cuda-samples/Samples/warpAggregatedAtomicsCG/warpAggregatedAtomicsCG.cu

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#include <stdio.h>
// includes, project
#include <helper_cuda.h>
#include <helper_functions.h>

#include <cuda_runtime.h>

#include <cooperative_groups.h>

namespace cg = cooperative_groups;

#define NUM_ELEMS 10000000
#define NUM_THREADS_PER_BLOCK 512

// warp-aggregated atomic increment
__device__ int atomicAggInc(int *counter) {
  cg::coalesced_group active = cg::coalesced_threads();

  int mask = active.ballot(1);
  // select the leader
  int leader = __ffs(mask) - 1;

  // leader does the update
  int res = 0;
  if (active.thread_rank() == leader) {
    res = atomicAdd(counter, __popc(mask));
  }

  // broadcast result
  res = active.shfl(res, leader);

  // each thread computes its own value
  return res + __popc(mask & ((1 << active.thread_rank()) - 1));
}

__global__ void filter_arr(int *dst, int *nres, const int *src, int n) {
  int id = threadIdx.x + blockIdx.x * blockDim.x;

  for (int i = id; i < n; i += gridDim.x * blockDim.x) {
    if (src[i] > 0) dst[atomicAggInc(nres)] = src[i];
  }
}

int main(int argc, char **argv) {
  int *data_to_filter, *filtered_data, nres = 0;
  int *d_data_to_filter, *d_filtered_data, *d_nres;

  data_to_filter = reinterpret_cast<int *>(malloc(sizeof(int) * NUM_ELEMS));

  // Generate input data.
  for (int i = 0; i < NUM_ELEMS; i++) {
    data_to_filter[i] = rand() % 20;
  }

  findCudaDevice(argc, (const char **)argv);

  checkCudaErrors(cudaMalloc(&d_data_to_filter, sizeof(int) * NUM_ELEMS));
  checkCudaErrors(cudaMalloc(&d_filtered_data, sizeof(int) * NUM_ELEMS));
  checkCudaErrors(cudaMalloc(&d_nres, sizeof(int)));

  checkCudaErrors(cudaMemcpy(d_data_to_filter, data_to_filter,
                             sizeof(int) * NUM_ELEMS, cudaMemcpyHostToDevice));
  checkCudaErrors(cudaMemset(d_nres, 0, sizeof(int)));

  dim3 dimBlock(NUM_THREADS_PER_BLOCK, 1, 1);
  dim3 dimGrid((NUM_ELEMS / NUM_THREADS_PER_BLOCK) + 1, 1, 1);

  filter_arr<<<dimGrid, dimBlock>>>(d_filtered_data, d_nres, d_data_to_filter,
                                    NUM_ELEMS);

  checkCudaErrors(
      cudaMemcpy(&nres, d_nres, sizeof(int), cudaMemcpyDeviceToHost));

  filtered_data = reinterpret_cast<int *>(malloc(sizeof(int) * nres));

  checkCudaErrors(cudaMemcpy(filtered_data, d_filtered_data, sizeof(int) * nres,
                             cudaMemcpyDeviceToHost));

  int *host_filtered_data =
      reinterpret_cast<int *>(malloc(sizeof(int) * NUM_ELEMS));

  // Generate host output with host filtering code.
  int host_flt_count = 0;
  for (int i = 0; i < NUM_ELEMS; i++) {
    if (data_to_filter[i] > 0) {
      host_filtered_data[host_flt_count++] = data_to_filter[i];
    }
  }

  printf("\nWarp Aggregated Atomics %s \n",
         host_flt_count == nres ? "PASSED" : "FAILED");

  checkCudaErrors(cudaFree(d_data_to_filter));
  checkCudaErrors(cudaFree(d_filtered_data));
  checkCudaErrors(cudaFree(d_nres));
  free(data_to_filter);
  free(filtered_data);
  free(host_filtered_data);
}