Document Number: P3323R1.
Date: 2024-11-18.
Reply to: Gonzalo Brito Gadeschi <gonzalob _at_ nvidia.com>.
Authors: Gonzalo Brito Gadeschi, Lewis Baker.
Audience: SG1.

cv-qualified types in atomic and atomic_ref

• cv-qualified types in atomic and atomic_ref
  • Summary
  • Motivation
  • Resolution for std::atomic
  • Resolution for std::atomic_ref

Changelog

R1 LWG review:

• Constraint notify_one and notify_all for is_const_v<T> is false.
• Minor wording changes.

Summary

Addresses LWG#4069 and LWG#3508 by clarifying that cv-qualified types are not supported by std::atomic<T> and specifying how these are supported by std::atomic_ref<T>.

Motivation

CWG#2094 made is_trivially_copyable_v<volatile ...-type> (integer, pointer, floating-point) true, leading to LWG#3508 and LWG#4069.

Supporting atomic_ref<volatile T> can be useful for atomically accessing objects of type T stored in shared-memory where the object was not created as an atomic<T>.

Resolution for std::atomic

std::atomic<...-type> specializations only apply for cv-unqualified types.

• Proposed resolution: restrict std::atomic<T> to types T for which
  same_as<T, remove_cv_t<T>> is true.
• Rationale: atomic<volatile int> use case is served by volatile atomic<int>, i.e., there is no need to support atomic<volatile T>.
• Impact: libstdc++ and libc++ can't compile atomic<volatile T> already. MSVC can, but usage is limited, e.g., because fetch_add only exists on specialization, not primary template.
• Proposed wording:

Modify [atomics.types.generic.general]:

The template argument for T shall meet the Cpp17CopyConstructible and Cpp17CopyAssignable requirements. The program is ill-formed if any of

1. is_trivially_copyable_v<T>,
2. is_copy_constructible_v<T>,
3. is_move_constructible_v<T>,
4. is_copy_assignable_v<T>,or
5. is_move_assignable_v<T>, or
6. same_as<T, remove_cv_t<T>>
   is false.

Resolution for std::atomic_ref

LWG#3508 also points out this problem, and indicates that for const-qualified types, it is not possible to implement atomic load or atomic read-modify-write operations.

std::atomic_ref<...-type> specializations only apply for cv-unqualified types.

• Proposed resolution: specify std::atomic_ref<T> for cv-qualified T by restricting support of volatile-qualified types to lock-free atomics and restricting support of const-qualified types to atomic read operations.
• Rationale: atomic_ref goal of improving concurrency support when interfacing with third-party types, which may be using volatile int for historical purposes, needs std::atomic_ref<volatile int>: the atomic_ref itself is not volatile, the data it references is.
• Impact: libstdc++ and libc++ (among others) would need to implement it.
• Wording:

Modify [atomics.ref.generic.general]:

namespace std {
  template<class T> struct atomic_ref {
  private:
    T* ptr;             // exposition only

  public:
    using value_type = Tremove_cv_t<T>;
    static constexpr size_t required_alignment = implementation-defined;

    static constexpr bool is_always_lock_free = implementation-defined;
    bool is_lock_free() const noexcept;

    explicit atomic_ref(T&);
    atomic_ref(const atomic_ref&) noexcept;
    atomic_ref& operator=(const atomic_ref&) = delete;

    void store(Tvalue_type, memory_order = memory_order::seq_cst) const noexcept;
    Tvalue_type operator=(Tvalue_type) const noexcept;
    Tvalue_type load(memory_order = memory_order::seq_cst) const noexcept;
    operator Tvalue_type() const noexcept;

    Tvalue_type exchange(Tvalue_type, memory_order = memory_order::seq_cst) 
      const noexcept;
    bool compare_exchange_weak(Tvalue_type&, Tvalue_type,
                               memory_order, memory_order) 
        const noexcept;
    bool compare_exchange_strong(Tvalue_type&, Tvalue_type,
                                 memory_order, memory_order)
        const noexcept;
    bool compare_exchange_weak(Tvalue_type&, Tvalue_type,
                               memory_order = memory_order::seq_cst) 
        const noexcept;
    bool compare_exchange_strong(Tvalue_type&, Tvalue_type,
                                 memory_order = memory_order::seq_cst)
        const noexcept;

    void wait(Tvalue_type, memory_order = memory_order::seq_cst) const noexcept;
    void notify_one() const noexcept;
    void notify_all() const noexcept;
  };
}

