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

src/x64/ic-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 319 matching lines @@
   __ CmpObjectType(key, LAST_UNIQUE_NAME_TYPE, map);
   __ j(above, not_unique);
   STATIC_ASSERT(LAST_UNIQUE_NAME_TYPE == FIRST_NONSTRING_TYPE);
   __ j(equal, &unique);
 
   // Is the string an array index, with cached numeric value?
   __ movl(hash, FieldOperand(key, Name::kHashFieldOffset));
   __ testl(hash, Immediate(Name::kContainsCachedArrayIndexMask));
   __ j(zero, index_string);  // The value in hash is used at jump target.
 
-  // Is the string internalized?
+  // Is the string internalized? We already know it's a string so a single
+  // bit test is enough.
   STATIC_ASSERT(kInternalizedTag != 0);
   __ testb(FieldOperand(map, Map::kInstanceTypeOffset),
            Immediate(kIsInternalizedMask));
   __ j(zero, not_unique);
 
   __ bind(&unique);
 }
 
 
 
@@ skipping 1373 matching lines @@
   Condition cc = (check == ENABLE_INLINED_SMI_CHECK)
       ? (*jmp_address == Assembler::kJncShortOpcode ? not_zero : zero)
       : (*jmp_address == Assembler::kJnzShortOpcode ? not_carry : carry);
   *jmp_address = static_cast<byte>(Assembler::kJccShortPrefix | cc);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64