Add C++2a synchronization support

Message ID 20200606002956.1512343-1-rodgert@appliantology.com
State Superseded
Headers show
Series
  • Add C++2a synchronization support
Related show

Commit Message

Thomas Rodgers June 6, 2020, 12:29 a.m.
Add support for -
        atomic wait/notify_one/notify_all
        counting_semaphore
        binary_semaphore
        latch

        * include/Makefile.am (bits_headers): Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/atomic_base.h (__atomic_base<_Itp>::wait): Define.
	(__atomic_base<_Itp>::notify_one): Likewise.
	(__atomic_base<_Itp>::notify_all): Likewise.
	(__atomic_base<_Ptp*>::wait): Likewise.
	(__atomic_base<_Ptp*>::notify_one): Likewise.
	(__atomic_base<_Ptp*>::notify_all): Likewise.
	(__atomic_impl::wait): Likewise.
	(__atomic_impl::notify_one): Likewise.
	(__atomic_impl::notify_all): Likewise.
	(__atomic_float<_Fp>::wait): Likewise.
	(__atomic_float<_Fp>::notify_one): Likewise.
	(__atomic_float<_Fp>::notify_all): Likewise.
	(__atomic_ref<_Tp>::wait): Likewise.
	(__atomic_ref<_Tp>::notify_one): Likewise.
	(__atomic_ref<_Tp>::notify_all): Likewise.
	(atomic_wait<_Tp>): Likewise.
	(atomic_wait_explicit<_Tp>): Likewise.
	(atomic_notify_one<_Tp>): Likewise.
	(atomic_notify_all<_Tp>): Likewise.
	* include/bits/atomic_wait.h: New file.
        * include/bits/atomic_timed_wait.h: New file.
        * include/bits/semaphore_base.h: New file.
	* include/std/atomic (atomic<bool>::wait): Define.
	(atomic<bool>::wait_one): Likewise.
	(atomic<bool>::wait_all): Likewise.
	(atomic<_Tp>::wait): Likewise.
	(atomic<_Tp>::wait_one): Likewise.
	(atomic<_Tp>::wait_all): Likewise.
	(atomic<_Tp*>::wait): Likewise.
	(atomic<_Tp*>::wait_one): Likewise.
	(atomic<_Tp*>::wait_all): Likewise.
        * include/std/latch: New file.
        * include/std/semaphore: New file.
        * include/std/version: Add __cpp_lib_semaphore and
        __cpp_lib_latch defines.
	* testsuite/29_atomic/atomic/wait_notify/atomic_refs.cc: New test.
	* testsuite/29_atomic/atomic/wait_notify/bool.cc: Likewise.
	* testsuite/29_atomic/atomic/wait_notify/integrals.cc: Likewise.
	* testsuite/29_atomic/atomic/wait_notify/floats.cc: Likewise.
	* testsuite/29_atomic/atomic/wait_notify/pointers.cc: Likewise.
	* testsuite/29_atomic/atomic/wait_notify/generic.h: New File.
        * testsuite/30_thread/semaphore/1.cc: New test.
        * testsuite/30_thread/semaphore/2.cc: Likewise.
        * testsuite/30_thread/semaphore/least_max_value_neg.cc: Likewise.
        * testsuite/30_thread/semaphore/try_acquire.cc: Likewise.
        * testsuite/30_thread/semaphore/try_acquire_for.cc: Likewise.
        * testsuite/30_thread/semaphore/try_acquire_futex.cc: Likewise.
        * testsuite/30_thread/semaphore/try_acquire_posix.cc: Likewise.
        * testsuite/30_thread/semaphore/try_acquire_until.cc: Likewise.
        * testsuite/30_thread/latch/1.cc: New test.
        * testsuite/30_thread/latch/2.cc: New test.
        * testsuite/30_thread/latch/3.cc: New test.
---
 libstdc++-v3/include/Makefile.am              |   5 +
 libstdc++-v3/include/Makefile.in              |   5 +
 libstdc++-v3/include/bits/atomic_base.h       | 161 +++++++++-
 libstdc++-v3/include/bits/atomic_timed_wait.h | 282 +++++++++++++++++
 libstdc++-v3/include/bits/atomic_wait.h       | 291 ++++++++++++++++++
 libstdc++-v3/include/bits/semaphore_base.h    | 272 ++++++++++++++++
 libstdc++-v3/include/std/atomic               |  61 ++++
 libstdc++-v3/include/std/latch                |  90 ++++++
 libstdc++-v3/include/std/semaphore            |  86 ++++++
 libstdc++-v3/include/std/version              |   2 +
 .../atomic/wait_notify/atomic_refs.cc         | 103 +++++++
 .../29_atomics/atomic/wait_notify/bool.cc     |  59 ++++
 .../29_atomics/atomic/wait_notify/floats.cc   |  32 ++
 .../29_atomics/atomic/wait_notify/generic.h   |  88 ++++++
 .../atomic/wait_notify/integrals.cc           |  56 ++++
 .../29_atomics/atomic/wait_notify/pointers.cc |  59 ++++
 libstdc++-v3/testsuite/30_threads/latch/1.cc  |  27 ++
 libstdc++-v3/testsuite/30_threads/latch/2.cc  |  27 ++
 libstdc++-v3/testsuite/30_threads/latch/3.cc  |  50 +++
 .../testsuite/30_threads/semaphore/1.cc       |  27 ++
 .../testsuite/30_threads/semaphore/2.cc       |  27 ++
 .../semaphore/least_max_value_neg.cc          |  28 ++
 .../30_threads/semaphore/try_acquire.cc       |  55 ++++
 .../30_threads/semaphore/try_acquire_for.cc   |  85 +++++
 .../30_threads/semaphore/try_acquire_futex.cc |  51 +++
 .../30_threads/semaphore/try_acquire_posix.cc | 169 ++++++++++
 .../30_threads/semaphore/try_acquire_until.cc |  94 ++++++
 27 files changed, 2291 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/include/bits/atomic_timed_wait.h
 create mode 100644 libstdc++-v3/include/bits/atomic_wait.h
 create mode 100644 libstdc++-v3/include/bits/semaphore_base.h
 create mode 100644 libstdc++-v3/include/std/latch
 create mode 100644 libstdc++-v3/include/std/semaphore
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/latch/1.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/latch/2.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/latch/3.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/1.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/2.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc
 create mode 100644 libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc

-- 
2.26.2

Comments

Aldy Hernandez via Gcc-patches July 8, 2020, 4:43 p.m. | #1
On 05/06/20 17:29 -0700, Thomas Rodgers wrote:
>Add support for -

>        atomic wait/notify_one/notify_all

>        counting_semaphore

>        binary_semaphore

>        latch

>

>        * include/Makefile.am (bits_headers): Add new header.

>	* include/Makefile.in: Regenerate.

>	* include/bits/atomic_base.h (__atomic_base<_Itp>::wait): Define.

>	(__atomic_base<_Itp>::notify_one): Likewise.

>	(__atomic_base<_Itp>::notify_all): Likewise.

>	(__atomic_base<_Ptp*>::wait): Likewise.

>	(__atomic_base<_Ptp*>::notify_one): Likewise.

>	(__atomic_base<_Ptp*>::notify_all): Likewise.

>	(__atomic_impl::wait): Likewise.

>	(__atomic_impl::notify_one): Likewise.

>	(__atomic_impl::notify_all): Likewise.

>	(__atomic_float<_Fp>::wait): Likewise.

>	(__atomic_float<_Fp>::notify_one): Likewise.

>	(__atomic_float<_Fp>::notify_all): Likewise.

>	(__atomic_ref<_Tp>::wait): Likewise.

>	(__atomic_ref<_Tp>::notify_one): Likewise.

>	(__atomic_ref<_Tp>::notify_all): Likewise.

>	(atomic_wait<_Tp>): Likewise.

>	(atomic_wait_explicit<_Tp>): Likewise.

>	(atomic_notify_one<_Tp>): Likewise.

>	(atomic_notify_all<_Tp>): Likewise.

>	* include/bits/atomic_wait.h: New file.

>        * include/bits/atomic_timed_wait.h: New file.

>        * include/bits/semaphore_base.h: New file.

>	* include/std/atomic (atomic<bool>::wait): Define.

>	(atomic<bool>::wait_one): Likewise.

>	(atomic<bool>::wait_all): Likewise.

>	(atomic<_Tp>::wait): Likewise.

>	(atomic<_Tp>::wait_one): Likewise.

>	(atomic<_Tp>::wait_all): Likewise.

>	(atomic<_Tp*>::wait): Likewise.

>	(atomic<_Tp*>::wait_one): Likewise.

>	(atomic<_Tp*>::wait_all): Likewise.

>        * include/std/latch: New file.

>        * include/std/semaphore: New file.

>        * include/std/version: Add __cpp_lib_semaphore and

>        __cpp_lib_latch defines.

>	* testsuite/29_atomic/atomic/wait_notify/atomic_refs.cc: New test.

>	* testsuite/29_atomic/atomic/wait_notify/bool.cc: Likewise.

>	* testsuite/29_atomic/atomic/wait_notify/integrals.cc: Likewise.

>	* testsuite/29_atomic/atomic/wait_notify/floats.cc: Likewise.

>	* testsuite/29_atomic/atomic/wait_notify/pointers.cc: Likewise.

>	* testsuite/29_atomic/atomic/wait_notify/generic.h: New File.

>        * testsuite/30_thread/semaphore/1.cc: New test.

>        * testsuite/30_thread/semaphore/2.cc: Likewise.

>        * testsuite/30_thread/semaphore/least_max_value_neg.cc: Likewise.

>        * testsuite/30_thread/semaphore/try_acquire.cc: Likewise.

>        * testsuite/30_thread/semaphore/try_acquire_for.cc: Likewise.

>        * testsuite/30_thread/semaphore/try_acquire_futex.cc: Likewise.

>        * testsuite/30_thread/semaphore/try_acquire_posix.cc: Likewise.

>        * testsuite/30_thread/semaphore/try_acquire_until.cc: Likewise.

>        * testsuite/30_thread/latch/1.cc: New test.

>        * testsuite/30_thread/latch/2.cc: New test.

>        * testsuite/30_thread/latch/3.cc: New test.



>diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am

>index 80aeb3f8959..b3ac1a3365f 100644

>--- a/libstdc++-v3/include/Makefile.am

>+++ b/libstdc++-v3/include/Makefile.am

>@@ -52,6 +52,7 @@ std_headers = \

> 	${std_srcdir}/iostream \

> 	${std_srcdir}/istream \

> 	${std_srcdir}/iterator \

>+	${std_srcdir}/latch\


Missing space before the backslash here.

> 	${std_srcdir}/limits \

> 	${std_srcdir}/list \

> 	${std_srcdir}/locale \


>--- a/libstdc++-v3/include/bits/atomic_base.h

>+++ b/libstdc++-v3/include/bits/atomic_base.h

>@@ -823,6 +851,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 					   int(__m1), int(__m2));

>       }

>

>+#if __cplusplus > 201703L

>+      _GLIBCXX_ALWAYS_INLINE void

>+      wait(__pointer_type __old, memory_order __m = memory_order_seq_cst) noexcept


This line should be < 80 cols.

>+      {

>@@ -911,6 +963,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 					 int(__success), int(__failure));

>       }

>

>+#if __cplusplus > 201703L

>+    template<typename _Tp>

>+      _GLIBCXX_ALWAYS_INLINE void

>+      wait(const _Tp* __ptr, _Val<_Tp> __old, memory_order __m = memory_order_seq_cst) noexcept


And this one.


>@@ -1164,6 +1242,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 				       __cmpexch_failure_order(__order));

>       }

>

>+      _GLIBCXX_ALWAYS_INLINE void

>+      wait(_Fp __old, memory_order __m = memory_order_seq_cst) const noexcept

>+      { __atomic_impl::wait(&_M_fp, __old, __m); }

>+

>+      // TODO add const volatile overload

>+

>+      _GLIBCXX_ALWAYS_INLINE void

>+      notify_one() const noexcept

>+      { __atomic_impl::notify_one(&_M_fp); }

>+

>+      // TODO add const volatile overload

>+

>+      _GLIBCXX_ALWAYS_INLINE void

>+      notify_all() const noexcept

>+      { __atomic_impl::notify_all(&_M_fp); }

>+

>+      // TODO add const volatile overload


Please add a newline after this comment.

>       value_type

>       fetch_add(value_type __i,

> 		memory_order __m = memory_order_seq_cst) noexcept

>@@ -1301,6 +1396,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 				       __cmpexch_failure_order(__order));

>       }

>

>+      _GLIBCXX_ALWAYS_INLINE void

>+      wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept

>+      { __atomic_impl::wait(_M_ptr, __old, __m); }

>+

>+      // TODO add const volatile overload

>+

>+      _GLIBCXX_ALWAYS_INLINE void

>+      notify_one() const noexcept

>+      { __atomic_impl::notify_one(_M_ptr); }

>+

>+      // TODO add const volatile overload

>+

>+      _GLIBCXX_ALWAYS_INLINE void

>+      notify_all() const noexcept

>+      { __atomic_impl::notify_all(_M_ptr); }

>+


The TODO comment seems to be missing here, and after some notify_all
cases below. Please either add one after every non-volatile function
that's missing a volatile overload, or just put one "TODO volatile
overloads of wait and notify_{one,all}" comment in each class.

>diff --git a/libstdc++-v3/include/bits/atomic_timed_wait.h b/libstdc++-v3/include/bits/atomic_timed_wait.h

>new file mode 100644

>index 00000000000..adef80aca61

>--- /dev/null

>+++ b/libstdc++-v3/include/bits/atomic_timed_wait.h

>@@ -0,0 +1,282 @@

>+// -*- C++ -*- header.

>+

>+// 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.

>+

>+// Under Section 7 of GPL version 3, you are granted additional

>+// permissions described in the GCC Runtime Library Exception, version

>+// 3.1, as published by the Free Software Foundation.

>+

>+// You should have received a copy of the GNU General Public License and

>+// a copy of the GCC Runtime Library Exception along with this program;

>+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

>+// <http://www.gnu.org/licenses/>.

>+

>+/** @file bits/atomic_timed_wait.h

>+ *  This is an internal header file, included by other library headers.

>+ *  Do not attempt to use it directly. @headername{atomic}

>+ */

>+

>+#ifndef _GLIBCXX_ATOMIC_TIMED_WAIT_H

>+#define _GLIBCXX_ATOMIC_TIMED_WAIT_H 1

>+

>+#pragma GCC system_header

>+

>+#include <bits/c++config.h>

>+#include <bits/functional_hash.h>

>+#include <bits/atomic_wait.h>

>+

>+#include <chrono>

>+

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+#include <sys/time.h>

>+#endif

>+

>+namespace std _GLIBCXX_VISIBILITY(default)

>+{

>+  _GLIBCXX_BEGIN_NAMESPACE_VERSION


No indentation for these BEGIN/END macros.

>+

>+  enum class __atomic_wait_status { __no_timeout, __timeout };


It seems a shame to have yet another status macro when we already have
cv_status and future_status which define enumerators with the same
names, but I suppose it's a bit confusing to reuse them. Maybe you
could just do:

using __atomic_wait_status = cv_status;

and then refer to __atomic_wait_status::no_timeout and
__atomic_wait_status::timeout, what do you think?

Even if it's a new distinct enum type, you could use no_timeout and
timeout as the enumerator names, because those are reserved names
anyway.

>+  namespace __detail

>+  {

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+    enum


I think unnamed enum types in headers cause problems for modules.

>+    {

>+      __futex_wait_bitset_private = __futex_wait_bitset | __futex_private_flag,

>+      __futex_wake_bitset_private = __futex_wake_bitset | __futex_private_flag,

>+      __futex_bitset_match_any = 0xffffffff

>+    };

>+

>+    using __platform_wait_clock_t = chrono::steady_clock;

>+

>+    template<typename _Duration>

>+      __atomic_wait_status

>+      __platform_wait_until_impl(__platform_wait_t* __addr, __platform_wait_t __val,

>+				 const chrono::time_point<__platform_wait_clock_t, _Duration>& __atime) noexcept

>+      {

>+	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);

>+	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

>+

>+	struct timespec __rt =

>+	{

>+	  static_cast<std::time_t>(__s.time_since_epoch().count()),

>+	  static_cast<long>(__ns.count())

>+	};

>+

>+	auto __e = syscall (SYS_futex, __addr, __futex_wait_bitset_private, __val, &__rt,

>+			    nullptr, __futex_bitset_match_any);

>+	if (__e && !(errno == EINTR || errno == EAGAIN || errno == ETIMEDOUT))

>+	    std::terminate();

>+	return (__platform_wait_clock_t::now() < __atime)

>+	       ? __atomic_wait_status::__no_timeout : __atomic_wait_status::__timeout;

>+      }

>+

>+    template<typename _Clock, typename _Duration>

>+      __atomic_wait_status

>+      __platform_wait_until(__platform_wait_t* __addr, __platform_wait_t __val,

>+			    const chrono::time_point<_Clock, _Duration>& __atime)

>+      {

>+	if constexpr (std::is_same_v<__platform_wait_clock_t, _Clock>)

>+	  {

>+	    return __platform_wait_until_impl(__addr, __val, __atime);


Since this is calling a free function with arguments of
program-defined types (the clock and duration), it needs to be
qualified (whereas the is_same_v above doesn't need to be qualified,
although that's harmless).

