Re: RISC

[Robert A Duff <bobduff@world.std.com>]

Laurent Guerby <guerby@acm.org> writes:

> Robert A Duff <bobduff@world.std.com> writes:
> > There are also a lot of things that rep clauses just can't express.  Eg,
> > TCP/IP protocols often have data structures defined like "a count byte,
> > followed by an array of sub-records" where the count gives the number of
> > sub-records, or the number of bytes including (or not including) the
> > count byte itself.  I would like to have a language that could describe
> > that sort of thing at a reasonably high level.
> 
> For the "non including" case, a discriminated record should do the job
> in Ada. (OK, it fails for more complicated stuff.)

If it's a count of bytes, a discrim record doesn't work.  The discrim
has to be a count of number of array elements.  Also, you can't control
where the array goes.  And if there's more data after the array, you're
in trouble.  And some compilers want to put dope fields in the midst of
discriminated records.  And some compilers allocate some record fields
as a pointer to a separately-allocated thing.

I actually think Ada takes the wrong approach by letting you (or making
you) specify individual bit numbers.  I would define a canonical record
layout algorithm, and a pragma that requests the canonical layout.  (Or
two canonical ones -- canonical tightly packed, and canonical not
crossing boundaries.  Or something like that.)  The default should be
that the compiler can do what it likes (which presumably is the most
efficient thing, subject to keeping compiler complexity within reason).

- Bob