[PATCH] libstdc++: Add support for C++20 barriers
Thomas Rodgers
rodgert@appliantology.com
Thu Dec 17 19:11:30 GMT 2020
From: Thomas Rodgers <trodgers@redhat.com>
Let's see if this one sticks...
Adds <barrier>
libstdc++/ChangeLog:
* doc/doxygen/user.cfg.in: Add new header.
* include/Makefile.am (std_headers): likewise.
* include/Makefile.in: Regenerate.
* include/precompiled/stdc++.h: Add new header.
* include/std/barrier: New file.
* include/std/version: Add __cpp_lib_barrier feature test macro.
* testsuite/30_thread/barrier/1.cc: New test.
* testsuite/30_thread/barrier/2.cc: Likewise.
* testsuite/30_thread/barrier/arrive_and_drop.cc: Likewise.
* testsuite/30_thread/barrier/arrive_and_wait.cc: Likewise.
* testsuite/30_thread/barrier/arrive.cc: Likewise.
* testsuite/30_thread/barrier/completion.cc: Likewise.
* testsuite/30_thread/barrier/max.cc: Likewise.
---
libstdc++-v3/doc/doxygen/user.cfg.in | 1 +
libstdc++-v3/include/Makefile.am | 1 +
libstdc++-v3/include/Makefile.in | 1 +
libstdc++-v3/include/precompiled/stdc++.h | 1 +
libstdc++-v3/include/std/barrier | 267 ++++++++++++++++++
libstdc++-v3/include/std/version | 3 +
.../testsuite/30_threads/barrier/1.cc | 27 ++
.../testsuite/30_threads/barrier/2.cc | 27 ++
.../testsuite/30_threads/barrier/arrive.cc | 48 ++++
.../30_threads/barrier/arrive_and_drop.cc | 46 +++
.../30_threads/barrier/arrive_and_wait.cc | 46 +++
.../30_threads/barrier/completion.cc | 53 ++++
.../testsuite/30_threads/barrier/max.cc | 40 +++
13 files changed, 561 insertions(+)
create mode 100644 libstdc++-v3/include/std/barrier
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/1.cc
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/2.cc
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/arrive.cc
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/arrive_and_drop.cc
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/arrive_and_wait.cc
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/completion.cc
create mode 100644 libstdc++-v3/testsuite/30_threads/barrier/max.cc
diff --git a/libstdc++-v3/doc/doxygen/user.cfg.in b/libstdc++-v3/doc/doxygen/user.cfg.in
index 2261d572efb..fb90db65e55 100644
--- a/libstdc++-v3/doc/doxygen/user.cfg.in
+++ b/libstdc++-v3/doc/doxygen/user.cfg.in
@@ -850,6 +850,7 @@ INPUT = @srcdir@/doc/doxygen/doxygroups.cc \
include/any \
include/array \
include/atomic \
+ include/barrier \
include/bit \
include/bitset \
include/charconv \
diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index 958dfea5a98..231f7c3ec8f 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -30,6 +30,7 @@ std_headers = \
${std_srcdir}/any \
${std_srcdir}/array \
${std_srcdir}/atomic \
+ ${std_srcdir}/barrier \
${std_srcdir}/bit \
${std_srcdir}/bitset \
${std_srcdir}/charconv \
diff --git a/libstdc++-v3/include/precompiled/stdc++.h b/libstdc++-v3/include/precompiled/stdc++.h
index 692fae76a42..424c0e277d1 100644
--- a/libstdc++-v3/include/precompiled/stdc++.h
+++ b/libstdc++-v3/include/precompiled/stdc++.h
@@ -134,6 +134,7 @@
#endif
#if __cplusplus > 201703L
+#include <barrier>
#include <bit>
#include <compare>
#include <concepts>
diff --git a/libstdc++-v3/include/std/barrier b/libstdc++-v3/include/std/barrier
new file mode 100644
index 00000000000..f3c819774bd
--- /dev/null
+++ b/libstdc++-v3/include/std/barrier
@@ -0,0 +1,267 @@
+// <barrier> -*- C++ -*-
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// This implementation is based on libcxx/include/barrier
+//===-- barrier.h --------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------===//
+
+/** @file include/atomic
+ * This is a Standard C++ Library header.