>+	  }

>+	else

>+	  {

>+	    const typename _Clock::time_point __c_entry = _Clock::now();

>+	    const __platform_wait_clock_t::time_point __s_entry =

>+		    __platform_wait_clock_t::now();

>+	    const auto __delta = __atime - __c_entry;

>+	    const auto __s_atime = __s_entry + __delta;

>+	    if (__platform_wait_until_impl(__addr, __val, __s_atime) == __atomic_wait_status::__no_timeout)

>+	      return __atomic_wait_status::__no_timeout;

>+

>+	    // We got a timeout when measured against __clock_t but

>+	    // we need to check against the caller-supplied clock

>+	    // to tell whether we should return a timeout.

>+	    if (_Clock::now() < __atime)

>+	      return __atomic_wait_status::__no_timeout;

>+	    return __atomic_wait_status::__timeout;

>+	  }

>+      }

>+#endif

>+

>+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT

>+    template<typename _Duration>

>+      __atomic_wait_status

>+      __cond_wait_until_impl(__gthread_cond_t* __cv,

>+	  unique_lock<mutex>& __lock,

>+	  const chrono::time_point<chrono::steady_clock, _Duration>& __atime)

>+      {

>+	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);

>+	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

>+

>+	__gthread_time_t __ts =

>+	  {

>+	    static_cast<std::time_t>(__s.time_since_epoch().count()),

>+	    static_cast<long>(__ns.count())

>+	  };

>+

>+	pthread_cond_clockwait(__cv, __lock.mutex()->native_handle(),

>+			       CLOCK_MONOTONIC,

>+			       &__ts);

>+	return (chrono::steady_clock::now() < __atime)

>+	       ? __atomic_wait_status::__no_timeout : __atomic_wait_status::__timeout;

>+      }

>+#endif

>+

>+      template<typename _Duration>

>+	__atomic_wait_status

>+	__cond_wait_until_impl(__gthread_cond_t* __cv,

>+	    unique_lock<std::mutex>& __lock,

>+	    const chrono::time_point<chrono::system_clock, _Duration>& __atime)

>+	{

>+	  auto __s = chrono::time_point_cast<chrono::seconds>(__atime);

>+	  auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

>+

>+	  __gthread_time_t __ts =

>+	  {

>+	    static_cast<std::time_t>(__s.time_since_epoch().count()),

>+	    static_cast<long>(__ns.count())

>+	  };

>+

>+	  __gthread_cond_timedwait(__cv, __lock.mutex()->native_handle(),

>+				   &__ts);

>+	  return (chrono::system_clock::now() < __atime)

>+		 ? __atomic_wait_status::__no_timeout

>+		 : __atomic_wait_status::__timeout;

>+	}

>+

>+      // return true if timeout

>+      template<typename _Clock, typename _Duration>

>+	__atomic_wait_status

>+	__cond_wait_until(__gthread_cond_t* __cv,

>+	    unique_lock<std::mutex>& __lock,

>+	    const chrono::time_point<_Clock, _Duration>& __atime)

>+	{

>+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT

>+	  using __clock_t = chrono::steady_clock;

>+#else

>+	  using __clock_t = chrono::system_clock;

>+#endif

>+	  const typename _Clock::time_point __c_entry = _Clock::now();

>+	  const __clock_t::time_point __s_entry = __clock_t::now();

>+	  const auto __delta = __atime - __c_entry;

>+	  const auto __s_atime = __s_entry + __delta;

>+	  if (__cond_wait_until_impl(__cv, __lock, __s_atime))


__cond_wait_until_impl should be qualified.

>+	    return __atomic_wait_status::__no_timeout;

>+	  // We got a timeout when measured against __clock_t but

>+	  // we need to check against the caller-supplied clock

>+	  // to tell whether we should return a timeout.

>+	  if (_Clock::now() < __atime)

>+	    return __atomic_wait_status::__no_timeout;

>+	  return __atomic_wait_status::__timeout;

>+	}

>+

>+    struct __timed_waiters : __waiters

>+    {

>+      template<typename _Clock, typename _Duration>

>+	__atomic_wait_status

>+	_M_do_wait_until(__platform_wait_t __version,

>+			 const chrono::time_point<_Clock, _Duration>& __atime)

>+	{

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+	  return __platform_wait_until(&_M_ver, __version, __atime);

>+#else

>+	  __platform_wait_t __cur = 0;

>+	  __waiters::__lock_t __l(_M_mtx);

>+	  while (__cur <= __version)

>+	    {

>+	      if (__cond_wait_until(&_M_cv, __l, __atime) == __atomic_wait_status::__timeout)


Qualify.

>+		return __atomic_wait_status::__timeout;

>+

>+	      __platform_wait_t __last = __cur;

>+	      __atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);

>+	      if (__cur < __last)

>+		break; // break the loop if version overflows

>+	    }

>+	  return __atomic_wait_status::__no_timeout;

>+#endif

>+	}

>+

>+      static __timed_waiters&

>+      _S_timed_for(void* __t)

>+      {

>+	static_assert(sizeof(__timed_waiters) == sizeof(__waiters));

>+	return (__timed_waiters&) __waiters::_S_for(__t);


I'd be more comfortable with a static_cast here.

>+      }

>+    };

>+  } // namespace __detail

>+

>+  template<typename _Tp, typename _Pred,

>+	   typename _Clock, typename _Duration>

>+    bool

>+    __atomic_wait_until(const _Tp* __addr, _Tp __old, _Pred __pred,

>+			const chrono::time_point<_Clock, _Duration>& __atime) noexcept

>+    {

>+      using namespace __detail;

>+

>+      if (std::__atomic_spin(__pred))

>+	return true;

>+

>+      auto& __w = __timed_waiters::_S_timed_for((void*)__addr);

>+      auto __version = __w._M_enter_wait();

>+      do

>+	{

>+	  __atomic_wait_status __res;

>+	  if constexpr (__platform_wait_uses_type<_Tp>)

>+	    {

>+	      __res = __platform_wait_until((__platform_wait_t*)(void*) __addr,

>+					    __old,

>+					    __atime);

>+	    }

>+	  else

>+	    {

>+	      __res = __w._M_do_wait_until(__version, __atime);

>+	    }

>+	  if (__res == __atomic_wait_status::__timeout)

>+	    return false;

>+	}

>+      while (!__pred() && __atime < _Clock::now());

>+      __w._M_leave_wait();

>+

>+      // if timed out, return false

>+      return (_Clock::now() < __atime);

>+    }

>+

>+  template<typename _Tp, typename _Pred,

>+	   typename _Rep, typename _Period>

>+    bool

>+    __atomic_wait_for(const _Tp* __addr, _Tp __old, _Pred __pred,

>+		      const chrono::duration<_Rep, _Period>& __rtime) noexcept

>+    {

>+      using namespace __detail;

>+

>+      if (std::__atomic_spin(__pred))

>+	return true;

>+

>+      if (!__rtime.count())

>+	return false; // no rtime supplied, and spin did not acquire

>+

>+      using __dur = chrono::steady_clock::duration;

>+      auto __reltime = chrono::duration_cast<__dur>(__rtime);

>+      if (__reltime < __rtime)

>+	++__reltime;

>+

>+

>+      return __atomic_wait_until(__addr, __old, std::move(__pred),

>+				 chrono::steady_clock::now() + __reltime);

>+    }

>+_GLIBCXX_END_NAMESPACE_VERSION

>+} // namespace std

>+#endif

>diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h

>new file mode 100644

>index 00000000000..92c1e2526ed

>--- /dev/null

>+++ b/libstdc++-v3/include/bits/atomic_wait.h

>@@ -0,0 +1,291 @@

>+// -*- C++ -*- header.

>+

>+// 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.

>+

>+// Under Section 7 of GPL version 3, you are granted additional

>+// permissions described in the GCC Runtime Library Exception, version

>+// 3.1, as published by the Free Software Foundation.

>+

>+// You should have received a copy of the GNU General Public License and

>+// a copy of the GCC Runtime Library Exception along with this program;

>+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

>+// <http://www.gnu.org/licenses/>.

>+

>+/** @file bits/atomic_wait.h

>+ *  This is an internal header file, included by other library headers.

>+ *  Do not attempt to use it directly. @headername{atomic}

>+ */

>+

>+#ifndef _GLIBCXX_ATOMIC_WAIT_H

>+#define _GLIBCXX_ATOMIC_WAIT_H 1

>+

>+#pragma GCC system_header

>+

>+#include <bits/c++config.h>

>+#include <bits/functional_hash.h>

>+#include <bits/gthr.h>

>+#include <bits/std_mutex.h>

>+#include <bits/unique_lock.h>

>+#include <ext/numeric_traits.h>

>+

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+#include <climits>

>+#include <unistd.h>

>+#include <syscall.h>

>+#endif

>+

>+#define _GLIBCXX_SPIN_COUNT_1 16

>+#define _GLIBCXX_SPIN_COUNT_2 12

>+


Yuck, do these have to be macros?

>+// TODO get this from Autoconf

>+#define _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE 1

>+

>+namespace std _GLIBCXX_VISIBILITY(default)

>+{

>+_GLIBCXX_BEGIN_NAMESPACE_VERSION

>+  namespace __detail

>+  {

>+    using __platform_wait_t = int;

>+

>+    inline constexpr

>+    auto __platform_wait_max_value =

>+		__gnu_cxx::__numeric_traits<__platform_wait_t>::__max;


You can usee the new __gnu_cxx::__int_traits alias here, since you
know __platform_wait_t is an integer, not a floating-point type. That
alias just saves instantiating __numeric_traits to figure out whether
to use __numeric_traits_integer or __numeric_traits_floating.

>+

>+    template<typename _Tp>

>+      inline constexpr bool __platform_wait_uses_type

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+	= is_same_v<remove_cv_t<_Tp>, __platform_wait_t>;

>+#else

>+	= false;

>+#endif

>+

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+    enum


See earlier comment about unnamed enum types.


>+    {

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE

>+      __futex_private_flag = 128,

>+#else

>+      __futex_private_flag = 0,

>+#endif

>+      __futex_wait = 0,

>+      __futex_wake = 1,

>+      __futex_wait_bitset = 9,

>+      __futex_wake_bitset = 10,

>+      __futex_wait_private = __futex_wait | __futex_private_flag,

>+      __futex_wake_private = __futex_wake | __futex_private_flag

>+    };

>+

>+    void

>+    __platform_wait(__platform_wait_t* __addr, __platform_wait_t __val) noexcept

>+    {

>+       auto __e = syscall (SYS_futex, __addr, __futex_wait_private, __val, nullptr);

>+       if (__e && !(errno == EINTR || errno == EAGAIN))

>+	 std::terminate();

>+    }

>+

>+    void

>+    __platform_notify(__platform_wait_t* __addr, bool __all) noexcept

>+    {

>+      syscall (SYS_futex, __addr, __futex_wake_private, __all ? INT_MAX : 1);

>+    }

>+#endif

>+

>+    struct __waiters

>+    {

>+      __platform_wait_t alignas(64) _M_ver = 0;

>+      __platform_wait_t alignas(64) _M_wait = 0;

>+

>+#ifndef _GLIBCXX_HAVE_LINUX_FUTEX

>+      using __lock_t = std::unique_lock<std::mutex>;

>+      mutable __lock_t::mutex_type _M_mtx;

>+

>+#  ifdef __GTHREAD_COND_INIT

>+      mutable __gthread_cond_t _M_cv = __GTHREAD_COND_INIT;


Not being able to use std::condition_variable here makes me sad.

>+      __waiters() noexcept = default;

>+#  else

>+      mutable __gthread_cond_t _M_cv;

>+      __waiters() noexcept

>+      {

>+	__GTHREAD_COND_INIT_FUNCTION(&_M_cond);

>+      }

>+#  endif

>+#endif

>+

>+      __platform_wait_t

>+      _M_enter_wait() noexcept

>+      {

>+	__platform_wait_t __res;

>+	__atomic_load(&_M_ver, &__res, __ATOMIC_ACQUIRE);

>+	__atomic_fetch_add(&_M_wait, 1, __ATOMIC_ACQ_REL);

>+	return __res;

>+      }

>+

>+      void

>+      _M_leave_wait() noexcept

>+      {

>+	__atomic_fetch_sub(&_M_wait, 1, __ATOMIC_ACQ_REL);

>+      }

>+

>+      void

>+      _M_do_wait(__platform_wait_t __version) noexcept

>+      {

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+	__platform_wait(&_M_ver, __version);

>+#else

>+	__platform_wait_t __cur = 0;

>+	while (__cur <= __version)

>+	  {

>+	    __waiters::__lock_t __l(_M_mtx);

>+	    auto __e = __gthread_cond_wait(&_M_cv, __l.mutex()->native_handle());

>+	    if (__e)

>+	      std::terminate();

>+	    __platform_wait_t __last = __cur;

>+	    __atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);

>+	    if (__cur < __last)

>+	      break; // break the loop if version overflows

>+	  }

>+#endif

>+      }

>+

>+      __platform_wait_t

>+      _M_waiting() const noexcept

>+	{

>+	  __platform_wait_t __res;

>+	  __atomic_load(&_M_wait, &__res, __ATOMIC_ACQUIRE);

>+	  return __res;

>+	}

>+

>+      void

>+      _M_notify(bool __all) noexcept

>+      {

>+	__atomic_fetch_add(&_M_ver, 1, __ATOMIC_ACQ_REL);

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+	__platform_notify(&_M_ver, __all);

>+#else

>+	auto __e = __gthread_cond_broadcast(&_M_cv);

>+	if (__e)

>+	  __throw_system_error(__e);

>+#endif

>+      }

>+

>+      static __waiters&

>+      _S_for(void* __t)

>+      {

>+	const unsigned char __mask = 0xf;

>+	static __waiters __w[__mask + 1];

>+

>+	auto __key = _Hash_impl::hash(__t) & __mask;

>+	return __w[__key];

>+      }

>+    };

>+

>+    struct __waiter

>+    {

>+      __waiters& _M_w;

>+      __platform_wait_t _M_version;

>+

>+      template<typename _Tp>

>+	__waiter(const _Tp* __addr) noexcept

>+	  : _M_w(__waiters::_S_for((void*) __addr))

>+	  , _M_version(_M_w._M_enter_wait())

>+	{ }

>+

>+      ~__waiter()

>+      { _M_w._M_leave_wait(); }

>+

>+      void _M_do_wait() noexcept

>+      { _M_w._M_do_wait(_M_version); }

>+    };

>+

>+    void

>+    __thread_relax() noexcept

>+    {

>+#if defined __i386__ || defined __x86_64__

>+      __builtin_ia32_pause();

>+#elif defined _GLIBCXX_USE_SCHED_YIELD

>+      __gthread_yield();

>+#endif


Should this and the identical code in <stop_token> be consolidated
somewhere?

>+    }

>+

>+    void

>+    __thread_yield() noexcept

>+   {

>+#if defined _GLIBCXX_USE_SCHED_YIELD

>+     __gthread_yield();

>+#endif

>+    }

>+

>+  } // namespace __detail

>+

>+  template<typename _Pred>

>+    bool

>+    __atomic_spin(_Pred __pred) noexcept

>+    {

>+      for (auto __i = 0; __i < _GLIBCXX_SPIN_COUNT_1; ++__i)

>+	{

>+	  if (__pred())

>+	    return true;

>+

>+	  if (__i < _GLIBCXX_SPIN_COUNT_2)

>+	    __detail::__thread_relax();

>+	  else

>+	    __detail::__thread_yield();

>+	}

>+      return false;

>+    }

>+

>+  template<typename _Tp, typename _Pred>

>+    void

>+    __atomic_wait(const _Tp* __addr, _Tp __old, _Pred __pred) noexcept

>+    {

>+      using namespace __detail;

>+      if (__atomic_spin(__pred))

>+	return;

>+

>+      __waiter __w(__addr);

>+      while (!__pred())

>+	{

>+	  if constexpr (__platform_wait_uses_type<_Tp>)

>+	    {

>+	      __platform_wait((__platform_wait_t*)(void*) __addr, __old);


This has an implicit conversion from _To to __platform_wait_t, is that
safe? Can we make it explicit with a static_cast?

>+	    }

>+	  else

>+	    {

>+	      // TODO support timed backoff when this can be moved into the lib

>+	      __w._M_do_wait();

>+	    }

>+	}

>+    }

>+

>+  template<typename _Tp>

>+    void

>+    __atomic_notify(const _Tp* __addr, bool __all) noexcept

>+    {

>+      using namespace __detail;

>+      auto& __w = __waiters::_S_for((void*)__addr);

>+      if (!__w._M_waiting())

>+	return;

>+

>+      if constexpr (__platform_wait_uses_type<_Tp>)

>+	{

>+	  __platform_notify((__platform_wait_t*)(void*) __addr, __all);

>+	}

>+      else

>+	{

>+	  __w._M_notify(__all);

>+	}

>+    }

>+_GLIBCXX_END_NAMESPACE_VERSION

>+} // namespace std

>+#endif

>diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h

>new file mode 100644

>index 00000000000..f0c4235d91c

>--- /dev/null

>+++ b/libstdc++-v3/include/bits/semaphore_base.h


I'll continue reviewing from here ASAP ...
Aldy Hernandez via Gcc-patches Aug. 3, 2020, 2:09 p.m. | #2
On 05/06/20 17:29 -0700, Thomas Rodgers wrote:
>diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h

>new file mode 100644

>index 00000000000..f0c4235d91c

>--- /dev/null

>+++ b/libstdc++-v3/include/bits/semaphore_base.h

>@@ -0,0 +1,272 @@

>+// -*- C++ -*- header.

>+

>+// 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.

>+

>+// Under Section 7 of GPL version 3, you are granted additional

>+// permissions described in the GCC Runtime Library Exception, version

>+// 3.1, as published by the Free Software Foundation.

