Hello World with CUDA

Writing Hello World in CUDA is bit difficult ( CUDA does not support strings)

But the following is a vector addition program which may be a good starting point


Simple code to add two vectors.(Blue colour)

#include "stdio.h"

__global__ void add_arrays_gpu( float *in1, float *in2, float *out, int Ntot)
{
int idx=blockIdx.x*blockDim.x+threadIdx.x;
if ( idx
out[idx]=in1[idx]+in2[idx];
}

int main()
{
/* pointers to host memory */
float *a, *b, *c;
/* pointers to device memory */
float *a_d, *b_d, *c_d;
int N=18;
int i;

/* Allocate arrays a, b and c on host*/
a = (float*) malloc(N*sizeof(float));
b = (float*) malloc(N*sizeof(float));
c = (float*) malloc(N*sizeof(float));

/* Allocate arrays a_d, b_d and c_d on device*/
cudaMalloc ((void **) &a_d, sizeof(float)*N);
cudaMalloc ((void **) &b_d, sizeof(float)*N);
cudaMalloc ((void **) &c_d, sizeof(float)*N);

/* Initialize arrays a and b */
for (i=0; i
{
a[i]= (float) i;
b[i]=-(float) i;
}


/* Copy data from host memory to device memory */
cudaMemcpy(a_d, a, sizeof(float)*N, cudaMemcpyHostToDevice);
cudaMemcpy(b_d, b, sizeof(float)*N, cudaMemcpyHostToDevice);

/* Compute the execution configuration */
int block_size=8;
dim3 dimBlock(block_size);
dim3 dimGrid ( (N/dimBlock.x) + (!(N%dimBlock.x)?0:1) );

/* Add arrays a and b, store result in c */
add_arrays_gpu<<>>(a_d, b_d, c_d, N);

/* Copy data from deveice memory to host memory */
cudaMemcpy(c, c_d, sizeof(float)*N, cudaMemcpyDeviceToHost);

/* Print c */
for (i=0; i
printf(" c[%d]=%f\n",i,c[i]);

/* Free the memory */
free(a); free(b); free(c);
cudaFree(a_d); cudaFree(b_d);cudaFree(c_d)

}

===========================
Running the Code
===========================

Copy the code in a file add_vector.cu
Compile it with nvcc: nvcc -o add_vector add_vector.cu
Run it: ./add_vector

If you don't have a Cuda capable GPU, compile it in emulation mode:
nvcc -deviceemu -o add_vector_emu add_vector.cu
Run it: ./add_vector_emu