P0020r3

• Align proposal with content of corresponding sections in N5131, 2016-07-15.

P0020r4

• Editorial, add hyphenation to "floating point"
• 2016-11-09 Issaquah SG1 decision: move to LEWG targeting C++20

P0020r5

• 2017-03-01 Kona LEWG review and consensus: move to C++20 IS
• Change remarks regarding floating point environment from note to normative.
• Query vendors regarding changing "should" to "shall" in regard to floating point environment. Response from two vendors supporting atomic floating point add operations in hardware was "do not change."
• Align with C++17 working draft atomic wording as per P0558r1
• Note: Section and paragraph numbers have not yet been updated to C++17 working draft new section numbering.

P0020r6

• 2017-11-10 Albuquerque LWG editorial update:
• Align with wording for [atomics.types.int] in N4700
• Add stable name [atomics.types.float]

Wording:

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

There are specializations of the atomic template for the floating-point types float, double, long double. For each such floating-point type floating-point , the specialization atomic< floating-point > provides additional atomic operations appropriate to floating-point types.

namespace std {

template<> struct atomic< floating-point > {

static constexpr bool is_always_lock_free = implementation-defined ;

bool is_lock_free() const volatile noexcept;

bool is_lock_free() const noexcept;

void store( floating-point , memory_order = memory_order_seq_cst ) volatile noexcept;

void store( floating-point , memory_order = memory_order_seq_cst ) noexcept;

floating-point load( memory_order = memory_order_seq_cst ) volatile noexcept;

floating-point load( memory_order = memory_order_seq_cst ) noexcept;

operator floating-point () volatile noexcept ;

operator floating-point () noexcept ;

floating-point exchange( floating-point , memory_order = memory_order_seq_cst ) volatile noexcept;

floating-point exchange( floating-point , memory_order = memory_order_seq_cst ) noexcept;

bool compare_exchange_weak( floating-point & , floating-point , memory_order , memory_order ) volatile noexcept;

bool compare_exchange_weak( floating-point & , floating-point , memory_order , memory_order ) noexcept;

bool compare_exchange_strong( floating-point & , floating-point , memory_order , memory_order ) volatile noexcept;

bool compare_exchange_strong( floating-point & , floating-point , memory_order , memory_order ) noexcept;

bool compare_exchange_weak( floating-point & , floating-point , memory_order = memory_order_seq_cst ) volatile noexcept;

bool compare_exchange_weak( floating-point & , floating-point , memory_order = memory_order_seq_cst ) noexcept;

bool compare_exchange_strong( floating-point &, floating-point , memory_order = memory_order_seq_cst ) volatile noexcept;

bool compare_exchange_strong( floating-point &, floating-point , memory_order = memory_order_seq_cst ) noexcept;

floating-point fetch_add( floating-point , memory_order = memory_order_seq_cst) volatile noexcept;

floating-point fetch_add( floating-point , memory_order = memory_order_seq_cst) noexcept;

floating-point fetch_sub( floating-point , memory_order = memory_order_seq_cst) volatile noexcept;

floating-point fetch_sub( floating-point , memory_order = memory_order_seq_cst) noexcept;

atomic() noexcept = default ;

constexpr atomic( floating-point ) noexcept ;

atomic( const atomic & ) = delete ;

atomic & operator = ( const atomic & ) = delete ;

atomic & operator = ( const atomic & ) volatile = delete ;

floating-point operator=( floating-point ) volatile noexcept ;

floating-point operator=( floating-point ) noexcept ;

floating-point operator+=( floating-point ) volatile noexcept;

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

floating-point operator-=( floating-point ) volatile noexcept;

floating-point operator-=( floating-point ) noexcept;

};

}

The atomic floating-point specializations are standard-layout structs. They each have a trivial default constructor and a trivial destructor.

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic addition and subtraction computations. The key, operator, and computation correspondence are identified in Table 138 — Atomic arithmetic computations.

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations (4.7).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remark: If the result is not a representable value for its type (5p4) the result is unspecified, but the operations otherwise have no undefined behavior. Atomic arithmetic operations on floating-point should conform to std::numeric_limits< floating-point > traits associated with the floating-point type (18.3.2). The floating-point environment (26.4) for atomic arithmetic operations on floating-point may be different than the calling thread's floating-point environment.

Effects: Equivalent to: return fetch_key (operand) op operand;

Remark: If the result is not a representable value for its type (5p4) the result is unspecified, but the operations otherwise have no undefined behavior. Atomic arithmetic operations on floating-point should conform to std::numeric_limits< floating-point > traits associated with the floating-point type (18.3.2). The floating-point environment (26.4) for atomic arithmetic operations on floating-point may be different than the calling thread's floating-point environment.