>+

>+// You should have received a copy of the GNU General Public License and

>+// a copy of the GCC Runtime Library Exception along with this program;

>+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

>+// <http://www.gnu.org/licenses/>.

>+

>+/** @file bits/semaphore_base.h

>+ *  This is an internal header file, included by other library headers.

>+ *  Do not attempt to use it directly. @headername{semaphore}

>+ */

>+

>+#ifndef _GLIBCXX_SEMAPHORE_BASE_H

>+#define _GLIBCXX_SEMAPHORE_BASE_H 1

>+

>+#pragma GCC system_header

>+

>+#include <bits/c++config.h>

>+#include <bits/atomic_base.h>

>+#include <bits/atomic_timed_wait.h>

>+

>+#if defined _POSIX_SEMAPHORES  && __has_include(<semaphore.h>)

>+#define _GLIBCXX_HAVE_POSIX_SEMAPHORE 1

>+#include <semaphore.h>

>+#endif

>+

>+#include <chrono>

>+#include <type_traits>

>+

>+namespace std _GLIBCXX_VISIBILITY(default)

>+{

>+_GLIBCXX_BEGIN_NAMESPACE_VERSION

>+

>+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE

>+  template<ptrdiff_t __least_max_value>

>+    struct __platform_semaphore

>+    {


I think we want to delete the copy constructor and copy assignment
operator for this type and __atomic_semaphore.

>+      using __clock_t = chrono::system_clock;

>+

>+      explicit __platform_semaphore(ptrdiff_t __count) noexcept

>+      {

>+	static_assert( __least_max_value <= SEM_VALUE_MAX, "");

>+	auto __e = sem_init(&_M_semaphore, 0, __count);

>+	if (__e)

>+	  std::terminate();


I don't think we should bother checking the return value. The failure
conditions are EINVAL if __count > SEM_VALUE_MAX, and ENOSYS if the
second argument is non-zero. Both conditions should be impossible.

>+      }

>+

>+      ~__platform_semaphore()

>+      {

>+	auto __e = sem_destroy(&_M_semaphore);

>+	if (__e)

>+	  std::terminate();


Likewise here. The only error condition is EINVAL, which is impossible
if _M_semaphore is a valid semaphore, which the constructor already
ensured.

If somehow _M_semaphore is invalid it means the user broke something
(e.g. by scribbling over *this) and we don't need to detect and handle
that case.

>+      }

>+

>+      _GLIBCXX_ALWAYS_INLINE void

>+      acquire() noexcept

>+      {

>+	auto __err = sem_wait(&_M_semaphore);

>+	if (__err)

>+	  std::terminate();


What about the EINTR case where the wait is interrupted by a signal?
I think that should retry, but it shouldn't terminate.

>+      template<typename _Duration>

>+	bool

>+	__try_acquire_until_impl(const chrono::time_point<__clock_t>& __atime) noexcept

>+	{

>+	  auto __s = chrono::time_point_cast<chrono::seconds>(__atime);

>+	  auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

>+

>+	  struct timespec __ts =

>+	  {

>+	    static_cast<std::time_t>(__s.time_since_epoch().count()),

>+	    static_cast<long>(__ns.count())

>+	  };

>+

>+	  auto __err = sem_timedwait(&_M_semaphore, &__ts);

>+	  if (__err && (errno == ETIMEDOUT))

>+	      return false;

>+	  else if (__err)

>+	      std::terminate();


I think this needs to handle both EINTR and EINVAL:

        EINVAL The value of abs_timeout.tv_nsecs is less than 0, or greater than or equal to 1000 million.

Alternatively, just return immediately without waiting for a time
before the epoch, i.e.  __abs_time < time_point<__clock_t>{}

Otherwise time_point(-20ns) will cause EINVAL and terminate.


>+	  return true;

>+	}

>+

>+      template<typename _Clock, typename _Duration>

>+	bool

>+	try_acquire_until(const chrono::time_point<_Clock, _Duration>& __atime) noexcept

>+	{

>+	  if constexpr (std::is_same<__clock_t, _Clock>::value)

>+	    {

>+	      return __try_acquire_until_impl(__atime);

>+	    }

>+	  else

>+	    {

>+	      const typename _Clock::time_point __c_entry = _Clock::now();

>+	      const __clock_t __s_entry = __clock_t::now();

>+	      const auto __delta = __atime - __c_entry;

>+	      const auto __s_atime = __s_entry + __delta;

>+	      if (__try_acquire_until_impl(__s_atime))

>+		return true;

>+

>+	      // We got a timeout when measured against __clock_t but

>+	      // we need to check against the caller-supplied clock

>+	      // to tell whether we should return a timeout.

>+	      return (_Clock::now() < __atime);

>+	    }

>+	}

>+

>+      template<typename _Rep, typename _Period>

>+	_GLIBCXX_ALWAYS_INLINE bool

>+	try_acquire_for(const chrono::duration<_Rep, _Period>& __rtime) noexcept

>+	{ return try_acquire_until(__clock_t::now() + __rtime); }

>+

>+      template<typename _Clock, typename _Duration>

>+	_GLIBCXX_ALWAYS_INLINE void

>+	release(ptrdiff_t __update) noexcept

>+	{

>+	  do

>+	    {

>+	      auto __err = sem_post(&_M_semaphore);

>+	      if (__err)

>+		std::terminate();

>+	    } while (--__update);

>+	}

>+

>+      private:

>+	sem_t _M_semaphore;

>+      };

>+#endif // _GLIBCXX_HAVE_POSIX_SEMAPHORE

>+

>+    template<typename _Tp>

>+      struct __atomic_semaphore

>+      {


Do we want a static_assert(is_integral_v<_Tp>) or similar?

>+	explicit __atomic_semaphore(_Tp __count)


Add noexcept?

>+	  : _M_a(__count)

>+	{ }

>+

>+	_GLIBCXX_ALWAYS_INLINE void

>+	acquire() noexcept

>+	{

>+	  auto const __pred = [this]

>+	    {

>+	      auto __old = __atomic_impl::load(&this->_M_a,

>+			      memory_order::acquire);


Would it be simpler to just use a local atomic_ref to manipulate _M_a
here, and in the other members performing atomic ops on that member?

>+	      if (__old == 0)

>+		return false;

>+	      return __atomic_impl::compare_exchange_strong(&this->_M_a,

>+			__old, __old - 1,

>+			memory_order::acquire,

>+			memory_order::release);

>+	    };

>+	  auto __old = __atomic_impl::load(&_M_a, memory_order_relaxed);

>+	  __atomic_wait(&_M_a, __old, __pred);


Strictly speaking, this __atomic_wait would violate the "all accesses
to that object shall exclusively occur through those atomic_ref
instances" rule. But we know that it's actually OK because
__atomic_wait is only performing atomic ops on the same location.

>+	}

>+

>+	bool

>+	try_acquire() noexcept

>+	{

>+	  auto __old = __atomic_impl::load(&_M_a, memory_order::acquire);

>+	  if (__old == 0)

>+	    return false;

>+

>+	  return __atomic_spin([this, &__old]

>+	    {

>+	      return __atomic_impl::compare_exchange_weak(&this->_M_a,

>+			__old, __old - 1,

>+			memory_order::acquire,

>+			memory_order::release);

>+	    });

>+	}

>+

>+	template<typename _Clock, typename _Duration>

>+	  _GLIBCXX_ALWAYS_INLINE bool

>+	  try_acquire_until(const chrono::time_point<_Clock, _Duration>& __atime) noexcept

>+	  {

>+	    auto const __pred = [this]

>+	      {

>+		auto __old = __atomic_impl::load(&this->_M_a,

>+				memory_order::acquire);

>+		if (__old == 0)

>+		  return false;

>+		return __atomic_impl::compare_exchange_strong(&this->_M_a,

>+				__old, __old - 1,

>+				memory_order::acquire,

>+				memory_order::release);

>+	      };

>+

>+	    auto __old = __atomic_impl::load(&_M_a, memory_order_relaxed);

>+	    return __atomic_wait_until(&_M_a, __old, __pred, __atime);

>+	}

>+

>+      template<typename _Rep, typename _Period>

>+	_GLIBCXX_ALWAYS_INLINE bool

>+	try_acquire_for(const chrono::duration<_Rep, _Period>& __rtime) noexcept

>+	{

>+	  auto const __pred = [this]

>+	    {

>+	      auto __old = __atomic_impl::load(&this->_M_a,

>+			      memory_order::acquire);

>+	      if (__old == 0)

>+		return false;

>+	      return  __atomic_impl::compare_exchange_strong(&this->_M_a,

>+			      __old, __old - 1,

>+			      memory_order::acquire,

>+			      memory_order::release);

>+	    };

>+

>+	  auto __old = __atomic_impl::load(&_M_a, memory_order_relaxed);

>+	  return __atomic_wait_for(&_M_a, __old, __pred, __rtime);

>+	}

>+

>+      _GLIBCXX_ALWAYS_INLINE void

>+      release(ptrdiff_t __update) noexcept

>+      {

>+	if (0 < __atomic_impl::fetch_add(&_M_a, __update, memory_order_release))

>+	  return;

>+	if (__update > 1)

>+	  __atomic_impl::notify_all(&_M_a);

>+	else

>+	  __atomic_impl::notify_one(&_M_a);

>+      }

>+

>+    private:

>+      alignas(__alignof__(_Tp)) _Tp _M_a;

>+    };

>+

>+#ifdef _GLIBCXX_REQUIRE_POSIX_SEMAPHORE


What is this case for? This macro seems to only exist for use by a
single testcase.

Is it supposed to be something users can set? If so, it needs to be
documented as ABI-breaking and as implying that counting_semaphore
cannot be use with counts higher than SEM_VALUE_MAX.

>+  template<ptrdiff_t __least_max_value>

>+    using __semaphore_base = __platform_semaphore<__least_max_value>;

>+#else

>+#  ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+  template<ptrdiff_t __least_max_value>

>+    using __semaphore_base = conditional_t<(

>+			      __least_max_value >= 0


This condition is redundant, we already know that counting_semaphore
has checked the value is non-negative.

>+				&& __least_max_value <= __detail::__platform_wait_max_value),

>+			      __atomic_semaphore<__detail::__platform_wait_t>,

>+			      __atomic_semaphore<ptrdiff_t>>;

>+

>+// __platform_semaphore

>+#  elif defined _GLIBCXX_HAVE_POSIX_SEMAPHORE

>+  template<ptrdiff_t __least_max_value>

>+    using __semaphore_base = conditional_t<(

>+			      __least_max_value >= 0


Redundant condition again.

>+				&& __least_max_value <= SEM_VALUE_MAX),

>+			      __platform_semaphore<__least_max_value>,

>+			      __atomic_semaphore<ptrdiff_t>>;

>+#  else

>+  template<ptrdiff_t __least_max_value>

>+    using __semaphore_base = __atomic_semaphore<ptrdiff_t>;

>+#  endif

>+#endif

>+

>+_GLIBCXX_END_NAMESPACE_VERSION

>+} // namespace std

>+

>+#endif

>diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic

>index a455286a784..3f18774031d 100644

>--- a/libstdc++-v3/include/std/atomic

>+++ b/libstdc++-v3/include/std/atomic

>@@ -163,6 +163,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

>     compare_exchange_strong(bool& __i1, bool __i2,

> 		    memory_order __m = memory_order_seq_cst) volatile noexcept

>     { return _M_base.compare_exchange_strong(__i1, __i2, __m); }

>+

>+#if __cplusplus > 201703L

>+    void wait(bool __old, memory_order __m = memory_order_seq_cst) const noexcept

>+    { _M_base.wait(__old, __m); }

>+

>+    // TODO add const volatile overload

>+

>+    void notify_one() const noexcept

>+    { _M_base.notify_one(); }

>+

>+    void notify_all() const noexcept

>+    { _M_base.notify_all(); }

>+#endif

>   };

>

> #if __cplusplus <= 201703L

>@@ -352,6 +365,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 		     memory_order __m = memory_order_seq_cst) volatile noexcept

>       { return compare_exchange_strong(__e, __i, __m,

>                                        __cmpexch_failure_order(__m)); }

>+#if __cplusplus > 201703L

>+    void wait(_Tp __old, memory_order __m = memory_order_seq_cst) noexcept

>+    { _M_i.wait(__old, __m); }


This can't be right. _M_i is of type _Tp in std::atomic<_Tp>, it doesn't 
have a wait member function.

If this compiles it suggests there are no tests for these members on
the primary template, e.g. something like:

void
test01()
{
   struct S { int i; };
   std:::atomic<S> s;
   s.wait();
}

>+

>+    // TODO add const volatile overload

>+

>+    void notify_one() const noexcept

>+    { _M_i.notify_one(); }

>+

>+    void notify_all() const noexcept

>+    { _M_i.notify_all(); }

>+#endif

>+

>     };

> #undef _GLIBCXX20_INIT

>

>@@ -590,6 +616,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 					    __cmpexch_failure_order(__m));

>       }

>

>+#if __cplusplus > 201703L

>+    void wait(__pointer_type __old, memory_order __m = memory_order_seq_cst) noexcept

>+    { _M_b.wait(__old, __m); }

>+

>+    // TODO add const volatile overload

>+

>+    void notify_one() const noexcept

>+    { _M_b.notify_one(); }

>+

>+    void notify_all() const noexcept

>+    { _M_b.notify_all(); }

>+#endif

>       __pointer_type

>       fetch_add(ptrdiff_t __d,

> 		memory_order __m = memory_order_seq_cst) noexcept

>@@ -1342,6 +1380,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

> 						     memory_order_seq_cst);

>     }

>

>+

>+#if __cplusplus > 201703L

>+  template<typename _Tp>

>+    inline void atomic_wait(const atomic<_Tp>* __a,


Newline after the return type.

>+	                    typename std::atomic<_Tp>::value_type __old) noexcept

>+    { __a->wait(__old); }

>+

>+  template<typename _Tp>

>+    inline void atomic_wait_explicit(const atomic<_Tp>* __a,


And here.

>+				     typename std::atomic<_Tp>::value_type __old,

>+				     std::memory_order __m) noexcept

>+    { __a->wait(__old, __m); }

>+

>+  template<typename _Tp>

>+    inline void atomic_notify_one(atomic<_Tp>* __a) noexcept

>+    { __a->notify_one(); }

>+

>+  template<typename _Tp>

>+    inline void atomic_notify_all(atomic<_Tp>* __a) noexcept

>+    { __a->notify_all(); }

>+

>+#endif // C++2a

>+

>   // Function templates for atomic_integral and atomic_pointer operations only.

>   // Some operations (and, or, xor) are only available for atomic integrals,

>   // which is implemented by taking a parameter of type __atomic_base<_ITp>*.

>diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch

>new file mode 100644

>index 00000000000..aa5299d9fdd

>--- /dev/null

>+++ b/libstdc++-v3/include/std/latch

>@@ -0,0 +1,90 @@

>+// <latch> -*- 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.

>+

>+// Under Section 7 of GPL version 3, you are granted additional

>+// permissions described in the GCC Runtime Library Exception, version

>+// 3.1, as published by the Free Software Foundation.

>+

>+// You should have received a copy of the GNU General Public License and

>+// a copy of the GCC Runtime Library Exception along with this program;

>+// see the files COPYING3 and COPYING.RUNTIME respectively.	 If not, see

>+// <http://www.gnu.org/licenses/>.

>+

>+/** @file include/latch

>+ *  This is a Standard C++ Library header.

>+ */

>+

>+#ifndef _GLIBCXX_LATCH

>+#define _GLIBCXX_LATCH

>+

>+#pragma GCC system_header

>+

>+#if __cplusplus > 201703L

>+#define __cpp_lib_latch 201907L

>+

>+#include <bits/atomic_base.h>

>+#include <ext/numeric_traits.h>

>+

>+namespace std _GLIBCXX_VISIBILITY(default)

>+{

>+_GLIBCXX_BEGIN_NAMESPACE_VERSION

>+

>+  class latch

>+  {

>+  public:

>+    static constexpr

>+    _GLIBCXX_ALWAYS_INLINE ptrdiff_t


I don't think we need _GLIBCXX_ALWAYS_INLINE here.

>+    max() noexcept

>+    { return __gnu_cxx::__numeric_traits<ptrdiff_t>::__max; }

>+

>+    constexpr explicit latch(ptrdiff_t __expected) : _M_a(__expected) { }


Add noexcept.

>+

>+    ~latch() = default;

>+    latch(const latch&) = delete;

>+    latch& operator=(const latch&) = delete;

>+

>+    _GLIBCXX_ALWAYS_INLINE void

>+    count_down(ptrdiff_t __update = 1)

>+    {

>+      auto const __old = __atomic_impl::fetch_sub(&_M_a, __update, memory_order::release);


It looks like this could use atomic_ref too, although there'd be less
benefit here.

>+      if (__old == __update)

>+	__atomic_impl::notify_all(&_M_a);

>+    }

>+

>+    _GLIBCXX_ALWAYS_INLINE bool

>+    try_wait() const noexcept

>+    { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }

>+

>+    _GLIBCXX_ALWAYS_INLINE void

>+    wait() const

>+    {

>+      auto const __old = __atomic_impl::load(&_M_a, memory_order::acquire);

>+      __atomic_wait(&_M_a, __old, [this] { return this->try_wait(); });

>+    }

>+

>+    _GLIBCXX_ALWAYS_INLINE void

>+    arrive_and_wait(ptrdiff_t __update = 1)

>+    {

>+      count_down();

>+      wait();

>+    }

>+

>+  private:

>+    alignas(__alignof__(ptrdiff_t)) ptrdiff_t _M_a;

>+  };

>+_GLIBCXX_END_NAMESPACE_VERSION

>+} // namespace

>+#endif // __cplusplus > 201703L

>+#endif // _GLIBCXX_LATCH

>diff --git a/libstdc++-v3/include/std/semaphore b/libstdc++-v3/include/std/semaphore

>new file mode 100644

>index 00000000000..90cf3244647

>--- /dev/null

>+++ b/libstdc++-v3/include/std/semaphore

>@@ -0,0 +1,86 @@

>+// <semaphore> -*- 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.

