{"id":2226180,"url":"http://patchwork.ozlabs.org/api/patches/2226180/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhub8n5f78.gcc.gcc-TEST.redi.15.1.2@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":"2026-04-22T10:25:11","name":"[v1,02/12] libstdc++: Allow unordered_set assignment to assign to existing nodes","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"2d99b451f7a3635f94eacff2a7a30072b9af82e1","submitter":{"id":93210,"url":"http://patchwork.ozlabs.org/api/people/93210/?format=json","name":"Jonathan Wakely via Sourceware Forge","email":"forge-bot+redi@forge-stage.sourceware.org"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhub8n5f78.gcc.gcc-TEST.redi.15.1.2@forge-stage.sourceware.org/mbox/","series":[{"id":500966,"url":"http://patchwork.ozlabs.org/api/series/500966/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=500966","date":"2026-04-22T10:25:11","name":"WIP: libstdc++: Refactor hash table code","version":1,"mbox":"http://patchwork.ozlabs.org/series/500966/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2226180/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2226180/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=38.145.34.32; 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server2.sourceware.org","From":"Jonathan Wakely via Sourceware Forge\n ","Date":"Wed, 22 Apr 2026 10:25:11 +0000","Subject":"[PATCH v1 02/12] libstdc++: Allow unordered_set assignment to assign\n to existing nodes","MIME-Version":"1.0","Content-Type":"text/plain; charset=UTF-8","Content-Transfer-Encoding":"8bit","To":"gcc-patches mailing list ","Message-ID":"\n ","X-Mailer":"batrachomyomachia","X-Requested-Reviewer":"fdumont","X-Pull-Request-Organization":"gcc","X-Pull-Request-Repository":"gcc-TEST","X-Pull-Request":"https://forge.sourceware.org/gcc/gcc-TEST/pulls/15","References":"\n ","In-Reply-To":"\n ","X-Patch-URL":"\n https://forge.sourceware.org/redi/gcc/commit/ddb8114a7c9e7454cfaf87e8f33ac806dcd8965d","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 , redi@gcc.gnu.org","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"From: Jonathan Wakely \n\nCurrently the _ReuseOrAllocNode::operator(Args&&...) function always\ndestroys the value stored in recycled nodes and constructs a new value.\n\nThe _ReuseOrAllocNode type is only ever used for implementing\nassignment, either from another unordered container of the same type, or\nfrom std::initializer_list. Consequently, the parameter pack\nArgs only ever consists of a single parameter or type const value_type&\nor value_type. We can replace the variadic parameter pack with a single\nforwarding reference parameter, and when the value_type is assignable\nfrom that type we can use assignment instead of destroying the existing\nvalue and then constructing a new one.\n\nUsing assignment is typically only possible for sets, because for maps\nthe value_type is std::pair and in most\ncases std::is_assignable_v is false.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/bits/hashtable_policy.h (_ReuseOrAllocNode::operator()):\n\tReplace parameter pack with a single parameter. Assign to\n\texisting value when possible.\n\t* testsuite/23_containers/unordered_multiset/allocator/move_assign.cc:\n\tAdjust expected count of operations.\n\t* testsuite/23_containers/unordered_set/allocator/move_assign.cc:\n\tLikewise.