KokkosTutorial05

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//                             Kokkos
//         Manycore Performance-Portable Multidimensional Arrays
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#include <Kokkos_Core.hpp>
#include <cstdio>
#include <cstdlib>
#include <cmath>

// Type of a one-dimensional length-N array of int.
typedef Kokkos::View<int*> view_type;
typedef view_type::HostMirror host_view_type;
// This is a "zero-dimensional" View, that is, a View of a single
// value (an int, in this case).  Access the value using operator()
// with no arguments: e.g., 'count()'.
//
// Zero-dimensional Views are useful for reduction results that stay
// resident in device memory, as well as for irregularly updated
// shared state.  We use it for the latter in this example.
typedef Kokkos::View<int> count_type;
typedef count_type::HostMirror host_count_type;


// Functor for finding a list of primes in a given set of numbers.  If
// run in parallel, the order of results is nondeterministic, because
// hardware atomic updates do not guarantee an order of execution.
struct findprimes {
  view_type data;
  view_type result;
  count_type count;

  findprimes (view_type data_, view_type result_, count_type count_) :
    data (data_), result (result_), count (count_)
  {}

  // Test if data(i) is prime.  If it is, increment the count of
  // primes (stored in the zero-dimensional View 'count') and add the
  // value to the current list of primes 'result'.
  KOKKOS_INLINE_FUNCTION
  void operator() (const int i) const {
    const int number = data(i); // the current number

    // Test all numbers from 3 to ceiling(sqrt(data(i))), to see if
    // they are factors of data(i).  It's not the most efficient prime
    // test, but it works.
    const int upper_bound = sqrt(1.0*number)+1;
    bool is_prime = !(number%2 == 0);
    int k = 3;
    while (k < upper_bound && is_prime) {
      is_prime = !(number%k == 0);
      k += 2; // don't have to test even numbers
    }

    if (is_prime) {
      // Use an atomic update both to update the current count of
      // primes, and to find a place in the current list of primes for
      // the new result.
      //
      // atomic_fetch_add results the _current_ count, but increments
      // it (by 1 in this case).  The current count of primes indexes
      // into the first unoccupied position of the 'result' array.
      const int idx = Kokkos::atomic_fetch_add (&count(), 1);
      result(idx) = number;
    }
  }

};

int main () {
  Kokkos::initialize ();

  srand (61391); // Set the random seed

  int nnumbers = 100000;
  view_type data ("RND", nnumbers);
  view_type result ("Prime", nnumbers);
  count_type count ("Count");

  host_view_type h_data = Kokkos::create_mirror_view (data);
  host_view_type h_result = Kokkos::create_mirror_view (result);
  host_count_type h_count = Kokkos::create_mirror_view (count);

  typedef view_type::size_type size_type;
  // Fill the 'data' array on the host with random numbers.  We assume
  // that they come from some process which is only implemented on the
  // host, via some library.  (That's true in this case.)
  for (size_type i = 0; i < data.dimension_0 (); ++i) {
    h_data(i) = rand () % nnumbers;
  }
  Kokkos::deep_copy (data, h_data); // copy from host to device

  Kokkos::parallel_for (data.dimension_0 (), findprimes (data, result, count));
  Kokkos::deep_copy (h_count, count); // copy from device to host

  printf ("Found %i prime numbers in %i random numbers\n", h_count(), nnumbers);
  Kokkos::finalize ();
}