>+

>+// Under Section 7 of GPL version 3, you are granted additional

>+// permissions described in the GCC Runtime Library Exception, version

>+// 3.1, as published by the Free Software Foundation.

>+

>+// You should have received a copy of the GNU General Public License and

>+// a copy of the GCC Runtime Library Exception along with this program;

>+// see the files COPYING3 and COPYING.RUNTIME respectively.	 If not, see

>+// <http://www.gnu.org/licenses/>.

>+

>+/** @file include/semaphore

>+ *  This is a Standard C++ Library header.

>+ */

>+

>+#ifndef _GLIBCXX_SEMAPHORE

>+#define _GLIBCXX_SEMAPHORE

>+

>+#pragma GCC system_header

>+

>+#if __cplusplus > 201703L

>+#define __cpp_lib_semaphore 201907L

>+#include <bits/semaphore_base.h>

>+#include <ext/numeric_traits.h>

>+

>+namespace std _GLIBCXX_VISIBILITY(default)

>+{

>+_GLIBCXX_BEGIN_NAMESPACE_VERSION

>+

>+  template<ptrdiff_t __least_max_value =

>+			__gnu_cxx::__numeric_traits<ptrdiff_t>::__max>

>+    class counting_semaphore

>+    {

>+      static_assert(__least_max_value >=0, "");


Add a space before the 0. Get rid of the empty string literal.

>+

>+      __semaphore_base<__least_max_value> _M_sem;


"base" seems a bit misleading here when it's a member not a base
class. Would __semaphore_impl be better?

>+

>+    public:

>+      explicit counting_semaphore(ptrdiff_t __desired) noexcept

>+	: _M_sem(__desired)

>+      { }

>+

>+      ~counting_semaphore() = default;

>+

>+      counting_semaphore(const counting_semaphore&) = delete;

>+      counting_semaphore& operator=(const counting_semaphore&) = delete;

>+

>+      static constexpr ptrdiff_t max() noexcept

>+      { return __least_max_value; }

>+

>+      void release(ptrdiff_t __update = 1)


We can add noexcept here, since we know neither of the
__semaphore_base types will throw. If you envision alternative
implementations in future, then it would be futureproof to use
noexcept(noexcept(_M_sem.release(1)))

>+      { _M_sem.release(__update); }

>+

>+      void acquire()


Same here.

>+      { _M_sem.acquire(); }

>+

>+      bool try_acquire() noexcept

>+      { return _M_sem.try_acquire(); }

>+

>+      template<class _Rep, class _Period>

>+	bool try_acquire_for(const std::chrono::duration<_Rep, _Period>& __rel_time)


Newline after the return type (the line is too long otherwise).

This one can potentially throw because of the duration arithmetic on
_Rep objects.

>+	{ return _M_sem.try_acquire_for(__rel_time); }

>+

>+      template<class _Clock, class _Duration>

>+	bool try_acquire_until(const std::chrono::time_point<_Clock, _Duration>& __abs_time)


Newline after the return type (I think it's still going to be too
long).

>+	{ return _M_sem.try_acquire_until(__abs_time); }

>+    };

>+

>+ using binary_semaphore = std::counting_semaphore<1>;

>+_GLIBCXX_END_NAMESPACE_VERSION

>+} // namespace

>+#endif // __cplusplus > 201703L

>+#endif // _GLIBCXX_SEMAPHORE

>diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version

>index c6bde2cfbda..f09da3344f7 100644

>--- a/libstdc++-v3/include/std/version

>+++ b/libstdc++-v3/include/std/version

>@@ -189,6 +189,8 @@

> #endif

> #define __cpp_lib_type_identity 201806L

> #define __cpp_lib_unwrap_ref 201811L

>+#define __cpp_lib_semaphore 201907L

>+#define __cpp_lib_latch 201907L


You're adding these macros in the freestanding section, and these
aren't freestanding headers. They need to be in the _GLIBCXX_HOSTED
block, and the macros need to be in alphabetical order.

> #if _GLIBCXX_HOSTED

> #undef __cpp_lib_array_constexpr

>diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc

>new file mode 100644

>index 00000000000..1ced9d44b20

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc

>@@ -0,0 +1,103 @@

>+// { dg-options "-std=gnu++2a -pthread -latomic -L../../libatomic/.libs" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+// 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/>.

>+

>+#include <atomic>

>+#include <thread>

>+#include <mutex>

>+#include <condition_variable>

>+#include <chrono>

>+#include <type_traits>

>+

>+#include <testsuite_hooks.h>

>+

>+template<typename Tp>

>+Tp check_wait_notify(Tp val1, Tp val2)

>+{

>+  using namespace std::literals::chrono_literals;

>+

>+  std::mutex m;

>+  std::condition_variable cv;

>+

>+  Tp aa = val1;

>+  std::atomic_ref<Tp> a(aa);

>+  std::thread t([&]

>+		{

>+		  cv.notify_one();

>+		  a.wait(val1);

>+		  if (a.load() != val2)

>+		    a = val1;

>+		});

>+  std::unique_lock<std::mutex> l(m);

>+  cv.wait(l);

>+  std::this_thread::sleep_for(100ms);

>+  a.store(val2);

>+  a.notify_one();

>+  t.join();

>+  return a.load();

>+}

>+

>+template<typename Tp,

>+	 bool = std::is_integral_v<Tp>

>+	 || std::is_floating_point_v<Tp>>

>+struct check;

>+

>+template<typename Tp>

>+struct check<Tp, true>

>+{

>+  check()

>+  {

>+    Tp a = 0;

>+    Tp b = 42;

>+    VERIFY(check_wait_notify(a, b) == b);

>+  }

>+};

>+

>+template<typename Tp>

>+struct check<Tp, false>

>+{

>+  check(Tp b)

>+  {

>+    Tp a;

>+    VERIFY(check_wait_notify(a, b) == b);

>+  }

>+};

>+

>+struct foo

>+{

>+  long a = 0;

>+  long b = 0;

>+

>+  foo& operator=(foo const&) = default;

>+

>+  friend bool

>+  operator==(foo const& rhs, foo const& lhs)

>+  { return rhs.a == lhs.a && rhs.b == lhs.b; }

>+};

>+

>+int

>+main ()

>+{

>+  check<long>();

>+  check<double>();

>+  check<foo>({42, 48});

>+  return 0;

>+}

>diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc

>new file mode 100644

>index 00000000000..b9fc063c66f

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc

>@@ -0,0 +1,59 @@

>+// { dg-options "-std=gnu++2a -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+// 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/>.

>+

>+#include <atomic>

>+#include <thread>

>+#include <mutex>

>+#include <condition_variable>

>+#include <type_traits>

>+#include <chrono>

>+

>+#include <testsuite_hooks.h>

>+

>+int

>+main ()

>+{

>+  using namespace std::literals::chrono_literals;

>+

>+  std::mutex m;

>+  std::condition_variable cv;

>+

>+  std::atomic<bool> a(false);

>+  std::atomic<bool> b(false);

>+  std::thread t([&]

>+		{

>+		  cv.notify_one();

>+		  a.wait(false);

>+		  if (a.load())

>+                  {

>+		    b.store(true);

>+                  }

>+		});

>+  std::unique_lock<std::mutex> l(m);

>+  cv.wait(l);

>+  std::this_thread::sleep_for(100ms);

>+  a.store(true);

>+  a.notify_one();

>+  t.join();

>+  VERIFY( b.load() );

>+  return 0;

>+}

>diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc

>new file mode 100644

>index 00000000000..1d032085752

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc

>@@ -0,0 +1,32 @@

>+// { dg-options "-std=gnu++2a -pthread -latomic -L../../libatomic/.libs" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+// 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/>.

>+

>+#include "generic.h"

>+

>+int

>+main ()

>+{

>+  check<float> f;

>+  check<double> d;

>+  check<long double> l;

>+  return 0;

>+}

>diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h

>new file mode 100644

>index 00000000000..0da374ece87

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h

>@@ -0,0 +1,88 @@

>+// -*- C++ -*- header.


This isn't a header. I don't think this modeline should be here.

The test seems to be missing any dg-options, target selectors etc.

>+// 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/>.

>+

>+#include <atomic>

>+#include <thread>

>+#include <mutex>

>+#include <condition_variable>

>+#include <chrono>

>+

>+#include <testsuite_hooks.h>

>+

>+template<typename Tp>

>+Tp check_wait_notify(Tp val1, Tp val2)

>+{

>+  using namespace std::literals::chrono_literals;

>+

>+  std::mutex m;

>+  std::condition_variable cv;

>+

>+  std::atomic<Tp> a(val1);

>+  std::thread t([&]

>+		{

>+		  cv.notify_one();

>+		  a.wait(val1);

>+		  if (a.load() != val2)

>+		    a = val1;

>+		});

>+  std::unique_lock<std::mutex> l(m);

>+  cv.wait(l);

>+  std::this_thread::sleep_for(100ms);

>+  a.store(val2);

>+  a.notify_one();

>+  t.join();

>+  return a.load();

>+}

>+

>+template<typename Tp>

>+Tp check_atomic_wait_notify(Tp val1, Tp val2)

>+{

>+  using namespace std::literals::chrono_literals;

>+

>+  std::mutex m;

>+  std::condition_variable cv;

>+

>+  std::atomic<Tp> a(val1);

>+  std::thread t([&]

>+		{

>+		  cv.notify_one();

>+		  std::atomic_wait(&a, val1);

>+		  if (a.load() != val2)

>+		    a = val1;

>+		});

>+  std::unique_lock<std::mutex> l(m);

>+  cv.wait(l);

>+  std::this_thread::sleep_for(100ms);

>+  a.store(val2);

>+  std::atomic_notify_one(&a);

>+  t.join();

>+  return a.load();

>+}

>+

>+template<typename Tp>

>+struct check

>+{

>+  check()

>+  {

>+    Tp a = 0;

>+    Tp b = 42;

>+    VERIFY(check_wait_notify(a, b) == b);

>+    VERIFY(check_atomic_wait_notify(a, b) == b);

>+  }

>+};

>diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc

>new file mode 100644

>index 00000000000..2afd19a7d14

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc

>@@ -0,0 +1,56 @@

>+// { dg-options "-std=gnu++2a -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+// 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/>.

>+

>+#include "generic.h"

>+

>+int

>+main ()

>+{

>+  // check<bool> bb;

>+  check<char> ch;

>+  check<signed char> sch;

>+  check<unsigned char> uch;

>+  check<short> s;

>+  check<unsigned short> us;

>+  check<int> i;

>+  check<unsigned int> ui;

>+  check<long> l;

>+  check<unsigned long> ul;

>+  check<long long> ll;

>+  check<unsigned long long> ull;

>+

>+  check<wchar_t> wch;

>+  check<char8_t> ch8;

>+  check<char16_t> ch16;

>+  check<char32_t> ch32;

>+

>+  check<int8_t> i8;

>+  check<int16_t> i16;

>+  check<int32_t> i32;

>+  check<int64_t> i64;

>+

>+  check<uint8_t> u8;

>+  check<uint16_t> u16;

>+  check<uint32_t> u32;

>+  check<uint64_t> u64;

>+  return 0;

>+}

>diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc

>new file mode 100644

>index 00000000000..8531bb2e788

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc

>@@ -0,0 +1,59 @@

>+// { dg-options "-std=gnu++2a -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+// 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/>.

>+

>+#include <atomic>

>+#include <thread>

>+#include <mutex>

>+#include <condition_variable>

>+#include <type_traits>

>+#include <chrono>

>+

>+#include <testsuite_hooks.h>

>+

>+int

>+main ()

>+{

>+  using namespace std::literals::chrono_literals;

>+

>+  std::mutex m;

>+  std::condition_variable cv;

>+

>+  long aa;

>+  long bb;

>+

>+  std::atomic<long*> a(nullptr);

>+  std::thread t([&]

>+		{

>+		  cv.notify_one();

>+		  a.wait(nullptr);

>+		  if (a.load() == &aa)

>+		    a.store(&bb);

>+		});

>+  std::unique_lock<std::mutex> l(m);

>+  cv.wait(l);

>+  std::this_thread::sleep_for(100ms);

>+  a.store(&aa);

>+  a.notify_one();

>+  t.join();

>+  VERIFY( a.load() == &bb);

>+  return 0;

>+}

>diff --git a/libstdc++-v3/testsuite/30_threads/latch/1.cc b/libstdc++-v3/testsuite/30_threads/latch/1.cc

>new file mode 100644

>index 00000000000..aa203cdf525

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/latch/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 <latch>

>+

>+#ifndef __cpp_lib_latch

>+# error "Feature-test macro for latch missing in <latch>"

>+#elif __cpp_lib_latch!= 201907L

>+# error "Feature-test macro for latch has wrong value in <latch>"

>+#endif

>diff --git a/libstdc++-v3/testsuite/30_threads/latch/2.cc b/libstdc++-v3/testsuite/30_threads/latch/2.cc

>new file mode 100644

>index 00000000000..756727f33b3

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/latch/2.cc

>@@ -0,0 +1,27 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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_latch

>+# error "Feature-test macro for latch missing in <version>"

>+#elif __cpp_lib_latch != 201907L

>+# error "Feature-test macro for latch has wrong value in <version>"

>+#endif

>diff --git a/libstdc++-v3/testsuite/30_threads/latch/3.cc b/libstdc++-v3/testsuite/30_threads/latch/3.cc

>new file mode 100644

>index 00000000000..10bb500d261

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/latch/3.cc

>@@ -0,0 +1,50 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.

>+//


Just 2020.

>+// 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 -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+//

>+#include <latch>

>+#include <atomic>

>+#include <thread>

>+#include <testsuite_hooks.h>

>+

>+int main()

>+{

>+  std::atomic<int> a(0);

>+

>+  std::latch l(3);

>+

>+  VERIFY( !l.try_wait() );

>+

>+  auto fn = [&]

>+  {

>+    ++a;

>+    l.count_down();

>+  };

>+

>+  std::thread t0(fn);

>+  std::thread t1(fn);

>+

>+  l.arrive_and_wait();

>+  t0.join();

>+  t1.join();

>+

>+  VERIFY( l.try_wait() );

>+}

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/1.cc b/libstdc++-v3/testsuite/30_threads/semaphore/1.cc

>new file mode 100644

>index 00000000000..1bbca687fc3

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/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 <semaphore>

>+

>+#ifndef __cpp_lib_semaphore

>+# error "Feature-test macro for semaphore missing in <semaphore>"

>+#elif __cpp_lib_semaphore != 201907L

>+# error "Feature-test macro for semaphore has wrong value in <semaphore>"

>+#endif

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/2.cc b/libstdc++-v3/testsuite/30_threads/semaphore/2.cc

>new file mode 100644

>index 00000000000..b96b8a59c64

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/2.cc

>@@ -0,0 +1,27 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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_semaphore

>+# error "Feature-test macro for semaphore missing in <version>"

>+#elif __cpp_lib_semaphore != 201907L

>+# error "Feature-test macro for semaphore has wrong value in <version>"

>+#endif

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc

>new file mode 100644

>index 00000000000..1ac9d261ca5

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc

>@@ -0,0 +1,28 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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 <semaphore>

>+

>+int main()

>+{

>+  std::counting_semaphore<-1> sem(2);

>+  return 0;

>+}

>+// { dg-error "static assertion failed" "" {  target *-*-* } 0 }

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc

>new file mode 100644

>index 00000000000..d38cef86cfc

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc

>@@ -0,0 +1,55 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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 -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+#include <semaphore>

>+#include <limits>

>+#include <cstddef>

>+#include <testsuite_hooks.h>

>+

>+void test01()

>+{

>+  std::counting_semaphore<10> s(3);

>+

>+  s.acquire();

>+  VERIFY( s.try_acquire() );

>+  VERIFY( s.try_acquire() );

>+  VERIFY( !s.try_acquire() );

>+  s.release();

>+  VERIFY( s.try_acquire() );

>+}

>+

>+void test02()

>+{

>+  std::binary_semaphore s(1);

>+

>+  s.acquire();

>+  VERIFY( !s.try_acquire() );

>+  s.release();

>+  VERIFY( s.try_acquire() );

>+}

>+

>+

>+int main()

>+{

>+  test01();

>+  test02();

>+}

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc

>new file mode 100644

>index 00000000000..965554a3c28

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc

>@@ -0,0 +1,85 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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 -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+#include <semaphore>

>+#include <chrono>

>+#include <thread>

>+#include <atomic>

>+#include <testsuite_hooks.h>

>+

>+void test01()

>+{

>+  using namespace std::chrono_literals;

>+  std::counting_semaphore<10> s(2);

>+  s.acquire();

>+

>+  auto const dur = 250ms;

>+  {

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( s.try_acquire_for(dur) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff < dur );

>+  }

>+

>+  {

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( !s.try_acquire_for(dur) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff >= dur );

>+  }

>+}

>+

>+void test02()

>+{

>+  using namespace std::chrono_literals;

>+  std::binary_semaphore s(1);

>+  std::atomic<int> a(0), b(0);

>+  std::thread t([&] {

>+    a.wait(0);

>+    auto const dur = 250ms;

>+    VERIFY( !s.try_acquire_for(dur) );

>+    b++;

>+    b.notify_one();

>+

>+    a.wait(1);

>+    VERIFY( s.try_acquire_for(dur) );

>+    b++;

>+    b.notify_one();

>+  });

>+  t.detach();

>+

>+  s.acquire();

>+  a++;

>+  a.notify_one();

>+  b.wait(0);

>+  s.release();

>+  a++;

>+  a.notify_one();

>+

>+  b.wait(1);

>+}

>+

>+int main()

>+{

>+  test01();

>+  test02();

>+}

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc

>new file mode 100644

>index 00000000000..5e05606e97f

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc

>@@ -0,0 +1,51 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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 -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+#include <semaphore>

>+#include <limits>

>+#include <cstddef>

>+#include <testsuite_hooks.h>

>+

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+void test01()

>+{

>+  // the implementation optimizes for values of least_max_t that can fit

>+  // in a futex, make sure we cover the case where least_max_t doesn't

>+  auto constexpr least_max_t = std::numeric_limits<std::ptrdiff_t>::max();

>+  std::counting_semaphore<least_max_t> s(3);

>+

>+  s.acquire();

>+  VERIFY( s.try_acquire() );

>+  VERIFY( s.try_acquire() );

>+  VERIFY( !s.try_acquire() );

>+  s.release();

>+  VERIFY( s.try_acquire() );

>+}

>+

>+#endif

>+

>+int main()

>+{

>+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>+  test01();

>+#endif

>+}

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc

>new file mode 100644

>index 00000000000..bf99fd3cf8f

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc

>@@ -0,0 +1,169 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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 -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+#include <semaphore>

>+#include <chrono>

>+#include <thread>

>+#include <atomic>

>+#include <testsuite_hooks.h>

>+

>+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE

>+  // The implementation supports posix as an implementation strategy

>+  // make sure we cover that case

>+#define _GLIBCXX_REQUIRE_POSIX_SEMAPHORE



This macro is defined too late to do anything. It would ned to be
defined before including <semaphore> to have any effect.

