1 // Copyright 2012 the V8 project authors. All rights reserved.
2 // Redistribution and use in source and binary forms, with or without
3 // modification, are permitted provided that the following conditions are
6 // * Redistributions of source code must retain the above copyright
7 // notice, this list of conditions and the following disclaimer.
8 // * Redistributions in binary form must reproduce the above
9 // copyright notice, this list of conditions and the following
10 // disclaimer in the documentation and/or other materials provided
11 // with the distribution.
12 // * Neither the name of Google Inc. nor the names of its
13 // contributors may be used to endorse or promote products derived
14 // from this software without specific prior written permission.
16 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
17 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
18 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
19 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
20 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
21 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
22 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
26 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
28 // The LazyInstance<Type, Traits> class manages a single instance of Type,
29 // which will be lazily created on the first time it's accessed. This class is
30 // useful for places you would normally use a function-level static, but you
31 // need to have guaranteed thread-safety. The Type constructor will only ever
32 // be called once, even if two threads are racing to create the object. Get()
33 // and Pointer() will always return the same, completely initialized instance.
35 // LazyInstance is completely thread safe, assuming that you create it safely.
36 // The class was designed to be POD initialized, so it shouldn't require a
37 // static constructor. It really only makes sense to declare a LazyInstance as
38 // a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
40 // LazyInstance is similar to Singleton, except it does not have the singleton
41 // property. You can have multiple LazyInstance's of the same type, and each
42 // will manage a unique instance. It also preallocates the space for Type, as
43 // to avoid allocating the Type instance on the heap. This may help with the
44 // performance of creating the instance, and reducing heap fragmentation. This
45 // requires that Type be a complete type so we can determine the size. See
46 // notes for advanced users below for more explanations.
49 // static LazyInstance<MyClass>::type my_instance = LAZY_INSTANCE_INITIALIZER;
50 // void SomeMethod() {
51 // my_instance.Get().SomeMethod(); // MyClass::SomeMethod()
53 // MyClass* ptr = my_instance.Pointer();
54 // ptr->DoDoDo(); // MyClass::DoDoDo
57 // Additionally you can override the way your instance is constructed by
58 // providing your own trait:
60 // struct MyCreateTrait {
61 // static void Construct(MyClass* allocated_ptr) {
62 // new (allocated_ptr) MyClass(/* extra parameters... */);
65 // static LazyInstance<MyClass, MyCreateTrait>::type my_instance =
66 // LAZY_INSTANCE_INITIALIZER;
69 // - This implementation of LazyInstance is NOT THREAD-SAFE by default. See
70 // ThreadSafeInitOnceTrait declared below for that.
71 // - Lazy initialization comes with a cost. Make sure that you don't use it on
72 // critical path. Consider adding your initialization code to a function
73 // which is explicitly called once.
75 // Notes for advanced users:
76 // LazyInstance can actually be used in two different ways:
78 // - "Static mode" which is the default mode since it is the most efficient
79 // (no extra heap allocation). In this mode, the instance is statically
80 // allocated (stored in the global data section at compile time).
81 // The macro LAZY_STATIC_INSTANCE_INITIALIZER (= LAZY_INSTANCE_INITIALIZER)
82 // must be used to initialize static lazy instances.
84 // - "Dynamic mode". In this mode, the instance is dynamically allocated and
85 // constructed (using new) by default. This mode is useful if you have to
86 // deal with some code already allocating the instance for you (e.g.
87 // OS::Mutex() which returns a new private OS-dependent subclass of Mutex).
88 // The macro LAZY_DYNAMIC_INSTANCE_INITIALIZER must be used to initialize
89 // dynamic lazy instances.
91 #ifndef V8_LAZY_INSTANCE_H_
92 #define V8_LAZY_INSTANCE_H_
99 #define LAZY_STATIC_INSTANCE_INITIALIZER { V8_ONCE_INIT, {} }
100 #define LAZY_DYNAMIC_INSTANCE_INITIALIZER { V8_ONCE_INIT, 0 }
102 // Default to static mode.
103 #define LAZY_INSTANCE_INITIALIZER LAZY_STATIC_INSTANCE_INITIALIZER
106 template <typename T>
107 struct LeakyInstanceTrait {
108 static void Destroy(T* /* instance */) {}
112 // Traits that define how an instance is allocated and accessed.
