Remove unnecessary NoBarrier_load instructions in store buffer.

src/heap/store-buffer.cc

Index: src/heap/store-buffer.cc
diff --git a/src/heap/store-buffer.cc b/src/heap/store-buffer.cc
index 7de8632b2ec71636f08ca6bb6470fbfd0c9f77e1..9c3c12c08d83751860a961732695deaeec41b5de 100644
--- a/src/heap/store-buffer.cc
+++ b/src/heap/store-buffer.cc
@@ -350,8 +350,7 @@ void StoreBuffer::VerifyPointers(LargeObjectSpace* space) {
         // When we are not in GC the Heap::InNewSpace() predicate
         // checks that pointers which satisfy predicate point into
         // the active semispace.
-        Object* object = reinterpret_cast<Object*>(
-            base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
+        Object* object = *slot;
         heap_->InNewSpace(object);
         slot_address += kPointerSize;
       }
@@ -382,8 +381,7 @@ void StoreBuffer::ProcessOldToNewSlot(Address slot_address,
                                       ObjectSlotCallback slot_callback,
                                       bool clear_maps) {
   Object** slot = reinterpret_cast<Object**>(slot_address);
-  Object* object = reinterpret_cast<Object*>(
-      base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
+  Object* object = *slot;
 
   // If the object is not in from space, it must be a duplicate store buffer
   // entry and the slot was already updated.
@@ -394,8 +392,7 @@ void StoreBuffer::ProcessOldToNewSlot(Address slot_address,
     // pointer. Clear the map field now lazily (during full GC).
     if (clear_maps) ClearDeadObject(heap_object);
     slot_callback(reinterpret_cast<HeapObject**>(slot), heap_object);
-    object = reinterpret_cast<Object*>(
-        base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
+    object = *slot;
     // If the object was in from space before and is after executing the
     // callback in to space, the object is still live.
     // Unfortunately, we do not know about the slot. It could be in a
@@ -440,6 +437,8 @@ void StoreBuffer::ClearInvalidStoreBufferEntries() {
   for (Address* current = old_start_; current < old_top_; current++) {
     Address addr = *current;
     Object** slot = reinterpret_cast<Object**>(*current);
+    // Use a NoBarrier_Load here since the slot can be in a dead object
+    // which may be touched by the concurrent sweeper thread.

[Jarin, 2015/03/10 13:32:25]

Why did not you update this one?

[Hannes Payer (out of office), 2015/03/10 13:51:17]

Because the stale pointers may be accessed concurrently by the sweeper thread.
After executing this method we have valid pointers in the store buffer and don't
have to worry about the sweeper thread anymore.

     Object* object = reinterpret_cast<Object*>(
         base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
     if (heap_->InNewSpace(object)) {
@@ -467,8 +466,7 @@ void StoreBuffer::ClearInvalidStoreBufferEntries() {
 void StoreBuffer::VerifyValidStoreBufferEntries() {
   for (Address* current = old_start_; current < old_top_; current++) {
     Object** slot = reinterpret_cast<Object**>(*current);
-    Object* object = reinterpret_cast<Object*>(
-        base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
+    Object* object = *slot;
     CHECK(heap_->InNewSpace(object));
     heap_->mark_compact_collector()->VerifyIsSlotInLiveObject(
         reinterpret_cast<HeapObject**>(slot),