1 /*
2 * Copyright (C) 2014 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include "java_lang_ref_Reference.h"
18
19 #include "nativehelper/jni_macros.h"
20
21 #include "gc/heap.h"
22 #include "gc/reference_processor.h"
23 #include "jni/jni_internal.h"
24 #include "mirror/object-inl.h"
25 #include "mirror/reference-inl.h"
26 #include "native_util.h"
27 #include "scoped_fast_native_object_access-inl.h"
28
29 namespace art HIDDEN {
30
Reference_getReferent(JNIEnv * env,jobject javaThis)31 static jobject Reference_getReferent(JNIEnv* env, jobject javaThis) {
32 ScopedFastNativeObjectAccess soa(env);
33 const ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
34 const ObjPtr<mirror::Object> referent =
35 Runtime::Current()->GetHeap()->GetReferenceProcessor()->GetReferent(soa.Self(), ref);
36 return soa.AddLocalReference<jobject>(referent);
37 }
38
Reference_refersTo0(JNIEnv * env,jobject javaThis,jobject o)39 static jboolean Reference_refersTo0(JNIEnv* env, jobject javaThis, jobject o) {
40 if (gUseReadBarrier && !kUseBakerReadBarrier) {
41 // Fall back to naive implementation that may block and needlessly preserve javaThis.
42 return env->IsSameObject(Reference_getReferent(env, javaThis), o);
43 }
44 ScopedFastNativeObjectAccess soa(env);
45 const ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
46 const ObjPtr<mirror::Object> other = soa.Decode<mirror::Reference>(o);
47 const ObjPtr<mirror::Object> referent = ref->template GetReferent<kWithoutReadBarrier>();
48 if (referent == other) {
49 return JNI_TRUE;
50 }
51 if (!gUseReadBarrier || referent.IsNull() || other.IsNull()) {
52 return JNI_FALSE;
53 }
54 // Explicitly handle the case in which referent is a from-space pointer. Don't use a
55 // read-barrier, since that could easily mark an object we no longer need and, since it creates
56 // new gray objects, may not be safe without blocking.
57 //
58 // Assume we're post flip in a GC. 'other' will always be a to-space reference. Thus the only
59 // remaining case in which we should return true is when 'referent' still points to from-space.
60 // ConcurrentCopying::Copy ensures that whenever a pointer to a to-space object is published,
61 // the forwarding pointer is also visible. Thus if 'other' and 'javaThis' refer to the same
62 // object, and we can ensure that the read of the forwarding pointer is ordered after the read
63 // of other, which ensured the forwarding pointer was set, then we're guaranteed to see the
64 // correct forwarding pointer, which should then match 'other'. This fence ensures that the
65 // forwarding pointer read is ordered with respect to the access to 'other':
66 atomic_thread_fence(std::memory_order_acquire);
67 // Note: On ARM and RISC-V, the above could be replaced by an asm fake-dependency hack to make
68 // referent appear to depend on other. That would be faster and uglier.
69 return gc::collector::ConcurrentCopying::GetFwdPtrUnchecked(referent.Ptr()) == other.Ptr() ?
70 JNI_TRUE : JNI_FALSE;
71 }
72
Reference_clearReferent(JNIEnv * env,jobject javaThis)73 static void Reference_clearReferent(JNIEnv* env, jobject javaThis) {
74 ScopedFastNativeObjectAccess soa(env);
75 const ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
76 Runtime::Current()->GetHeap()->GetReferenceProcessor()->ClearReferent(ref);
77 }
78
79 static JNINativeMethod gMethods[] = {
80 FAST_NATIVE_METHOD(Reference, getReferent, "()Ljava/lang/Object;"),
81 FAST_NATIVE_METHOD(Reference, clearReferent, "()V"),
82 FAST_NATIVE_METHOD(Reference, refersTo0, "(Ljava/lang/Object;)Z"),
83 };
84
register_java_lang_ref_Reference(JNIEnv * env)85 void register_java_lang_ref_Reference(JNIEnv* env) {
86 REGISTER_NATIVE_METHODS("java/lang/ref/Reference");
87 }
88
89 } // namespace art
90