1. Revision History
[P0019r8]

SG1 straw poll to forward to LWG for inclusion in IS20
SF F N A SA 11 14 2 1 1 SA: Reported that the ThreadSanitizer team potentially opposed this paper

20180605 ThreadSanitizer team is not opposed, SA would now vote N.
A: Believes that a TS is a better ship vehicle.


20180605 SG1
Concern about
atomic_ref
of something that is not always lock freeConsensus for no change. No objections.

Many editorial changes requested.
[P0019r7]

Update to reflect Jacksonville LWG review

Update to reference resolution of padding bits from [P0528r2]

Add a note clarifying that
atomic_ref
might not be lock free even ifatomic
is lock free 
Add wording for all member functions and specializations (in previous version only the constructor had wording)

Added reference implementation

targeted towards IS20

Convert to bikeshed

20171107 Albuquerque LEWG review

Settle on name
atomic_ref

Split out
atomic_ref<T[]>
into a separate paper, apply editorial changes accordingly 
Restore copy constructor; not assignment operator

add Throws: Nothing to constructor but do not add noexcept

Remove wrapping terminology

Address problem of CAS on
atomic_ref<T>
whereT
is a struct containing padding bits 
With these revisions move to LWG


20170301 Kona LEWG review

Merge in P0440 Floating Point Atomic View because LEWG consensus to move P0020 Floating Point Atomic to C++20 IS

Rename from
atomic_view
andatomic_array_view
; authors' selectionatomic_ref<T>
andatomic_ref<T[]>
, other name suggestedatomic_wrapper
. 
Remove
constexpr
qualification from default constructor because this qualification constrains implementations and does not add apparent value.


Remove default constructor, copy constructor, and assignment operator for tighter alignment with
atomic<T>
and prevent empty references. 
Revise syntax to align with [P0558r1], Resolving atomic
base class inconsistencies 
Recommend feature next macro

wrapper constructor strengthen requires clause and omit throws clause

Note types must be trivially copyable, as required for all atomics

20161109 Issaquah SG1 decision: move to LEWG targeting Concurrency TS V2

Align proposal with content of corresponding sections in N5131, 20160715.

Remove the one root wrapping constructor requirement from
atomic_array_view
. 
Other minor revisions responding to feedback from SG1 @ Oulu.
2. Overview
This paper proposes an extension to the atomic operations library [atomics] to allow atomic operations to apply to nonatomic objects. As required by [atomics.types.generic] the value type T must be trivially copyable.
This paper includes atomic floating point capability defined in [P0020r5].
Note: A reference implementation](https://github.com/ORNL/cppproposalspub/blob/master/P0019/atomic_ref.hpp) is available that works on compilers which support the GNU atomic builtin functions including recent versions of g++, icpc, and clang++. end note
This paper is currently targeting C++20.
3. Motivation
3.1. Atomic Operations on a Single Nonatomic Object
An atomic reference is used to perform atomic operations on a referenced nonatomic object. The intent is for atomic reference to provide the bestperforming implementation of atomic operations for the nonatomic object type. All atomic operations performed through an atomic reference on a referenced nonatomic object are atomic with respect to any other atomic reference that references the same object, as defined by equality of pointers to that object. The intent is for atomic operations to directly update the referenced object. An atomic reference constructor might acquire a resource, such as a lock from a collection of addresssharded locks, to perform atomic operations. Such atomic reference objects are not lock free and not address free. When such a resource is necessary, subsequent copy and move constructors and assignment operators might reduce overhead by copying or moving the previously acquired resource as opposed to reacquiring that resource.
Introducing concurrency within legacy code might require replacing operations on existing nonatomic objects with atomic operations such that the nonatomic object cannot be replaced with an atomic object.
An object could be heavily used nonatomically in welldefined phases of an application. Forcing such objects to be exclusively atomic would incur an unnecessary performance penalty.
3.2. Atomic Operations on Members of a Very Large Array
Highperformance computing (HPC) applications use very large arrays. Computations with these arrays typically have distinct phases that allocate and initialize members of the array, update members of the array, and read members of the array. Parallel algorithms for initialization (e.g., zero fill) have nonconflicting access when assigning member values. Parallel algorithms for updates have conflicting access to members which must be guarded by atomic operations. Parallel algorithms with readonly access require bestperforming streaming read access, random read access, vectorization, or other guaranteed nonconflicting HPC pattern.
4. Referenceability Constraints
An object referenced by an atomic reference must satisfy possibly architecturespecific constraints. For example, the object might need to be properly aligned in memory or might not be allowed to reside in GPU register memory. We do not enumerate all potential constraints or specify behavior when these constraints are violated. It is a qualityofimplementation issue to generate appropriate information when constraints are violated.
Note: Whether an implementation of atomic<T>
is lock free, does not
necessarily constrain whether the corresponding implementation of atomic_ref<T>
is lock free. end note
5. Concern with atomic<T>
and padding bits in T
A concern has been discussed for atomic<T>
where T
is a class type that
contains padding bits and how construction and compare_exchange
operations
are effected by the value of those padding bits. We require that the
resolution of padding bits follow [P0528r2].
6. Naming atomic reference of T
20180315 Suggested that atomic reference of T be named atomic_ref<T>
instead of atomic<T&>
?
Result of SG1 poll
SF  F  N  A  SA 

