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GET /api/1.2/patches/2226288/?format=api
{ "id": 2226288, "url": "http://patchwork.ozlabs.org/api/1.2/patches/2226288/?format=api", "web_url": "http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.13@forge-stage.sourceware.org/", "project": { "id": 17, "url": "http://patchwork.ozlabs.org/api/1.2/projects/17/?format=api", "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": "<bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.13@forge-stage.sourceware.org>", "list_archive_url": null, "date": "2026-04-22T10:44:28", "name": "[v1,13/16] libstdc++: Fix std::barrier for constant initialization [PR118395]", "commit_ref": null, "pull_url": null, "state": "new", "archived": false, "hash": "f079cbfcdab08629bf1a947f9220df7a3478c91f", "submitter": { "id": 93210, "url": "http://patchwork.ozlabs.org/api/1.2/people/93210/?format=api", "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.31.1.13@forge-stage.sourceware.org/mbox/", "series": [ { "id": 500987, "url": "http://patchwork.ozlabs.org/api/1.2/series/500987/?format=api", "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/2226288/comments/", "check": "pending", "checks": "http://patchwork.ozlabs.org/api/patches/2226288/checks/", "tags": {}, "related": [], "headers": { "Return-Path": "<gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org>", "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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Wed, 22 Apr 2026 10:46:09 +0000 (GMT)", "from forge-stage.sourceware.org (localhost [IPv6:::1])\n (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n key-exchange x25519 server-signature ECDSA (prime256v1) server-digest SHA256)\n (No client certificate requested)\n by forge-stage.sourceware.org (Postfix) with ESMTPS id 338B642BBC\n for <gcc-patches@gcc.gnu.org>; Wed, 22 Apr 2026 10:46:02 +0000 (UTC)" ], "DKIM-Filter": [ "OpenDKIM Filter v2.11.0 sourceware.org 30C2140A0CCA", "OpenDKIM Filter v2.11.0 sourceware.org 5F9F84015E99" ], "DMARC-Filter": "OpenDMARC Filter v1.4.2 sourceware.org 5F9F84015E99", "ARC-Filter": "OpenARC Filter v1.0.0 sourceware.org 5F9F84015E99", "ARC-Seal": "i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1776854769; cv=none;\n b=XyQz0cp3InRjlJ0PQLSg6XYvdoiNlt2I4stzO6hymvV2jZvxpSQoj0V0sI28ugfrMsf4ibeYV1Rn7F6NOJQREcUK9cdiQXL2x6EjGwfexIlMIiAq1fcpMga5tE6IVLDecb631TlVLqtYCtfHBV+7i1VK4+7WSGRpEv1PTD3aHec=", "ARC-Message-Signature": "i=1; a=rsa-sha256; d=sourceware.org; s=key;\n t=1776854769; c=relaxed/simple;\n bh=j893D4en4ro1e83rhBqb9u0vy48nEVBg5+LCDVYbe9E=;\n h=From:Date:Subject:To:Message-ID;\n b=q1aePFUlmFoPc8e1D5W2ay9BU74oC+W/6tS9IFXgKJ3vl2D0G5XxzWSIvxiZFkuXL2FivZRh4CZgXi2AcNTBA56eFpzOTQDeSPSNFIibwm8GuaA20CSc0jJkqFbNr70klp1BLoWsQljblIwOOhf482s/+eiUm8xeUsQ/kFHIfPA=", "ARC-Authentication-Results": "i=1; server2.sourceware.org", "From": "Jonathan Wakely via Sourceware Forge\n <forge-bot+redi@forge-stage.sourceware.org>", "Date": "Wed, 22 Apr 2026 10:44:28 +0000", "Subject": "[PATCH v1 13/16] libstdc++: Fix std::barrier for constant\n initialization [PR118395]", "To": "gcc-patches mailing list <gcc-patches@gcc.gnu.org>", "Message-ID": "\n <bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.13@forge-stage.sourceware.org>", "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 <bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.0@forge-stage.sourceware.org>", "In-Reply-To": "\n <bmm.hhubrmqub2.gcc.gcc-TEST.redi.31.1.0@forge-stage.sourceware.org>", "X-Patch-URL": "\n https://forge.sourceware.org/redi/gcc/commit/9a90c36a05d87e9ba6684b688f3c429790e38ecd", "X-BeenThere": "gcc-patches@gcc.gnu.org", "X-Mailman-Version": "2.1.30", "Precedence": "list", "List-Id": "Gcc-patches mailing list <gcc-patches.gcc.gnu.org>", "List-Unsubscribe": "<https://gcc.gnu.org/mailman/options/gcc-patches>,\n <mailto:gcc-patches-request@gcc.gnu.org?subject=unsubscribe>", "List-Archive": "<https://gcc.gnu.org/pipermail/gcc-patches/>", "List-Post": "<mailto:gcc-patches@gcc.gnu.org>", "List-Help": "<mailto:gcc-patches-request@gcc.gnu.org?subject=help>", "List-Subscribe": "<https://gcc.gnu.org/mailman/listinfo/gcc-patches>,\n <mailto:gcc-patches-request@gcc.gnu.org?subject=subscribe>", "Reply-To": "gcc-patches mailing list <gcc-patches@gcc.gnu.org>, redi@gcc.gnu.org", "Errors-To": "gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org" }, "content": "From: Jonathan Wakely <jwakely@redhat.com>\n\nThe std::barrier constructor should be constexpr, which means we need to\ndefer the dynamic allocation if the constructor is called during\nconstant-initialization. We can defer it to the first call to\nbarrier::arrive, using compare-and-swap on an atomic<T*> (instead of the\nunique_ptr<T[]> currently used).