1   Revision History

2   Description / Motivation

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 (P0019) 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 and potentially must acquire resources such as a lock in order to perform atomic operations on that object. Construction of atomic_ref has potentially large overhead which could be performed once for an entire span.

The proposed mdspan (P0009) allows for additional property parameters to be specified. Collections of property parameters form an ontology, described in P0900, where the property-values are organized into orthogonal property-types.

We propose the atomic_access property-value for span and mdspan such that

• all references to mdspan elements are atomic_ref
• potential overhead associated with constructing atomic_ref occurs only once when an mdspan object is constructed.

3   Proposal

Modify mdspan specification:

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {

  template< class ArrayType , typename ... Properties >
  struct mdspan {
    // revision
    // using reference = element_type & ; // remove
    using reference = /* implementation defined */ ;
  };

}}}

Add to [views] a new [views.atomics] section.

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {

  struct atomic_access ;

}}}

The atomic_access property-value may appear at most once in the Properties... parameter pack of span or mdpsan. Let S be an instantiation of span or mdspan with Properties... parameter pack containing atomic_access. Let R be atomic_ref< S ::element_type> .

Requires:

• S ::element_type satisfies requirements for atomic_ref< S ::element_type>,
• S constructors also require R constructors' requirements for member objects of S.

Effects:

• S ::reference is atomic_ref< S ::element_type>,
• S iterator types dereference to atomic_ref< S ::element_type>

4   Application to span

When the span library is updated to accept properties (P0546) the same revision should be applied to accept the atomic_access property-value.

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {

  template< class ArrayType , typename ... Properties >
  struct span {
    // revision
    // using reference = element_type & ; // remove
    using reference = /* implementation defined */ ;
  };

}}}