sycl_ext_oneapi_weak_object.asciidoc

sycl_ext_oneapi_weak_object

Notice

Copyright © 2022-2022 Intel Corporation. All rights reserved.

Khronos® is a registered trademark and SYCL™ and SPIR™ are trademarks of The Khronos Group Inc. OpenCL™ is a trademark of Apple Inc. used by permission by Khronos.

Contact

To report problems with this extension, please open a new issue at:

https://github.com/intel/llvm/issues

Dependencies

This extension is written against the SYCL 2020 revision 6 specification. All references below to the "core SYCL specification" or to section numbers in the SYCL specification refer to that revision.

Status

This extension is implemented and fully supported by DPC++.

Overview

SYCL 2020 has a number of classes with common reference semantics, which mean that copies of objects of these classes will reference the same objects and will be considered the same. As a result, these objects will only be fully destroyed when all copies have been destroyed. However, there is currently no way to have a reference to an object with these semantics without keeping the object alive.

This extension adds the notion of a "weak" SYCL object, represented by the weak_object class. A weak object holds a reference to a SYCL object with common reference semantics of the specified type but will not keep the underlying object alive. A weak object is considered expired if the underlying object is no longer alive. If a weak object has not expired, a copy of the underlying object can be created by calling either lock or try_lock.

Additionally this extension adds the notion of owner-based ordering to allow for use of SYCL object and their weak variants in order-oriented data structures and operations.

Specification

Feature test macro

This extension provides a feature-test macro as described in the core SYCL specification. An implementation supporting this extension must predefine the macro SYCL_EXT_ONEAPI_WEAK_OBJECT to one of the values defined in the table below. Applications can test for the existence of this macro to determine if the implementation supports this feature, or applications can test the macro’s value to determine which of the extension’s features the implementation supports.

Value	Description
1	Initial version of this extension.

The new weak_object class

This extension adds the following class:

namespace sycl {
namespace ext {
namespace oneapi {

template <typename SyclObject>
class weak_object {
public:
  using object_type = SyclObject;

  constexpr weak_object() noexcept;
  weak_object(const SyclObject &SYCLObj) noexcept;
  weak_object(const weak_object &Other) noexcept;
  weak_object(weak_object &&Other) noexcept;

  weak_object &operator=(const SyclObject &SYCLObj) noexcept;
  weak_object &operator=(const weak_object &Other) noexcept;
  weak_object &operator=(weak_object &&Other) noexcept;

  void reset() noexcept;
  void swap(weak_object &Other) noexcept;

  bool expired() const noexcept;
  std::optional<SyclObject> try_lock() const noexcept;
  SyclObject lock() const;

  bool owner_before(const weak_object &Other) const noexcept;
  bool owner_before(const SyclObject &Other) const noexcept;
};

} // namespace oneapi
} // namespace ext
} // namespace sycl

The methods of the new weak_object class have the following semantics:

Member Function	Description
constexpr weak_object() noexcept;	Constructor for creating an empty weak_object.
weak_object(const SyclObject &SYCLObj) noexcept; weak_object(const weak_object &Other) noexcept;	Copy constructor.
weak_object(weak_object &&Other) noexcept;	Move constructor.
weak_object &operator=(const SyclObject &SYCLObj) noexcept; weak_object &operator=(const weak_object &Other) noexcept;	Copy assignment operator.
weak_object &operator=(weak_object &&Other) noexcept;	Move assignment operator.
void reset() noexcept;	Releases the reference to the underlying SYCL object.
void swap(weak_object &Other) noexcept;	Exchanges the reference to the underlying SYCL object with the one held by Other.
bool expired() const noexcept;	Returns true if the underlying SYCL object has been destroyed or its destruction is imminent. Otherwise returns false.
std::optional<SyclObject> try_lock() const noexcept;	Attempts to return the underlying SYCL object. Returns std::nullopt if the weak_object is empty or if expired() returns true.
SyclObject lock() const;	Returns the underlying SYCL object. Throws an exception with the errc::invalid error code if the weak_object is empty or if expired() returns true.
bool owner_before(const weak_object &Other) const noexcept; bool owner_before(const SyclObject &Other) const noexcept;	Checks whether this weak_object precedes Other in the implementation-defined owner-based order. The order is defined such that two objects defining this ordering compare equivalent if both are weak_object without an underlying SYCL object or if both reference the same SYCL object.

Additionally the following members are added to the members in SYCL classes with common reference semantics:

namespace sycl {

// Where T is a SYCL type with common reference semantics.
class T {
  ...

public:
  ...

  bool ext_oneapi_owner_before(const ext::oneapi::weak_object<T> &Other) const noexcept;
  bool ext_oneapi_owner_before(const T &Other) const noexcept;
};

} // namespace sycl

These new methods have the following semantics:

Member Function	Description
bool ext_oneapi_owner_before(const ext::oneapi::weak_object<T> &Other) const noexcept; bool ext_oneapi_owner_before(const T &Other) const noexcept;	Checks whether this SYCL object precedes Other in the implementation-defined owner-based order. The order is defined such that two objects defining this ordering compare equivalent if both are weak_object without an underlying SYCL object or if both reference the same SYCL object.

The owner_less function object is added with the following specializations:

namespace sycl {
namespace ext {
namespace oneapi {

template <typename SyclObject> struct owner_less;

// Where T is a SYCL type with common reference semantics.
template <> struct owner_less<T> {
  bool operator()(const T &lhs, const T &rhs) const noexcept;
  bool operator()(const weak_object<T> &lhs,
                  const weak_object<T> &rhs) const noexcept;
  bool operator()(const T &lhs, const weak_object<T> &rhs) const noexcept;
  bool operator()(const weak_object<T> &lhs, const T &rhs) const noexcept;
};

} // namespace oneapi
} // namespace ext
} // namespace sycl

The operator overloads of the new owner_less function object have the following semantics:

Member Function	Description
bool operator()(const T &lhs, const T &rhs) const noexcept; bool operator()(const weak_object<T> &lhs, const weak_object<T> &rhs) const noexcept; bool operator()(const T &lhs, const weak_object<T> &rhs) const noexcept; bool operator()(const weak_object<T> &lhs, const T &rhs) const noexcept;	Compares lhs and rhs using owner-based semantics, similar to calling owner_before on weak_object or ext_oneapi_owner_before on SYCL objects. lhs and rhs are equivalent if they both reference the same SYCL object or if they are both empty weak_object instances.

The weak_object class, the ext_oneapi_owner_before member functions and the owner_less function object type must not be used in device code.