Re: How do you bitwise operations in Ada '83 and '95

["Randy Brukardt" <randy@rrsoftware.com>]

"Marius Amado-Alves" <marius@amado-alves.info> wrote in message
news:mailman.43.1153460313.30988.comp.lang.ada@ada-france.org...
...
> > Which is much more likely to ruin the alignment of the bit-field
components
> > (an important part of maximum performance).
>
> For records. For a 64-bit unit it does not matter for the "<"
> functions where the fields are.

That is only true on a 64-bit machine. On more typical 32-bit machines, you
are doing double precision math to extract a few bits from the middle.
That's going to be more expensive.

> >> And the expressions are not
> >> complex. Once you get the masks and magnitudes right, it's simply
> >>
> >> Part := (Whole and Mask) / Magnitude.
> >
> > But that's for each component. Surely you do other operations (like
> > difference in seconds) where the expressions are  far more complex?
>
> Actually, in this case, no. I leave it to Ada.Calendar. Convert,
> compute, convert. This limitation is documented.

Sounds nearly useless. Especially as Ada.Calendar.Time is a 64-bit type on
most compilers (but I realize you can't depend on that).

Even so, I'd expect that your equivelent of Time_Of would use a pretty
complex expression.

> > All-in-all, I prefer the record representation.
>
> For Calendar_64 I specifically wanted a 64-bit string, it was a
> requirement.

Yes, but that has nothing to do with whether it is represented as a record
or not. Just put a size clause on your record:

    for My_Time'Size use 64;

If you need an untyped blob for some reason, its easy enough to use an
Unchecked_Conversion. But it's often easy to avoid the need to have it
untyped in the first place -- we did that in many places in the Win32 calls
that underlie Claw.

> If I was to implement Ada.Calendar I think I would use a record too,
> because it is a private type by definition, so I would have to write
> the "<" functions anyway.
>
> > Janus/Ada Calendar.Time implementation is a 64-bit record type
>
> Just curious: is it aligned properly i.e. if you uncheckedly convert
> to a 64-bit modular will the modular "<" functions work properly i.e.
> represent the relations between the dates properly?

I think so, but it is a moot point: Janus/Ada doesn't (currently) support
any 64 bit integer or modular types. So you'd have to convert to a
Storage_Array or Stream_Array or similar.

> Of course a Calendar_64 for a specific compiler/platform could take
> advantage of the respective Ada.Calendar representation.

Yes, of course.

                               Randy.