1. 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]).
2. The program is ill-formed if is_trivially_copyable_v<T> is false.
3. The lifetime ([basic.life]) of an object referenced by *ptr shall exceed the lifetime of all atomic_refs that reference the object. While any atomic_ref instances exist that 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.
4. 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 1: Atomic operations or the atomic_ref constructor can acquire a shared resource, such as a lock associated with the referenced object, to enable atomic operations to be applied to the referenced object. — end note]
5. The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

Modify [atomics.ref.ops] as follows:

33.5.7.2 Operations [atomics.ref.ops]

static constexpr size_t required_alignment;

1. The alignment required for an object to be referenced by an atomic reference, which is at least alignof(T).
2. [Note 1: Hardware could require an object referenced by an atomic_ref to have stricter alignment ([basic.align]) than other objects of type T. Further, whether operations on an atomic_ref are lock-free could depend on the alignment of the referenced object. For example, lock-free operations on std​::​complex<double> could be supported only if aligned to 2*alignof(double). — end note]

static constexpr bool is_always_lock_free;

3. The static data member is_always_lock_free is true if the atomic_ref type's operations are always lock-free, and false otherwise.

bool is_lock_free() const noexcept;

4. Returns: true if operations on all objects of the type atomic_ref<T> are lock-free, false otherwise.

atomic_ref(T& obj);

5. Preconditions: The referenced object is aligned to required_alignment.
6. Postconditions: *this references obj.
7. Throws: Nothing.

atomic_ref(const atomic_ref& ref) noexcept;

8. Postconditions: *this references the object referenced by ref.

void store(Tvalue_type desired, memory_order order = memory_order::seq_cst) const noexcept

9. Constraints: is_const_v<T> is false.
10. Preconditions: order is memory_order​::​relaxed, memory_order​::​release, or memory_order​::​seq_cst.
11. Effects: Atomically replaces the value referenced by *ptr with the value of desired. Memory is affected according to the value of order.

Tvalue_type operator=(Tvalue_type desired) const noexcept;

12. Constraints: is_const_v<T> is false.
13. Effects: Equivalent to:

  store(desired);
  return desired;

Tvalue_type load(memory_order order = memory_order::seq_cst) const noexcept;

14. Preconditions: order is memory_order​::​relaxed, memory_order​::​consume, memory_order​::​acquire, or memory_order​::​seq_cst.
15. Effects: Memory is affected according to the value of order.
16. Returns: Atomically returns the value referenced by *ptr.

operator Tvalue_type() const noexcept;

15. Effects: Equivalent to: return load();

Tvalue_type exchange(Tvalue_type desired, memory_order order = memory_order::seq_cst) const noexcept;

16. Constraints: is_const_v<T> is false.
17. Effects: Atomically replaces the value referenced by *ptr with desired. Memory is affected according to the value of order. This operation is an atomic read-modify-write operation ([intro.multithread]).
18. Returns: Atomically returns the value referenced by *ptr immediately before the effects.

bool compare_exchange_weak(Tvalue_type& expected, Tvalue_type desired,
                           memory_order success, memory_order failure) const noexcept;

bool compare_exchange_strong(Tvalue_type& expected, Tvalue_type desired,
                             memory_order success, memory_order failure) const noexcept;

bool compare_exchange_weak(Tvalue_type& expected, Tvalue_type desired,
                           memory_order order = memory_order::seq_cst) const noexcept;

bool compare_exchange_strong(Tvalue_type& expected, Tvalue_type desired,
                             memory_order order = memory_order::seq_cst) const noexcept;

19. Constraints: is_const_v<T> is false.
20. Preconditions: failure is memory_order​::​relaxed, memory_order​::​consume, memory_order​::​acquire, or memory_order​::​seq_cst.
21. Effects: Retrieves the value in expected. It then atomically compares the value representation of 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 value in expected is replaced by the value read from the value referenced by *ptr during the atomic comparison. If the operation returns true, these operations are atomic read-modify-write operations ([intro.races]) on the value referenced by *ptr. Otherwise, these operations are atomic load operations on that memory.
22. Returns: The result of the comparison.
23. Remarks: A weak compare-and-exchange 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 2: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note]

void wait(Tvalue_type old, memory_order order = memory_order::seq_cst) const noexcept;

