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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Forcing GNAT to use 32-bit load/store instructions on ARM?
Date: Tue, 01 Jul 2014 18:18:10 +0100
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Date: 2014-07-01T18:18:10+01:00
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Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> writes:

> On 2014-06-30, Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> wrote:
>> On 06/30/2014 03:11 PM, daniel.dmk@googlemail.com wrote:
>>>
>>> Does anyone know how I can force GNAT to generate the appropriate
>>> instructions?
>>
>> If you need specific machine instructions, you should use a
>> machine-code insertion.
>>
>
> The issue here is that Pragma Atomic should have given him the guarantee
> he needs.
>
> There is a place for specific assembly language insertions (for example,
> on ARM when you need to access the CPSR register), but that should not
> be required here.

Just for fun I tried this on Intel (x86_64) with GNAT GPL 2014 and GCC
4.9.0 on this:

   procedure Atom is

      type R is record
         A : Boolean;
         B : Boolean;
      end record with Size => 32;
      for R use record
         A at 0 range 0 .. 0;
         B at 0 range 31 .. 31;
      end record;

      V : R with Atomic, Import, Convention => C, External_Name => "foo";

   begin

      V := (A => True, B => True);
      V.A := False;

   end Atom;

and found that the compiler did a reaonable thing:

        movl    _T3b.2569(%rip), %edx
        movq    _foo@GOTPCREL(%rip), %rax
        movl    %edx, (%rax)
        mfence
        movq    _foo@GOTPCREL(%rip), %rax
        movl    (%rax), %edx
        andl    $-2, %edx
        movl    %edx, (%rax)

        .const
        .align 2
_T3b.2569:
        .byte   1
        .space 2
        .byte   128