Prefill allocated objects with one word fillers if we do not use allocation folding.

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 5102 matching lines @@
     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_ = static_cast<HAllocate::Flags>(flags_ | ALLOCATE_DOUBLE_ALIGNED);
+    }
+    // We have to fill the allocated object with one word fillers if we do
+    // not use allocation folding.

[titzer, 2013/08/22 12:32:56]

This comment would be a little more useful if it explained what part of the
system invariants would be violated if we didn't do this.

+    if (!FLAG_use_gvn || !FLAG_use_allocation_folding) {
+      flags_ = static_cast<HAllocate::Flags>(flags_ | PREFILL_WITH_FILLER);
     }
   }
 
   Flags flags_;
   Handle<Map> known_initial_map_;
 };
 
 
 class HInnerAllocatedObject: public HTemplateInstruction<1> {
  public:
@@ skipping 1653 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_