{"id":2226663,"url":"http://patchwork.ozlabs.org/api/patches/2226663/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhunvhjcqm.gcc.gcc-TEST.peppe.41.1.1@forge-stage.sourceware.org/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null,"list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"","list_archive_url":null,"date":"2026-04-22T18:03:45","name":"[v1,1/1] libstdc++: optimize weak_ptr converting constructor/assignment","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"550c3ee5c0bad5b9d456c0fffcf670d13f80e516","submitter":{"id":93222,"url":"http://patchwork.ozlabs.org/api/people/93222/?format=json","name":"peppe via Sourceware Forge","email":"forge-bot+peppe@forge-stage.sourceware.org"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhunvhjcqm.gcc.gcc-TEST.peppe.41.1.1@forge-stage.sourceware.org/mbox/","series":[{"id":501065,"url":"http://patchwork.ozlabs.org/api/series/501065/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=501065","date":"2026-04-22T18:03:44","name":"WIP: libstdc++: use is_virtual_base_of in std::weak_ptr operations","version":1,"mbox":"http://patchwork.ozlabs.org/series/501065/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2226663/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2226663/checks/","tags":{},"related":[],"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 spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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server2.sourceware.org","From":"peppe via Sourceware Forge ","Date":"Wed, 22 Apr 2026 18:03:45 +0000","Subject":"[PATCH v1 1/1] libstdc++: optimize weak_ptr converting\n constructor/assignment","To":"gcc-patches mailing list ","Message-ID":"\n ","X-Mailer":"batrachomyomachia","X-Pull-Request-Organization":"gcc","X-Pull-Request-Repository":"gcc-TEST","X-Pull-Request":"https://forge.sourceware.org/gcc/gcc-TEST/pulls/41","References":"\n ","In-Reply-To":"\n ","X-Patch-URL":"\n https://forge.sourceware.org/peppe/gcc-TEST/commit/a081ba7fda52fe2a74923166a4e97976292a9155","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"gcc-patches mailing list ,\n peppe@noreply.localhost","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"From: Giuseppe D'Angelo \n\nConverting a weak_ptr to a weak_ptr requires calling\nlock() on the source object in the general case.\n\nAlthough the source weak_ptr does contain a raw pointer to\nDerived, we can't just get it and (up)cast it to Base, as that will\ndereference the pointer in case Base is a virtual base class of Derived.\n\nWe don't know if the managed object is still alive, and therefore if\nthis operation is safe to do; we therefore temporarily lock() the source\nweak_ptr, do the cast using the resulting shared_ptr, and then discard\nthis shared_ptr. Simply checking the strong counter isn't sufficient,\nbecause if multiple threads are involved then we'd have a race / TOCTOU\nproblem; the object may get destroyed after we check the strong counter\nand before we cast the pointer.\n\nHowever lock() is not necessary if we know that Base is *not* a virtual\nbase class of Derived; in this case we can avoid the relatively\nexpensive call to lock() and just cast the pointer. This commit uses\nthe newly added builtin to detect this case and optimize std::weak_ptr's\nconverting constructors and assignment operations.