+ */
+
+#ifndef _GLIBCXX_BARRIER
+#define _GLIBCXX_BARRIER 1
+
+#pragma GCC system_header
+
+#if __cplusplus > 201703L
+#include <bits/atomic_base.h>
+#if __cpp_lib_atomic_wait && __cpp_aligned_new
+#include <bits/functional_hash.h>
+#include <bits/std_thread.h>
+
+#include <array>
+#include <memory>
+
+#define __cpp_lib_barrier 201907L
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+ struct __empty_completion
+ {
+ _GLIBCXX_ALWAYS_INLINE void
+ operator()() noexcept
+ { }
+ };
+
+/*
+
+The default implementation of __tree_barrier is a classic tree barrier.
+
+It looks different from literature pseudocode for two main reasons:
+ 1. Threads that call into std::barrier functions do not provide indices,
+ so a numbering step is added before the actual barrier algorithm,
+ appearing as an N+1 round to the N rounds of the tree barrier.
+ 2. A great deal of attention has been paid to avoid cache line thrashing
+ by flattening the tree structure into cache-line sized arrays, that
+ are indexed in an efficient way.
+
+*/
+
+ enum class __barrier_phase_t : unsigned char { };
+
+ template<typename _CompletionF>
+ class __tree_barrier
+ {
+ using __atomic_phase_ref_t = std::__atomic_ref<__barrier_phase_t>;
+ using __atomic_phase_const_ref_t = std::__atomic_ref<const __barrier_phase_t>;
+ static constexpr auto __phase_alignment =
+ __atomic_phase_ref_t::required_alignment;
+
+ using __tickets_t = std::array<__barrier_phase_t, 64>;
+ struct alignas(64) /* naturally-align the heap state */ __state_t
+ {
+ alignas(__phase_alignment) __tickets_t __tickets;
+
+ /*
+ struct __ticket_t
+ {
+ __atomic_base<__barrier_phase_t> __phase
+ = { static_cast<__barrier_phase_t>(0) };
+ };
+ std::array<__ticket_t, 64> __tickets;
+ */
+
+ };
+
+ ptrdiff_t _M_expected;
+ // unique_ptr<char[]> _M_state_allocation;
+ // __state_t* _M_state;
+ unique_ptr<__state_t[]> _M_state;
+ __atomic_base<ptrdiff_t> _M_expected_adjustment;
+ _CompletionF _M_completion;
+
+ /*
+ using __atomic_phase_ref_t = std::__atomic_ref<__barrier_phase_t>;
+ using __atomic_phase_const_ref_t = std::__atomic_ref<const __barrier_phase_t>;
+ static constexpr size_t __phase_alignment =
+ __atomic_phase_ref_t::required_alignment;
+ */
+ alignas(__phase_alignment) __barrier_phase_t _M_phase;
+
+ bool
+ _M_arrive(__barrier_phase_t __old_phase)
+ {
+ const auto __old_phase_val = static_cast<unsigned char>(__old_phase);
+ const auto __half_step =
+ static_cast<__barrier_phase_t>(__old_phase_val + 1);
+ const auto __full_step =
+ static_cast<__barrier_phase_t>(__old_phase_val + 2);
+
+ size_t __current_expected = _M_expected;
+ std::hash<std::thread::id>__hasher;
+ size_t __current = __hasher(std::this_thread::get_id())
+ % ((_M_expected + 1) >> 1);
+
+ for (int __round = 0; ; ++__round)
+ {
+ if (__current_expected <= 1)
+ return true;
+ size_t const __end_node = ((__current_expected + 1) >> 1),
+ __last_node = __end_node - 1;
+ for ( ; ; ++__current)
+ {
+ if (__current == __end_node)
+ __current = 0;
+ auto __expect = __old_phase;
+ __atomic_phase_ref_t __phase(_M_state[__current]
+ .__tickets[__round]);
+ if (__current == __last_node && (__current_expected & 1))
+ {
+ if (__phase.compare_exchange_strong(__expect, __full_step,
+ memory_order_acq_rel))
+ break; // I'm 1 in 1, go to next __round
+ }
+ else if (__phase.compare_exchange_strong(__expect, __half_step,
+ memory_order_acq_rel))
+ {
+ return false; // I'm 1 in 2, done with arrival
+ }
+ else if (__expect == __half_step)
+ {
+ if (__phase.compare_exchange_strong(__expect, __full_step,
+ memory_order_acq_rel))
+ break; // I'm 2 in 2, go to next __round
+ }
+ }
+ __current_expected = __last_node + 1;
+ __current >>= 1;
+ }
+ }
+
+ public:
+ using arrival_token = __barrier_phase_t;
+
+ static constexpr ptrdiff_t
+ max() noexcept
+ { return __PTRDIFF_MAX__; }
+
+ __tree_barrier(ptrdiff_t __expected, _CompletionF __completion)
+ : _M_expected(__expected), _M_expected_adjustment(0),
