Incremental mode now works for x64. The only difference

src/ia32/stub-cache-ia32.cc

 // Copyright 2011 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 1529 matching lines @@
       __ RecordWrite(ebx, edx, ecx, kDontSaveFPRegs, OMIT_REMEMBERED_SET);
 
       // Restore receiver to edx as finish sequence assumes it's here.
       __ mov(edx, Operand(esp, (argc + 1) * kPointerSize));
 
       // Increment element's and array's sizes.
       __ add(FieldOperand(ebx, FixedArray::kLengthOffset),
              Immediate(Smi::FromInt(kAllocationDelta)));
       __ mov(FieldOperand(edx, JSArray::kLengthOffset), eax);
 
-      // Elements are in new space, so write barrier is not required.
       __ ret((argc + 1) * kPointerSize);
     }
 
     __ bind(&call_builtin);
     __ TailCallExternalReference(
         ExternalReference(Builtins::c_ArrayPush, isolate()),
         argc + 1,
         1);
   }
 
@@ skipping 2308 matching lines @@
       masm->isolate()->builtins()->KeyedStoreIC_MissForceGeneric();
   __ jmp(ic_force_generic, RelocInfo::CODE_TARGET);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32