------- Comment #2 from codemasterhs at yahoo dot de  2009-04-03 08:30 -------
Subject: Re:  wrong assumption of clobbered registers of inline
 assembly

The code is for my loader/os and so I can´t and do not want to use the 
builtin functions.

Your 1st example works, but I find it a bit strange that I have to write 
such code only to tell gcc, that 2 registers are clobbered! Your 2nd 
example doesn´t work, because of the plus, this gives me an error.

I think the better way would be either gcc assumes that edi and ecx are 
clobbered (which isn´t so) or I can specify that in the clobbered list 
(which is not possible)!

jakub at gcc dot gnu dot org wrote:
> ------- Comment #1 from jakub at gcc dot gnu dot org  2009-04-03 08:11 -------
> User error, if you don't tell gcc that %edi is clobbered, obviously it can
> assume it hasn't.
> As rep stosl modifies both %ecx and %edi, you can write e.g.:
> int tmp1, tmp2;
> asm volatile("xor %%eax,%%eax\n\trep stosl" : "=c" (tmp1), "=D" (tmp2) : "0"
> (count), "1" (dst) : "%eax", "cc", "memory");
> or:
> asm volatile("xor %%eax,%%eax\n\trep stosl" : "+c" (count), "+D" (dst) : :
> "%eax", "cc", "memory");
> if you don't mind that the count and dst variables will change.
> Also note that __builtin_memset (dst, '\0', count); will in most cases
> result in more optimal code for your CPU, hardcoding these in assembly is
> usually a bad idea.
>
>
>   


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39620