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Execution Policies

Execution Policies#

C++ Standard Aligned Execution Policies#

oneDPL has the set of execution policies semantically aligned with the C++ Standard, 6th edition (C++20):

// Defined in <oneapi/dpl/execution>

namespace oneapi {
  namespace dpl {
    namespace execution {

      class sequenced_policy { /*unspecified*/ };
      class parallel_policy { /*unspecified*/ };
      class parallel_unsequenced_policy { /*unspecified*/ };
      class unsequenced_policy { /*unspecified*/ };

      inline constexpr sequenced_policy seq { /*unspecified*/ };
      inline constexpr parallel_policy par { /*unspecified*/ };
      inline constexpr parallel_unsequenced_policy par_unseq { /*unspecified*/ };
      inline constexpr unsequenced_policy unseq { /*unspecified*/ };

    }
  }
}

See “Execution policies” in the C++ Standard for more information.

Note

The std::is_execution_policy type trait resolves to std::false_type for oneDPL execution policies. Implementations and programs should instead use the oneDPL type trait.

Device Execution Policies#

A device execution policy represents a SYCL device and queue to run oneDPL algorithms.

// Defined in <oneapi/dpl/execution>

namespace oneapi {
  namespace dpl {
    namespace execution {

      template <typename KernelName = /*unspecified*/>
      class device_policy;

      inline const device_policy<> dpcpp_default;

      template <typename KernelName = /*unspecified*/>
      device_policy<KernelName>
      make_device_policy( sycl::queue );

      template <typename KernelName = /*unspecified*/>
      device_policy<KernelName>
      make_device_policy( sycl::device );

      template <typename NewKernelName, typename OldKernelName>
      device_policy<NewKernelName>
      make_device_policy( const device_policy<OldKernelName>& = dpcpp_default );

    }
  }
}

device_policy Class#

template <typename KernelName = /*unspecified*/>
class device_policy
{
public:
    using kernel_name = KernelName;

    device_policy();
    template <typename OtherName>
    device_policy( const device_policy<OtherName>& );
    explicit device_policy( sycl::queue );
    explicit device_policy( sycl::device );

    sycl::queue queue() const;
    operator sycl::queue() const;
};

An object of the device_policy type is associated with a sycl::queue that is used to run algorithms on a SYCL device. When an algorithm runs with device_policy it is capable of processing SYCL buffers (passed via oneapi::dpl::begin/end), data in the host memory and data in Unified Shared Memory (USM), including device USM. Data placed in the host memory and USM can be passed to oneDPL algorithms as pointers and random access iterators. The way to transfer data from the host memory to a device and back is unspecified; per-element data movement to/from a temporary storage is a possible valid implementation.

The KernelName template parameter, also aliased as kernel_name within the class template, is to explicitly provide a name for SYCL kernels executed by an algorithm the policy is passed to.

The device_policy type is copy-constructible, copy-assignable, move-constructible and move-assignable. A policy instance constructed as or assigned a copy of another instance is associated with a sycl::queue that compares equal, as defined by SYCL, to the queue of that other instance. A policy instance constructed as or assigned a move of another instance is associated with the queue of that other instance without copying it. It is unspecified whether the moved-from policy instance is associated with any queue after the move.

device_policy()

Construct a policy object associated with a queue created with the default device selector.

template <typename OtherName>
device_policy( const device_policy<OtherName>& policy )

Construct a policy object as if by copying policy and changing the kernel name to kernel_name defined for the new policy.

explicit device_policy( sycl::queue queue )

Construct a policy object associated with the given queue.

explicit device_policy( sycl::device device )

Construct a policy object associated with a queue created for the given device.

sycl::queue queue() const

Return the queue the policy is associated with.

operator sycl::queue() const

Allow implicit conversion of the policy to a sycl::queue object.

Predefined Device Policy#

dpcpp_default is a predefined execution policy object to run algorithms on the default SYCL device. It is a global immutable (const) instance of type device_policy<>. [Note: dpcpp_default can be copied but cannot be moved. – end note]

make_device_policy Function#

The make_device_policy function templates simplify device_policy creation.

template <typename KernelName = /*unspecified*/>
device_policy<KernelName>
make_device_policy( sycl::queue queue )

Return a policy object associated with queue, with a kernel name possibly provided as the template argument, otherwise unspecified.

template <typename KernelName = /*unspecified*/>
device_policy<KernelName>
make_device_policy( sycl::device device )

Return a policy object to run algorithms on device, with a kernel name possibly provided as the template argument, otherwise unspecified.

template <typename NewKernelName, typename OldKernelName>
device_policy<NewKernelName>
make_device_policy( const device_policy<OldKernelName>& policy = dpcpp_default )

Return a policy object constructed from policy, with a new kernel name provided as the template argument. If no policy object is provided, the new policy is constructed from dpcpp_default.

Execution Policy Type Trait#

oneDPL provides type trait utilities to detect its execution policy types at compile time for the purpose of function overload resolution:

// Defined in <oneapi/dpl/execution>

namespace oneapi {
  namespace dpl {

    template <class T>
    struct is_execution_policy { /*see below*/ };

    template <class T>
    inline constexpr bool is_execution_policy_v = oneapi::dpl::is_execution_policy<T>::value;

    namespace execution {

      template <class T>
      struct is_execution_policy { /*see below*/ };

      template <class T>
      inline constexpr bool is_execution_policy_v = oneapi::dpl::execution::is_execution_policy<T>::value;

    }
  }
}

oneapi::dpl::is_execution_policy and oneapi::dpl::execution::is_execution_policy must be treated as name aliases to the same class template. It is unspecified, which namespace contains the class template definition and its specializations.

Note

The oneapi::dpl::execution::is_execution_policy class originated in the oneDPL specification version 1.0, while oneapi::dpl::is_execution_policy has been added later to better align with the C++ standard.

For new code, use of the type trait utilities in namespace oneapi::dpl is highly recommended. Those in namespace oneapi::dpl::execution are provided for backward compatibility and may be deprecated in the future.

is_execution_policy<T> must have the characteristics of std::true_type if T is one of the above specified or implementation-defined oneDPL execution policy types, otherwise it must have the characteristics of std::false_type. Following the C++ Standard, is_execution_policy<T> does not automatically strip references and cv-qualifiers from its template argument. [Note: Use it with std::decay_t<T> or similar type transformation utilities. – end note]

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