How to copy data from unsigned int to ulong4 in CUDA - c++

Question

.h file:
#define VECTOR_SIZE 1024
.cpp file:
int main ()
{
unsigned int* A;
A = new unsigned int [VECTOR_SIZE];
CopyToDevice (A);
}
.cu file:
void CopyToDevice (unsigned int *A)
{
ulong4 *UA
unsigned int VectorSizeUlong4 = VECTOR_SIZE / 4;
unsigned int VectorSizeBytesUlong4 = VectorSizeUlong4 * sizeof(ulong4);
cudaMalloc( (void**)&UA, VectorSizeBytesUlong4 );
// how to use cudaMemcpy to copy data from A to UA?
// I tried to do the following but it gave access violation error:
for (int i=0; i<VectorSizeUlong4; ++i)
{
UA[i].x = A[i*4 + 0];
UA[i].y = A[i*4 + 1];
UA[i].z = A[i*4 + 2];
UA[i].w = A[i*4 + 3];
}
// I also tried to copy *A to device and then work on it instead going back to CPU to access *A every time but this did not work again
}

Answer 1

The CUDA ulong4 is a 16 byte aligned structure defined as
struct __builtin_align__(16) ulong4
{
unsigned long int x, y, z, w;
};
this means that the stream of four consecutive 32 bit unsigned source integers you want to use to populate a stream of ulong4 are the same size. The simplest solution is contained right in the text on the image you posted - just cast (either implicitly or explicitly) the unsigned int pointer to a ulong4 pointer, use cudaMemcpydirectly on the host and device memory, and pass the resulting device pointer to whatever kernel function you have that requires a ulong4 input. Your device transfer function could look something like:
ulong4* CopyToDevice (unsigned int* A)
{
ulong4 *UA, *UA_h;
size_t VectorSizeUlong4 = VECTOR_SIZE / 4;
size_t VectorSizeBytesUlong4 = VectorSizeUlong4 * sizeof(ulong4);
cudaMalloc( (void**)&UA, VectorSizeBytesUlong4);
UA_h = reinterpret_cast<ulong4*>(A); // not necessary but increases transparency
cudaMemcpy(UA, UA_h, VectorSizeBytesUlong4);
return UA;
}
[Usual disclaimer: written in browser, not tested or compiled, use at own risk]

Answer 2

This should raise all alarm bells:
cudaMalloc( (void**)&UA, VectorSizeBytesUlong4 );
// ...
UA[i].x = A[i*4 + 0];
You are allocating UA on the device and then use it in host code. Don't ever do that. You will need to use cudaMemcpy to copy arrays to the device. This tutorial shows you a basic program that uses cudaMemcpy to copy things over. The length argument to cudaMemcpy is the length of your array in bytes. And in your case that is VECTOR_SIZE * sizeof(unsigned int).