) and (!b |-> fail_p) then why not
  (first_match(b_p) |-> pass_p) and (!first_match(b_p) |-> fail_p)
 This would make it easier for messages. ]
For error messages, the if() structure allows for checking of the failure of
a sequence or property and subsequent failing of the property and provision
of a correct error message.

   a |->  if (##[1:10] b) 1 else 0, $error("Did not receive b within 10
cycles.")

Here if the sequence (##[1:10] b) fails then the else branch is taken which
yields 0
and executes the $error() routine. This routine shall replace any implicit
$error()
routine call (when there is no fail statement in an action block.) If a fail
statement
is included in an action block, any $error() (or fatal or warning) calls
shall override
the error routine from the property or sequence.

Messages shall not be attached to the success of a sequence. Only a boolean
expression
may preceed a message statement. When a sequence is expected to fail,
inversion of the
sequence followed by a message will suffice.

    a |->  not ##[1:10] b |-> 0, $error("Did not receive b within 10
cycles.");

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