// // Copyright 2017 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. // // ----------------------------------------------------------------------------- // File: cycleclock.h // ----------------------------------------------------------------------------- // // This header file defines a `CycleClock`, which yields the value and frequency // of a cycle counter that increments at a rate that is approximately constant. // // NOTE: // // The cycle counter frequency is not necessarily related to the core clock // frequency and should not be treated as such. That is, `CycleClock` cycles are // not necessarily "CPU cycles" and code should not rely on that behavior, even // if experimentally observed. // // An arbitrary offset may have been added to the counter at power on. // // On some platforms, the rate and offset of the counter may differ // slightly when read from different CPUs of a multiprocessor. Usually, // we try to ensure that the operating system adjusts values periodically // so that values agree approximately. If you need stronger guarantees, // consider using alternate interfaces. // // The CPU is not required to maintain the ordering of a cycle counter read // with respect to surrounding instructions. #ifndef ABSL_BASE_INTERNAL_CYCLECLOCK_H_ #define ABSL_BASE_INTERNAL_CYCLECLOCK_H_ #include <cstdint> #include "absl/base/config.h" namespace absl { ABSL_NAMESPACE_BEGIN namespace base_internal { // ----------------------------------------------------------------------------- // CycleClock // ----------------------------------------------------------------------------- class CycleClock { public: // CycleClock::Now() // // Returns the value of a cycle counter that counts at a rate that is // approximately constant. static int64_t Now(); // CycleClock::Frequency() // // Returns the amount by which `CycleClock::Now()` increases per second. Note // that this value may not necessarily match the core CPU clock frequency. static double Frequency(); private: CycleClock() = delete; // no instances CycleClock(const CycleClock&) = delete; CycleClock& operator=(const CycleClock&) = delete; }; using CycleClockSourceFunc = int64_t (*)(); class CycleClockSource { private: // CycleClockSource::Register() // // Register a function that provides an alternate source for the unscaled CPU // cycle count value. The source function must be async signal safe, must not // call CycleClock::Now(), and must have a frequency that matches that of the // unscaled clock used by CycleClock. A nullptr value resets CycleClock to use // the default source. static void Register(CycleClockSourceFunc source); }; } // namespace base_internal ABSL_NAMESPACE_END } // namespace absl #endif // ABSL_BASE_INTERNAL_CYCLECLOCK_H_