Thrust: why always host code is executed in spite of __CUDA_ARCH__ - cuda

Question

I try to define two branches in code: one for CUDA execution and the other - without it (with future OMP in mind). But when I use macro __CUDA_ARCH__ it looks as if always the host code is executed. But I supposed that Thrust by default use CUDA (and branch for device code). What's wrong with my code?
Here it is:
#include <thrust/transform.h>
#include <thrust/functional.h>
#include <thrust/iterator/counting_iterator.h>
#include <stdio.h>
struct my_op
{
my_op(int init_const) : constanta(init_const) {}
__host__ __device__ int operator()(const int &x) const
{
#if defined(__CUDA_ARCH__)
return 2 * x * constanta; // never executed - why?
#else
return x * constanta; // always executed
#endif
}
private:
int constanta;
};
int main()
{
int data[7] = { 0, 0, 0, 0, 0, 0, 0 };
thrust::counting_iterator<int> first(10);
thrust::counting_iterator<int> last = first + 7;
int init_value = 1;
my_op op(init_value);
thrust::transform(first, last, data, op);
for each (int el in data)
std::cout << el << " ";
std::cout << std::endl;
}
I expect that "transform" will define vector as multiplied by 2*constanta but I see that host code is used - the output is "10 11 12 13 14 15 16", not "20 22 24 26 28 30 32" (as expected).
Why?

Answer 1

Thrust is choosing the host path because one of your data items supplied to the thrust transform operation is in host memory:
thrust::transform(first, last, data, op);
^^^^
If you want a thrust algorithm to operate on the device, generally speaking all the container data you pass to/from must also reside in device memory.
Here's a modification to your code that demonstrates that thrust will follow the device path if we replace data with a device-resident container:
$ cat t13.cu
#include <thrust/transform.h>
#include <thrust/functional.h>
#include <thrust/iterator/counting_iterator.h>
#include <thrust/device_vector.h>
#include <stdio.h>
struct my_op
{
my_op(int init_const) : constanta(init_const) {}
__host__ __device__ int operator()(const int &x) const
{
#if defined(__CUDA_ARCH__)
return 2 * x * constanta; // never executed - why?
#else
return x * constanta; // always executed
#endif
}
private:
int constanta;
};
int main()
{
// int data[7] = { 0, 0, 0, 0, 0, 0, 0 };
thrust::counting_iterator<int> first(10);
thrust::counting_iterator<int> last = first + 7;
thrust::device_vector<int> d_data(7);
int init_value = 1;
my_op op(init_value);
thrust::transform(first, last, d_data.begin(), op);
for (int el = 0; el < 7; el++) {
int dat = d_data[el];
std::cout << dat << " "; }
std::cout << std::endl;
}
$ nvcc -arch=sm_61 -o t13 t13.cu
$ ./t13
20 22 24 26 28 30 32
$
You may want to read the thrust quick start guide to learn about thrust algorithm dispatch.