CUDA double/float woes CUDA not denoting double prec types?

I am having a strange problem with CUDA. Anytime I use a double type in my kernel function(s) things go very very wrong…

If I change ‘VARTYPE’ to float everything works.

I am using a 8600GT card which only has compute compatibility of 1.1. Both in this thread and the CUDA programming guide…

there is talk about a conversion for my device. I was under the impression that this was done automatically and did not have to worry about this. Now it seems I do. But no matter what kind of converts I try to do for the sample code below I do not get a sensible answer? What can I do except for converting everything in my application to floats?

See the sample code below:

#include <stdio.h>

#include <cutil.h>

#define VARTYPE double

//#define VARTYPE float

__global__ void test(VARTYPE *data) {

	int id  = threadIdx.x + threadIdx.y * gridDim.x;

	data[id] = id;


#define NB 512

int main() {

	dim3 blockSize(NB, 1);


	VARTYPE *dbg;


	CUDA_SAFE_CALL(cudaMalloc((void**)&dbg, sizeof(*dbg) * NB));

	test<<<1, blockSize>>>(dbg);

	CUDA_SAFE_CALL(cudaMemcpy(res, dbg, sizeof(*res) * NB, cudaMemcpyDeviceToHost));

	CUT_CHECK_ERROR("1. kernel");


	for (int i = 0; i != NB; i++) printf("%3d %f\n", i + 1, res[i]);




You can use the double type for local variables in your device functions, and they will be demoted to floats, but the same is not true for copying host arrays to the device like you are trying to do. The cudaMalloc and cudaMemcpy functions don’t think about the type of data you are dealing with, they simply allocate and copy bytes.

For what you are trying to do, I don’t think there’s any option but to convert your input data to float on the host before sending it to the GPU.