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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.
// The implementation of CycleClock::Frequency.
// NOTE: only i386 and x86_64 have been well tested.
// PPC, sparc, alpha, and ia64 are based on
// with modifications by m3b. See also
#include "absl/base/internal/cycleclock.h"
#include <atomic>
#include <chrono> // NOLINT(build/c++11)
#include "absl/base/internal/unscaledcycleclock.h"
namespace absl {
namespace base_internal {
namespace {
#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;
// In debug mode use a different shift to discourage depending on a
// particular shift value.
static constexpr int32_t kShift = 2;
static constexpr double kFrequencyScale = 1.0 / (1 << kShift);
static std::atomic<CycleClockSourceFunc> cycle_clock_source;
CycleClockSourceFunc LoadCycleClockSource() {
// 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;
// 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);
} // namespace
int64_t CycleClock::Now() {
auto fn = LoadCycleClockSource();
if (fn == nullptr) {
return base_internal::UnscaledCycleClock::Now() >> kShift;
return fn() >> kShift;
double CycleClock::Frequency() {
return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();
void CycleClockSource::Register(CycleClockSourceFunc source) {
// Corresponds to the load(std::memory_order_acquire) in LoadCycleClockSource., std::memory_order_release);
int64_t CycleClock::Now() {
return std::chrono::duration_cast<std::chrono::nanoseconds>(
double CycleClock::Frequency() {
return 1e9;
} // namespace base_internal
} // namespace absl