Allow linker initialization of lazy instance

base/lazy_instance.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // 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.
 // When the instance is constructed it is registered with AtExitManager.  The
 // destructor will be called on program exit.
 //
 // 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 base::LinkerInitialized constructor.
 //
 // 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.
 //
 // Example usage:
-//   static LazyInstance<MyClass> my_instance(base::LINKER_INITIALIZED);
+//   static LazyInstance<MyClass> my_instance = LINKER_ZERO_INITIALIZED;

[Nico, 2011/11/11 20:21:56]

Is this needed? Aren't statics implicitly zero initialized? (See the definition
of LINKER_INITIALIZED for why that's there)

[joth, 2011/11/14 16:58:23]

Just needed as a double check to prove to the caller the instance really is POD
initializer-list initializable

 //   void SomeMethod() {
 //     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
 //
 //     MyClass* ptr = my_instance.Pointer();
 //     ptr->DoDoDo();  // MyClass::DoDoDo
 //   }
 
 #ifndef BASE_LAZY_INSTANCE_H_
 #define BASE_LAZY_INSTANCE_H_
 #pragma once
@@ skipping 34 matching lines @@
   static const bool kRegisterOnExit = false;
   static const bool kAllowedToAccessOnNonjoinableThread = true;
 
   static Type* New(void* instance) {
     return DefaultLazyInstanceTraits<Type>::New(instance);
   }
   static void Delete(Type* instance) {
   }
 };
 
-// We pull out some of the functionality into a non-templated base, so that we
-// can implement the more complicated pieces out of line in the .cc file.
-class BASE_EXPORT LazyInstanceHelper {
- protected:
-  enum {
-    STATE_EMPTY    = 0,
-    STATE_CREATING = 1,
-    STATE_CREATED  = 2
-  };
+// We pull out some of the functionality into a non-templated functions, so we
+// we can implement the more complicated pieces out of line in the .cc file.
+namespace internal {
 
-  explicit LazyInstanceHelper(LinkerInitialized /*unused*/) {/* state_ is 0 */}
+// Our AtomicWord doubles as a spinlock, where a value of
+// kBeingCreatedMarker means the spinlock is being held for creation.
+static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
 
+// Declaring a destructor (even if it's empty) will cause MSVC to register a
+// static initializer to register the empty destructor with atexit().
+
+// Check if instance needs to be created. If so return true otherwise
+// if another thread has beat us, wait for instance to be created and
+// return false.
+BASE_EXPORT bool NeedsLazyInstance(base::subtle::AtomicWord* state);
+
+// After creating an instance, call this to register the dtor to be called
+// at program exit and to update the atomic state to hold the |new_instance|
+BASE_EXPORT void CompleteLazyInstance(base::subtle::AtomicWord* state,
+                                      base::subtle::AtomicWord new_instance,
+                                      void* lazy_instance,
+                                      void (*dtor)(void*));
+
+}  // namespace internal
+
+#define LINKER_ZERO_INITIALIZED {0}
+
+template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
+class LazyInstance {
+ public:
   // Declaring a destructor (even if it's empty) will cause MSVC to register a
   // static initializer to register the empty destructor with atexit().
-
-  // A destructor is intentionally not defined.  If we were to say
-  // ~LazyInstanceHelper() { }
   // Even though it's empty, a destructor will still be generated.
   // In order for the constructor to be called for static variables,
   // it will be registered as a callback at runtime with AtExit().
   // We don't want this, so we don't declare a destructor at all,
   // effectively keeping the type POD (at least in terms of
   // initialization and destruction).
-
-  // Check if instance needs to be created. If so return true otherwise
-  // if another thread has beat us, wait for instance to be created and
-  // return false.
-  bool NeedsInstance();
-
-  // After creating an instance, call this to register the dtor to be called
-  // at program exit and to update the state to STATE_CREATED.
-  void CompleteInstance(void* instance, void (*dtor)(void*));
-
-  base::subtle::Atomic32 state_;
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(LazyInstanceHelper);
-};
-
-template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
-class LazyInstance : public LazyInstanceHelper {
- public:
-  explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { }
-
-  // Declaring a destructor (even if it's empty) will cause MSVC to register a
-  // static initializer to register the empty destructor with atexit().
-  // Refer to the destructor-related comment in LazyInstanceHelper.
   // ~LazyInstance() {}
 
   Type& Get() {
     return *Pointer();
   }
 
   Type* Pointer() {
 #ifndef NDEBUG
     // Avoid making TLS lookup on release builds.
     if (!Traits::kAllowedToAccessOnNonjoinableThread)
       base::ThreadRestrictions::AssertSingletonAllowed();
 #endif
 
     // We will hopefully have fast access when the instance is already created.
-    // Since a thread sees state_ != STATE_CREATED at most once,
+    // Since a thread sees private_instance_ == 0 or Creating at most once,

[willchan no longer on Chromium, 2011/11/11 22:18:21]

You mean kLazyInstanceStateCreating instead of just Creating, right?

