| Project: | ISO JTC1/SC22/WG21: Programming Language C++ |
|---|---|
| Number: | P0020r4 |
| Date: | 2016-11-09 |
| Reply-to: | hcedwar@sandia.gov |
| Author: | H. Carter Edwards |
| Contact: | hcedwar@sandia.gov |
| Author: | Hans Boehm |
| Contact: | hboehm@google.com |
| Author: | Olivier Giroux |
| Contact: | ogiroux@nvidia.com |
| Author: | JF Bastien |
| Contact: | jfbastien@apple.com |
| Author: | James Reus |
| Contact: | reus1@llnl.gov |
| Audience: | Library Evolution |
| URL: | https://github.com/kokkos/ISO-CPP-Papers/blob/master/P0020.rst |
- Align proposal with content of corresponding sections in N5131, 2016-07-15.
- Editorial, add hyphenation to "floating point"
- 2016-11-09 Issaquah SG1 decision: move to LEWG targeting C++20
This paper proposes an extension to the atomic operations library [atomics] for atomic addition on an object conforming to the atomic<T> where T is a floating-point type (N5131 3.9.1p8).
This paper does not include proposed extension of the named atomic type (N5131 Tables 134 and 135), or by implication extension of the C _Atomic qualification for floating-point types.
The capability for atomic addition on floating-point types critical for high performance computing (HPC) applications. The need is for extension of atomic and atomic_view (P0019) for floating-point types.
namespace std {template<> stuct atomic< floating-point >;// In the following declarations, atomic-floating-point is atomic<T>.floating-point atomic_fetch_add( volatile atomic-floating-point*, floating-point ) noexcept ;floating-point atomic_fetch_add( atomic-floating-point*, floating-point ) noexcept ;floating-point atomic_fetch_add_explicit( volatile atomic-floating-point*, floating-point , memory_order ) noexcept ;floating-point atomic_fetch_add_explicit( atomic-floating-point*, floating-point , memory_order ) noexcept ;floating-point atomic_fetch_sub( volatile atomic-floating-point*, floating-point ) noexcept ;floating-point atomic_fetch_sub( atomic-floating-point*, floating-point ) noexcept ;floating-point atomic_fetch_sub_explicit( volatile atomic-floating-point*, floating-point , memory_order ) noexcept ;floating-point atomic_fetch_sub_explicit( atomic-floating-point*, floating-point , memory_order ) noexcept ;}
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;};
In the declarations of these functions and function template specializations, the name floating-point refers to a floating-point type and the name atomic-floating-point refers to atomic<floating-point>.
regarding arithmetic operations
update 29.6.5p31 Remark
Remark: For signed integer types, arithmetic is defined to use two’s complement representation. Thereand there are no undefined results. For floating-point types, if the result is not mathematically defined or not in the range of representable values for its type (5p4) the result is unspecified, but the operations otherwise have no undefined behavior. [Note: 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. - end note] For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.