I want to allocate a 2D array in shared memory in CUDA. I know that to allocate a 1D shared memory array you have to pass the size per block as a parameter to the kernel.
I also know that it is impossible to create an actual 2D array dynamically in shared memory.
However I was wondering if this could be done if one of the dimensions is known.
extern __shared__ int array[COMPILE_TIME_SIZE][];
Can this be done? If so how do I pass the size of the second dimension?
Doing exactly so, with the known dimension in the first place (highest order - first square bracket entry) is not possible, as the compiler may not implement addressing properly.
However, it is possible to do so setting the second parameter only at compile time. Here is an example code:
extern __shared__ int shared2Darray[][17] ;
__global__ void kernel(int* output)
{
shared2Darray[threadIdx.y][threadIdx.x] = threadIdx.x + 2*threadIdx.y ;
__syncthreads();
output [threadIdx.y * blockDim.x + threadIdx.x] = shared2Darray[threadIdx.y][threadIdx.x] ;
__syncthreads();
}
int main()
{
int* h_output, *d_output ;
cudaMalloc(&d_output, 16*16*sizeof(int));
kernel<<<1, dim3(16,16,1), 16*17*sizeof(int)>>> (d_output) ;
h_output = new int[16*16] ;
cudaMemcpy (h_output, d_output, 16*16*sizeof(int), cudaMemcpyDeviceToHost) ;
cudaDeviceReset();
for (int x = 0 ; x < 16 ; ++x)
{
for (int y = 0 ; y < 16 ; ++y)
{
if (h_output[y*16+x] != x+2*y)
printf ("ERROR\n");
}
}
printf ("DONE\n");
delete[] h_output ;
return 0 ;
}
The size of the array is defined by the shared memory parameter in the triple angled bracket notation. Hence, the size of the second dimension is deduced by dividing the shared memory size in bytes by the size in bytes of a single entry.
