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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// 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.
#include <atomic>
#include <cstdint>
#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/internal/unscaledcycleclock.h"
namespace absl {
namespace base_internal {
using CycleClockSourceFunc = int64_t (*)();
// -----------------------------------------------------------------------------
// CycleClock
// -----------------------------------------------------------------------------
class CycleClock {
// 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();
static CycleClockSourceFunc LoadCycleClockSource();
#ifdef NDEBUG
// Not debug mode and the UnscaledCycleClock frequency is the CPU
// frequency. Scale the CycleClock to prevent overflow if someone
// tries to represent the time as cycles since the Unix epoch.
static constexpr int32_t kShift = 1;
// Not debug mode and the UnscaledCycleClock isn't operating at the
// raw CPU frequency. There is no need to do any scaling, so don't
// needlessly sacrifice precision.
static constexpr int32_t kShift = 0;
#else // NDEBUG
// In debug mode use a different shift to discourage depending on a
// particular shift value.
static constexpr int32_t kShift = 2;
#endif // NDEBUG
static constexpr double kFrequencyScale = 1.0 / (1 << kShift);
ABSL_CONST_INIT static std::atomic<CycleClockSourceFunc> cycle_clock_source_;
CycleClock() = delete; // no instances
CycleClock(const CycleClock&) = delete;
CycleClock& operator=(const CycleClock&) = delete;
friend class CycleClockSource;
class CycleClockSource {
// 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);
inline CycleClockSourceFunc CycleClock::LoadCycleClockSource() {
#if !defined(__x86_64__)
// Optimize for the common case (no callback) by first doing a relaxed load;
// this is significantly faster on non-x86 platforms.
if (cycle_clock_source_.load(std::memory_order_relaxed) == nullptr) {
return nullptr;
#endif // !defined(__x86_64__)
// This corresponds to the store(std::memory_order_release) in
// CycleClockSource::Register, and makes sure that any updates made prior to
// registering the callback are visible to this thread before the callback
// is invoked.
return cycle_clock_source_.load(std::memory_order_acquire);
// Accessing globals in inlined code in Window DLLs is problematic.
#ifndef _WIN32
inline int64_t CycleClock::Now() {
auto fn = LoadCycleClockSource();
if (fn == nullptr) {
return base_internal::UnscaledCycleClock::Now() >> kShift;
return fn() >> kShift;
inline double CycleClock::Frequency() {
return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();
} // namespace base_internal
} // namespace absl