\n\nAlso add precondition checks to the constructor and arrive member\nfunction. Also implement the proposed resolution of LWG 3898.\n\nlibstdc++-v3/ChangeLog:\n\n\tPR libstdc++/118395\n\tPR libstdc++/108974\n\tPR libstdc++/98749\n\t* include/std/barrier (__tree_barrier): Use default\n\tmember-initializers. Change _M_state member from\n\tunique_ptr<__state_t[]> to atomic<__state_t*>. Add\n\tno_unique_address attribute to _M_completion.\n\t(__tree_barrier::_M_arrive): Load value from _M_state.\n\t(__tree_barrier::_M_invoke_completion): New member function to\n\tensure a throwing completion function will terminate, as\n\tproposed in LWG 3898.\n\t(__tree_barrier::max): Reduce by one to avoid overflow.\n\t(__tree_barrier::__tree_barrier): Add constexpr. Qualify call to\n\tstd::move. Remove mem-initializers made unnecessary by default\n\tmember-initializers. Add precondition check. Only allocate state\n\tarray if not constant evaluated.\n\t(__tree_barrier::arrive): Add precondition check. Do deferred\n\tinitialization of _M_state if needed.\n\t(barrier): Add static_assert, as proposed in LWG 3898.\n\t(barrier::barrier): Add constexpr.\n\t* testsuite/30_threads/barrier/cons.cc: New test.\n\t* testsuite/30_threads/barrier/lwg3898.cc: New test.\n---\n libstdc++-v3/include/std/barrier | 57 ++++++++++++++-----\n .../testsuite/30_threads/barrier/cons.cc | 6 ++\n .../testsuite/30_threads/barrier/lwg3898.cc | 45 +++++++++++++++\n 3 files changed, 93 insertions(+), 15 deletions(-)\n create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/cons.cc\n create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/lwg3898.cc", "diff": "diff --git a/libstdc++-v3/include/std/barrier b/libstdc++-v3/include/std/barrier\nindex 62b03d0223f4..9c1de411f9ce 100644\n--- a/libstdc++-v3/include/std/barrier\n+++ b/libstdc++-v3/include/std/barrier\n@@ -96,11 +96,11 @@ It looks different from literature pseudocode for two main reasons:\n };\n \n ptrdiff_t _M_expected;\n- unique_ptr<__state_t[]> _M_state;\n- __atomic_base<ptrdiff_t> _M_expected_adjustment;\n- _CompletionF _M_completion;\n+ __atomic_base<__state_t*> _M_state{nullptr};\n+ __atomic_base<ptrdiff_t> _M_expected_adjustment{0};\n+ [[no_unique_address]] _CompletionF _M_completion;\n \n- alignas(__phase_alignment) __barrier_phase_t _M_phase;\n+ alignas(__phase_alignment) __barrier_phase_t _M_phase{};\n \n bool\n _M_arrive(__barrier_phase_t __old_phase, size_t __current)\n@@ -114,6 +114,8 @@ It looks different from literature pseudocode for two main reasons:\n \tsize_t __current_expected = _M_expected;\n \t__current %= ((_M_expected + 1) >> 1);\n \n+\t__state_t* const __state = _M_state.load(memory_order_relaxed);\n+\n \tfor (int __round = 0; ; ++__round)\n \t {\n \t if (__current_expected <= 1)\n@@ -125,7 +127,7 @@ It looks different from literature pseudocode for two main reasons:\n \t\tif (__current == __end_node)\n \t\t __current = 0;\n \t\tauto __expect = __old_phase;\n-\t\t__atomic_phase_ref_t __phase(_M_state[__current]\n+\t\t__atomic_phase_ref_t __phase(__state[__current]\n \t\t\t\t\t\t.