0  7  11  3  1 
Those against atomic<T&>
raised the concern that it allows dangerous
errors to creep into generic code, which requires users to be aware of this
edge case to avoid. Also, after an atomic<T>
is constructed it does not
have data races with other objects, while an atomic reference of T does.
Furthermore atomic<T&>
does not have volatile member functions.
Consequently, atomic<T&>
is a specialization of atomic<T>
with weaker
guarantees.
The arguments for atomic<T&>
is that it is more concise and reduces the
vocabulary terms that a user needs to know.
We decided to keep the name of an atomic reference of T as atomic_ref<T>
for two reasons. First, using the name atomic_ref<T>
removes any possibility of impacting existing generic code which uses atomic<T>
. Second, when trying to create wording for atomic<T&>
the
specializations had a distinct vector<bool>
feel where each
specialization needed to walk back from guarantees made by the primary
template. In particular, the atomic<T&>
specializations would be unable
to use the phrase "Descriptions are provided below only for members that differ from the primary template".
7. Future Work

Rewrite
atomic<T>
in terms ofatomic_ref<T>
?Result of SG1 poll
SF F N A SA 4 3 13 0 0 [P0019r8] duplicates much of the wording of
atomic<T>
and it is desirable to reduce this overhead. However, rewritingatomic<T>
would require revisiting the contentious issue ofatomic<T>
having an exposition only member of typeT
. While there is strong support for this rewrite in SG1 and LWG, we are limiting the scope of this paper to atomic reference to reduce unnecessary conflicts. 
The free functions and macros intended for C compatibility are deliberately omitted from this paper, but can be added if/when a need for them arises.
8. Proposal
The proposed changes are relative to the working draft of the standard as of [N4727].
Text in blockquotes is not proposed wordingThe � character is used to denote a placeholder section number which the editor shall determine.
[ Note: Most of the wording in this paper is duplicated from the wording foratomic<T>
. A future paper is needed to defineatomic<T>
in terms ofatomic_ref<T>
and improve the current wording for atomic objects. end note]
Apply the following changes to 32.2.� [atomics.syn]:
namespace std { // 3.� atomic ref template<class T> struct atomic_ref; // 3.� atomic ref partial specialization for pointers template<class T> struct atomic_ref<T*>; }
Add a new subsection [atomics.ref.generic] before [atomics.types.generic]
Class template atomic_ref
template<class T> struct atomic_ref { private: T* ptr; // exposition only public: using value_type = T; static constexpr bool is_always_lock_free = implementationdefined; static constexpr size_t required_alignment = implementationdefined; atomic_ref() = delete; atomic_ref& operator=(const atomic_ref&) = delete; explicit atomic_ref(T&); atomic_ref(const atomic_ref&) noexcept; T operator=(T) const noexcept; operator T() const noexcept; bool is_lock_free() const noexcept; void store(T, memory_order = memory_order_seq_cst) const noexcept; T load(memory_order = memory_order_seq_cst) const noexcept; T exchange(T, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_weak(T&, T, memory_order, memory_order) const noexcept; bool compare_exchange_strong(T&, T, memory_order, memory_order) const noexcept; bool compare_exchange_weak(T&, T, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_strong(T&, T, memory_order = memory_order_seq_cst) const noexcept; };
An atomic_ref
object applies atomic operations [atomics.general] to the object referenced by *ptr
such that, for the lifetime [basic.life]
of the atomic_ref
object, the object referenced by *ptr
is an atomic object [intro.races].
The template argument for T
shall be trivially copyable [basic.types].
The lifetime [basic.life] of an object referenced by *ptr
shall exceed the
lifetime of all atomic_ref
s that reference the object. While any atomic_ref
instances exist which reference the *ptr
object all accesses to
that object shall exclusively occur through those atomic_ref
instances.
No subobject of the object referenced by atomic_ref
shall be concurrently
referenced by any other atomic_ref
object.
Atomic operations applied to an object through a referencing atomic_ref
are atomic with respect to atomic operations applied through any other atomic_ref
referencing the same object.
[Note: Atomic operations or the atomic_ref
constructor could acquire a shared resource,
such as a lock associated with the referenced object, to enable atomic operations applied to
the referenced object.  end note]
Add a new subsubsection [atomics.ref.operations] after [atomics.ref.generic]
Operations on atomic types
static constexpr bool is_always_lockfree;
The static data member is_always_lock_free
is true
