libstdc++: Add support for C++20 barriers

Message ID 20201217191130.3049887-1-rodgert@appliantology.com
State Superseded
Headers show
Series
  • libstdc++: Add support for C++20 barriers
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Commit Message

Thomas Rodgers Dec. 17, 2020, 7:11 p.m.
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

-- 
2.26.2

Patch

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, "");
+}