strange behavior while compiling code with asm()

strange behavior while compiling code with asm()

[milouz]

Hi all,

There's an unexpected behavior I can't understand when I compile some
Ada code with Asm() directive. In the following example, code compile
with no problem :

  ...
  procedure Main is
      Foo : Integer;
  begin
      Foo := 1;       -- Foo is initialized here !
      System.Machine_Code.Asm(
          "movl %0, %%eax",
          Inputs => Integer'Asm_Input("m", Foo),
          Volatile => True
     );
  ...

Why no trying to initialize `Foo' variale during its declaration ? The
code is exactly the same except for `Foo' :

  procedure Main is
      Foo : Integer := 1;     -- Foo is initialized here !
  begin


But then, compilation fails with that message : "error: memory input 0
is not directly addressable"

Someone have an explanation ?

Re: strange behavior while compiling code with asm()

[Simon Wright]

milouz <a.michelizza@gmail.com> writes:

>   procedure Main is
>       Foo : Integer;
>   begin
>       Foo := 1;       -- Foo is initialized here !
>       System.Machine_Code.Asm(
>           "movl %0, %%eax",
>           Inputs => Integer'Asm_Input("m", Foo),
>           Volatile => True
>      );
>   ...
>
> Why no trying to initialize `Foo' variale during its declaration ? The
> code is exactly the same except for `Foo' :
>
>   procedure Main is
>       Foo : Integer := 1;     -- Foo is initialized here !
>   begin
>
>
> But then, compilation fails with that message : "error: memory input 0
> is not directly addressable"

Here (GCC 4.6.0) I get

   gnatmake -c -u -f milouz.adb
   gcc -c milouz.adb
   milouz.adb: In function 'Milouz':
   milouz.adb:13:23: warning: use of memory input without lvalue in asm operand 0 is deprecated [enabled by default]

but this goes away if instead of "m" I use "g". I suppose that the first
form places Foo in memory, while the second places it in a register,
which won't work with the "m" constraint
(http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Simple-Constraints.html)

Re: strange behavior while compiling code with asm()

[milouz]

Thank you very much for the explanation and the useful link !
It's very clear now :-D