+ _M_completion(move(__completion)),
+ _M_phase(static_cast<__barrier_phase_t>(0))
+ {
+ size_t const __count = (_M_expected + 1) >> 1;
+
+ _M_state = std::make_unique<__state_t[]>(__count);
+ }
+
+ [[nodiscard]] arrival_token
+ arrive(ptrdiff_t __update)
+ {
+ __atomic_phase_ref_t __phase(_M_phase);
+ const auto __old_phase = __phase.load(memory_order_relaxed);
+ const auto __cur = static_cast<unsigned char>(__old_phase);
+ for(; __update; --__update)
+ {
+ if(_M_arrive(__old_phase))
+ {
+ _M_completion();
+ _M_expected += _M_expected_adjustment.load(memory_order_relaxed);
+ _M_expected_adjustment.store(0, memory_order_relaxed);
+ auto __new_phase = static_cast<__barrier_phase_t>(__cur + 2);
+ __phase.store(__new_phase, memory_order_release);
+ __phase.notify_all();
+ }
+ }
+ return __old_phase;
+ }
+
+ void
+ wait(arrival_token&& __old_phase) const
+ {
+ __atomic_phase_const_ref_t __phase(_M_phase);
+ auto const __test_fn = [=, this]
+ {
+ return __phase.load(memory_order_acquire) != __old_phase;
+ };
+ std::__atomic_wait(&_M_phase, __old_phase, __test_fn);
+ }
+
+ void
+ arrive_and_drop()
+ {
+ _M_expected_adjustment.fetch_sub(1, memory_order_relaxed);
+ (void)arrive(1);
+ }
+ };
+
+ template<typename _CompletionF = __empty_completion>
+ class barrier
+ {
+ // Note, we may introduce a "central" barrier algorithm at some point
+ // for more space constrained targets
+ using __algorithm_t = __tree_barrier<_CompletionF>;
+ __algorithm_t _M_b;
+
+ public:
+ using arrival_token = typename __tree_barrier<_CompletionF>::arrival_token;
+
+ static constexpr ptrdiff_t
+ max() noexcept
+ { return __algorithm_t::max(); }
+
+ explicit barrier(ptrdiff_t __count,
+ _CompletionF __completion = _CompletionF())
+ : _M_b(__count, std::move(__completion))
+ { }
+
+ barrier(barrier const&) = delete;
+ barrier& operator=(barrier const&) = delete;
+
+ [[nodiscard]] arrival_token
+ arrive(ptrdiff_t __update = 1)
+ { return _M_b.arrive(__update); }
+
+ void
+ wait(arrival_token&& __phase) const
+ { _M_b.wait(std::move(__phase)); }
+
+ void
+ arrive_and_wait()
+ { wait(arrive()); }
+
+ void
+ arrive_and_drop()
+ { _M_b.arrive_and_drop(); }
+ };
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
+#endif // _GLIBCXX_HAS_GTHREADS
+#endif // __cplusplus > 201703L
+#endif // _GLIBCXX_BARRIER
+
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index e4a8bed52ab..07d17433c5b 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -200,6 +200,9 @@
#if defined _GLIBCXX_HAS_GTHREADS || defined _GLIBCXX_HAVE_LINUX_FUTEX
# define __cpp_lib_atomic_wait 201907L
#endif
+#if __cpp_lib_atomic_wait
+#define __cpp_lib_barrier 201907L
+#endif
#define __cpp_lib_bind_front 201907L
#if __has_builtin(__builtin_bit_cast)
# define __cpp_lib_bit_cast 201806L
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/1.cc b/libstdc++-v3/testsuite/30_threads/barrier/1.cc
new file mode 100644
index 00000000000..0b38160a58b
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/1.cc
@@ -0,0 +1,27 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <barrier>
+
+#ifndef __cpp_lib_barrier
+# error "Feature-test macro for barrier missing in <barrier>"
+#elif __cpp_lib_barrier != 201907L
+# error "Feature-test macro for barrier has wrong value in <barrier>"
+#endif
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/2.cc b/libstdc++-v3/testsuite/30_threads/barrier/2.cc
new file mode 100644
index 00000000000..1d8d83639e0
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/2.cc
@@ -0,0 +1,27 @@
+// Copyright (C) 2019-2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <version>
+
+#ifndef __cpp_lib_barrier
+# error "Feature-test macro for barrier missing in <version>"
+#elif __cpp_lib_barrier != 201907L
+# error "Feature-test macro for barrier has wrong value in <version>"
+#endif
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/arrive.cc b/libstdc++-v3/testsuite/30_threads/barrier/arrive.cc