22. Preconditions: order is memory_order​::​relaxed, memory_order​::​consume, memory_order​::​acquire, or memory_order​::​seq_cst.
23. Effects: Repeatedly performs the following steps, in order:
    (23.1) Evaluates load(order) and compares its value representation for equality against that of old.
    (23.2) If they compare unequal, returns.
    (23.3) Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.
24. Remarks: This function is an atomic waiting operation ([atomics.wait]) on atomic object *ptr.

void notify_one() const noexcept;

25. Constraints: is_const_v<T> is false.
26. Effects: Unblocks the execution of at least one atomic waiting operation on *ptr that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.
27. Remarks: This function is an atomic notifying operation ([atomics.wait]) on atomic object *ptr.

void notify_all() const noexcept;

28. Constraints: is_const_v<T> is false.
29. Effects: Unblocks the execution of all atomic waiting operations on *ptr that are eligible to be unblocked ([atomics.wait]) by this call.
30. Remarks: This function is an atomic notifying operation ([atomics.wait]) on atomic object *ptr.

Modify [atomics.ref.int]:

33.5.7.3 Specializations for integral types[atomics.ref.int]

1. There are specializations of the atomic_ref class template for all integral types except cvboolthe integral types char, signed char, unsigned char, short, unsigned short, int, unsigned int, long, unsigned long, long long, unsigned long long, char8_t, char16_t, char32_t, wchar_t, and any other types needed by the typedefs in the header <cstdint>. For each such type integral-type, the specialization atomic_ref<integral-type> provides additional atomic operations appropriate to integral types.
   [Note 1: The specialization atomic_ref<bool> uses the primary template ([atomics.ref.generic]). — end note]
2. The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std {
  template<> struct atomic_ref<integral-type> {
  private:
    integral-type* ptr;         // exposition only

  public:
    using value_type = remove_cv_t<integral-type>;
    using difference_type = value_type;
    static constexpr size_t required_alignment = implementation-defined;

    static constexpr bool is_always_lock_free = implementation-defined;
    bool is_lock_free() const noexcept;

    explicit atomic_ref(integral-type&);
    atomic_ref(const atomic_ref&) noexcept;
    atomic_ref& operator=(const atomic_ref&) = delete;

    void store(integral-typevalue_type, memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type operator=(integral-typevalue_type) const noexcept;
    integral-typevalue_type load(memory_order = memory_order::seq_cst) const noexcept;
    operator integral-typevalue_type() const noexcept;

    integral-typevalue_type exchange(integral-typevalue_type,
                           memory_order = memory_order::seq_cst) const noexcept;
    bool compare_exchange_weak(integral-typevalue_type&, integral-typevalue_type,
                               memory_order, memory_order) const noexcept;
    bool compare_exchange_strong(integral-typevalue_type&, integral-typevalue_type,
                                 memory_order, memory_order) const noexcept;
    bool compare_exchange_weak(integral-typevalue_type&, integral-typevalue_type,
                               memory_order = memory_order::seq_cst) const noexcept;
    bool compare_exchange_strong(integral-typevalue_type&, integral-typevalue_type,
                                 memory_order = memory_order::seq_cst) const noexcept;

    integral-typevalue_type fetch_add(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type fetch_sub(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type fetch_and(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type fetch_or(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type fetch_xor(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type fetch_max(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;
    integral-typevalue_type fetch_min(integral-typevalue_type,
                            memory_order = memory_order::seq_cst) const noexcept;

    integral-typevalue_type operator++(int) const noexcept;
    integral-typevalue_type operator--(int) const noexcept;
    integral-typevalue_type operator++() const noexcept;
    integral-typevalue_type operator--() const noexcept;
    integral-typevalue_type operator+=(integral-typevalue_type) const noexcept;
    integral-typevalue_type operator-=(integral-typevalue_type) const noexcept;
    integral-typevalue_type operator&=(integral-typevalue_type) const noexcept;
    integral-typevalue_type operator|=(integral-typevalue_type) const noexcept;
    integral-typevalue_type operator^=(integral-typevalue_type) const noexcept;

    void wait(integral-typevalue_type, memory_order = memory_order::seq_cst) const noexcept;
    void notify_one() const noexcept;
    void notify_all() const noexcept;
  };
}

3. Descriptions are provided below only for members that differ from the primary template.
4. The following operations perform arithmetic computations. The correspondence among key, operator, and computation is specified in Table 148.

