Re: Ada 95 Numerics questions for the experts

[jeffrey.m.creem@lmco.com (Jeff Creem)]

In article <01bcb2f2$b512f700$928871a5@dhoos>, "David C. Hoos, Sr."
<david.c.hoos.sr@ada95.com> wrote:

>Just a couple of points, viz.:
>
>  1.  The DEC Ada compiler for VAXen allowed biased rep clauses, as well.
>       It's gratifying that gnat chose to implement this also.
>
>  2.  The problem with your packed array example, however, is that what you
>       are asking the compiler to do is to force array elements which are
>smaller than
>       one byte to straddle byte boundaries.  No compiler likes to do this,
>as far as I
>       know. (Record components straddling byte boundaries, are another
>matter, and they



But VADS, Apex and GNAT all do handle this (essentially)


>       are handled quite nicely without any ugly source code).
>       Simply using pragma Pack instead of a "for 'size use 24" clause will
>result in your
>       8-element x 3-bit array occupying 32 bits, which is not bad at all.
>       Undoubtedly, the extra code required to mask and shift the varying
>amounts as

Nope. The AdaMajic front end (Green hills in this case) rounds the
components to 8 bits even in the precense of a pragma pack
you get a 64 Byte array.

>       a function of the array index would substantially exceed the one
>byte of array
>       size saved.  Besides, even if the compiler did give you your 24-bit
>array, it would
>       probably skip to the next 32-bit boundary for the next object in
>memory, anyway.


Actually in the real code that the example came from, the array was an
 element of a record with another field. The total length of the record
was then 32 bits.


>       Even gnat will not give you a 24-bit array for this case (at least
>the WinNT version 3.09).
>       

It gives you a 32 bit array (which Is what I would expect) but does allow
you to stick it in a record with another 8 bits and have a structure which
is a total of 32 bits long.. Again very nice