Clear next map word when folding allocations into js arrays.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5108 matching lines @@
     ALLOCATE_IN_OLD_POINTER_SPACE = 1 << 2,
     ALLOCATE_DOUBLE_ALIGNED = 1 << 3,
     PREFILL_WITH_FILLER = 1 << 4
   };
 
   HAllocate(HValue* context,
             HValue* size,
             HType type,
             PretenureFlag pretenure_flag,
             InstanceType instance_type)
-      : HTemplateInstruction<2>(type) {
+      : HTemplateInstruction<2>(type),
+        clear_next_map_word_(false) {
     SetOperandAt(0, context);
     SetOperandAt(1, size);
     set_representation(Representation::Tagged());
     SetFlag(kTrackSideEffectDominators);
     SetGVNFlag(kChangesNewSpacePromotion);
     SetGVNFlag(kDependsOnNewSpacePromotion);
     flags_ = pretenure_flag == TENURED
         ? (Heap::TargetSpaceId(instance_type) == OLD_POINTER_SPACE
             ? ALLOCATE_IN_OLD_POINTER_SPACE : ALLOCATE_IN_OLD_DATA_SPACE)
         : ALLOCATE_IN_NEW_SPACE;
     if (instance_type == FIXED_DOUBLE_ARRAY_TYPE) {
       flags_ = static_cast<HAllocate::Flags>(flags_ |
           ALLOCATE_DOUBLE_ALIGNED);
     }
   }
 
   Flags flags_;
   Handle<Map> known_initial_map_;
+  bool clear_next_map_word_;

[mvstanton, 2013/08/26 10:01:27]

Actually, do you need this boolean, since you handle things directly during the
dominance walk.

[Hannes Payer (out of office), 2013/08/26 11:51:07]

Yes, otherwise would create n stores, where n is the number of folding
operations.

 };
 
 
 class HInnerAllocatedObject V8_FINAL : public HTemplateInstruction<1> {
  public:
   static HInnerAllocatedObject* New(Zone* zone,
                                     HValue* context,
                                     HValue* value,
                                     int offset,
                                     HType type = HType::Tagged()) {
@@ skipping 1635 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_