Let relational comparison operators quickly compare a string with itself.

src/x64/codegen-x64.cc

 // Copyright 2010 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 9059 matching lines @@
     // rax must be unchanged on jump to not_identical.
 
     if (never_nan_nan_ && (cc_ == equal)) {
       __ Set(rax, EQUAL);
       __ ret(0);
     } else {
       Label heap_number;
       // If it's not a heap number, then return equal for (in)equality operator.
       __ Cmp(FieldOperand(rdx, HeapObject::kMapOffset),
              Factory::heap_number_map());
-      if (cc_ == equal) {
-        __ j(equal, &heap_number);
-        __ Set(rax, EQUAL);
-        __ ret(0);
-      } else {
-        // Identical objects must still be converted to primitive for < and >.
-        __ j(not_equal, &not_identical);
+      __ j(equal, &heap_number);
+      if (cc_ != equal) {
+        // Call runtime on identical JSObjects.  Otherwise return equal.
+        __ CmpObjectType(rax, FIRST_JS_OBJECT_TYPE, rcx);
+        __ j(above_equal, &not_identical);
       }
+      __ Set(rax, EQUAL);
+      __ ret(0);
 
       __ bind(&heap_number);
       // It is a heap number, so return  equal if it's not NaN.
       // For NaN, return 1 for every condition except greater and
       // greater-equal.  Return -1 for them, so the comparison yields
       // false for all conditions except not-equal.
       __ Set(rax, EQUAL);
       __ movsd(xmm0, FieldOperand(rdx, HeapNumber::kValueOffset));
       __ ucomisd(xmm0, xmm0);
       __ setcc(parity_even, rax);
@@ skipping 2953 matching lines @@
 }
 
 #endif
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64