[2/3] libstdc++: Simplify std::shared_ptr internals

  Now that we don't use the EBO we can flatten the class layout by
removing some _Impl classes.

libstdc++-v3/ChangeLog:

	* include/bits/out_ptr.h (out_ptr_t::_Impl::~_Impl): Adjust
	access to shared_ptr internals.
	* include/bits/shared_ptr_base.h (_Sp_counted_deleter): Remove
	_Impl class and replace _M_impl with the data members it
	contained.
	(_Sp_counted_ptr_inplace): Likewise.
---

Tested x86_64-linux.

 libstdc++-v3/include/bits/out_ptr.h         |  2 +-
 libstdc++-v3/include/bits/shared_ptr_base.h | 60 ++++++---------------
 2 files changed, 18 insertions(+), 44 deletions(-)
  

On Tue, May 5, 2026 at 5:06 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> Now that we don't use the EBO we can flatten the class layout by
> removing some _Impl classes.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/out_ptr.h (out_ptr_t::_Impl::~_Impl): Adjust
>         access to shared_ptr internals.
>         * include/bits/shared_ptr_base.h (_Sp_counted_deleter): Remove
>         _Impl class and replace _M_impl with the data members it
>         contained.
>         (_Sp_counted_ptr_inplace): Likewise.
> ---
>
> Tested x86_64-linux.
>
>  libstdc++-v3/include/bits/out_ptr.h         |  2 +-
>  libstdc++-v3/include/bits/shared_ptr_base.h | 60 ++++++---------------
>  2 files changed, 18 insertions(+), 44 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/out_ptr.h
> b/libstdc++-v3/include/bits/out_ptr.h
> index cc5a8a3adacb..f5fdc6cf910f 100644
> --- a/libstdc++-v3/include/bits/out_ptr.h
> +++ b/libstdc++-v3/include/bits/out_ptr.h
> @@ -257,7 +257,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>             auto& __pi = _M_smart._M_refcount._M_pi;
>
>             if (_Sp __ptr = _M_smart.get())
> -             static_cast<_Scd*>(__pi)->_M_impl._M_ptr = __ptr;
> +             static_cast<_Scd*>(__pi)->_M_ptr = __ptr;
>             else // Destroy the control block manually without invoking
> deleter.
>               std::__exchange(__pi, nullptr)->_M_destroy();
>           }
> diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h
> b/libstdc++-v3/include/bits/shared_ptr_base.h
> index 3a966eddc06c..cf1be953af2c 100644
> --- a/libstdc++-v3/include/bits/shared_ptr_base.h
> +++ b/libstdc++-v3/include/bits/shared_ptr_base.h
> @@ -547,32 +547,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<typename _Ptr, typename _Deleter, typename _Alloc,
> _Lock_policy _Lp>
>      class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>
>      {
> -      class _Impl
> -      {
> -       _Sp_ebo_helper<_Deleter> _M_d;
> -       _Sp_ebo_helper<_Alloc>   _M_a;
> -
> -      public:
> -       _Impl(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
> -       : _M_d{std::move(__d)}, _M_a{__a}, _M_ptr(__p)
> -       { }
> -
> -       _Deleter& _M_del() noexcept { return _M_d._M_obj; }
> -       _Alloc& _M_alloc() noexcept { return _M_a._M_obj; }
> -
> -       _Ptr _M_ptr;
> -      };
> -
>      public:
>        using __allocator_type = __alloc_rebind<_Alloc,
> _Sp_counted_deleter>;
>
>        // __d(__p) must not throw.
>        _Sp_counted_deleter(_Ptr __p, _Deleter __d) noexcept
> -      : _M_impl(__p, std::move(__d), _Alloc()) { }
> +      : _M_del{std::move(__d)}, _M_alloc{}, _M_ptr(__p) { }
>
>        // __d(__p) must not throw.
>        _Sp_counted_deleter(_Ptr __p, _Deleter __d, const _Alloc& __a)
> noexcept
> -      : _M_impl(__p, std::move(__d), __a) { }
> +      : _M_del{std::move(__d)}, _M_alloc{__a}, _M_ptr(__p) { }
>
>  #pragma GCC diagnostic push // PR tree-optimization/122197
>  #pragma GCC diagnostic ignored "-Wfree-nonheap-object"
> @@ -582,12 +566,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>        virtual void
>        _M_dispose() noexcept
> -      { _M_impl._M_del()(_M_impl._M_ptr); }
> +      { _M_del._M_obj(_M_ptr); }
>
>        virtual void
>        _M_destroy() noexcept
>        {
> -       __allocator_type __a(_M_impl._M_alloc());
> +       __allocator_type __a(_M_alloc._M_obj);
>         __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
>         this->~_Sp_counted_deleter();
>        }
> @@ -598,19 +582,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #if __cpp_rtti
>         // _GLIBCXX_RESOLVE_LIB_DEFECTS
>         // 2400. shared_ptr's get_deleter() should use addressof()
> -        return __ti == typeid(_Deleter)
> -         ? std::__addressof(_M_impl._M_del())
> -         : nullptr;
> -#else
> -        return nullptr;
> +       if (__ti == typeid(_Deleter))
> +         return std::__addressof(_M_del._M_obj);
>  #endif
> +       return nullptr;
>        }
>
>      private:
>  #ifdef __glibcxx_out_ptr
>        template<typename, typename, typename...> friend class out_ptr_t;
>  #endif
> -      _Impl _M_impl;
> +
> +      _Sp_ebo_helper<_Deleter> _M_del;
> +      _Sp_ebo_helper<_Alloc>   _M_alloc;
>
As in previous patch you need [[no_unique_address]] on this member.

