[Bug target/54087] __atomic_fetch_add does not use xadd instruction

[drepper.fsp at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54087

--- Comment #4 from Ulrich Drepper <drepper.fsp at gmail dot com> 2012-08-02 
14:33:19 UTC ---
One more data point.  In a micro-benchmark which uses realistic code used in
production the change from

   __sync_sub_and_fetch(var, constant)

to

   __sync_add_and_fetch(var, -constant)

lead to a 10% to 27% improvement in performance.  The cmpxchg use with the
necessary initial load and I->S cache transition really kills performance when
memory is highly contested.