Avoid a strict aliasing issue in LazyInstance.

base/lazy_instance.cc

 // 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.
 
 #include "base/lazy_instance.h"
 
 #include "base/at_exit.h"
 #include "base/atomicops.h"
 #include "base/dynamic_annotations.h"
 #include "base/basictypes.h"
 #include "base/platform_thread.h"
 
 namespace base {
 
-void LazyInstanceHelper::EnsureInstance(void* instance,
-                                        void (*ctor)(void*),
-                                        void (*dtor)(void*)) {
+bool LazyInstanceHelper::NeedsInstance() {
   // 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) {
-    // Created the instance in the space provided by |instance|.
-    ctor(instance);
-
-    // 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(&LazyInstanceHelper::ResetState,
-                                          this);
-
-    // Make sure that the lazily instantiated object will get destroyed at exit.
-    base::AtExitManager::RegisterCallback(dtor, instance);
+    // Caller must create instance
+    return true;
   } else {
     // It's either in the process of being created, or already created.  Spin.
     while (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
       PlatformThread::YieldCurrentThread();
   }
+
+  // Someone else created the instance.
+  return false;
+}
+
+void LazyInstanceHelper::CompleteInstance(void* instance, void (*dtor)(void*)) {
+  // 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(&LazyInstanceHelper::ResetState,
+                                        this);
+
+  // Make sure that the lazily instantiated object will get destroyed at exit.
+  base::AtExitManager::RegisterCallback(dtor, instance);
 }
 
 // static
 void LazyInstanceHelper::ResetState(void* helper) {
   reinterpret_cast<LazyInstanceHelper*>(helper)->state_ = STATE_EMPTY;
 }
 
 }  // namespace base