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

src/runtime.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 4603 matching lines @@
   return Heap::AllocateRawTwoByteString(length);
 }
 
 
 template <>
 MaybeObject* AllocateRawString<SeqAsciiString>(int length) {
   return Heap::AllocateRawAsciiString(length);
 }
 
 
-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++);
     if (sizeof(Char) > 1u && static_cast<unsigned>(c) >= kQuoteTableLength) {
       quoted_length++;
     } else {
       quoted_length += JsonQuoteLengths[static_cast<unsigned>(c)];
     }
   }
   MaybeObject* new_alloc = AllocateRawString<StringType>(quoted_length);
   Object* new_object;
   if (!new_alloc->ToObject(&new_object)) {
     return new_alloc;
   }
   StringType* new_string = StringType::cast(new_object);
 
   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++);
     if (sizeof(Char) > 1u && static_cast<unsigned>(c) >= kQuoteTableLength) {
       *(write_cursor++) = c;
     } else {
       int len = JsonQuoteLengths[static_cast<unsigned>(c)];
       const char* replacement = JsonQuotes +
           static_cast<unsigned>(c) * kJsonQuotesCharactersPerEntry;
       for (int i = 0; i < len; i++) {
         *write_cursor++ = *replacement++;
       }
     }
   }
   *(write_cursor++) = '"';
   return new_string;
 }
 
 
-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);
   Object* new_object;
   if (!new_alloc->ToObject(&new_object)) {
     return new_alloc;
   }
   if (!Heap::new_space()->Contains(new_object)) {
     // Even if our string is small enough to fit in new space we still have to
     // 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));
 
   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();
   const Char* end = read_cursor + length;
   while (read_cursor < end) {
     Char c = *(read_cursor++);
     if (sizeof(Char) > 1u && static_cast<unsigned>(c) >= kQuoteTableLength) {
       *(write_cursor++) = c;
     } else {
       int len = JsonQuoteLengths[static_cast<unsigned>(c)];
@@ skipping 30 matching lines @@
   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>(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;
 
   CONVERT_CHECKED(String, s, args[0]);
   CONVERT_SMI_CHECKED(radix, args[1]);
 
   s->TryFlatten();
 
   RUNTIME_ASSERT(radix == 0 || (2 <= radix && radix <= 36));
@@ skipping 6065 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal