MultiThreaded Demo:

- fixing various race conditions throughout (usage of static vars, etc)
 - addition of a few lightweight mutexes (which are compiled out by default)
 - slight code rearrangement in discreteDynamicsWorld to facilitate multithreading
 - PoolAllocator::allocate() can now be called when pool is full without
     crashing (null pointer returned)
 - PoolAllocator allocate and freeMemory, are OPTIONALLY threadsafe
     (default is un-threadsafe)
 - CollisionDispatcher no longer checks if the pool allocator is full
     before calling allocate(), instead it just calls allocate() and
     checks if the return is null -- this avoids a race condition
 - SequentialImpulseConstraintSolver OPTIONALLY uses different logic in
     getOrInitSolverBody() to avoid a race condition with kinematic bodies
 - addition of 2 classes which together allow simulation islands to be run
   in parallel:
    - btSimulationIslandManagerMt
    - btDiscreteDynamicsWorldMt
 - MultiThreadedDemo example in the example browser demonstrating use of
   OpenMP, Microsoft PPL, and Intel TBB
 - use multithreading for other demos
 - benchmark demo: add parallel raycasting

src/LinearMath/btThreads.h

@@ -0,0 +1,76 @@
+/*
+Copyright (c) 2003-2014 Erwin Coumans  http://bullet.googlecode.com
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the use of this software.
+Permission is granted to anyone to use this software for any purpose, 
+including commercial applications, and to alter it and redistribute it freely, 
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+*/
+
+
+
+#ifndef BT_THREADS_H
+#define BT_THREADS_H
+
+#include "btScalar.h" // has definitions like SIMD_FORCE_INLINE
+
+///
+/// btSpinMutex -- lightweight spin-mutex implemented with atomic ops, never puts
+///               a thread to sleep because it is designed to be used with a task scheduler
+///               which has one thread per core and the threads don't sleep until they
+///               run out of tasks. Not good for general purpose use.
+///
+class btSpinMutex
+{
+    int mLock;
+
+public:
+    btSpinMutex()
+    {
+        mLock = 0;
+    }
+    void lock();
+    void unlock();
+    bool tryLock();
+};
+
+#if BT_THREADSAFE
+
+// for internal Bullet use only
+SIMD_FORCE_INLINE void btMutexLock( btSpinMutex* mutex )
+{
+    mutex->lock();
+}
+
+SIMD_FORCE_INLINE void btMutexUnlock( btSpinMutex* mutex )
+{
+    mutex->unlock();
+}
+
+SIMD_FORCE_INLINE bool btMutexTryLock( btSpinMutex* mutex )
+{
+    return mutex->tryLock();
+}
+
+// for internal use only
+bool btIsMainThread();
+unsigned int btGetCurrentThreadIndex();
+const unsigned int BT_MAX_THREAD_COUNT = 64;
+
+#else
+
+// for internal Bullet use only
+// if BT_THREADSAFE is undefined or 0, should optimize away to nothing
+SIMD_FORCE_INLINE void btMutexLock( btSpinMutex* ) {}
+SIMD_FORCE_INLINE void btMutexUnlock( btSpinMutex* ) {}
+SIMD_FORCE_INLINE bool btMutexTryLock( btSpinMutex* ) {return true;}
+
+#endif
+
+
+#endif //BT_THREADS_H