| // Copyright 2017 The Abseil Authors. |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // https://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| // The implementation of CycleClock::Frequency. |
| // |
| // NOTE: only i386 and x86_64 have been well tested. |
| // PPC, sparc, alpha, and ia64 are based on |
| // http://peter.kuscsik.com/wordpress/?p=14 |
| // with modifications by m3b. See also |
| // https://setisvn.ssl.berkeley.edu/svn/lib/fftw-3.0.1/kernel/cycle.h |
| |
| #include "absl/base/internal/cycleclock.h" |
| |
| #include <atomic> |
| #include <chrono> // NOLINT(build/c++11) |
| |
| #include "absl/base/internal/unscaledcycleclock.h" |
| |
| namespace absl { |
| ABSL_NAMESPACE_BEGIN |
| namespace base_internal { |
| |
| #if ABSL_USE_UNSCALED_CYCLECLOCK |
| |
| namespace { |
| |
| #ifdef NDEBUG |
| #ifdef ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY |
| // Not debug mode and the UnscaledCycleClock frequency is the CPU |
| // frequency. Scale the CycleClock to prevent overflow if someone |
| // tries to represent the time as cycles since the Unix epoch. |
| static constexpr int32_t kShift = 1; |
| #else |
| // Not debug mode and the UnscaledCycleClock isn't operating at the |
| // raw CPU frequency. There is no need to do any scaling, so don't |
| // needlessly sacrifice precision. |
| static constexpr int32_t kShift = 0; |
| #endif |
| #else |
| // In debug mode use a different shift to discourage depending on a |
| // particular shift value. |
| static constexpr int32_t kShift = 2; |
| #endif |
| |
| static constexpr double kFrequencyScale = 1.0 / (1 << kShift); |
| static std::atomic<CycleClockSourceFunc> cycle_clock_source; |
| |
| CycleClockSourceFunc LoadCycleClockSource() { |
| // Optimize for the common case (no callback) by first doing a relaxed load; |
| // this is significantly faster on non-x86 platforms. |
| if (cycle_clock_source.load(std::memory_order_relaxed) == nullptr) { |
| return nullptr; |
| } |
| // This corresponds to the store(std::memory_order_release) in |
| // CycleClockSource::Register, and makes sure that any updates made prior to |
| // registering the callback are visible to this thread before the callback is |
| // invoked. |
| return cycle_clock_source.load(std::memory_order_acquire); |
| } |
| |
| } // namespace |
| |
| int64_t CycleClock::Now() { |
| auto fn = LoadCycleClockSource(); |
| if (fn == nullptr) { |
| return base_internal::UnscaledCycleClock::Now() >> kShift; |
| } |
| return fn() >> kShift; |
| } |
| |
| double CycleClock::Frequency() { |
| return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency(); |
| } |
| |
| void CycleClockSource::Register(CycleClockSourceFunc source) { |
| // Corresponds to the load(std::memory_order_acquire) in LoadCycleClockSource. |
| cycle_clock_source.store(source, std::memory_order_release); |
| } |
| |
| #else |
| |
| int64_t CycleClock::Now() { |
| return std::chrono::duration_cast<std::chrono::nanoseconds>( |
| std::chrono::steady_clock::now().time_since_epoch()) |
| .count(); |
| } |
| |
| double CycleClock::Frequency() { |
| return 1e9; |
| } |
| |
| #endif |
| |
| } // namespace base_internal |
| ABSL_NAMESPACE_END |
| } // namespace absl |