libstdc++-v3/include/bits/atomic_futex.h - rust-lang/gcc - Git at Google

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

 // Copyright (C) 2015-2021 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_futex.h
  *  This is an internal header file, included by other library headers.
  *  Do not attempt to use it directly.
  */

 #ifndef _GLIBCXX_ATOMIC_FUTEX_H
 #define _GLIBCXX_ATOMIC_FUTEX_H 1

 #pragma GCC system_header

 #include <bits/c++config.h>
 #include <atomic>
 #include <chrono>
 #if ! (defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1)
 #include <mutex>
 #include <condition_variable>
 #endif

 #ifndef _GLIBCXX_ALWAYS_INLINE
 #define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
 #endif

 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION

 #ifdef _GLIBCXX_HAS_GTHREADS
 #if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
   struct __atomic_futex_unsigned_base
   {
     // __s and __ns are measured against CLOCK_REALTIME. Returns false
     // iff a timeout occurred.
     bool
     _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
 	chrono::seconds __s, chrono::nanoseconds __ns);

     // __s and __ns are measured against CLOCK_MONOTONIC. Returns
     // false iff a timeout occurred.
     bool
     _M_futex_wait_until_steady(unsigned *__addr, unsigned __val,
 	bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns);

     // This can be executed after the object has been destroyed.
     static void _M_futex_notify_all(unsigned* __addr);
   };

   template <unsigned _Waiter_bit = 0x80000000>
   class __atomic_futex_unsigned : __atomic_futex_unsigned_base
   {
     typedef chrono::steady_clock __clock_t;

     // This must be lock-free and at offset 0.
     atomic<unsigned> _M_data;

   public:
     explicit
     __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
     { }

     _GLIBCXX_ALWAYS_INLINE unsigned
     _M_load(memory_order __mo)
     {
       return _M_data.load(__mo) & ~_Waiter_bit;
     }

   private:
     // If a timeout occurs, returns a current value after the timeout;
     // otherwise, returns the operand's value if equal is true or a different
     // value if equal is false.
     // The assumed value is the caller's assumption about the current value
     // when making the call.
     // __s and __ns are measured against CLOCK_REALTIME.
     unsigned
     _M_load_and_test_until(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo, bool __has_timeout,
 	chrono::seconds __s, chrono::nanoseconds __ns)
     {
       for (;;)
 	{
 	  // Don't bother checking the value again because we expect the caller
 	  // to have done it recently.
 	  // memory_order_relaxed is sufficient because we can rely on just the
 	  // modification order (store_notify uses an atomic RMW operation too),
 	  // and the futex syscalls synchronize between themselves.
 	  _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
 	  bool __ret = _M_futex_wait_until((unsigned*)(void*)&_M_data,
 					   __assumed | _Waiter_bit,
 					   __has_timeout, __s, __ns);
 	  // Fetch the current value after waiting (clears _Waiter_bit).
 	  __assumed = _M_load(__mo);
 	  if (!__ret || ((__operand == __assumed) == __equal))
 	    return __assumed;
 	  // TODO adapt wait time
 	}
     }

     // If a timeout occurs, returns a current value after the timeout;
     // otherwise, returns the operand's value if equal is true or a different
     // value if equal is false.
     // The assumed value is the caller's assumption about the current value
     // when making the call.
     // __s and __ns are measured against CLOCK_MONOTONIC.
     unsigned
     _M_load_and_test_until_steady(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo, bool __has_timeout,
 	chrono::seconds __s, chrono::nanoseconds __ns)
     {
       for (;;)
 	{
 	  // Don't bother checking the value again because we expect the caller
 	  // to have done it recently.
 	  // memory_order_relaxed is sufficient because we can rely on just the
 	  // modification order (store_notify uses an atomic RMW operation too),
 	  // and the futex syscalls synchronize between themselves.
 	  _M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
 	  bool __ret = _M_futex_wait_until_steady((unsigned*)(void*)&_M_data,
 					   __assumed | _Waiter_bit,
 					   __has_timeout, __s, __ns);
 	  // Fetch the current value after waiting (clears _Waiter_bit).
 	  __assumed = _M_load(__mo);
 	  if (!__ret || ((__operand == __assumed) == __equal))
 	    return __assumed;
 	  // TODO adapt wait time
 	}
     }

     // Returns the operand's value if equal is true or a different value if
     // equal is false.
     // The assumed value is the caller's assumption about the current value
     // when making the call.
     unsigned
     _M_load_and_test(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo)
     {
       return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
 				    false, {}, {});
     }

