Re: matching constraints in asm operands question

[Richard Henderson <rth at redhat dot com>]

On Tue, Mar 01, 2005 at 11:50:04PM -0500, Peter Barada wrote:
> 
> >> which seems to work, but I'm really concerned about the manuals
> >> warning of the input and output operads being in seperate places.
> >>
> >> Which form is correct?
> >
> >static __inline__ void atomic_inc(atomic_t *v)
> >{
> >	__asm__ __volatile__("addql #1,%0" : "+m" (*v));
> >}
> >
> >Works just fine, every where I know of.  It is the same as you last
> >example also.
> 
> Ugh, in the hopes of simplifying the example, I made it somewhat trivial...
> 
> static __inline__ void atomic_add(atomic_t *v, int i)
> {
> 	__asm__ __volatile__("addl %2,%0" : "=m" (*v) : "m" (*v), "d" (i));
> }
> 
> Is that correct?  And if so, then isn't the documentation wrong?

This is correct, as is a version using "+m" (*v) : "d"(i),
though of course the %2 gets moved to %1 in that case.

Using "m" multiple times is correct because "m" specifies a specific
address, and the compiler cannot change the address.  It can change
the *form* of the address (%a0 vs 20(%a0,%d1*4), etc), but that's a
different prospect.


r~