tmp/tmpwo0wx6s8/{from.md → to.md}
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| 1 |
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###### Overview <a id="mdspan.accessor.aligned.overview">[[mdspan.accessor.aligned.overview]]</a>
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``` cpp
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namespace std {
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template<class ElementType, size_t ByteAlignment>
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struct aligned_accessor {
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using offset_policy = default_accessor<ElementType>;
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using element_type = ElementType;
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using reference = ElementType&;
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using data_handle_type = ElementType*;
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static constexpr size_t byte_alignment = ByteAlignment;
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constexpr aligned_accessor() noexcept = default;
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template<class OtherElementType, size_t OtherByteAlignment>
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constexpr aligned_accessor(
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aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;
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template<class OtherElementType>
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constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept;
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template<class OtherElementType>
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constexpr operator default_accessor<OtherElementType>() const noexcept;
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constexpr reference access(data_handle_type p, size_t i) const noexcept;
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constexpr typename offset_policy::data_handle_type offset(
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data_handle_type p, size_t i) const noexcept;
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};
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}
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```
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*Mandates:*
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- `byte_alignment` is a power of two, and
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- `byte_alignment >= alignof(ElementType)` is `true`.
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`aligned_accessor` meets the accessor policy requirements.
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`ElementType` is required to be a complete object type that is neither
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| 40 |
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an abstract class type nor an array type.
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Each specialization of `aligned_accessor` is a trivially copyable type
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that models `semiregular`.
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\[`0`, n) is an accessible range for an object `p` of type
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`data_handle_type` and an object of type `aligned_accessor` if and only
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if
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- \[`p`, `p + `n) is a valid range, and,
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- if n is greater than zero, then
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`is_sufficiently_aligned<byte_alignment>(p)` is `true`.
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[*Example 1*:
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The following function `compute` uses `is_sufficiently_aligned` to check
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| 56 |
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whether a given `mdspan` with `default_accessor` has a data handle with
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| 57 |
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sufficient alignment to be used with
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| 58 |
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`aligned_accessor<float, 4 * sizeof(float)>`. If so, the function
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dispatches to a function `compute_using_fourfold_overalignment` that
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requires fourfold over-alignment of arrays, but can therefore use
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hardware-specific instructions, such as four-wide SIMD (Single
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Instruction Multiple Data) instructions. Otherwise, `compute` dispatches
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| 63 |
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to a possibly less optimized function
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`compute_without_requiring_overalignment` that has no over-alignment
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| 65 |
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requirement.
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| 66 |
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``` cpp
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void compute_using_fourfold_overalignment(
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mdspan<float, dims<1>, layout_right, aligned_accessor<float, 4 * alignof(float)>> x);
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| 70 |
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void compute_without_requiring_overalignment(
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mdspan<float, dims<1>, layout_right> x);
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void compute(mdspan<float, dims<1>> x) {
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constexpr auto byte_alignment = 4 * sizeof(float);
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auto accessor = aligned_accessor<float, byte_alignment>{};
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auto x_handle = x.data_handle();
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| 78 |
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if (is_sufficiently_aligned<byte_alignment>(x_handle)) {
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compute_using_fourfold_overalignment(mdspan{x_handle, x.mapping(), accessor});
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} else {
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compute_without_requiring_overalignment(x);
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}
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}
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```
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— *end example*]
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