Some string optimizations:

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 3609 matching lines @@
   Condition is_smi = masm_->CheckSmi(value.reg());
   destination()->false_target()->Branch(is_smi);
   // It is a heap object - get map.
   // Check if the object is a JS array or not.
   __ CmpObjectType(value.reg(), JS_ARRAY_TYPE, kScratchRegister);
   value.Unuse();
   destination()->Split(equal);
 }
 
 
+void CodeGenerator::GenerateIsRegExp(ZoneList<Expression*>* args) {
+  ASSERT(args->length() == 1);
+  Load(args->at(0));
+  Result value = frame_->Pop();
+  value.ToRegister();
+  ASSERT(value.is_valid());
+  Condition is_smi = masm_->CheckSmi(value.reg());
+  destination()->false_target()->Branch(is_smi);
+  // It is a heap object - get map.
+  // Check if the object is a regexp.
+  __ CmpObjectType(value.reg(), JS_REGEXP_TYPE, kScratchRegister);
+  value.Unuse();
+  destination()->Split(equal);
+}
+
+
 void CodeGenerator::GenerateIsObject(ZoneList<Expression*>* args) {
   // This generates a fast version of:
   // (typeof(arg) === 'object' || %_ClassOf(arg) == 'RegExp')
   ASSERT(args->length() == 1);
   Load(args->at(0));
   Result obj = frame_->Pop();
   obj.ToRegister();
   Condition is_smi = masm_->CheckSmi(obj.reg());
   destination()->false_target()->Branch(is_smi);
 
@@ skipping 5834 matching lines @@
   // Call the function from C++.
   return FUNCTION_CAST<ModuloFunction>(buffer);
 }
 
 #endif
 
 
 #undef __
 
 } }  // namespace v8::internal