parallel solver: various changes

- threading: adding btSequentialImpulseConstraintSolverMt
 - task scheduler: added parallelSum so that parallel solver can compute residuals
 - CommonRigidBodyMTBase: add slider for solver least squares residual and allow multithreading without needing OpenMP, TBB, or PPL
 - taskScheduler: don't wait for workers to sleep/signal at the end of each parallel block
 - parallel solver: convertContacts split into an allocContactConstraints and setupContactConstraints stage, the latter of which is done in parallel
 - parallel solver: rolling friction is now interleaved along with normal friction
 - parallel solver: batchified split impulse solving + some cleanup
 - parallel solver: sorting batches from largest to smallest
 - parallel solver: added parallel batch creation
 - parallel solver: added warmstartingWriteBackContacts func + other cleanup
 - task scheduler: truncate low bits to preserve determinism with parallelSum
 - parallel solver: reducing dynamic mem allocs and trying to parallelize more of the batch setup
 - parallel solver: parallelize updating constraint batch ids for merging
 - parallel solver: adding debug visualization
 - task scheduler: make TBB task scheduler parallelSum deterministic
 - parallel solver: split batch gen code into separate file; allow selection of batch gen method
 - task scheduler: add sleepWorkerThreadsHint() at end of simulation
 - parallel solver: added grain size per phase
 - task Scheduler: fix for strange threading issue; also no need for main thread to wait for workers to sleep
 - base constraint solver: break out joint setup into separate function for profiling/overriding
 - parallel solver: allow different batching method for contacts vs joints
 - base constraint solver: add convertJoint and convertBodies to make it possible to parallelize joint and body conversion
 - parallel solver: convert joints and bodies in parallel now
 - parallel solver: speed up batch creation with run-length encoding
 - parallel solver: batch gen: run-length expansion in parallel; collect constraint info in parallel
 - parallel solver: adding spatial grid batching method
 - parallel solver: enhancements to spatial grid batching
 - sequential solver: moving code for writing back into functions that derived classes can call
 - parallel solver: do write back of bodies and joints in parallel
 - parallel solver: removed all batching methods except for spatial grid (others were ineffective)
 - parallel solver: added 2D or 3D grid batching options; and a bit of cleanup
 - move btDefaultTaskScheduler into LinearMath project

src/LinearMath/btThreads.h

@@ -107,6 +107,17 @@ public:
     virtual void forLoop( int iBegin, int iEnd ) const = 0;
 };
 
+//
+// btIParallelSumBody -- subclass this to express work that can be done in parallel
+//                       and produces a sum over all loop elements
+//
+class btIParallelSumBody
+{
+public:
+    virtual ~btIParallelSumBody() {}
+    virtual btScalar sumLoop( int iBegin, int iEnd ) const = 0;
+};
+
 //
 // btITaskScheduler -- subclass this to implement a task scheduler that can dispatch work to
 //                     worker threads
@@ -122,6 +133,8 @@ public:
     virtual int getNumThreads() const = 0;
     virtual void setNumThreads( int numThreads ) = 0;
     virtual void parallelFor( int iBegin, int iEnd, int grainSize, const btIParallelForBody& body ) = 0;
+    virtual btScalar parallelSum( int iBegin, int iEnd, int grainSize, const btIParallelSumBody& body ) = 0;
+    virtual void sleepWorkerThreadsHint() {}  // hint the task scheduler that we may not be using these threads for a little while
 
     // internal use only
     virtual void activate();
@@ -143,6 +156,9 @@ btITaskScheduler* btGetTaskScheduler();
 // get non-threaded task scheduler (always available)
 btITaskScheduler* btGetSequentialTaskScheduler();
 
+// create a default task scheduler (Win32 or pthreads based)
+btITaskScheduler* btCreateDefaultTaskScheduler();
+
 // get OpenMP task scheduler (if available, otherwise returns null)
 btITaskScheduler* btGetOpenMPTaskScheduler();
 
@@ -156,5 +172,9 @@ btITaskScheduler* btGetPPLTaskScheduler();
 //                 (iterations may be done out of order, so no dependencies are allowed)
 void btParallelFor( int iBegin, int iEnd, int grainSize, const btIParallelForBody& body );
 
+// btParallelSum -- call this to dispatch work like a for-loop, returns the sum of all iterations
+//                 (iterations may be done out of order, so no dependencies are allowed)
+btScalar btParallelSum( int iBegin, int iEnd, int grainSize, const btIParallelSumBody& body );
+
 
 #endif