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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Examining individual bytes of an integer
Date: Tue, 16 Oct 2018 17:33:01 -0500
Organization: JSA Research & Innovation
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Xref: reader02.eternal-september.org comp.lang.ada:54608
Date: 2018-10-16T17:33:01-05:00
List-Id: <comp.lang.ada>

"AdaMagica" <christ-usch.grein@t-online.de> wrote in message 
news:35f53cd9-4979-49b8-a5df-2c1cf0764507@googlegroups.com...
> Am Montag, 15. Oktober 2018 22:18:16 UTC+2 schrieb Randy Brukardt:
>> "Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message
>> news:g2k5fbF9thnU1@mid.individual.net...
>> > On 18-10-15 13:18 , AdaMagica wrote:
>> >> There is the old Dewar rule that no compiler is doing nonsense. It
>> >> would be a very unfriendly compiler to not follow this advice.
>> >
>> > As I remember it, Dewar's rule says that the Ada *RM* does not specify
>> > nonsense, meaning that if some reading of the RM seems to be nonsense, 
>> > it
>> > is the wrong reading.
>>
>> That's correct.
>>
>> > I don't remember Dewar saying anything similar about compilers.
>>
>> Me either. Compilers do nonsense all of the time (usually not on purpose,
>> though).
>
> OK, I stay corrected. But I still claim that it would be extremely unfair 
> of a compiler not to follow this advice (A'Address = A (A'First)'Address 
> with increasing addresses for further components).
>
> I'm apt to try a wager that Randy agrees. Praps I'm gonna lose again... 
> Tant pis!

You would. For Janus/Ada, that's true only for statically constrained 
arrays. For an unconstrained array, the 'Address points at the array 
descriptor, and for dynamically constrained array, it points at the indirect 
address. That's the model for the entire compiler: all objects are 
statically sized, and if needed, the actual data is found by indirection.

Because of the Implementation Advice. I've considered changing 'Address to 
point at the first data item. But that then does not work for address 
clauses for unconstrained array types (those have to point at the 
descriptor). Nor does it work for code that needs to look at the descriptor 
(not much of that, of course). Given that pretty much no one should be using 
'Address anyway, this hasn't been an issue in practice, so it's not a high 
priority to deal with.

                                   Randy.