[PATCH v1] libstdc++: Improve and cleanup mdspan related code.

[Tomasz Kaminski]

On Tue, Sep 30, 2025 at 12:57 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> The improvement is that in __index_type_cast, we don't need to check at
> runtime if we know that _IndexType is smaller than _OIndexType.
>
> The cleanup is whitespace (overlength lines) in <mdspan> and
> de-uglifying a variable in test code.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (__mdspan::__index_type_cast): Optimize by
>         skipping a __glibcxx_assert if it's know at compile-time.
>         * testsuite/23_containers/mdspan/int_like.h: Rename _M_i to
>         value.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM with the two whitespace changes reverted. I can do that, and add it on
top
of mdspan series I have locally.


>  libstdc++-v3/include/std/mdspan               | 25 +++++++++++++------
>  .../testsuite/23_containers/mdspan/int_like.h | 12 ++++-----
>  2 files changed, 23 insertions(+), 14 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index d0a60b2d183..8efd168bcf0 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -77,11 +77,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>         if constexpr (std::is_integral_v<_OIndexType>)
>           {
> -           __glibcxx_assert(cmp_less_equal(__other,
> -               __gnu_cxx::__int_traits<_IndexType>::__max));
> +           constexpr _IndexType __index_type_max
> +             = __gnu_cxx::__int_traits<_IndexType>::__max;
> +           constexpr _OIndexType __oindex_type_max
> +             = __gnu_cxx::__int_traits<_OIndexType>::__max;
> +
> +           if constexpr (__index_type_max < __oindex_type_max)
> +             __glibcxx_assert(cmp_less_equal(__other, __index_type_max));
> +
>             if constexpr (std::is_signed_v<_OIndexType>)
> -             __glibcxx_assert(__other >= 0);
> -           return std::move(__other);
> +             __glibcxx_assert(__other >=  0);
> +           return static_cast<_IndexType>(std::move(__other));
>           }
>         else
>           {
> @@ -510,7 +516,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        __fwd_prod(const _Extents& __exts, size_t __begin, size_t __end)
> noexcept
>        {
>         size_t __sta_prod = [__begin, __end] {
> -         span<const size_t> __sta_exts =
> __static_extents<_Extents>(__begin, __end);
> +         span<const size_t> __sta_exts =
> __static_extents<_Extents>(__begin,
> +
> __end);
>
Here, I prefer slightly overlong line, our hard limit is 100, and the 80 is
more guideline,
but I think the code was more readable before.


>           size_t __ret = 1;
>           for(auto __ext : __sta_exts)
>             if (__ext != dynamic_extent)
> @@ -726,7 +733,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>      template<typename _Layout, typename _Mapping>
>        concept __mapping_of =
> -       is_same_v<typename _Layout::template mapping<typename
> _Mapping::extents_type>,
> +       is_same_v<typename _Layout::template mapping<
> +                   typename _Mapping::extents_type>,
>
Same here.

>                   _Mapping>;
>
>      template<template<size_t> typename _Layout, typename _Mapping>
> @@ -981,7 +989,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<class _RightPaddedMapping>
>         requires __mdspan::__is_right_padded_mapping<_RightPaddedMapping>
>                  && is_constructible_v<extents_type,
> -                                      typename
> _RightPaddedMapping::extents_type>
> +                     typename _RightPaddedMapping::extents_type>
>         constexpr
>         explicit(!is_convertible_v<typename
> _RightPaddedMapping::extents_type,
>                                    extents_type>)
> @@ -994,7 +1002,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>           if constexpr (__rank > 1)
>             {
> -             if constexpr (extents_type::static_extent(__rank - 1) !=
> dynamic_extent
> +             if constexpr (extents_type::static_extent(__rank - 1)
> +                           != dynamic_extent
>
Same here.
>
>                   && __ostride_sta != dynamic_extent)
>                 static_assert(extents_type::static_extent(__rank - 1)
>                     == __ostride_sta);
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> index 310dd8ddf20..e9172c13455 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> @@ -15,7 +15,7 @@ template<CustomIndexKind Kind>
>    public:
>      explicit
>      CustomIndexType(int i)
> -    : _M_i(i)
> +    : value(i)
>      { }
>
>      CustomIndexType() = delete;
> @@ -31,25 +31,25 @@ template<CustomIndexKind Kind>
>      constexpr
>      operator int() const noexcept
>      requires (Kind == CustomIndexKind::Const)
> -    { return _M_i; }
> +    { return value; }
>
>      constexpr
>      operator int() const
>      requires (Kind == CustomIndexKind::Throwing)
> -    { return _M_i; }
> +    { return value; }
>
>      constexpr
>      operator int() noexcept
>      requires (Kind == CustomIndexKind::Mutating)
> -    { return _M_i; }
> +    { return value; }
>
>      constexpr
>      operator int() && noexcept
>      requires (Kind == CustomIndexKind::RValue)
> -    { return _M_i; }
> +    { return value; }
>
>    private:
> -    int _M_i;
> +    int value;
>    };
>
>  using IntLike = CustomIndexType<CustomIndexKind::Const>;
> --
> 2.50.1
>
>
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