Return the length as smi.

src/x64/stub-cache-x64.cc

 // 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 1533 matching lines @@
       // Check if we could survive without allocation.
       __ cmpl(rax, rcx);
       __ j(greater, &call_builtin);
 
       __ movq(rcx, Operand(rsp, argc * kPointerSize));
       __ StoreNumberToDoubleElements(
           rcx, rdi, rax, xmm0, &call_builtin, argc * kDoubleSize);
 
       // Save new length.
       __ Integer32ToSmiField(FieldOperand(rdx, JSArray::kLengthOffset), rax);
+      __ Integer32ToSmi(rax, rax);  // Return new length as smi.
       __ ret((argc + 1) * kPointerSize);
 
       __ bind(&with_write_barrier);
 
       __ movq(rbx, FieldOperand(rdx, HeapObject::kMapOffset));
 
       if (FLAG_smi_only_arrays  && !FLAG_trace_elements_transitions) {
         Label fast_object, not_fast_object;
         __ CheckFastObjectElements(rbx, &not_fast_object, Label::kNear);
         __ jmp(&fast_object);
@@ skipping 2571 matching lines @@
     __ jmp(ic_slow, RelocInfo::CODE_TARGET);
   }
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64