Re: [PATCH] libstdc++: Add C++2a synchronization support

Jonathan Wakely via Gcc-patches Mon, 28 Sep 2020 06:26:05 -0700

On 11/09/20 16:58 -0700, Thomas Rodgers wrote:

From: Thomas Rodgers <trodg...@redhat.com>


This patch supercedes both the Add C++2a synchronization support patch
being replied to *and* the patch adding wait/notify_* to atomic_flag.

Add support for -
 * atomic_flag::wait/notify_one/notify_all
 * atomic::wait/notify_one/notify_all
 * counting_semaphore
 * binary_semaphore
 * latch

libstdc++-v3/ChangeLog:

        * include/Makefile.am (bits_headers): Add new header.
        * include/Makefile.in: Regenerate.
        * include/bits/atomic_base.h (__atomic_flag::wait): Define.
        (__atomic_flag::notify_one): Likewise.
        (__atomic_flag::notify_all): Likewise.
        (__atomic_base<_Itp>::wait): Likewise.
        (__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.cc: Liekwise.
        * testsuite/29_atomic/atomic/wait_notify/generic.h: New File.
        * testsuite/29_atomics/atomic_flag/wait_notify/1.cc: New test.
        * 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_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       | 195 +++++++++++-
libstdc++-v3/include/bits/atomic_timed_wait.h | 281 ++++++++++++++++
libstdc++-v3/include/bits/atomic_wait.h       | 301 ++++++++++++++++++
libstdc++-v3/include/bits/semaphore_base.h    | 283 ++++++++++++++++
libstdc++-v3/include/std/atomic               |  73 +++++
libstdc++-v3/include/std/latch                |  90 ++++++
libstdc++-v3/include/std/semaphore            |  92 ++++++
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.cc  |  31 ++
.../29_atomics/atomic/wait_notify/generic.h   | 160 ++++++++++
.../atomic/wait_notify/integrals.cc           |  65 ++++
.../29_atomics/atomic/wait_notify/pointers.cc |  59 ++++
.../29_atomics/atomic_flag/wait_notify/1.cc   |  61 ++++
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          |  30 ++
.../30_threads/semaphore/try_acquire.cc       |  55 ++++
.../30_threads/semaphore/try_acquire_for.cc   |  85 +++++
.../30_threads/semaphore/try_acquire_posix.cc | 153 +++++++++
.../30_threads/semaphore/try_acquire_until.cc |  94 ++++++
28 files changed, 2471 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.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/29_atomics/atomic_flag/wait_notify/1.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_posix.cc
create mode 100644 
libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc

diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index c9df9a9d6c6..9b5b6ed0005 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 \
@@ -101,6 +103,8 @@ 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 \
@@ -175,6 +179,7 @@ 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 2cdd2bd6cae..dd4db926592 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;
@@ -226,6 +229,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      __atomic_load(&_M_i, &__v, int(__m));
      return __v == __GCC_ATOMIC_TEST_AND_SET_TRUEVAL;
    }
+
+    _GLIBCXX_ALWAYS_INLINE void
+    wait(bool __old,
+       memory_order __m = memory_order_seq_cst) const noexcept
+    {
+      std::__atomic_wait(&_M_i, __old,
+                        [__m, this, __old]()
+                        { return this->test(__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++20

    _GLIBCXX_ALWAYS_INLINE void
@@ -576,6 +602,31 @@ _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
@@ -845,6 +896,31 @@ _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
@@ -933,6 +1009,33 @@ _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
@@ -1186,6 +1289,24 @@ _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
@@ -1323,6 +1444,24 @@ _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); }
+
+      // TODO add const volatile overload
+
    private:
      _Tp* _M_ptr;
    };
@@ -1418,6 +1557,24 @@ _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); }
+
+      // TODO add const volatile overload
+
      value_type
      fetch_add(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
@@ -1573,6 +1730,24 @@ _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); }
+
+      // TODO add const volatile overload
+
      value_type
      fetch_add(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
@@ -1682,6 +1857,24 @@ _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); }
+
+      // TODO add const volatile overload
+
      _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..2f57356b366
--- /dev/null
+++ b/libstdc++-v3/include/bits/atomic_timed_wait.h
@@ -0,0 +1,281 @@
+// -*- 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
+    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,
+                             
static_cast<int>(__futex_wait_flags::__wait_bitset_private),
+                             __val, &__rt, nullptr,
+                             
static_cast<int>(__futex_wait_flags::__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;


Newline before the ': __atomic_wait_status::timeout' to keep it
fitting in 80 columns.

