1 // Copyright (c) 2012 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // For atomic operations on reference counts, see atomic_refcount.h. 6 // For atomic operations on sequence numbers, see atomic_sequence_num.h. 7 8 // The routines exported by this module are subtle. If you use them, even if 9 // you get the code right, it will depend on careful reasoning about atomicity 10 // and memory ordering; it will be less readable, and harder to maintain. If 11 // you plan to use these routines, you should have a good reason, such as solid 12 // evidence that performance would otherwise suffer, or there being no 13 // alternative. You should assume only properties explicitly guaranteed by the 14 // specifications in this file. You are almost certainly _not_ writing code 15 // just for the x86; if you assume x86 semantics, x86 hardware bugs and 16 // implementations on other archtectures will cause your code to break. If you 17 // do not know what you are doing, avoid these routines, and use a Mutex. 18 // 19 // It is incorrect to make direct assignments to/from an atomic variable. 20 // You should use one of the Load or Store routines. The NoBarrier 21 // versions are provided when no barriers are needed: 22 // NoBarrier_Store() 23 // NoBarrier_Load() 24 // Although there are currently no compiler enforcement, you are encouraged 25 // to use these. 26 // 27 28 #ifndef BASE_ATOMICOPS_H_ 29 #define BASE_ATOMICOPS_H_ 30 31 #include <stdint.h> 32 33 // Small C++ header which defines implementation specific macros used to 34 // identify the STL implementation. 35 // - libc++: captures __config for _LIBCPP_VERSION 36 // - libstdc++: captures bits/c++config.h for __GLIBCXX__ 37 #include <cstddef> 38 39 #include "base/base_export.h" 40 #include "build/build_config.h" 41 42 #if defined(OS_WIN) && defined(ARCH_CPU_64_BITS) 43 // windows.h #defines this (only on x64). This causes problems because the 44 // public API also uses MemoryBarrier at the public name for this fence. So, on 45 // X64, undef it, and call its documented 46 // (http://msdn.microsoft.com/en-us/library/windows/desktop/ms684208.aspx) 47 // implementation directly. 48 #undef MemoryBarrier 49 #endif 50 51 namespace base { 52 namespace subtle { 53 54 typedef int32_t Atomic32; 55 #ifdef ARCH_CPU_64_BITS 56 // We need to be able to go between Atomic64 and AtomicWord implicitly. This 57 // means Atomic64 and AtomicWord should be the same type on 64-bit. 58 #if defined(__ILP32__) || defined(OS_NACL) 59 // NaCl's intptr_t is not actually 64-bits on 64-bit! 60 // http://code.google.com/p/nativeclient/issues/detail?id=1162 61 typedef int64_t Atomic64; 62 #else 63 typedef intptr_t Atomic64; 64 #endif 65 #endif 66 67 // Use AtomicWord for a machine-sized pointer. It will use the Atomic32 or 68 // Atomic64 routines below, depending on your architecture. 69 typedef intptr_t AtomicWord; 70 71 // Atomically execute: 72 // result = *ptr; 73 // if (*ptr == old_value) 74 // *ptr = new_value; 75 // return result; 76 // 77 // I.e., replace "*ptr" with "new_value" if "*ptr" used to be "old_value". 78 // Always return the old value of "*ptr" 79 // 80 // This routine implies no memory barriers. 81 Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr, 82 Atomic32 old_value, 83 Atomic32 new_value); 84 85 // Atomically store new_value into *ptr, returning the previous value held in 86 // *ptr. This routine implies no memory barriers. 87 Atomic32 NoBarrier_AtomicExchange(volatile Atomic32* ptr, Atomic32 new_value); 88 89 // Atomically increment *ptr by "increment". Returns the new value of 90 // *ptr with the increment applied. This routine implies no memory barriers. 91 Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr, Atomic32 increment); 92 93 Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr, 94 Atomic32 increment); 95 96 // These following lower-level operations are typically useful only to people 97 // implementing higher-level synchronization operations like spinlocks, 98 // mutexes, and condition-variables. They combine CompareAndSwap(), a load, or 99 // a store with appropriate memory-ordering instructions. "Acquire" operations 100 // ensure that no later memory access can be reordered ahead of the operation. 101 // "Release" operations ensure that no previous memory access can be reordered 102 // after the operation. "Barrier" operations have both "Acquire" and "Release" 103 // semantics. A MemoryBarrier() has "Barrier" semantics, but does no memory 104 // access. 105 Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr, 106 Atomic32 old_value, 107 Atomic32 new_value); 108 Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr, 109 Atomic32 old_value, 110 Atomic32 new_value); 111 112 void MemoryBarrier(); 113 void NoBarrier_Store(volatile Atomic32* ptr, Atomic32 value); 114 void Acquire_Store(volatile Atomic32* ptr, Atomic32 value); 115 void Release_Store(volatile Atomic32* ptr, Atomic32 value); 116 117 Atomic32 NoBarrier_Load(volatile const Atomic32* ptr); 118 Atomic32 Acquire_Load(volatile const Atomic32* ptr); 119 Atomic32 Release_Load(volatile const Atomic32* ptr); 120 121 // 64-bit atomic operations (only available on 64-bit processors). 122 #ifdef ARCH_CPU_64_BITS 123 Atomic64 NoBarrier_CompareAndSwap(volatile Atomic64* ptr, 124 Atomic64 old_value, 125 Atomic64 new_value); 126 Atomic64 NoBarrier_AtomicExchange(volatile Atomic64* ptr, Atomic64 new_value); 127 Atomic64 NoBarrier_AtomicIncrement(volatile Atomic64* ptr, Atomic64 increment); 128 Atomic64 Barrier_AtomicIncrement(volatile Atomic64* ptr, Atomic64 increment); 129 130 Atomic64 Acquire_CompareAndSwap(volatile Atomic64* ptr, 131 Atomic64 old_value, 132 Atomic64 new_value); 133 Atomic64 Release_CompareAndSwap(volatile Atomic64* ptr, 134 Atomic64 old_value, 135 Atomic64 new_value); 136 void NoBarrier_Store(volatile Atomic64* ptr, Atomic64 value); 137 void Acquire_Store(volatile Atomic64* ptr, Atomic64 value); 138 void Release_Store(volatile Atomic64* ptr, Atomic64 value); 139 Atomic64 NoBarrier_Load(volatile const Atomic64* ptr); 140 Atomic64 Acquire_Load(volatile const Atomic64* ptr); 141 Atomic64 Release_Load(volatile const Atomic64* ptr); 142 #endif // ARCH_CPU_64_BITS 143 144 } // namespace subtle 145 } // namespace base 146 147 #if defined(OS_WIN) 148 // TODO(jfb): The MSVC header includes windows.h, which other files end up 149 // relying on. Fix this as part of crbug.com/559247. 150 # include "base/atomicops_internals_x86_msvc.h" 151 #else 152 # include "base/atomicops_internals_portable.h" 153 #endif 154 155 // On some platforms we need additional declarations to make 156 // AtomicWord compatible with our other Atomic* types. 157 #if defined(OS_MACOSX) || defined(OS_OPENBSD) 158 #include "base/atomicops_internals_atomicword_compat.h" 159 #endif 160 161 #endif // BASE_ATOMICOPS_H_ 162