if the atomic_ref
type’s
operations are always lockfree, and false
otherwise.
static constexpr size_t required_alignment;
The required alignment of an object to be referenced by an atomic reference,
which is at least alignof(T)
.
[Note: Hardware could require that an object to be referenced
by an atomic_ref
have stricter alignment [basic.align] than other objects
of type T
. Further, whether operations on an atomic_ref
are
lockfree could depend on the alignment of the referenced object. For example,
lockfree operations on std::complex<double>
could be supported
only if aligned to 2*alignof(double)
.  end note ]
atomic_ref(T& obj);
Requires: The referenced object shall be aligned to required_alignment
.
Effects: Constructs an atomic reference that references the object.
Throws: Nothing.
atomic_ref(const atomic_ref& ref) noexcept;
Effects: Construct an atomic reference that references the object referenced by ref
.
T operator=(T desired) const noexcept;
Effects: Equivalent to:
store(desired); return desired;
operator T() const noexcept;
Effects: Equivalent to: return load();
bool is_lock_free() const noexcept;
Returns: true
if the object’s operations are lockfree, false
otherwise.
void store(T desired, memory_order order = memory_order_seq_cst) const noexcept;
Requires: The order
argument shall not be memory_order_consume
, memory_order_acquire
, nor memory_order_acq_rel
.
Effects: Atomically replaces the value referenced by *ptr
with the value of desired
. Memory is affected according to the value of order
.
T load(memory_order order = memory_order_seq_cst) const noexcept;
Requires: The order
argument shall not be memory_order_release
nor memory_order_acq_rel
.
Effects: Memory is affected according to the value of order
.
Returns: Atomically returns the value referenced by *ptr
.
T exchange(T desired, memory_order order = memory_order_seq_cst) noexcept;
Effects: Atomically replaces the value referenced by *ptr
with desired
.
Memory is affected according to the value of order
. These operations are
atomic readmodifywrite operations [intro.multithread].
Returns: Atomically returns the value referenced by *ptr
immediately before
the effects.
bool compare_exchange_weak(T& expected, T desired, memory_order success, memory_order failure) const noexcept;
bool compare_exchange_strong(T& expected, T desired, memory_order success, memory_order failure) const noexcept;
bool compare_exchange_weak(T& expected, T desired, memory_order order = memory_order_seq_cst) const noexcept;
bool compare_exchange_strong(T& expected, T desired, memory_order order = memory_order_seq_cst) const noexcept;
Requires: The failure argument shall not be memory_order_release
nor memory_order_acq_rel
.
Effects: Retrieves the value in expected
. It then atomically compares the
value referenced by *ptr
for equality with that previously
retrieved from expected
, and if true
, replaces the value referenced by *ptr
with that in desired
. If and only if the comparison is true
, memory is affected according to the value of success
, and if the
comparison is false
, memory is affected according to the value of failure
.
When only one memory_order
argument is supplied, the value of success
is order
, and the value of failure
is order
except that a value of memory_order_acq_rel
shall be replaced by the value memory_order_acquire
and a value of memory_order_release
shall be replaced by the value memory_order_relaxed
. If and only if the comparison is false
then, after the
atomic operation, the contents of the memory in expected
are replaced by the
value read from the value referenced by *ptr
during the atomic comparison.
If the operation returns true
, these operations are atomic readmodifywrite
operations [intro.races] on the value referenced by *ptr
. Otherwise, these
operations are atomic load operations on that memory.
Returns: The result of the comparison.
Remarks: A weak compareandexchange operation may fail spuriously. That is,
even when the contents of memory referred to by expected and ptr
are equal,
it may return false
and store back to expected the same memory contents that
were originally there. [ Note: This spurious failure enables implementation
of compareandexchange on a broader class of machines, e.g., loadlocked
storeconditional machines. A consequence of spurious failure is that nearly
all uses of weak compareandexchange will be in a loop. When a
compareandexchange is in a loop, the weak version will yield better
performance on some platforms. When a weak compareandexchange would require
a loop and a strong one would not, the strong one is preferable. — end note ]
Add a new subsubsection [atomics.ref.int] following the [atomics.ref.operations] subsubsection
Specializations for integral types
There are specializations of the atomic template for the integral types char
, signed char