+      }
+
+    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 (is_same_v<__platform_wait_clock_t, _Clock>)
+         {
+           return std::__detail::__platform_wait_until_impl(__addr, __val, 
__atime);


I askjed for this to be qualified, but just __detail:: is enough, you
don't need std::__detail. That should make the line fit in 80 columns.

There are still dozens of lines that don't fit in 80 cols in this
revision of the patch.

+         }
+       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 (std::__detail::__platform_wait_until_impl(__addr, __val, 
__s_atime) ==


Just __detail:: again for the qualification. It doesn't need to be
std::__detail::.

Line breaks should be before operators please, i.e.

   value_with_a_long_name
     == bar

or:

   long_condition
     && another_condition

It's much easier to find the operator when it's at the start of the
line, at a predictable indentation, rather than wandering off on the
right somewhere.

+                   __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 (std::__detail::__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 (std::__detail::__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 static_cast<__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..21ec3d38c94
--- /dev/null
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -0,0 +1,301 @@
+// -*- 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
+
+
+// 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;
+
+    constexpr auto __atomic_spin_count_1 = 16;
+    constexpr auto __atomic_spin_count_2 = 12;
+
+    inline constexpr
+    auto __platform_wait_max_value =
+               __gnu_cxx::__int_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 class __futex_wait_flags : int
+    {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE
+      __private_flag = 128,
+#else
+      __private_flag = 0,
+#endif
+      __wait = 0,
+      __wake = 1,
+      __wait_bitset = 9,
+      __wake_bitset = 10,
+      __wait_private = __wait | __private_flag,
+      __wake_private = __wake | __private_flag,
+      __wait_bitset_private = __wait_bitset | __private_flag,
+      __wake_bitset_private = __wake_bitset | __private_flag,
+      __bitset_match_any = -1
+    };
+
+    template<typename _Tp>
+      void
+      __platform_wait(const _Tp* __addr, __platform_wait_t __val) noexcept
+      {
+        auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
+                             
static_cast<int>(__futex_wait_flags::__wait_private),
+                               __val, nullptr);
+        if (__e && !(errno == EINTR || errno == EAGAIN))
+          std::terminate();


This treats EINTR and EGAIN as a spurious wakeup, right?

So callers cannot assume that this function has actually waited for
the value to change. I'll come back to that below.

+    struct __waiters
+    {
+      __platform_wait_t alignas(64) _M_ver = 0;
+      __platform_wait_t alignas(64) _M_wait = 0;


The alignment-specifier needs to come before the type name:

a.cc:3:7: warning: attribute ignored [-Wattributes]
    3 |   int alignas(64) i = 0;
      |       ^~~~~~~
a.cc:3:7: note: an attribute that appertains to a type-specifier is ignored

+      __platform_wait_t
+      _M_waiting() const noexcept


Should this function return a bool?
It looks like the only caller of it just tests if it's zero.

If it means "number of waiters" rather than "there are threads
waiting" then I think it should be renamed to make that more clear
(and similarly for _M_wait, which sounds like a boolean flag not a
count).

+       {
+         __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);


This will terminate since the function is noexcept, which is fine.
Maybe we should consistently use __throw_system_error(__e) which
allows the verbose terminate handler to print a message:

terminate called after throwing an instance of 'std::system_error'
  what():  Resource temporarily unavailable
Aborted (core dumped)

So the other places that currently call std::terminate() should maybe
change to __throw_system_error(__e). What do you think?

+#endif
+      }
+
+      static __waiters&
+      _S_for(const 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(static_cast<const 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


Should this take its argument by (non-const) reference?

+    {
+      for (auto __i = 0; __i < __detail::__atomic_spin_count_1; ++__i)
+       {
+         if (__pred())
+           return true;
+
+         if (__i < __detail::__atomic_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(__addr, __old);


If this gets a spurious wakeup due to EINTR or EAGAIN then we'll loop
and test the predicate again.

