[PATCH v3 4/5] libstdc++: Implement __mdspan::__size.

[Tomasz Kaminski]

On Fri, Jul 4, 2025 at 10:34 AM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> The current code uses __mdspan::__fwd_prod(__exts, __rank) to express
> computing the size of an extent. This commit adds an function __mdspan::
> __size(__exts) to express the idea more directly.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (__mdspan::__size): New function.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM

>  libstdc++-v3/include/std/mdspan | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 1d6cdc93d80..7e970c2b905 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -398,6 +398,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        __rev_prod(const _Extents& __exts, size_t __r) noexcept
>        { return __exts_prod(__exts, __r + 1, __exts.rank()); }
>
> +    template<typename _Extents>
> +      constexpr typename _Extents::index_type
> +      __size(const _Extents& __exts) noexcept
> +      { return __fwd_prod(__exts, __exts.rank()); }
> +
>      template<typename _IndexType, size_t... _Counts>
>        auto __build_dextents_type(integer_sequence<size_t, _Counts...>)
>         -> extents<_IndexType, ((void) _Counts, dynamic_extent)...>;
> @@ -591,7 +596,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>        constexpr index_type
>        required_span_size() const noexcept
> -      { return __mdspan::__fwd_prod(_M_extents, extents_type::rank()); }
> +      { return __mdspan::__size(_M_extents); }
>
>        template<__mdspan::__valid_index_type<index_type>... _Indices>
>         requires (sizeof...(_Indices) == extents_type::rank())
> @@ -730,7 +735,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>        constexpr index_type
>        required_span_size() const noexcept
> -      { return __mdspan::__fwd_prod(_M_extents, extents_type::rank()); }
> +      { return __mdspan::__size(_M_extents); }
>
>        template<__mdspan::__valid_index_type<index_type>... _Indices>
>         requires (sizeof...(_Indices) == extents_type::rank())
> @@ -986,8 +991,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>         if constexpr (!is_always_exhaustive())
>           {
> -           constexpr auto __rank = extents_type::rank();
> -           auto __size = __mdspan::__fwd_prod(_M_extents, __rank);
> +           auto __size = __mdspan::__size(_M_extents);
>             if(__size > 0)
>               return __size == required_span_size();
>           }
> --
> 2.49.0
>
>
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