>+void test01()

>+{

>+  std::counting_semaphore<10> s(3);

>+

>+  s.acquire();

>+  VERIFY( s.try_acquire() );

>+  VERIFY( s.try_acquire() );

>+  VERIFY( !s.try_acquire() );

>+  s.release();

>+  VERIFY( s.try_acquire() );

>+}

>+

>+void test02()

>+{

>+  using namespace std::chrono_literals;

>+  std::counting_semaphore<10> s(2);

>+  s.acquire();

>+

>+  auto const dur = 250ms;

>+  {

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( s.try_acquire_for(dur) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff < dur );

>+  }

>+

>+  {

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( !s.try_acquire_for(dur) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff >= dur );

>+  }

>+}

>+

>+void test03()

>+{

>+  using namespace std::chrono_literals;

>+  std::binary_semaphore s(1);

>+  std::atomic<int> a(0), b(0);

>+  std::thread t([&] {

>+    a.wait(0);

>+    auto const dur = 250ms;

>+    VERIFY( !s.try_acquire_for(dur) );

>+    b++;

>+    b.notify_one();

>+

>+    a.wait(1);

>+    VERIFY( s.try_acquire_for(dur) );

>+    b++;

>+    b.notify_one();

>+  });

>+  t.detach();

>+

>+  s.acquire();

>+  a++;

>+  a.notify_one();

>+  b.wait(0);

>+  s.release();

>+  a++;

>+  a.notify_one();

>+

>+  b.wait(1);

>+}

>+

>+void test04()

>+{

>+  using namespace std::chrono_literals;

>+  std::counting_semaphore<10> s(2);

>+  s.acquire();

>+

>+  auto const dur = 250ms;

>+  {

>+    auto const at = std::chrono::system_clock::now() + dur;

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( s.try_acquire_until(at) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff < dur );

>+  }

>+

>+  {

>+    auto const at = std::chrono::system_clock::now() + dur;

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( !s.try_acquire_until(at) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff >= dur );

>+  }

>+}

>+

>+void test05()

>+{

>+  using namespace std::chrono_literals;

>+  std::binary_semaphore s(1);

>+  std::atomic<int> a(0), b(0);

>+  std::thread t([&] {

>+    a.wait(0);

>+    auto const dur = 250ms;

>+    {

>+      auto const at = std::chrono::system_clock::now() + dur;

>+      VERIFY( !s.try_acquire_until(at) );

>+

>+      b++;

>+      b.notify_one();

>+    }

>+

>+    a.wait(1);

>+    {

>+      auto const at = std::chrono::system_clock::now() + dur;

>+      VERIFY( s.try_acquire_until(at) );

>+    }

>+    b++;

>+    b.notify_one();

>+  });

>+  t.detach();

>+

>+  s.acquire();

>+  a++;

>+  a.notify_one();

>+  b.wait(0);

>+  s.release();

>+  a++;

>+  a.notify_one();

>+

>+  b.wait(1);

>+}

>+#endif

>+

>+int main()

>+{

>+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE

>+  test01();

>+  test02();

>+  test03();

>+  test04();

>+  test05();

>+#endif

>+}

>diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc

>new file mode 100644

>index 00000000000..cc67c5c0bf0

>--- /dev/null

>+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc

>@@ -0,0 +1,94 @@

>+// Copyright (C) 2019-2020 Free Software Foundation, Inc.


Just 2020.

>+//

>+// 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 -pthread" }

>+// { dg-do run { target c++2a } }

>+// { dg-require-effective-target pthread }

>+// { dg-require-gthreads "" }

>+

>+#include <semaphore>

>+#include <chrono>

>+#include <thread>

>+#include <atomic>

>+#include <testsuite_hooks.h>

>+

>+void test01()

>+{

>+  using namespace std::chrono_literals;

>+  std::counting_semaphore<10> s(2);

>+  s.acquire();

>+

>+  auto const dur = 250ms;

>+  {

>+    auto const at = std::chrono::system_clock::now() + dur;

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( s.try_acquire_until(at) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff < dur );

>+  }

>+

>+  {

>+    auto const at = std::chrono::system_clock::now() + dur;

>+    auto const t0 = std::chrono::steady_clock::now();

>+    VERIFY( !s.try_acquire_until(at) );

>+    auto const diff = std::chrono::steady_clock::now() - t0;

>+    VERIFY( diff >= dur );

>+  }

>+}

>+

>+void test02()

>+{

>+  using namespace std::chrono_literals;

>+  std::binary_semaphore s(1);

>+  std::atomic<int> a(0), b(0);

>+  std::thread t([&] {

>+    a.wait(0);

>+    auto const dur = 250ms;

>+    {

>+      auto const at = std::chrono::system_clock::now() + dur;

>+      VERIFY( !s.try_acquire_until(at) );

>+

>+      b++;

>+      b.notify_one();

>+    }

>+

>+    a.wait(1);

>+    {

>+      auto const at = std::chrono::system_clock::now() + dur;

>+      VERIFY( s.try_acquire_until(at) );

>+    }

>+    b++;

>+    b.notify_one();

>+  });

>+  t.detach();

>+

>+  s.acquire();

>+  a++;

>+  a.notify_one();

>+  b.wait(0);

>+  s.release();

>+  a++;

>+  a.notify_one();

>+

>+  b.wait(1);

>+}

>+

>+int main()

>+{

>+  test01();

>+  test02();

>+}

>-- 

>2.26.2

>
Aldy Hernandez via Gcc-patches Aug. 3, 2020, 8:19 p.m. | #3
On 03/08/20 15:09 +0100, Jonathan Wakely wrote:
>On 05/06/20 17:29 -0700, Thomas Rodgers wrote:

>>diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h

>>new file mode 100644

>>index 00000000000..f0c4235d91c

>>--- /dev/null

>>+++ b/libstdc++-v3/include/bits/semaphore_base.h

>>@@ -0,0 +1,272 @@

>>+// -*- C++ -*- header.

>>+

>>+// 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.

>>+

>>+// Under Section 7 of GPL version 3, you are granted additional

>>+// permissions described in the GCC Runtime Library Exception, version

>>+// 3.1, as published by the Free Software Foundation.

>>+

>>+// You should have received a copy of the GNU General Public License and

>>+// a copy of the GCC Runtime Library Exception along with this program;

>>+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

>>+// <http://www.gnu.org/licenses/>.

>>+

>>+/** @file bits/semaphore_base.h

>>+ *  This is an internal header file, included by other library headers.

>>+ *  Do not attempt to use it directly. @headername{semaphore}

>>+ */

>>+

>>+#ifndef _GLIBCXX_SEMAPHORE_BASE_H

>>+#define _GLIBCXX_SEMAPHORE_BASE_H 1

>>+

>>+#pragma GCC system_header

>>+

>>+#include <bits/c++config.h>

>>+#include <bits/atomic_base.h>

>>+#include <bits/atomic_timed_wait.h>

>>+

>>+#if defined _POSIX_SEMAPHORES  && __has_include(<semaphore.h>)

>>+#define _GLIBCXX_HAVE_POSIX_SEMAPHORE 1

>>+#include <semaphore.h>

>>+#endif

>>+

>>+#include <chrono>

>>+#include <type_traits>

>>+

>>+namespace std _GLIBCXX_VISIBILITY(default)

>>+{

>>+_GLIBCXX_BEGIN_NAMESPACE_VERSION

>>+

>>+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE

>>+  template<ptrdiff_t __least_max_value>

>>+    struct __platform_semaphore


Why is this a template?

It means we'll instantiate identical code for counting_semaphore<3>
and counting_semaphore<4>, but they're distinct types that will bloat
the binary.

Can't we just have a single __platform_semaphore type for all max
values, and just make sure we don't instantiate it for values larger
than SEM_VALUE_MAX?

>>+    {

>

>I think we want to delete the copy constructor and copy assignment

>operator for this type and __atomic_semaphore.

>

>>+      using __clock_t = chrono::system_clock;

>>+

>>+      explicit __platform_semaphore(ptrdiff_t __count) noexcept

>>+      {

>>+	static_assert( __least_max_value <= SEM_VALUE_MAX, "");


I think it would be useful for __platform_semaphore to define a static
constexpr member like:

     static constexpr ptrdiff_t _S_max = SEM_VALUE_MAX;

And then __atomic_semaphore could define:

     static constexpr _Tp _S_max = __gnu_cxx::__int_traits<_Tp>::__max;

And then the alias __semaphore_base could be defined as:

#if _GLIBCXX_HAVE_LINUX_FUTEX && ! _GLIBCXX_REQUIRE_POSIX_SEMAPHORE
   // Use futex if available and user didn't force use of POSIX:
   using __fast_semaphore_base = __atomic_semaphore<__platform_wait_t>;
#elif _GLIBCXX_HAVE_POSIX_SEMAPHORE
   // Otherwise, use POSIX if available:
   using __fast_semaphore_base = __platform_semaphore<SEM_VALUE_MAX>;
#else
// Otherwise, use atomics:
   using __fast_semaphore_base = __atomic_semaphore<ptrdiff_t>;
#endif

   template<ptrdiff_t __least_max_value>
     using __semaphore_base = conditional_t<
       __least_max_value <= __fast_semaphore_base::_S_max,
       __fast_semaphore_base, __atomic_semaphore<ptrdiff_t>;

Would that make sense?

Do the comments above reflect the intent?


>>+#ifdef _GLIBCXX_REQUIRE_POSIX_SEMAPHORE

>

>What is this case for? This macro seems to only exist for use by a

>single testcase.

>

>Is it supposed to be something users can set? If so, it needs to be

>documented as ABI-breaking and as implying that counting_semaphore

>cannot be use with counts higher than SEM_VALUE_MAX.

>

>>+  template<ptrdiff_t __least_max_value>

>>+    using __semaphore_base = __platform_semaphore<__least_max_value>;

>>+#else

>>+#  ifdef _GLIBCXX_HAVE_LINUX_FUTEX

>>+  template<ptrdiff_t __least_max_value>

>>+    using __semaphore_base = conditional_t<(

>>+			      __least_max_value >= 0

>

>This condition is redundant, we already know that counting_semaphore

>has checked the value is non-negative.

>

>>+				&& __least_max_value <= __detail::__platform_wait_max_value),

>>+			      __atomic_semaphore<__detail::__platform_wait_t>,

>>+			      __atomic_semaphore<ptrdiff_t>>;

>>+

>>+// __platform_semaphore

>>+#  elif defined _GLIBCXX_HAVE_POSIX_SEMAPHORE

>>+  template<ptrdiff_t __least_max_value>

>>+    using __semaphore_base = conditional_t<(

>>+			      __least_max_value >= 0

>

>Redundant condition again.

>

>>+				&& __least_max_value <= SEM_VALUE_MAX),

>>+			      __platform_semaphore<__least_max_value>,

>>+			      __atomic_semaphore<ptrdiff_t>>;

>>+#  else

>>+  template<ptrdiff_t __least_max_value>

>>+    using __semaphore_base = __atomic_semaphore<ptrdiff_t>;

>>+#  endif

>>+#endif

>>+

>>+_GLIBCXX_END_NAMESPACE_VERSION

>>+} // namespace std

>>+

>>+#endif

Patch

diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index 80aeb3f8959..b3ac1a3365f 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -52,6 +52,7 @@  std_headers = \
 	${std_srcdir}/iostream \
 	${std_srcdir}/istream \
 	${std_srcdir}/iterator \
+	${std_srcdir}/latch\
 	${std_srcdir}/limits \
 	${std_srcdir}/list \
 	${std_srcdir}/locale \
@@ -69,6 +70,7 @@  std_headers = \
 	${std_srcdir}/ratio \
 	${std_srcdir}/regex \
 	${std_srcdir}/scoped_allocator \
+	${std_srcdir}/semaphore \
 	${std_srcdir}/set \
 	${std_srcdir}/shared_mutex \
 	${std_srcdir}/span \
@@ -100,6 +102,8 @@  bits_headers = \
 	${bits_srcdir}/allocated_ptr.h \
 	${bits_srcdir}/allocator.h \
 	${bits_srcdir}/atomic_base.h \
+	${bits_srcdir}/atomic_wait.h \
+	${bits_srcdir}/atomic_timed_wait.h \
 	${bits_srcdir}/atomic_futex.h \
 	${bits_srcdir}/basic_ios.h \
 	${bits_srcdir}/basic_ios.tcc \
@@ -174,6 +178,7 @@  bits_headers = \
 	${bits_srcdir}/regex_compiler.tcc \
 	${bits_srcdir}/regex_executor.h \
 	${bits_srcdir}/regex_executor.tcc \
+	${bits_srcdir}/semaphore_base.h \
 	${bits_srcdir}/shared_ptr.h \
 	${bits_srcdir}/shared_ptr_atomic.h \
 	${bits_srcdir}/shared_ptr_base.h \
diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index 3b66b040976..68d9e7e3756 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -37,6 +37,10 @@ 
 #include <bits/atomic_lockfree_defines.h>
 #include <bits/move.h>
 
+#if __cplusplus > 201703L
+#include <bits/atomic_wait.h>
+#endif
+
 #ifndef _GLIBCXX_ALWAYS_INLINE
 #define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
 #endif
@@ -134,7 +138,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __ret;
     }
 
-
   // Base types for atomics.
   template<typename _IntTp>
     struct __atomic_base;
@@ -562,6 +565,31 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       __cmpexch_failure_order(__m));
       }
 
+#if __cplusplus > 201703L
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(__int_type __old,
+	  memory_order __m = memory_order_seq_cst) const noexcept
+      {
+	std::__atomic_wait(&_M_i, __old,
+			   [__m, this, __old]()
+			   { return this->load(__m) != __old; });
+      }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { std::__atomic_notify(&_M_i, false); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { std::__atomic_notify(&_M_i, true); }
+
+      // TODO add const volatile overload
+#endif // C++2a
+
       _GLIBCXX_ALWAYS_INLINE __int_type
       fetch_add(__int_type __i,
 		memory_order __m = memory_order_seq_cst) noexcept
@@ -823,6 +851,30 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					   int(__m1), int(__m2));
       }
 
+#if __cplusplus > 201703L
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(__pointer_type __old, memory_order __m = memory_order_seq_cst) noexcept
+      {
+	std::__atomic_wait(&_M_p, __old,
+		      [__m, this, __old]()
+		      { return this->load(__m) != __old; });
+      }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { std::__atomic_notify(&_M_p, false); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { std::__atomic_notify(&_M_p, true); }
+
+      // TODO add const volatile overload
+#endif // C++2a
+
       _GLIBCXX_ALWAYS_INLINE __pointer_type
       fetch_add(ptrdiff_t __d,
 		memory_order __m = memory_order_seq_cst) noexcept
@@ -911,6 +963,32 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					 int(__success), int(__failure));
       }
 
+#if __cplusplus > 201703L
+    template<typename _Tp>
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(const _Tp* __ptr, _Val<_Tp> __old, memory_order __m = memory_order_seq_cst) noexcept
+      {
+	std::__atomic_wait(__ptr, __old,
+	    [=]() { return load(__ptr, __m) == __old; });
+      }
+
+      // TODO add const volatile overload
+
+    template<typename _Tp>
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one(const _Tp* __ptr) noexcept
+      { std::__atomic_notify(__ptr, false); }
+
+      // TODO add const volatile overload
+
+    template<typename _Tp>
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all(const _Tp* __ptr) noexcept
+      { std::__atomic_notify(__ptr, true); }
+
+      // TODO add const volatile overload
+#endif // C++2a
+
     template<typename _Tp>
       _GLIBCXX_ALWAYS_INLINE _Tp
       fetch_add(_Tp* __ptr, _Diff<_Tp> __i, memory_order __m) noexcept
@@ -1164,6 +1242,23 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       __cmpexch_failure_order(__order));
       }
 
