[{"id":3687014,"web_url":"http://patchwork.ozlabs.org/comment/3687014/","msgid":"","list_archive_url":null,"date":"2026-05-06T12:02:38","subject":"Re: [PATCH v2 2/3] libstdc++: Simplify std::shared_ptr internals","submitter":{"id":90409,"url":"http://patchwork.ozlabs.org/api/people/90409/","name":"Tomasz Kamiński","email":"tkaminsk@redhat.com"},"content":"On Wed, May 6, 2026 at 12:14 PM Jonathan Wakely wrote:\n\n> Now that we don't use the EBO we can flatten the class layout by\n> removing some _Impl classes.\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/bits/out_ptr.h (out_ptr_t::_Impl::~_Impl): Adjust\n> access to shared_ptr internals.\n> * include/bits/shared_ptr_base.h (_Sp_counted_deleter): Remove\n> _Impl class and replace _M_impl with the data members it\n> contained.\n> (_Sp_counted_ptr_inplace): Likewise.\n> ---\n>\n> v2: Use the attribute on the _M_del and _M_alloc members.\n>\nLGTM. I see now why you want [[no_unique_address(cond)]], because\nwith that we could simply have _M_alloc member of type _Alloc.\n\n>\n> Tested x86_64-linux.\n>\n> libstdc++-v3/include/bits/out_ptr.h | 2 +-\n> libstdc++-v3/include/bits/shared_ptr_base.h | 60 ++++++---------------\n> 2 files changed, 18 insertions(+), 44 deletions(-)\n>\n> diff --git a/libstdc++-v3/include/bits/out_ptr.h\n> b/libstdc++-v3/include/bits/out_ptr.h\n> index cc5a8a3adacb..f5fdc6cf910f 100644\n> --- a/libstdc++-v3/include/bits/out_ptr.h\n> +++ b/libstdc++-v3/include/bits/out_ptr.h\n> @@ -257,7 +257,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> auto& __pi = _M_smart._M_refcount._M_pi;\n>\n> if (_Sp __ptr = _M_smart.get())\n> - static_cast<_Scd*>(__pi)->_M_impl._M_ptr = __ptr;\n> + static_cast<_Scd*>(__pi)->_M_ptr = __ptr;\n> else // Destroy the control block manually without invoking\n> deleter.\n> std::__exchange(__pi, nullptr)->_M_destroy();\n> }\n> diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h\n> b/libstdc++-v3/include/bits/shared_ptr_base.h\n> index 3ab73f6e4a0d..01fd57180ecc 100644\n> --- a/libstdc++-v3/include/bits/shared_ptr_base.h\n> +++ b/libstdc++-v3/include/bits/shared_ptr_base.h\n> @@ -547,32 +547,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> template _Lock_policy _Lp>\n> class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>\n> {\n> - class _Impl\n> - {\n> - [[__no_unique_address__]] _Sp_ebo_helper<_Deleter> _M_d;\n> - [[__no_unique_address__]] _Sp_ebo_helper<_Alloc> _M_a;\n> -\n> - public:\n> - _Impl(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept\n> - : _M_d{std::move(__d)}, _M_a{__a}, _M_ptr(__p)\n> - { }\n> -\n> - _Deleter& _M_del() noexcept { return _M_d._M_obj; }\n> - _Alloc& _M_alloc() noexcept { return _M_a._M_obj; }\n> -\n> - _Ptr _M_ptr;\n> - };\n> -\n> public:\n> using __allocator_type = __alloc_rebind<_Alloc,\n> _Sp_counted_deleter>;\n>\n> // __d(__p) must not throw.\n> _Sp_counted_deleter(_Ptr __p, _Deleter __d) noexcept\n> - : _M_impl(__p, std::move(__d), _Alloc()) { }\n> + : _M_del{std::move(__d)}, _M_alloc{}, _M_ptr(__p) { }\n>\n> // __d(__p) must not throw.