Re: Ada 95 Numerics questions for the experts

[bobduff@world.std.com (Robert A Duff)]

In article <dewar.873149493@merv>, Robert Dewar <dewar@merv.cs.nyu.edu> wrote:
>Note that GNAT whereever possible follows the advice in the RM even if
>it causes portability troubles. A common example is the following
>
>  type arr8 is array (1..8) of boolean;
>  for arr8'size use 8;
>
>The RM permits the size clause to cause implicit packing, but recommends
>against it.

It would be pretty hard for the RM to do anything more than "recommend"
here.  After all, the compiler is allowed to use 1-bit components all
the time (even if that's not a good idea), or to use 1-bit components if
the program is compiled on a the first Tuesday of the month (an even
worse idea!), or to follow the phase of the moon.  Given that, how could
the RM do more than "recommend"?

>... VADS at least permitted this implicit packing (even though
>there was an AI that recommended against it).

One can't blame VADS *too* much.  The AI appeared after VADS had
implemented some particular mechanism.  Ada 95 merely codified the AI
into the RM.

>Consequently in GNAT, we reject the above, and require an explicit pragma
>Pack.

Or, presumably, a "for arr8'Component_Size use 1;".  Good.

>... The workdaround for existing code here is easy, and the code is
>arguably clearer with the pragma there in any case.
>
>However, if you have code that depends on close packed 3 bit components,
>you are completely up the creak if your compiler does not support it!

And, also unfortunately, the RM doesn't nail down what 3-bit components
would mean.  On a 32-bit machine, it might mean 10 3-bit components per
word, with a 2-bit wastage in each word, or it might mean 3-bit
components, crossing word boundaries, with no wastage.  Sigh.

I don't feel *too* bad about that -- other languages with similar
features (eg packed arrays in Pascal) are even less specific.

- Bob