cuda-samples/cpp/0_Introduction/vectorAdd/vectorAdd.cu

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/**
 * Vector addition: C = A + B.
 *
 * This sample is a very basic sample that implements element by element
 * vector addition. It is the same as the sample illustrating Chapter 2
 * of the programming guide with some additions like error checking.
 */

#include <cuda_runtime_api.h>
#include <memory.h>
#include <cstdlib>
#include <ctime>
#include <stdio.h>
#include <cuda/cmath>
/**
 * CUDA Kernel Device code
 *
 * Computes the vector addition of A and B into C. The 3 vectors have the same
 * number of elements. This exmple shows Vector addition using Unified memory.
 */


__global__ void vecAdd(float* A, float* B, float* C, int vectorLength)
{
    int workIndex = threadIdx.x + blockIdx.x*blockDim.x;
    if(workIndex < vectorLength)
    {
        C[workIndex] = A[workIndex] + B[workIndex];
    }
}

void initArray(float* A, int length)
{
     std::srand(std::time({}));
    for(int i=0; i<length; i++)
    {
        A[i] = rand() / (float)RAND_MAX;
    }
}

void serialVecAdd(float* A, float* B, float* C,  int length)
{
    for(int i=0; i<length; i++)
    {
        C[i] = A[i] + B[i];
    }
}

bool vectorApproximatelyEqual(float* A, float* B, int length, float epsilon=0.00001)
{
    for(int i=0; i<length; i++)
    {
        if(fabs(A[i] -B[i]) > epsilon)
        {
            printf("Index %d mismatch: %f != %f", i, A[i], B[i]);
            return false;
        }
    }
    return true;
}

int main(int argc, char** argv)
{
    int vectorLength = 1024;
    if(argc >=2)
    {
        vectorLength = std::atoi(argv[1]);
    }

    //unified-memory-example-begin

    // Pointers to memory vectors
    float* A = nullptr;
    float* B = nullptr;
    float* C = nullptr;
    float* comparisonResult = (float*)malloc(vectorLength*sizeof(float));

    // Use unified memory to allocate buffers
    cudaMallocManaged(&A, vectorLength*sizeof(float));
    cudaMallocManaged(&B, vectorLength*sizeof(float));
    cudaMallocManaged(&C, vectorLength*sizeof(float));

    // Initialize vectors on the host
    initArray(A, vectorLength);
    initArray(B, vectorLength);

    // Launch the kernel. Unified memory will make sure A, B, and C are
    // accessible to the GPU
    int threads = 256;
    int blocks = cuda::ceil_div(vectorLength, threads);
    vecAdd<<<blocks, threads>>>(A, B, C, vectorLength);
    // Wait for the kernel to complete execution
    cudaDeviceSynchronize();

    // Perform computation serially on CPU for comparison
    serialVecAdd(A, B, comparisonResult, vectorLength);

    // Confirm that CPU and GPU got the same answer
    if(vectorApproximatelyEqual(C, comparisonResult, vectorLength))
    {
        printf("Unified Memory: CPU and GPU answers match\n");
    }
    else
    {
        printf("Unified Memory: Error - CPU and GPU answers do not match\n");
    }

    // Clean Up
    cudaFree(A);
    cudaFree(B);
    cudaFree(C);
    free(comparisonResult);

    //unified-memory-example-end	

    return 0;
}