Implement Reference.refersTo()
Avoid Reference.get(), so that we
a) Don't block for the GC when weak ref access is disabled.
b) Don't preserve a referent just because we compared to it.
Strengthen ConcurrentCopying::Copy memory ordering guarantees to make
this possible. In the process, weaken the existing fence to a
constructor fence, which should sometimes be slightly cheaper than
a release fence, which will be implemented as a full fence on ARM.
While looking at forwarding reference memory ordering, removed a
long identified unnecessary seq_cst memory access in
IsNullOrMarkedHeapReference.
Test: boot AOSP, atest ReferenceTest in libcore
Bug: 172573708
Change-Id: Ie716df684be94006353a54226f85ea93a7254b6c
diff --git a/runtime/native/java_lang_ref_Reference.cc b/runtime/native/java_lang_ref_Reference.cc
index b241b1d..f23010b 100644
--- a/runtime/native/java_lang_ref_Reference.cc
+++ b/runtime/native/java_lang_ref_Reference.cc
@@ -36,6 +36,37 @@
return soa.AddLocalReference<jobject>(referent);
}
+static jboolean Reference_refersTo0(JNIEnv* env, jobject javaThis, jobject o) {
+ if (kUseReadBarrier && !kUseBakerReadBarrier) {
+ // Fall back to naive implementation that may block and needlessly preserve javaThis.
+ return env->IsSameObject(Reference_getReferent(env, javaThis), o);
+ }
+ ScopedFastNativeObjectAccess soa(env);
+ const ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
+ const ObjPtr<mirror::Object> other = soa.Decode<mirror::Reference>(o);
+ const ObjPtr<mirror::Object> referent = ref->template GetReferent<kWithoutReadBarrier>();
+ if (referent == other) {
+ return JNI_TRUE;
+ }
+ if (!kUseReadBarrier || referent.IsNull() || other.IsNull()) {
+ return JNI_FALSE;
+ }
+ // Explicitly handle the case in which referent is a from-space pointer. Don't use a
+ // read-barrier, since that could easily mark an object we no longer need and, since it
+ // creates new gray objects, may not be safe without blocking.
+ //
+ // ConcurrentCopying::Copy ensure that whenever a pointer to a to_space object is published,
+ // the forwarding pointer is also visible. We need that guarantee to ensure that if referent
+ // == other and referent is in from-space, then referent has a forwarding pointer. In order to
+ // use that guarantee, we need to ensure that the forwarding pointer is loaded after we
+ // retrieved other. Hence this fence:
+ atomic_thread_fence(std::memory_order_acquire);
+ // Note: On ARM, the above could be replaced by an asm fake-dependency hack to make
+ // referent appear to depend on other. That would be faster and uglier.
+ return gc::collector::ConcurrentCopying::GetFwdPtrUnchecked(referent.Ptr()) == other.Ptr() ?
+ JNI_TRUE : JNI_FALSE;
+}
+
static void Reference_clearReferent(JNIEnv* env, jobject javaThis) {
ScopedFastNativeObjectAccess soa(env);
const ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
@@ -45,6 +76,7 @@
static JNINativeMethod gMethods[] = {
FAST_NATIVE_METHOD(Reference, getReferent, "()Ljava/lang/Object;"),
FAST_NATIVE_METHOD(Reference, clearReferent, "()V"),
+ FAST_NATIVE_METHOD(Reference, refersTo0, "(Ljava/lang/Object;)Z"),
};
void register_java_lang_ref_Reference(JNIEnv* env) {