Specialize JSON parser to only check for SequentialAsciiString once.

src/runtime.cc

 // Copyright 2011 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 8462 matching lines @@
   if (!global->IsJSGlobalObject()) return isolate->heap()->null_value();
   return JSGlobalObject::cast(global)->global_receiver();
 }
 
 
 RUNTIME_FUNCTION(MaybeObject*, Runtime_ParseJson) {
   HandleScope scope(isolate);
   ASSERT_EQ(1, args.length());
   CONVERT_ARG_CHECKED(String, source, 0);
 
-  Handle<Object> result = JsonParser::Parse(source);
+  source = Handle<String>(source->TryFlattenGetString());
+  // Optimized fast case where we only have ascii characters.
+  Handle<Object> result;
+  if (source->IsSeqAsciiString()) {
+    result = JsonParser<true>::Parse(source);
+  } else {
+    result = JsonParser<false>::Parse(source);
+  }
   if (result.is_null()) {
     // Syntax error or stack overflow in scanner.
     ASSERT(isolate->has_pending_exception());
     return Failure::Exception();
   }
   return *result;
 }
 
 
 bool CodeGenerationFromStringsAllowed(Isolate* isolate,
@@ skipping 3858 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal