1 Revision History

• Revision 0 submitted 2024-07-14

  • LEWG reviewed this version 2024-10-15, and voted to forward the [mdspan.sub] changes (with the [mdspan.syn], i.e., the “accept user-defined pair types” part, removed) to LWG. LEWG also asked that we increment the submdspan feature test macro __cpp_lib_submdspan from its current value (202403L, set by adoption of P2642R6 (“Padded mdspan layouts”) into the Working Draft for C++26).

• Revision 1 to be submitted for the 2024-10-16 mailing

  • Remove “user-defined pair types as slices” feature. We will make this a separate paper.

  • Increment __cpp_lib_submdspan from its current value (202403L).

  • Implement suggestion during LEWG wording review to define a “wording macro” for all the slice specifier types that have unit stride. We call it unit-stride slice for M, where M is a layout mapping type. (The definition only depends on an index_type, but the wording is most natural when it depends on the layout mapping type for which submdspan-mapping-impl is a private member function.)

2 Abstract

We propose to change submdspan_mapping for the following layouts’ layout mappings:

• layout_left,
• layout_right,
• layout_left_padded, and
• layout_right_padded,

so that a strided_slice slice with compile-time unit stride results in the returned mapping having the same layout as if the slice were a pair of integers. This preserves compile-time optimization information for common layouts.

This change needs to be merged into the Working Draft before C++26. Otherwise, it would be a breaking change.

3 Motivation and design discussion

3.1 Vectorization example

Suppose that one wants to vectorize a 1-D array copy operation using mdspan and aligned_accessor (P2897). One has a copy_8_floats function that optimizes the special case of copying a contiguous array of 8 floats, where the start of the array is aligned to 8 * sizeof(float) (32) bytes. In practice, plenty of libraries exist to optimize 1-D array copy. This is just an example that simplifies the use cases for explicit 8-wide SIMD enough to show in a brief proposal.

template<class ElementType, size_t ext, size_t byte_alignment>
using aligned_array_view = mdspan<ElementType,
  extents<int, ext>, layout_right,
  aligned_accessor<ElementType, byte_alignment>>;

void
copy_8_floats(aligned_array_view<const float, 8, 32> src,
  aligned_array_view<float, 8, 32> dst)
{
  // One would instead use a hardware instruction for aligned copy,
  // or a "simd" or "unroll" pragma.
  for (int k = 0; k < 8; ++k) {
    dst[k] = src[k];
  }
}

The natural next step would be to use copy_8_floats to implement copying 1-D float arrays by the usual “strip-mining” approach.

template<class ElementType>
using array_view = mdspan<ElementType, dims<1, int>>;

void slow_copy(array_view<const float> src, array_view<float> dst)
{
  assert(src.extent(0) == dst.extent(0));
  for (int k = 0; k < src.extent(0); ++k) {
    dst[k] = src[k];
  }
}

template<size_t vector_length>
void strip_mined_copy(
  aligned_array_view<const float, dynamic_extent,
    vector_length * sizeof(float)> src,
  aligned_array_view<      float, dynamic_extent,
    vector_length * sizeof(float)> dst)
{
  assert(src.extent(0) == dst.extent(0));
  assert(src.extent(0) % vector_length == 0);

  for (int beg = 0; beg < src.extent(0); beg += vector_length) {
    constexpr auto one = std::integral_constant<int, 1>{};
    constexpr auto vec_len = std::integral_constant<int, vector_length>{};

    // Using strided_slice lets the extent be a compile-time constant.
    // tuple{beg, beg + vector_length} would result in dynamic_extent.
    constexpr auto vector_slice =
      strided_slice{.offset=dst_lower, .extent=vector_length, .stride=one};

    // PROBLEM: Current wording makes this always layout_stride,
    // but we know that it could be layout_right.
    auto src_slice = submdspan(src, vector_slice);
    auto dst_slice = submdspan(dst, vector_slice);

    copy_8_floats(src_slice, dst_slice);
  }
}

void copy(array_view<const float> src, array_view<float> dst)
{
  assert(src.extent(0) == dst.extent(0));
  constexpr int vector_length = 8;

  // Handle possibly unaligned prefix of less than vector_length elements. 
  auto aligned_starting_index = [](auto* ptr) {
    constexpr auto v = static_cast<unsigned>(vector_length);
    auto ptr_value = reinterpret_cast<uintptr_t>(ptr_value);
    auto remainder = ptr_value % v;
    return static_cast<int>(ptr_value + (v - remainder) % v);
  };
  int src_beg = aligned_starting_index(src.data());
  int dst_beg = aligned_starting_index(dst.data());
  if (src_beg != dst_beg) {
    slow_copy(src, dst);
    return;
  }
  slow_copy(submdspan(src, tuple{0, src_beg}),
    submdspan(dst, tuple{0, dst_beg}));

  // Strip-mine the aligned vector_length segments of the array.
  int src_end = (src.size() / vector_length) * vector_length;
  int dst_end = (dst.size() / vector_length) * vector_length;
  strip_mined_copy<8>(submdspan(src, tuple{src_beg, src_end}),
    submdspan(dst, tuple{dst_beg, dst_end}));

  // Handle suffix of less than vector_length elements.
  slow_copy(submdspan(src, tuple{src_end, src.extent(0)}),
    submdspan(dst, tuple{dst_end, dst.extent(0)}));
}

The strip_mined_copy function must use strided_slice to get slices of 8 elements at a time, rather than tuple. This ensures that the resulting extent is a compile-time constant 8, even though the slice starts at a run-time index beg.

3.2 Issues with C++ Working Draft

The current C++ Working Draft has two issues that hinder optimization of the above code.

1. The above submdspan results always have layout_stride, even though we know that they are contiguous and thus should have layout_right.

2. The submdspan operations in strip_mined_copy should result in aligned_accessor with 32-byte alignment, but instead give default_accessor. This is because aligned_accessor’s offset member function takes the offset as a size_t. This discards compile-time information, namely that the offset can be expressed as the product of some integer and the overalignment factor, where the overalignment factor is known at compile time.

3.3 What this proposal does and doesn’t change

This proposal fixes (1) for all layouts currently in the Working Draft that have a submdspan_mapping customization: layout_left, layout_right, layout_left_padded, and layout_right_padded. We can do that without breaking changes, as long as this proposal is merged before C++26 is finalized. After that, merging the proposal would be a breaking change.

This proposal does not fix (2), because that would require a breaking change to both the layout mapping requirements and the accessor requirements, and because it would complicate both of them quite a bit.

3.4 Why we don’t try to fix aligned_accessor::offset

Regarding (2), [mdspan.submdspan.submdspan] 6 says that submdspan(src, slices...) has effects equivalent to the following.

auto sub_map_offset = submdspan_mapping(src.mapping(), slices...);
return mdspan(src.accessor().offset(src.data(), sub_map_offset.offset),
              sub_map_offset.mapping,
              AccessorPolicy::offset_policy(src.accessor()));

The problem is AccessorPolicy::offset_policy(src.accessor()). The type offset_policy is the wrong type in this case, default_accessor<const float> instead of aligned_accessor<const float, 32>. If we want an offset with suitable compile-time alignment to have a different accessor type, then we would need at least the following changes.

1. The Standard Library would need a new type that represents the product of a compile-time integer (that is, an integral-constant-like type) and a “run-time” integer (an integral-not-bool type). It would need overloaded arithmetic operators that preserve this product form as much as possible. For example, 8x + 4 for a run-time integer x should result in 4y where y = 2x + 1 is a run-time integer.

2. At least the Standard layout mappings’ operator() would need to compute with these types and return them if possible. The layout mapping requirements would thus need to change, as currently operator() must return index_type (see [[mdspan.layout.reqmts]] 7).

3. aligned_accessor::offset would need an overload taking a type that expresses the product of a compile-time integer (of suitable alignment) and a run-time integer. The accessor requirements [[mdspan.accessor.reqmts]] may also need to change to permit this.

4. The definition of submdspan would need some way to get the accessor type corresponding to the new offset overload, instead of aligned_accessor::offset_policy (which in this case is default_accessor).

The work-around is to convert the result of submdspan by hand to use the desired accessor. In the above copy example, one would replace the line

    copy_8_floats(src_slice, dst_slice);

with the following, that depends on aligned_accessor’s explicit constructor from default_accessor.

    copy_8_floats(aligned_array_view<const float, 8, 32>{src},
      aligned_array_view<float, 8, 32>{dst});

Given that this work-around is easy to do, should only be needed for a few special cases, and avoids a big design change to the accessor policy requirements, we don’t propose trying to fix this issue in the C++ Working Draft.

4 Implementation experience

Daisy Hollman’s original implementation of submdspan implemented strided slices in this way.

5 Desired ship vehicle

C++26 / IS.

6 Wording

Text in blockquotes is not proposed wording, but rather instructions for generating proposed wording.

6.1 Increment __cpp_lib_submdspan feature test macro

In [version.syn], increase the value of the __cpp_lib_submdspan macro by replacing YYYMML below with the integer literal encoding the appropriate year (YYYY) and month (MM).

#define __cpp_lib_submdspan YYYYMML // also in <mdspan>

6.2 Change existing layout mappings’ submdspan_mapping results

Append the following to the end of [mdspan.sub.map.common].

9 Given a layout mapping type M, a type S is a unit-stride slice for M if

(9.1) S is a specialization of strided_slice where S::stride_type models integral-constant-like and S::stride_type::value equals 1,

(9.2) S models index-pair-like<M::index_type>, or

(9.3) is_convertible_v<S, full_extent_t> is true.

Throughout [mdspan.sub], wherever the text says

“S_k models index-pair-like<index_type> or is_convertible_v<S_k, full_extent_t> is true,”

replace it with

“S_k is a unit-stride slice for decltype(*this).”

Apply the analogous transformation if the text says S_p or S₀, but is otherwise the same. Make this set of changes in the following places.

• [mdspan.sub.map.left] (1.3.2), (1.4), (1.4.1), and (1.4.3);

• [mdspan.sub.map.right] (1.3.2), (1.4), (1.4.1), and (1.4.3);

• [mdspan.sub.map.leftpad] (1.3.2), (1.4), (1.4.1), and (1.4.3); and

• [mdspan.sub.map.rightpad] (1.3.2), (1.4), (1.4.1), and (1.4.3).