\n\nApart from non-virtual bases, there's also another couple of interesting\ncases where we can also avoid locking. Specifically:\n\n1) converting a weak_ptr to a weak_ptr;\n2) converting a weak_ptr to a weak_ptr or similar.\n\nSince this logic is going to be used by multiple places, I've\ncentralized it in a new static helper.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/bits/shared_ptr_base.h (__weak_ptr): Avoid calling\n\tlock() when converting or assigning a weak_ptr to\n\ta weak_ptr in case Base is not a virtual base of Derived.\n\tThis logic is centralized in _S_safe_upcast, called by the\n\tvarious converting constructors/assignment operators.\n\t(_S_safe_upcast): New helper function.\n\t* testsuite/20_util/weak_ptr/cons/virtual_bases.cc: New test.\n---\n libstdc++-v3/include/bits/shared_ptr_base.h | 52 ++++++++++--\n .../20_util/weak_ptr/cons/virtual_bases.cc | 80 +++++++++++++++++++\n 2 files changed, 125 insertions(+), 7 deletions(-)\n create mode 100644 libstdc++-v3/testsuite/20_util/weak_ptr/cons/virtual_bases.cc","diff":"diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h\nindex b4be1b49e4da..5f3ae33848a6 100644\n--- a/libstdc++-v3/include/bits/shared_ptr_base.h\n+++ b/libstdc++-v3/include/bits/shared_ptr_base.h\n@@ -1986,6 +1986,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n template\n class __weak_ptr\n {\n+ public:\n+ using element_type = typename remove_extent<_Tp>::type;\n+\n+ private:\n template\n \tusing _Compatible = typename\n \t enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;\n@@ -1994,9 +1998,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n template\n \tusing _Assignable = _Compatible<_Yp, __weak_ptr&>;\n \n- public:\n- using element_type = typename remove_extent<_Tp>::type;\n+#pragma GCC diagnostic push\n+#pragma GCC diagnostic ignored \"-Wc++17-extensions\" // if constexpr\n+ // Helper for construction/assignment:\n+ template\n+\tstatic element_type* _S_safe_upcast(const __weak_ptr<_Yp, _Lp>& __r)\n+\t{\n+\t // We know that _Yp and _Tp are compatible, that is, either\n+\t // _Yp* is convertible to _Tp* or _Yp is U[N] and _Tp is U cv [].\n+\n+\t // If _Yp is the same as _Tp after removing extents and cv\n+\t // qualifications, there's no pointer adjustments to do. This\n+\t // also allows us to support incomplete types.\n+\t using _At = typename remove_cv::type>::type;\n+\t using _Bt = typename remove_cv::type>::type;\n+\t if constexpr (is_same<_At, _Bt>::value)\n+\t return __r._M_ptr;\n+\t // If they're not the same type, but they're both scalars,\n+\t // we again don't need any adjustment. This allows us to support e.g.\n+\t // pointers to a differently cv qualified type X.\n+\t else if constexpr (__and_, is_scalar<_Bt>>::value)\n+\t return __r._M_ptr;\n+#if _GLIBCXX_USE_BUILTIN_TRAIT(__builtin_is_virtual_base_of)\n+\t // If _Tp is not a virtual base class of _Yp, the pointer\n+\t // conversion does not require dereferencing __r._M_ptr; just\n+\t // rely on the implicit conversion.\n+\t else if constexpr (!__builtin_is_virtual_base_of(_Tp, _Yp))\n+\t return __r._M_ptr;\n+#endif\n+\t // Expensive path; must lock() and do the pointer conversion while\n+\t // a shared_ptr keeps the pointee alive (because we may need\n+\t // to dereference).