Introduce new runtime function to make join with lower memory usage.

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 5777 matching lines @@
     SeqTwoByteString* answer = SeqTwoByteString::cast(object);
     StringBuilderConcatHelper(special,
                               answer->GetChars(),
                               fixed_array,
                               array_length);
     return answer;
   }
 }
 
 
+template <typename sinkchar>
+static inline void StringBuilderJoinHelper(String* separator,
+                                           sinkchar* sink,
+                                           FixedArray* fixed_array,
+                                           int array_length) {
+  ASSERT(array_length > 0);
+  String* s = String::cast(fixed_array->get(0));
+  int position = s->length();
+  String::WriteToFlat(s, sink, 0, position);
+
+  int separator_length = separator->length();
+
+  for (int i = 1; i < array_length; i++) {
+    String::WriteToFlat(separator, sink + position, 0, separator_length);
+    position += separator_length;
+
+    String* element = String::cast(fixed_array->get(i));
+    int element_length = element->length();
+    String::WriteToFlat(element, sink + position, 0, element_length);
+    position += element_length;
+  }
+}
+
+
+static MaybeObject* Runtime_StringBuilderJoin(Arguments args) {
+  NoHandleAllocation ha;
+  ASSERT(args.length() == 3);
+  CONVERT_CHECKED(JSArray, array, args[0]);
+  if (!args[1]->IsSmi()) {
+    Top::context()->mark_out_of_memory();
+    return Failure::OutOfMemoryException();
+  }
+  int array_length = Smi::cast(args[1])->value();
+  CONVERT_CHECKED(String, separator, args[2]);
+
+  if (!array->HasFastElements()) {
+    return Top::Throw(Heap::illegal_argument_symbol());
+  }
+  FixedArray* fixed_array = FixedArray::cast(array->elements());
+  if (fixed_array->length() < array_length) {
+    array_length = fixed_array->length();
+  }
+
+  if (array_length == 0) {
+    return Heap::empty_string();
+  } else if (array_length == 1) {
+    Object* first = fixed_array->get(0);
+    if (first->IsString()) return first;
+  }
+
+  int separator_length = separator->length();
+  int max_nof_separators =
+      (String::kMaxLength + separator_length - 1) / separator_length;
+  if (max_nof_separators < (array_length - 1)) {
+      Top::context()->mark_out_of_memory();
+      return Failure::OutOfMemoryException();
+  }
+  int position = (array_length - 1) * separator_length;
+  bool ascii = separator->HasOnlyAsciiChars();

[sandholm, 2011/02/14 15:23:04]

You will always need a TwoByteString.
If everything is ascii then the %_FastAsciiArrayJoin call in the array.js file
would have produced an ascii result already.
I.e. you can safely remove the ascii variable and checks and inline the
StringBuilderJoinHelper function.

+  for (int i = 0; i < array_length; i++) {
+    int increment = 0;
+    String* element = String::cast(fixed_array->get(i));
+    int element_length = element->length();
+    increment = element_length;
+    if (ascii && !element->HasOnlyAsciiChars()) {
+      ascii = false;
+    }
+    if (increment > String::kMaxLength - position) {
+      Top::context()->mark_out_of_memory();
+      return Failure::OutOfMemoryException();
+    }
+    position += increment;
+  }
+
+  int length = position;
+  Object* object;
+
+  if (ascii) {
+    { MaybeObject* maybe_object = Heap::AllocateRawAsciiString(length);
+      if (!maybe_object->ToObject(&object)) return maybe_object;
+    }
+    SeqAsciiString* answer = SeqAsciiString::cast(object);
+    StringBuilderJoinHelper(separator,
+                            answer->GetChars(),
+                            fixed_array,
+                            array_length);
+    return answer;
+  } else {
+    { MaybeObject* maybe_object = Heap::AllocateRawTwoByteString(length);
+      if (!maybe_object->ToObject(&object)) return maybe_object;
+    }
+    SeqTwoByteString* answer = SeqTwoByteString::cast(object);
+    StringBuilderJoinHelper(separator,
+                            answer->GetChars(),
+                            fixed_array,
+                            array_length);
+    return answer;
+  }
+}
+
+
 static MaybeObject* Runtime_NumberOr(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 2);
 
   CONVERT_NUMBER_CHECKED(int32_t, x, Int32, args[0]);
   CONVERT_NUMBER_CHECKED(int32_t, y, Int32, args[1]);
   return Heap::NumberFromInt32(x | y);
 }
 
 
@@ skipping 5290 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