+           }
+         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..ed127a7a953
--- /dev/null
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -0,0 +1,283 @@
+// -*- 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 __has_include(<semaphore.h>)
+#define _GLIBCXX_HAVE_POSIX_SEMAPHORE 1
+#include <semaphore.h>
+#endif
+
+#include <chrono>
+#include <type_traits>
+
+#include <iostream>
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  struct __platform_semaphore
+  {
+    using __clock_t = chrono::system_clock;
+    static constexpr ptrdiff_t _S_max = SEM_VALUE_MAX;
+
+    explicit __platform_semaphore(ptrdiff_t __count) noexcept
+    {
+      sem_init(&_M_semaphore, 0, __count);
+    }
+
+    __platform_semaphore(const __platform_semaphore&) = delete;
+    __platform_semaphore& operator=(const __platform_semaphore&) = delete;
+
+    ~__platform_semaphore()
+    { sem_destroy(&_M_semaphore); }
+
+    _GLIBCXX_ALWAYS_INLINE void
+    _M_acquire() noexcept
+    {
+      for (;;)
+       {
+         auto __err = sem_wait(&_M_semaphore);
+         if (__err && (errno == EINTR))
+           continue;
+         else if (__err)
+           std::terminate();
+         else
+           break;
+       }
+    }
+
+    _GLIBCXX_ALWAYS_INLINE void
+    _M_release(std::ptrdiff_t __update) noexcept
+    {
+      for(; __update != 0; --__update)
+       {
+          auto __err = sem_post(&_M_semaphore);
+          if (__err)
+            std::terminate();
+       }
+    }
+
+    bool
+    _M_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())
+      };
+
+      for (;;)
+       {
+         auto __err = sem_timedwait(&_M_semaphore, &__ts);
+         if (__err && (errno == EINTR))
+             continue;
+         else if (__err && (errno == ETIMEDOUT))
+             return false;
+         else if (__err && (errno == EINVAL))
+             return false; // caller supplied an invalid __atime
+         else if (__err)
+             std::terminate();
+         else
+           break;
+       }
+      return true;
+    }
+
+    template<typename _Clock, typename _Duration>
+      bool
+      _M_try_acquire_until(const chrono::time_point<_Clock, _Duration>& 
__atime) noexcept
+      {
+       if constexpr (std::is_same<__clock_t, _Clock>::value)
+         {
+           return _M_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 (_M_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
+      _M_try_acquire_for(const chrono::duration<_Rep, _Period>& __rtime) 
noexcept
+      { return _M_try_acquire_until(__clock_t::now() + __rtime); }
+
+    private:
+      sem_t _M_semaphore;
+    };
+#endif // _GLIBCXX_HAVE_POSIX_SEMAPHORE
+
+    template<typename _Tp>
+      struct __atomic_semaphore
+      {
+       static_assert(std::is_integral_v<_Tp>);
+       static constexpr ptrdiff_t _S_max = __gnu_cxx::__int_traits<_Tp>::__max;


Do we need a check that ptrdiff_t can represent this value?
__atomic_semaphore<size_t> would overflow here.

+
+       explicit __atomic_semaphore(_Tp __count) noexcept
+         : _M_a(__count)
+       { }


This should use __glibcxx_assert to check the preconditions:

__glibcxx_assert(__count >= 0 && __count <= max());

+
+       __atomic_semaphore(const __atomic_semaphore&) = delete;
+       __atomic_semaphore& operator=(const __atomic_semaphore&) = delete;
+
+       _GLIBCXX_ALWAYS_INLINE void
+       _M_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
+       _M_try_acquire() noexcept
+       {
+         auto __old = __atomic_impl::load(&_M_a, memory_order::acquire);
+         if (__old == 0)
+           return false;
+
+         return __atomic_spin([this, &__old]
+           {


Doesn't the check for __old==0 need to be inside the predicate?