integral-typevalue_type fetch_key(integral-typevalue_type operand,
  memory_order order = memory_order::seq_cst) const noexcept;

5. Constraints: is_const_v<integral-type> is false.
6. 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 read-modify-write operations ([intro.races]).
7. Returns: Atomically, the value referenced by *ptr immediately before the effects.
8. Remarks: Except for fetch_max and fetch_min, for signed integer types the result is as if the object value and parameters were converted to their corresponding unsigned types, the computation performed on those types, and the result converted back to the signed type.
   [Note 2: There are no undefined results arising from the computation. — end note]
9. For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and min algorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.

integral-typevalue_type  operator op=(integral-typevalue_type  operand) const noexcept;

10. Constraints: is_const_v<integral-type> is false.
11. Effects: Equivalent to: return fetch_key(operand) op operand;

Modify [atomics.ref.float]:

33.5.7.4 Specializations for floating-point types[atomics.ref.float]

1. There are specializations of the atomic_ref class template for all cv-unqualified floating-point types. For each such type floating-point-type, the specialization atomic_ref<floating-point> provides additional atomic operations appropriate to floating-point types.
2. The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std {
  template<> struct atomic_ref<floating-point-type> {
  private:
    floating-point-type* ptr;   // exposition only

  public:
    using value_type = remove_cv_t<floating-point-type>;
    using difference_type = value_type;
    static constexpr size_t required_alignment = implementation-defined;

    static constexpr bool is_always_lock_free = implementation-defined;
    bool is_lock_free() const noexcept;

    explicit atomic_ref(floating-point-type&);
    atomic_ref(const atomic_ref&) noexcept;
    atomic_ref& operator=(const atomic_ref&) = delete;

    void store(floating-point-typevalue_type, memory_order = memory_order::seq_cst) const noexcept;
    floating-point-typevalue_type operator=(floating-point-typevalue_type) const noexcept;
    floating-point-typevalue_type load(memory_order = memory_order::seq_cst) const noexcept;
    operator floating-point-typevalue_type() const noexcept;

    floating-point-typevalue_type exchange(floating-point-typevalue_type,
                                 memory_order = memory_order::seq_cst) const noexcept;
    bool compare_exchange_weak(floating-point-typevalue_type&, floating-point-typevalue_type,
                               memory_order, memory_order) const noexcept;
    bool compare_exchange_strong(floating-point-typevalue_type&, floating-point-typevalue_type,
                                 memory_order, memory_order) const noexcept;
    bool compare_exchange_weak(floating-point-typevalue_type&, floating-point-typevalue_type,
                               memory_order = memory_order::seq_cst) const noexcept;
    bool compare_exchange_strong(floating-point-typevalue_type&, floating-point-typevalue_type,
                                 memory_order = memory_order::seq_cst) const noexcept;

    floating-point-typevalue_type fetch_add(floating-point-typevalue_type,
                                  memory_order = memory_order::seq_cst) const noexcept;
    floating-point-typevalue_type fetch_sub(floating-point-typevalue_type,
                                  memory_order = memory_order::seq_cst) const noexcept;

    floating-point-typevalue_type operator+=(floating-point-typevalue_type) const noexcept;
    floating-point-typevalue_type operator-=(floating-point-typevalue_type) const noexcept;

    void wait(floating-point-typevalue_type, memory_order = memory_order::seq_cst) const noexcept;
    void notify_one() const noexcept;
    void notify_all() const noexcept;
  };
}

3. Descriptions are provided below only for members that differ from the primary template.
4. The following operations perform arithmetic computations. The correspondence among key, operator, and computation is specified in Table 148.

floating-point-typevalue_type fetch_key(floating-point-typevalue_type operand,
                          memory_order order = memory_order::seq_cst) const noexcept;

5. Constraints: is_const_v<floating-point-type> is false.
6. 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 read-modify-write operations ([intro.races]).
7. Returns: Atomically, the value referenced by *ptr immediately before the effects.
8. 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 floating-point-type should conform to the std​::​numeric_limits<floating-point-typevalue_type> traits associated with the floating-point type ([limits.syn]). The floating-point environment ([cfenv]) for atomic arithmetic operations on floating-point-type may be different than the calling thread's floating-point environment.

floating-point-typevalue_type operator op=(floating-point-typevalue_type operand) const noexcept;

9. Constraints: is_const_v<floating-point-type> is false.
10. Effects: Equivalent to: return fetch_key(operand) op operand;

