Abseil Common Libraries (C++) (grcp 依赖)
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91 lines
2.7 KiB
91 lines
2.7 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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// http://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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// The implementation of CycleClock::Frequency. |
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// |
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// NOTE: only i386 and x86_64 have been well tested. |
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// PPC, sparc, alpha, and ia64 are based on |
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// http://peter.kuscsik.com/wordpress/?p=14 |
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// with modifications by m3b. See also |
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// https://setisvn.ssl.berkeley.edu/svn/lib/fftw-3.0.1/kernel/cycle.h |
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#include "absl/base/internal/cycleclock.h" |
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#include <atomic> |
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#include <chrono> // NOLINT(build/c++11) |
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#include "absl/base/internal/unscaledcycleclock.h" |
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namespace absl { |
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namespace base_internal { |
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#if ABSL_USE_UNSCALED_CYCLECLOCK |
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namespace { |
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#ifdef NDEBUG |
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#ifdef ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY |
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// Not debug mode and the UnscaledCycleClock frequency is the CPU |
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// frequency. Scale the CycleClock to prevent overflow if someone |
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// tries to represent the time as cycles since the Unix epoch. |
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static constexpr int32_t kShift = 1; |
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#else |
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// Not debug mode and the UnscaledCycleClock isn't operating at the |
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// raw CPU frequency. There is no need to do any scaling, so don't |
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// needlessly sacrifice precision. |
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static constexpr int32_t kShift = 0; |
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#endif |
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#else |
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// In debug mode use a different shift to discourage depending on a |
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// particular shift value. |
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static constexpr int32_t kShift = 2; |
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#endif |
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static constexpr double kFrequencyScale = 1.0 / (1 << kShift); |
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static std::atomic<CycleClockSourceFunc> cycle_clock_source; |
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} // namespace |
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int64_t CycleClock::Now() { |
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auto fn = cycle_clock_source.load(std::memory_order_relaxed); |
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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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double CycleClock::Frequency() { |
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return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency(); |
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} |
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void CycleClockSource::Register(CycleClockSourceFunc source) { |
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cycle_clock_source.store(source, std::memory_order_relaxed); |
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} |
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#else |
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int64_t CycleClock::Now() { |
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return std::chrono::duration_cast<std::chrono::nanoseconds>( |
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std::chrono::steady_clock::now().time_since_epoch()) |
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.count(); |
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} |
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double CycleClock::Frequency() { |
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return 1e9; |
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} |
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#endif |
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} // namespace base_internal |
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} // namespace absl
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