cuda-samples/Samples/0_Introduction/simpleCooperativeGroups/simpleCooperativeGroups.cu

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/**
 *
 * This sample is a simple code that illustrates basic usage of
 * cooperative groups within the thread block. The code launches a single
 * thread block, creates a cooperative group of all threads in the block,
 * and a set of tiled partition cooperative groups. For each, it uses a
 * generic reduction function to calculate the sum of all the ranks in
 * that group. In each case the result is printed, together with the
 * expected answer (which is calculated using the analytical formula
 * (n-1)*n)/2, noting that the ranks start at zero).
 *
 */

#include <stdio.h>
#include <cooperative_groups.h>

using namespace cooperative_groups;

/**
 * CUDA device function
 *
 * calculates the sum of val across the group g. The workspace array, x,
 * must be large enough to contain g.size() integers.
 */
__device__ int sumReduction(thread_group g, int *x, int val) {
  // rank of this thread in the group
  int lane = g.thread_rank();

  // for each iteration of this loop, the number of threads active in the
  // reduction, i, is halved, and each active thread (with index [lane])
  // performs a single summation of it's own value with that
  // of a "partner" (with index [lane+i]).
  for (int i = g.size() / 2; i > 0; i /= 2) {
    // store value for this thread in temporary array
    x[lane] = val;

    // synchronize all threads in group
    g.sync();

    if (lane < i)
      // active threads perform summation of their value with
      // their partner's value
      val += x[lane + i];

    // synchronize all threads in group
    g.sync();
  }

  // master thread in group returns result, and others return -1.
  if (g.thread_rank() == 0)
    return val;
  else
    return -1;
}

/**
 * CUDA kernel device code
 *
 * Creates cooperative groups and performs reductions
 */
__global__ void cgkernel() {
  // threadBlockGroup includes all threads in the block
  thread_block threadBlockGroup = this_thread_block();
  int threadBlockGroupSize = threadBlockGroup.size();

  // workspace array in shared memory required for reduction
  extern __shared__ int workspace[];

  int input, output, expectedOutput;

  // input to reduction, for each thread, is its' rank in the group
  input = threadBlockGroup.thread_rank();

  // expected output from analytical formula (n-1)(n)/2
  // (noting that indexing starts at 0 rather than 1)
  expectedOutput = (threadBlockGroupSize - 1) * threadBlockGroupSize / 2;

  // perform reduction
  output = sumReduction(threadBlockGroup, workspace, input);

  // master thread in group prints out result
  if (threadBlockGroup.thread_rank() == 0) {
    printf(
        " Sum of all ranks 0..%d in threadBlockGroup is %d (expected %d)\n\n",
        (int)threadBlockGroup.size() - 1, output, expectedOutput);

    printf(" Now creating %d groups, each of size 16 threads:\n\n",
           (int)threadBlockGroup.size() / 16);
  }

  threadBlockGroup.sync();

  // each tiledPartition16 group includes 16 threads
  thread_block_tile<16> tiledPartition16 =
      tiled_partition<16>(threadBlockGroup);

  // This offset allows each group to have its own unique area in the workspace
  // array
  int workspaceOffset =
      threadBlockGroup.thread_rank() - tiledPartition16.thread_rank();

  // input to reduction, for each thread, is its' rank in the group
  input = tiledPartition16.thread_rank();

  // expected output from analytical formula (n-1)(n)/2
  // (noting that indexing starts at 0 rather than 1)
  expectedOutput = 15 * 16 / 2;

  // Perform reduction
  output = sumReduction(tiledPartition16, workspace + workspaceOffset, input);

  // each master thread prints out result
  if (tiledPartition16.thread_rank() == 0)
    printf(
        "   Sum of all ranks 0..15 in this tiledPartition16 group is %d "
        "(expected %d)\n",
        output, expectedOutput);

  return;
}

/**
 * Host main routine
 */
int main() {
  // Error code to check return values for CUDA calls
  cudaError_t err;

  // Launch the kernel

  int blocksPerGrid = 1;
  int threadsPerBlock = 64;

  printf("\nLaunching a single block with %d threads...\n\n", threadsPerBlock);

  // we use the optional third argument to specify the size
  // of shared memory required in the kernel
  cgkernel<<<blocksPerGrid, threadsPerBlock, threadsPerBlock * sizeof(int)>>>();
  err = cudaDeviceSynchronize();

  if (err != cudaSuccess) {
    fprintf(stderr, "Failed to launch kernel (error code %s)!\n",
            cudaGetErrorString(err));
    exit(EXIT_FAILURE);
  }

  printf("\n...Done.\n\n");

  return 0;
}