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(_Fp __old, memory_order __m = memory_order_seq_cst) const noexcept
+      { __atomic_impl::wait(&_M_fp, __old, __m); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { __atomic_impl::notify_one(&_M_fp); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { __atomic_impl::notify_all(&_M_fp); }
+
+      // TODO add const volatile overload
       value_type
       fetch_add(value_type __i,
 		memory_order __m = memory_order_seq_cst) noexcept
@@ -1301,6 +1396,22 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       __cmpexch_failure_order(__order));
       }
 
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
+      { __atomic_impl::wait(_M_ptr, __old, __m); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { __atomic_impl::notify_one(_M_ptr); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { __atomic_impl::notify_all(_M_ptr); }
+
     private:
       _Tp* _M_ptr;
     };
@@ -1396,6 +1507,22 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       __cmpexch_failure_order(__order));
       }
 
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
+      { __atomic_impl::wait(_M_ptr, __old, __m); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { __atomic_impl::notify_one(_M_ptr); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { __atomic_impl::notify_all(_M_ptr); }
+
       value_type
       fetch_add(value_type __i,
 		memory_order __m = memory_order_seq_cst) const noexcept
@@ -1551,6 +1678,22 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       __cmpexch_failure_order(__order));
       }
 
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(_Fp __old, memory_order __m = memory_order_seq_cst) const noexcept
+      { __atomic_impl::wait(_M_ptr, __old, __m); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { __atomic_impl::notify_one(_M_ptr); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { __atomic_impl::notify_all(_M_ptr); }
+
       value_type
       fetch_add(value_type __i,
 		memory_order __m = memory_order_seq_cst) const noexcept
@@ -1660,6 +1803,22 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       __cmpexch_failure_order(__order));
       }
 
+      _GLIBCXX_ALWAYS_INLINE void
+      wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
+      { __atomic_impl::wait(_M_ptr, __old, __m); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_one() const noexcept
+      { __atomic_impl::notify_one(_M_ptr); }
+
+      // TODO add const volatile overload
+
+      _GLIBCXX_ALWAYS_INLINE void
+      notify_all() const noexcept
+      { __atomic_impl::notify_all(_M_ptr); }
+
       _GLIBCXX_ALWAYS_INLINE value_type
       fetch_add(difference_type __d,
 		memory_order __m = memory_order_seq_cst) const noexcept
diff --git a/libstdc++-v3/include/bits/atomic_timed_wait.h b/libstdc++-v3/include/bits/atomic_timed_wait.h
new file mode 100644
index 00000000000..adef80aca61
--- /dev/null
+++ b/libstdc++-v3/include/bits/atomic_timed_wait.h
@@ -0,0 +1,282 @@ 
+// -*- C++ -*- header.
+
+// 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/atomic_timed_wait.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{atomic}
+ */
+
+#ifndef _GLIBCXX_ATOMIC_TIMED_WAIT_H
+#define _GLIBCXX_ATOMIC_TIMED_WAIT_H 1
+
+#pragma GCC system_header
+
+#include <bits/c++config.h>
+#include <bits/functional_hash.h>
+#include <bits/atomic_wait.h>
+
+#include <chrono>
+
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+#include <sys/time.h>
+#endif
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+  _GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  enum class __atomic_wait_status { __no_timeout, __timeout };
+
+  namespace __detail
+  {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+    enum
+    {
+      __futex_wait_bitset_private = __futex_wait_bitset | __futex_private_flag,
+      __futex_wake_bitset_private = __futex_wake_bitset | __futex_private_flag,
+      __futex_bitset_match_any = 0xffffffff
+    };
+
+    using __platform_wait_clock_t = chrono::steady_clock;
+
+    template<typename _Duration>
+      __atomic_wait_status
+      __platform_wait_until_impl(__platform_wait_t* __addr, __platform_wait_t __val,
+				 const chrono::time_point<__platform_wait_clock_t, _Duration>& __atime) noexcept
+      {
+	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
+	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
+
+	struct timespec __rt =
+	{
+	  static_cast<std::time_t>(__s.time_since_epoch().count()),
+	  static_cast<long>(__ns.count())
+	};
+
+	auto __e = syscall (SYS_futex, __addr, __futex_wait_bitset_private, __val, &__rt,
+			    nullptr, __futex_bitset_match_any);
+	if (__e && !(errno == EINTR || errno == EAGAIN || errno == ETIMEDOUT))
+	    std::terminate();
+	return (__platform_wait_clock_t::now() < __atime)
+	       ? __atomic_wait_status::__no_timeout : __atomic_wait_status::__timeout;
+      }
+
+    template<typename _Clock, typename _Duration>
+      __atomic_wait_status
+      __platform_wait_until(__platform_wait_t* __addr, __platform_wait_t __val,
+			    const chrono::time_point<_Clock, _Duration>& __atime)
+      {
+	if constexpr (std::is_same_v<__platform_wait_clock_t, _Clock>)
+	  {
+	    return __platform_wait_until_impl(__addr, __val, __atime);
+	  }
+	else
+	  {
+	    const typename _Clock::time_point __c_entry = _Clock::now();
+	    const __platform_wait_clock_t::time_point __s_entry =
+		    __platform_wait_clock_t::now();
+	    const auto __delta = __atime - __c_entry;
+	    const auto __s_atime = __s_entry + __delta;
+	    if (__platform_wait_until_impl(__addr, __val, __s_atime) == __atomic_wait_status::__no_timeout)
+	      return __atomic_wait_status::__no_timeout;
+
+	    // We got a timeout when measured against __clock_t but
+	    // we need to check against the caller-supplied clock
+	    // to tell whether we should return a timeout.
+	    if (_Clock::now() < __atime)
+	      return __atomic_wait_status::__no_timeout;
+	    return __atomic_wait_status::__timeout;
+	  }
+      }
+#endif
+
+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+    template<typename _Duration>
+      __atomic_wait_status
+      __cond_wait_until_impl(__gthread_cond_t* __cv,
+	  unique_lock<mutex>& __lock,
+	  const chrono::time_point<chrono::steady_clock, _Duration>& __atime)
+      {
+	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
+	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
+
+	__gthread_time_t __ts =
+	  {
+	    static_cast<std::time_t>(__s.time_since_epoch().count()),
+	    static_cast<long>(__ns.count())
+	  };
+
+	pthread_cond_clockwait(__cv, __lock.mutex()->native_handle(),
+			       CLOCK_MONOTONIC,
+			       &__ts);
+	return (chrono::steady_clock::now() < __atime)
+	       ? __atomic_wait_status::__no_timeout : __atomic_wait_status::__timeout;
+      }
+#endif
+
+      template<typename _Duration>
+	__atomic_wait_status
+	__cond_wait_until_impl(__gthread_cond_t* __cv,
+	    unique_lock<std::mutex>& __lock,
+	    const chrono::time_point<chrono::system_clock, _Duration>& __atime)
+	{
+	  auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
+	  auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
+
+	  __gthread_time_t __ts =
+	  {
+	    static_cast<std::time_t>(__s.time_since_epoch().count()),
+	    static_cast<long>(__ns.count())
+	  };
+
+	  __gthread_cond_timedwait(__cv, __lock.mutex()->native_handle(),
+				   &__ts);
+	  return (chrono::system_clock::now() < __atime)
+		 ? __atomic_wait_status::__no_timeout
+		 : __atomic_wait_status::__timeout;
+	}
+
+      // return true if timeout
+      template<typename _Clock, typename _Duration>
+	__atomic_wait_status
+	__cond_wait_until(__gthread_cond_t* __cv,
+	    unique_lock<std::mutex>& __lock,
+	    const chrono::time_point<_Clock, _Duration>& __atime)
+	{
+#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
+	  using __clock_t = chrono::steady_clock;
+#else
+	  using __clock_t = chrono::system_clock;
+#endif
+	  const typename _Clock::time_point __c_entry = _Clock::now();
+	  const __clock_t::time_point __s_entry = __clock_t::now();
+	  const auto __delta = __atime - __c_entry;
+	  const auto __s_atime = __s_entry + __delta;
+	  if (__cond_wait_until_impl(__cv, __lock, __s_atime))
+	    return __atomic_wait_status::__no_timeout;
+	  // We got a timeout when measured against __clock_t but
+	  // we need to check against the caller-supplied clock
+	  // to tell whether we should return a timeout.
+	  if (_Clock::now() < __atime)
+	    return __atomic_wait_status::__no_timeout;
+	  return __atomic_wait_status::__timeout;
+	}
+
+    struct __timed_waiters : __waiters
+    {
+      template<typename _Clock, typename _Duration>
+	__atomic_wait_status
+	_M_do_wait_until(__platform_wait_t __version,
+			 const chrono::time_point<_Clock, _Duration>& __atime)
+	{
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+	  return __platform_wait_until(&_M_ver, __version, __atime);
+#else
+	  __platform_wait_t __cur = 0;
+	  __waiters::__lock_t __l(_M_mtx);
+	  while (__cur <= __version)
+	    {
+	      if (__cond_wait_until(&_M_cv, __l, __atime) == __atomic_wait_status::__timeout)
+		return __atomic_wait_status::__timeout;
+
+	      __platform_wait_t __last = __cur;
+	      __atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);
+	      if (__cur < __last)
+		break; // break the loop if version overflows
+	    }
+	  return __atomic_wait_status::__no_timeout;
+#endif
+	}
+
+      static __timed_waiters&
+      _S_timed_for(void* __t)
+      {
+	static_assert(sizeof(__timed_waiters) == sizeof(__waiters));
+	return (__timed_waiters&) __waiters::_S_for(__t);
+      }
+    };
+  } // namespace __detail
+
+  template<typename _Tp, typename _Pred,
+	   typename _Clock, typename _Duration>
+    bool
+    __atomic_wait_until(const _Tp* __addr, _Tp __old, _Pred __pred,
+			const chrono::time_point<_Clock, _Duration>& __atime) noexcept
+    {
+      using namespace __detail;
+
+      if (std::__atomic_spin(__pred))
+	return true;
+
+      auto& __w = __timed_waiters::_S_timed_for((void*)__addr);
+      auto __version = __w._M_enter_wait();
+      do
+	{
+	  __atomic_wait_status __res;
+	  if constexpr (__platform_wait_uses_type<_Tp>)
+	    {
+	      __res = __platform_wait_until((__platform_wait_t*)(void*) __addr,
+					    __old,
+					    __atime);
+	    }
+	  else
+	    {
+	      __res = __w._M_do_wait_until(__version, __atime);
+	    }
+	  if (__res == __atomic_wait_status::__timeout)
+	    return false;
+	}
+      while (!__pred() && __atime < _Clock::now());
+      __w._M_leave_wait();
+
+      // if timed out, return false
+      return (_Clock::now() < __atime);
+    }
+
+  template<typename _Tp, typename _Pred,
+	   typename _Rep, typename _Period>
+    bool
+    __atomic_wait_for(const _Tp* __addr, _Tp __old, _Pred __pred,
+		      const chrono::duration<_Rep, _Period>& __rtime) noexcept
+    {
+      using namespace __detail;
+
+      if (std::__atomic_spin(__pred))
+	return true;
+
+      if (!__rtime.count())
+	return false; // no rtime supplied, and spin did not acquire
+
+      using __dur = chrono::steady_clock::duration;
+      auto __reltime = chrono::duration_cast<__dur>(__rtime);
+      if (__reltime < __rtime)
+	++__reltime;
+
+
+      return __atomic_wait_until(__addr, __old, std::move(__pred),
+				 chrono::steady_clock::now() + __reltime);
+    }
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+#endif
diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
new file mode 100644
index 00000000000..92c1e2526ed
--- /dev/null
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -0,0 +1,291 @@ 
+// -*- C++ -*- header.
+
+// 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/atomic_wait.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{atomic}
+ */
+
+#ifndef _GLIBCXX_ATOMIC_WAIT_H
+#define _GLIBCXX_ATOMIC_WAIT_H 1
+
+#pragma GCC system_header
+
+#include <bits/c++config.h>
+#include <bits/functional_hash.h>
+#include <bits/gthr.h>
+#include <bits/std_mutex.h>
+#include <bits/unique_lock.h>
+#include <ext/numeric_traits.h>
+
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+#include <climits>
+#include <unistd.h>
+#include <syscall.h>
+#endif
+
+#define _GLIBCXX_SPIN_COUNT_1 16
+#define _GLIBCXX_SPIN_COUNT_2 12
+
+// TODO get this from Autoconf
+#define _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE 1
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+  namespace __detail
+  {
+    using __platform_wait_t = int;
+
+    inline constexpr
+    auto __platform_wait_max_value =
+		__gnu_cxx::__numeric_traits<__platform_wait_t>::__max;
+
+    template<typename _Tp>
+      inline constexpr bool __platform_wait_uses_type
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+	= is_same_v<remove_cv_t<_Tp>, __platform_wait_t>;
+#else
+	= false;
+#endif
+
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+    enum
+    {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE
+      __futex_private_flag = 128,
+#else
+      __futex_private_flag = 0,
+#endif
+      __futex_wait = 0,
+      __futex_wake = 1,
+      __futex_wait_bitset = 9,
+      __futex_wake_bitset = 10,
+      __futex_wait_private = __futex_wait | __futex_private_flag,
+      __futex_wake_private = __futex_wake | __futex_private_flag
+    };
+
+    void
+    __platform_wait(__platform_wait_t* __addr, __platform_wait_t __val) noexcept
+    {
+       auto __e = syscall (SYS_futex, __addr, __futex_wait_private, __val, nullptr);
+       if (__e && !(errno == EINTR || errno == EAGAIN))
+	 std::terminate();
+    }
+
+    void
+    __platform_notify(__platform_wait_t* __addr, bool __all) noexcept
+    {
+      syscall (SYS_futex, __addr, __futex_wake_private, __all ? INT_MAX : 1);
+    }
+#endif
+
+    struct __waiters
+    {
+      __platform_wait_t alignas(64) _M_ver = 0;
+      __platform_wait_t alignas(64) _M_wait = 0;
+
+#ifndef _GLIBCXX_HAVE_LINUX_FUTEX
+      using __lock_t = std::unique_lock<std::mutex>;
+      mutable __lock_t::mutex_type _M_mtx;
+
+#  ifdef __GTHREAD_COND_INIT
+      mutable __gthread_cond_t _M_cv = __GTHREAD_COND_INIT;
+      __waiters() noexcept = default;
+#  else
+      mutable __gthread_cond_t _M_cv;
+      __waiters() noexcept
+      {
+	__GTHREAD_COND_INIT_FUNCTION(&_M_cond);
+      }
+#  endif
+#endif
+
+      __platform_wait_t
+      _M_enter_wait() noexcept
+      {
+	__platform_wait_t __res;
+	__atomic_load(&_M_ver, &__res, __ATOMIC_ACQUIRE);
+	__atomic_fetch_add(&_M_wait, 1, __ATOMIC_ACQ_REL);
+	return __res;
+      }
+
+      void
+      _M_leave_wait() noexcept
+      {
+	__atomic_fetch_sub(&_M_wait, 1, __ATOMIC_ACQ_REL);
+      }
+
+      void
+      _M_do_wait(__platform_wait_t __version) noexcept
+      {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+	__platform_wait(&_M_ver, __version);
+#else
+	__platform_wait_t __cur = 0;
+	while (__cur <= __version)
+	  {
+	    __waiters::__lock_t __l(_M_mtx);
+	    auto __e = __gthread_cond_wait(&_M_cv, __l.mutex()->native_handle());
+	    if (__e)
+	      std::terminate();
+	    __platform_wait_t __last = __cur;
+	    __atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);
+	    if (__cur < __last)
+	      break; // break the loop if version overflows
+	  }
+#endif
+      }
+
+      __platform_wait_t
+      _M_waiting() const noexcept
+	{
+	  __platform_wait_t __res;
+	  __atomic_load(&_M_wait, &__res, __ATOMIC_ACQUIRE);
+	  return __res;
+	}
+
+      void
+      _M_notify(bool __all) noexcept
+      {
+	__atomic_fetch_add(&_M_ver, 1, __ATOMIC_ACQ_REL);
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+	__platform_notify(&_M_ver, __all);
+#else
+	auto __e = __gthread_cond_broadcast(&_M_cv);
+	if (__e)
+	  __throw_system_error(__e);
+#endif
+      }
+
+      static __waiters&
+      _S_for(void* __t)
+      {
+	const unsigned char __mask = 0xf;
+	static __waiters __w[__mask + 1];
+
+	auto __key = _Hash_impl::hash(__t) & __mask;
+	return __w[__key];
+      }
+    };
+
+    struct __waiter
+    {
+      __waiters& _M_w;
+      __platform_wait_t _M_version;
+
+      template<typename _Tp>
+	__waiter(const _Tp* __addr) noexcept
+	  : _M_w(__waiters::_S_for((void*) __addr))
+	  , _M_version(_M_w._M_enter_wait())
+	{ }
+
+      ~__waiter()
+      { _M_w._M_leave_wait(); }
+
+      void _M_do_wait() noexcept
+      { _M_w._M_do_wait(_M_version); }
+    };
+
+    void
+    __thread_relax() noexcept
+    {
+#if defined __i386__ || defined __x86_64__
+      __builtin_ia32_pause();
+#elif defined _GLIBCXX_USE_SCHED_YIELD
+      __gthread_yield();
+#endif
+    }
+
+    void
+    __thread_yield() noexcept
+   {
+#if defined _GLIBCXX_USE_SCHED_YIELD
+     __gthread_yield();
+#endif
+    }
+
+  } // namespace __detail
+
+  template<typename _Pred>
+    bool
+    __atomic_spin(_Pred __pred) noexcept
+    {
+      for (auto __i = 0; __i < _GLIBCXX_SPIN_COUNT_1; ++__i)
+	{
+	  if (__pred())
+	    return true;
+
+	  if (__i < _GLIBCXX_SPIN_COUNT_2)
+	    __detail::__thread_relax();
+	  else
+	    __detail::__thread_yield();
+	}
+      return false;
+    }
+
+  template<typename _Tp, typename _Pred>
+    void
+    __atomic_wait(const _Tp* __addr, _Tp __old, _Pred __pred) noexcept
+    {
+      using namespace __detail;
+      if (__atomic_spin(__pred))
+	return;
+
+      __waiter __w(__addr);
+      while (!__pred())
+	{
+	  if constexpr (__platform_wait_uses_type<_Tp>)
+	    {
+	      __platform_wait((__platform_wait_t*)(void*) __addr, __old);
+	    }
+	  else
+	    {
+	      // TODO support timed backoff when this can be moved into the lib
+	      __w._M_do_wait();
+	    }
+	}
+    }
+
+  template<typename _Tp>
+    void
+    __atomic_notify(const _Tp* __addr, bool __all) noexcept
+    {
+      using namespace __detail;
+      auto& __w = __waiters::_S_for((void*)__addr);
+      if (!__w._M_waiting())
+	return;
+
+      if constexpr (__platform_wait_uses_type<_Tp>)
+	{
+	  __platform_notify((__platform_wait_t*)(void*) __addr, __all);
+	}
+      else
+	{
+	  __w._M_notify(__all);
+	}
+    }
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+#endif
diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
new file mode 100644
index 00000000000..f0c4235d91c
--- /dev/null
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -0,0 +1,272 @@ 
+// -*- C++ -*- header.
+
+// 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/semaphore_base.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{semaphore}
+ */
+
+#ifndef _GLIBCXX_SEMAPHORE_BASE_H
+#define _GLIBCXX_SEMAPHORE_BASE_H 1
+
+#pragma GCC system_header
+
+#include <bits/c++config.h>
+#include <bits/atomic_base.h>
+#include <bits/atomic_timed_wait.h>
+
+#if defined _POSIX_SEMAPHORES  && __has_include(<semaphore.h>)
+#define _GLIBCXX_HAVE_POSIX_SEMAPHORE 1
+#include <semaphore.h>
+#endif
+
+#include <chrono>
+#include <type_traits>
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  template<ptrdiff_t __least_max_value>
+    struct __platform_semaphore
+    {
+      using __clock_t = chrono::system_clock;
+
+      explicit __platform_semaphore(ptrdiff_t __count) noexcept
+      {
+	static_assert( __least_max_value <= SEM_VALUE_MAX, "");
+	auto __e = sem_init(&_M_semaphore, 0, __count);
+	if (__e)
+	  std::terminate();
+      }
+
+      ~__platform_semaphore()
+      {
+	auto __e = sem_destroy(&_M_semaphore);
+	if (__e)
+	  std::terminate();
+      }
+
+      _GLIBCXX_ALWAYS_INLINE void
+      acquire() noexcept
+      {
+	auto __err = sem_wait(&_M_semaphore);
+	if (__err)
+	  std::terminate();
+      }
+
+      template<typename _Duration>
+	bool
+	__try_acquire_until_impl(const chrono::time_point<__clock_t>& __atime) noexcept
+	{
+	  auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
+	  auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
+
+	  struct timespec __ts =
+	  {
+	    static_cast<std::time_t>(__s.time_since_epoch().count()),
+	    static_cast<long>(__ns.count())
+	  };
+
+	  auto __err = sem_timedwait(&_M_semaphore, &__ts);
+	  if (__err && (errno == ETIMEDOUT))
+	      return false;
+	  else if (__err)
+	      std::terminate();
+	  return true;
+	}
+
+      template<typename _Clock, typename _Duration>
+	bool
+	try_acquire_until(const chrono::time_point<_Clock, _Duration>& __atime) noexcept
+	{
+	  if constexpr (std::is_same<__clock_t, _Clock>::value)
+	    {
+	      return __try_acquire_until_impl(__atime);
+	    }
+	  else
+	    {
+	      const typename _Clock::time_point __c_entry = _Clock::now();
+	      const __clock_t __s_entry = __clock_t::now();
+	      const auto __delta = __atime - __c_entry;
+	      const auto __s_atime = __s_entry + __delta;
+	      if (__try_acquire_until_impl(__s_atime))
+		return true;
+
+	      // We got a timeout when measured against __clock_t but
+	      // we need to check against the caller-supplied clock
+	      // to tell whether we should return a timeout.
+	      return (_Clock::now() < __atime);
+	    }
+	}
+
+      template<typename _Rep, typename _Period>
+	_GLIBCXX_ALWAYS_INLINE bool
+	try_acquire_for(const chrono::duration<_Rep, _Period>& __rtime) noexcept
+	{ return try_acquire_until(__clock_t::now() + __rtime); }
+
+      template<typename _Clock, typename _Duration>
+	_GLIBCXX_ALWAYS_INLINE void
+	release(ptrdiff_t __update) noexcept
+	{
+	  do
+	    {
+	      auto __err = sem_post(&_M_semaphore);
+	      if (__err)
+		std::terminate();
+	    } while (--__update);
+	}
+
+      private:
+	sem_t _M_semaphore;
+      };
+#endif // _GLIBCXX_HAVE_POSIX_SEMAPHORE
+
+    template<typename _Tp>
+      struct __atomic_semaphore
+      {
+	explicit __atomic_semaphore(_Tp __count)
+	  : _M_a(__count)
+	{ }
+
+	_GLIBCXX_ALWAYS_INLINE void
+	acquire() noexcept
+	{
+	  auto const __pred = [this]
+	    {
+	      auto __old = __atomic_impl::load(&this->_M_a,
+			      memory_order::acquire);
+	      if (__old == 0)
+		return false;
+	      return __atomic_impl::compare_exchange_strong(&this->_M_a,
+			__old, __old - 1,
+			memory_order::acquire,
+			memory_order::release);
+	    };
+	  auto __old = __atomic_impl::load(&_M_a, memory_order_relaxed);
+	  __atomic_wait(&_M_a, __old, __pred);
+	}
+
+	bool
+	try_acquire() noexcept
+	{
+	  auto __old = __atomic_impl::load(&_M_a, memory_order::acquire);
+	  if (__old == 0)
+	    return false;
+
+	  return __atomic_spin([this, &__old]
+	    {
+	      return __atomic_impl::compare_exchange_weak(&this->_M_a,
+			__old, __old - 1,
+			memory_order::acquire,
+			memory_order::release);
+	    });
+	}
+
+	template<typename _Clock, typename _Duration>
+	  _GLIBCXX_ALWAYS_INLINE bool
+	  try_acquire_until(const chrono::time_point<_Clock, _Duration>& __atime) noexcept
+	  {
+	    auto const __pred = [this]
+	      {
+		auto __old = __atomic_impl::load(&this->_M_a,
+				memory_order::acquire);
+		if (__old == 0)
+		  return false;
+		return __atomic_impl::compare_exchange_strong(&this->_M_a,
+				__old, __old - 1,
+				memory_order::acquire,
+				memory_order::release);
+	      };
+
+	    auto __old = __atomic_impl::load(&_M_a, memory_order_relaxed);
+	    return __atomic_wait_until(&_M_a, __old, __pred, __atime);
+	}
+
+      template<typename _Rep, typename _Period>
+	_GLIBCXX_ALWAYS_INLINE bool
+	try_acquire_for(const chrono::duration<_Rep, _Period>& __rtime) noexcept
+	{
+	  auto const __pred = [this]
+	    {
+	      auto __old = __atomic_impl::load(&this->_M_a,
+			      memory_order::acquire);
+	      if (__old == 0)
+		return false;
+	      return  __atomic_impl::compare_exchange_strong(&this->_M_a,
+			      __old, __old - 1,
+			      memory_order::acquire,
+			      memory_order::release);
+	    };
+
+	  auto __old = __atomic_impl::load(&_M_a, memory_order_relaxed);
+	  return __atomic_wait_for(&_M_a, __old, __pred, __rtime);
+	}
+
+      _GLIBCXX_ALWAYS_INLINE void
+      release(ptrdiff_t __update) noexcept
+      {
+	if (0 < __atomic_impl::fetch_add(&_M_a, __update, memory_order_release))
+	  return;
+	if (__update > 1)
+	  __atomic_impl::notify_all(&_M_a);
+	else
+	  __atomic_impl::notify_one(&_M_a);
+      }
+
+    private:
+      alignas(__alignof__(_Tp)) _Tp _M_a;
+    };
+
+#ifdef _GLIBCXX_REQUIRE_POSIX_SEMAPHORE
+  template<ptrdiff_t __least_max_value>
+    using __semaphore_base = __platform_semaphore<__least_max_value>;
+#else
+#  ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+  template<ptrdiff_t __least_max_value>
+    using __semaphore_base = conditional_t<(
+			      __least_max_value >= 0
+				&& __least_max_value <= __detail::__platform_wait_max_value),
+			      __atomic_semaphore<__detail::__platform_wait_t>,
+			      __atomic_semaphore<ptrdiff_t>>;
+
+// __platform_semaphore
+#  elif defined _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  template<ptrdiff_t __least_max_value>
+    using __semaphore_base = conditional_t<(
+			      __least_max_value >= 0
+				&& __least_max_value <= SEM_VALUE_MAX),
+			      __platform_semaphore<__least_max_value>,
+			      __atomic_semaphore<ptrdiff_t>>;
+#  else
+  template<ptrdiff_t __least_max_value>
+    using __semaphore_base = __atomic_semaphore<ptrdiff_t>;
+#  endif
+#endif
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+
+#endif
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index a455286a784..3f18774031d 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -163,6 +163,19 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     compare_exchange_strong(bool& __i1, bool __i2,
 		    memory_order __m = memory_order_seq_cst) volatile noexcept
     { return _M_base.compare_exchange_strong(__i1, __i2, __m); }
+
+#if __cplusplus > 201703L
+    void wait(bool __old, memory_order __m = memory_order_seq_cst) const noexcept
+    { _M_base.wait(__old, __m); }
+
+    // TODO add const volatile overload
+
+    void notify_one() const noexcept
+    { _M_base.notify_one(); }
+
+    void notify_all() const noexcept
+    { _M_base.notify_all(); }
+#endif
   };
 