__tickets[__round]);\n \t\tif (__current == __last_node && (__current_expected & 1))\n \t\t {\n@@ -150,36 +152,59 @@ It looks different from literature pseudocode for two main reasons:\n \t }\n }\n \n+ // _GLIBCXX_RESOLVE_LIB_DEFECTS\n+ // 3898. Possibly unintended preconditions for completion functions\n+ void _M_invoke_completion() noexcept { _M_completion(); }\n+\n public:\n using arrival_token = __barrier_phase_t;\n \n static constexpr ptrdiff_t\n max() noexcept\n- { return __PTRDIFF_MAX__; }\n+ { return __PTRDIFF_MAX__ - 1; }\n \n+ constexpr\n __tree_barrier(ptrdiff_t __expected, _CompletionF __completion)\n-\t : _M_expected(__expected), _M_expected_adjustment(0),\n-\t _M_completion(move(__completion)),\n-\t _M_phase(static_cast<__barrier_phase_t>(0))\n+ : _M_expected(__expected), _M_completion(std::move(__completion))\n {\n-\tsize_t const __count = (_M_expected + 1) >> 1;\n+\t__glibcxx_assert(__expected >= 0 && __expected <= max());\n \n-\t_M_state = std::make_unique<__state_t[]>(__count);\n+\tif (!std::is_constant_evaluated())\n+\t {\n+\t size_t const __count = (_M_expected + 1) >> 1;\n+\t _M_state.store(new __state_t[__count], memory_order_release);\n+\t }\n }\n \n [[nodiscard]] arrival_token\n arrive(ptrdiff_t __update)\n {\n+\t__glibcxx_assert(__update > 0);\n+\t// FIXME: Check that update is less than or equal to the expected count\n+\t// for the current barrier phase.\n+\n \tstd::hash<std::thread::id> __hasher;\n \tsize_t __current = __hasher(std::this_thread::get_id());\n \t__atomic_phase_ref_t __phase(_M_phase);\n \tconst auto __old_phase = __phase.load(memory_order_relaxed);\n \tconst auto __cur = static_cast<unsigned char>(__old_phase);\n-\tfor(; __update; --__update)\n+\n+\tif (__cur == 0 && !_M_state.load(memory_order_relaxed)) [[unlikely]]\n \t {\n-\t if(_M_arrive(__old_phase, __current))\n+\t size_t const __count = (_M_expected + 1) >> 1;\n+\t auto __p = make_unique<__state_t[]>(__count);\n+\t __state_t* __val = nullptr;\n+\t if (_M_state.compare_exchange_strong(__val, __p.get(),\n+\t\t\t\t\t\t memory_order_seq_cst,\n+\t\t\t\t\t\t memory_order_acquire))\n+\t __p.release();\n+\t }\n+\n+\tfor (; __update; --__update)\n+\t {\n+\t if (_M_arrive(__old_phase, __current))\n \t {\n-\t\t_M_completion();\n+\t\t_M_invoke_completion();\n \t\t_M_expected += _M_expected_adjustment.load(memory_order_relaxed);\n \t\t_M_expected_adjustment.store(0, memory_order_relaxed);\n \t\tauto __new_phase = static_cast<__barrier_phase_t>(__cur + 2);\n@@ -208,6 +233,8 @@ It looks different from literature pseudocode for two main reasons:\n template<typename _CompletionF = __empty_completion>\n class barrier\n {\n+ static_assert(is_invocable_v<_CompletionF&>);\n+\n // Note, we may introduce a \"central\" barrier algorithm at some point\n // for more space constrained targets\n using __algorithm_t = __tree_barrier<_CompletionF>;\n@@ -232,7 +259,7 @@ It looks different from literature pseudocode for two main reasons:\n max() noexcept\n { return __algorithm_t::max(); }\n \n- explicit\n+ constexpr explicit\n barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())\n : _M_b(__count, std::move(__completion))\n { }\ndiff --git a/libstdc++-v3/testsuite/30_threads/barrier/cons.cc b/libstdc++-v3/testsuite/30_threads/barrier/cons.cc\nnew file mode 100644\nindex 000000000000..0b805143ef00\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/30_threads/barrier/cons.cc\n@@ -0,0 +1,6 @@\n+// { dg-do compile { target c++20 } }\n+\n+#include <barrier>\n+\n+// PR 118395 Constructor of std::barrier is not constexpr\n+constinit std::barrier<> b(std::barrier<>::max());\ndiff --git a/libstdc++-v3/testsuite/30_threads/barrier/lwg3898.cc b/libstdc++-v3/testsuite/30_threads/barrier/lwg3898.cc\nnew file mode 100644\nindex 000000000000..e3160dc16584\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/30_threads/barrier/lwg3898.cc\n@@ -0,0 +1,45 @@\n+// { dg-do run { target c++20 } }\n+// { dg-require-effective-target gthreads }\n+\n+#include <barrier>\n+#include <exception>\n+#include <cstdlib>\n+#if !_GLIBCXX_USE_C99_STDLIB && defined _GLIBCXX_HAVE_UNISTD_H\n+# include <unistd.h>\n+#endif\n+\n+void handle_terminate()\n+{\n+#if _GLIBCXX_USE_C99_STDLIB\n+ std::_Exit(0);\n+#elif defined _GLIBCXX_HAVE_UNISTD_H\n+ _exit(0);\n+#else\n+ std::exit(0);\n+#endif\n+}\n+\n+struct F\n+{\n+ void operator()()\n+ {\n+ std::set_terminate(handle_terminate);\n+ throw 1;\n+ }\n+};\n+\n+void\n+test_lwg3898()\n+{\n+ std::barrier<F> b(1, F{});\n+ // This should call the terminate handler and exit with zero status:\n+ b.arrive_and_wait();\n+ // Should not reach here:\n+ std::abort();\n+}\n+\n+int\n+main()\n+{\n+ test_lwg3898();\n+}\n", "prefixes": [ "v1", "13/16" ] }