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

Before, the LazyInstance::buf_ was 4-byte aligned on x64, which is wrong.

WHY?! I thought buf_ is the first member of LazyInstance?!
NO! LazyInstance inherits LazyInstanceHelper, sizeof(LIH) = 4.

Then, buf_ is given to placement new.

As a result, the LazyInstance<Type> instances are all 4-byte aligned on x64.
This may break some stuff like SSE-based optimizations assuming the instance is
8-bytes aligned (fair assumption).
Also, if Type contains a bunch of std::vector/hash_map's, their pointers occupy
two half-words and Valgrind doesn't traverse to their data, reporting a false leak.

BUG=64930

Review URL: http://codereview.chromium.org/8366041

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@106763 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 6 insertions, 1 deletions

diff --git a/base/lazy_instance.h b/base/lazy_instance.h
index c6785f3..1471335 100644
--- a/base/lazy_instance.h
+++ b/base/lazy_instance.h
@@ -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 @@ 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();
@@ -186,8 +191,8 @@ class LazyInstance : public LazyInstanceHelper {
     base::subtle::Release_Store(&me->state_, STATE_EMPTY);
   }
 
-  int8 buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
   Type *instance_;
+  int8 buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
 
   DISALLOW_COPY_AND_ASSIGN(LazyInstance);
 };