Further improve old space allocation write barrier elimination.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 011b4080e59999f30b5ce56a40341b7628b325d4..e4e3131c37c064e800981797dca814e63d5e3987 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -5721,19 +5721,20 @@ inline bool ReceiverObjectNeedsWriteBarrier(HValue* object,
     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 object
+    // is the new space dominator and the value is not in new space.

[Hannes Payer (out of office), 2014/05/18 18:54:58]

new_space_dominator should be called dominator, otherwise the code does not fit
the description. Moreover the part about  the new space dominator should be
mentioned earlier.

[Benedikt Meurer, 2014/05/19 06:17:51]

Done.

+    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).
+    // Likewise we don't need a write barrier if we store a value was previously
+    // allocated in old space.

[Hannes Payer (out of office), 2014/05/18 18:54:58]

broken comment.

[Benedikt Meurer, 2014/05/19 06:17:51]

Done.

     while (value->IsInnerAllocatedObject()) {
       value = HInnerAllocatedObject::cast(value)->base_object();
     }
-    return object != value;
+    if (value->IsAllocate() &&
+        HAllocate::cast(value)->IsOldDataSpaceAllocation()) {
+      return false;
+    }

[Hannes Payer (out of office), 2014/05/18 18:54:58]

Why not
if (value->IsAllocate() &&
    (HAllocate::cast(value)->IsOldDataSpaceAllocation() ||
        HAllocate::cast(value)->IsOldPointerSpaceAllocation())

[Benedikt Meurer, 2014/05/19 06:17:51]

Done.

   }
   return true;
 }