On Sat, 18 Aug 2007, Segher Boessenkool wrote:

quoted

#define forget(a)	__asm__ __volatile__ ("" :"=m" (a) :"m" (a))

[ This is exactly equivalent to using "+m" in the constraints, as recently
  explained on a GCC list somewhere, in response to the patch in my bitops
  series a few weeks back where I thought "+m" was bogus. ]

[It wasn't explained on a GCC list in response to your patch, as
far as I can see -- if I missed it, please point me to an archived
version of it].

http://gcc.gnu.org/ml/gcc-patches/2007-07/msg01758.html

is a follow-up in the thread on the gcc-patches@gcc.gnu.org mailing list,
which began with:

http://gcc.gnu.org/ml/gcc-patches/2007-07/msg01677.html

that was posted by Jan Kubicka, as he quotes in that initial posting,
after I had submitted:

http://lkml.org/lkml/2007/7/23/252

which was a (wrong) patch to "rectify" what I thought was the "bogus"
"+m" constraint, as per the quoted extract from gcc docs (that was
given in that (wrong) patch's changelog).

That's when _I_ came to know how GCC interprets "+m", but probably
this has been explained on those lists multiple times. Who cares,
anyway?

One last time: it isn't equivalent on older (but still supported
by Linux) versions of GCC.  Current versions of GCC allow it, but
it has no documented behaviour at all, so use it at your own risk.

True.