> +      _Ptr _M_ptr;
>      };
>
>    // helpers for make_shared / allocate_shared
> @@ -640,25 +625,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
>      class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>
>      {
> -      class _Impl
> -      {
> -       _Sp_ebo_helper<_Alloc>  _M_a;
> -
> -      public:
> -       explicit _Impl(_Alloc __a) noexcept : _M_a{std::move(__a)} { }
> -
> -       _Alloc& _M_alloc() noexcept { return _M_a._M_obj; }
> -
> -       __gnu_cxx::__aligned_buffer<__remove_cv_t<_Tp>> _M_storage;
> -      };
> -
>      public:
>        using __allocator_type = __alloc_rebind<_Alloc,
> _Sp_counted_ptr_inplace>;
>
>        // Alloc parameter is not a reference so doesn't alias anything in
> __args
>        template<typename... _Args>
>         _Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
> -       : _M_impl(__a)
> +       : _M_alloc{__a}
>         {
>           // _GLIBCXX_RESOLVE_LIB_DEFECTS
>           // 2070.  allocate_shared should use
> allocator_traits<A>::construct
> @@ -674,14 +647,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        virtual void
>        _M_dispose() noexcept
>        {
> -       allocator_traits<_Alloc>::destroy(_M_impl._M_alloc(), _M_ptr());
> +       allocator_traits<_Alloc>::destroy(_M_alloc._M_obj, _M_ptr());
>        }
>
>        // Override because the allocator needs to know the dynamic type
>        virtual void
>        _M_destroy() noexcept
>        {
> -       __allocator_type __a(_M_impl._M_alloc());
> +       __allocator_type __a(_M_alloc._M_obj);
>         __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
>         this->~_Sp_counted_ptr_inplace();
>        }
> @@ -711,9 +684,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>
>        __remove_cv_t<_Tp>*
> -      _M_ptr() noexcept { return _M_impl._M_storage._M_ptr(); }
> +      _M_ptr() noexcept { return _M_storage._M_ptr(); }
>
> -      _Impl _M_impl;
> +      _Sp_ebo_helper<_Alloc> _M_alloc;
> +      __gnu_cxx::__aligned_buffer<__remove_cv_t<_Tp>> _M_storage;
>      };
>
>  #ifdef __glibcxx_smart_ptr_for_overwrite // C++ >= 20 && HOSTED
> --
> 2.54.0
>
>