Abseil Common Libraries (C++) (grcp 依赖)
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90 lines
3.2 KiB
90 lines
3.2 KiB
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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 <cstdint> |
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namespace absl { |
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namespace base_internal { |
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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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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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}; |
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using CycleClockSourceFunc = int64_t (*)(); |
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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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} // namespace base_internal |
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} // namespace absl |
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#endif // ABSL_BASE_INTERNAL_CYCLECLOCK_H_
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