//

// 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 <atomic>

#include <cstdint>

#include "absl/base/attributes.h"

#include "absl/base/config.h"

#include "absl/base/internal/cycleclock_config.h"

#include "absl/base/internal/unscaledcycleclock.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

namespace base_internal {

using CycleClockSourceFunc = int64_t (*)();

// -----------------------------------------------------------------------------

// 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:

#if ABSL_USE_UNSCALED_CYCLECLOCK

  static CycleClockSourceFunc LoadCycleClockSource();

  static constexpr int32_t kShift = kCycleClockShift;

  static constexpr double kFrequencyScale = kCycleClockFrequencyScale;

  ABSL_CONST_INIT static std::atomic<CycleClockSourceFunc> cycle_clock_source_;

#endif  //  ABSL_USE_UNSCALED_CYCLECLOC

  CycleClock() = delete;  // no instances

  CycleClock(const CycleClock&) = delete;

  CycleClock& operator=(const CycleClock&) = delete;

  friend class CycleClockSource;

};

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);

};

#if ABSL_USE_UNSCALED_CYCLECLOCK

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;

}

#endif

inline double CycleClock::Frequency() {

  return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();

}

#endif  // ABSL_USE_UNSCALED_CYCLECLOCK

}  // namespace base_internal

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_CYCLECLOCK_H_