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95 lines
3.3 KiB
95 lines
3.3 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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#include "absl/base/config.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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// -----------------------------------------------------------------------------
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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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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_BASE_INTERNAL_CYCLECLOCK_H_
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