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1 : // Copyright 2012 the V8 project 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 : // The LazyInstance<Type, Traits> class manages a single instance of Type,
6 : // which will be lazily created on the first time it's accessed. This class is
7 : // useful for places you would normally use a function-level static, but you
8 : // need to have guaranteed thread-safety. The Type constructor will only ever
9 : // be called once, even if two threads are racing to create the object. Get()
10 : // and Pointer() will always return the same, completely initialized instance.
11 : //
12 : // LazyInstance is completely thread safe, assuming that you create it safely.
13 : // The class was designed to be POD initialized, so it shouldn't require a
14 : // static constructor. It really only makes sense to declare a LazyInstance as
15 : // a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
16 : //
17 : // LazyInstance is similar to Singleton, except it does not have the singleton
18 : // property. You can have multiple LazyInstance's of the same type, and each
19 : // will manage a unique instance. It also preallocates the space for Type, as
20 : // to avoid allocating the Type instance on the heap. This may help with the
21 : // performance of creating the instance, and reducing heap fragmentation. This
22 : // requires that Type be a complete type so we can determine the size. See
23 : // notes for advanced users below for more explanations.
24 : //
25 : // Example usage:
26 : // static LazyInstance<MyClass>::type my_instance = LAZY_INSTANCE_INITIALIZER;
27 : // void SomeMethod() {
28 : // my_instance.Get().SomeMethod(); // MyClass::SomeMethod()
29 : //
30 : // MyClass* ptr = my_instance.Pointer();
31 : // ptr->DoDoDo(); // MyClass::DoDoDo
32 : // }
33 : //
34 : // Additionally you can override the way your instance is constructed by
35 : // providing your own trait:
36 : // Example usage:
37 : // struct MyCreateTrait {
38 : // static void Construct(void* allocated_ptr) {
39 : // new (allocated_ptr) MyClass(/* extra parameters... */);
40 : // }
41 : // };
42 : // static LazyInstance<MyClass, MyCreateTrait>::type my_instance =
43 : // LAZY_INSTANCE_INITIALIZER;
44 : //
45 : // WARNINGS:
46 : // - This implementation of LazyInstance IS THREAD-SAFE by default. See
47 : // SingleThreadInitOnceTrait if you don't care about thread safety.
48 : // - Lazy initialization comes with a cost. Make sure that you don't use it on
49 : // critical path. Consider adding your initialization code to a function
50 : // which is explicitly called once.
51 : //
52 : // Notes for advanced users:
53 : // LazyInstance can actually be used in two different ways:
54 : //
55 : // - "Static mode" which is the default mode since it is the most efficient
56 : // (no extra heap allocation). In this mode, the instance is statically
57 : // allocated (stored in the global data section at compile time).
58 : // The macro LAZY_STATIC_INSTANCE_INITIALIZER (= LAZY_INSTANCE_INITIALIZER)
59 : // must be used to initialize static lazy instances.
60 : //
61 : // - "Dynamic mode". In this mode, the instance is dynamically allocated and
62 : // constructed (using new) by default. This mode is useful if you have to
63 : // deal with some code already allocating the instance for you (e.g.
64 : // OS::Mutex() which returns a new private OS-dependent subclass of Mutex).
65 : // The macro LAZY_DYNAMIC_INSTANCE_INITIALIZER must be used to initialize
66 : // dynamic lazy instances.
67 :
68 : #ifndef V8_BASE_LAZY_INSTANCE_H_
69 : #define V8_BASE_LAZY_INSTANCE_H_
70 :
71 : #include "src/base/macros.h"
72 : #include "src/base/once.h"
73 :
74 : namespace v8 {
75 : namespace base {
76 :
77 : #define LAZY_STATIC_INSTANCE_INITIALIZER { V8_ONCE_INIT, { {} } }
78 : #define LAZY_DYNAMIC_INSTANCE_INITIALIZER { V8_ONCE_INIT, 0 }
79 :
80 : // Default to static mode.
81 : #define LAZY_INSTANCE_INITIALIZER LAZY_STATIC_INSTANCE_INITIALIZER
82 :
83 :
84 : template <typename T>
85 : struct LeakyInstanceTrait {
86 : static void Destroy(T* /* instance */) {}
87 : };
88 :
89 :
90 : // Traits that define how an instance is allocated and accessed.
91 :
92 :
93 : template <typename T>
94 : struct StaticallyAllocatedInstanceTrait {
95 : // 16-byte alignment fallback to be on the safe side here.
