Inline assembler + register values

[Georg-Johann Lay]

Konstantin Vladimirov schrieb:
> Hi,
> 
> 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?
> 
> Thanks for responses in advance.

There was quite long discussion on the topic last August starting at
http://gcc.gnu.org/ml/gcc/2011-08/msg00023.html
with resonable conclusions like
http://gcc.gnu.org/ml/gcc/2011-08/msg00067.html

I don't know what the current state of implementation is or if anyone is 
working on it or planned/plans to improve the matter.

Johann