RE: Attributes and constant expressions

From: J. Bhasker (jbhasker@cadence.com)
Date: Wed Oct 24 2001 - 09:01:02 PDT

Precedence: bulk

Paul:

Just a comment: Attributes are not free standing, i.e. they are not a
statement
by themselves, but are always associated with a statement/module/expression.
So trying
to implement synthesis_on/off using attributes would limit its flexibility
as compared
to where all you can place the on/off directives today.

- bhasker 

--

J. Bhasker
Cadence Design Systems
7535 Windsor Drive, Suite A200, Allentown, PA 18195
(610) 398-6312, (610) 530-7985(fax), jbhasker@cadence.com

<p>-----Original Message-----
From: owner-btf@boyd.com [mailto:owner-btf@boyd.com]On Behalf Of Paul
Graham
Sent: Wednesday, October 24, 2001 11:26 AM
To: stuart@sutherland-hdl.com
Cc: btf@boyd.com
Subject: Re: Attributes and constant expressions

<p>Precedence: bulk

> I believe the standard very clealy states that a genvar is only valid at
> elaboration time.  It therefore would have no value during simulation
time,
> and therefore should not be a legal expression in an attribute.

No, a reference to a genvar results in a value at simulation time.
Otherwise, the following would make no sense:

    genvar i;
    generate for (i = 0; i < 8; i = i + 1)
      begin : blah
        assign q[i] = d[i];
      end
    endgenerate

When the simulator executes, there must be some meaning for q[i] and d[i].
While the genvar data structure may exist only during elaboration, its value
remains for simulation.  So it is a perfectly good, simulation-time
constant.

My original question was really to find out whether attributes were intended
to be more like preprocessor-style directives or vhdl attributes.
Preprocessor directives in C support only the simplest of expressions, while
vhdl attributes can have any complex constant value (including calls to
constant functions).  It seems the intent is be like vhdl.

I also was wondering how to implement attributes in my parser.  If
attributes are simple, preprocessor-like directives, then a simple data
structure holding, say, a string and integer would be sufficient for the
parser to store an attribute's value.

Also, since attributes are intended in part to provide a vendor-independent
way of specifying synthesis directives, I wondered how some commonly used
synthesis directives could be specified using attributes.  One very common
directive is:

    // ambit synthesis off
    ...
    // ambit synthesis on

Suppose this were translated into an attribute:

    (* synthesis = 0 *)
    ...
    (* synthesis = 1 *)

Now suppose that the expression depends on a parameter.  Then synthesis
could be enabled or disabled in a module based on some defparam in another
module.  Thinking about this gave me a headache.

Paul

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