Optimize JSON stringify by allowing QuoteJSONString to prefix with a comma.

src/runtime.cc

Index: src/runtime.cc
===================================================================
--- src/runtime.cc	(revision 6322)
+++ src/runtime.cc	(working copy)
@@ -4621,12 +4621,12 @@
 }
 
 
-template <typename Char, typename StringType>
+template <typename Char, typename StringType, bool comma>
 static MaybeObject* SlowQuoteJsonString(Vector<const Char> characters) {

[Erik Corry, 2011/01/14 14:42:57]

I think it would be nice to know what the performance hit of having this as a
normal argument is, before blowing this code up with a factor of 2.

[sandholm, 2011/01/16 21:08:43]

Having this as a normal argument makes the change regress performance on JSON
stringify rather than improve it.

   int length = characters.length();
   const Char* read_cursor = characters.start();
   const Char* end = read_cursor + length;
-  const int kSpaceForQuotes = 2;
+  const int kSpaceForQuotes = 2 + (comma ? 1 :0);
   int quoted_length = kSpaceForQuotes;
   while (read_cursor < end) {
     Char c = *(read_cursor++);
@@ -4645,8 +4645,9 @@
 
   Char* write_cursor = reinterpret_cast<Char*>(
       new_string->address() + SeqAsciiString::kHeaderSize);
+  if (comma) *(write_cursor++) = ',';
   *(write_cursor++) = '"';
-
+  
   read_cursor = characters.start();
   while (read_cursor < end) {
     Char c = *(read_cursor++);
@@ -4666,14 +4667,14 @@
 }
 
 
-template <typename Char, typename StringType>
+template <typename Char, typename StringType, bool comma>
 static MaybeObject* QuoteJsonString(Vector<const Char> characters) {
   int length = characters.length();
   Counters::quote_json_char_count.Increment(length);
-  const int kSpaceForQuotes = 2;
+  const int kSpaceForQuotes = 2 + (comma ? 1 :0);
   int worst_case_length = length * kJsonQuoteWorstCaseBlowup + kSpaceForQuotes;
   if (worst_case_length > kMaxGuaranteedNewSpaceString) {
-    return SlowQuoteJsonString<Char, StringType>(characters);
+    return SlowQuoteJsonString<Char, StringType, comma>(characters);
   }
 
   MaybeObject* new_alloc = AllocateRawString<StringType>(worst_case_length);
@@ -4686,7 +4687,7 @@
     // handle it being allocated in old space as may happen in the third
     // attempt.  See CALL_AND_RETRY in heap-inl.h and similar code in
     // CEntryStub::GenerateCore.
-    return SlowQuoteJsonString<Char, StringType>(characters);
+    return SlowQuoteJsonString<Char, StringType, comma>(characters);
   }
   StringType* new_string = StringType::cast(new_object);
   ASSERT(Heap::new_space()->Contains(new_string));
@@ -4694,6 +4695,7 @@
   STATIC_ASSERT(SeqTwoByteString::kHeaderSize == SeqAsciiString::kHeaderSize);
   Char* write_cursor = reinterpret_cast<Char*>(
       new_string->address() + SeqAsciiString::kHeaderSize);
+  if (comma) *(write_cursor++) = ',';
   *(write_cursor++) = '"';
 
   const Char* read_cursor = characters.start();
@@ -4744,14 +4746,33 @@
     ASSERT(str->IsFlat());
   }
   if (str->IsTwoByteRepresentation()) {
-    return QuoteJsonString<uc16, SeqTwoByteString>(str->ToUC16Vector());
+    return QuoteJsonString<uc16, SeqTwoByteString, false>(str->ToUC16Vector());
   } else {
-    return QuoteJsonString<char, SeqAsciiString>(str->ToAsciiVector());
+    return QuoteJsonString<char, SeqAsciiString, false>(str->ToAsciiVector());
   }
 }
 
 
+static MaybeObject* Runtime_QuoteJSONStringComma(Arguments args) {
+  NoHandleAllocation ha;
+  CONVERT_CHECKED(String, str, args[0]);
+  if (!str->IsFlat()) {
+    MaybeObject* try_flatten = str->TryFlatten();
+    Object* flat;
+    if (!try_flatten->ToObject(&flat)) {
+      return try_flatten;
+    }
+    str = String::cast(flat);
+    ASSERT(str->IsFlat());
+  }
+  if (str->IsTwoByteRepresentation()) {
+    return QuoteJsonString<uc16, SeqTwoByteString, true>(str->ToUC16Vector());
+  } else {
+    return QuoteJsonString<char, SeqAsciiString, true>(str->ToAsciiVector());
+  }
+}
 
+
 static MaybeObject* Runtime_StringParseInt(Arguments args) {
   NoHandleAllocation ha;