...
A 2007 mailing list post Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints by Linus which describes that "+m" as an asm output constraint is acceptable. This is in conflict with Extended Asm in the GCC manual, which Linus claims needs to be updated.
http://gcc.gnu.org/viewcvs/trunk/gcc/doc/extend.texi?r1=88547&r2=88999
Change to gcc Extended Asm docs (Revision 88999 of gcc/doc/extend.texi, Modified Wed Oct 13 19:14:44 2004 UTC by Dale Johannesen <dalej@apple.com>):
* doc/extend.texi (Extended Asm): Rewrite asm volatile description.