Add ALIGNAS and ALIGNOF macros to ensure proper alignment of StaticMemorySingletonTraits

base/lazy_instance.h

 // Copyright (c) 2012 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 24 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/memory/aligned_memory.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/threading/thread_restrictions.h"
 
 // LazyInstance uses its own struct initializer-list style static
 // initialization, as base's LINKER_INITIALIZED requires a constructor and on
 // some compilers (notably gcc 4.4) this still ends up needing runtime
 // initialization.
 #define LAZY_INSTANCE_INITIALIZER {0}
 
 namespace base {
 
 template <typename Type>
 struct DefaultLazyInstanceTraits {
   static const bool kRegisterOnExit = true;
   static const bool kAllowedToAccessOnNonjoinableThread = false;
 
   static Type* New(void* instance) {
-    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) % sizeof(instance), 0u)
+    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 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(Type* instance) {
     // Explicitly call the destructor.
     instance->~Type();
@@ skipping 75 matching lines @@
     // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
     // at most once, the load is taken out of NeedsInstance() as a fast-path.
     // The load has acquire memory ordering as a thread which sees
     // private_instance_ > creating needs to acquire visibility over
     // the associated data (private_buf_). Pairing Release_Store is in
     // CompleteLazyInstance().
     subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);
     if (!(value & kLazyInstanceCreatedMask) &&
         internal::NeedsLazyInstance(&private_instance_)) {
       // Create the instance in the space provided by |private_buf_|.
-      value = reinterpret_cast<subtle::AtomicWord>(Traits::New(private_buf_));
+      value = reinterpret_cast<subtle::AtomicWord>(
+          Traits::New(private_buf_.void_data()));
       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 CompleteLazyInstance(...) happens before "return instance()" below.
     // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).
     ANNOTATE_HAPPENS_AFTER(&private_instance_);
     return instance();
   }
 
   bool operator==(Type* p) {
     switch (subtle::NoBarrier_Load(&private_instance_)) {
       case 0:
         return p == NULL;
       case internal::kLazyInstanceStateCreating:
-        return static_cast<int8*>(static_cast<void*>(p)) == private_buf_;
+        return static_cast<void*>(p) == private_buf_.void_data();
       default:
         return p == instance();
     }
   }
 
   // Effectively private: member data is only public to allow the linker to
-  // statically initialize it. DO NOT USE FROM OUTSIDE THIS CLASS.
+  // statically initialize it and to maintain a POD class. DO NOT USE FROM
+  // OUTSIDE THIS CLASS.
 
-  // Note this must use AtomicWord, not Atomic32, to ensure correct alignment
-  // of |private_buf_| on 64 bit architectures. (This member must be first to
-  // allow the syntax used in LAZY_INSTANCE_INITIALIZER to work correctly.)
   subtle::AtomicWord private_instance_;
-  int8 private_buf_[sizeof(Type)];  // Preallocated space for the Type instance.
+  // Preallocated space for the Type instance.
+  base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
 
  private:
   Type* instance() {
     return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&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());
-    subtle::Release_Store(&me->private_instance_, 0);
+    subtle::NoBarrier_Store(&me->private_instance_, 0);
   }
 };
 
 }  // namespace base
 
 #endif  // BASE_LAZY_INSTANCE_H_