Improve array to string conversion.

src/objects.cc

 // Copyright 2012 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 7604 matching lines @@
     // Isolate the array index form the full hash field.
     *index = (kArrayIndexHashMask & field) >> kHashShift;
     return true;
   } else {
     StringInputBuffer buffer(this);
     return ComputeArrayIndex(&buffer, index, length());
   }
 }
 
 
+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.
   ASSERT(length > 0);
   ASSERT(length <= String::kMaxArrayIndexSize);
   ASSERT(TenToThe(String::kMaxCachedArrayIndexLength) <
          (1 << String::kArrayIndexValueBits));
 
   value <<= String::kHashShift;
   value |= length << String::kArrayIndexHashLengthShift;
@@ skipping 6244 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal