fc8dc48020
git-subtree-dir: third_party/abseil_cpp git-subtree-mainline:ffb2ae54begit-subtree-split:768eb2ca28
107 lines
3.3 KiB
C++
107 lines
3.3 KiB
C++
// 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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// 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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ABSL_NAMESPACE_BEGIN
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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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CycleClockSourceFunc LoadCycleClockSource() {
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// Optimize for the common case (no callback) by first doing a relaxed load;
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// this is significantly faster on non-x86 platforms.
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if (cycle_clock_source.load(std::memory_order_relaxed) == nullptr) {
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return nullptr;
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}
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// This corresponds to the store(std::memory_order_release) in
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// CycleClockSource::Register, and makes sure that any updates made prior to
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// registering the callback are visible to this thread before the callback is
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// invoked.
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return cycle_clock_source.load(std::memory_order_acquire);
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}
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} // namespace
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int64_t CycleClock::Now() {
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auto fn = LoadCycleClockSource();
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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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// Corresponds to the load(std::memory_order_acquire) in LoadCycleClockSource.
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cycle_clock_source.store(source, std::memory_order_release);
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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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ABSL_NAMESPACE_END
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} // namespace absl
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