Otherwise consider the case where _M_a=1 initially. Two threads call
try_acquire() concurrently. Both see that __old==0 is false, so both
call __atomic_spin with this CAS "predicate" (*). The first thread
replaces the value with 0, the second thread fails the CAS because the
value changed, but it replaces __old with the new value and tries
again (because __atomic_spin loops) and this time it succeeds,
changing the value to -1. The second thread should have had no effect
and returned immediately.

(*) I find it a bit disconcerting that these predicates are not const,
they modify the variable, which implies a precondition that the
predicate is never evaluated again after it returns true.

+             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
+         _M_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
+       _M_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
+      _M_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;


I'd prefer a better name than _M_a here.

The standard calls it "counter" so _M_counter maybe?

+    };
+
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX && !_GLIBCXX_REQUIRE_POSIX_SEMAPHORE


I still don't like this macro, but nevermind.

Could you add a comment saying that the macro can be used to force the
use of POSIX sem_t but that doing so alters the ABI?

+  // Use futex if available and didn't force use of POSIX
+  using __fast_semaphore = __atomic_semaphore<__detail::__platform_wait_t>;
+#elif _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  using __fast_semaphore = __platform_semaphore;
+#else
+  using __fast_semaphore = __atomic_semaphore<ptrdiff_t>;
+#endif
+
+template<ptrdiff_t __least_max_value>
+  using __semaphore_impl = conditional_t<
+               (__least_max_value > 1),
+               conditional_t<
+                   (__least_max_value <= __fast_semaphore::_S_max),
+                   __fast_semaphore,
+                   __atomic_semaphore<ptrdiff_t>>,
+               __fast_semaphore>;
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+
+#endif
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 1a304261fe7..c15909d9ccb 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
@@ -363,6 +376,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                     memory_order __m = memory_order_seq_cst) volatile noexcept
      { return compare_exchange_strong(__e, __i, __m,
                                       __cmpexch_failure_order(__m)); }


Blank line here before the next member function please.

+#if __cplusplus > 201703L
+    void wait(_Tp __old, memory_order __m = memory_order_seq_cst) const 
noexcept
+    {
+      std::__atomic_wait(&_M_i, __old,
+                        [__m, this, __old]
+                        {
+                          const auto __v = this->load(__m);
+                          // TODO make this ignore padding bits when we can do 
that
+                          return __builtin_memcmp(&__old, &__v, sizeof(_Tp)) 
!= 0;
+                        });
+    }
+
+    // TODO add const volatile overload
+
+    void notify_one() const noexcept
+    { std::__atomic_notify(&_M_i, false); }
+
+    void notify_all() const noexcept
+    { std::__atomic_notify(&_M_i, true); }
+#endif
+
    };
#undef _GLIBCXX20_INIT

@@ -601,6 +635,18 @@ _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
@@ -1353,6 +1399,33 @@ _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..bd06db5aa7f
--- /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 ptrdiff_t
+    max() noexcept
+    { return __gnu_cxx::__numeric_traits<ptrdiff_t>::__max; }


You can use __gnu_cxx::__int_traits<ptrdiff_t> here, since we already
know ptrdiff_t is an integer type and so don't need to do the
selection between __numeric_traits for integers or floats.

+    constexpr explicit latch(ptrdiff_t __expected) noexcept
+      : _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);


This line is too long,

+      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(); });


This function is not noexcept, but it only calls noexcept functions.
That suggests either __atomic_wait should not be noexcept, or this one
could be.

+    }
+
+    _GLIBCXX_ALWAYS_INLINE void
+    arrive_and_wait(ptrdiff_t __update = 1)
+    {
+      count_down();


This needs to pass the __udpate parameter to count_down.

This suggests we're missing a test using arrive_and_wait(2) or greater
values.

+      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..865d6c4aecb
--- /dev/null
+++ b/libstdc++-v3/include/std/semaphore
@@ -0,0 +1,92 @@
+// <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>


This can use __gnu_cxx::__int_traits too.

