70 lines
2.6 KiB
C++
70 lines
2.6 KiB
C++
// Copyright (c) 2011 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// This is a low level implementation of atomic semantics for reference
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// counting. Please use base/memory/ref_counted.h directly instead.
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#ifndef BASE_ATOMIC_REF_COUNT_H_
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#define BASE_ATOMIC_REF_COUNT_H_
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#include <atomic>
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namespace base {
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class AtomicRefCount {
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public:
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constexpr AtomicRefCount() : ref_count_(0) {}
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explicit constexpr AtomicRefCount(int initial_value)
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: ref_count_(initial_value) {}
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// Increment a reference count.
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// Returns the previous value of the count.
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int Increment() { return Increment(1); }
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// Increment a reference count by "increment", which must exceed 0.
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// Returns the previous value of the count.
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int Increment(int increment) {
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return ref_count_.fetch_add(increment, std::memory_order_relaxed);
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}
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// Decrement a reference count, and return whether the result is non-zero.
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// Insert barriers to ensure that state written before the reference count
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// became zero will be visible to a thread that has just made the count zero.
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bool Decrement() {
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// TODO(jbroman): Technically this doesn't need to be an acquire operation
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// unless the result is 1 (i.e., the ref count did indeed reach zero).
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// However, there are toolchain issues that make that not work as well at
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// present (notably TSAN doesn't like it).
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return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
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}
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// Return whether the reference count is one. If the reference count is used
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// in the conventional way, a refrerence count of 1 implies that the current
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// thread owns the reference and no other thread shares it. This call
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// performs the test for a reference count of one, and performs the memory
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// barrier needed for the owning thread to act on the object, knowing that it
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// has exclusive access to the object.
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bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; }
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// Return whether the reference count is zero. With conventional object
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// referencing counting, the object will be destroyed, so the reference count
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// should never be zero. Hence this is generally used for a debug check.
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bool IsZero() const {
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return ref_count_.load(std::memory_order_acquire) == 0;
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}
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// Returns the current reference count (with no barriers). This is subtle, and
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// should be used only for debugging.
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int SubtleRefCountForDebug() const {
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return ref_count_.load(std::memory_order_relaxed);
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}
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private:
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std::atomic_int ref_count_;
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};
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} // namespace base
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#endif // BASE_ATOMIC_REF_COUNT_H_
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