Re: Standard Ada Preprocessor

["Robert I. Eachus" <rieachus@comcast.net>]

Marin David Condic wrote:

> A simple case: Suppose you are developing for an embedded box but you 
> want to do some simple "Smoke Testing" on a PC. For the embedded box, 
> you need a rep clause for some register. For the PC you're dummying that 
> up with something and don't want a rep clause at all. (Or the PC 
> compiler barfs on the rep clause that the embedded compiler accepts.) It 
> would be nice to say "If compiling for the PC with Aonix, use this code 
> (no rep), but if compiling for the target with the Gnat compiler, go use 
> that code..."

You are familar with RM 13.1(22) right?  "An implementation need not 
support representation items containing nonstatic expressions, except 
that an implementation should support a representation item for a given 
entity if each nonstatic expression in the representation item is a name 
that statically denotes a constant declared before the entity."

I know it is Implemenation Advice, but especially for Address 
representation specifications it is almost required of any decent 
compiler that they support at least this.  If you don't understand how 
to read it look at the "Rosen trick":

function Random(Gen: Generator) return Float is
    Local: Generator;
    for Local'Address use Gen'Address;
begin
    ....
end Random;

The value of Gen'Address is definitely non-static.  But it is easy for a 
compiler to "get it right" in this case because the value of 
Local'Address each time the declaration of Local is elaborated.

What has this got to do with Marin's complaint?  He thinks that invalid 
representation clauses must be illegal.  If they are non-static they 
can't be illegal because the value to be set at run-time is wrong. 
Either the compiler will raise Program_Error at run-time or execution of 
the invalid representation clause will make the program erroneous.  See 
for example RM 13.3(13): "If an Address is specified, it is the 
programmer's responsibility to ensure that the address is valid; 
otherwise, program execution is erroneous."

Since the address in the representation clause need not be static, your 
program can set it at run-time depending on the hardware involved.  You 
mean you have never done that?  I have code with address clauses that 
depend on system calls made at run-time.

So loosen up a little Marin, and try writing non-static representation 
clauses.  Remember 13.1(22) that I referenced above, and that 'Size for 
objects (but not subtypes) must be static, RM 13.3(41). But you should 
be able to do a lot more with Rep clauses than you have done so far. ;-)

-- 
                                           Robert I. Eachus

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