Add a read barrier for weak roots in monitors.

A weak root requires a read barrier for the to-space invariant to hold
because the object pointed to by a weak root can't be marked/forwarded
like the one pointed to by a strong root (GC does not know if it's
alive or not at that point) and because, without a read barrier, a
mutator could access it and obtain a from-space reference, which would
violate the to-space invariant.

TODO: do similar for the other types of weak roots.

Bug: 12687968
Change-Id: I563a0fa4f875e0c21ac96f57696959454e13b15a

1 files changed, 15 insertions, 0 deletions

diff --git a/runtime/read_barrier-inl.h b/runtime/read_barrier-inl.h
index 88e2f8f..4302c9e 100644
--- a/runtime/read_barrier-inl.h
+++ b/runtime/read_barrier-inl.h
@@ -43,6 +43,21 @@ inline MirrorType* ReadBarrier::Barrier(
   }
 }
 
+template <typename MirrorType, ReadBarrierOption kReadBarrierOption>
+inline MirrorType* ReadBarrier::BarrierForWeakRoot(MirrorType* ref) {
+  UNUSED(ref);
+  const bool with_read_barrier = kReadBarrierOption == kWithReadBarrier;
+  if (with_read_barrier && kUseBakerReadBarrier) {
+    // To be implemented.
+    return ref;
+  } else if (with_read_barrier && kUseBrooksReadBarrier) {
+    // To be implemented.
+    return ref;
+  } else {
+    return ref;
+  }
+}
+
 }  // namespace art
 
 #endif  // ART_RUNTIME_READ_BARRIER_INL_H_