Add 'third_party/abseil_cpp/' from commit '768eb2ca2857342673fcd462792ce04b8bac3fa3' r/781

git-subtree-dir: third_party/abseil_cpp
git-subtree-mainline: ffb2ae54beb5796cd408fbe15d2d2da09ff37adf
git-subtree-split: 768eb2ca2857342673fcd462792ce04b8bac3fa3

1 files changed, 107 insertions, 0 deletions

diff --git a/third_party/abseil_cpp/absl/base/internal/cycleclock.cc b/third_party/abseil_cpp/absl/base/internal/cycleclock.cc
new file mode 100644
index 000000000000..0e65005b8914
--- /dev/null
+++ b/third_party/abseil_cpp/absl/base/internal/cycleclock.cc
@@ -0,0 +1,107 @@
+// 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