, unsigned char
, short
, unsigned short
, int
, unsigned int
, long
, unsigned long
, long long
, unsigned long long
, char16_t
, char32_t
, wchar_t
, and
any other types needed by the typedefs in the header <cstdint>
. For each such
integral type integral, the specialization atomic_ref<integral>
provides additional
atomic operations appropriate to integral types. [ Note: For the specialization atomic_ref<bool>
, see [atomics.ref.generic]. — end note ]
template<> struct atomic_ref<integral> { private: integral* ptr; // exposition only public: using value_type = integral; using difference_type = value_type; static constexpr bool is_always_lock_free = implementationdefined; static constexpr size_t required_alignment = implementationdefined; atomic_ref() = delete; atomic_ref& operator = (const atomic_ref&) = delete; explicit atomic_ref(integral&); atomic_ref(const atomic_ref&) noexcept; integral operator=(integral) const noexcept; operator integral () const noexcept; bool is_lock_free() const noexcept; void store(integral, memory_order = memory_order_seq_cst) const noexcept; integral load(memory_order = memory_order_seq_cst) const noexcept; integral exchange(integral, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_weak(integral&, integral, memory_order, memory_order) const noexcept; bool compare_exchange_strong(integral&, integral, memory_order, memory_order) const noexcept; bool compare_exchange_weak(integral&, integral, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_strong(integral&, integral, memory_order = memory_order_seq_cst) const noexcept; integral fetch_add(integral, memory_order = memory_order_seq_cst) const noexcept; integral fetch_sub(integral, memory_order = memory_order_seq_cst) const noexcept; integral fetch_and(integral, memory_order = memory_order_seq_cst) const noexcept; integral fetch_or(integral, memory_order = memory_order_seq_cst) const noexcept; integral fetch_xor(integral, memory_order = memory_order_seq_cst) const noexcept; integral operator++(int) const noexcept; integral operator(int) const noexcept; integral operator++() const noexcept; integral operator() const noexcept; integral operator+=(integral) const noexcept; integral operator=(integral) const noexcept; integral operator&=(integral) const noexcept; integral operator=(integral) const noexcept; integral operator^=(integral) const noexcept; };
Descriptions are provided below only for members that differ from the primary template.
The following operations perform arithmetic computations. The key, operator, and computation correspondence are identified in Table 130 [atomics.types.int].
integral fetch_key(integral operand, memory_order order = memory_order_seq_cst) const noexcept;
Effects: Atomically replaces the value referenced by *ptr
with the result
of the computation applied to the value referenced by *ptr
and the given operand
. Memory is affected according to the value of order
. These operations
are atomic readmodifywrite operations [intro.races].
Returns: Atomically, the value referenced by *ptr
immediately before the
effects.
Remarks: For signed integer types, arithmetic is defined to use two’s complement representation. There are no undefined results.
integral operator op=(integral operand) const noexcept;
Effects: Equivalent to: return fetch_key(operand) op operand;
Add a new subsubsection [atomics.ref.float] following the [atomics.ref.int] subsubsection
Specializations for floatingpoint types
There are specializations of the atomic_ref
template for the floatingpoint types float
, double
, and long double
. For each such floatingpoint type floatingpoint, the specialization atomic_ref<floatingpoint>
provides additional atomic operations appropriate to floatingpoint types.
template<> struct atomic_ref<floatingpoint> { private: floatingpoint* ptr; // exposition only public: using value_type = floatingpoint; using difference_type = value_type; static constexpr bool is_always_lock_free = implementationdefined; static constexpr size_t required_alignment = implementationdefined; atomic_ref() = delete; atomic_ref& operator = (const atomic_ref&) = delete; explicit atomic_ref(floatingpoint&); atomic_ref(const atomic_ref&) noexcept; floatingpoint operator=(floatingpoint) noexcept; operator floatingpoint () const noexcept; bool is_lock_free() const noexcept; void store(floatingpoint, memory_order = memory_order_seq_cst) const noexcept; floatingpoint load(memory_order = memory_order_seq_cst) const noexcept; floatingpoint exchange(floatingpoint, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_weak(floatingpoint&, floatingpoint, memory_order, memory_order) const noexcept; bool compare_exchange_strong(floatingpoint&, floatingpoint, memory_order, memory_order) const noexcept; bool compare_exchange_weak(floatingpoint&, floatingpoint, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_strong(floatingpoint&, floatingpoint, memory_order = memory_order_seq_cst) const noexcept; floatingpoint fetch_add(floatingpoint, memory_order = memory_order_seq_cst) const noexcept; floatingpoint fetch_sub(floatingpoint, memory_order = memory_order_seq_cst) const noexcept; floatingpoint operator+=(floatingpoint) const noexcept; floatingpoint operator=(floatingpoint) const noexcept; };
