How to create an array of cl::sycl::buffers? - c++

Question

I am using the Xilinx's triSYCL github implementation,https://github.com/triSYCL/triSYCL.
I am trying to create a design with 100 producer/consumer to read/write from 100 pipes.
What I am not sure of is, How to create an array of cl::sycl::buffer and initialize it using std::iota.
Here is my code:
constexpr size_t T=6;
constexpr size_t n_threads=100;
cl::sycl::buffer<float, n_threads> a { T };
for (int i=0; i<n_threads; i++)
{
auto ba = a[i].get_access<cl::sycl::access::mode::write>();
// Initialize buffer a with increasing integer numbers starting at 0
std::iota(ba.begin(), ba.end(), i*T);
}
And I am getting the following error:
error: no matching function for call to ‘cl::sycl::buffer<float, 2>::buffer(<brace-enclosed initializer list>)’
cl::sycl::buffer<float, n_threads> a { T };
I am new to C++ programming. So I am not able to figure out the exact way to do this.

Answer 1

There are 2 points I think cause the issue you are currently having:
The 2nd template argument in the buffer object definition should be the dimensionality of the buffer (count of dimensions, should be 1, 2 or 3), not the dimensions themselves.
The constructor for the buffer should contain either the actual dimensions of the buffer, or the data that you want the buffer to have and the dimensions. To pass the dimensions, you need to pass a cl::sycl::range object to the constructor
As I understand you are trying to initialize a buffer of dimensionality 1 and with dimensions { 100, 1, 1 }. To do this, the definition of a should change to:
cl::sycl::buffer < float, 1 > a(cl::sycl::range< 1 >(n_threads));
Also, as the dimensionality can be deduced from the range template parameter, thus you can achieve the same effect with:
cl::sycl::buffer< float > a (cl::sycl::range< 1 >(n_threads));
As for initializing the buffer with std::iota, you have 3 options:
Use an array to initialize the data with the iota usage and pass them to the sycl buffer (case A),
Use the accessor to write to the buffer directly for host - CPU only (case B), or
Use an accessor with a parallel_for for execution on either host or an OpenCL device (case C).
Accessors should not be used as iterators (with .begin(), .end())
Case A:
std::vector<float> data(n_threads); // or std::array<float, n_threads> data;
std::iota(data.begin(), data.end(), 0); // this will create the data { 0, 1, 2, 3, ... }
cl::sycl::buffer<float> a(data.data(), cl::sycl::range<1>(n_threads));
// The data in a are already initialized, you can create an accessor to use them directly
Case B:
cl::sycl::buffer<float> a(cl::sycl::range<1>(n_threads));
{
auto ba = a.get_access<cl::sycl::access::mode::write>();
for(size_t i=0; i< n_threads; i++) {
ba[i] = i;
}
}
Case C:
cl::sycl::buffer<float> a(cl::sycl::range<1>(n_threads));
cl::sycl::queue q{cl::sycl::default_selector()}; // create a command queue for host or device execution
q.Submit([&](cl::sycl::handler& cgh) {
auto ba = a.get_access<cl::sycl::access::mode::write>();
cgh.parallel_for<class kernel_name>([=](cl::sycl::id<1> i){
ba[i] = i.get(0);
});
});
q.wait_and_throw(); // wait until kernel execution completes
Also check chapter 4.8 of the SYCL 1.2.1 spec https://www.khronos.org/registry/SYCL/specs/sycl-1.2.1.pdf as it has an example for iota

Answer 2

Disclaimer: triSYCL is a research project for now. Please use ComputeCpp for anything serious. :-)
If you really need arrays of buffer, I guess you can use something similar to Is there a way I can create an array of cl::sycl::pipe?
As a variant, you can use a std::vector<cl::sycl::buffer<float>> or std::array<cl::sycl::buffer<float>, n_threads> and initialize with a loop from a cl::sycl::buffer<float> { T }.