SYCL named function objects compile error ( Needs keyword "const" for operator () )

[Sidarth]

Hello,

I am trying to compile a simple SYCL program which runs parallel for using "Named function objects" instead of  lambdas. I get a compile error which ultimately is resolved if I add "const" to the end of the overloaded () operator and I am not sure why that is the case. Could someone please point out why the "const" qualifier is needed and if there are any ways around it ?

#include <iostream>
#include <sycl/sycl.hpp>

#define SIZE 10
#define DIV 12.3

using namespace sycl;

class test_kernel
{
public:
   test_kernel(int* xx, const double yy):x(xx), y(yy)
   {}

   void operator() (nd_item<3> itm)
   {
      int idx = itm.get_global_id(0);
      if(idx < SIZE)
        x[idx] = y/12.3;
   }

private:
   int* x;
   double y;

};

int main()
{
    queue m_que{default_selector_v};
    range<3> global_range(SIZE,1,1);
    range<3> local_range(SIZE,1,1);
    std::cout << "Device Name: " << m_que.get_device().get_info<info::device::name>() << std::endl;
    
    double val = DIV * 1000;
    int* x = malloc_shared<int>(SIZE, m_que);
    
    for(int i =0; i<SIZE; i++)
    {
        x[i] = 10;
    }
    
    m_que.parallel_for(sycl::nd_range<3>{global_range,local_range}, test_kernel{x, val}).wait();
    
    for(int i = 0; i < SIZE; i++)
    {
        if(x[i] != 1000)
        {
            std::cout << "x[ " << i << "] = " << x[i] << " instead of " << int(val/DIV) << std::endl;
        }
    }
    
    free(x, m_que);
}

 
Here is the compile command and error:

snarayanan@crane:builds$ icpx -fsycl -o submit_to_gpu_complex ../src/submit_to_gpu_complex.cpp
In file included from ../src/submit_to_gpu_complex.cpp:2:
In file included from /usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/sycl.hpp:16:
In file included from /usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/backend.hpp:27:
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:1540:5: error: no matching function for call to object of type 'const test_kernel'
1540 |     KernelFunc(detail::Builder::getElement(detail::declptr<ElementType>()));
|     ^~~~~~~~~~
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:1617:10: note: in instantiation of function template specialization 'sycl::handler::kernel_parallel_for<test_kernel, sycl::nd_item<3>, test_kernel>' requested here
1617 |       h->kernel_parallel_for<TypesToForward..., Props...>(Args...);
|          ^
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:1703:29: note: in instantiation of function template specialization 'sycl::handler::KernelPropertiesUnpackerImpl<>::kernel_parallel_for_unpack<test_kernel, sycl::nd_item<3>, test_kernel, test_kernel>' requested here
1703 |           Unpacker.template kernel_parallel_for_unpack<KernelName, ElementType,
|                             ^
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:1699:5: note: in instantiation of function template specialization 'sycl::handler::unpack<test_kernel, test_kernel, sycl::ext::oneapi::experimental::properties<std::tuple<>>, false, (lambda at /usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:1702:21)>' requested here
1699 |     unpack<KernelName, KernelType, PropertiesT,
|     ^
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:1373:5: note: in instantiation of function template specialization 'sycl::handler::kernel_parallel_for_wrapper<test_kernel, sycl::nd_item<3>, test_kernel, sycl::ext::oneapi::experimental::properties<std::tuple<>>>' requested here
1373 |     kernel_parallel_for_wrapper<NameT, TransformedArgType, KernelType,
|     ^
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/handler.hpp:2370:5: note: in instantiation of function template specialization 'sycl::handler::parallel_for_impl<sycl::detail::auto_name, test_kernel, 3, sycl::ext::oneapi::experimental::properties<std::tuple<>>>' requested here
2370 |     parallel_for_impl<KernelName>(Range, Properties, std::move(KernelFunc));
|     ^
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/queue.hpp:2504:24: note: in instantiation of function template specialization 'sycl::handler::parallel_for<sycl::detail::auto_name, test_kernel, sycl::ext::oneapi::experimental::properties<std::tuple<>>, 3>' requested here
2504 |           CGH.template parallel_for<KernelName>(Range, Properties, Rest...);
|                        ^
/usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../../include/sycl/queue.hpp:2519:12: note: in instantiation of function template specialization 'sycl::queue::parallel_for<sycl::detail::auto_name, 3, sycl::ext::oneapi::experimental::properties<std::tuple<>>, test_kernel &>' requested here
2519 |     return parallel_for<KernelName>(
|            ^
../src/submit_to_gpu_complex.cpp:43:11: note: in instantiation of function template specialization 'sycl::queue::parallel_for<sycl::detail::auto_name, 3, test_kernel>' requested here
43 |     m_que.parallel_for(sycl::nd_range<3>{global_range,local_range}, test_kernel{x, val}).wait();
|           ^
../src/submit_to_gpu_complex.cpp:15:9: note: candidate function not viable: 'this' argument has type 'const test_kernel', but method is not marked const
15 |    void operator() (nd_item<3> itm)
|         ^
1 error generated.

 DPC++ compiler version:

snarayanan@crane:builds$ icpx --version
Intel(R) oneAPI DPC++/C++ Compiler 2024.1.0 (2024.1.0.20240308)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler
Configuration file: /usr/people/shared/tools/rocky/9/intel_oneapi/2024.1.0/compiler/2024.1/bin/compiler/../icpx.cfg

[Alex_Y_Intel]

I'm escalating your question to our internal team for further investigation.