general tbb issue for calculating fibonacci numbers - c++

Question

I came across the tbb template below as an example of task-based programming for calculating the sum of fibonacci numbers in c++. But when I run it I get a value of 1717986912 which can't be the case. The output should be 3. What am I doing wrong?
class FibTask: public task
{
public:
const long n;
long * const sum;
FibTask( long n_, long* sum_ ) : n(n_), sum(sum_) {}
task* execute( )
{
// Overrides virtual function task::execute
if( n < 0)
{
return 0;
}
else
{
long x, y;
FibTask& a = *new( allocate_child( ) ) FibTask(n-1,&x);
FibTask& b = *new( allocate_child( ) ) FibTask(n-2,&y);
// Set ref_count to "two children plus one for the wait".
set_ref_count(3);
// Start b running.
spawn( b );
// Start a running and wait for all children (a and b).
spawn_and_wait_for_all( a );
// Do the sum
*sum = x+y;
}
return NULL;
}
long ParallelFib( long n )
{
long sum;
FibTask& a = *new(task::allocate_root( )) FibTask(n,&sum);
task::spawn_root_and_wait(a);
return sum;
}
};
long main(int argc, char** argv)
{
FibTask * obj = new FibTask(3,0);
long b = obj->ParallelFib(3);
std::cout << b;
return 0;
}

Answer 1

The cutoff is messed here. It must be 2 at least. E.g.:
if( n<2 ) {
*sum = n;
return NULL;
}
The original example also uses SerialFib as showed here http://www.threadingbuildingblocks.org/docs/help/tbb_userguide/Simple_Example_Fibonacci_Numbers.htm
The inefficient method for calculating Fibonacci numbers using inefficient blocking style technique will be even more inefficient without call to SerialFib().
WARNING: Please note that this example is intended just to demonstrate this particular low-level TBB API and this particular way of using it. It is not intended for reuse unless you are really sure why you are doing this.
Modern high-level API (though, still for the inefficient Fibonacci algorithm) would look like this:
int Fib(int n) {
if( n<CUTOFF ) { // 2 is minimum
return fibSerial(n);
} else {
int x, y;
tbb::parallel_invoke([&]{x=Fib(n-1);}, [&]{y=Fib(n-2);});
return x+y;
}
}