While modern pthread mutexes are very fast, they still incur a call to an
external DSO and overhead of the generality and features of pthread mutexes.
Most mutexes in mesa only needs lock/unlock, and the idea here is that we can
inline the atomic operation and make the fast case just two intructions.
Mutexes are subtle and finicky to implement, so we carefully copy the
implementation from Ulrich Dreppers well-written and well-reviewed paper:

  "Futexes Are Tricky"
  http://www.akkadia.org/drepper/futex.pdf



We implement "mutex3", which gives us a mutex that has no syscalls on
uncontended lock or unlock.  Further, the uncontended case boils down to a
cmpxchg and an untaken branch and the uncontended unlock is just a locked decr
and an untaken branch.  We use __builtin_expect() to indicate that contention
is unlikely so that gcc will put the contention code out of the main code
flow.

A fast mutex only supports lock/unlock, can't be recursive or used with
condition variables.  We keep the pthread mutex implementation around as
for the few places where we use condition variables or recursive locking.
For platforms or compilers where futex and atomics aren't available,
simple_mtx_t falls back to the pthread mutex.

The pthread mutex lock/unlock overhead shows up on benchmarks for CPU bound
applications.  Most CPU bound cases are helped and some of our internal
bind_buffer_object heavy benchmarks gain up to 10%.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>