Abseil Common Libraries (C++) (grcp 依赖)
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96 lines
3.1 KiB
96 lines
3.1 KiB
// 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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// UnscaledCycleClock |
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// An UnscaledCycleClock yields the value and frequency of a cycle counter |
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// that increments at a rate that is approximately constant. |
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// This class is for internal use only, you should consider using CycleClock |
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// instead. |
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// |
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// Notes: |
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// The cycle counter frequency is not necessarily the core clock frequency. |
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// That is, CycleCounter cycles are not necessarily "CPU cycles". |
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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_UNSCALEDCYCLECLOCK_H_ |
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#define ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_ |
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#include <cstdint> |
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#if defined(__APPLE__) |
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#include <TargetConditionals.h> |
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#endif |
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#include "absl/base/config.h" |
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#include "absl/base/internal/unscaledcycleclock_config.h" |
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#if ABSL_USE_UNSCALED_CYCLECLOCK |
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namespace absl { |
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ABSL_NAMESPACE_BEGIN |
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namespace time_internal { |
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class UnscaledCycleClockWrapperForGetCurrentTime; |
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} // namespace time_internal |
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namespace base_internal { |
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class CycleClock; |
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class UnscaledCycleClockWrapperForInitializeFrequency; |
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class UnscaledCycleClock { |
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private: |
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UnscaledCycleClock() = delete; |
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// Return 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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// Return the how much UnscaledCycleClock::Now() increases per second. |
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// This is not necessarily the core CPU clock frequency. |
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// It may be the nominal value report by the kernel, rather than a measured |
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// value. |
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static double Frequency(); |
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// Allowed users |
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friend class base_internal::CycleClock; |
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friend class time_internal::UnscaledCycleClockWrapperForGetCurrentTime; |
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friend class base_internal::UnscaledCycleClockWrapperForInitializeFrequency; |
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}; |
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#if defined(__x86_64__) |
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inline int64_t UnscaledCycleClock::Now() { |
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uint64_t low, high; |
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__asm__ volatile("rdtsc" : "=a"(low), "=d"(high)); |
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return static_cast<int64_t>((high << 32) | low); |
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} |
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#endif |
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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_USE_UNSCALED_CYCLECLOCK |
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#endif // ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_
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