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December 20, 2011 17:36
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Minimal CUDA example (with helpful comments).
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#include <stdio.h> | |
// | |
// Nearly minimal CUDA example. | |
// Compile with: | |
// | |
// nvcc -o example example.cu | |
// | |
#define N 1000 | |
// | |
// A function marked __global__ | |
// runs on the GPU but can be called from | |
// the CPU. | |
// | |
// This function multiplies the elements of an array | |
// of ints by 2. | |
// | |
// The entire computation can be thought of as running | |
// with one thread per array element with blockIdx.x | |
// identifying the thread. | |
// | |
// The comparison i<N is because often it isn't convenient | |
// to have an exact 1-1 correspondence between threads | |
// and array elements. Not strictly necessary here. | |
// | |
// Note how we're mixing GPU and CPU code in the same source | |
// file. An alternative way to use CUDA is to keep | |
// C/C++ code separate from CUDA code and dynamically | |
// compile and load the CUDA code at runtime, a little | |
// like how you compile and load OpenGL shaders from | |
// C/C++ code. | |
// | |
__global__ | |
void add(int *a, int *b) { | |
int i = blockIdx.x; | |
if (i<N) { | |
b[i] = 2*a[i]; | |
} | |
} | |
int main() { | |
// | |
// Create int arrays on the CPU. | |
// ('h' stands for "host".) | |
// | |
int ha[N], hb[N]; | |
// | |
// Create corresponding int arrays on the GPU. | |
// ('d' stands for "device".) | |
// | |
int *da, *db; | |
cudaMalloc((void **)&da, N*sizeof(int)); | |
cudaMalloc((void **)&db, N*sizeof(int)); | |
// | |
// Initialise the input data on the CPU. | |
// | |
for (int i = 0; i<N; ++i) { | |
ha[i] = i; | |
} | |
// | |
// Copy input data to array on GPU. | |
// | |
cudaMemcpy(da, ha, N*sizeof(int), cudaMemcpyHostToDevice); | |
// | |
// Launch GPU code with N threads, one per | |
// array element. | |
// | |
add<<<N, 1>>>(da, db); | |
// | |
// Copy output array from GPU back to CPU. | |
// | |
cudaMemcpy(hb, db, N*sizeof(int), cudaMemcpyDeviceToHost); | |
for (int i = 0; i<N; ++i) { | |
printf("%d\n", hb[i]); | |
} | |
// | |
// Free up the arrays on the GPU. | |
// | |
cudaFree(da); | |
cudaFree(db); | |
return 0; | |
} |
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Thank you so much for this; truly helped a lot with my homework.