+    class counting_semaphore
+    {
+      static_assert(__least_max_value >= 0);
+
+      __semaphore_impl<__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) noexcept(noexcept(_M_sem._M_release(1)))
+      { _M_sem._M_release(__update); }
+
+      void
+      acquire() noexcept(noexcept(_M_sem._M_acquire()))
+      { _M_sem._M_acquire(); }
+
+      bool
+      try_acquire() noexcept(noexcept(_M_sem._M_try_acquire()))
+      { return _M_sem._M_try_acquire(); }
+
+      template<class _Rep, class _Period>


typename, not class

+       bool
+       try_acquire_for(const std::chrono::duration<_Rep, _Period>& __rtime)
+       { return _M_sem._M_try_acquire_for(__rtime); }
+
+      template<class _Clock, class _Dur>


typename, not class

+       bool
+       try_acquire_until(const std::chrono::time_point<_Clock, _Dur>& __atime)
+       { return _M_sem._M_try_acquire_until(__atime); }
+    };
+
+ 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 d5d42ed0a72..b9c7c6c62a8 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -216,12 +216,14 @@
#ifdef _GLIBCXX_HAS_GTHREADS
# define __cpp_lib_jthread 201911L
#endif
+#define __cpp_lib_latch 201907L
#define __cpp_lib_list_remove_return_type 201806L
#define __cpp_lib_math_constants 201907L
#define __cpp_lib_polymorphic_allocator 201902L
#if __cpp_lib_concepts
# define __cpp_lib_ranges 201911L
#endif
+#define __cpp_lib_semaphore 201907L
#define __cpp_lib_shift 201806L
#define __cpp_lib_span 202002L
#define __cpp_lib_ssize 201902L
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


This file should be under testsuite/29_atomics/atomic_ref somewhere,
not under testsuite/29_atomics/atomic

@@ -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


And this should be under testsuite/29_atomics/atomic_float

Please take another look at the dir structure under
testsuite/29_atomics/ and locate the new tests with the relevant
types. testsuite/29_atomics/atomic/ is for the generic std::atomic
template.

