cycleclock.h
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1 //
2 // Copyright 2017 The Abseil Authors.
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 //
8 // https://www.apache.org/licenses/LICENSE-2.0
9 //
10 // Unless required by applicable law or agreed to in writing, software
11 // distributed under the License is distributed on an "AS IS" BASIS,
12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 // See the License for the specific language governing permissions and
14 // limitations under the License.
15 //
16 
17 // -----------------------------------------------------------------------------
18 // File: cycleclock.h
19 // -----------------------------------------------------------------------------
20 //
21 // This header file defines a `CycleClock`, which yields the value and frequency
22 // of a cycle counter that increments at a rate that is approximately constant.
23 //
24 // NOTE:
25 //
26 // The cycle counter frequency is not necessarily related to the core clock
27 // frequency and should not be treated as such. That is, `CycleClock` cycles are
28 // not necessarily "CPU cycles" and code should not rely on that behavior, even
29 // if experimentally observed.
30 //
31 // An arbitrary offset may have been added to the counter at power on.
32 //
33 // On some platforms, the rate and offset of the counter may differ
34 // slightly when read from different CPUs of a multiprocessor. Usually,
35 // we try to ensure that the operating system adjusts values periodically
36 // so that values agree approximately. If you need stronger guarantees,
37 // consider using alternate interfaces.
38 //
39 // The CPU is not required to maintain the ordering of a cycle counter read
40 // with respect to surrounding instructions.
41 
42 #ifndef ABSL_BASE_INTERNAL_CYCLECLOCK_H_
43 #define ABSL_BASE_INTERNAL_CYCLECLOCK_H_
44 
45 #include <cstdint>
46 
47 namespace absl {
48 namespace base_internal {
49 
50 // -----------------------------------------------------------------------------
51 // CycleClock
52 // -----------------------------------------------------------------------------
53 class CycleClock {
54  public:
55  // CycleClock::Now()
56  //
57  // Returns the value of a cycle counter that counts at a rate that is
58  // approximately constant.
59  static int64_t Now();
60 
61  // CycleClock::Frequency()
62  //
63  // Returns the amount by which `CycleClock::Now()` increases per second. Note
64  // that this value may not necessarily match the core CPU clock frequency.
65  static double Frequency();
66 
67  private:
68  CycleClock() = delete; // no instances
69  CycleClock(const CycleClock&) = delete;
70  CycleClock& operator=(const CycleClock&) = delete;
71 };
72 
73 using CycleClockSourceFunc = int64_t (*)();
74 
76  private:
77  // CycleClockSource::Register()
78  //
79  // Register a function that provides an alternate source for the unscaled CPU
80  // cycle count value. The source function must be async signal safe, must not
81  // call CycleClock::Now(), and must have a frequency that matches that of the
82  // unscaled clock used by CycleClock. A nullptr value resets CycleClock to use
83  // the default source.
84  static void Register(CycleClockSourceFunc source);
85 };
86 
87 } // namespace base_internal
88 } // namespace absl
89 
90 #endif // ABSL_BASE_INTERNAL_CYCLECLOCK_H_
int64_t(*)( CycleClockSourceFunc)
Definition: cycleclock.h:73
Definition: algorithm.h:29
CycleClock & operator=(const CycleClock &)=delete


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autogenerated on Wed Jun 19 2019 19:19:56