cef/include/base/cef_atomic_ref_count.h

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4.8 KiB
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// Copyright (c) 2014 Marshall A. Greenblatt. Portions copyright (c) 2011
// Google Inc. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// * Neither the name of Google Inc. nor the name Chromium Embedded
// Framework nor the names of its contributors may be used to endorse
// or promote products derived from this software without specific prior
// written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
// This is a low level implementation of atomic semantics for reference
// counting. Please use cef_ref_counted.h directly instead.
//
// The Chromium implementation includes annotations to avoid some false
// positives when using data race detection tools. Annotations are not
// currently supported by the CEF implementation.
#ifndef CEF_INCLUDE_BASE_CEF_ATOMIC_REF_COUNT_H_
#define CEF_INCLUDE_BASE_CEF_ATOMIC_REF_COUNT_H_
#pragma once
#if defined(USING_CHROMIUM_INCLUDES)
// When building CEF include the Chromium header directly.
#include "base/atomic_ref_count.h"
#else // !USING_CHROMIUM_INCLUDES
// The following is substantially similar to the Chromium implementation.
// If the Chromium implementation diverges the below implementation should be
// updated to match.
#include <atomic>
namespace base {
class AtomicRefCount {
public:
constexpr AtomicRefCount() : ref_count_(0) {}
explicit constexpr AtomicRefCount(int initial_value)
: ref_count_(initial_value) {}
///
/// Increment a reference count.
/// Returns the previous value of the count.
///
int Increment() { return Increment(1); }
///
/// Increment a reference count by "increment", which must exceed 0.
/// Returns the previous value of the count.
///
int Increment(int increment) {
return ref_count_.fetch_add(increment, std::memory_order_relaxed);
}
///
/// Decrement a reference count, and return whether the result is non-zero.
/// Insert barriers to ensure that state written before the reference count
/// became zero will be visible to a thread that has just made the count zero.
///
bool Decrement() {
// TODO(jbroman): Technically this doesn't need to be an acquire operation
// unless the result is 1 (i.e., the ref count did indeed reach zero).
// However, there are toolchain issues that make that not work as well at
// present (notably TSAN doesn't like it).
return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
}
///
/// Return whether the reference count is one. If the reference count is used
/// in the conventional way, a refrerence count of 1 implies that the current
/// thread owns the reference and no other thread shares it. This call
/// performs the test for a reference count of one, and performs the memory
/// barrier needed for the owning thread to act on the object, knowing that it
/// has exclusive access to the object.
///
bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; }
///
/// Return whether the reference count is zero. With conventional object
/// referencing counting, the object will be destroyed, so the reference count
/// should never be zero. Hence this is generally used for a debug check.
///
bool IsZero() const {
return ref_count_.load(std::memory_order_acquire) == 0;
}
///
/// Returns the current reference count (with no barriers). This is subtle,
/// and should be used only for debugging.
///
int SubtleRefCountForDebug() const {
return ref_count_.load(std::memory_order_relaxed);
}
private:
std::atomic_int ref_count_;
};
} // namespace base
#endif // !USING_CHROMIUM_INCLUDES
#endif // CEF_INCLUDE_BASE_CEF_ATOMIC_REF_COUNT_H_