Expose a hint which communicates that string might be written many times.

src/api.cc

 // Copyright 2009 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 2623 matching lines @@
   return Utils::OpenHandle(this)->length();
 }
 
 
 int String::Utf8Length() const {
   if (IsDeadCheck("v8::String::Utf8Length()")) return 0;
   return Utils::OpenHandle(this)->Utf8Length();
 }
 
 
-int String::WriteUtf8(char* buffer, int capacity, int* nchars_ref) const {
+int String::WriteUtf8(char* buffer,
+                      int capacity,
+                      int* nchars_ref,
+                      WriteHints hints) const {
   if (IsDeadCheck("v8::String::WriteUtf8()")) return 0;
   LOG_API("String::WriteUtf8");
   ENTER_V8;
   i::Handle<i::String> str = Utils::OpenHandle(this);
   StringTracker::RecordWrite(str);
+  if (hints & 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();
+  }
   write_input_buffer.Reset(0, *str);
   int len = str->length();
   // Encode the first K - 3 bytes directly into the buffer since we
   // know there's room for them.  If no capacity is given we copy all
   // of them here.
   int fast_end = capacity - (unibrow::Utf8::kMaxEncodedSize - 1);
   int i;
   int pos = 0;
   int nchars = 0;
   for (i = 0; i < len && (capacity == -1 || pos < fast_end); i++) {
@@ skipping 21 matching lines @@
       }
     }
   }
   if (nchars_ref != NULL) *nchars_ref = nchars;
   if (i == len && (capacity == -1 || pos < capacity))
     buffer[pos++] = '\0';
   return pos;
 }
 
 
-int String::WriteAscii(char* buffer, int start, int length) const {
+int String::WriteAscii(char* buffer,
+                       int start,
+                       int length,
+                       WriteHints hints) const {
   if (IsDeadCheck("v8::String::WriteAscii()")) return 0;
   LOG_API("String::WriteAscii");
   ENTER_V8;
   ASSERT(start >= 0 && length >= -1);
   i::Handle<i::String> str = Utils::OpenHandle(this);
   StringTracker::RecordWrite(str);
-  // Flatten the string for efficiency.  This applies whether we are
-  // using StringInputBuffer or Get(i) to access the characters.
-  str->TryFlatten();
+  if (hints & 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();
+  }
   int end = length;
   if ( (length == -1) || (length > str->length() - start) )
     end = str->length() - start;
   if (end < 0) return 0;
   write_input_buffer.Reset(start, *str);
   int i;
   for (i = 0; i < end; i++) {
     char c = static_cast<char>(write_input_buffer.GetNext());
     if (c == '\0') c = ' ';
     buffer[i] = c;
   }
   if (length == -1 || i < length)
     buffer[i] = '\0';
   return i;
 }
 
 
-int String::Write(uint16_t* buffer, int start, int length) const {
+int String::Write(uint16_t* buffer,
+                  int start,
+                  int length,
+                  WriteHints hints) const {
   if (IsDeadCheck("v8::String::Write()")) return 0;
   LOG_API("String::Write");
   ENTER_V8;
   ASSERT(start >= 0 && length >= -1);
   i::Handle<i::String> str = Utils::OpenHandle(this);
   StringTracker::RecordWrite(str);
+  if (hints & 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();
+  }
   int end = length;
   if ( (length == -1) || (length > str->length() - start) )
     end = str->length() - start;
   if (end < 0) return 0;
   i::String::WriteToFlat(*str, buffer, start, end);
   if (length == -1 || end < length)
     buffer[end] = '\0';
   return end;
 }
 
 
-void v8::String::Flatten() {
-  if (IsDeadCheck("v8::String::Flatten()")) return;
-  i::Handle<i::String> str = Utils::OpenHandle(this);
-  i::FlattenString(str);
-}
-
-
 bool v8::String::IsExternal() const {
   EnsureInitialized("v8::String::IsExternal()");
   i::Handle<i::String> str = Utils::OpenHandle(this);
   return i::StringShape(*str).IsExternalTwoByte();
 }
 
 
 bool v8::String::IsExternalAscii() const {
   EnsureInitialized("v8::String::IsExternalAscii()");
   i::Handle<i::String> str = Utils::OpenHandle(this);
@@ skipping 1460 matching lines @@
 
 
 char* HandleScopeImplementer::Iterate(ObjectVisitor* v, char* storage) {
   HandleScopeImplementer* thread_local =
       reinterpret_cast<HandleScopeImplementer*>(storage);
   thread_local->IterateThis(v);
   return storage + ArchiveSpacePerThread();
 }
 
 } }  // namespace v8::internal