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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Bit manipulation
Date: 2000/11/13
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In article <slrn90vl0k.l5.lutz@taranis.iks-jena.de>,
  lutz@iks-jena.de (Lutz Donnerhacke) wrote:
> * Redryder wrote:
> >I need to swap the bits in a 32-bit word.
>
> type word32 is 0 .. 2**32-1;
>
> procedure swap32 (i : in word32) returns word32 is
>    type  lowrec is record  data : word32;  end record;
>    type highrec is record  data : word32;  end record;
>    for  lowrec'Bit_Order use Low_Order_First;
>    for highrec'Bit_Order use High_Order_First;
>
>    function l2h is new Ada.Unchecked_Conversion (lowrec,
highrec);
>
>    low  :  lowrec := (i);
>    high : highrec := l2h(low);
> begin
>    return high;
> end swap32;
>
> It's up to your compiler to find out what's a effictive way to
> implement this.

No, this is completely wrong. Once again, it is *really* a good
idea to actually try out something before posting it here.

Bit_Order confuses many people, since certainly it *sounds*
as though it might do what you expected, but most certainly
it does *not* do anything of the kind.

In fact the solution above is doubly wrong, because the use of
unchecked conversion ensures that nothing actually happens at
all!

You clearly did not compile the code, or you would have seen
other errors as follows:

 1. procedure when you meant function
 2. returns when you meant return
 3. positional aggregate with only one component
 4. wrong return type

All in all, these few lines of code contain a rather high
density of errors, and certainly do not inform this discussion.

It does not take much effort to try out any code you post. I
always do, because even for a simple program, I do not trust
myself to avoid silly mistakes :-)

In any case, read the GNAT documentation to understand what
the real situation here is. The operable sentence from the
GNAT Reference Manual is

  In the case where the non-standard value is specified, the
  effect is to renumber bits within each bit, but the ordering
  of bytes is not affected.

let alone ordering of bits!!!!


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