Further improve old space allocation write barrier elimination.

Stores to old space allocations require no write barriers if the
object is the new space dominator and the value is not in new space.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/290173003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21360 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.h

@@ -5702,7 +5702,7 @@ inline bool StoringValueNeedsWriteBarrier(HValue* value) {
 
 inline bool ReceiverObjectNeedsWriteBarrier(HValue* object,
                                             HValue* value,
-                                            HValue* new_space_dominator) {
+                                            HValue* dominator) {
   while (object->IsInnerAllocatedObject()) {
     object = HInnerAllocatedObject::cast(object)->base_object();
   }
@@ -5714,26 +5714,27 @@ inline bool ReceiverObjectNeedsWriteBarrier(HValue* object,
     // Stores to external references require no write barriers
     return false;
   }
-  if (object != new_space_dominator) return true;
-  if (object->IsAllocate()) {
-    // Stores to new space allocations require no write barriers if the object
-    // is the new space dominator.
+  // We definitely need a write barrier unless the object is the allocation
+  // dominator.
+  if (object == dominator && object->IsAllocate()) {
+    // Stores to new space allocations require no write barriers.
     if (HAllocate::cast(object)->IsNewSpaceAllocation()) {
       return false;
     }
-    // Storing a map or an immortal immovable object requires no write barriers
-    // if the object is the new space dominator.
-    if (value->IsConstant() &&
-        (HConstant::cast(value)->IsMap() ||
-         HConstant::cast(value)->ImmortalImmovable())) {
+    // Stores to old space allocations require no write barriers if the value is
+    // a constant provably not in new space.
+    if (value->IsConstant() && HConstant::cast(value)->NotInNewSpace()) {
       return false;
     }
-    // Likewise we don't need a write barrier if we store a value that
-    // originates from the same allocation (via allocation folding).
+    // Stores to old space allocations require no write barriers if the value is
+    // an old space allocation.
     while (value->IsInnerAllocatedObject()) {
       value = HInnerAllocatedObject::cast(value)->base_object();
     }
-    return object != value;
+    if (value->IsAllocate() &&
+        !HAllocate::cast(value)->IsNewSpaceAllocation()) {
+      return false;
+    }
   }
   return true;
 }