 #if __cplusplus <= 201703L
@@ -352,6 +365,19 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		     memory_order __m = memory_order_seq_cst) volatile noexcept
       { return compare_exchange_strong(__e, __i, __m,
                                        __cmpexch_failure_order(__m)); }
+#if __cplusplus > 201703L
+    void wait(_Tp __old, memory_order __m = memory_order_seq_cst) noexcept
+    { _M_i.wait(__old, __m); }
+
+    // TODO add const volatile overload
+
+    void notify_one() const noexcept
+    { _M_i.notify_one(); }
+
+    void notify_all() const noexcept
+    { _M_i.notify_all(); }
+#endif
+
     };
 #undef _GLIBCXX20_INIT
 
@@ -590,6 +616,18 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					    __cmpexch_failure_order(__m));
       }
 
+#if __cplusplus > 201703L
+    void wait(__pointer_type __old, memory_order __m = memory_order_seq_cst) noexcept
+    { _M_b.wait(__old, __m); }
+
+    // TODO add const volatile overload
+
+    void notify_one() const noexcept
+    { _M_b.notify_one(); }
+
+    void notify_all() const noexcept
+    { _M_b.notify_all(); }
+#endif
       __pointer_type
       fetch_add(ptrdiff_t __d,
 		memory_order __m = memory_order_seq_cst) noexcept
@@ -1342,6 +1380,29 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 						     memory_order_seq_cst);
     }
 
+
+#if __cplusplus > 201703L
+  template<typename _Tp>
+    inline void atomic_wait(const atomic<_Tp>* __a,
+	                    typename std::atomic<_Tp>::value_type __old) noexcept
+    { __a->wait(__old); }
+
+  template<typename _Tp>
+    inline void atomic_wait_explicit(const atomic<_Tp>* __a,
+				     typename std::atomic<_Tp>::value_type __old,
+				     std::memory_order __m) noexcept
+    { __a->wait(__old, __m); }
+
+  template<typename _Tp>
+    inline void atomic_notify_one(atomic<_Tp>* __a) noexcept
+    { __a->notify_one(); }
+
+  template<typename _Tp>
+    inline void atomic_notify_all(atomic<_Tp>* __a) noexcept
+    { __a->notify_all(); }
+
+#endif // C++2a
+
   // Function templates for atomic_integral and atomic_pointer operations only.
   // Some operations (and, or, xor) are only available for atomic integrals,
   // which is implemented by taking a parameter of type __atomic_base<_ITp>*.
diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch
new file mode 100644
index 00000000000..aa5299d9fdd
--- /dev/null
+++ b/libstdc++-v3/include/std/latch
@@ -0,0 +1,90 @@ 
+// <latch> -*- 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.	 If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file include/latch
+ *  This is a Standard C++ Library header.
+ */
+
+#ifndef _GLIBCXX_LATCH
+#define _GLIBCXX_LATCH
+
+#pragma GCC system_header
+
+#if __cplusplus > 201703L
+#define __cpp_lib_latch 201907L
+
+#include <bits/atomic_base.h>
+#include <ext/numeric_traits.h>
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  class latch
+  {
+  public:
+    static constexpr
+    _GLIBCXX_ALWAYS_INLINE ptrdiff_t
+    max() noexcept
+    { return __gnu_cxx::__numeric_traits<ptrdiff_t>::__max; }
+
+    constexpr explicit latch(ptrdiff_t __expected) : _M_a(__expected) { }
+
+    ~latch() = default;
+    latch(const latch&) = delete;
+    latch& operator=(const latch&) = delete;
+
+    _GLIBCXX_ALWAYS_INLINE void
+    count_down(ptrdiff_t __update = 1)
+    {
+      auto const __old = __atomic_impl::fetch_sub(&_M_a, __update, memory_order::release);
+      if (__old == __update)
+	__atomic_impl::notify_all(&_M_a);
+    }
+
+    _GLIBCXX_ALWAYS_INLINE bool
+    try_wait() const noexcept
+    { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
+
+    _GLIBCXX_ALWAYS_INLINE void
+    wait() const
+    {
+      auto const __old = __atomic_impl::load(&_M_a, memory_order::acquire);
+      __atomic_wait(&_M_a, __old, [this] { return this->try_wait(); });
+    }
+
+    _GLIBCXX_ALWAYS_INLINE void
+    arrive_and_wait(ptrdiff_t __update = 1)
+    {
+      count_down();
+      wait();
+    }
+
+  private:
+    alignas(__alignof__(ptrdiff_t)) ptrdiff_t _M_a;
+  };
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
+#endif // __cplusplus > 201703L
+#endif // _GLIBCXX_LATCH
diff --git a/libstdc++-v3/include/std/semaphore b/libstdc++-v3/include/std/semaphore
new file mode 100644
index 00000000000..90cf3244647
--- /dev/null
+++ b/libstdc++-v3/include/std/semaphore
@@ -0,0 +1,86 @@ 
+// <semaphore> -*- 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.	 If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file include/semaphore
+ *  This is a Standard C++ Library header.
+ */
+
+#ifndef _GLIBCXX_SEMAPHORE
+#define _GLIBCXX_SEMAPHORE
+
+#pragma GCC system_header
+
+#if __cplusplus > 201703L
+#define __cpp_lib_semaphore 201907L
+#include <bits/semaphore_base.h>
+#include <ext/numeric_traits.h>
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  template<ptrdiff_t __least_max_value =
+			__gnu_cxx::__numeric_traits<ptrdiff_t>::__max>
+    class counting_semaphore
+    {
+      static_assert(__least_max_value >=0, "");
+
+      __semaphore_base<__least_max_value> _M_sem;
+
+    public:
+      explicit counting_semaphore(ptrdiff_t __desired) noexcept
+	: _M_sem(__desired)
+      { }
+
+      ~counting_semaphore() = default;
+
+      counting_semaphore(const counting_semaphore&) = delete;
+      counting_semaphore& operator=(const counting_semaphore&) = delete;
+
+      static constexpr ptrdiff_t max() noexcept
+      { return __least_max_value; }
+
+      void release(ptrdiff_t __update = 1)
+      { _M_sem.release(__update); }
+
+      void acquire()
+      { _M_sem.acquire(); }
+
+      bool try_acquire() noexcept
+      { return _M_sem.try_acquire(); }
+
+      template<class _Rep, class _Period>
+	bool try_acquire_for(const std::chrono::duration<_Rep, _Period>& __rel_time)
+	{ return _M_sem.try_acquire_for(__rel_time); }
+
+      template<class _Clock, class _Duration>
+	bool try_acquire_until(const std::chrono::time_point<_Clock, _Duration>& __abs_time)
+	{ return _M_sem.try_acquire_until(__abs_time); }
+    };
+
+ using binary_semaphore = std::counting_semaphore<1>;
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
+#endif // __cplusplus > 201703L
+#endif // _GLIBCXX_SEMAPHORE
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index c6bde2cfbda..f09da3344f7 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -189,6 +189,8 @@ 
 #endif
 #define __cpp_lib_type_identity 201806L
 #define __cpp_lib_unwrap_ref 201811L
+#define __cpp_lib_semaphore 201907L
+#define __cpp_lib_latch 201907L
 
