Reduce unnecessary read barriers in GC

Removed read barrier from IsUnprocessed, DequeuePendingReference,
EnqueueReference, and a few other places.

Hard to tell if GC time goes down.

EAAC:
Before GC slow path count: 254857
After GC slow path count: 1005

Bug: 30162165
Bug: 12687968

Test: test-art-host, volantis boot with CC

Change-Id: Ic2add3a9b1e1d7561b0b167f2218b10f8dbff76c

1 files changed, 5 insertions, 3 deletions

diff --git a/runtime/gc/reference_queue.cc b/runtime/gc/reference_queue.cc
index 6088a43ab1..62625c41b4 100644
--- a/runtime/gc/reference_queue.cc
+++ b/runtime/gc/reference_queue.cc
@@ -44,7 +44,9 @@ void ReferenceQueue::EnqueueReference(mirror::Reference* ref) {
     // 1 element cyclic queue, ie: Reference ref = ..; ref.pendingNext = ref;
     list_ = ref;
   } else {
-    mirror::Reference* head = list_->GetPendingNext();
+    // The list is owned by the GC, everything that has been inserted must already be at least
+    // gray.
+    mirror::Reference* head = list_->GetPendingNext<kWithoutReadBarrier>();
     DCHECK(head != nullptr);
     ref->SetPendingNext(head);
   }
@@ -54,14 +56,14 @@ void ReferenceQueue::EnqueueReference(mirror::Reference* ref) {
 
 mirror::Reference* ReferenceQueue::DequeuePendingReference() {
   DCHECK(!IsEmpty());
-  mirror::Reference* ref = list_->GetPendingNext();
+  mirror::Reference* ref = list_->GetPendingNext<kWithoutReadBarrier>();
   DCHECK(ref != nullptr);
   // Note: the following code is thread-safe because it is only called from ProcessReferences which
   // is single threaded.
   if (list_ == ref) {
     list_ = nullptr;
   } else {
-    mirror::Reference* next = ref->GetPendingNext();
+    mirror::Reference* next = ref->GetPendingNext<kWithoutReadBarrier>();
     list_->SetPendingNext(next);
   }
   ref->SetPendingNext(nullptr);