Reduced frequency of blocks for concurrent gc

Immediately return for references that are marked before reference
processing without blocking. Soft references are kept in the queue until
the reference processor stops preserving, after which, all marked
references are removed. Finalizer references will still block on get().

Bug: 15471830
Change-Id: I588fcaef40b79ed7c95a4aa7f4fc2e17ee0c288f

1 files changed, 11 insertions, 10 deletions

diff --git a/runtime/gc/reference_queue.cc b/runtime/gc/reference_queue.cc
index caacef5cf0..3910c297a2 100644
--- a/runtime/gc/reference_queue.cc
+++ b/runtime/gc/reference_queue.cc
@@ -160,22 +160,23 @@ void ReferenceQueue::EnqueueFinalizerReferences(ReferenceQueue& cleared_referenc
   }
 }
 
-void ReferenceQueue::PreserveSomeSoftReferences(IsMarkedCallback* preserve_callback, void* arg) {
-  ReferenceQueue cleared;
-  while (!IsEmpty()) {
-    mirror::Reference* ref = DequeuePendingReference();
+void ReferenceQueue::ForwardSoftReferences(IsMarkedCallback* preserve_callback,
+                                                void* arg) {
+  if (UNLIKELY(IsEmpty())) {
+    return;
+  }
+  mirror::Reference* const head = list_;
+  mirror::Reference* ref = head;
+  do {
     mirror::Object* referent = ref->GetReferent<kWithoutReadBarrier>();
     if (referent != nullptr) {
       mirror::Object* forward_address = preserve_callback(referent, arg);
-      if (forward_address == nullptr) {
-        // Either the reference isn't marked or we don't wish to preserve it.
-        cleared.EnqueuePendingReference(ref);
-      } else if (forward_address != referent) {
+      if (forward_address != nullptr && forward_address != referent) {
         ref->SetReferent<false>(forward_address);
       }
     }
-  }
-  list_ = cleared.GetList();
+    ref = ref->GetPendingNext();
+  } while (LIKELY(ref != head));
 }
 
 }  // namespace gc