From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM
	autolearn=ham autolearn_force=no version=3.4.4
X-Google-Thread: 103376,40bf3d9b43d88cff,start
X-Google-Attributes: gid103376,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news3.google.com!news.germany.com!newsfeed01.sul.t-online.de!t-online.de!newsfeed.arcor.de!newsspool1.arcor-online.net!news.arcor.de.POSTED!not-for-mail
From: Tomas Cernaj <tcernaj.WITHOUTTHIS@gmx.de>
Subject: Access to fixed memory location
Newsgroups: comp.lang.ada
User-Agent: Pan/0.129 (Benson & Hedges Moscow Gold)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Date: 30 Nov 2007 19:32:29 GMT
Message-ID: <4750654d$0$13115$9b4e6d93@newsspool2.arcor-online.net>
Organization: Arcor
NNTP-Posting-Date: 30 Nov 2007 20:32:29 CET
NNTP-Posting-Host: cd7355a9.newsspool2.arcor-online.net
X-Trace: 
 DXC=f00X`Y1Uj<N[kmHKHnaEnMA9EHlD;3YcB4Fo<]lROoRA4nDHegD_]RESI?@\0<;fVDkd=52>SHf;Bg=1B;EnI3JDLR[O15[\ACO6h7]UAPH2FD
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news1.google.com comp.lang.ada:18682
Date: 2007-11-30T20:32:29+01:00
List-Id: <comp.lang.ada>

Hello everyone,

is there any way to tell the Ada compiler that an Address attribute 
should be constant? I want to use it for accessing fixed memory 
locations, and without the Address attribute being constant the compiler 
won't optimize away an indirect access.

I couldn't find anything in the RM or GNAT documentation... The point is 
that I want to use the 'Address construct on the AVR target, which is an 
8-bit processor, so the indirect access is quite expensive.

I suppose that was a bit unclear, so here's an example: :)
I have the following declarations in a package spec file:

    type T_Type is array (0 .. 7) of Boolean;
    pragma Pack (T_Type);
    for T_Type'Size use 8;
    
    package TA is new System.Address_To_Access_Conversions (T_Type);
    X : constant access T_Type
        := TA.To_Pointer (System.Storage_Elements.To_Address (50));
    
    Y : T_Type;
    for Y'Address use System.Storage_Elements.To_Address (51);
    pragma Import (Ada, Y);

Note that X is something like a "T_Type * const X" in C.
Compiling the following code

        X (4) := True;
        Y (5) := True;

with GNAT (gcc-4.2 -O -S) yields this assembler output (x86-64):

        orb     $16, 50			; X (4) := True
        movq    testpkg__y(%rip), %rdx	; Y (5) := True
        movzbl  (%rdx), %eax
        orl     $32, %eax
        movb    %al, (%rdx)

That's because the compiler can not assume that Y'Address will not be 
changed.

Thank you for your help,
Tomas