Eliminate RawObject::class_ field entirely.

runtime/vm/scavenger.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/scavenger.h"
 
 #include "vm/dart.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
 #include "vm/stack_frame.h"
 #include "vm/verifier.h"
 #include "vm/visitor.h"
 
 namespace dart {
 
+// Scavenger uses RawObject::kFreeBit to distinguish forwaded and non-forwarded
+// objects because scavenger can never encounter free list element during
+// evacuation and thus all objects scavenger encounters have
+// kFreeBit cleared.
 enum {
-  kForwardingMask = 3,
-  kNotForwarded = 1,  // Tagged pointer.
-  kForwarded = 3,  // Tagged pointer and forwarding bit set.
+  kForwardingMask = 1,
+  kNotForwarded = 0,
+  kForwarded = 1,
 };
 
 
 static inline bool IsForwarding(uword header) {
   uword bits = header & kForwardingMask;
   ASSERT((bits == kNotForwarded) || (bits == kForwarded));
   return bits == kForwarded;
 }
 
 
@@ skipping 119 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(ScavengerWeakVisitor);
 };
 
 
 Scavenger::Scavenger(Heap* heap, intptr_t max_capacity, uword object_alignment)
     : heap_(heap),
       object_alignment_(object_alignment),
       count_(0),
       scavenging_(false) {
+  // Verify assumptions about the first word in objects that scavenger is going

[Ivan Posva, 2012/06/06 13:42:11]

objects which the scavenger

+  // to use for forwarding pointers.
+  ASSERT(Object::tags_offset() == 0);
+  ASSERT(kForwardingMask == (1 << RawObject::kFreeBit));
+
   // Allocate the virtual memory for this scavenge heap.
   space_ = VirtualMemory::Reserve(max_capacity);
   ASSERT(space_ != NULL);
 
   // Allocate the entire space at the beginning.
   space_->Commit(false);
 
   // Setup the semi spaces.
   uword semi_space_size = space_->size() / 2;
   ASSERT((semi_space_size & (VirtualMemory::PageSize() - 1)) == 0);
@@ skipping 217 matching lines @@
     OS::PrintErr(" done.\n");
   }
 
   count_++;
   // Done scavenging. Reset the marker.
   ASSERT(scavenging_);
   scavenging_ = false;
 }
 
 }  // namespace dart