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TMessagesProj/jni/voip/webrtc/absl/base/internal/unscaledcycleclock.h

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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
+//
+//      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.
+//
+// UnscaledCycleClock
+//    An UnscaledCycleClock yields the value and frequency of a cycle counter
+//    that increments at a rate that is approximately constant.
+//    This class is for internal use only, you should consider using CycleClock
+//    instead.
+//
+// Notes:
+// The cycle counter frequency is not necessarily the core clock frequency.
+// That is, CycleCounter cycles are not necessarily "CPU cycles".
+//
+// An arbitrary offset may have been added to the counter at power on.
+//
+// On some platforms, the rate and offset of the counter may differ
+// slightly when read from different CPUs of a multiprocessor.  Usually,
+// we try to ensure that the operating system adjusts values periodically
+// so that values agree approximately.   If you need stronger guarantees,
+// consider using alternate interfaces.
+//
+// The CPU is not required to maintain the ordering of a cycle counter read
+// with respect to surrounding instructions.
+
+#ifndef ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_
+#define ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_
+
+#include <cstdint>
+
+#if defined(__APPLE__)
+#include <TargetConditionals.h>
+#endif
+
+#include "absl/base/config.h"
+#include "absl/base/internal/unscaledcycleclock_config.h"
+
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace time_internal {
+class UnscaledCycleClockWrapperForGetCurrentTime;
+}  // namespace time_internal
+
+namespace base_internal {
+class CycleClock;
+class UnscaledCycleClockWrapperForInitializeFrequency;
+
+class UnscaledCycleClock {
+ private:
+  UnscaledCycleClock() = delete;
+
+  // Return the value of a cycle counter that counts at a rate that is
+  // approximately constant.
+  static int64_t Now();
+
+  // Return the how much UnscaledCycleClock::Now() increases per second.
+  // This is not necessarily the core CPU clock frequency.
+  // It may be the nominal value report by the kernel, rather than a measured
+  // value.
+  static double Frequency();
+
+  // Allowed users
+  friend class base_internal::CycleClock;
+  friend class time_internal::UnscaledCycleClockWrapperForGetCurrentTime;
+  friend class base_internal::UnscaledCycleClockWrapperForInitializeFrequency;
+};
+
+#if defined(__x86_64__)
+
+inline int64_t UnscaledCycleClock::Now() {
+  uint64_t low, high;
+  __asm__ volatile("rdtsc" : "=a"(low), "=d"(high));
+  return static_cast<int64_t>((high << 32) | low);
+}
+
+#elif defined(__aarch64__)
+
+// System timer of ARMv8 runs at a different frequency than the CPU's.
+// The frequency is fixed, typically in the range 1-50MHz.  It can be
+// read at CNTFRQ special register.  We assume the OS has set up
+// the virtual timer properly.
+inline int64_t UnscaledCycleClock::Now() {
+  int64_t virtual_timer_value;
+  asm volatile("mrs %0, cntvct_el0" : "=r"(virtual_timer_value));
+  return virtual_timer_value;
+}
+
+#endif
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_USE_UNSCALED_CYCLECLOCK
+
+#endif  // ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_