VexCL vector of structs? - c++

Question

So I know that it is possible to use custom types with OpenCL. But I haven't been able to use them with VexCL. Creating a device vector of structs works fine, but I can't perform any operations.
As I haven't found any examples using custom types with VexCL my question is is that even possible? Thanks in advance.

Answer 1

VexCL does not support operations with vectors of structs out of the box. You will need to help it a bit. First, you need to tell VexCL how to spell the type name of the struct. Let's say you have the following struct defined on the host side:
struct point2d {
double x;
double y;
};
You need to provide a specification of the vex::type_name_impl struct that will generate a string corresponding to the type name of the struct. Remember that the code you are generating is C99:
namespace vex {
template <> struct type_name_impl<point2d> {
static std::string get() { return "struct point2d"; }
};
}
You will also need to make sure every generated kernel knows about your struct. This may be achieved with vex::push_program_header() function after the VexCL context has been initialized:
vex::push_program_header(ctx, "struct point2d { double x; double y; };");
This will allow you to declare vectors of the struct, and to pass the vectors to custom functions. That should be general enough. Here is the complete example:
#include <vexcl/vexcl.hpp>
// Host-side definition of the struct.
struct point2d {
double x, y;
};
// We need this for code generation.
namespace vex {
template <>
struct type_name_impl<point2d> {
static std::string get() { return "struct point2d"; }
};
}
int main() {
const size_t n = 16;
vex::Context ctx(vex::Filter::Env);
std::cout << ctx << std::endl;
// After this, every kernel will have the struct declaration in header:
vex::push_program_header(ctx, "struct point2d { double x; double y; };");
// Now we may define vectors of the struct:
vex::vector<point2d> x(ctx, n);
vex::vector<double> y(ctx, n);
// We won't be able to use the vectors in any expressions except for
// custom functions, but that should be enough:
VEX_FUNCTION(point2d, init, (double, x)(double, y),
struct point2d p = {x, y}; return p;
);
VEX_FUNCTION(double, dist, (point2d, p),
return sqrt(p.x * p.x + p.y * p.y);
);
x = init(3,4);
y = dist(x);
std::cout << y << std::endl;
}
And here is the kernel that will be generated for the assignment operation of y = dist(x);:
struct point2d { double x; double y; };
double dist
(
struct point2d p
)
{
return sqrt(p.x * p.x + p.y * p.y);
}
kernel void vexcl_vector_kernel
(
ulong n,
global double * prm_1,
global struct point2d * prm_2
)
{
for(ulong idx = get_global_id(0); idx < n; idx += get_global_size(0))
{
prm_1[idx] = dist( prm_2[idx] );
}
}