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Subject: Re: Bitmanipulation in Ada
Newsgroups: comp.lang.ada
References: <Xns9549E5E9CFFB1BerndSpechgmxcom@151.189.20.10>
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From: "Ed Falis" <falis@verizon.net>
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Date: Thu, 19 Aug 2004 17:44:09 GMT
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Date: 2004-08-19T17:44:09+00:00
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On 19 Aug 2004 17:30:17 GMT, Bernd Specht <Bernd.Specht@gmx.com> wrote

> EF wrote:
>> In all implementations of which I'm aware (and I've been in the Ada
>> compiler business for over 20 years) no code would be generated for an
>> unchecked conversion where the source and target have the same size.
>
> Sorry, but that's absolutely *nonsense*

> You agree that "I" and "B" are located at different adresses? Do you
> *really* think that you need *no* code to move data from one address to
> another??? That's wrong! You need at least *one* instruction, and this  
> can
> be time consuming - especially if the data is missaligned and/or the code
> runs in a multiprocessor environment.

I believe your original complaint was about cost of conversion, not of  
assignment - and this is what I was responding to.  The conversion costs  
nothing (in the implementations I'm aware of).  If you assign, you of  
course have at least one instruction, regardless of the types.  If the  
conversion appeared as part of an expression being evaluated, there would  
be no overhead related to it.

Of course, if you insist on separately-named entities, then try:

with Ada.Unchecked_Conversion;
procedure P is
    type Byte_Array is array (1 .. Integer'Size) of Boolean;
    pragma Pack (Byte_Array);
    for Byte_Array'Size use Integer'Size;
    function To_Byte_Array is new
       Ada.Unchecked_Conversion (Source => Integer, Target => Byte_Array);
    I : Integer := 42;
    B : Byte_Array renames To_Byte_Array (I);
begin
    null;
end P;


Compile some code and take a look at the generated assembly language.

- Ed