Refactor and improve GC root handling

Changed GcRoot to use compressed references. Changed root visiting to
use virtual functions instead of function pointers. Changed root visting
interface to be an array of roots instead of a single root at a time.
Added buffered root marking helper to avoid dispatch overhead.

Root marking seems a bit faster on EvaluateAndApplyChanges due to batch
marking. Pause times unaffected.

Mips64 is untested but might work, maybe.

Before:
MarkConcurrentRoots: Sum: 67.678ms 99% C.I. 2us-664.999us Avg: 161.138us Max: 671us

After:
MarkConcurrentRoots: Sum: 54.806ms 99% C.I. 2us-499.986us Avg: 136.333us Max: 602us

Bug: 19264997

Change-Id: I0a71ebb5928f205b9b3f7945b25db6489d5657ca

1 files changed, 22 insertions, 0 deletions

diff --git a/runtime/mirror/object_reference.h b/runtime/mirror/object_reference.h
index b63d13d602..5edda8b346 100644
--- a/runtime/mirror/object_reference.h
+++ b/runtime/mirror/object_reference.h
@@ -43,6 +43,11 @@ class MANAGED ObjectReference {
 
   void Clear() {
     reference_ = 0;
+    DCHECK(IsNull());
+  }
+
+  bool IsNull() const {
+    return reference_ == 0;
   }
 
   uint32_t AsVRegValue() const {
@@ -86,6 +91,23 @@ class MANAGED HeapReference : public ObjectReference<kPoisonHeapReferences, Mirr
       : ObjectReference<kPoisonHeapReferences, MirrorType>(mirror_ptr) {}
 };
 
+// Standard compressed reference used in the runtime. Used for StackRefernce and GC roots.
+template<class MirrorType>
+class MANAGED CompressedReference : public mirror::ObjectReference<false, MirrorType> {
+ public:
+  CompressedReference<MirrorType>() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
+      : mirror::ObjectReference<false, MirrorType>(nullptr) {}
+
+  static CompressedReference<MirrorType> FromMirrorPtr(MirrorType* p)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    return CompressedReference<MirrorType>(p);
+  }
+
+ private:
+  CompressedReference<MirrorType>(MirrorType* p) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
+      : mirror::ObjectReference<false, MirrorType>(p) {}
+};
+
 }  // namespace mirror
 }  // namespace art