KokkosTutorial03

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//                             Kokkos
//         Manycore Performance-Portable Multidimensional Arrays
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//
// First Kokkos::View (multidimensional array) example:
//   1. Start up Kokkos
//   2. Allocate a Kokkos::View
//   3. Execute a parallel_for and a parallel_reduce over that View's data
//   4. Shut down Kokkos
//
// Compare this example to 03_simple_view_lambda, which uses C++11
// lambdas to define the loop bodies of the parallel_for and
// parallel_reduce.
//

#include <Kokkos_Core.hpp>
#include <cstdio>

// A Kokkos::View is an array of zero or more dimensions.  The number
// of dimensions is specified at compile time, as part of the type of
// the View.  This array has two dimensions.  The first one
// (represented by the asterisk) is a run-time dimension, and the
// second (represented by [3]) is a compile-time dimension.  Thus,
// this View type is an N x 3 array of type double, where N is
// specified at run time in the View's constructor.
//
// The first dimension of the View is the dimension over which it is
// efficient for Kokkos to parallelize.
typedef Kokkos::View<double*[3]> view_type;

// parallel_for functor that fills the View given to its constructor.
// The View must already have been allocated.
struct InitView {
  view_type a;

  // Views have "view semantics."  This means that they behave like
  // pointers, not like std::vector.  Their copy constructor and
  // operator= only do shallow copies.  Thus, you can pass View
  // objects around by "value"; they won't do a deep copy unless you
  // explicitly ask for a deep copy.
  InitView (view_type a_) :
    a (a_)
  {}

  // Fill the View with some data.  The parallel_for loop will iterate
  // over the View's first dimension N.
  KOKKOS_INLINE_FUNCTION
  void operator () (const int i) const {
    // Acesss the View just like a Fortran array.  The layout depends
    // on the View's memory space, so don't rely on the View's
    // physical memory layout unless you know what you're doing.
    a(i,0) = 1.0*i;
    a(i,1) = 1.0*i*i;
    a(i,2) = 1.0*i*i*i;
  }
};

// Reduction functor that reads the View given to its constructor.
struct ReduceFunctor {
  view_type a;

  // Constructor takes View by "value"; this does a shallow copy.
  ReduceFunctor (view_type a_) : a (a_) {}

  // If you write a functor to do a reduction, you must specify the
  // type of the reduction result via a public 'value_type' typedef.
  typedef double value_type;

  KOKKOS_INLINE_FUNCTION
  void operator() (int i, double &lsum) const {
    lsum += a(i,0)*a(i,1)/(a(i,2)+0.1);
  }
};

int main (int argc, char* argv[]) {
  Kokkos::initialize (argc, argv);
  const int N = 10;

  // Allocate the View.  The first dimension is a run-time parameter
  // N.  We set N = 10 here.  The second dimension is a compile-time
  // parameter, 3.  We don't specify it here because we already set it
  // by declaring the type of the View.
  //
  // Views get initialized to zero by default.  This happens in
  // parallel, using the View's memory space's default execution
  // space.  Parallel initialization ensures first-touch allocation.
  // There is a way to shut off default initialization.
  //
  // You may NOT allocate a View inside of a parallel_{for, reduce,
  // scan}.  Treat View allocation as a "thread collective."
  //
  // The string "A" is just the label; it only matters for debugging.
  // Different Views may have the same label.
  view_type a ("A", N);

  Kokkos::parallel_for (N, InitView (a));
  double sum = 0;
  Kokkos::parallel_reduce (N, ReduceFunctor (a), sum);
  printf ("Result: %lf\n", sum);
  Kokkos::finalize ();
}