Adding a fast path for parsing index keys.

src/json-parser.h

 // 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 283 matching lines @@
   Handle<JSFunction> object_constructor(
       isolate()->native_context()->object_function());
   Handle<JSObject> json_object =
       isolate()->factory()->NewJSObject(object_constructor);
   ASSERT_EQ(c0_, '{');
 
   AdvanceSkipWhitespace();
   if (c0_ != '}') {
     do {
       if (c0_ != '"') return ReportUnexpectedCharacter();
-      Handle<String> key = ParseJsonSymbol();
-      if (key.is_null() || c0_ != ':') return ReportUnexpectedCharacter();
-      AdvanceSkipWhitespace();
-      Handle<Object> value = ParseJsonValue();
-      if (value.is_null()) return ReportUnexpectedCharacter();
 
-      uint32_t index;
-      if (key->AsArrayIndex(&index)) {
+      int position = position_;

[Yang, 2012/10/18 13:45:32]

maybe something more descriptive than "position", maybe "start_pos" so that it
becomes clear that you are resetting position_.

[Toon Verwaest, 2012/10/18 14:15:26]

Done.

+      Advance();
+
+      int digits = 0;
+      uint32_t index = 0;
+      while (c0_ >= '0' && c0_ <= '9') {
+        index = index * 10 + c0_ - '0';
+        digits++;
+        Advance();
+      }
+
+      if (digits > 0 && c0_ == '"' && digits < 10) {

[Yang, 2012/10/18 13:45:32]

Why 10?
The logic of calculating array index (in StringHasher::AddCharacter and
String::ComputeArrayIndex) compares the intermediate result to kMaxUInt32/10 (we
should put kMaxUInt32/10 in there instead of the hardcoded literal, too). Maybe
we can just inline that logic here, so that we don't have to track the number of
digits. This way, we could also save the check for array index in the slow case
(l. 339).

+        AdvanceSkipWhitespace();
+
+        if (c0_ != ':') return ReportUnexpectedCharacter();
+        AdvanceSkipWhitespace();
+        Handle<Object> value = ParseJsonValue();
+        if (value.is_null()) return ReportUnexpectedCharacter();
+
         JSObject::SetOwnElement(json_object, index, value, kNonStrictMode);
-      } else if (key->Equals(isolate()->heap()->Proto_symbol())) {
-        prototype = value;
       } else {
-        if (JSObject::TryTransitionToField(json_object, key)) {
-          json_object->FastPropertyAtPut(current_index++, *value);
+        position_ = position;
+#ifdef DEBUG
+        c0_ = '"';
+#endif
+
+        Handle<String> key = ParseJsonSymbol();
+        if (key.is_null()) return ReportUnexpectedCharacter();

[Yang, 2012/10/18 13:45:32]

Keeping it as
if (key.is_null() || c0_ != ':') return ReportUnexpectedCharacter();
seems alright.

[Toon Verwaest, 2012/10/18 14:15:26]

Done.

+
+        if (c0_ != ':') return ReportUnexpectedCharacter();
+        AdvanceSkipWhitespace();
+        Handle<Object> value = ParseJsonValue();
+        if (value.is_null()) return ReportUnexpectedCharacter();
+
+        if (key->AsArrayIndex(&index)) {
+          JSObject::SetOwnElement(json_object, index, value, kNonStrictMode);
+        } else if (key->Equals(isolate()->heap()->Proto_symbol())) {
+          prototype = value;
         } else {
-          JSObject::SetLocalPropertyIgnoreAttributes(
-              json_object, key, value, NONE);
+          if (JSObject::TryTransitionToField(json_object, key)) {
+            json_object->FastPropertyAtPut(current_index++, *value);
+          } else {
+            JSObject::SetLocalPropertyIgnoreAttributes(
+                json_object, key, value, NONE);
+          }
         }
       }
     } while (MatchSkipWhiteSpace(','));
     if (c0_ != '}') {
       return ReportUnexpectedCharacter();
     }
     if (!prototype.is_null()) SetPrototype(json_object, prototype);
   }
   AdvanceSkipWhitespace();
   return json_object;
@@ skipping 318 matching lines @@
   }
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_