Re: Using pointers with inline assembly in Ada

[Simon Wright <simon@pushface.org>]

NiGHTS <nights@unku.us> writes:

> This was my first experiment which produces a memory access error. In
> this early version I'm just trying to write to the first element.
>
> declare
>          type ff is array (0 .. 10) of Unsigned_32;
>          pragma Pack(ff);
>          Flags : aliased ff := (others => 0);
>          Flag_Address : System.Address := Flags'Address;
> begin
>         Asm (   "movl %0, %%eax" &
>                 "movl $1, (%%eax)" ,             
>                 Inputs   => System.Address'Asm_Input ("g", Flag_Address),
>                 Clobber  => "eax",
>                 Volatile => true
>         );
>         Put_Line ("Output:" & Flags(0)'Img);
> end;

I got an access error as well (macOS, GCC 12.1.0, 64 bits).

Eventually, it turned out that the problem was that eax is a 32-bit
register. (the compiler used rdx, and the assembler told me that
movl %rdx, %eax wasn't allowed).

This is my working code; Flags is volatile, because otherwise the
compiler doesn't realise (at -O2) that Flags (0) has been touched.

ALso, note the Inputs line!

====
with System.Machine_Code; use System.Machine_Code;
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
procedure Nights is
   type Ff is array (0 .. 10) of Unsigned_32;
   Flags : aliased Ff := (others => 0) with Volatile;
begin
   Asm (
        "movq %0, %%rax" & ASCII.LF & ASCII.HT
          & "movl $1, (%%rax)",
        Inputs   => System.Address'Asm_Input ("g", Flags (Flags'First)'Address),
        Clobber  => "rax",
        Volatile => True
       );
   Put_Line ("Output:" & Flags(0)'Img);
end Nights;