114 // TODO(kalmard): __alignof__ is only defined for GCC > 4.2. Fix alignment issue
115 // on MIPS with other compilers.
116 #if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 2))
117 #define LAZY_ALIGN(x) __attribute__((aligned(__alignof__(x))))
119 #define LAZY_ALIGN(x)
122 template <typename T>
123 struct StaticallyAllocatedInstanceTrait {
124 typedef char StorageType[sizeof(T)] LAZY_ALIGN(T);
126 static T* MutableInstance(StorageType* storage) {
127 return reinterpret_cast<T*>(storage);
130 template <typename ConstructTrait>
131 static void InitStorageUsingTrait(StorageType* storage) {
132 ConstructTrait::Construct(MutableInstance(storage));
139 template <typename T>
140 struct DynamicallyAllocatedInstanceTrait {
141 typedef T* StorageType;
143 static T* MutableInstance(StorageType* storage) {
147 template <typename CreateTrait>
148 static void InitStorageUsingTrait(StorageType* storage) {
149 *storage = CreateTrait::Create();
154 template <typename T>
155 struct DefaultConstructTrait {
156 // Constructs the provided object which was already allocated.
157 static void Construct(T* allocated_ptr) {
158 new(allocated_ptr) T();
163 template <typename T>
164 struct DefaultCreateTrait {
171 struct ThreadSafeInitOnceTrait {
172 template <typename Function, typename Storage>
173 static void Init(OnceType* once, Function function, Storage storage) {
174 CallOnce(once, function, storage);
179 // Initialization trait for users who don't care about thread-safety.
180 struct SingleThreadInitOnceTrait {
181 template <typename Function, typename Storage>
182 static void Init(OnceType* once, Function function, Storage storage) {
183 if (*once == ONCE_STATE_UNINITIALIZED) {
185 *once = ONCE_STATE_DONE;
191 // TODO(pliard): Handle instances destruction (using global destructors).
192 template <typename T, typename AllocationTrait, typename CreateTrait,
193 typename InitOnceTrait, typename DestroyTrait /* not used yet. */>
194 struct LazyInstanceImpl {
196 typedef typename AllocationTrait::StorageType StorageType;
199 static void InitInstance(StorageType* storage) {
200 AllocationTrait::template InitStorageUsingTrait<CreateTrait>(storage);
206 // Casts to void* are needed here to avoid breaking strict aliasing
208 reinterpret_cast<void(*)(void*)>(&InitInstance), // NOLINT
209 reinterpret_cast<void*>(&storage_));
215 return AllocationTrait::MutableInstance(&storage_);
218 const T& Get() const {
220 return *AllocationTrait::MutableInstance(&storage_);
223 mutable OnceType once_;
224 // Note that the previous field, OnceType, is an AtomicWord which guarantees
225 // 4-byte alignment of the storage field below. If compiling with GCC (>4.2),
226 // the LAZY_ALIGN macro above will guarantee correctness for any alignment.
227 mutable StorageType storage_;
231 template <typename T,
232 typename CreateTrait = DefaultConstructTrait<T>,
233 typename InitOnceTrait = SingleThreadInitOnceTrait,
234 typename DestroyTrait = LeakyInstanceTrait<T> >
235 struct LazyStaticInstance {
236 typedef LazyInstanceImpl<T, StaticallyAllocatedInstanceTrait<T>,
237 CreateTrait, InitOnceTrait, DestroyTrait> type;
241 template <typename T,
242 typename CreateTrait = DefaultConstructTrait<T>,
243 typename InitOnceTrait = SingleThreadInitOnceTrait,
244 typename DestroyTrait = LeakyInstanceTrait<T> >
245 struct LazyInstance {
246 // A LazyInstance is a LazyStaticInstance.
247 typedef typename LazyStaticInstance<T, CreateTrait, InitOnceTrait,
248 DestroyTrait>::type type;
252 template <typename T,
253 typename CreateTrait = DefaultCreateTrait<T>,
254 typename InitOnceTrait = SingleThreadInitOnceTrait,
255 typename DestroyTrait = LeakyInstanceTrait<T> >
256 struct LazyDynamicInstance {
257 typedef LazyInstanceImpl<T, DynamicallyAllocatedInstanceTrait<T>,
258 CreateTrait, InitOnceTrait, DestroyTrait> type;
261 } } // namespace v8::internal
263 #endif // V8_LAZY_INSTANCE_H_