Add CallOnce() and simple LazyInstance implementation.

src/lazy-instance.h

+// Copyright 2012 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// The LazyInstance<Type, Traits> class manages a single instance of Type,
+// which will be lazily created on the first time it's accessed.  This class is
+// useful for places you would normally use a function-level static, but you
+// need to have guaranteed thread-safety.  The Type constructor will only ever
+// be called once, even if two threads are racing to create the object.  Get()
+// and Pointer() will always return the same, completely initialized instance.
+//
+// LazyInstance is completely thread safe, assuming that you create it safely.
+// The class was designed to be POD initialized, so it shouldn't require a
+// static constructor.  It really only makes sense to declare a LazyInstance as
+// a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
+//
+// LazyInstance is similar to Singleton, except it does not have the singleton
+// property.  You can have multiple LazyInstance's of the same type, and each
+// will manage a unique instance.  It also preallocates the space for Type, as
+// to avoid allocating the Type instance on the heap.  This may help with the
+// performance of creating the instance, and reducing heap fragmentation.  This
+// requires that Type be a complete type so we can determine the size. See
+// notes for advanced users below for more explanations.
+//
+// Example usage:
+//   static LazyInstance<MyClass>::type my_instance = LAZY_INSTANCE_INITIALIZER;
+//   void SomeMethod() {
+//     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
+//
+//     MyClass* ptr = my_instance.Pointer();
+//     ptr->DoDoDo();  // MyClass::DoDoDo
+//   }
+//
+// Additionally you can override the way your instance is constructed by
+// providing your own trait:
+// Example usage:
+//   struct MyCreateTrait {
+//     static void Construct(MyClass* allocated_ptr) {
+//       new (allocated_ptr) MyClass(/* extra parameters... */);
+//     }
+//   };
+//   static LazyInstance<MyClass, MyCreateTrait>::type my_instance =
+//      LAZY_INSTANCE_INITIALIZER;
+//
+// Notes for advanced users:
+// LazyInstance can actually be used in two different ways:
+//
+// - "Static mode" which is the default mode since it is the most efficient
+//   (no extra heap allocation). In this mode, the instance is statically
+//   allocated (stored in the global data section at compile time).
+//   The macro LAZY_STATIC_INSTANCE_INITIALIZER (= LAZY_INSTANCE_INITIALIZER)
+//   must be used to initialize static lazy instances.
+//
+// - "Dynamic mode". In this mode, the instance is dynamically allocated and
+//   constructed (using new) by default. This mode is useful if you have to
+//   deal with some code already allocating the instance for you (e.g.
+//   OS::Mutex() which returns a new private OS-dependent subclass of Mutex).
+//   The macro LAZY_DYNAMIC_INSTANCE_INITIALIZER must be used to initialize
+//   dynamic lazy instances.
+
+#ifndef V8_LAZY_INSTANCE_H_
+#define V8_LAZY_INSTANCE_H_
+
+#include "once.h"
+
+namespace v8 {
+namespace internal {
+
+#define LAZY_STATIC_INSTANCE_INITIALIZER { V8_ONCE_INIT, {} }
+#define LAZY_DYNAMIC_INSTANCE_INITIALIZER { V8_ONCE_INIT, 0 }
+
+// Default to static mode.
+#define LAZY_INSTANCE_INITIALIZER LAZY_STATIC_INSTANCE_INITIALIZER
+
+
+template <typename T>
+struct LeakyInstanceTrait {
+  static void Destroy(T* /* instance */) {}
+};
+
+
+// Traits that define how an instance is allocated and accessed.
+
+template <typename T>
+struct StaticallyAllocatedInstanceTrait {
+  typedef char StorageType[sizeof(T)];
+
+  static T* MutableInstance(StorageType* storage) {
+    return reinterpret_cast<T*>(storage);
+  }
+
+  template <typename ConstructTrait>
+  static void InitStorageUsingTrait(StorageType* storage) {
+    ConstructTrait::Construct(MutableInstance(storage));
+  }
+};
+
+
+template <typename T>
+struct DynamicallyAllocatedInstanceTrait {
+  typedef T* StorageType;
+
+  static T* MutableInstance(StorageType* storage) {
+    return *storage;
+  }
+
+  template <typename CreateTrait>
+  static void InitStorageUsingTrait(StorageType* storage) {
+    *storage = CreateTrait::Create();
+  }
+};
+
+
+template <typename T>
+struct DefaultConstructTrait {
+  // Constructs the provided object which was already allocated.
+  static void Construct(T* allocated_ptr) {
+    new(allocated_ptr) T();
+  }
+};
+
+
+template <typename T>
+struct DefaultCreateTrait {
+  static T* Create() {
+    return new T();
+  }
+};
+
+
+// TODO(pliard): Handle instances destruction (using global destructors).
+template <typename T, typename AllocationTrait, typename CreateTrait,
+          typename DestroyTrait  /* not used yet. */ >
+struct LazyInstanceImpl {
+ public:
+  typedef typename AllocationTrait::StorageType StorageType;
+
+ private:
+  static void InitInstance(StorageType* storage) {
+    AllocationTrait::template InitStorageUsingTrait<CreateTrait>(storage);
+  }
+
+  void Init() const {
+    CallOnce(&once_, &InitInstance, &storage_);
+  }
+
+ public:
+  T* Pointer() {
+    Init();
+    return AllocationTrait::MutableInstance(&storage_);
+  }
+
+  const T& Get() const {
+    Init();
+    return *AllocationTrait::MutableInstance(&storage_);
+  }
+
+  mutable OnceType once_;
+  // Note that the previous field, OnceType, is an AtomicWord which guarantees
+  // the correct alignment of the storage field below.
+  mutable StorageType storage_;
+};
+
+
+template <typename T,
+          typename CreateTrait = DefaultConstructTrait<T>,
+          typename DestroyTrait = LeakyInstanceTrait<T> >
+struct LazyStaticInstance {
+  typedef LazyInstanceImpl<T, StaticallyAllocatedInstanceTrait<T>, CreateTrait,
+      DestroyTrait> type;
+};
+
+
+template <typename T,
+          typename CreateTrait = DefaultConstructTrait<T>,
+          typename DestroyTrait = LeakyInstanceTrait<T> >
+struct LazyInstance {
+  // A LazyInstance is a LazyStaticInstance.
+  typedef typename LazyStaticInstance<T, CreateTrait, DestroyTrait>::type type;
+};
+
+
+template <typename T,
+          typename CreateTrait = DefaultConstructTrait<T>,
+          typename DestroyTrait = LeakyInstanceTrait<T> >
+struct LazyDynamicInstance {
+  typedef LazyInstanceImpl<T, DynamicallyAllocatedInstanceTrait<T>, CreateTrait,
+      DestroyTrait> type;
+};
+
+} }  // namespace v8::internal
+
+#endif  // V8_LAZY_INSTANCE_H_