unscaledcycleclock.h
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1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // UnscaledCycleClock
16 // An UnscaledCycleClock yields the value and frequency of a cycle counter
17 // that increments at a rate that is approximately constant.
18 // This class is for internal / whitelisted use only, you should consider
19 // using CycleClock instead.
20 //
21 // Notes:
22 // The cycle counter frequency is not necessarily the core clock frequency.
23 // That is, CycleCounter cycles are not necessarily "CPU cycles".
24 //
25 // An arbitrary offset may have been added to the counter at power on.
26 //
27 // On some platforms, the rate and offset of the counter may differ
28 // slightly when read from different CPUs of a multiprocessor. Usually,
29 // we try to ensure that the operating system adjusts values periodically
30 // so that values agree approximately. If you need stronger guarantees,
31 // consider using alternate interfaces.
32 //
33 // The CPU is not required to maintain the ordering of a cycle counter read
34 // with respect to surrounding instructions.
35 
36 #ifndef ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_
37 #define ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_
38 
39 #include <cstdint>
40 
41 #if defined(__APPLE__)
42 #include <TargetConditionals.h>
43 #endif
44 
45 #include "absl/base/port.h"
46 
47 // The following platforms have an implementation of a hardware counter.
48 #if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__) || \
49  defined(__powerpc__) || defined(__ppc__) || \
50  defined(_M_IX86) || defined(_M_X64)
51 #define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 1
52 #else
53 #define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 0
54 #endif
55 
56 // The following platforms often disable access to the hardware
57 // counter (through a sandbox) even if the underlying hardware has a
58 // usable counter. The CycleTimer interface also requires a *scaled*
59 // CycleClock that runs at atleast 1 MHz. We've found some Android
60 // ARM64 devices where this is not the case, so we disable it by
61 // default on Android ARM64.
62 #if defined(__native_client__) || \
63  (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || \
64  (defined(__ANDROID__) && defined(__aarch64__))
65 #define ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT 0
66 #else
67 #define ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT 1
68 #endif
69 
70 // UnscaledCycleClock is an optional internal feature.
71 // Use "#if ABSL_USE_UNSCALED_CYCLECLOCK" to test for its presence.
72 // Can be overridden at compile-time via -DABSL_USE_UNSCALED_CYCLECLOCK=0|1
73 #if !defined(ABSL_USE_UNSCALED_CYCLECLOCK)
74 #define ABSL_USE_UNSCALED_CYCLECLOCK \
75  (ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION && \
76  ABSL_USE_UNSCALED_CYCLECLOCK_DEFAULT)
77 #endif
78 
79 #if ABSL_USE_UNSCALED_CYCLECLOCK
80 
81 // This macro can be used to test if UnscaledCycleClock::Frequency()
82 // is NominalCPUFrequency() on a particular platform.
83 #if (defined(__i386__) || defined(__x86_64__) || \
84  defined(_M_IX86) || defined(_M_X64))
85 #define ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
86 #endif
87 
88 namespace absl {
89 namespace time_internal {
90 class UnscaledCycleClockWrapperForGetCurrentTime;
91 } // namespace time_internal
92 
93 namespace base_internal {
94 class CycleClock;
95 class UnscaledCycleClockWrapperForInitializeFrequency;
96 
97 class UnscaledCycleClock {
98  private:
99  UnscaledCycleClock() = delete;
100 
101  // Return the value of a cycle counter that counts at a rate that is
102  // approximately constant.
103  static int64_t Now();
104 
105  // Return the how much UnscaledCycleClock::Now() increases per second.
106  // This is not necessarily the core CPU clock frequency.
107  // It may be the nominal value report by the kernel, rather than a measured
108  // value.
109  static double Frequency();
110 
111  // Whitelisted friends.
112  friend class base_internal::CycleClock;
113  friend class time_internal::UnscaledCycleClockWrapperForGetCurrentTime;
114  friend class base_internal::UnscaledCycleClockWrapperForInitializeFrequency;
115 };
116 
117 } // namespace base_internal
118 } // namespace absl
119 
120 #endif // ABSL_USE_UNSCALED_CYCLECLOCK
121 
122 #endif // ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_
Time Now()
Definition: clock.cc:37
Definition: algorithm.h:29


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autogenerated on Wed Jun 19 2019 19:19:58