RE: [sv-bc] Trimming whitespace from macro actuals

>From: "Brad Pierce" <Brad.Pierce@synopsys.com>

>I looked through every occurrence of "white space" and "whitespace" in
>Draft 3a, but did not find the following rule --
>
>> The LRM says that whitespace at the beginning and end of the arguments
>is removed.

Strange.  I thought I recalled this rule being quoted recently (by Shalom
in reference to the proposal for 1571?), but I cannot find it in the LRM
or my email.  I remember being slightly surprised by the rule, since white
space didn't matter in the past.  Apparently I was hallucinating the entire
thing.

So now I don't know what happens to white space.  The LRM says that the
actuals are "expressions" rather than arbitrary text, which might have
some bearing on this.  However, I expect that in practice, implementations
allow fairly arbitrary text.  So it is hard to argue that leading or
trailing white space should be removed on the grounds that it is not part
of the "expression".

I think we do have to assume that white space terminating an escaped
identifier is part of the actual.  It is both part of the arbitrary text
of the actual, and if the actual is an expression, it is part of the
text of that expression.

>Nevertheless, your analysis of the problem is useful and interesting.  I
>agree with you that Verilog's \-and-whitespace escape brackets make the
>problem more complicated than C had to deal with. 

Based on the current LRM specification of how '' works, I conclude that
you cannot use it to paste together escaped identifiers.  The LRM does
not describe '' as pasting tokens together.  It just acts as a lexical
token delimiter that goes away after macro expansion.  This allows a
macro formal to be recognized as a token, but to end up immediately
adjacent to other text after expansion.  This is similar to the old trick
in K&R C code of putting a comment between two tokens in a macro to glue
them together.

So you can't just use escaped identifiers in the actuals and expect them
to work like normal identifiers that have the escaped names.  You have to
be consider their lexical form and what happens to that during the
substitution and expansion.

I conclude that

  assign `paste(\7@ ,8) = 1'b1;
  
expands into

  assign \7@ 8 = 1'b1;
  
and

  assign `paste(\7@ ,\8@ ) = 1'b1;
  
expands into

  assign \7@ \8@  = 1'b1;

both of which will produce a syntax error.
  
So you cannot write a paste macro that works on escaped identifiers.
This is annoying, but I guess it isn't a huge deal.


Steven Sharp
sharp@cadence.com


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