\n+\t else\n+\t return __r.lock().get();\n+\t}\n+#pragma GCC diagnostic pop\n \n+ public:\n constexpr __weak_ptr() noexcept\n : _M_ptr(nullptr), _M_refcount()\n { }\n@@ -2021,8 +2059,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n // in multithreaded programs __r._M_ptr may be invalidated at any point.\n template>\n \t__weak_ptr(const __weak_ptr<_Yp, _Lp>& __r) noexcept\n-\t: _M_refcount(__r._M_refcount)\n- { _M_ptr = __r.lock().get(); }\n+\t: _M_ptr(_S_safe_upcast(__r)), _M_refcount(__r._M_refcount)\n+ { }\n \n template>\n \t__weak_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept\n@@ -2035,7 +2073,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n \n template>\n \t__weak_ptr(__weak_ptr<_Yp, _Lp>&& __r) noexcept\n-\t: _M_ptr(__r.lock().get()), _M_refcount(std::move(__r._M_refcount))\n+\t: _M_ptr(_S_safe_upcast(__r)), _M_refcount(std::move(__r._M_refcount))\n { __r._M_ptr = nullptr; }\n \n __weak_ptr&\n@@ -2045,7 +2083,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n \t_Assignable<_Yp>\n \toperator=(const __weak_ptr<_Yp, _Lp>& __r) noexcept\n \t{\n-\t _M_ptr = __r.lock().get();\n+\t _M_ptr = _S_safe_upcast(__r);\n \t _M_refcount = __r._M_refcount;\n \t return *this;\n \t}\n@@ -2070,7 +2108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n \t_Assignable<_Yp>\n \toperator=(__weak_ptr<_Yp, _Lp>&& __r) noexcept\n \t{\n-\t _M_ptr = __r.lock().get();\n+\t _M_ptr = _S_safe_upcast(__r);\n \t _M_refcount = std::move(__r._M_refcount);\n \t __r._M_ptr = nullptr;\n \t return *this;\ndiff --git a/libstdc++-v3/testsuite/20_util/weak_ptr/cons/virtual_bases.cc b/libstdc++-v3/testsuite/20_util/weak_ptr/cons/virtual_bases.cc\nnew file mode 100644\nindex 000000000000..ac3e4bce9da5\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/20_util/weak_ptr/cons/virtual_bases.cc\n@@ -0,0 +1,80 @@\n+// { dg-do run { target c++11 } }\n+\n+#include \n+#include \n+\n+struct BaseBase { virtual ~BaseBase() = default; };\n+struct Base : BaseBase { virtual ~Base() = default; };\n+struct Derived1 : Base { virtual ~Derived1() = default; };\n+struct Derived2 : virtual Base { virtual ~Derived2() = default; };\n+struct Derived3 : virtual Base { virtual ~Derived3() = default; };\n+struct Derived4 : Derived2, Derived3 { virtual ~Derived4() = default; };\n+struct Derived5 : Derived4 { virtual ~Derived5() = default; };\n+\n+template\n+void test01()\n+{\n+ std::shared_ptr ptr(new T);\n+ VERIFY(ptr);\n+\n+ std::weak_ptr wptr1 = ptr;\n+ VERIFY(wptr1.lock());\n+\n+ std::weak_ptr wptr2 = ptr;\n+ VERIFY(wptr2.lock());\n+\n+ std::weak_ptr wptr3 = wptr1;\n+ VERIFY(wptr3.lock());\n+\n+ std::weak_ptr wptr4 = ptr;\n+ VERIFY(wptr4.lock());\n+\n+ std::weak_ptr wptr5 = std::move(wptr1);\n+ VERIFY(wptr5.lock());\n+\n+ ptr.reset();\n+\n+ VERIFY(!wptr1.lock());\n+ VERIFY(!wptr2.lock());\n+ VERIFY(!wptr3.lock());\n+ VERIFY(!wptr4.lock());\n+ VERIFY(!wptr5.lock());\n+}\n+\n+template\n+void test02()\n+{\n+ std::shared_ptr ptr(new T);\n+ VERIFY(ptr);\n+\n+ std::weak_ptr wptr1 = ptr;\n+ VERIFY(wptr1.lock());\n+\n+ std::weak_ptr wptr2 = ptr;\n+ VERIFY(wptr2.lock());\n+\n+ ptr.reset();\n+\n+ std::weak_ptr wptr3 = wptr1;\n+ std::weak_ptr wptr4 = wptr1;\n+ std::weak_ptr wptr5 = std::move(wptr1);\n+\n+ VERIFY(!wptr1.lock());\n+ VERIFY(!wptr2.lock());\n+ VERIFY(!wptr3.lock());\n+ VERIFY(!wptr4.lock());\n+ VERIFY(!wptr5.lock());\n+}\n+\n+int main()\n+{\n+ test01();\n+ test01();\n+ test01();\n+ test01();\n+\n+ test02();\n+ test02();\n+ test02();\n+ test02();\n+}\n","prefixes":["v1","1/1"]}