Make the placement-new buffer in LazyInstance<Type> aligned.

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.
@@ skipping 23 matching lines @@
 
 #ifndef BASE_LAZY_INSTANCE_H_
 #define BASE_LAZY_INSTANCE_H_
 #pragma once
 
 #include <new>  // For placement new.
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/logging.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/thread_restrictions.h"
 
 namespace base {
 
 template <typename Type>
 struct DefaultLazyInstanceTraits {
   static const bool kAllowedToAccessOnNonjoinableThread = false;
 
   static Type* New(void* instance) {
+    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) % sizeof(instance), 0u)
+        << ": Bad boy, the buffer passed to placement new is not aligned!\n"
+        "This may break some stuff like SSE-based optimizations assuming the "
+        "<Type> objects are word aligned.";
     // Use placement new to initialize our instance in our preallocated space.
     // The parenthesis is very important here to force POD type initialization.
     return new (instance) Type();
   }
   static void Delete(void* instance) {
     // Explicitly call the destructor.
     reinterpret_cast<Type*>(instance)->~Type();
   }
 };
 
@@ skipping 115 matching lines @@
   // threaded, so don't use atomic operations.
   // 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);
   }
 
+  Type *instance_;
   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_