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

base/lazy_instance.h

Index: base/lazy_instance.h
===================================================================
--- base/lazy_instance.h	(revision 106713)
+++ base/lazy_instance.h	(working copy)
@@ -41,6 +41,7 @@
 #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"
 
@@ -51,6 +52,10 @@
   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();
@@ -186,8 +191,8 @@
     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);
 };