Improve WriteUtf8 and WriteAscii.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index ac4f07fd5e1e8e4ab9ca4b4eeaf1aa1bf9f5c728..2548c06fdd0bf6146558f3c39c899bea0b31832b 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -3642,6 +3642,11 @@ int String::WriteUtf8(char* buffer,
   LOG_API(isolate, "String::WriteUtf8");
   ENTER_V8(isolate);
   i::Handle<i::String> str = Utils::OpenHandle(this);
+  if (options & HINT_MANY_WRITES_EXPECTED) {
+    // Flatten the string for efficiency.  This applies whether we are
+    // using StringInputBuffer or Get(i) to access the characters.
+    FlattenString(str);
+  }

[Yang, 2011/10/25 16:21:45]

Flatten as early as possible. FlattenString does nothing else than WriteToFlat.
This way, the WriteToFlat in the Ascii case will be merely a copy from the
already flattened string.

   if (str->IsAsciiRepresentation()) {
     int len;
     if (capacity == -1) {
@@ -3661,11 +3666,7 @@ int String::WriteUtf8(char* buffer,
 
   i::StringInputBuffer& write_input_buffer = *isolate->write_input_buffer();
   isolate->string_tracker()->RecordWrite(str);
-  if (options & HINT_MANY_WRITES_EXPECTED) {
-    // Flatten the string for efficiency.  This applies whether we are
-    // using StringInputBuffer or Get(i) to access the characters.
-    FlattenString(str);
-  }
+
   write_input_buffer.Reset(0, *str);
   int len = str->length();
   // Encode the first K - 3 bytes directly into the buffer since we
@@ -3723,8 +3724,24 @@ int String::WriteAscii(char* buffer,
   if (options & HINT_MANY_WRITES_EXPECTED) {
     // Flatten the string for efficiency.  This applies whether we are
     // using StringInputBuffer or Get(i) to access the characters.
-    str->TryFlatten();
+    FlattenString(str);
   }
+  if (str->IsAsciiRepresentation()) {

[Yang, 2011/10/25 16:21:45]

Same thing as WriteUtf8. WriteToFlat is faster than InputBuffer/ReadBlock
because in case of a convoluted cons string, ReadBlock has to hunt down the
right part to read from after every 1024 characters (buffer size).

[Erik Corry, 2011/10/25 19:21:22]

This doesn't do the same with embedded nulls as the previous WriteAscii code. 
That makes me think that our test coverage of WriteAscii is not good enough. 
The treatment of embedded null characters is pretty bogus though, which makes me
wonder if anyone uses this function.

+    int len;
+    if (length == -1) {
+      length = str->length() + 1;
+      len = str->length();
+    } else {
+      len = i::Min(length, str->length());
+    }
+    i::String::WriteToFlat(*str, buffer, start, len);
+    if (!(options & NO_NULL_TERMINATION) && length > len) {
+      buffer[len] = '\0';
+      return len + 1;
+    }
+    return len;
+  }
+
   int end = length;
   if ( (length == -1) || (length > str->length() - start) )
     end = str->length() - start;
@@ -3756,7 +3773,7 @@ int String::Write(uint16_t* buffer,
   if (options & HINT_MANY_WRITES_EXPECTED) {
     // Flatten the string for efficiency.  This applies whether we are
     // using StringInputBuffer or Get(i) to access the characters.
-    str->TryFlatten();
+    FlattenString(str);
   }
   int end = start + length;
   if ((length == -1) || (length > str->length() - start) )