Another question re asm constraints

[dw <limegreensocks at yahoo dot com>]

Does specifying the + operator on outputs remove the need for listing 
the variable as an input?  For example:

    __CRT_INLINE SHORT AddTest(SHORT *Addend)
    {
      SHORT ret = 1;
      __asm__ __volatile__("lock\n\txaddw %0,%1 "
    : "+r" (ret), "+m" (*Addend)
    : "0" (ret)
    : "cc");
      return ret + 1;
    }

Is the "0" constraint accomplishing anything useful here (such as 
ensuring the 1 is in the register)?  Or is it just advertising the fact 
that I don't fully understand how constraints work?

And more importantly, is the "+m" enough to ensure that any existing 
values for Addend have been written to memory?  Or do I need to add "m" 
(*Addend) as an input?

I've run some tests, and as best I can tell, neither input constraint is 
required.  But I don't rule out the possibility that that's just an 
artifact of my test code.  I can say for sure that "1" (*Addend) is the 
wrong answer (compile error).

Thanks.