 #if _GLIBCXX_HOSTED
 #undef __cpp_lib_array_constexpr
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc
new file mode 100644
index 00000000000..1ced9d44b20
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/atomic_refs.cc
@@ -0,0 +1,103 @@ 
+// { dg-options "-std=gnu++2a -pthread -latomic -L../../libatomic/.libs" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+// 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/>.
+
+#include <atomic>
+#include <thread>
+#include <mutex>
+#include <condition_variable>
+#include <chrono>
+#include <type_traits>
+
+#include <testsuite_hooks.h>
+
+template<typename Tp>
+Tp check_wait_notify(Tp val1, Tp val2)
+{
+  using namespace std::literals::chrono_literals;
+
+  std::mutex m;
+  std::condition_variable cv;
+
+  Tp aa = val1;
+  std::atomic_ref<Tp> a(aa);
+  std::thread t([&]
+		{
+		  cv.notify_one();
+		  a.wait(val1);
+		  if (a.load() != val2)
+		    a = val1;
+		});
+  std::unique_lock<std::mutex> l(m);
+  cv.wait(l);
+  std::this_thread::sleep_for(100ms);
+  a.store(val2);
+  a.notify_one();
+  t.join();
+  return a.load();
+}
+
+template<typename Tp,
+	 bool = std::is_integral_v<Tp>
+	 || std::is_floating_point_v<Tp>>
+struct check;
+
+template<typename Tp>
+struct check<Tp, true>
+{
+  check()
+  {
+    Tp a = 0;
+    Tp b = 42;
+    VERIFY(check_wait_notify(a, b) == b);
+  }
+};
+
+template<typename Tp>
+struct check<Tp, false>
+{
+  check(Tp b)
+  {
+    Tp a;
+    VERIFY(check_wait_notify(a, b) == b);
+  }
+};
+
+struct foo
+{
+  long a = 0;
+  long b = 0;
+
+  foo& operator=(foo const&) = default;
+
+  friend bool
+  operator==(foo const& rhs, foo const& lhs)
+  { return rhs.a == lhs.a && rhs.b == lhs.b; }
+};
+
+int
+main ()
+{
+  check<long>();
+  check<double>();
+  check<foo>({42, 48});
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc
new file mode 100644
index 00000000000..b9fc063c66f
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/bool.cc
@@ -0,0 +1,59 @@ 
+// { dg-options "-std=gnu++2a -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+// 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/>.
+
+#include <atomic>
+#include <thread>
+#include <mutex>
+#include <condition_variable>
+#include <type_traits>
+#include <chrono>
+
+#include <testsuite_hooks.h>
+
+int
+main ()
+{
+  using namespace std::literals::chrono_literals;
+
+  std::mutex m;
+  std::condition_variable cv;
+
+  std::atomic<bool> a(false);
+  std::atomic<bool> b(false);
+  std::thread t([&]
+		{
+		  cv.notify_one();
+		  a.wait(false);
+		  if (a.load())
+                  {
+		    b.store(true);
+                  }
+		});
+  std::unique_lock<std::mutex> l(m);
+  cv.wait(l);
+  std::this_thread::sleep_for(100ms);
+  a.store(true);
+  a.notify_one();
+  t.join();
+  VERIFY( b.load() );
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc
new file mode 100644
index 00000000000..1d032085752
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/floats.cc
@@ -0,0 +1,32 @@ 
+// { dg-options "-std=gnu++2a -pthread -latomic -L../../libatomic/.libs" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+// 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/>.
+
+#include "generic.h"
+
+int
+main ()
+{
+  check<float> f;
+  check<double> d;
+  check<long double> l;
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h
new file mode 100644
index 00000000000..0da374ece87
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h
@@ -0,0 +1,88 @@ 
+// -*- C++ -*- header.
+
+// 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/>.
+
+#include <atomic>
+#include <thread>
+#include <mutex>
+#include <condition_variable>
+#include <chrono>
+
+#include <testsuite_hooks.h>
+
+template<typename Tp>
+Tp check_wait_notify(Tp val1, Tp val2)
+{
+  using namespace std::literals::chrono_literals;
+
+  std::mutex m;
+  std::condition_variable cv;
+
+  std::atomic<Tp> a(val1);
+  std::thread t([&]
+		{
+		  cv.notify_one();
+		  a.wait(val1);
+		  if (a.load() != val2)
+		    a = val1;
+		});
+  std::unique_lock<std::mutex> l(m);
+  cv.wait(l);
+  std::this_thread::sleep_for(100ms);
+  a.store(val2);
+  a.notify_one();
+  t.join();
+  return a.load();
+}
+
+template<typename Tp>
+Tp check_atomic_wait_notify(Tp val1, Tp val2)
+{
+  using namespace std::literals::chrono_literals;
+
+  std::mutex m;
+  std::condition_variable cv;
+
+  std::atomic<Tp> a(val1);
+  std::thread t([&]
+		{
+		  cv.notify_one();
+		  std::atomic_wait(&a, val1);
+		  if (a.load() != val2)
+		    a = val1;
+		});
+  std::unique_lock<std::mutex> l(m);
+  cv.wait(l);
+  std::this_thread::sleep_for(100ms);
+  a.store(val2);
+  std::atomic_notify_one(&a);
+  t.join();
+  return a.load();
+}
+
+template<typename Tp>
+struct check
+{
+  check()
+  {
+    Tp a = 0;
+    Tp b = 42;
+    VERIFY(check_wait_notify(a, b) == b);
+    VERIFY(check_atomic_wait_notify(a, b) == b);
+  }
+};
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc
new file mode 100644
index 00000000000..2afd19a7d14
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc
@@ -0,0 +1,56 @@ 
+// { dg-options "-std=gnu++2a -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+// 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/>.
+
+#include "generic.h"
+
+int
+main ()
+{
+  // check<bool> bb;
+  check<char> ch;
+  check<signed char> sch;
+  check<unsigned char> uch;
+  check<short> s;
+  check<unsigned short> us;
+  check<int> i;
+  check<unsigned int> ui;
+  check<long> l;
+  check<unsigned long> ul;
+  check<long long> ll;
+  check<unsigned long long> ull;
+
+  check<wchar_t> wch;
+  check<char8_t> ch8;
+  check<char16_t> ch16;
+  check<char32_t> ch32;
+
+  check<int8_t> i8;
+  check<int16_t> i16;
+  check<int32_t> i32;
+  check<int64_t> i64;
+
+  check<uint8_t> u8;
+  check<uint16_t> u16;
+  check<uint32_t> u32;
+  check<uint64_t> u64;
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc
new file mode 100644
index 00000000000..8531bb2e788
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/pointers.cc
@@ -0,0 +1,59 @@ 
+// { dg-options "-std=gnu++2a -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+// 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/>.
+
+#include <atomic>
+#include <thread>
+#include <mutex>
+#include <condition_variable>
+#include <type_traits>
+#include <chrono>
+
+#include <testsuite_hooks.h>
+
+int
+main ()
+{
+  using namespace std::literals::chrono_literals;
+
+  std::mutex m;
+  std::condition_variable cv;
+
+  long aa;
+  long bb;
+
+  std::atomic<long*> a(nullptr);
+  std::thread t([&]
+		{
+		  cv.notify_one();
+		  a.wait(nullptr);
+		  if (a.load() == &aa)
+		    a.store(&bb);
+		});
+  std::unique_lock<std::mutex> l(m);
+  cv.wait(l);
+  std::this_thread::sleep_for(100ms);
+  a.store(&aa);
+  a.notify_one();
+  t.join();
+  VERIFY( a.load() == &bb);
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/30_threads/latch/1.cc b/libstdc++-v3/testsuite/30_threads/latch/1.cc
new file mode 100644
index 00000000000..aa203cdf525
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/latch/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 <latch>
+
+#ifndef __cpp_lib_latch
+# error "Feature-test macro for latch missing in <latch>"
+#elif __cpp_lib_latch!= 201907L
+# error "Feature-test macro for latch has wrong value in <latch>"
+#endif
diff --git a/libstdc++-v3/testsuite/30_threads/latch/2.cc b/libstdc++-v3/testsuite/30_threads/latch/2.cc
new file mode 100644
index 00000000000..756727f33b3
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/latch/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_latch
+# error "Feature-test macro for latch missing in <version>"
+#elif __cpp_lib_latch != 201907L
+# error "Feature-test macro for latch has wrong value in <version>"
+#endif
diff --git a/libstdc++-v3/testsuite/30_threads/latch/3.cc b/libstdc++-v3/testsuite/30_threads/latch/3.cc
new file mode 100644
index 00000000000..10bb500d261
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/latch/3.cc
@@ -0,0 +1,50 @@ 
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+//
+#include <latch>
+#include <atomic>
+#include <thread>
+#include <testsuite_hooks.h>
+
+int main()
+{
+  std::atomic<int> a(0);
+
+  std::latch l(3);
+
+  VERIFY( !l.try_wait() );
+
+  auto fn = [&]
+  {
+    ++a;
+    l.count_down();
+  };
+
+  std::thread t0(fn);
+  std::thread t1(fn);
+
+  l.arrive_and_wait();
+  t0.join();
+  t1.join();
+
+  VERIFY( l.try_wait() );
+}
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/1.cc b/libstdc++-v3/testsuite/30_threads/semaphore/1.cc
new file mode 100644
index 00000000000..1bbca687fc3
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/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 <semaphore>
+
+#ifndef __cpp_lib_semaphore
+# error "Feature-test macro for semaphore missing in <semaphore>"
+#elif __cpp_lib_semaphore != 201907L
+# error "Feature-test macro for semaphore has wrong value in <semaphore>"
+#endif
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/2.cc b/libstdc++-v3/testsuite/30_threads/semaphore/2.cc
new file mode 100644
index 00000000000..b96b8a59c64
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/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_semaphore
+# error "Feature-test macro for semaphore missing in <version>"
+#elif __cpp_lib_semaphore != 201907L
+# error "Feature-test macro for semaphore has wrong value in <version>"
+#endif
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc
new file mode 100644
index 00000000000..1ac9d261ca5
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc
@@ -0,0 +1,28 @@ 
+// 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 <semaphore>
+
+int main()
+{
+  std::counting_semaphore<-1> sem(2);
+  return 0;
+}
+// { dg-error "static assertion failed" "" {  target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc
new file mode 100644
index 00000000000..d38cef86cfc
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc
@@ -0,0 +1,55 @@ 
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <semaphore>
+#include <limits>
+#include <cstddef>
+#include <testsuite_hooks.h>
+
+void test01()
+{
+  std::counting_semaphore<10> s(3);
+
+  s.acquire();
+  VERIFY( s.try_acquire() );
+  VERIFY( s.try_acquire() );
+  VERIFY( !s.try_acquire() );
+  s.release();
+  VERIFY( s.try_acquire() );
+}
+
+void test02()
+{
+  std::binary_semaphore s(1);
+
+  s.acquire();
+  VERIFY( !s.try_acquire() );
+  s.release();
+  VERIFY( s.try_acquire() );
+}
+
+
+int main()
+{
+  test01();
+  test02();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc
new file mode 100644
index 00000000000..965554a3c28
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc
@@ -0,0 +1,85 @@ 
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <semaphore>
+#include <chrono>
+#include <thread>
+#include <atomic>
+#include <testsuite_hooks.h>
+
+void test01()
+{
+  using namespace std::chrono_literals;
+  std::counting_semaphore<10> s(2);
+  s.acquire();
+
+  auto const dur = 250ms;
+  {
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( s.try_acquire_for(dur) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff < dur );
+  }
+
+  {
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( !s.try_acquire_for(dur) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff >= dur );
+  }
+}
+
+void test02()
+{
+  using namespace std::chrono_literals;
+  std::binary_semaphore s(1);
+  std::atomic<int> a(0), b(0);
+  std::thread t([&] {
+    a.wait(0);
+    auto const dur = 250ms;
+    VERIFY( !s.try_acquire_for(dur) );
+    b++;
+    b.notify_one();
+
+    a.wait(1);
+    VERIFY( s.try_acquire_for(dur) );
+    b++;
+    b.notify_one();
+  });
+  t.detach();
+
+  s.acquire();
+  a++;
+  a.notify_one();
+  b.wait(0);
+  s.release();
+  a++;
+  a.notify_one();
+
+  b.wait(1);
+}
+
+int main()
+{
+  test01();
+  test02();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc
new file mode 100644
index 00000000000..5e05606e97f
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_futex.cc
@@ -0,0 +1,51 @@ 
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <semaphore>
+#include <limits>
+#include <cstddef>
+#include <testsuite_hooks.h>
+
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+void test01()
+{
+  // the implementation optimizes for values of least_max_t that can fit
+  // in a futex, make sure we cover the case where least_max_t doesn't
+  auto constexpr least_max_t = std::numeric_limits<std::ptrdiff_t>::max();
+  std::counting_semaphore<least_max_t> s(3);
+
+  s.acquire();
+  VERIFY( s.try_acquire() );
+  VERIFY( s.try_acquire() );
+  VERIFY( !s.try_acquire() );
+  s.release();
+  VERIFY( s.try_acquire() );
+}
+
+#endif
+
+int main()
+{
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+  test01();
+#endif
+}
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc
new file mode 100644
index 00000000000..bf99fd3cf8f
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc
@@ -0,0 +1,169 @@ 
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <semaphore>
+#include <chrono>
+#include <thread>
+#include <atomic>
+#include <testsuite_hooks.h>
+
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  // The implementation supports posix as an implementation strategy
+  // make sure we cover that case
+#define _GLIBCXX_REQUIRE_POSIX_SEMAPHORE
+
+void test01()
+{
+  std::counting_semaphore<10> s(3);
+
+  s.acquire();
+  VERIFY( s.try_acquire() );
+  VERIFY( s.try_acquire() );
+  VERIFY( !s.try_acquire() );
+  s.release();
+  VERIFY( s.try_acquire() );
+}
+
+void test02()
+{
+  using namespace std::chrono_literals;
+  std::counting_semaphore<10> s(2);
+  s.acquire();
+
+  auto const dur = 250ms;
+  {
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( s.try_acquire_for(dur) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff < dur );
+  }
+
+  {
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( !s.try_acquire_for(dur) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff >= dur );
+  }
+}
+
+void test03()
+{
+  using namespace std::chrono_literals;
+  std::binary_semaphore s(1);
+  std::atomic<int> a(0), b(0);
+  std::thread t([&] {
+    a.wait(0);
+    auto const dur = 250ms;
+    VERIFY( !s.try_acquire_for(dur) );
+    b++;
+    b.notify_one();
+
+    a.wait(1);
+    VERIFY( s.try_acquire_for(dur) );
+    b++;
+    b.notify_one();
+  });
+  t.detach();
+
+  s.acquire();
+  a++;
+  a.notify_one();
+  b.wait(0);
+  s.release();
+  a++;
+  a.notify_one();
+
+  b.wait(1);
+}
+
+void test04()
+{
+  using namespace std::chrono_literals;
+  std::counting_semaphore<10> s(2);
+  s.acquire();
+
+  auto const dur = 250ms;
+  {
+    auto const at = std::chrono::system_clock::now() + dur;
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( s.try_acquire_until(at) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff < dur );
+  }
+
+  {
+    auto const at = std::chrono::system_clock::now() + dur;
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( !s.try_acquire_until(at) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff >= dur );
+  }
+}
+
+void test05()
+{
+  using namespace std::chrono_literals;
+  std::binary_semaphore s(1);
+  std::atomic<int> a(0), b(0);
+  std::thread t([&] {
+    a.wait(0);
+    auto const dur = 250ms;
+    {
+      auto const at = std::chrono::system_clock::now() + dur;
+      VERIFY( !s.try_acquire_until(at) );
+
+      b++;
+      b.notify_one();
+    }
+
+    a.wait(1);
+    {
+      auto const at = std::chrono::system_clock::now() + dur;
+      VERIFY( s.try_acquire_until(at) );
+    }
+    b++;
+    b.notify_one();
+  });
+  t.detach();
+
+  s.acquire();
+  a++;
+  a.notify_one();
+  b.wait(0);
+  s.release();
+  a++;
+  a.notify_one();
+
+  b.wait(1);
+}
+#endif
+
+int main()
+{
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  test01();
+  test02();
+  test03();
+  test04();
+  test05();
+#endif
+}
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc
new file mode 100644
index 00000000000..cc67c5c0bf0
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc
@@ -0,0 +1,94 @@ 
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <semaphore>
+#include <chrono>
+#include <thread>
+#include <atomic>
+#include <testsuite_hooks.h>
+
+void test01()
+{
+  using namespace std::chrono_literals;
+  std::counting_semaphore<10> s(2);
+  s.acquire();
+
+  auto const dur = 250ms;
+  {
+    auto const at = std::chrono::system_clock::now() + dur;
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( s.try_acquire_until(at) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff < dur );
+  }
+
+  {
+    auto const at = std::chrono::system_clock::now() + dur;
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( !s.try_acquire_until(at) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff >= dur );
+  }
+}
+
+void test02()
+{
+  using namespace std::chrono_literals;
+  std::binary_semaphore s(1);
+  std::atomic<int> a(0), b(0);
+  std::thread t([&] {
+    a.wait(0);
+    auto const dur = 250ms;
+    {
+      auto const at = std::chrono::system_clock::now() + dur;
+      VERIFY( !s.try_acquire_until(at) );
+
+      b++;
+      b.notify_one();
+    }
+
+    a.wait(1);
+    {
+      auto const at = std::chrono::system_clock::now() + dur;
+      VERIFY( s.try_acquire_until(at) );
+    }
+    b++;
+    b.notify_one();
+  });
+  t.detach();
+
+  s.acquire();
+  a++;
+  a.notify_one();
+  b.wait(0);
+  s.release();
+  a++;
+  a.notify_one();
+
+  b.wait(1);
+}
+
+int main()
+{
+  test01();
+  test02();
+}