[joth, 2011/11/14 16:58:23]

Done.

     // the load is taken out of NeedsInstance() as a fast-path.
     // The load has acquire memory ordering as a thread which sees
-    // state_ == STATE_CREATED needs to acquire visibility over
-    // the associated data (buf_). Pairing Release_Store is in
+    // private_instance_ > creating needs to acquire visibility over
+    // the associated data (private_buf_). Pairing Release_Store is in
     // CompleteInstance().
-    if ((base::subtle::Acquire_Load(&state_) != STATE_CREATED) &&
-        NeedsInstance()) {
-      // Create the instance in the space provided by |buf_|.
-      instance_ = Traits::New(buf_);
-      CompleteInstance(this, Traits::kRegisterOnExit ? OnExit : NULL);
+    base::subtle::AtomicWord value =

[willchan no longer on Chromium, 2011/11/11 22:18:21]

No need to use base:: from within the base namespace.

[joth, 2011/11/14 16:58:23]

Done. here & many other places in .h and .cc

+        base::subtle::Acquire_Load(&private_instance_);
+    if ((value == 0 || value == internal::kLazyInstanceStateCreating) &&

[willchan no longer on Chromium, 2011/11/11 22:18:21]

I'm not sure I understand why we changed away from using the state machine.
We're doing two compares here now (0 and kLazyInstanceStateCreating) rather than
a single != STATE_CREATED. Why? Are you making a space/time tradeoff here,
combining the state_ and instance_ variables into a single private_instance_
variable?

[joth, 2011/11/14 16:58:23]

Yep memory reduction plus consistency with Singleton.
but I remove the double comparison (by adding a bitwise and). 

+        internal::NeedsLazyInstance(&private_instance_)) {
+      // Create the instance in the space provided by |private_buf_|.
+      value = reinterpret_cast<base::subtle::AtomicWord>(
+          Traits::New(private_buf_));
+      internal::CompleteLazyInstance(&private_instance_, value, this,
+                                     Traits::kRegisterOnExit ? OnExit : NULL);
     }
 
     // This annotation helps race detectors recognize correct lock-less
     // synchronization between different threads calling Pointer().
     // We suggest dynamic race detection tool that "Traits::New" above
-    // and CompleteInstance(...) happens before "return instance_" below.
+    // and CompleteInstance(...) happens before "return instance()" below.
     // See the corresponding HAPPENS_BEFORE in CompleteInstance(...).
-    ANNOTATE_HAPPENS_AFTER(&state_);
-    return instance_;
+    ANNOTATE_HAPPENS_AFTER(&private_instance_);
+    return instance();
   }
 
   bool operator==(Type* p) {
-    switch (base::subtle::NoBarrier_Load(&state_)) {
-      case STATE_EMPTY:
+    switch (base::subtle::NoBarrier_Load(&private_instance_)) {
+      case 0:
         return p == NULL;
-      case STATE_CREATING:
-        return static_cast<int8*>(static_cast<void*>(p)) == buf_;
-      case STATE_CREATED:
-        return p == instance_;
+      case internal::kLazyInstanceStateCreating:
+        return static_cast<int8*>(static_cast<void*>(p)) == private_buf_;
       default:
-        return false;
+        return p == instance();
     }
   }
 
+  // Effectively private: member data is only public to allow the linker to

[Nico, 2011/11/11 20:21:56]

Sentence cut off

[joth, 2011/11/11 21:15:36]

Done.

+  // Note this must use AtomicWord, not Atomic32, to ensure correct alignment of
+  // |private_buf_| on 64 bit arcchitectures.

[Nico, 2011/11/11 20:21:56]

spello arcchitectures

[joth, 2011/11/11 21:15:36]

Done.

+  base::subtle::AtomicWord private_instance_;
+  // statically initialize it. DO NOT USE FROM OUTSIDE THIS CLASS.
+  int8 private_buf_[sizeof(Type)];  // Preallocated space for the Type instance.
+
  private:
+  Type* instance() { return reinterpret_cast<Type*>(private_instance_); }
+
   // Adapter function for use with AtExit.  This should be called single
   // threaded, so don't synchronize across threads.
   // Calling OnExit while the instance is in use by other threads is a mistake.
   static void OnExit(void* lazy_instance) {
     LazyInstance<Type, Traits>* me =
         reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
-    Traits::Delete(me->instance_);
-    me->instance_ = NULL;
-    base::subtle::Release_Store(&me->state_, STATE_EMPTY);
+    Traits::Delete(me->instance());
+    base::subtle::Release_Store(&me->private_instance_, 0);
   }
-
-  Type *instance_;
-  int8 buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
-
-  DISALLOW_COPY_AND_ASSIGN(LazyInstance);
 };
 
 }  // namespace base
 
 #endif  // BASE_LAZY_INSTANCE_H_