CUDA数组求和&求最大值

1 求和

#include <cstdio>
#include <iostream>
#include <chrono>

#define BLOCK_DIM 1024
#define REDUCE_SCALE 4


__global__ void sum_gpu(int* const arr, int* sum, int n) {
    __shared__ volatile int local_sum[BLOCK_DIM];

    int temp_sum = 0;
    for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < n; i += gridDim.x * blockDim.x) {
        temp_sum += arr[i];
    }
    local_sum[threadIdx.x] = temp_sum;
    __syncthreads();

    for (int j = blockDim.x >> 1; j > 32; j >>= 1) {
        if (threadIdx.x < j) local_sum[threadIdx.x] += local_sum[threadIdx.x + j];
        __syncthreads();
    }

    if (threadIdx.x < 32) {
        local_sum[threadIdx.x] += local_sum[threadIdx.x + 32];
        local_sum[threadIdx.x] += local_sum[threadIdx.x + 16];
        local_sum[threadIdx.x] += local_sum[threadIdx.x + 8];
        local_sum[threadIdx.x] += local_sum[threadIdx.x + 4];
        local_sum[threadIdx.x] += local_sum[threadIdx.x + 2];
        local_sum[threadIdx.x] += local_sum[threadIdx.x + 1];
    }

    sum[blockIdx.x] = local_sum[0];
}


int main() {
    int n = 1<<24;
    int* arr_host = (int*)malloc(sizeof(int) * n);
    for (int i = 0; i < n; i++) {
        arr_host[i] = std::rand() % 4;
    }

    int* arr_dev;
    cudaMalloc(&arr_dev, sizeof(int)*n);
    cudaMemcpy(arr_dev, arr_host, sizeof(int)*n, cudaMemcpyHostToDevice);

    int gridDim = n/BLOCK_DIM/REDUCE_SCALE;
    int* sum_dev;
    cudaMalloc(&sum_dev, sizeof(int) * gridDim);

    auto start = std::chrono::steady_clock::now();
    sum_gpu<<<gridDim, BLOCK_DIM>>>(arr_dev, sum_dev, n);
    auto end = std::chrono::steady_clock::now();
    auto dt = end - start;
    std::cout << "time: " << (double)dt.count()/(1000000) << "ms" << std::endl;

    int* sum_host = (int*)malloc(sizeof(int) * gridDim);
    cudaMemcpy(sum_host, sum_dev, sizeof(int) * gridDim, cudaMemcpyDeviceToHost);

    int final_sum = 0;
    for (int i = 0; i < gridDim; i++) {
        final_sum += sum_host[i];
    }

    printf("sum: %d\n", final_sum);
}

2 求最大值

#include <cstdio>
#include <iostream>
#include <chrono>

#define BLOCK_DIM 1024
#define REDUCE_SCALE 4


__global__ void sum_gpu(int* const arr, int* sum, int n) {
    __shared__ volatile int local_sum[BLOCK_DIM];

    int temp_sum = arr[0];
    for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < n; i += gridDim.x * blockDim.x) {
        if (temp_sum < arr[i]) temp_sum = arr[i];
    }
    local_sum[threadIdx.x] = temp_sum;
    __syncthreads();

    for (int j = blockDim.x >> 1; j > 32; j >>= 1) {
        if (threadIdx.x < j) {
            if (local_sum[threadIdx.x] < local_sum[threadIdx.x + j]) local_sum[threadIdx.x] = local_sum[threadIdx.x + j];
        }
        __syncthreads();
    }

    if (threadIdx.x < 32) {
        if (local_sum[threadIdx.x] < local_sum[threadIdx.x + 32]) local_sum[threadIdx.x] = local_sum[threadIdx.x + 32];
        if (local_sum[threadIdx.x] < local_sum[threadIdx.x + 16]) local_sum[threadIdx.x] = local_sum[threadIdx.x + 16];
        if (local_sum[threadIdx.x] < local_sum[threadIdx.x + 8]) local_sum[threadIdx.x] = local_sum[threadIdx.x + 8];
        if (local_sum[threadIdx.x] < local_sum[threadIdx.x + 4]) local_sum[threadIdx.x] = local_sum[threadIdx.x + 4];
        if (local_sum[threadIdx.x] < local_sum[threadIdx.x + 2]) local_sum[threadIdx.x] = local_sum[threadIdx.x + 2];
        if (local_sum[threadIdx.x] < local_sum[threadIdx.x + 1]) local_sum[threadIdx.x] = local_sum[threadIdx.x + 1];
    }

    sum[blockIdx.x] = local_sum[0];
}


int main() {
    // 创建数组
    int n = 1<<24;
    int* arr_host = (int*)malloc(sizeof(int) * n);
    for (int i = 0; i < n; i++) {
        arr_host[i] = i;
    }

    int* arr_dev;
    int* sum_dev;
    int gridDim = n/BLOCK_DIM/REDUCE_SCALE;
    cudaMalloc(&arr_dev, sizeof(int)*n);
    cudaMalloc(&sum_dev, sizeof(int) * gridDim);
    cudaMemcpy(arr_dev, arr_host, sizeof(int)*n, cudaMemcpyHostToDevice);

    auto start = std::chrono::steady_clock::now();
    sum_gpu<<<gridDim, BLOCK_DIM>>>(arr_dev, sum_dev, n);
    auto end = std::chrono::steady_clock::now();
    auto dt = end - start;
    std::cout << "time: " << (double)dt.count()/(1000000) << "ms" << std::endl;

    int* sum_host = (int*)malloc(sizeof(int) * gridDim);
    cudaMemcpy(sum_host, sum_dev, sizeof(int) * gridDim, cudaMemcpyDeviceToHost);

    int final_sum = sum_host[0];
    for (int i = 1; i < gridDim; i++) {
        if (final_sum < sum_host[i]) final_sum = sum_host[i];
    }
    printf("sum: %d\n", final_sum);
    
    free(arr_host);
    cudaFree(arr_dev);
    cudaFree(sum_dev);
    free(sum_host);

	return 0;
}