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[EDIT] FILE: atomicity.h

// Support for atomic operations -*- C++ -*-

// Copyright (C) 2004-2018 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 ext/atomicity.h
 *  This file is a GNU extension to the Standard C++ Library.
 */

#ifndef _GLIBCXX_ATOMICITY_H
#define _GLIBCXX_ATOMICITY_H	1

#pragma GCC system_header

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

namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

// Functions for portable atomic access.
  // To abstract locking primitives across all thread policies, use:
  // __exchange_and_add_dispatch
  // __atomic_add_dispatch
#ifdef _GLIBCXX_ATOMIC_BUILTINS
  static inline _Atomic_word 
  __exchange_and_add(volatile _Atomic_word* __mem, int __val)
  { return __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }

static inline void
  __atomic_add(volatile _Atomic_word* __mem, int __val)
  { __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
#else
  _Atomic_word
  __attribute__ ((__unused__))
  __exchange_and_add(volatile _Atomic_word*, int) throw ();

void
  __attribute__ ((__unused__))
  __atomic_add(volatile _Atomic_word*, int) throw ();
#endif

static inline _Atomic_word
  __exchange_and_add_single(_Atomic_word* __mem, int __val)
  {
    _Atomic_word __result = *__mem;
    *__mem += __val;
    return __result;
  }

static inline void
  __atomic_add_single(_Atomic_word* __mem, int __val)
  { *__mem += __val; }

static inline _Atomic_word
  __attribute__ ((__unused__))
  __exchange_and_add_dispatch(_Atomic_word* __mem, int __val)
  {
#ifdef __GTHREADS
    if (__gthread_active_p())
      return __exchange_and_add(__mem, __val);
    else
      return __exchange_and_add_single(__mem, __val);
#else
    return __exchange_and_add_single(__mem, __val);
#endif
  }

static inline void
  __attribute__ ((__unused__))
  __atomic_add_dispatch(_Atomic_word* __mem, int __val)
  {
#ifdef __GTHREADS
    if (__gthread_active_p())
      __atomic_add(__mem, __val);
    else
      __atomic_add_single(__mem, __val);
#else
    __atomic_add_single(__mem, __val);
#endif
  }

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

// Even if the CPU doesn't need a memory barrier, we need to ensure
// that the compiler doesn't reorder memory accesses across the
// barriers.
#ifndef _GLIBCXX_READ_MEM_BARRIER
#define _GLIBCXX_READ_MEM_BARRIER __atomic_thread_fence (__ATOMIC_ACQUIRE)
#endif
#ifndef _GLIBCXX_WRITE_MEM_BARRIER
#define _GLIBCXX_WRITE_MEM_BARRIER __atomic_thread_fence (__ATOMIC_RELEASE)
#endif

#endif

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