The check for internalized strings relied on the fact that we had less

src/x64/macro-assembler-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 2290 matching lines @@
   andl(scratch2, Immediate(kFlatAsciiStringMask));
   // Interleave the bits to check both scratch1 and scratch2 in one test.
   ASSERT_EQ(0, kFlatAsciiStringMask & (kFlatAsciiStringMask << 3));
   lea(scratch1, Operand(scratch1, scratch2, times_8, 0));
   cmpl(scratch1,
        Immediate(kFlatAsciiStringTag + (kFlatAsciiStringTag << 3)));
   j(not_equal, on_fail, near_jump);
 }
 
 
+template<class T>
+static void JumpIfNotUniqueNameHelper(MacroAssembler* masm,
+                                      T operand_or_register,
+                                      Label* not_unique_name,
+                                      Label::Distance distance) {
+  STATIC_ASSERT(((SYMBOL_TYPE - 1) & kIsInternalizedMask) == kInternalizedTag);
+  masm->cmpb(operand_or_register, Immediate(kInternalizedTag));
+  masm->j(less, not_unique_name, distance);
+  masm->cmpb(operand_or_register, Immediate(SYMBOL_TYPE));
+  masm->j(greater, not_unique_name, distance);
+}
+
+
+void MacroAssembler::JumpIfNotUniqueName(Operand operand,
+                                         Label* not_unique_name,
+                                         Label::Distance distance) {
+  JumpIfNotUniqueNameHelper<Operand>(this, operand, not_unique_name, distance);
+}
+
+
+void MacroAssembler::JumpIfNotUniqueName(Register reg,
+                                         Label* not_unique_name,
+                                         Label::Distance distance) {
+  JumpIfNotUniqueNameHelper<Register>(this, reg, not_unique_name, distance);
+}
+
 
 void MacroAssembler::Move(Register dst, Register src) {
   if (!dst.is(src)) {
     movq(dst, src);
   }
 }
 
 
 void MacroAssembler::Move(Register dst, Handle<Object> source) {
   AllowDeferredHandleDereference smi_check;
@@ skipping 2380 matching lines @@
   j(greater, &no_info_available);
   CompareRoot(MemOperand(scratch_reg, -AllocationSiteInfo::kSize),
               Heap::kAllocationSiteInfoMapRootIndex);
   bind(&no_info_available);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64