     // If a timeout occurs, returns a current value after the timeout;
     // otherwise, returns the operand's value if equal is true or a different
     // value if equal is false.
     // The assumed value is the caller's assumption about the current value
     // when making the call.
     template<typename _Dur>
     unsigned
     _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo,
 	const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
     {
       auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
       auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
       // XXX correct?
       return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
 	  true, __s.time_since_epoch(), __ns);
     }

     template<typename _Dur>
     unsigned
     _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo,
 	const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
     {
       auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
       auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
       // XXX correct?
       return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
 	  true, __s.time_since_epoch(), __ns);
     }

   public:

     _GLIBCXX_ALWAYS_INLINE unsigned
     _M_load_when_not_equal(unsigned __val, memory_order __mo)
     {
       unsigned __i = _M_load(__mo);
       if ((__i & ~_Waiter_bit) != __val)
 	return (__i & ~_Waiter_bit);
       // TODO Spin-wait first.
       return _M_load_and_test(__i, __val, false, __mo);
     }

     _GLIBCXX_ALWAYS_INLINE void
     _M_load_when_equal(unsigned __val, memory_order __mo)
     {
       unsigned __i = _M_load(__mo);
       if ((__i & ~_Waiter_bit) == __val)
 	return;
       // TODO Spin-wait first.
       _M_load_and_test(__i, __val, true, __mo);
     }

     // Returns false iff a timeout occurred.
     template<typename _Rep, typename _Period>
       _GLIBCXX_ALWAYS_INLINE bool
       _M_load_when_equal_for(unsigned __val, memory_order __mo,
 	  const chrono::duration<_Rep, _Period>& __rtime)
       {
 	using __dur = typename __clock_t::duration;
 	return _M_load_when_equal_until(__val, __mo,
 		    __clock_t::now() + chrono::__detail::ceil<__dur>(__rtime));
       }

     // Returns false iff a timeout occurred.
     template<typename _Clock, typename _Duration>
       _GLIBCXX_ALWAYS_INLINE bool
       _M_load_when_equal_until(unsigned __val, memory_order __mo,
 	  const chrono::time_point<_Clock, _Duration>& __atime)
       {
 	typename _Clock::time_point __c_entry = _Clock::now();
 	do {
 	  const __clock_t::time_point __s_entry = __clock_t::now();
 	  const auto __delta = __atime - __c_entry;
 	  const auto __s_atime = __s_entry +
 	      chrono::__detail::ceil<__clock_t::duration>(__delta);
 	  if (_M_load_when_equal_until(__val, __mo, __s_atime))
 	    return true;
 	  __c_entry = _Clock::now();
 	} while (__c_entry < __atime);
 	return false;
       }

     // Returns false iff a timeout occurred.
     template<typename _Duration>
     _GLIBCXX_ALWAYS_INLINE bool
     _M_load_when_equal_until(unsigned __val, memory_order __mo,
 	const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
     {
       unsigned __i = _M_load(__mo);
       if ((__i & ~_Waiter_bit) == __val)
 	return true;
       // TODO Spin-wait first.  Ignore effect on timeout.
       __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
       return (__i & ~_Waiter_bit) == __val;
     }

     // Returns false iff a timeout occurred.
     template<typename _Duration>
     _GLIBCXX_ALWAYS_INLINE bool
     _M_load_when_equal_until(unsigned __val, memory_order __mo,
 	const chrono::time_point<std::chrono::steady_clock, _Duration>& __atime)
     {
       unsigned __i = _M_load(__mo);
       if ((__i & ~_Waiter_bit) == __val)
 	return true;
       // TODO Spin-wait first.  Ignore effect on timeout.
       __i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
       return (__i & ~_Waiter_bit) == __val;
     }

     _GLIBCXX_ALWAYS_INLINE void
     _M_store_notify_all(unsigned __val, memory_order __mo)
     {
       unsigned* __futex = (unsigned *)(void *)&_M_data;
       if (_M_data.exchange(__val, __mo) & _Waiter_bit)
 	_M_futex_notify_all(__futex);
     }
   };

 #else // ! (_GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1)

   // If futexes are not available, use a mutex and a condvar to wait.
   // Because we access the data only within critical sections, all accesses
   // are sequentially consistent; thus, we satisfy any provided memory_order.
   template <unsigned _Waiter_bit = 0x80000000>
   class __atomic_futex_unsigned
   {
     typedef chrono::system_clock __clock_t;

     unsigned _M_data;
     mutex _M_mutex;
     condition_variable _M_condvar;

   public:
     explicit
     __atomic_futex_unsigned(unsigned __data) : _M_data(__data)
     { }

