Re: Bitmanipulation in Ada

[Bernd.Specht@gmx.com (Bernd Specht)]

Ludovic Brenta <ludovic.brenta@insalien.org> wrote in
news:87k6vwrwym.fsf@insalien.org: 

>  (Bernd Specht) writes:
>> Hi, 

Thanks.

>> 2. I did not found shift and rotate operations. Did I miss something
>> or are there really none? 
> 
> These operations are usually carried out in hardware for certain sizes
> of modular integers only.  Consequently, these operations are defined
> in package Interfaces (see RM 3.5.4(31) and B.2).  Package Interfaces
> defines the following subprograms:
> 
>    function Shift_Left  (Value  : Unsigned_n;
>                          Amount : Natural) return Unsigned_n;
>    function Shift_Right (Value  : Unsigned_n;
>                          Amount : Natural) return Unsigned_n;
>    function Shift_Right_Arithmetic (Value  : Unsigned_n;
>                                     Amount : Natural)
>      return Unsigned_n;
>    function Rotate_Left  (Value  : Unsigned_n;
>                           Amount : Natural) return Unsigned_n;
>    function Rotate_Right (Value  : Unsigned_n;
>                           Amount : Natural) return Unsigned_n;


Ok. Is there a reason why it is not (directly) available in the language 
itself like the and/or/xor or the >> and << in C?
 
>> 3. When I want treat a value both as an integer and as a boolean
>> array, how can i do this? In pascal I would use a tagless record like 
>> Such overlay structures are not valid with Ada, so what do instead?
> 
> Have you considered a variant record?
> 
> type Ov (T : Boolean) is record
>    case T is
>       when True => I : Integer;
>       when False => B : Byte_Array;
>    end T;
> end record;

I found, that (at least GNAT) different storage locations are used for i and 
b.
 
> If what you really want is to convert an integer to a byte array, use
> Unchecked_Conversion:

OK, but this would result in an assignment operation (a memory move on 
maschine code level). What I want is a real "overlay" (same storage location 
used for both), so reading the value would not need extra instructions.