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// Copyright 2022 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <atomic>
#include <type_traits>
namespace ipcz {
// MonitoredAtomic is a trivial wrapper around around an atomic unsigned
// integral value, with the high bit reserved for primitive communication
// between one producer and any number of concurrent consumers of the value.
// Consumers can atomically query the value while simultaneously signaling that
// they want to be notified about the next time the value changes. Producers can
// atomically update the value while simulataneously querying (and resetting)
// the consumer's interest in being notified about the change.
template <typename T>
class MonitoredAtomic {
static_assert(std::is_integral_v<T> && std::is_unsigned_v<T>,
"MonitoredAtomic requires an unsigned integral type");
struct State {
T value;
bool monitored;
static constexpr T kMaxValue = std::numeric_limits<T>::max() >> 1;
static constexpr T kMonitorBit = kMaxValue + 1;
MonitoredAtomic() noexcept = default;
explicit MonitoredAtomic(T value) noexcept : value_(value) {}
// Returns a best-effort snapshot of the most recent underlying value. If
// `monitor` is true in `options`, then the stored value is also atomically
// flagged for monitoring.
struct QueryOptions {
bool monitor;
State Query(const QueryOptions& options) {
T value = value_.load(std::memory_order_relaxed);
while (options.monitor && !IsMonitored(value) &&
!value_.compare_exchange_weak(value, Monitored(value),
std::memory_order_relaxed)) {
return {.value = Unmonitored(value), .monitored = IsMonitored(value)};
// Stores a new underlying value, resetting the monitor bit if it was set.
// Returns a boolean indicating whether the monitor bit was set.
[[nodiscard]] bool UpdateValueAndResetMonitor(T value) {
T old_value = value_.load(std::memory_order_relaxed);
while (value != old_value &&
!value_.compare_exchange_weak(old_value, value,
std::memory_order_relaxed)) {
return IsMonitored(old_value);
static bool IsMonitored(T value) { return (value & kMonitorBit) != 0; }
static T Monitored(T value) { return value | kMonitorBit; }
static T Unmonitored(T value) { return value & kMaxValue; }
std::atomic<T> value_{0};
} // namespace ipcz