Improve SeqString::Truncate for latest allocated strings.

src/objects.cc

 // Copyright 2013 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 7608 matching lines @@
     if ((field & kIsNotArrayIndexMask) != 0) return false;
     // Isolate the array index form the full hash field.
     *index = (kArrayIndexHashMask & field) >> kHashShift;
     return true;
   } else {
     return ComputeArrayIndex(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;
 }
 
 
 AllocationSiteInfo* AllocationSiteInfo::FindForJSObject(JSObject* object) {
   // Currently, AllocationSiteInfo objects are only allocated immediately
   // after JSArrays in NewSpace, and detecting whether a JSArray has one
   // involves carefully checking the object immediately after the JSArray
   // (if there is one) to see if it's an AllocationSiteInfo.
   if (FLAG_track_allocation_sites && object->GetHeap()->InNewSpace(object)) {
     Address ptr_end = (reinterpret_cast<Address>(object) - kHeapObjectTag) +
@@ skipping 6660 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