Another question re asm constraints

[Ian Lance Taylor]

On Mon, Mar 25, 2013 at 2:51 PM, dw <limegreensocks@yahoo.com> wrote:
> 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

No.

> And more importantly, is the "+m" enough to ensure that any existing values
> for Addend have been written to memory?

Yes, at least from the compiler's perspective (there may be pending
writes from another processor that the compiler knows nothing about).

Ian