Issaquah (Feb 2023) P2642 presentation

P2642 (padded mdspan layouts)

Use cases

1. Overaligned access to the beginning of each contiguous segment of elements

2. Representing column-major or row-major matrices whose stride is greater than the number of rows

   • "Pitched allocations*

   • Critical use case for matrix algorithms; the layout of BLAS and LAPACK

Use case for padding_stride=0 (at compile time)

Leftmost (for layout_left_padded; rightmost for layout_right_padded)
extent is zero at compile time: extents<Integral, 0, ...>.

template<int Value>
using Int = integral_constant<int, Value>;

layout_left::mapping<extents<int, 0, 10>> mapping{};
mdspan m{nullptr, mapping};
auto m_sub = submdspan(m, tuple{Int<0>{}, Int<0>{}}, full_extent);

Before P2642, m_sub would have mapping type
layout_stride::mapping<extents<int, 0, 10>>.

After P2642, m_sub would have mapping type
layout_left_padded<0>::mapping<extents<int, 0, 10>>.

Errata

• .extents() returns by value, not by const reference

  • [mdspan.layout.reqmts] specifies that .extents() returns const extents_type&, but layout_{left,right}_padded::mapping::extents in P2642 returns extents_type.

  • layout_{left,right}_padded::mapping does not store the user's extents object

    • It stores the "padded extents"

    • Effectively a "layout_{left,right} of a bigger array"

  • Fix: Change to return const extents_type&; rely on lifetime extension

• Specification of layout_left_padded<ps>::mapping(const layout_left_padded<ps>::mapping<OtherExtents>&) converting constructors and operator== may need adjustments, so that the spec as written can deduce the input padding_stride template argument. (See comments below.)

  • Consider adding a public member to layout_{left,right}_padded::mapping so users can get the padding_stride template argument without needing to write a traits class

Implementation

PR 237 in https://github.com/kokkos/mdspan

Start by reading the P2642 use cases and errata in this issue, then review implementation PR 237 in the kokkos/mdspan repository. The issue discusses presentation material and specification corrections, but does not identify files, tests, or a clear completion condition.