Descriptions are provided below only for members that differ from the primary template.
The following operations perform arithmetic computations. The key, operator, and computation correspondence are identified in Table 130 [atomics.types.int].
floatingpoint fetch_key(floatingpoint operand, memory_order order = memory_order_seq_cst) const noexcept;
Effects: Atomically replaces the value referenced by *ptr
with the result
of the computation applied to the value referenced by *ptr
and the given operand
. Memory is affected according to the value of order
. These operations
are atomic readmodifywrite operations [intro.races].
Returns: Atomically, the value referenced by *ptr
immediately before the
effects.
Remarks: If the result is not a representable value for its type [expr.pre] the
result is unspecified, but the operations otherwise have no undefined
behavior. Atomic arithmetic operations on floatingpoint should conform to
the std::numeric_limits<floatingpoint>
traits associated with the floatingpoint type [limits.syn]. The floatingpoint environment [cfenv] for atomic
arithmetic operations on floatingpoint may be different than the calling
thread’s floatingpoint environment.
floatingpoint operator op=(floatingpoint operand) const noexcept;
Effects: Equivalent to: return fetch_key(operand) op operand;
Add a new subsubsection [atomics.ref.pointer] following the [atomics.ref.float] subsubsection
Partial specialization for pointers
template<class T> struct atomic_ref<T*> { private: T** ptr; // exposition only public: using value_type = T*; using difference_type = ptrdiff_t; static constexpr bool is_always_lock_free = implementationdefined; static constexpr size_t required_alignment = implementationdefined; atomic_ref() = delete; atomic_ref& operator = (const atomic_ref&) = delete; explicit atomic_ref(T*&); atomic_ref(const atomic_ref&) noexcept; T* operator=(T*) const noexcept; operator T* () const noexcept; bool is_lock_free() const noexcept; void store(T*, memory_order = memory_order_seq_cst) const noexcept; T* load(memory_order = memory_order_seq_cst) const noexcept; T* exchange(T*, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_weak(T*&, T*, memory_order, memory_order) const noexcept; bool compare_exchange_strong(T*&, T*, memory_order, memory_order) const noexcept; bool compare_exchange_weak(T*&, T*, memory_order = memory_order_seq_cst) const noexcept; bool compare_exchange_strong(T*&, T*, memory_order = memory_order_seq_cst) const noexcept; T* fetch_add(difference_type, memory_order = memory_order_seq_cst) const noexcept; T* fetch_sub(difference_type, memory_order = memory_order_seq_cst) const noexcept; T* operator++(int) const noexcept; T* operator(int) const noexcept; T* operator++() const noexcept; T* operator() const noexcept; T* operator+=(difference_type) const noexcept; T* operator=(difference_type) const noexcept; };
Descriptions are provided below only for members that differ from the primary template.
The following operations perform arithmetic computations. The key, operator, and computation correspondence are identified in Table 130 [atomics.types.pointer].
T* fetch_key(difference_type operand, memory_order order = memory_order_seq_cst) const noexcept;
Requires: T shall be an object type, otherwise the program is illformed
Effects: Atomically replaces the value referenced by *ptr
with the result
of the computation applied to the value referenced by *ptr
and the given operand
. Memory is affected according to the value of order
. These operations
are atomic readmodifywrite operations [intro.races].
Returns: Atomically, the value referenced by *ptr
immediately before the
effects.
Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.
T* operator op=(difference_type operand) const noexcept;
Effects: Equivalent to: return fetch_key(operand) op operand;
Add a new subsubsection [atomics.ref.memberops] following the [atomics.ref.pointer] subsubsection
Member operators of atomic_ref
common to integers and
pointers to objects
T* operator++(int) const noexcept;
Effects: Equivalent to: return fetch_add(1);
T* operator(int) const noexcept;
Effects: Equivalent to: return fetch_sub(1);
T* operator++() const noexcept;
Effects: Equivalent to: return fetch_add(1) + 1;
T* operator(int) const noexcept;
Effects: Equivalent to: return fetch_sub(1)  1;
9. Feature Testing
The __cpp_lib_atomic_ref
feature test macro should be added.