Support for custom kernels vector_view

[jkelling]

It would be nice, if one could use vector views with custom kernels.

When calling such a function one needs to pass pointers instead of vectors. The vex::vectors provides operator()(int d) which provides the pointer to to the portion of the array on device d. vex::vector_view also has such an operator, but what it returns does not seem to be a valid pointer to the underlying data: when I tried to access a vectors wrapped in a view this way the kernel just crashed. Unfortunately I could not find any documentation on what vex::vector_view::operator()(int) is supposed to do. (i my case the sliced ranged is continuous (not stride), so pointer to the data can be computed).

[ddemidov]

It is not possible to send a vector_view to a custom kernel as a pointer, because a vector_view is an expression that is evaluated on the fly by vexcl.

You can assign the expression to a temporary vector, and get the pointer to that.

Or, instead of a custom kernel, you could use custom function that returns void:

#include <vexcl/vexcl.hpp>

int main() {
    vex::Context ctx(vex::Filter::Env);

    vex::vector<int> x(ctx, 16);
    vex::vector<int> y(ctx, 8);

    x = vex::element_index();

    vex::slicer<2> slice(vex::extents[8][2]);

    VEX_FUNCTION(void, assign, (int, idx)(int*, out)(int, in),
            out[idx] = in;
    );

    vex::eval(
            assign(vex::element_index(), raw_pointer(y), slice[vex::_][0](x)),
            y.queue_list(),
            y.partition());

    std::cout << "y = " << y << std::endl;
}

Here the function gets view of x in parameter in, and writes it to output.

[jkelling]

It is good to know, that a VEX_FUNCTION can use a view. Maybe I could use this in some places.

• in one place I need to use a kernel, because using one thread per item would not be efficient. The code uses one warp block per item and performs a reduction over threads in a warp before writing the item
• In another place, I use a kernel just because it allows me include a device function in with the code. This function is called by the kernel and is inserted at runtime. There does not seem to be a way to define a VEX_FUNCTION from a string at runtime. Some really ugly macros would be required to do it at compile time.

I guess, vexcl will insert the VEX_FUNCTION as a device function into some generic kernel which handles access to the view and call the function for each item. Would it be possible to expose this interface to custom kernels?

[ddemidov]

There does not seem to be a way to define a VEX_FUNCTION from a string at runtime.

There is VEX_FUNCTION_S:

https://github.com/ddemidov/vexcl/blob/192137a2c1f74de97dc2859d8af76b822a9a4dd6/vexcl/function.hpp#L205-L210

Would it be possible to expose this interface to custom kernels?

You can get function definition from a VEX_FUNCTION as a string using define() method:

#include <vexcl/vexcl.hpp>

int main() {
    vex::Context ctx(vex::Filter::Env);

    VEX_FUNCTION(void, assign, (int, idx)(int*, out)(int, in),
            out[idx] = in;
    );

    vex::backend::source_generator src(ctx.queue(0));
    assign.define(src);

    std::cout << src.str() << std::endl;
}

With an OpenCL backend, this results in:

void assign
(
  int idx,
  global int * out,
  int in
)
{
  out[idx] = in;
}

[jkelling]

There is VEX_FUNCTION_S: "The body of the function is passed as a string literal or a static string expression."

Ok, apparently, in this macro body can be anything that can be ostreamed, even expressions like

begin << middle << end

(not sure if this is meant by "static string expression"). Thanks. However, this still only works with literals. It would not work if e.g. middle was a local variable in calling function, as this code is inserted into a member function of the generated class. Also: I need the function object to be a member of some class, not just a local variable, which means I need to know of the object I will get before using VEX_FUNCTION_S, so I can declare the variable. All of this works with a custom kernel.

You can get function definition from a VEX_FUNCTION as a string using define() method:

This only returns the function definition, but I know, that I can find the generated kernel in vexcl's staging area. What I would like to have is a documented interface that defines how a vector_view is passed to a kernel and how it should be accessed.

[ddemidov]

not sure if this is meant by "static string expression"

This should be an expression that always returns the same string. Also, VEX_FUNCTION macro unpacks into something like

struct <name>_function_type { ... } const <name>;

so it can be included into your classes like this:

#include <vexcl/vexcl.hpp>

struct test {
    static std::string body() {
        return "return x;";
    }

    VEX_FUNCTION_S(int, foo, (int, x), body());
};

int main() {
    vex::Context ctx(vex::Filter::Env);

    test t;
    vex::backend::source_generator src(ctx.queue(0));
    t.foo.define(src);

    std::cout << src.str() << std::endl;
}

This only returns the function definition, but I know, that I can find the generated kernel in vexcl's staging area

You could just prepend the function definition to your kernel definition:

    VEX_FUNCTION(int, foo, (int, x), return x;);

    vex::backend::source_generator src(ctx.queue(0));
    foo.define(src);

    vex::backend::kernel test(ctx.queue(0), src.str() + VEX_STRINGIZE_SOURCE(
        kernel void test(ulong n, global int *x) {
            for(ulong i = get_global_id(0); i < n; i += get_global_size(0)) {
                x[i] = foo(x[i]);
            }
        }), "test");

What I would like to have is a documented interface that defines how a vector_view is passed to a kernel and how it should be accessed.

That is really not possible. vector_view is potentially a complex expression (something like slice[_](3 * cos(x)) is perfectly acceptable), so there is no generic way to pass the results to a custom kernel without a temporary vector.