Improve WriteUtf8 and WriteAscii.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index ac4f07fd5e1e8e4ab9ca4b4eeaf1aa1bf9f5c728..5c6b6e8a0f36b767ed364d98d5989764acca98c2 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);
+  }
   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 +3724,21 @@ 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()) {
+    if (length == -1) length = str->length() + 1;
+    int len = i::Min(length, str->length() - start);
+    i::String::WriteToFlat(*str, buffer, start, start + len);
+    for (int i = 0; i < len; i++) {
+      if (buffer[i] == '\0') buffer[i] = ' ';
+    }
+    if (!(options & NO_NULL_TERMINATION) && length > len) {
+      buffer[len] = '\0';
+    }
+    return len;
   }
+
   int end = length;
   if ( (length == -1) || (length > str->length() - start) )
     end = str->length() - start;
@@ -3756,7 +3770,7 @@ 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) )