Initial Commit

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diff --git a/absl/base/internal/cycleclock.cc b/absl/base/internal/cycleclock.cc
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+// 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
+//
+//      http://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 <chrono>  // NOLINT(build/c++11)
+
+#include "absl/base/internal/unscaledcycleclock.h"
+
+namespace absl {
+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);
+
+}  // namespace
+
+int64_t CycleClock::Now() {
+  return base_internal::UnscaledCycleClock::Now() >> kShift;
+}
+
+double CycleClock::Frequency() {
+  return kFrequencyScale * base_internal::UnscaledCycleClock::Frequency();
+}
+
+#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
+}  // namespace absl