Modify [atomics.ref.pointer]:

33.5.7.5 Partial specialization for pointers[atomics.ref.pointer]

1. There are specializations of the atomic_ref class template for all pointer-to-object types. For each such type pointer-type, the specialization atomic_ref<pointer-type> provides additional atomic operations appropriate to pointer types.
2. The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std {
  template<class T> struct atomic_ref<T*pointer-type> {
  private:
    T*pointer-type* ptr;        // exposition only

  public:
    using value_type = T*remove_cv_t<pointer-type>;
    using difference_type = ptrdiff_t;
    static constexpr size_t required_alignment = implementation-defined;

    static constexpr bool is_always_lock_free = implementation-defined;
    bool is_lock_free() const noexcept;

    explicit atomic_ref(T*pointer-type&);
    atomic_ref(const atomic_ref&) noexcept;
    atomic_ref& operator=(const atomic_ref&) = delete;

    void store(T*value_type, memory_order = memory_order::seq_cst) const noexcept;
    T*value_type operator=(T*value_type) const noexcept;
    T*value_type load(memory_order = memory_order::seq_cst) const noexcept;
    operator T*value_type() const noexcept;

    T*value_type exchange(T*value_type, memory_order = memory_order::seq_cst) const noexcept;
    bool compare_exchange_weak(T*value_type&, T*value_type,
                               memory_order, memory_order) const noexcept;
    bool compare_exchange_strong(T*value_type&, T*value_type,
                                 memory_order, memory_order) const noexcept;
    bool compare_exchange_weak(T*value_type&, T*value_type,
                               memory_order = memory_order::seq_cst) const noexcept;
    bool compare_exchange_strong(T*value_type&, T*value_type,
                                 memory_order = memory_order::seq_cst) const noexcept;

    T*value_type fetch_add(difference_type, memory_order = memory_order::seq_cst) const noexcept;
    T*value_type fetch_sub(difference_type, memory_order = memory_order::seq_cst) const noexcept;
    T*value_type fetch_max(T*value_type, memory_order = memory_order::seq_cst) const noexcept;
    T*value_type fetch_min(T*value_type, memory_order = memory_order::seq_cst) const noexcept;

    T*value_type operator++(int) const noexcept;
    T*value_type operator--(int) const noexcept;
    T*value_type operator++() const noexcept;
    T*value_type operator--() const noexcept;
    T*value_type operator+=(difference_type) const noexcept;
    T*value_type operator-=(difference_type) const noexcept;

    void wait(T*value_type, memory_order = memory_order::seq_cst) const noexcept;
    void notify_one() const noexcept;
    void notify_all() const noexcept;
  };
}

3. Descriptions are provided below only for members that differ from the primary template.
4. The following operations perform arithmetic computations. The correspondence among key, operator, and computation is specified in Table 149.

T*value_type fetch_key(difference_type operand, memory_order order = memory_order::seq_cst) const noexcept;

5. Constraints: is_const_v<pointer-type> is false.
6. Mandates: Tremove_pointer_t<pointer-type> is a complete object type.
7. 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 read-modify-write operations ([intro.races]).
8. Returns: Atomically, the value referenced by *ptr immediately before the effects.
9. Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.
10. For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and min algorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.
    [Note 1: If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]). — end note]

T*value_type operator op=(difference_type operand) const noexcept;

11. Constraints: is_const_v<pointer-type> is false.
12. Effects: Equivalent to: return fetch_key(operand) op operand;

Modify [atomics.ref.memop]:

33.5.7.6 Member operators common to integers and pointers to objects[atomics.ref.memop]

1. Let referred-type be pointer-type for the specializations in [atomics.ref.pointer] and be integral-type for the specializations in [atomics.ref.int].

value_type operator++(int) const noexcept;

2. Constraints: is_const_v<referred-type> is false.
3. Effects: Equivalent to: return fetch_add(1);

value_type operator--(int) const noexcept;

4. Constraints: is_const_v<referred-type> is false.
5. Effects: Equivalent to: return fetch_sub(1);

value_type operator++() const noexcept;

6. Constraints: is_const_v<referred-type> is false.
7. Effects: Equivalent to: return fetch_add(1) + 1;

value_type operator--() const noexcept;

8. Constraints: is_const_v<referred-type> is false.
9. Effects: Equivalent to: return fetch_sub(1) - 1;