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From: lutz@iks-jena.de (Lutz Donnerhacke)
Subject: Re: Bit manipulation
Date: 2000/11/13
Message-ID: <slrn90vl0k.l5.lutz@taranis.iks-jena.de>#1/1
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Date: 2000-11-13T00:00:00+00:00
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* 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.
Some architectures may use the endianess swap prefix opcode.

Other question: GNAT 3.12p has problems with efficent code of a simple
rotation. How to solve it portable?

with Ada.Unchecked_Conversion;

procedure binary_rotation is
  wordsize : constant := 16;
  
  type word is mod 2**wordsize;
  
  type bitfield is array(1..wordsize) of boolean;
  pragma Pack(bitfield); -- the following clause will not work without pragma
  for bitfield'Size use wordsize;
  
  function w2b is new Ada.Unchecked_Conversion (word, bitfield);
  function b2w is new Ada.Unchecked_Conversion (bitfield, word);
  
  subtype offset is Natural range 1 .. wordsize-1;
  function rotate (b : bitfield; s : offset) return bitfield is
  begin
     return b(b'first .. b'last - s) & b(b'last - s + 1 .. b'last);
  end rotate;
begin
  null;
end binary_rotation;