Re: GNAT 4.4.5 Record Bit_Order Endian issues

[Stephen Leake <stephen_leake@stephe-leake.org>]

Simon Wright <simon@pushface.org> writes:

> Stephen Leake <stephen_leake@stephe-leake.org> writes:
>
>> awdorrin <awdorrin@gmail.com> writes:
>>
>>> I have a record with specification definition that starts, using big endian notation, with:
>>>
>>> for TRACK_RECORD_TYPE use
>>>  record
>>>   TRACK_ID at 0 range 0 .. 11;
>>>   TRACK_ID_CNT at 1 range 4 .. 9;
>>>    ...
>>>
>>>
>>> The meaning being that the first 12 bits hold a track id and the next
>>> 6 bits hold the Count value.
>>
>> If you are allocating consecutive  bits, then do this:
>>
>> for TRACK_RECORD_TYPE use
>>    record
>>      TRACK_ID     at 0 range  0 .. 11;
>>      TRACK_ID_CNT at 0 range 12 .. 17;
>>      ...
>
> I don't see why this makes a difference? 

The best explanation I have seen is in
http://www.ada-auth.org/ai-files/grab_bag/bitorder.pdf

That introduces the concept of "machine scalar", which helps a lot.

The problem is what does "the first twelve bits" mean, when the
"natural" size of a chunk of memory is 8 bits?

In this code:

   TRACK_ID at 0 range 0 .. 11;
   TRACK_ID_CNT at 1 range 4 .. 9;

if Storage_Unit = 8, the two fields overlap. I can't figure out a
storage arrangement that makes these fields contiguous; can you draw a
picture? Changing to a compiler/target with a different Storage_Size
changes the meaning.

In this code: 

      TRACK_ID     at 0 range  0 .. 11;
      TRACK_ID_CNT at 0 range 12 .. 17;

it's obvious that the fields are contiguous, and it's up to the compiler
to ensure that happens.

Note that gnat agrees with me:

> OP's way:

> awdorrin.ads:13:26: info: reverse bit order in machine scalar of length 16
> awdorrin.ads:13:26: info: little-endian range for component "Track_Id" is 4 .. 15
> awdorrin.ads:15:06: component "Track_Id_Cnt" overlaps "Track_Id" at line 13
> awdorrin.ads:15:30: info: reverse bit order in machine scalar of length 16
> awdorrin.ads:15:30: info: little-endian range for component "Track_Id_Cnt" is 6 .. 11
> gnatmake: "awdorrin.ads" compilation error

This says that the first code specifies overlapping fields, which is not
what you want.

> Your way:

> awdorrin.ads:13:26: info: reverse bit order in machine scalar of length 32
> awdorrin.ads:13:26: info: little-endian range for component "Track_Id" is 20 .. 31
> awdorrin.ads:14:30: info: reverse bit order in machine scalar of length 32
> awdorrin.ads:14:30: info: little-endian range for component "Track_Id_Cnt" is 14 .. 19

This says the two fields occupy the 16 bits numbered 31 .. 19, and are
consecutive.

Why do you say this is buggy?

-- 
-- Stephe