p0860R1
Atomic Access Property for mdspan

1. Revision History

[P0860r1]

• Convert to bikeshed

• Remove span

• Conform to mdspan accessor concept

[P0860r0]

• 2017-11-08 Albuquerque LEWG feedback for P0009, P0019, and P0564 Extract atomic array reference from P0019 and create a separate paper

2. Overview

High performance 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 non-conflicting access when assigning member values. Parallel algorithms for updates have conflicting access to members which must be guarded by atomic operations. Parallel algorithms with read-only access require best-performing streaming read access, random read access, vectorization, or other guaranteed non-conflicting HPC pattern.

An atomic_ref [P0019r7] is used to perform atomic operations on the non-atomic members of the referenced array. Construction of an atomic_ref for a non-atomic object requires the non-atomic object satisfy several conditions.

We propose the atomic_accessor accessor for mdspan [P0009r6] such that all references to elements are atomic_ref.

3. Proposal

Add the following Accessor Policy to section 3.7 of [P0009r6]

struct atomic_accessor
{
  template <typename T>
  struct accessor
  {
    using pointer      = T*;
    using reference    = atomic_ref<T>;

    constexpr reference operator()(pointer p, ptrdiff_t i) const noexcept;
  };
};

The template argument for T shall be trivially copyable [basic.types].

constexpr reference operator()(pointer p, ptrdiff_t i) const noexcept ;

Requires: p[i] is aligned to the required_alignment of atomic_ref [atomic.ref.generic]

Effects: Equivalent to return atomic_ref<T>(p[i]);