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.cpp

@@ -0,0 +1,230 @@
+/*
+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.
+*/
+
+
+#include "btThreads.h"
+
+//
+// Lightweight spin-mutex based on atomics
+// Using ordinary system-provided mutexes like Windows critical sections was noticeably slower
+// presumably because when it fails to lock at first it would sleep the thread and trigger costly
+// context switching.
+// 
+
+#if BT_THREADSAFE
+
+#if __cplusplus >= 201103L
+
+// for anything claiming full C++11 compliance, use C++11 atomics
+// on GCC or Clang you need to compile with -std=c++11
+#define USE_CPP11_ATOMICS 1
+
+#elif defined( _MSC_VER )
+
+// on MSVC, use intrinsics instead
+#define USE_MSVC_INTRINSICS 1
+
+#elif defined( __GNUC__ ) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
+
+// available since GCC 4.7 and some versions of clang
+// todo: check for clang
+#define USE_GCC_BUILTIN_ATOMICS 1
+
+#elif defined( __GNUC__ ) && (__GNUC__ == 4 && __GNUC_MINOR__ >= 1)
+
+// available since GCC 4.1
+#define USE_GCC_BUILTIN_ATOMICS_OLD 1
+
+#endif
+
+
+#if USE_CPP11_ATOMICS
+
+#include <atomic>
+#include <thread>
+
+#define THREAD_LOCAL_STATIC thread_local static
+
+bool btSpinMutex::tryLock()
+{
+    std::atomic<int>* aDest = reinterpret_cast<std::atomic<int>*>(&mLock);
+    int expected = 0;
+    return std::atomic_compare_exchange_weak_explicit( aDest, &expected, int(1), std::memory_order_acq_rel, std::memory_order_acquire );
+}
+
+void btSpinMutex::lock()
+{
+    // note: this lock does not sleep the thread.
+    while (! tryLock())
+    {
+        // spin
+    }
+}
+
+void btSpinMutex::unlock()
+{
+    std::atomic<int>* aDest = reinterpret_cast<std::atomic<int>*>(&mLock);
+    std::atomic_store_explicit( aDest, int(0), std::memory_order_release );
+}
+
+
+#elif USE_MSVC_INTRINSICS
+
+#define WIN32_LEAN_AND_MEAN
+
+#include <windows.h>
+#include <intrin.h>
+
+#define THREAD_LOCAL_STATIC __declspec( thread ) static
+
+
+bool btSpinMutex::tryLock()
+{
+    volatile long* aDest = reinterpret_cast<long*>(&mLock);
+    return ( 0 == _InterlockedCompareExchange( aDest, 1, 0) );
+}
+
+void btSpinMutex::lock()
+{
+    // note: this lock does not sleep the thread
+    while (! tryLock())
+    {
+        // spin
+    }
+}
+
+void btSpinMutex::unlock()
+{
+    volatile long* aDest = reinterpret_cast<long*>( &mLock );
+    _InterlockedExchange( aDest, 0 );
+}
+
+#elif USE_GCC_BUILTIN_ATOMICS
+
+#define THREAD_LOCAL_STATIC static __thread
+
+
+bool btSpinMutex::tryLock()
+{
+    int expected = 0;
+    bool weak = false;
+    const int memOrderSuccess = __ATOMIC_ACQ_REL;
+    const int memOrderFail = __ATOMIC_ACQUIRE;
+    return __atomic_compare_exchange_n(&mLock, &expected, int(1), weak, memOrderSuccess, memOrderFail);
+}
+
+void btSpinMutex::lock()
+{
+    // note: this lock does not sleep the thread
+    while (! tryLock())
+    {
+        // spin
+    }
+}
+
+void btSpinMutex::unlock()
+{
+    __atomic_store_n(&mLock, int(0), __ATOMIC_RELEASE);
+}
+
+#elif USE_GCC_BUILTIN_ATOMICS_OLD
+
+
+#define THREAD_LOCAL_STATIC static __thread
+
+bool btSpinMutex::tryLock()
+{
+    return __sync_bool_compare_and_swap(&mLock, int(0), int(1));
+}
+
+void btSpinMutex::lock()
+{
+    // note: this lock does not sleep the thread
+    while (! tryLock())
+    {
+        // spin
+    }
+}
+
+void btSpinMutex::unlock()
+{
+    // write 0
+    __sync_fetch_and_and(&mLock, int(0));
+}
+
+#else //#elif USE_MSVC_INTRINSICS
+
+#error "no threading primitives defined -- unknown platform"
+
+#endif  //#else //#elif USE_MSVC_INTRINSICS
+
+
+struct ThreadsafeCounter
+{
+    unsigned int mCounter;
+    btSpinMutex mMutex;
+
+    ThreadsafeCounter() {mCounter=0;}
+
+    unsigned int getNext()
+    {
+        // no need to optimize this with atomics, it is only called ONCE per thread!
+        mMutex.lock();
+        unsigned int val = mCounter++;
+        mMutex.unlock();
+        return val;
+    }
+};
+
+static ThreadsafeCounter gThreadCounter;
+
+
+// return a unique index per thread, starting with 0 and counting up
+unsigned int btGetCurrentThreadIndex()
+{
+    const unsigned int kNullIndex = ~0U;
+    THREAD_LOCAL_STATIC unsigned int sThreadIndex = kNullIndex;
+    if ( sThreadIndex == kNullIndex )
+    {
+        sThreadIndex = gThreadCounter.getNext();
+    }
+    return sThreadIndex;
+}
+
+bool btIsMainThread()
+{
+    return btGetCurrentThreadIndex() == 0;
+}
+
+#else // #if BT_THREADSAFE
+
+// These should not be called ever
+void btSpinMutex::lock()
+{
+    btAssert(!"unimplemented btSpinMutex::lock() called");
+}
+
+void btSpinMutex::unlock()
+{
+    btAssert(!"unimplemented btSpinMutex::unlock() called");
+}
+
+bool btSpinMutex::tryLock()
+{
+    btAssert(!"unimplemented btSpinMutex::tryLock() called");
+    return true;
+}
+
+#endif // #else // #if BT_THREADSAFE
+