     _GLIBCXX_ALWAYS_INLINE unsigned
     _M_load(memory_order __mo)
     {
       unique_lock<mutex> __lock(_M_mutex);
       return _M_data;
     }

     _GLIBCXX_ALWAYS_INLINE unsigned
     _M_load_when_not_equal(unsigned __val, memory_order __mo)
     {
       unique_lock<mutex> __lock(_M_mutex);
       while (_M_data == __val)
 	_M_condvar.wait(__lock);
       return _M_data;
     }

     _GLIBCXX_ALWAYS_INLINE void
     _M_load_when_equal(unsigned __val, memory_order __mo)
     {
       unique_lock<mutex> __lock(_M_mutex);
       while (_M_data != __val)
 	_M_condvar.wait(__lock);
     }

     template<typename _Rep, typename _Period>
       _GLIBCXX_ALWAYS_INLINE bool
       _M_load_when_equal_for(unsigned __val, memory_order __mo,
 	  const chrono::duration<_Rep, _Period>& __rtime)
       {
 	unique_lock<mutex> __lock(_M_mutex);
 	return _M_condvar.wait_for(__lock, __rtime,
 				   [&] { return _M_data == __val;});
       }

     template<typename _Clock, typename _Duration>
       _GLIBCXX_ALWAYS_INLINE bool
       _M_load_when_equal_until(unsigned __val, memory_order __mo,
 	  const chrono::time_point<_Clock, _Duration>& __atime)
       {
 	unique_lock<mutex> __lock(_M_mutex);
 	return _M_condvar.wait_until(__lock, __atime,
 				     [&] { return _M_data == __val;});
       }

     _GLIBCXX_ALWAYS_INLINE void
     _M_store_notify_all(unsigned __val, memory_order __mo)
     {
       unique_lock<mutex> __lock(_M_mutex);
       _M_data = __val;
       _M_condvar.notify_all();
     }
   };

 #endif // _GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1
 #endif // _GLIBCXX_HAS_GTHREADS

 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std

 #endif
	// -- C++ -- header.

	// Copyright (C) 2015-2021 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_futex.h
	* This is an internal header file, included by other library headers.
	* Do not attempt to use it directly.
	*/

	#ifndef _GLIBCXX_ATOMIC_FUTEX_H
	#define _GLIBCXX_ATOMIC_FUTEX_H 1

	#pragma GCC system_header

	#include <bits/c++config.h>
	#include <atomic>
	#include <chrono>
	#if ! (defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1)
	#include <mutex>
	#include <condition_variable>
	#endif

	#ifndef _GLIBCXX_ALWAYS_INLINE
	#define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
	#endif

	namespace std _GLIBCXX_VISIBILITY(default)
	{
	_GLIBCXX_BEGIN_NAMESPACE_VERSION

	#ifdef _GLIBCXX_HAS_GTHREADS
	#if defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1
	struct __atomic_futex_unsigned_base
	{
	// __s and __ns are measured against CLOCK_REALTIME. Returns false
	// iff a timeout occurred.
	bool
	_M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout,
	chrono::seconds __s, chrono::nanoseconds __ns);

	// __s and __ns are measured against CLOCK_MONOTONIC. Returns
	// false iff a timeout occurred.
	bool
	_M_futex_wait_until_steady(unsigned *__addr, unsigned __val,
	bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns);

	// This can be executed after the object has been destroyed.
	static void _M_futex_notify_all(unsigned* __addr);
	};

	template <unsigned _Waiter_bit = 0x80000000>
	class __atomic_futex_unsigned : __atomic_futex_unsigned_base
	{
	typedef chrono::steady_clock __clock_t;

	// This must be lock-free and at offset 0.
	atomic<unsigned> _M_data;

	public:
	explicit
	__atomic_futex_unsigned(unsigned __data) : _M_data(__data)
	{ }

