Assigning an item element

SYCL development
1

In the SYCL 1.2.1 spec, the SYCL item class includes the following operator definition:

size_t &operator[](int dimension);

This is described as returning the constituent id l-value. In ComputeCpp 1.1.2’s item_base.h I can only see:

size_t operator[](int dimension) const;

This means the following (say) will not compile:

#include <CL/sycl.hpp>

namespace kernels { class K; }

int main(int argc, char *argv[])
{
  using namespace cl::sycl;
  queue q;

  q.submit([&](handler &cgh) {
    cgh.parallel_for<kernels::K>(range<3>{4,4,4}, [=](item<3> ix) {
      ix[0] = 0;
    });
  });

  return 0;
}

Indeed it’s unlikely that such an assignment would be a good idea, but it seems to be allowed in the specification. Do you think the spec will change on this point?

2

Hi Paul,

So the id and range classes do allow you to modify the elements via the subscript operator if the object is non-const. However, the intention of SYCL 1.2.1 is that the id and range objects retrieved via the kernel invocation API are read-only, so the item subscript operator and get_id member function return by value.

I believe the function you mentioned above is on the id and range class but not on the item class.

The example above could be changed to:

q.submit([&](handler &cgh) {
  cgh.parallel_for<kernels::K>(range<3>{4,4,4}, [=](item<3> ix) {
    auto id = ix.get_id();
    id[0] = 0;
  });
});

However this would not modify the id within the item object, just the local copy.

I hope this helps.

Gordon

3

Isn’t the operator[] method I mentioned included on listing line 13 on page 150 as a member of item in the current (1.2.1) spec?
Thanks,
Paul

4

Hi Paul,

The item class does have the subscript operator you mention, though it returns by-value, so you cannot assign to the result.

The subscript operator on the id class has the same operator but returns size_t & so you can modify it as you describe.

I noticed the page number you mentioned didn’t quite line up with what I have, perhaps you have a slightly older version of the spec, we have been releasing regular updates with errata fixes. You can find the latest in the Khronos registry here: https://www.khronos.org/registry/SYCL/specs/sycl-1.2.1.pdf.

I hope this helps,

Gordon

5

Hi Gordon,

Thanks. Yes, I was looking at SYCL 1.2.1 Revision 3 from July 19, 2018. I see that Revision 5 is available since April 18 this year, and the operator[] definition is changed as you describe.

Cheers,
Paul