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, 8 insertions, 4 deletions

diff --git a/runtime/class_linker_test.cc b/runtime/class_linker_test.cc
index 3e727e7..3f6c5a0 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -358,7 +358,8 @@ class ClassLinkerTest : public CommonRuntimeTest {
       const char* descriptor = dex.GetTypeDescriptor(type_id);
       AssertDexFileClass(class_loader, descriptor);
     }
-    class_linker_->VisitRoots(TestRootVisitor, nullptr, kVisitRootFlagAllRoots);
+    TestRootVisitor visitor;
+    class_linker_->VisitRoots(&visitor, kVisitRootFlagAllRoots);
     // Verify the dex cache has resolution methods in all resolved method slots
     mirror::DexCache* dex_cache = class_linker_->FindDexCache(dex);
     mirror::ObjectArray<mirror::ArtMethod>* resolved_methods = dex_cache->GetResolvedMethods();
@@ -367,9 +368,12 @@ class ClassLinkerTest : public CommonRuntimeTest {
     }
   }
 
-  static void TestRootVisitor(mirror::Object** root, void*, const RootInfo&) {
-    EXPECT_TRUE(*root != nullptr);
-  }
+  class TestRootVisitor : public SingleRootVisitor {
+   public:
+    void VisitRoot(mirror::Object* root, const RootInfo& info ATTRIBUTE_UNUSED) OVERRIDE {
+      EXPECT_TRUE(root != nullptr);
+    }
+  };
 };
 
 struct CheckOffset {