Inline assembler + register values

[Andrew Haley]

On 01/24/2012 11:35 AM, Konstantin Vladimirov wrote:
>
> This problem is backend independent, so I build reproduction in x86
> backend. Consider code:
>
> int func(int x);
>
> int test(int x, int *data)
> {
>   int retval;
>   register int *buffer asm ("eax");
>
>   buffer = data;
>   retval = func(x);
>   __asm__ __volatile__ (".internal_label _t." "4096"
>                         ".%0.%1.%2:;nop"::"ri" (buffer), "ri" (4137), "ri" (4));
>   return retval;
> }
>
> here nop will be replaced for something, that essentially uses %eax,
> but it is not actually required for minimal reproduction.
>
> being compiled with gcc (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch
> revision 160292]
>
> with following options:
>
> gcc -m32 -O2 -S -fomit-frame-pointer reprox86.c
>
> yields assembler:
>
>   .type test, @function
> test:
>   subl  $28, %esp
>   movl  32(%esp), %eax
>   movl  %eax, (%esp)
>   call  func                           // <--- here eax is clobbered
>   .internal_label _t.4096.%eax.$4137.$4:;nop 0
>   addl  $28, %esp
>   ret
>
> That looks quite odd. I have a question -- why compiler doesn't save &
> restore buffer (i.e. %eax) value around function call, when we
> specifically pointed to %eax usage in the inline assembler
> instruction?

Well, hold on: you've said that you want buffer to live in register
EAX for the entire duration of test().  GCC assumes that you know what
you are doing, know that EAX is call-clobbered, and are using EAX
because you want to pass arguments to func() in EAX.  In other words,
it's not a bug, it's a feature.

This is exactly how register asm variables are used to do system calls
in GNU/Linux.

If you want to pass a value in EAX to an asm, do this:

int test(int x, int *data)
{
  int retval;
  retval = func(x);
  __asm__ __volatile__ (".internal_label _t." "4096"
                        ".%0.%1.%2:;nop"::"a" (data), "ri" (4137), "ri" (4));
  return retval;
}

Andrew.