	_GLIBCXX_ALWAYS_INLINE unsigned
	_M_load(memory_order __mo)
	{
	return _M_data.load(__mo) & ~_Waiter_bit;
	}

	private:
	// If a timeout occurs, returns a current value after the timeout;
	// otherwise, returns the operand's value if equal is true or a different
	// value if equal is false.
	// The assumed value is the caller's assumption about the current value
	// when making the call.
	// __s and __ns are measured against CLOCK_REALTIME.
	unsigned
	_M_load_and_test_until(unsigned __assumed, unsigned __operand,
	bool __equal, memory_order __mo, bool __has_timeout,
	chrono::seconds __s, chrono::nanoseconds __ns)
	{
	for (;;)
	{
	// Don't bother checking the value again because we expect the caller
	// to have done it recently.
	// memory_order_relaxed is sufficient because we can rely on just the
	// modification order (store_notify uses an atomic RMW operation too),
	// and the futex syscalls synchronize between themselves.
	_M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
	bool __ret = _M_futex_wait_until((unsigned)(void)&_M_data,
	__assumed \| _Waiter_bit,
	__has_timeout, __s, __ns);
	// Fetch the current value after waiting (clears _Waiter_bit).
	__assumed = _M_load(__mo);
	if (!__ret \|\| ((__operand == __assumed) == __equal))
	return __assumed;
	// TODO adapt wait time
	}
	}

	// If a timeout occurs, returns a current value after the timeout;
	// otherwise, returns the operand's value if equal is true or a different
	// value if equal is false.
	// The assumed value is the caller's assumption about the current value
	// when making the call.
	// __s and __ns are measured against CLOCK_MONOTONIC.
	unsigned
	_M_load_and_test_until_steady(unsigned __assumed, unsigned __operand,
	bool __equal, memory_order __mo, bool __has_timeout,
	chrono::seconds __s, chrono::nanoseconds __ns)
	{
	for (;;)
	{
	// Don't bother checking the value again because we expect the caller
	// to have done it recently.
	// memory_order_relaxed is sufficient because we can rely on just the
	// modification order (store_notify uses an atomic RMW operation too),
	// and the futex syscalls synchronize between themselves.
	_M_data.fetch_or(_Waiter_bit, memory_order_relaxed);
	bool __ret = _M_futex_wait_until_steady((unsigned)(void)&_M_data,
	__assumed \| _Waiter_bit,
	__has_timeout, __s, __ns);
	// Fetch the current value after waiting (clears _Waiter_bit).
	__assumed = _M_load(__mo);
	if (!__ret \|\| ((__operand == __assumed) == __equal))
	return __assumed;
	// TODO adapt wait time
	}
	}

	// Returns the operand's value if equal is true or a different value if
	// equal is false.
	// The assumed value is the caller's assumption about the current value
	// when making the call.
	unsigned
	_M_load_and_test(unsigned __assumed, unsigned __operand,
	bool __equal, memory_order __mo)
	{
	return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
	false, {}, {});
	}

	// If a timeout occurs, returns a current value after the timeout;
	// otherwise, returns the operand's value if equal is true or a different
	// value if equal is false.
	// The assumed value is the caller's assumption about the current value
	// when making the call.
	template<typename _Dur>
	unsigned
	_M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
	bool __equal, memory_order __mo,
	const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
	{
	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
	// XXX correct?
	return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
	true, __s.time_since_epoch(), __ns);
	}

	template<typename _Dur>
	unsigned
	_M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
	bool __equal, memory_order __mo,
	const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
	{
	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
	// XXX correct?
	return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
	true, __s.time_since_epoch(), __ns);
	}

	public:

	_GLIBCXX_ALWAYS_INLINE unsigned
	_M_load_when_not_equal(unsigned __val, memory_order __mo)
	{
	unsigned __i = _M_load(__mo);
	if ((__i & ~_Waiter_bit) != __val)
	return (__i & ~_Waiter_bit);
	// TODO Spin-wait first.
	return _M_load_and_test(__i, __val, false, __mo);
	}

	_GLIBCXX_ALWAYS_INLINE void
	_M_load_when_equal(unsigned __val, memory_order __mo)
	{
	unsigned __i = _M_load(__mo);
	if ((__i & ~_Waiter_bit) == __val)
	return;
	// TODO Spin-wait first.
	_M_load_and_test(__i, __val, true, __mo);
	}

	// Returns false iff a timeout occurred.
	template<typename _Rep, typename _Period>
	_GLIBCXX_ALWAYS_INLINE bool
	_M_load_when_equal_for(unsigned __val, memory_order __mo,
	const chrono::duration<_Rep, _Period>& __rtime)
	{
	using __dur = typename __clock_t::duration;
	return _M_load_when_equal_until(__val, __mo,
	__clock_t::now() + chrono::__detail::ceil<__dur>(__rtime));
	}

