[PATCH v2] libstdc++: Improve and cleanup mdspan related code.

[Tomasz Kaminski]

On Wed, Oct 8, 2025 at 10:37 AM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Fri, 03 Oct 2025 at 12:48 +0200, Tomasz Kamiński wrote:
> >From: Luc Grosheintz <luc.grosheintz@gmail.com>
> >
> >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.
> >
> >Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
> >Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> >---
> >v2 revers few whitespace changes.
> >
> > libstdc++-v3/include/std/mdspan                  | 16 +++++++++++-----
> > .../testsuite/23_containers/mdspan/int_like.h    | 12 ++++++------
> > 2 files changed, 17 insertions(+), 11 deletions(-)
> >
> >diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> >index ce7e4178f67..469d4c3cb6c 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));
>
> Do we need the std::move here, if we know that __other has an integral
> type?
>
Good catch. The `std::move` was applied out of habit here. I will change
that before
submitting.

>
>
>
> >         }
> >       else
> >         {
> >@@ -981,7 +987,7 @@ _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>)
> >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>;
> >--
> >2.51.0
> >
> >
>
>
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