[PATCH v2 2/2] libstdc++: Simplify precomputed partial products in <mdspan>.

[Tomasz Kaminski]

On Mon, Aug 11, 2025 at 10:16 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> Prior to this commit, the partial products of static extents in <mdspan>
> was done in a loop that calls a function that computes the partial
> product. The complexity is quadratic in the rank.
>
> This commit removes the quadratic complexity.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (__static_prod): Delete.
>         (__fwd_partial_prods): Compute at compile-time in O(rank), not
>         O(rank**2).
>         (__rev_partial_prods): Ditto.
>         (__size): Inline __static_prod.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM.
Thank you for adjusting my preferences on the style here.

>  libstdc++-v3/include/std/mdspan | 44 +++++++++++++++++----------------
>  1 file changed, 23 insertions(+), 21 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 8f974257e96..4db4fd98b43 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -256,27 +256,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        __static_extents() noexcept
>        { return _Extents::_S_storage::_S_static_extents(); }
>
> -    template<array _Extents>
> -      consteval size_t
> -      __static_prod(size_t __begin, size_t __end)
> -      {
> -       size_t __prod = 1;
> -       for(size_t __i = __begin; __i < __end; ++__i)
> -         {
> -           auto __ext = _Extents[__i];
> -           __prod *= __ext == dynamic_extent ? size_t(1) : __ext;
> -         }
> -       return __prod;
> -      }
> -
>      // Pre-compute: \prod_{i = 0}^r _Extents[i], for r = 0,..., n
> (exclusive)
>      template<array _Extents>
>        constexpr auto __fwd_partial_prods = [] consteval
>         {
>           constexpr size_t __rank = _Extents.size();
>           std::array<size_t, __rank> __ret;
> -         for(size_t __r = 0; __r < __rank; ++__r)
> -           __ret[__r] = __static_prod<_Extents>(0, __r);
> +         size_t __prod = 1;
> +         for (size_t __r = 0; __r < __rank; ++__r)
> +           {
> +             __ret[__r] = __prod;
> +             if (size_t __ext = _Extents[__r]; __ext != dynamic_extent)
> +               __prod *= __ext;
> +           }
>           return __ret;
>         }();
>
> @@ -286,8 +278,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         {
>           constexpr size_t __rank = _Extents.size();
>           std::array<size_t, __rank> __ret;
> -         for(size_t __r = 0; __r < __rank; ++__r)
> -           __ret[__r] = __static_prod<_Extents>(__r + 1, __rank);
> +         size_t __prod = 1;
> +         for (size_t __r = __rank; __r > 0; --__r)
> +           {
> +             __ret[__r - 1] = __prod;
> +             if (size_t __ext = _Extents[__r - 1]; __ext !=
> dynamic_extent)
> +               __prod *= __ext;
> +           }
>           return __ret;
>         }();
>
> @@ -517,10 +514,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        constexpr typename _Extents::index_type
>        __size(const _Extents& __exts) noexcept
>        {
> -       constexpr auto& __sta_exts = __static_extents<_Extents>();
> -       constexpr size_t __rank = _Extents::rank();
> -       constexpr size_t __sta_prod = __static_prod<__sta_exts>(0, __rank);
> -       return __extents_prod(__exts, __sta_prod, 0, __rank);
> +       constexpr size_t __sta_prod = [] {
> +         span<const size_t> __sta_exts = __static_extents<_Extents>();
> +         size_t __ret = 1;
> +         for(auto __ext : __sta_exts)
> +           if (__ext != dynamic_extent)
> +             __ret *= __ext;
> +         return __ret;
> +       }();
> +       return __extents_prod(__exts, __sta_prod, 0, _Extents::rank());
>        }
>
>      template<typename _IndexType, size_t... _Counts>
> --
> 2.50.0
>
>
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