Adding a fast path for parsing index keys.

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index 0b8cb769ca679676b85b3dd8884500a480f23800..5adc57664986bf275379a6fb4de8951824084bea 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -301,23 +301,52 @@ Handle<Object> JsonParser<seq_ascii>::ParseJsonObject() {
   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(','));