Re: [sv-bc] query regarding $bits function

>From: Surya Pratik Saha <spsaha@cal.interrasystems.com>

>If the 'side-effect' is not concern, then I am not clear about the 
>reason behind the restriction.

I don't know all the reasoning, but I can speculate on some of it.

1. Allowing functions without side effects would require defining what
that means.  The definition for constant functions is inadequate, as
there are things allowed in constant functions that would constitute
side effects if done at runtime.  For example, $display is ignored in
a constant function, but not at runtime.  Any call to a static function
has side effects, since it modifies the arguments and return value, which
are static variables accessible from elsewhere.

2. Allowing functions without side effects would require implementations
to do an analysis of the function and its entire call graph to determine
legality.  This is undesirable.  Disallowing the function is simple.

3. Why would you want to evaluate a function just to find out the
width of the result it would produce, and then throw that result away?
If you are planning to follow it by evaluating again and keeping the
result the second time, then this is wasteful.  You should evaluate it
to get the result, and then ask for the width of that.  Evaluation takes
time.

In short, you would have to present a reasonable use for this before the
extra complexity would make sense.

> $bits can not evaluate width for any 
>dynamic variables without evaluating it. For e.g.
>
>string s;
>initial begin
>    s = "abc";
>    i = $bits(s);
>end
>
>Is it possible to evaluate the width of 's' without evaluating it? I 
>don't think so.

Here the evaluation is so simple that it may not be easy to see that
it is not being evaluated.  But $bits does not necessarily need to
fetch the value to get the width.  If an implementation stores strings
as a size plus a data array, all $bits has to do is fetch the size, not
the data.  So it is not evaluating 's'.

To make it clearer, here is an example with a more complex expression:

  i = $bits({s,s});
  
Here the result is 2 times the width of s.  There is no need to evaluate
the expression by constructing the concatenation of s with itself to
determine that.

Steven Sharp
sharp@cadence.com


-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.