Second review, and changes I have done locally:
* replaced auto with span<const size_t> when saving __static_exts to avoid
making copy of array
* in __contains_zero for array keeping static size is not important, so
just rating dynamically sized span.
I will post update patch on this thread soon.
diff --git a/libstdc++-v3/include/std/mdspan
b/libstdc++-v3/include/std/mdspan
index 7b73df8550e..08e6b8ff518 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -355,7 +355,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp, size_t _Nm>
consteval bool
__contains_zero(const array<_Tp, _Nm>& __exts) noexcept
- { return __contains_zero(span<const _Tp, _Nm>{__exts}); }
+ { return __contains_zero(span<const _Tp>(__exts)); }
template<typename _Extents>
constexpr bool
@@ -388,9 +388,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
size_t __ret = 1;
if constexpr (_Extents::rank_dynamic() != _Extents::rank())
{
- auto __sta_exts = __static_extents<_Extents>();
- __ret = __static_extents_prod(span{__sta_exts.data() + __begin,
- __sta_exts.data() + __end});
+ std::span<const size_t> __sta_exts =
__static_extents<_Extents>();
+ __ret = __static_extents_prod(__sta_exts.subspan(__begin,
+ __end -
__begin));
if (__ret == 0)
return 0;
}
@@ -483,7 +483,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
consteval _IndexType
__static_quotient(_IndexType __nom =
numeric_limits<_IndexType>::max())
{
- auto __sta_exts = __static_extents<_Extents>();
+ std::span<const size_t> __sta_exts = __static_extents<_Extents>();
for (auto __factor : __sta_exts)
{
On Sun, Aug 3, 2025 at 11:02 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
> In mdspan related code involving static extents, often the IndexType is
> part of the template parameters, even though it's not needed.
>
> This commit extracts the parts of _ExtentsStorage not related to
> IndexType into a separate class _StaticExtents.
>
> It also prefers passing the array of static extents, instead of the
> whole extents object where possible.
>
> The size of an object file compiled with -O2 that instantiates
> Layout::mapping<extents<IndexType, Indices...>::stride
> Layout::mapping<extents<IndexType, Indices...>::required_span_size
> for the product of
> - eight IndexTypes
> - three Layouts,
> - nine choices of Indices...
> decreases by 19% from 69.2kB to 55.8kB.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (_StaticExtents): Extract non IndexType
> related code from _ExtentsStorage.
> (_ExtentsStorage): Use _StaticExtents.
> (__static_extents): Return reference to NTTP of _StaticExtents.
> (__contains_zero): New overload.
>
> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> ---
> libstdc++-v3/include/std/mdspan | 54 ++++++++++++++++++++++-----------
> 1 file changed, 36 insertions(+), 18 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 5e79d4bfb59..7b73df8550e 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -49,19 +49,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
> _GLIBCXX_BEGIN_NAMESPACE_VERSION
> namespace __mdspan
> {
> - template<typename _IndexType, array _Extents>
> - class _ExtentsStorage
> + template<array _Extents>
> + class _StaticExtents
> {
> public:
> static consteval bool
> _S_is_dyn(size_t __ext) noexcept
> { return __ext == dynamic_extent; }
>
> - template<typename _OIndexType>
> - static constexpr _IndexType
> - _S_int_cast(const _OIndexType& __other) noexcept
> - { return _IndexType(__other); }
> -
> static constexpr size_t _S_rank = _Extents.size();
>
> // For __r in [0, _S_rank], _S_dynamic_index[__r] is the number
> @@ -99,6 +94,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> static constexpr size_t
> _S_static_extent(size_t __r) noexcept
> { return _Extents[__r]; }
> + };
> +
> + template<typename _IndexType, array _Extents>
> + class _ExtentsStorage : public _StaticExtents<_Extents>
> + {
> + private:
> + using _S_base = _StaticExtents<_Extents>;
> +
> + public:
> + using _S_base::_S_rank;
> + using _S_base::_S_rank_dynamic;
> + using _S_base::_S_dynamic_index;
> + using _S_base::_S_dynamic_index_inv;
> +
> + template<typename _OIndexType>
> + static constexpr _IndexType
> + _S_int_cast(const _OIndexType& __other) noexcept
> + { return _IndexType(__other); }
>
> constexpr _IndexType
> _M_extent(size_t __r) const noexcept
> @@ -157,9 +170,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> { return __exts[__i]; });
> }
>
> - static constexpr span<const size_t>
> - _S_static_extents(size_t __begin, size_t __end) noexcept
> - { return {_Extents.data() + __begin, _Extents.data() + __end}; }
> + static constexpr const array<size_t, _S_rank>&
> + _S_static_extents() noexcept
> + { return _Extents; }
>
> constexpr span<const _IndexType>
> _M_dynamic_extents(size_t __begin, size_t __end) const noexcept
> @@ -185,10 +198,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> || _Extent <= numeric_limits<_IndexType>::max();
>
> template<typename _Extents>
> - constexpr span<const size_t>
> - __static_extents(size_t __begin = 0, size_t __end =
> _Extents::rank())
> - noexcept
> - { return _Extents::_S_storage::_S_static_extents(__begin, __end); }
> + constexpr const array<size_t, _Extents::rank()>&
> + __static_extents() noexcept
> + { return _Extents::_S_storage::_S_static_extents(); }
>
> template<typename _Extents>
> constexpr span<const typename _Extents::index_type>
> @@ -314,8 +326,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
>
> private:
> - friend span<const size_t>
> - __mdspan::__static_extents<extents>(size_t, size_t);
> + friend const array<size_t, rank()>&
> + __mdspan::__static_extents<extents>();
>
> friend span<const index_type>
> __mdspan::__dynamic_extents<extents>(const extents&, size_t,
> size_t);
> @@ -340,6 +352,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return false;
> }
>
> + template<typename _Tp, size_t _Nm>
> + consteval bool
> + __contains_zero(const array<_Tp, _Nm>& __exts) noexcept
> + { return __contains_zero(span<const _Tp, _Nm>{__exts}); }
> +
> template<typename _Extents>
> constexpr bool
> __empty(const _Extents& __exts) noexcept
> @@ -371,8 +388,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> size_t __ret = 1;
> if constexpr (_Extents::rank_dynamic() != _Extents::rank())
> {
> - auto __sta_exts = __static_extents<_Extents>(__begin, __end);
> - __ret = __static_extents_prod(__sta_exts);
> + auto __sta_exts = __static_extents<_Extents>();
> + __ret = __static_extents_prod(span{__sta_exts.data() + __begin,
> + __sta_exts.data() + __end});
> if (__ret == 0)
> return 0;
> }
> --
> 2.50.0
>
>
