Access to fixed memory location

[Tomas Cernaj <tcernaj.WITHOUTTHIS@gmx.de>]

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