Refactor lazy_instance to avoid a strict aliasing problem with gcc 4.4.

base/lazy_instance.h

 // Copyright (c) 2008 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.
@@ skipping 17 matching lines @@
 //   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_
 
+#include "base/at_exit.h"
 #include "base/atomicops.h"
 #include "base/basictypes.h"
 #include "base/dynamic_annotations.h"
+#include "base/platform_thread.h"
 
 namespace base {
 
 template <typename Type>
 struct DefaultLazyInstanceTraits {
-  static void New(void* instance) {
+  static Type* New(void* instance) {
     // Use placement new to initialize our instance in our preallocated space.
     // The parenthesis is very important here to force POD type initialization.
-    new (instance) Type();
+    return new (instance) Type();
   }
   static void Delete(void* instance) {
     // Explicitly call the destructor.
     reinterpret_cast<Type*>(instance)->~Type();
   }
 };
 
-// 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 LazyInstanceHelper {
- protected:
-  enum {
-    STATE_EMPTY    = 0,
-    STATE_CREATING = 1,
-    STATE_CREATED  = 2
-  };
-
-  explicit LazyInstanceHelper(LinkerInitialized x) { /* state_ is 0 */ }
+template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
+class LazyInstance {
+ public:
+  explicit LazyInstance(LinkerInitialized x) { }
   // Declaring a destructor (even if it's empty) will cause MSVC to register a
   // static initializer to register the empty destructor with atexit().
 
-  // Make sure that instance is created, creating or waiting for it to be
-  // created if neccessary.  Constructs with |ctor| in the space provided by
-  // |instance| and registers dtor for destruction at program exit.
-  void EnsureInstance(void* instance, void (*ctor)(void*), void (*dtor)(void*));
-
-  base::subtle::Atomic32 state_;
-
- private:
-  // Resets state of |helper| to STATE_EMPTY so that it can be reused.
-  // Not thread safe.
-  static void ResetState(void* helper);
-
-  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().
-
   Type& Get() {
     return *Pointer();
   }
 
   Type* Pointer() {
-    Type* instance = reinterpret_cast<Type*>(&buf_);
-
     // We will hopefully have fast access when the instance is already created.
     if (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
-      EnsureInstance(instance, Traits::New, Traits::Delete);
+      EnsureInstance();
 
     // This annotation helps race detectors recognize correct lock-less
     // synchronization between different threads calling Pointer().
     // We suggest dynamic race detection tool that
-    // "ctor(instance)" in EnsureInstance(...) happens before
-    // "return instance" in Pointer().
-    // See the corresponding HAPPENS_BEFORE in EnsureInstance(...).
+    // "Traits::New(buf_)" in EnsureInstance() happens before
+    // "return instance_" in Pointer().
+    // See the corresponding HAPPENS_BEFORE in EnsureInstance().
     ANNOTATE_HAPPENS_AFTER(&state_);
 
-    return instance;
+    return instance_;
   }
 
  private:
+  enum {
+    STATE_EMPTY    = 0,
+    STATE_CREATING = 1,
+    STATE_CREATED  = 2
+  };
+
+  // Make sure that instance is created, creating or waiting for it to be
+  // created if neccessary.  Constructs with Traits::New in the space provided
+  // by buf_ and registers Traits::Delete for destruction at program exit.
+  void EnsureInstance() {
+    // Try to create the instance, if we're the first, will go from EMPTY
+    // to CREATING, otherwise we've already been beaten here.
+    if (base::subtle::Acquire_CompareAndSwap(
+      &state_, STATE_EMPTY, STATE_CREATING) == STATE_EMPTY) {
+      // Create the instance in the space provided by |buf_|.
+      instance_ = Traits::New(buf_);
+
+      // See the comment to the corresponding HAPPENS_AFTER in Pointer().
+      ANNOTATE_HAPPENS_BEFORE(&state_);
+
+      // Instance is created, go from CREATING to CREATED.
+      base::subtle::Release_Store(&state_, STATE_CREATED);
+
+      // Allow reusing the LazyInstance (reset it to the initial state). This
+      // makes possible calling all AtExit callbacks between tests. Assumes that
+      // no other threads execute when AtExit callbacks are processed.
+      base::AtExitManager::RegisterCallback(&ResetState, this);
+
+      // Make sure that the lazily instantiated object will get destroyed at exit.
+      base::AtExitManager::RegisterCallback(Traits::Delete, instance_);
+    } else {
+      // It's either in the process of being created, or already created.  Spin.
+      while (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
+        PlatformThread::YieldCurrentThread();
+    }
+  }
+
+  // Resets state of |lazyinstance| to STATE_EMPTY so that it can be reused.
+  // Not thread safe.
+  static void ResetState(void* lazyinstance) {
+    reinterpret_cast<LazyInstance*>(lazyinstance)->state_ = STATE_EMPTY;
+  }
+
+  base::subtle::Atomic32 state_;
   int8 buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
+  Type* instance_;
 
   DISALLOW_COPY_AND_ASSIGN(LazyInstance);
 };
 
 }  // namespace base
 
 #endif  // BASE_LAZY_INSTANCE_H_