[PATCH] libstdc++: Optimize __make_comp_proj and __make_pred_proj for empty types

[Jonathan Wakely]

On Thu, 12 Jun 2025, 16:56 Patrick Palka, <ppalka@redhat.com> wrote:

> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> I'm sure if introducing a new overload is preferable to
> introducing a constexpr if branch in the existing overload.
>

I think a constexpr if branch in the existing functions is cheaper. We need
to check the traits either way, but we can avoid doing overload resolution.



> -- >8 --
>
> When composing a comparator/predicate and projection function, if both
> are stateless (and default constructible) then we don't need to capture
> them by reference; we can just return a captureless lambda and materialize
> them on the spot.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/ranges_algo.h (__detail::__make_comp_proj): New
>         more specialized overload returning a stateless lambda if both
>         the comparator/predicate and projection are stateless.
>         (__detail::__make_pred_proj): Likewise.
> ---
>  libstdc++-v3/include/bits/ranges_algo.h | 27 +++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> b/libstdc++-v3/include/bits/ranges_algo.h
> index 819316fa79c3..7dc84d639fbd 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -61,6 +61,21 @@ namespace ranges
>         };
>        }
>
> +    template<typename _Comp, typename _Proj>
> +      requires is_empty_v<_Comp> && is_default_constructible_v<_Comp>
> +       && is_empty_v<_Proj> && is_default_constructible_v<_Proj>
> +      constexpr auto
> +      __make_comp_proj(_Comp& __comp, _Proj& __proj)
> +      {
> +       return [] (auto&& __lhs, auto&& __rhs) -> bool {
> +         using _TL = decltype(__lhs);
> +         using _TR = decltype(__rhs);
> +         return std::__invoke(_Comp{},
> +                              std::__invoke(_Proj{},
> std::forward<_TL>(__lhs)),
> +                              std::__invoke(_Proj{},
> std::forward<_TR>(__rhs)));
> +       };
> +      }
> +
>      template<typename _Pred, typename _Proj>
>        constexpr auto
>        __make_pred_proj(_Pred& __pred, _Proj& __proj)
> @@ -70,6 +85,18 @@ namespace ranges
>                                std::__invoke(__proj,
> std::forward<_Tp>(__arg)));
>         };
>        }
> +
> +    template<typename _Pred, typename _Proj>
> +      requires is_empty_v<_Pred> && is_default_constructible_v<_Pred>
> +       && is_empty_v<_Proj> && is_default_constructible_v<_Pred>
> +      constexpr auto
> +      __make_pred_proj(_Pred& __pred, _Proj& __proj)
> +      {
> +       return [] <typename _Tp> (_Tp&& __arg) -> bool {
> +         return std::__invoke(_Pred{},
> +                              std::__invoke(_Proj{},
> std::forward<_Tp>(__arg)));
> +       };
> +      }
>    } // namespace __detail
>
>    struct __all_of_fn
> --
> 2.50.0.rc2
>
>
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