Further improve old space allocation write barrier elimination.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HYDROGEN_INSTRUCTIONS_H_
 #define V8_HYDROGEN_INSTRUCTIONS_H_
 
 #include "v8.h"
 
 #include "allocation.h"
@@ skipping 5684 matching lines @@
 
 inline bool StoringValueNeedsWriteBarrier(HValue* value) {
   return !value->type().IsBoolean()
       && !value->type().IsSmi()
       && !(value->IsConstant() && HConstant::cast(value)->ImmortalImmovable());
 }
 
 
 inline bool ReceiverObjectNeedsWriteBarrier(HValue* object,
                                             HValue* value,
-                                            HValue* new_space_dominator) {
+                                            HValue* dominator) {
   while (object->IsInnerAllocatedObject()) {
     object = HInnerAllocatedObject::cast(object)->base_object();
   }
   if (object->IsConstant() && HConstant::cast(object)->IsCell()) {
     return false;
   }
   if (object->IsConstant() &&
       HConstant::cast(object)->HasExternalReferenceValue()) {
     // 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
+    // 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).
     while (value->IsInnerAllocatedObject()) {

[Hannes Payer (out of office), 2014/05/19 09:43:45]

Please add a comment what is going on here.

       value = HInnerAllocatedObject::cast(value)->base_object();
     }
-    return object != value;
+    if (value->IsAllocate() &&
+        !HAllocate::cast(value)->IsNewSpaceAllocation()) {
+      return false;
+    }
   }
   return true;
 }
 
 
 class HStoreGlobalCell V8_FINAL : public HUnaryOperation {
  public:
   DECLARE_INSTRUCTION_FACTORY_P3(HStoreGlobalCell, HValue*,
                                  Handle<PropertyCell>, PropertyDetails);
 
@@ skipping 1971 matching lines @@
   virtual bool IsDeletable() const V8_OVERRIDE { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_