On Sat, 18 Aug 2007, Segher Boessenkool wrote:

quoted text
> > #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?


quoted text
> 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.
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