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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Rep specs
Date: 1999/06/04
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 <3756CA64.A9EF5BA3@hso.link.com>
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Date: 1999-06-04T00:00:00+00:00
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In article <3756CA64.A9EF5BA3@hso.link.com>,
  "Samuel T. Harris" <sam_harris@hso.link.com> wrote:
> With Ada 95,
> I simply need say "fully implements the systems programming
> annex" and I sleep well knowing my bases are covered.
>
> --
> Samuel T. Harris, Principal Engineer
> Raytheon, Scientific and Technical Systems
> "If you can make it, We can fake it!"


Hmmm! In practice we find that very few large customers can live
with the small subset of rep clauses required to be implemented
by the systems programming annex, and a lot of our development
work has been to add capabilities in this area, in response to
customer requirements. Some examples:

 1. Tight packing for other than 1,2,4,8 bits

 2. Size clauses on records and arrays

 3. Integer fields across storage boundaries

 4. Arrays indexed by enumeration types with holes are compact

 5. Record fields on other than natural alignment boundary

There are many others. None of these are required by the SPA.
Sam, are you really sure you stick strictly to the SPA subset?
If so, you are, in our experience an unusual user. As I say,
most of our users, especially those with legacy Ada 83 code,
require far more.

Indeed I would argue that the Ada 83 RM *required* a lot more,
since it required the compiler to accept much more. In Ada 83,
the only allowed restriction was:

  An implementation may limit its acceptance of representation
  clauses to those that can be handled simply by the underlying
  hardware.

Now of course this is vague, since simply is not defined. On
the other hand, it is clear that all the Ada 95 SPA requirements
are indeed in this category. It is also clear that many things
meet this requirement which are NOT required by the Ada 95 SPA.
So in that sense, the requirements, while being more precise,
are actually weaker.

And if you think that is just a theoretical issue, consider that
all major Ada 83 compilers went far beyond the current Ada 95
requirements in practice.






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