RE: compiler directive with formal arguments

We'll try. I'd be glad to hear specific suggestions.

Shalom

> -----Original Message-----
> From: owner-verilog-ams@server.eda.org [mailto:owner-verilog-
> ams@server.eda.org] On Behalf Of Muranyi, Arpad
> Sent: Friday, November 10, 2006 1:19 AM
> To: verilog-ams
> Subject: RE: compiler directive with formal arguments
> 
> Shalom,
> 
> Thank you very much for your detailed explanation.
> I think I understand it now.  Would it be possible
> to make the LRM somewhat easier to understand (or
> am I the only one who took this long to get it)?
> 
> Thanks,
> 
> Arpad
> ==================================================
> 
> -----Original Message-----
> From: Bresticker, Shalom
> Sent: Tuesday, November 07, 2006 1:30 AM
> To: Muranyi, Arpad; 'Geoffrey.Coram'
> Cc: 'Marq Kole'; 'verilog-ams'
> Subject: RE: compiler directive with formal arguments
> 
> Hi,
> 
> Let me try to clarify things.
> 
> > -----Original Message-----
> > From: Muranyi, Arpad
> > Sent: Monday, November 06, 2006 10:06 PM
> > To: Geoffrey.Coram; Bresticker, Shalom
> > Cc: Marq Kole; verilog-ams
> > Subject: RE: compiler directive with formal arguments
> >
> > Hello everyone,
> >
> > I need to add a few of my thoughts to this subject on text macros.
> >
> > For one, the example below uses the double quotes around the macro
> > text, which is incorrect considering what is shown in "will print".
> > As I was studying section 11.3 in LRM v2.2 (p. 239), I was surprised
> > to find out that macro text and string are two different things.
> > Strings are surrounded by double quotes, but the macro text is NOT!
> > I thought this was an inconsistency, but some Verilog-AMS gurus from
> > a vendor convinced me that this is the way it is (and should be).
> >
> > So the example below should have said:
> >
> >   `define H(x) "Hello, x"
> >   $display(`H(world));
> > will print
> >   "Hello, x"
> >
> > including the double quotes!
> [SB]
> What happens is that where the macro is used, the actual argument is
> substituted as is as a textual substitution. Then the macro definition
> with the actual arguments is text substituted for the macro call. This
> is really a pre-compilation stage.
> 
> However, there is an exception. A string literal is a single token
from
> the point of view of the pre-processor. The textual substitution of
> macros occurs after the parser has turned the source text into a
> sequence of tokens. Substitution does not occur within a string
literal.
> That is what Mantis 1119 comes to clarify, because it is not specified
> in 1364-2005.
> 
> So you get the following:
> 
> $display(`H(world));
> 
> turns into
> 
> $display("Hello, x");
> 
> This prints simply
> 
> Hello, x
> 
> without quotes.
> 
> If the macro had been defined as
> 
> Hello, x
> 
> without quotes, then `Hello(world) would give us
> 
> $display(Hello, world);
> 
> At this stage, the compiler has no idea what 'Hello' and 'world' are.
> 
> If 'Hello' and 'world' are variables or parameters, for example, this
> will be legal. If not, it will be a syntax error. But the same macro
> expansion occurs in both cases.
> 
> 
> > This raises another question in the LRM regarding arguments.  The
LRM
> > says the following in this section:
> >
> > "The macro text can be any arbitrary text specified on the same line
> as
> > the text macro
> > name, but shall not begin with __VAMS_ to avoid conflicts with
> > predefined macros in
> > Section 11.7."
> >
> > The "any arbitrary text" means that the character "(" at the
beginning
> > of
> > the macro text  can be part of the text.  On the other hand, if
> > argument(s)
> > are used, they will be enclosed by parentheses.  How is the compiler
> > going
> > to decide that an opening "(" character is an argument, or part of
the
> > macro
> > text?  You may say that if there is no space between the
> > text_macro_identifier
> > and the first "(" character then it is an argument, but if there is
a
> > space
> > then it is part of the macro text.  However, this is not spelled out
> in
> > the
> > LRM.
> [SB] As mentioned by Geoffrey, this is specified in 1364-2005.
> 
> >  In addition, what am I supposed to do if I want to start my macro
> > text
> > with a space or tab character?  Theoretically that should also be
> > allowed,
> > since "any arbitrary text" is allowed in the macro text, right?
> 
> [SB] You really can't start a macro with white space. It will be
> stripped out by the compiler.
> 
> >
> > Another question I have on this subject is what the argument type
can
> > be.
> > The LRM example on p. 241 uses a literal for the argument:
> >
> > //define an adc with variable delay
> > `define var_adc(dly) adc #(dly)
> > // Given the above macro the following uses
> > `var_adc(2) g121 (q21, n10, n11);
> > `var_adc(5) g122 (q22, n10, n11);
> > // shall result in:
> > adc #(2) g121 (q21, n10, n11);
> > adc #(5) g122 (q22, n10, n11);
> >
> > Is it possible to use a variable or parameter instead, like this:
> >
> > parameter integer MyNumber = 2;
> > `define var_adc(dly) adc #(dly)
> > `var_adc(MyNumber) g121 (q21, n10, n11);
> > // and expect this result?
> > adc #(2) g121 (q21, n10, n11);
> 
> [SB] Yes. As I said, pure textual substitution occurs. So
> 
> `var_adc(MyNumber) g121 (q21, n10, n11);
> 
> expands to
> 
> adc #(MyNumber) g121 (q21, n10, n11);
> 
> Note that the substitution of 2 for MyNumber occurs later.
> So that if you use `var_adc(MyNumber) in different places, it can
expand
> to different values, if MyNumber has a different value in each place.
> 
> >
> > Take this further one step, can we use string parameters or
variables
> > and expect the compiler to expand it as if it was macro text?
> [SB] Sure. As I said, the preprocessor just takes what is written as
an
> argument in the macro call, and substitutes it in for the formal
> argument, without substituting a parameter's value for the parameter
> name, etc.
> 
>   The
> > example
> > below is interesting from this perspective also:
> >
> >   $display(`H(world));
> >
> > Note that the argument world is not enclosed by double quotes.  Why?
> > What
> > is the type of this argument?  Is it a string?  If not, do we have a
> > magic
> > type perhaps called "text" which doesn't get enclosed by double
> quotes?
> 
> [SB] That is maybe a good way of putting it. There is a Mantis item
open
> to clarify this. It is confusing because the LRM says that the actual
> argument is an expression, but in reality that restriction does not
> exist. The argument can be either with quotes or without, as you wish.
> It will be substituted as is into the text, with quotes or without,
> respectively.
> 
> >
> > But what if the argument is a string variable or parameter?  Is that
> > going
> > to get expanded in the macro text and become part of the expanded
> macro
> > text?
> > For example, can I write:
> >
> > parameter string MacroArg = "world";
> > `define H(x) Hello, x
> > $display(`H(MacroArg));
> >
> > and expect it to print:
> >   Hello, world
> >
> > Or is it going to print:
> >   Hello, "world"
> >
> > with the double quotes?  Or will (should) the compiler even take
this?
> [SB] This would turn into
> 
> $display(Hello, "world");
> 
> Hope this helps.
> 
> Shalom