Re: [PATCH 1/3] libstdc++: Remove std::bind_front specialization for no bound args

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 12 Sept 2023 at 13:46, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Mon, 11 Sep 2023, Patrick Palka wrote:
>
> > This specialization for the case of no bound args, added by
> > r13-4214-gcbd05ca5ab1231, seems to be mostly obsoleted by
> > r13-5033-ge2eab3c4edb6aa which added [[no_unique_address]] to the
> > main template's data members.  And the compile time advantage of
> > avoiding an empty tuple and index_sequence seems minimal.  Removing this
> > specialization also means we don't have to fix the PR111327 bug in
> > another place.
>
> FWIW I don't feel strongly about removing this specialization.  If we
> keep it We'd at least be able to reuse it for std::bind_back, and it
> wouldn't be hard to fix the PR111327 bug in its implementation.

Yeah, I'm ambivalent. But since you've got a patch to fix 111327 ready
which doesn't include this specialization, let's remove it.

The empty std::tuple is at least already explicitly specialized, so I
agree its overhead probably isn't very significant.

OK for trunk. I'm not sure if we should change it in gcc-13 now though.

>
> >
> >       PR libstdc++/111327
> >
> > libstdc++-v3/ChangeLog:
> >
> >       * include/std/functional (_Bind_front0): Remove.
> >       (_Bind_front_t): Adjust.
> > ---
> >  libstdc++-v3/include/std/functional | 63 +----------------------------
> >  1 file changed, 1 insertion(+), 62 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
> > index 60d4d1f3dd2..7d1b890bb4e 100644
> > --- a/libstdc++-v3/include/std/functional
> > +++ b/libstdc++-v3/include/std/functional
> > @@ -996,69 +996,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >        [[no_unique_address]] std::tuple<_BoundArgs...> _M_bound_args;
> >      };
> >
> > -  // Avoid the overhead of an empty tuple<> if there are no bound args.
> > -  template<typename _Fd>
> > -    struct _Bind_front0
> > -    {
> > -      static_assert(is_move_constructible_v<_Fd>);
> > -
> > -      // First parameter is to ensure this constructor is never used
> > -      // instead of the copy/move constructor.
> > -      template<typename _Fn>
> > -     explicit constexpr
> > -     _Bind_front0(int, _Fn&& __fn)
> > -     noexcept(is_nothrow_constructible_v<_Fd, _Fn>)
> > -     : _M_fd(std::forward<_Fn>(__fn))
> > -     { }
> > -
> > -      _Bind_front0(const _Bind_front0&) = default;
> > -      _Bind_front0(_Bind_front0&&) = default;
> > -      _Bind_front0& operator=(const _Bind_front0&) = default;
> > -      _Bind_front0& operator=(_Bind_front0&&) = default;
> > -      ~_Bind_front0() = default;
> > -
> > -      template<typename... _CallArgs>
> > -     constexpr
> > -     invoke_result_t<_Fd&, _CallArgs...>
> > -     operator()(_CallArgs&&... __call_args) &
> > -     noexcept(is_nothrow_invocable_v<_Fd&, _CallArgs...>)
> > -     { return std::invoke(_M_fd, std::forward<_CallArgs>(__call_args)...); }
> > -
> > -      template<typename... _CallArgs>
> > -     constexpr
> > -     invoke_result_t<const _Fd&, _CallArgs...>
> > -     operator()(_CallArgs&&... __call_args) const &
> > -     noexcept(is_nothrow_invocable_v<const _Fd&, _CallArgs...>)
> > -     { return std::invoke(_M_fd, std::forward<_CallArgs>(__call_args)...); }
> > -
> > -      template<typename... _CallArgs>
> > -     constexpr
> > -     invoke_result_t<_Fd, _CallArgs...>
> > -     operator()(_CallArgs&&... __call_args) &&
> > -     noexcept(is_nothrow_invocable_v<_Fd, _CallArgs...>)
> > -     {
> > -       return std::invoke(std::move(_M_fd),
> > -                          std::forward<_CallArgs>(__call_args)...);
> > -     }
> > -
> > -      template<typename... _CallArgs>
> > -     constexpr
> > -     invoke_result_t<const _Fd, _CallArgs...>
> > -     operator()(_CallArgs&&... __call_args) const &&
> > -     noexcept(is_nothrow_invocable_v<const _Fd, _CallArgs...>)
> > -     {
> > -       return std::invoke(std::move(_M_fd),
> > -                          std::forward<_CallArgs>(__call_args)...);
> > -     }
> > -
> > -    private:
> > -      [[no_unique_address]] _Fd _M_fd;
> > -    };
> > -
> >    template<typename _Fn, typename... _Args>
> > -    using _Bind_front_t
> > -      = __conditional_t<sizeof...(_Args) == 0, _Bind_front0<decay_t<_Fn>>,
> > -                     _Bind_front<decay_t<_Fn>, decay_t<_Args>...>>;
> > +    using _Bind_front_t = _Bind_front<decay_t<_Fn>, decay_t<_Args>...>;
> >
> >    /** Create call wrapper by partial application of arguments to function.
> >     *
> > --
> > 2.42.0.158.g94e83dcf5b
> >
> >
>