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, 6 insertions, 8 deletions

diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index a767cf086f..3f67f9e72d 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -344,16 +344,14 @@ uint32_t Dbg::instrumentation_events_ = 0;
 // Breakpoints.
 static std::vector<Breakpoint> gBreakpoints GUARDED_BY(Locks::breakpoint_lock_);
 
-void DebugInvokeReq::VisitRoots(RootCallback* callback, void* arg, const RootInfo& root_info) {
-  receiver.VisitRootIfNonNull(callback, arg, root_info);  // null for static method call.
-  klass.VisitRoot(callback, arg, root_info);
-  method.VisitRoot(callback, arg, root_info);
+void DebugInvokeReq::VisitRoots(RootVisitor* visitor, const RootInfo& root_info) {
+  receiver.VisitRootIfNonNull(visitor, root_info);  // null for static method call.
+  klass.VisitRoot(visitor, root_info);
+  method.VisitRoot(visitor, root_info);
 }
 
-void SingleStepControl::VisitRoots(RootCallback* callback, void* arg, const RootInfo& root_info) {
-  if (method_ != nullptr) {
-    callback(reinterpret_cast<mirror::Object**>(&method_), arg, root_info);
-  }
+void SingleStepControl::VisitRoots(RootVisitor* visitor, const RootInfo& root_info) {
+  visitor->VisitRootIfNonNull(reinterpret_cast<mirror::Object**>(&method_), root_info);
 }
 
 void SingleStepControl::AddDexPc(uint32_t dex_pc) {