96 : struct V8_ALIGNAS(T, 16) StorageType {
97 : char x[sizeof(T)];
98 : };
99 :
100 : STATIC_ASSERT(V8_ALIGNOF(StorageType) >= V8_ALIGNOF(T));
101 :
102 : static T* MutableInstance(StorageType* storage) {
103 : return reinterpret_cast<T*>(storage);
104 : }
105 :
106 : template <typename ConstructTrait>
107 : static void InitStorageUsingTrait(StorageType* storage) {
108 94966 : ConstructTrait::Construct(storage);
109 : }
110 : };
111 :
112 :
113 : template <typename T>
114 : struct DynamicallyAllocatedInstanceTrait {
115 : typedef T* StorageType;
116 :
117 : static T* MutableInstance(StorageType* storage) {
118 : return *storage;
119 : }
120 :
121 : template <typename CreateTrait>
122 : static void InitStorageUsingTrait(StorageType* storage) {
123 206 : *storage = CreateTrait::Create();
124 : }
125 : };
126 :
127 :
128 : template <typename T>
129 : struct DefaultConstructTrait {
130 : // Constructs the provided object which was already allocated.
131 405740 : static void Construct(void* allocated_ptr) { new (allocated_ptr) T(); }
132 : };
133 :
134 :
135 : template <typename T>
136 : struct DefaultCreateTrait {
137 : static T* Create() {
138 : return new T();
139 : }
140 : };
141 :
142 :
143 : struct ThreadSafeInitOnceTrait {
144 : template <typename Function, typename Storage>
145 : static void Init(OnceType* once, Function function, Storage storage) {
146 33298645 : CallOnce(once, function, storage);
147 : }
148 : };
149 :
150 :
151 : // Initialization trait for users who don't care about thread-safety.
152 : struct SingleThreadInitOnceTrait {
153 : template <typename Function, typename Storage>
154 : static void Init(OnceType* once, Function function, Storage storage) {
155 : if (*once == ONCE_STATE_UNINITIALIZED) {
156 : function(storage);
157 : *once = ONCE_STATE_DONE;
158 : }
159 : }
160 : };
161 :
162 :
163 : // TODO(pliard): Handle instances destruction (using global destructors).
164 : template <typename T, typename AllocationTrait, typename CreateTrait,
165 : typename InitOnceTrait, typename DestroyTrait /* not used yet. */>
166 : struct LazyInstanceImpl {
167 : public:
168 : typedef typename AllocationTrait::StorageType StorageType;
169 :
170 : private:
171 432478 : static void InitInstance(StorageType* storage) {
172 : AllocationTrait::template InitStorageUsingTrait<CreateTrait>(storage);
173 432478 : }
174 :
175 : void Init() const {
176 : InitOnceTrait::Init(
177 : &once_,
178 : // Casts to void* are needed here to avoid breaking strict aliasing
179 : // rules.
180 : reinterpret_cast<void(*)(void*)>(&InitInstance), // NOLINT
181 2668 : reinterpret_cast<void*>(&storage_));
182 : }
183 :
184 : public:
185 2668 : T* Pointer() {
186 : Init();
187 2668 : return AllocationTrait::MutableInstance(&storage_);
188 : }
189 :
190 : const T& Get() const {
191 : Init();
192 : return *AllocationTrait::MutableInstance(&storage_);
193 : }
194 :
195 : mutable OnceType once_;
196 : // Note that the previous field, OnceType, is an AtomicWord which guarantees
197 : // 4-byte alignment of the storage field below. If compiling with GCC (>4.2),
198 : // the LAZY_ALIGN macro above will guarantee correctness for any alignment.
199 : mutable StorageType storage_;
200 : };
201 :
202 :
203 : template <typename T,
204 : typename CreateTrait = DefaultConstructTrait<T>,
205 : typename InitOnceTrait = ThreadSafeInitOnceTrait,
206 : typename DestroyTrait = LeakyInstanceTrait<T> >
207 : struct LazyStaticInstance {
208 : typedef LazyInstanceImpl<T, StaticallyAllocatedInstanceTrait<T>,
209 : CreateTrait, InitOnceTrait, DestroyTrait> type;
210 : };
211 :
212 :
213 : template <typename T,
214 : typename CreateTrait = DefaultConstructTrait<T>,
215 : typename InitOnceTrait = ThreadSafeInitOnceTrait,
216 : typename DestroyTrait = LeakyInstanceTrait<T> >
217 : struct LazyInstance {
218 : // A LazyInstance is a LazyStaticInstance.
219 : typedef typename LazyStaticInstance<T, CreateTrait, InitOnceTrait,
220 : DestroyTrait>::type type;
221 : };
222 :
223 :
224 : template <typename T,
225 : typename CreateTrait = DefaultCreateTrait<T>,
226 : typename InitOnceTrait = ThreadSafeInitOnceTrait,
227 : typename DestroyTrait = LeakyInstanceTrait<T> >
228 : struct LazyDynamicInstance {
229 : typedef LazyInstanceImpl<T, DynamicallyAllocatedInstanceTrait<T>,
230 : CreateTrait, InitOnceTrait, DestroyTrait> type;
231 : };
232 :
233 : } // namespace base
234 : } // namespace v8
235 :
236 : #endif // V8_BASE_LAZY_INSTANCE_H_
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