Improve array to string conversion.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 324b10ce2066c5fc0c24fd485813f895557b089c..86b7e844ec0681068b85720cd4291a165f8d6401 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7622,6 +7622,30 @@ bool String::SlowAsArrayIndex(uint32_t* index) {
 }
 
 
+void SeqString::Truncate(int new_length) {
+  int string_size, allocated_string_size;
+  ASSERT(new_length < length() && new_length > 0);
+  if (IsSeqOneByteString()) {
+    allocated_string_size = SeqOneByteString::SizeFor(length());
+    string_size = SeqOneByteString::SizeFor(new_length);
+  } else {
+    allocated_string_size = SeqTwoByteString::SizeFor(length());
+    string_size = SeqTwoByteString::SizeFor(new_length);
+  }
+
+  int delta = allocated_string_size - string_size;
+  set_length(new_length);
+
+  // String sizes are pointer size aligned, so that we can use filler objects
+  // that are a multiple of pointer size.
+  Address end_of_string = address() + string_size;
+  GetHeap()->CreateFillerObjectAt(end_of_string, delta);
+  if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
+    MemoryChunk::IncrementLiveBytesFromMutator(address(), -delta);
+  }
+}
+
+
 uint32_t StringHasher::MakeArrayIndexHash(uint32_t value, int length) {
   // For array indexes mix the length into the hash as an array index could
   // be zero.