	// Returns false iff a timeout occurred.
	template<typename _Clock, typename _Duration>
	_GLIBCXX_ALWAYS_INLINE bool
	_M_load_when_equal_until(unsigned __val, memory_order __mo,
	const chrono::time_point<_Clock, _Duration>& __atime)
	{
	typename _Clock::time_point __c_entry = _Clock::now();
	do {
	const __clock_t::time_point __s_entry = __clock_t::now();
	const auto __delta = __atime - __c_entry;
	const auto __s_atime = __s_entry +
	chrono::__detail::ceil<__clock_t::duration>(__delta);
	if (_M_load_when_equal_until(__val, __mo, __s_atime))
	return true;
	__c_entry = _Clock::now();
	} while (__c_entry < __atime);
	return false;
	}

	// Returns false iff a timeout occurred.
	template<typename _Duration>
	_GLIBCXX_ALWAYS_INLINE bool
	_M_load_when_equal_until(unsigned __val, memory_order __mo,
	const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
	{
	unsigned __i = _M_load(__mo);
	if ((__i & ~_Waiter_bit) == __val)
	return true;
	// TODO Spin-wait first. Ignore effect on timeout.
	__i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
	return (__i & ~_Waiter_bit) == __val;
	}

	// Returns false iff a timeout occurred.
	template<typename _Duration>
	_GLIBCXX_ALWAYS_INLINE bool
	_M_load_when_equal_until(unsigned __val, memory_order __mo,
	const chrono::time_point<std::chrono::steady_clock, _Duration>& __atime)
	{
	unsigned __i = _M_load(__mo);
	if ((__i & ~_Waiter_bit) == __val)
	return true;
	// TODO Spin-wait first. Ignore effect on timeout.
	__i = _M_load_and_test_until_impl(__i, __val, true, __mo, __atime);
	return (__i & ~_Waiter_bit) == __val;
	}

	_GLIBCXX_ALWAYS_INLINE void
	_M_store_notify_all(unsigned __val, memory_order __mo)
	{
	unsigned* __futex = (unsigned )(void )&_M_data;
	if (_M_data.exchange(__val, __mo) & _Waiter_bit)
	_M_futex_notify_all(__futex);
	}
	};

	#else // ! (_GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1)

	// If futexes are not available, use a mutex and a condvar to wait.
	// Because we access the data only within critical sections, all accesses
	// are sequentially consistent; thus, we satisfy any provided memory_order.
	template <unsigned _Waiter_bit = 0x80000000>
	class __atomic_futex_unsigned
	{
	typedef chrono::system_clock __clock_t;

	unsigned _M_data;
	mutex _M_mutex;
	condition_variable _M_condvar;

	public:
	explicit
	__atomic_futex_unsigned(unsigned __data) : _M_data(__data)
	{ }

	_GLIBCXX_ALWAYS_INLINE unsigned
	_M_load(memory_order __mo)
	{
	unique_lock<mutex> __lock(_M_mutex);
	return _M_data;
	}

	_GLIBCXX_ALWAYS_INLINE unsigned
	_M_load_when_not_equal(unsigned __val, memory_order __mo)
	{
	unique_lock<mutex> __lock(_M_mutex);
	while (_M_data == __val)
	_M_condvar.wait(__lock);
	return _M_data;
	}

	_GLIBCXX_ALWAYS_INLINE void
	_M_load_when_equal(unsigned __val, memory_order __mo)
	{
	unique_lock<mutex> __lock(_M_mutex);
	while (_M_data != __val)
	_M_condvar.wait(__lock);
	}

	template<typename _Rep, typename _Period>
	_GLIBCXX_ALWAYS_INLINE bool
	_M_load_when_equal_for(unsigned __val, memory_order __mo,
	const chrono::duration<_Rep, _Period>& __rtime)
	{
	unique_lock<mutex> __lock(_M_mutex);
	return _M_condvar.wait_for(__lock, __rtime,
	[&] { return _M_data == __val;});
	}

	template<typename _Clock, typename _Duration>
	_GLIBCXX_ALWAYS_INLINE bool
	_M_load_when_equal_until(unsigned __val, memory_order __mo,
	const chrono::time_point<_Clock, _Duration>& __atime)
	{
	unique_lock<mutex> __lock(_M_mutex);
	return _M_condvar.wait_until(__lock, __atime,
	[&] { return _M_data == __val;});
	}

	_GLIBCXX_ALWAYS_INLINE void
	_M_store_notify_all(unsigned __val, memory_order __mo)
	{
	unique_lock<mutex> __lock(_M_mutex);
	_M_data = __val;
	_M_condvar.notify_all();
	}
	};

	#endif // _GLIBCXX_HAVE_LINUX_FUTEX && ATOMIC_INT_LOCK_FREE > 1
	#endif // _GLIBCXX_HAS_GTHREADS

	_GLIBCXX_END_NAMESPACE_VERSION
	} // namespace std

	#endif