{"id":2226286,"url":"http://patchwork.ozlabs.org/api/1.2/patches/2226286/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.12@forge-stage.sourceware.org/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/1.2/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":"2026-04-22T10:44:27","name":"[v1,12/16] libstdc++: Optimise std::latch::arrive_and_wait","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"9e45f3c0e20e7eaa31157a43c7198ed3cb78977d","submitter":{"id":93210,"url":"http://patchwork.ozlabs.org/api/1.2/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.hhubrmqub2.gcc.gcc-TEST.redi.31.1.12@forge-stage.sourceware.org/mbox/","series":[{"id":500987,"url":"http://patchwork.ozlabs.org/api/1.2/series/500987/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=500987","date":"2026-04-22T10:44:17","name":"atomic wait/notify ABI stabilization","version":1,"mbox":"http://patchwork.ozlabs.org/series/500987/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2226286/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2226286/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":"Jonathan Wakely via Sourceware Forge\n ","Date":"Wed, 22 Apr 2026 10:44:27 +0000","Subject":"[PATCH v1 12/16] libstdc++: Optimise std::latch::arrive_and_wait","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/31","References":"\n ","In-Reply-To":"\n ","X-Patch-URL":"\n https://forge.sourceware.org/redi/gcc/commit/26f3d12d57c02ae4771c946d0734fefebba3ecc1","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\nWe don't need to wait if we know the counter has reached zero.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/std/latch (latch::arrive_and_wait): Optimise.\n---\n libstdc++-v3/include/std/latch | 17 +++++++++++++++--\n 1 file changed, 15 insertions(+), 2 deletions(-)","diff":"diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch\nindex 0a5bb0fdc6c4..f0e9850730ce 100644\n--- a/libstdc++-v3/include/std/latch\n+++ b/libstdc++-v3/include/std/latch\n@@ -101,8 +101,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n _GLIBCXX_ALWAYS_INLINE void\n arrive_and_wait(ptrdiff_t __update = 1) noexcept\n {\n- count_down(__update);\n- wait();\n+ // The standard specifies this functions as count_down(update); wait();\n+ // but we combine those two calls into one and avoid the wait() if we\n+ // know the counter reached zero.\n+\n+ __glibcxx_assert(__update >= 0 && __update <= max());\n+ // Use acq_rel here because an omitted wait() would have used acquire:\n+ auto const __old = __atomic_impl::fetch_sub(&_M_a, __update,\n+\t\t\t\t\t\t memory_order::acq_rel);\n+ if (std::cmp_equal(__old, __update))\n+\t__atomic_impl::notify_all(&_M_a);\n+ else\n+\t{\n+\t __glibcxx_assert(std::cmp_less(__update, __old));\n+\t wait();\n+\t}\n }\n \n private:\n","prefixes":["v1","12/16"]}