.. SPDX-FileCopyrightText: 2019-2022 Intel Corporation .. SPDX-FileCopyrightText: Contributors to the oneAPI Specification project. .. .. SPDX-License-Identifier: CC-BY-4.0 Execution Policies ------------------ C++ Standard Aligned Execution Policies +++++++++++++++++++++++++++++++++++++++ oneDPL has the set of execution policies semantically aligned with the `C++ Standard`_, 6th edition (C++20): .. code:: cpp // Defined in 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 :ref:`oneDPL type trait `. Device Execution Policies +++++++++++++++++++++++++ A device execution policy represents a `SYCL`_ device and queue to run oneDPL algorithms. .. code:: cpp // Defined in namespace oneapi { namespace dpl { namespace execution { template class device_policy; inline const device_policy<> dpcpp_default; template device_policy make_device_policy( sycl::queue ); template device_policy make_device_policy( sycl::device ); template device_policy make_device_policy( const device_policy& = dpcpp_default ); } } } device_policy Class ^^^^^^^^^^^^^^^^^^^ .. code:: cpp template class device_policy { public: using kernel_name = KernelName; device_policy(); template device_policy( const device_policy& ); 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`` or other :doc:`buffer wrappers `), 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, including :doc:`oneDPL 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. .. code:: cpp device_policy() Construct a policy object associated with a queue created with the default device selector. .. code:: cpp template device_policy( const device_policy& policy ) Construct a policy object as if by copying ``policy`` and changing the kernel name to ``kernel_name`` defined for the new policy. .. code:: cpp explicit device_policy( sycl::queue queue ) Construct a policy object associated with the given queue. .. code:: cpp explicit device_policy( sycl::device device ) Construct a policy object associated with a queue created for the given device. .. code:: cpp sycl::queue queue() const Return the queue the policy is associated with. .. code:: cpp 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. .. code:: cpp template device_policy 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. .. code:: cpp template device_policy 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. .. code:: cpp template device_policy make_device_policy( const device_policy& 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``. .. _exec-policy-type-trait: 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: .. code:: cpp // Defined in namespace oneapi { namespace dpl { template struct is_execution_policy { /*see below*/ }; template inline constexpr bool is_execution_policy_v = oneapi::dpl::is_execution_policy::value; namespace execution { template struct is_execution_policy { /*see below*/ }; template inline constexpr bool is_execution_policy_v = oneapi::dpl::execution::is_execution_policy::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`` 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`` does not automatically strip references and cv-qualifiers from its template argument. [*Note*: Use it with ``std::decay_t`` or similar type transformation utilities. -- *end note*] .. _`C++ Standard`: https://isocpp.org/std/the-standard .. _`SYCL`: https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html