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 ABSL_NAMESPACE_BEGIN 90 namespace time_internal { 91 class UnscaledCycleClockWrapperForGetCurrentTime; 92 } // namespace time_internal 93 94 namespace base_internal { 95 class CycleClock; 96 class UnscaledCycleClockWrapperForInitializeFrequency; 97 98 class UnscaledCycleClock { 99 private: 100 UnscaledCycleClock() = delete; 101 102 // Return the value of a cycle counter that counts at a rate that is 103 // approximately constant. 104 static int64_t Now(); 105 106 // Return the how much UnscaledCycleClock::Now() increases per second. 107 // This is not necessarily the core CPU clock frequency. 108 // It may be the nominal value report by the kernel, rather than a measured 109 // value. 110 static double Frequency(); 111 112 // Whitelisted friends. 113 friend class base_internal::CycleClock; 114 friend class time_internal::UnscaledCycleClockWrapperForGetCurrentTime; 115 friend class base_internal::UnscaledCycleClockWrapperForInitializeFrequency; 116 }; 117 118 } // namespace base_internal 119 ABSL_NAMESPACE_END 120 } // namespace absl 121 122 #endif // ABSL_USE_UNSCALED_CYCLECLOCK 123 124 #endif // ABSL_BASE_INTERNAL_UNSCALEDCYCLECLOCK_H_ 125