Improve SeqString::Truncate for latest allocated strings.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index dbbec582228aad3e6d7b610d2a51d44205461ab9..5b70c98f28f77b55dd80579af00eeeefc7c63a30 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7626,33 +7626,42 @@ bool String::SlowAsArrayIndex(uint32_t* index) {
 }
 
 
-String* SeqString::Truncate(int new_length) {
-  Heap* heap = GetHeap();
-  if (new_length <= 0) return heap->empty_string();
+void SeqString::Truncate(int new_length) {
+  ASSERT_LT(0, new_length);

[Hannes Payer (out of office), 2013/03/21 12:48:30]

new_length = 0 should be possible

 
-  int string_size, allocated_string_size;
+  int new_size, old_size;
   int old_length = length();
-  if (old_length <= new_length) return this;
+  if (old_length <= new_length) return;
 
   if (IsSeqOneByteString()) {
-    allocated_string_size = SeqOneByteString::SizeFor(old_length);
-    string_size = SeqOneByteString::SizeFor(new_length);
+    old_size = SeqOneByteString::SizeFor(old_length);
+    new_size = SeqOneByteString::SizeFor(new_length);
   } else {
-    allocated_string_size = SeqTwoByteString::SizeFor(old_length);
-    string_size = SeqTwoByteString::SizeFor(new_length);
+    old_size = SeqTwoByteString::SizeFor(old_length);
+    new_size = SeqTwoByteString::SizeFor(new_length);
   }
 
-  int delta = allocated_string_size - string_size;
+  int delta = old_size - new_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;
-  heap->CreateFillerObjectAt(end_of_string, delta);
+  Address start_of_string = address();
+  ASSERT_OBJECT_ALIGNED(start_of_string);
+  ASSERT_OBJECT_ALIGNED(start_of_string + new_size);
+
+  Heap* heap = GetHeap();
+  if (heap->InNewSpace(start_of_string) &&
+      heap->new_space()->allocation_info_.top == start_of_string + old_size) {
+    // Last allocated object in new space.  Simply lower allocation top.
+    heap->new_space()->allocation_info_.top = start_of_string + new_size;

[Hannes Payer (out of office), 2013/03/21 12:48:30]

I think you also want to adjust MemoryChunk::IncrementLiveBytesFromMutator, as
it was before.

[Yang, 2013/03/21 14:18:34]

I'm doing this summarily below.

+  } else {
+    // String sizes are pointer size aligned, so that we can use filler objects
+    // that are a multiple of pointer size.
+    heap->CreateFillerObjectAt(start_of_string + new_size, delta);

[Hannes Payer (out of office), 2013/03/21 12:48:30]

Do we also have to adjust  MemoryChunk::IncrementLiveBytesFromMutator here?

[Yang, 2013/03/21 14:18:34]

I'm doing this summarily below.

+  }
+
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
-    MemoryChunk::IncrementLiveBytesFromMutator(address(), -delta);
+    MemoryChunk::IncrementLiveBytesFromMutator(start_of_string, -delta);
   }
-  return this;
 }