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//@HEADER
// ************************************************************************
//
// Kokkos v. 4.0
// Copyright (2022) National Technology & Engineering
// Solutions of Sandia, LLC (NTESS).
//
// Under the terms of Contract DE-NA0003525 with NTESS,
// the U.S. Government retains certain rights in this software.
//
// Part of Kokkos, under the Apache License v2.0 with LLVM Exceptions.
// See https://kokkos.org/LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//@HEADER
#include <Kokkos_Core.hpp>
#include <cstdio>
//
// "Hello world" parallel_for example:
// 1. Start up Kokkos
// 2. Execute a parallel for loop in the default execution space,
// using a functor to define the loop body
// 3. Shut down Kokkos
//
// Try comparing this example to 01_hello_world_lambda, which uses
// C++11 lambdas (anonymous functions) to define the loop body of the
// parallel_for. That makes the code much more concise and readable.
// On the other hand, breaking out the loop body into an explicit
// functor makes it easier to test the loop independently of the
// parallel pattern.
//
// Functor that defines the parallel_for's loop body.
//
// A "functor" is just a class or struct with a public operator()
// instance method.
struct hello_world {
// If a functor has an "execution_space" (or "execution_space", for
// backwards compatibility) public alias, parallel_* will only run
// the functor in that execution space. That's a good way to mark a
// functor as specific to an execution space. If the functor lacks
// this alias, parallel_for will run it in the default execution
// space, unless you tell it otherwise (that's an advanced topic;
// see "execution policies").
// The functor's operator() defines the loop body. It takes an
// integer argument which is the parallel for loop index. Other
// arguments are possible; see the "hierarchical parallelism" part
// of the tutorial.
//
// The operator() method must be const, and must be marked with the
// KOKKOS_INLINE_FUNCTION macro. If building with CUDA, this macro
// will mark your method as suitable for running on the CUDA device
// (as well as on the host). If not building with CUDA, the macro
// is unnecessary but harmless.
KOKKOS_INLINE_FUNCTION
void operator()(const int i) const {
Kokkos::printf("Hello from i = %i\n", i);
}
};
int main(int argc, char* argv[]) {
// You must call initialize() before you may call Kokkos.
//
// With no arguments, this initializes the default execution space
// (and potentially its host execution space) with default
// parameters. You may also pass in argc and argv, analogously to
// MPI_Init(). It reads and removes command-line arguments that
// start with "--kokkos-".
Kokkos::initialize(argc, argv);
// Print the name of Kokkos' default execution space.
printf("Hello World on Kokkos execution space %s\n",
Kokkos::DefaultExecutionSpace::name());
// Run the above functor on the default Kokkos execution space in
// parallel, with a parallel for loop count of 15.
//
// The Kokkos::DefaultExecutionSpace alias gives the default
// execution space. Depending on how Kokkos was configured, this
// could be OpenMP, Threads, Cuda, Serial, or even some other
// execution space.
//
// The following line of code would look like this in OpenMP:
//
// #pragma omp parallel for
// for (int i = 0; i < 15; ++i) {
// printf ("Hello from i = %i\n", i);
// }
//
// You may notice that the printed numbers do not print out in
// order. Parallel for loops may execute in any order.
Kokkos::parallel_for("HelloWorld", 15, hello_world());
// You must call finalize() after you are done using Kokkos.
Kokkos::finalize();
}
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