buffer::get_access<access::mode::read> should be marked const.

[j-stephan]

The Problem

Calling get_access in read-only mode requires the buffer to be non-const since get_access itself is never marked as const. This is a problem for const data structures which encapsulate a SYCL buffer. It also contradicts const-correctness which is strongly advocated for by the C++ community. Finally it is counterintuitive for the programmer.

Example

#include <CL/sycl.hpp>

int main()
{
    auto queue = cl::sycl::queue{};

    const auto in = cl::sycl::buffer<int, 1>{1024};
    auto out = cl::sycl::buffer<int, 1>{1024};

    queue.submit([&](cl::sycl::handler& cgh)
    {
        // candidate function not viable: method is not marked const
        auto in_acc = in.get_access<cl::sycl::access::mode::read>(cgh);
        auto out_acc = out.get_access<cl::sycl::access::mode::write>(cgh);

        cgh.copy(in_acc, out_acc);
    });

    return 0;
}

Use case

Any const data structure encapsulating a SYCL buffer.

Possible solutions

1. Mark get_access const for access::mode::read. I'd be happy to open a pull-request for this if needed.
2. Add wording to get_access's specification which explains why it cannot be marked const.

[keryell]

This seems reasonable and the working group is looking at the details.

But we agreed that you can make a pull request. Thanks!

[AerialMantis]

SYCL working group:
The get_access member function could potentially be marked const, however, implementations may modify the state of the buffer even if it is const. This definitely requires clarification, so the working group will investigate this further.

[keryell]

That means having mutable stuff inside the buffer, which is actually done in triSYCL I think for some members.

[fraggamuffin]

Mar 22
looking for more feedback from implementers.

If feedback is ok then will
@j-stephan please do a PR along the line of solution 1:
Mark get_access const for access::mode::read. I'd be happy to open a pull-request for this if needed.

[fraggamuffin]

if we change here, we should also change accessor constructors since they don't take const ref, just a non-const

[LukeTheWalker]

Any update on this?

[keryell]

Any update on this?

No.