\n> _Sp_counted_deleter(_Ptr __p, _Deleter __d, const _Alloc& __a)\n> noexcept\n> - : _M_impl(__p, std::move(__d), __a) { }\n> + : _M_del{std::move(__d)}, _M_alloc{__a}, _M_ptr(__p) { }\n>\n> #pragma GCC diagnostic push // PR tree-optimization/122197\n> #pragma GCC diagnostic ignored \"-Wfree-nonheap-object\"\n> @@ -582,12 +566,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n>\n> virtual void\n> _M_dispose() noexcept\n> - { _M_impl._M_del()(_M_impl._M_ptr); }\n> + { _M_del._M_obj(_M_ptr); }\n>\n> virtual void\n> _M_destroy() noexcept\n> {\n> - __allocator_type __a(_M_impl._M_alloc());\n> + __allocator_type __a(_M_alloc._M_obj);\n> __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };\n> this->~_Sp_counted_deleter();\n> }\n> @@ -598,19 +582,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> #if __cpp_rtti\n> // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> // 2400. shared_ptr's get_deleter() should use addressof()\n> - return __ti == typeid(_Deleter)\n> - ? std::__addressof(_M_impl._M_del())\n> - : nullptr;\n> -#else\n> - return nullptr;\n> + if (__ti == typeid(_Deleter))\n> + return std::__addressof(_M_del._M_obj);\n> #endif\n> + return nullptr;\n> }\n>\n> private:\n> #ifdef __glibcxx_out_ptr\n> template friend class out_ptr_t;\n> #endif\n> - _Impl _M_impl;\n> +\n> + [[__no_unique_address__]] _Sp_ebo_helper<_Deleter> _M_del;\n> + [[__no_unique_address__]] _Sp_ebo_helper<_Alloc> _M_alloc;\n> + _Ptr _M_ptr;\n> };\n>\n> // helpers for make_shared / allocate_shared\n> @@ -640,25 +625,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> template\n> class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>\n> {\n> - class _Impl\n> - {\n> - [[__no_unique_address__]] _Sp_ebo_helper<_Alloc> _M_a;\n> -\n> - public:\n> - explicit _Impl(_Alloc __a) noexcept : _M_a{std::move(__a)} { }\n> -\n> - _Alloc& _M_alloc() noexcept { return _M_a._M_obj; }\n> -\n> - __gnu_cxx::__aligned_buffer<__remove_cv_t<_Tp>> _M_storage;\n> - };\n> -\n> public:\n> using __allocator_type = __alloc_rebind<_Alloc,\n> _Sp_counted_ptr_inplace>;\n>\n> // Alloc parameter is not a reference so doesn't alias anything in\n> __args\n> template\n> _Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)\n> - : _M_impl(__a)\n> + : _M_alloc{__a}\n> {\n> // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> // 2070. allocate_shared should use\n> allocator_traits::construct\n> @@ -674,14 +647,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> virtual void\n> _M_dispose() noexcept\n> {\n> - allocator_traits<_Alloc>::destroy(_M_impl._M_alloc(), _M_ptr());\n> + allocator_traits<_Alloc>::destroy(_M_alloc._M_obj, _M_ptr());\n> }\n>\n> // Override because the allocator needs to know the dynamic type\n> virtual void\n> _M_destroy() noexcept\n> {\n> - __allocator_type __a(_M_impl._M_alloc());\n> + __allocator_type __a(_M_alloc._M_obj);\n> __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };\n> this->~_Sp_counted_ptr_inplace();\n> }\n> @@ -711,9 +684,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> }\n>\n> __remove_cv_t<_Tp>*\n> - _M_ptr() noexcept { return _M_impl._M_storage._M_ptr(); }\n> + _M_ptr() noexcept { return _M_storage._M_ptr(); }\n>\n> - _Impl _M_impl;\n> + [[__no_unique_address__]] _Sp_ebo_helper<_Alloc> _M_alloc;\n> + __gnu_cxx::__aligned_buffer<__remove_cv_t<_Tp>> _M_storage;\n> };\n>\n> #ifdef __glibcxx_smart_ptr_for_overwrite // C++ >= 20 && HOSTED\n> --\n> 2.54.0\n>\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=czhr3fLT;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=czhr3fLT","sourceware.org; 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