Coverage Report

Created: 2024-09-23 06:29

/src/abseil-cpp/absl/base/internal/cycleclock.h
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//
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// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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//      https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// -----------------------------------------------------------------------------
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// File: cycleclock.h
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// -----------------------------------------------------------------------------
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//
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// This header file defines a `CycleClock`, which yields the value and frequency
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// of a cycle counter that increments at a rate that is approximately constant.
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//
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// NOTE:
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//
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// The cycle counter frequency is not necessarily related to the core clock
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// frequency and should not be treated as such. That is, `CycleClock` cycles are
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// not necessarily "CPU cycles" and code should not rely on that behavior, even
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// if experimentally observed.
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//
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// An arbitrary offset may have been added to the counter at power on.
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//
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// On some platforms, the rate and offset of the counter may differ
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// slightly when read from different CPUs of a multiprocessor. Usually,
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// we try to ensure that the operating system adjusts values periodically
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// so that values agree approximately.   If you need stronger guarantees,
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// consider using alternate interfaces.
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//
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// The CPU is not required to maintain the ordering of a cycle counter read
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// with respect to surrounding instructions.
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#ifndef ABSL_BASE_INTERNAL_CYCLECLOCK_H_
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#define ABSL_BASE_INTERNAL_CYCLECLOCK_H_
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#include <atomic>
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#include <cstdint>
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#include "absl/base/attributes.h"
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#include "absl/base/config.h"
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#include "absl/base/internal/cycleclock_config.h"
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#include "absl/base/internal/unscaledcycleclock.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace base_internal {
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using CycleClockSourceFunc = int64_t (*)();
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// -----------------------------------------------------------------------------
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// CycleClock
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// -----------------------------------------------------------------------------
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class CycleClock {
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 public:
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  // CycleClock::Now()
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  //
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  // Returns the value of a cycle counter that counts at a rate that is
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  // approximately constant.
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  static int64_t Now();
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  // CycleClock::Frequency()
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  //
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  // Returns the amount by which `CycleClock::Now()` increases per second. Note
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  // that this value may not necessarily match the core CPU clock frequency.
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  static double Frequency();
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 private:
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#if ABSL_USE_UNSCALED_CYCLECLOCK
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  static CycleClockSourceFunc LoadCycleClockSource();
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  static constexpr int32_t kShift = kCycleClockShift;
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  static constexpr double kFrequencyScale = kCycleClockFrequencyScale;
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  ABSL_CONST_INIT static std::atomic<CycleClockSourceFunc> cycle_clock_source_;
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#endif  //  ABSL_USE_UNSCALED_CYCLECLOC
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  CycleClock() = delete;  // no instances
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  CycleClock(const CycleClock&) = delete;
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  CycleClock& operator=(const CycleClock&) = delete;
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  friend class CycleClockSource;
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};
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class CycleClockSource {
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 private:
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  // CycleClockSource::Register()
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  //
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  // Register a function that provides an alternate source for the unscaled CPU
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  // cycle count value. The source function must be async signal safe, must not
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  // call CycleClock::Now(), and must have a frequency that matches that of the
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  // unscaled clock used by CycleClock. A nullptr value resets CycleClock to use
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  // the default source.
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  static void Register(CycleClockSourceFunc source);
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};
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#if ABSL_USE_UNSCALED_CYCLECLOCK
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inline CycleClockSourceFunc CycleClock::LoadCycleClockSource() {
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#if !defined(__x86_64__)
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  // Optimize for the common case (no callback) by first doing a relaxed load;
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  // this is significantly faster on non-x86 platforms.
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  if (cycle_clock_source_.load(std::memory_order_relaxed) == nullptr) {
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    return nullptr;
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  }
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#endif  // !defined(__x86_64__)
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  // This corresponds to the store(std::memory_order_release) in
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  // CycleClockSource::Register, and makes sure that any updates made prior to
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  // registering the callback are visible to this thread before the callback
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  // is invoked.
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  return cycle_clock_source_.load(std::memory_order_acquire);
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}
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// Accessing globals in inlined code in Window DLLs is problematic.
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#ifndef _WIN32
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inline int64_t CycleClock::Now() {
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  auto fn = LoadCycleClockSource();
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  if (fn == nullptr) {
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    return base_internal::UnscaledCycleClock::Now() >> kShift;
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  }
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  return fn() >> kShift;
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}
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#endif
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inline double CycleClock::Frequency() {
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  return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();
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}
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#endif  // ABSL_USE_UNSCALED_CYCLECLOCK
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}  // namespace base_internal
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ABSL_NAMESPACE_END
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}  // namespace absl
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#endif  // ABSL_BASE_INTERNAL_CYCLECLOCK_H_