Added updated task launch implementation that now tracks task groups.

Within each function that launches tasks, we now can easily track which
tasks that function launched, so that the sync at the end of the function
can just sync on the tasks launched by that function (not all tasks
launched by all functions.)

Implementing this led to a rework of the task system API that ispc generates
code to call; the example task systems in examples/tasksys.cpp have been
updated to conform to this API.  (The updated API is also documented in
the ispc user's guide.)

As part of this, "launch[n]" syntax was added to launch a number of tasks
in a single launch statement, rather than requiring a loop over 'n' to
launch n tasks.

This commit thus fixes issue #84 (enhancement to launch multiple tasks from
a single launch statement) as well as issue #105 (recursive task launches
were broken).

ctx.h

@@ -210,15 +210,8 @@ public:
         i32. */
     llvm::Value *I1VecToBoolVec(llvm::Value *b);
 
-    /** Emit code to call the user-supplied ISPCMalloc function to
-        allocate space for an object of thee given type.  Returns the
-        pointer value returned by the ISPCMalloc call. */
-    llvm::Value *EmitMalloc(LLVM_TYPE_CONST llvm::Type *ty, int align = 0);
-
-    /** Emit code to call the user-supplied ISPCFree function, passing it
-        the given pointer to storage previously allocated by an
-        EmitMalloc() call. */
-    void EmitFree(llvm::Value *ptr);
+    /** Returns the size of the given type. */
+    llvm::Value *SizeOf(LLVM_TYPE_CONST llvm::Type *ty);
 
     /** If the user has asked to compile the program with instrumentation,
         this inserts a callback to the user-supplied instrumentation
@@ -399,7 +392,10 @@ public:
     /** Launch an asynchronous task to run the given function, passing it
         he given argument values. */
     llvm::Instruction *LaunchInst(llvm::Function *callee, 
-                                  std::vector<llvm::Value *> &argVals);
+                                  std::vector<llvm::Value *> &argVals,
+                                  llvm::Value *launchCount);
+
+    void SyncInst();
 
     llvm::Instruction *ReturnInst();
     /** @} */
@@ -489,6 +485,11 @@ private:
     /** True if a 'launch' statement has been encountered in the function. */
     bool launchedTasks;
 
+    /** This is a pointer to a void * that is passed to the ISPCLaunch(),
+        ISPCAlloc(), and ISPCSync() routines as a handle to the group ot
+        tasks launched from the current function. */
+    llvm::Value *launchGroupHandlePtr;
+
     llvm::Value *pointerVectorToVoidPointers(llvm::Value *value);
     static void addGSMetadata(llvm::Instruction *inst, SourcePos pos);
     bool ifsInLoopAllUniform() const;