\n\nReviewed-by: François Dumont \n---\n libstdc++-v3/include/bits/hashtable_policy.h | 37 +++++++++++++------\n .../allocator/move_assign.cc | 5 ++-\n .../unordered_set/allocator/move_assign.cc | 10 +++--\n 3 files changed, 35 insertions(+), 17 deletions(-)","diff":"diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h\nindex b5f837e60619..7a3c66c37fd6 100644\n--- a/libstdc++-v3/include/bits/hashtable_policy.h\n+++ b/libstdc++-v3/include/bits/hashtable_policy.h\n@@ -172,24 +172,39 @@ namespace __detail\n ~_ReuseOrAllocNode()\n { _M_h._M_deallocate_nodes(_M_nodes); }\n \n- template\n+#pragma GCC diagnostic push\n+#pragma GCC diagnostic ignored \"-Wc++17-extensions\" // if constexpr\n+ template\n \t__node_ptr\n-\toperator()(_Args&&... __args)\n+\toperator()(_Arg&& __arg)\n \t{\n \t if (!_M_nodes)\n-\t return _M_h._M_allocate_node(std::forward<_Args>(__args)...);\n+\t return _M_h._M_allocate_node(std::forward<_Arg>(__arg));\n+\n+\t using value_type = typename _NodeAlloc::value_type::value_type;\n \n \t __node_ptr __node = _M_nodes;\n-\t _M_nodes = _M_nodes->_M_next();\n-\t __node->_M_nxt = nullptr;\n-\t auto& __a = _M_h._M_node_allocator();\n-\t __node_alloc_traits::destroy(__a, __node->_M_valptr());\n-\t _NodePtrGuard<__hashtable_alloc, __node_ptr> __guard { _M_h, __node };\n-\t __node_alloc_traits::construct(__a, __node->_M_valptr(),\n-\t\t\t\t\t std::forward<_Args>(__args)...);\n-\t __guard._M_ptr = nullptr;\n+\t if constexpr (is_assignable::value)\n+\t {\n+\t __node->_M_v() = std::forward<_Arg>(__arg);\n+\t _M_nodes = _M_nodes->_M_next();\n+\t __node->_M_nxt = nullptr;\n+\t }\n+\t else\n+\t {\n+\t _M_nodes = _M_nodes->_M_next();\n+\t __node->_M_nxt = nullptr;\n+\t auto& __a = _M_h._M_node_allocator();\n+\t __node_alloc_traits::destroy(__a, __node->_M_valptr());\n+\t _NodePtrGuard<__hashtable_alloc, __node_ptr>\n+\t\t__guard{ _M_h, __node };\n+\t __node_alloc_traits::construct(__a, __node->_M_valptr(),\n+\t\t\t\t\t std::forward<_Arg>(__arg));\n+\t __guard._M_ptr = nullptr;\n+\t }\n \t return __node;\n \t}\n+#pragma GCC diagnostic pop\n \n private:\n __node_ptr _M_nodes;\ndiff --git a/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc b/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc\nindex 50608ec443ff..6d00354902ee 100644\n--- a/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc\n+++ b/libstdc++-v3/testsuite/23_containers/unordered_multiset/allocator/move_assign.cc\n@@ -46,8 +46,9 @@ void test01()\n VERIFY( 1 == v1.get_allocator().get_personality() );\n VERIFY( 2 == v2.get_allocator().get_personality() );\n \n- VERIFY( counter_type::move_count == 1 );\n- VERIFY( counter_type::destructor_count == 2 );\n+ VERIFY( counter_type::move_count == 0 );\n+ // 1 element in v1 destroyed.\n+ VERIFY( counter_type::destructor_count == 1 );\n }\n \n void test02()\ndiff --git a/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc b/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc\nindex 677ea67d0ea7..6be70022705a 100644\n--- a/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc\n+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/allocator/move_assign.cc\n@@ -52,8 +52,9 @@ void test01()\n VERIFY( 1 == v1.get_allocator().get_personality() );\n VERIFY( 2 == v2.get_allocator().get_personality() );\n \n- VERIFY( counter_type::move_count == 1 );\n- VERIFY( counter_type::destructor_count == 2 );\n+ VERIFY( counter_type::move_count == 0 );\n+ // 1 element in v1 destroyed.\n+ VERIFY( counter_type::destructor_count == 1 );\n }\n \n // Check there's nothing left allocated or constructed.\n@@ -130,8 +131,9 @@ void test03()\n VERIFY( 1 == v1.get_allocator().get_personality() );\n VERIFY( 2 == v2.get_allocator().get_personality() );\n \n- VERIFY( counter_type::move_count == 1 );\n- VERIFY( counter_type::destructor_count == i + 1 );\n+ VERIFY( counter_type::move_count == 0 );\n+ // (i - 1) elements in v2 destroyed, and 1 element in v1 destroyed.\n+ VERIFY( counter_type::destructor_count == i );\n }\n \n // Check there's nothing left allocated or constructed.\n","prefixes":["v1","02/12"]}