Re: GNAT 4.4.5 Record Bit_Order Endian issues

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

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"AdaMagica" <christ-usch.grein@t-online.de> wrote in message 
news:9dc6d5fe-3382-44dd-8460-e042bd6b808e@googlegroups.com...
On Saturday, January 19, 2013 2:43:03 AM UTC+1, Stephen Leake wrote:
...
>> Then 13.5.2 talks about the Position attribute.
>> Which just retreives the mysterious "position" number, it doesn't say
>> what it _means_.

Huh? 13.5.2(2/2) says in part:
"denotes Denotes the same value as R.C'Address � R'Address". That makes it
pretty clear what it represents (for the Default_Bit_Order).

For the non-default bit order, it just is whatever you specify (and that's 
all we can say,
because it *doesn't* have a clear physical meaning - it defines the machine 
scalars to use,
which makes it a totally confusing mess if you try to work the other way).

>> > I agree that the RM is confusing in this area.
>> > It's partly my fault.  :-(
>> Ok. What paragraphs do we add? (in five years; better late than never :)

I don't think there is anything that would help. We surely tried when we 
created the non-default bit order stuff.

>The AARM helps, but this is of course not a binding standard:
>
>13.5.2(2.a/2) Ramification: {AI95-00133-01} Thus, for the default bit 
>order,
>R.C'Position is the offset of C in storage elements from the beginning of 
>the object,
>where the first storage element of an object is numbered zero. R'Address +
>R.C'Position = R.C'Address. For record extensions, the offset is not 
>measured
>from the beginning of the extension part, but from the beginning of the 
>whole object,
>as usual.
>(2.b) In �R.C'Address � R'Address�, the "�" operator is the one in
>System.Storage_Elements that takes two Addresses and returns a 
>Storage_Offset.
>
>Why isn't this in the RM itself? An oversight?

It's not necessary, it follows directly from the definition of "-", 
'Address, and 'Position as related above, for the default bit order case. In 
the non-default bit order, (2.a/2) isn't even true, so it wouldn't help to 
add such a statement in that case. ('Position defines the machine scalars to 
use in that case, and the meaning defies any simple explanation -- it's 
roughly the same as the meaning in the default-bit-order case, but not 
exactly. The RM can never lie.)

                                                   Randy.