Re: 64-bit integers in Ada

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Victor Giddings <victor.giddings@ois.com> writes:

> David Rasmussen <david.rasmussen@gmx.spam.egg.sausage.and.spam.net> wrote 
> in news:3CE3978F.6070704@gmx.spam.egg.sausage.and.spam.net:
> 
> > I understand that I can easily use an integer in Ada that has exactly 64 
> > bits. But are there any guarantees that such a type would be mapped to 
> > native integers on 64-bit machines or to a reasonable double 32-bit 
> > implementation on 32-bit machines?

In the language standard, there is no "guarantee" of anything related to
efficiency.  I think that's true of pretty much all languages, and I
don't see how one could do better.

>... At least, are compilers ok at this in 
> > real life?

Yes.  Compiler writers don't deliberately go out of their way to harm
efficiency.

Note that the RM does not require support for 64-bit integers
(unfortunately, IMHO), and there are compilers that do not support
64-bit integers.  But if the compiler *does* support 64-bit integers,
I see no reason to suspect that it wouldn't do so in the obviously
efficient way (native 64-bit ints on a 64-bit machine, or a pair of
32-bit ints on a 32-bit machine).

> > /David
> 
> Try using (or deriving from) Interfaces.Integer_64 or Integer.Unsigned_64. 
> Admittedly, this requires 2 steps on the part of the compiler developer. 1) 
> actually support the 64-bit integer type. 2) to put it in Interfaces (as 
> required by B.2(7)). However, we rely on this in our CORBA product 
> implementation and have been making sure that the compiler vendors are 
> adding these types when they are supported. 

I don't see the point of this advice.  If you say "type T is range
-2**63..2**63-1;", I don't see any reason why T would be more or less
efficient than Interfaces.Integer_64.  In fact, I would think they would
have identical representation, and arithmetic ops would use identical
machine code.

> As of now, I know of only one compiler that supports 64-bit integers and 
> doesn't define Interface.Integer_64. That is to be remedied very soon.

- Bob