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 5703 matching lines @@
     // 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.
     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;
 }
 
 
 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_