@@ -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.cc 
b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.cc
new file mode 100644
index 00000000000..d15b9c86ae6
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.cc
@@ -0,0 +1,31 @@
+// { 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 ()
+{
+  struct S{ int i; };
+  check<S> check_s{S{0},S{42}};
+  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..a319e8b60a6
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/generic.h
@@ -0,0 +1,160 @@
+// 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 <chrono>
+#include <condition_variable>
+#include <concepts>
+#include <mutex>
+#include <thread>
+
+#include <testsuite_hooks.h>
+
+#include <iostream>
+
+template<typename Tp>
+Tp check_wait_notify(Tp val1, Tp val2)
+  requires std::equality_comparable<Tp>
+{
+  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_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);
+                 auto v = a.load();
+                 // TODO this needs to zero padding bits when we can do that
+                 if (__builtin_memcmp(&v, &val2, sizeof(Tp)) != 0)
+                   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)
+  requires std::equality_comparable<Tp>
+{
+  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>
+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);
+                 auto v = a.load();
+                 // TODO this needs to zero padding bits when we can do that
+                 if (__builtin_memcmp(&v, &val2, sizeof(Tp)) != 0)
+                   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)
+  {
+    if constexpr (std::equality_comparable<Tp>)
+    {
+      VERIFY( check_wait_notify(a, b) == b);
+      VERIFY( check_atomic_wait_notify(a, b) == b);
+    }
+    else
+    {
+      {
+       // TODO this needs to zero padding bits when we can do that
+       auto v = check_wait_notify(a, b);
+       VERIFY( __builtin_memcmp(&v, &b, sizeof(Tp)) == 0 );
+      }
+
+      {
+       // TODO this needs to zero padding bits when we can do that
+       auto v = check_atomic_wait_notify(a, b);
+       VERIFY( __builtin_memcmp(&v, &b, sizeof(Tp)) == 0);
+      }
+    }
+  }
+};
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..115cb79a040
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/integrals.cc
@@ -0,0 +1,65 @@
+// { 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"
+
+void
+test01()
+{
+  struct S{ int i; };
+  std::atomic<S> s;
+
+  s.wait(S{42});
+}
+
+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/29_atomics/atomic_flag/wait_notify/1.cc 
b/libstdc++-v3/testsuite/29_atomics/atomic_flag/wait_notify/1.cc
new file mode 100644
index 00000000000..6de7873ecc2
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_flag/wait_notify/1.cc
@@ -0,0 +1,61 @@
+// { 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 <chrono>
+#include <condition_variable>
+#include <concepts>
+#include <mutex>
+#include <thread>
+
+#include <testsuite_hooks.h>
+
+int
+main()
+{
+  using namespace std::literals::chrono_literals;
+
+  std::mutex m;
+  std::condition_variable cv;
+
+  std::atomic_flag a;
+  std::atomic_flag b;
+  std::thread t([&]
+               {
+                 cv.notify_one();
+                 a.wait(false);
+                  b.test_and_set();
+                  b.notify_one();
+               });
+
+  std::unique_lock<std::mutex> l(m);
+  cv.wait(l);
+  std::this_thread::sleep_for(100ms);
+  a.test_and_set();
+  a.notify_one();
+  b.wait(false);
+  t.join();
+
+  VERIFY( a.test() );
+  VERIFY( b.test() );
+  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..318a859ee21
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/latch/2.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 <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..cf1a31f996b
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/latch/3.cc
@@ -0,0 +1,50 @@
+// 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 -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..98743f5e27c
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/2.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 <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..d74cfad53e9
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value_neg.cc
@@ -0,0 +1,30 @@
+// 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 } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#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..25280441d07
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire.cc
@@ -0,0 +1,55 @@
+// 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 -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..3f450e74661
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_for.cc
@@ -0,0 +1,85 @@
+// 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 -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_posix.cc 
b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc
new file mode 100644
index 00000000000..13bd7487d56
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_posix.cc
@@ -0,0 +1,153 @@
+// 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 -pthread" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <semaphore>
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+#include <chrono>
+#include <thread>
+#include <atomic>
+#include <testsuite_hooks.h>
+
+void test01()
+{
+  using namespace std::chrono_literals;
+  std::__platform_semaphore s(2);
+  s._M_acquire();
+
+  auto const dur = 250ms;
+  {
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( s._M_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._M_try_acquire_for(dur) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff >= dur );
+  }
+}
+
+void test02()
+{
+  using namespace std::chrono_literals;
+  std::__platform_semaphore s(1);
+  std::atomic<int> a(0), b(0);
+  std::thread t([&] {
+    a.wait(0);
+    auto const dur = 250ms;
+    VERIFY( !s._M_try_acquire_for(dur) );
+    b++;
+    b.notify_one();
+
+    a.wait(1);
+    VERIFY( s._M_try_acquire_for(dur) );
+    b++;
+    b.notify_one();
+  });
+  t.detach();
+
+  s._M_acquire();
+  a++;
+  a.notify_one();
+  b.wait(0);
+  s._M_release(1);
+  a++;
+  a.notify_one();
+
+  b.wait(1);
+}
+
+void test03()
+{
+  using namespace std::chrono_literals;
+  std::__platform_semaphore s(2);
+  s._M_acquire();
+
+  auto const dur = 250ms;
+  {
+    auto const at = std::chrono::system_clock::now() + dur;
+    auto const t0 = std::chrono::steady_clock::now();
+    VERIFY( s._M_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._M_try_acquire_until(at) );
+    auto const diff = std::chrono::steady_clock::now() - t0;
+    VERIFY( diff >= dur );
+  }
+}
+
+void test04()
+{
+  using namespace std::chrono_literals;
+  std::__platform_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._M_try_acquire_until(at) );
+
+      b++;
+      b.notify_one();
+    }
+
+    a.wait(1);
+    {
+      auto const at = std::chrono::system_clock::now() + dur;
+      VERIFY( s._M_try_acquire_until(at) );
+    }
+    b++;
+    b.notify_one();
+  });
+  t.detach();
+
+  s._M_acquire();
+  a++;
+  a.notify_one();
+  b.wait(0);
+  s._M_release(1);
+  a++;
+  a.notify_one();
+
+  b.wait(1);
+}
+#endif
+
+int main()
+{
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+  test01();
+  test02();
+  test03();
+  test04();
+#endif
+  return 0;
+}
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..af7ab7bac39
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc
@@ -0,0 +1,94 @@
+// 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 -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

Re: [PATCH] libstdc++: Add C++2a synchronization support

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