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From: Per Sandberg <per.sandberg@bredband.net>
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Newsgroups: comp.lang.ada
Subject: Re: Problem with representation clause in gnat 4.3
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Date: Sat, 04 Oct 2008 10:10:39 +0000
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Date: 2008-10-04T10:10:39+00:00
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Hi
Think this has to do with the implicit alignment of the record, in the 
first case the integer (integers has a default alignment of 4) 
components are aligned to word boundaries breaking their default 
alignment resulting in an alignment of 2 for the record as whole.
In the second case the integers are aligned to dword boundaries in the 
record resulting in a propagation of the default alignment of 4 for the 
integers the record.

So if you add an alignment clause "for Compound'Alignment use 2" the 
compiler will do what you want.

Haven't tested it since I don't run "Debian Lenny".

/Hope it helps
/Per

Michael Bode wrote:
> Hi,
> 
> I've a problem with a representation that I don't understand. And I'm
> pretty sure older versions of gnat than 4.3 had a different behaviour.
> 
> This stuff compiles as I expect with gnat 4.3 (on Debian Lenny):
> 
> with Ada.Text_Io;
> 
> procedure Align is
> 
>    type Compound is record
>       A : Boolean;
>       B : Integer;
>       C : Integer;
>    end record;
>    for Compound use record
>       A at 0 range 0 .. 0;
>       B at 2 range 0 .. 31;
>       C at 6 range 0 .. 31;
>    end record;
> 
>    type C_Array is array (Natural range <>) of Compound;
>    for C_Array'Component_Size use 80;
> 
>    Size : Natural := Compound'Size;
> begin
>    Ada.Text_Io.Put_Line ("Compound has size" & Natural'Image(Size));
> end Align;
> 
> ~/myprogs/test $ gnatmake align
> gcc-4.3 -c align.adb
> gnatbind -x align.ali
> gnatlink align.ali
> ~/myprogs/test $ ./align 
> Compound has size 80
> ~/myprogs/test $ 
> 
> Now if I move component A of the record to the end as in
> ....
>    type Compound is record
>       B : Integer;
>       C : Integer;
>       A : Boolean;
>    end record;
>    for Compound use record
>       B at 0 range 0 .. 31;
>       C at 4 range 0 .. 31;
>       A at 8 range 0 .. 0;  
>    end record;
> ....
> 
> ~/myprogs/test $ gnatmake align
> gcc-4.3 -c align.adb
> align.adb:16:09: component size for "C_Array" too small, minimum allowed is 96
> gnatmake: "align.adb" compilation error
> ~/myprogs/test $ 
> 
> And if i remove the line 'for C_Array'Component_Size use 80;' so the
> program compiles I get
> 
> ~/myprogs/test $ gnatmake align
> gcc-4.3 -c align.adb
> gnatbind -x align.ali
> gnatlink align.ali
> ~/myprogs/test $ ./align 
> Compound has size 65
> 
> So we see Compound should definitivly fit into 80 bits.
> 
> Obviously gnat wants to align to DWORD boundaries in the 2nd case but
> not in the 1st. Why and what can I do about it?