matching constraints in asm operands question

[Andrew Pinski]

On Mar 5, 2005, at 12:24 AM, amylaar@spamcop.net wrote:

>> static __inline__ void atomic_add(atomic_t *v, int i)
>> {
>> 	__asm__ __volatile__("addl %1,%0" : "+m" (*v) : "d" (i));
>> }
>>
>> Then the compiler complains with:
>>
>> /asm/atomic.h:33: warning: read-write constraint does not allow a 
>> register
>>
>> So is the warning wrong?
>
> Yes, the warning is wrong, and the text in the manual about '+' is also
> nonsense.  Support for '+' is asms was specifically introduced to make
> it safe to have read-write memory operands.  Jason, the point of using 
> '+'
> is that the matched parts start out as the same, and the compiler is
> supposed to keep them the same.  We initially did this with shared rtl,
> but IIRC we changed cse since, but still the same premise holds: the
> compiler is supposed to keep both parts of the match in sync.  We
> can guarantee this for '+', but we can't for matching constraints,
> and this was documented properly till you till you changed extend.texi
> in December 2003.

The warning has been removed from later versions of gcc.

-- Pinski