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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: GNAT 4.8 atomic access to 64-bit objects
Date: Fri, 15 Nov 2013 22:33:37 +0100
Organization: cbb software GmbH
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Date: 2013-11-15T22:33:37+01:00
List-Id: <comp.lang.ada>

On Fri, 15 Nov 2013 20:25:28 +0100, Georg Bauhaus wrote:

> On 15.11.13 09:44, Dmitry A. Kazakov wrote:
>> On Thu, 14 Nov 2013 21:34:26 +0100, Ludovic Brenta wrote:
> 
>>> I'd suggest you use 64-bit floating-point registers instead; i386
>>> processors have them, I think.  That has been a useful trick for a
>>> decade or so :)
>>
>> Using unchecked union or unchecked conversion?
> 
> FTR, a different 32 bit implementation (Ada 95) does not support a binary
> modulus of 64, and also, while Unchecked_Conversion passes the value 42
> from an integer register to a FPT register and back as 42, idly trying
>    type Fake is new  Long_Float;
>    pragma Atomic (Fake);
> gives
>   LRM:C.6(10), Indivisible read/update not supported for given subtype, pragma Atomic ignored

Try this:

with Interfaces;
with Ada.Unchecked_Conversion;
with Ada.Text_IO;

procedure Test is
   type T is mod 2**64;
   type Atomic_T is new Interfaces.IEEE_Float_64;
   function Load is new Ada.Unchecked_Conversion (Atomic_T, T);
   function Store is new Ada.Unchecked_Conversion (T, Atomic_T);
   X : Atomic_T;
   pragma Atomic (X);
begin
   X := Store (123);
   Ada.Text_IO.Put_Line (T'Image (Load (X)));
   X := Store (Load (X) + 1);
   Ada.Text_IO.Put_Line (T'Image (Load (X)));
end Test;

The code generated looks horrific.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de