// Copyright 2021 The Abseil Authors // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // https://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. #ifndef ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_ #define ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_ #include <atomic> #include <cstdint> #include "absl/base/config.h" namespace absl { ABSL_NAMESPACE_BEGIN namespace cord_internal { // CordzUpdateTracker tracks counters for Cord update methods. // // The purpose of CordzUpdateTracker is to track the number of calls to methods // updating Cord data for sampled cords. The class internally uses 'lossy' // atomic operations: Cord is thread-compatible, so there is no need to // synchronize updates. However, Cordz collection threads may call 'Value()' at // any point, so the class needs to provide thread safe access. // // This class is thread-safe. But as per above comments, all non-const methods // should be used single-threaded only: updates are thread-safe but lossy. class CordzUpdateTracker { public: // Tracked update methods. enum MethodIdentifier { kUnknown, kAppendBuffer, kAppendCord, kAppendExternalMemory, kAppendString, kAssignCord, kAssignString, kClear, kConstructorCord, kConstructorString, kCordReader, kFlatten, kGetAppendRegion, kMakeCordFromExternal, kMoveAppendCord, kMoveAssignCord, kMovePrependCord, kPrependBuffer, kPrependCord, kPrependString, kRemovePrefix, kRemoveSuffix, kSubCord, // kNumMethods defines the number of entries: must be the last entry. kNumMethods, }; // Constructs a new instance. All counters are zero-initialized. constexpr CordzUpdateTracker() noexcept : values_{} {} // Copy constructs a new instance. CordzUpdateTracker(const CordzUpdateTracker& rhs) noexcept { *this = rhs; } // Assigns the provided value to this instance. CordzUpdateTracker& operator=(const CordzUpdateTracker& rhs) noexcept { for (int i = 0; i < kNumMethods; ++i) { values_[i].store(rhs.values_[i].load(std::memory_order_relaxed), std::memory_order_relaxed); } return *this; } // Returns the value for the specified method. int64_t Value(MethodIdentifier method) const { return values_[method].load(std::memory_order_relaxed); } // Increases the value for the specified method by `n` void LossyAdd(MethodIdentifier method, int64_t n = 1) { auto& value = values_[method]; value.store(value.load(std::memory_order_relaxed) + n, std::memory_order_relaxed); } // Adds all the values from `src` to this instance void LossyAdd(const CordzUpdateTracker& src) { for (int i = 0; i < kNumMethods; ++i) { MethodIdentifier method = static_cast<MethodIdentifier>(i); if (int64_t value = src.Value(method)) { LossyAdd(method, value); } } } private: // Until C++20 std::atomic is not constexpr default-constructible, so we need // a wrapper for this class to be constexpr constructible. class Counter : public std::atomic<int64_t> { public: constexpr Counter() noexcept : std::atomic<int64_t>(0) {} }; Counter values_[kNumMethods]; }; } // namespace cord_internal ABSL_NAMESPACE_END } // namespace absl #endif // ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_