new file mode 100644
index 00000000000..df70587db5b
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/arrive.cc
@@ -0,0 +1,48 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-gthreads "" }
+// { dg-additional-options "-pthread" { target pthread } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// This implementation is based on libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <barrier>
+#include <thread>
+
+int main(int, char**)
+{
+ std::barrier<> b(2);
+
+ auto tok = b.arrive();
+ std::thread t([&](){
+ (void)b.arrive();
+ });
+ b.wait(std::move(tok));
+ t.join();
+
+ auto tok2 = b.arrive(2);
+ b.wait(std::move(tok2));
+}
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/arrive_and_drop.cc b/libstdc++-v3/testsuite/30_threads/barrier/arrive_and_drop.cc
new file mode 100644
index 00000000000..746a8f470e3
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/arrive_and_drop.cc
@@ -0,0 +1,46 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-gthreads "" }
+// { dg-additional-options "-pthread" { target pthread } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// This implementation is based on libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <barrier>
+#include <thread>
+
+int main(int, char**)
+{
+ std::barrier<> b(2);
+
+ std::thread t([&](){
+ b.arrive_and_drop();
+ });
+
+ b.arrive_and_wait();
+ b.arrive_and_wait();
+ t.join();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/arrive_and_wait.cc b/libstdc++-v3/testsuite/30_threads/barrier/arrive_and_wait.cc
new file mode 100644
index 00000000000..509986f3b78
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/arrive_and_wait.cc
@@ -0,0 +1,46 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-gthreads "" }
+// { dg-additional-options "-pthread" { target pthread } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// This implementation is based on libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <barrier>
+#include <thread>
+
+int main(int, char**)
+{
+ std::barrier<> b(2);
+
+ std::thread t([&](){
+ for(int i = 0; i < 10; ++i)
+ b.arrive_and_wait();
+ });
+ for(int i = 0; i < 10; ++i)
+ b.arrive_and_wait();
+ t.join();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/completion.cc b/libstdc++-v3/testsuite/30_threads/barrier/completion.cc
new file mode 100644
index 00000000000..c7997d9d7fb
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/completion.cc
@@ -0,0 +1,53 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-gthreads "" }
+// { dg-additional-options "-pthread" { target pthread } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// This implementation is based on libcxx/test/std/thread/thread.barrier/completion.pass.cpp
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <barrier>
+#include <thread>
+
+#include <testsuite_hooks.h>
+
+int main(int, char**)
+{
+ int x = 0;
+ auto comp = [&] { x += 1; };
+ std::barrier<decltype(comp)> b(2, comp);
+
+ std::thread t([&](){
+ for(int i = 0; i < 10; ++i)
+ b.arrive_and_wait();
+ });
+
+ for(int i = 0; i < 10; ++i)
+ b.arrive_and_wait();
+
+ VERIFY( x == 10 );
+ t.join();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/barrier/max.cc b/libstdc++-v3/testsuite/30_threads/barrier/max.cc
new file mode 100644
index 00000000000..278bc3fdd10
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/barrier/max.cc
@@ -0,0 +1,40 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-gthreads "" }
+// { dg-additional-options "-pthread" { target pthread } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// This implementation is based on libcxx/test/std/thread/thread.barrier/max.pass.cpp
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <barrier>
+#include <thread>
+
+int main(int, char**)
+{
+ static_assert(std::barrier<>::max() > 0, "");
+ auto l = [](){};
+ static_assert(std::barrier<decltype(l)>::max() > 0, "");
+}
--
2.26.2
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