--- a/src/hotspot/share/prims/jvmtiTagMap.cpp	Mon Jan 08 15:09:18 2018 +0100
+++ b/src/hotspot/share/prims/jvmtiTagMap.cpp	Mon Jan 08 15:12:05 2018 +0100
@@ -30,6 +30,7 @@
 #include "code/codeCache.hpp"
 #include "jvmtifiles/jvmtiEnv.hpp"
 #include "memory/resourceArea.hpp"
+#include "oops/access.inline.hpp"
 #include "oops/instanceMirrorKlass.hpp"
 #include "oops/objArrayKlass.hpp"
 #include "oops/objArrayOop.inline.hpp"
@@ -52,10 +53,6 @@
 #include "runtime/vm_operations.hpp"
 #include "services/serviceUtil.hpp"
 #include "utilities/macros.hpp"
-#if INCLUDE_ALL_GCS
-#include "gc/g1/g1SATBCardTableModRefBS.hpp"
-#include "gc/parallel/parallelScavengeHeap.hpp"
-#endif // INCLUDE_ALL_GCS
 
 // JvmtiTagHashmapEntry
 //
@@ -78,22 +75,31 @@
   }
 
   // constructor
-  JvmtiTagHashmapEntry(oop object, jlong tag)         { init(object, tag); }
+  JvmtiTagHashmapEntry(oop object, jlong tag) { init(object, tag); }
 
  public:
 
   // accessor methods
-  inline oop object() const                           { return _object; }
-  inline oop* object_addr()                           { return &_object; }
-  inline jlong tag() const                            { return _tag; }
+  inline oop* object_addr() { return &_object; }
+  inline oop object()       { return RootAccess<ON_PHANTOM_OOP_REF>::oop_load(object_addr()); }
+  // Peek at the object without keeping it alive. The returned object must be
+  // kept alive using a normal access if it leaks out of a thread transition from VM.
+  inline oop object_peek()  {
+    return RootAccess<ON_PHANTOM_OOP_REF | AS_NO_KEEPALIVE>::oop_load(object_addr());
+  }
+  inline jlong tag() const  { return _tag; }
 
   inline void set_tag(jlong tag) {
     assert(tag != 0, "can't be zero");
     _tag = tag;
   }
 
-  inline JvmtiTagHashmapEntry* next() const             { return _next; }
-  inline void set_next(JvmtiTagHashmapEntry* next)      { _next = next; }
+  inline bool equals(oop object) {
+    return object == object_peek();
+  }
+
+  inline JvmtiTagHashmapEntry* next() const        { return _next; }
+  inline void set_next(JvmtiTagHashmapEntry* next) { _next = next; }
 };
 
 
@@ -211,7 +217,7 @@
       JvmtiTagHashmapEntry* entry = _table[i];
       while (entry != NULL) {
         JvmtiTagHashmapEntry* next = entry->next();
-        oop key = entry->object();
+        oop key = entry->object_peek();
         assert(key != NULL, "jni weak reference cleared!!");
         unsigned int h = hash(key, new_size);
         JvmtiTagHashmapEntry* anchor = new_table[h];
@@ -304,7 +310,7 @@
     unsigned int h = hash(key);
     JvmtiTagHashmapEntry* entry = _table[h];
     while (entry != NULL) {
-      if (entry->object() == key) {
+      if (entry->equals(key)) {
          return entry;
       }
       entry = entry->next();
@@ -345,7 +351,7 @@
     JvmtiTagHashmapEntry* entry = _table[h];
     JvmtiTagHashmapEntry* prev = NULL;
     while (entry != NULL) {
-      if (key == entry->object()) {
+      if (entry->equals(key)) {
         break;
       }
       prev = entry;
@@ -1535,16 +1541,12 @@
   void do_entry(JvmtiTagHashmapEntry* entry) {
     for (int i=0; i<_tag_count; i++) {
       if (_tags[i] == entry->tag()) {
+        // The reference in this tag map could be the only (implicitly weak)
+        // reference to that object. If we hand it out, we need to keep it live wrt
+        // SATB marking similar to other j.l.ref.Reference referents. This is
+        // achieved by using a phantom load in the object() accessor.
         oop o = entry->object();
         assert(o != NULL && Universe::heap()->is_in_reserved(o), "sanity check");
-#if INCLUDE_ALL_GCS
-        if (UseG1GC) {
-          // The reference in this tag map could be the only (implicitly weak)
-          // reference to that object. If we hand it out, we need to keep it live wrt
-          // SATB marking similar to other j.l.ref.Reference referents.
-          G1SATBCardTableModRefBS::enqueue(o);
-        }
-#endif
         jobject ref = JNIHandles::make_local(JavaThread::current(), o);
         _object_results->append(ref);
         _tag_results->append((uint64_t)entry->tag());
@@ -3363,10 +3365,8 @@
     while (entry != NULL) {
       JvmtiTagHashmapEntry* next = entry->next();
 
-      oop* obj = entry->object_addr();
-
       // has object been GC'ed
-      if (!is_alive->do_object_b(entry->object())) {
+      if (!is_alive->do_object_b(entry->object_peek())) {
         // grab the tag
         jlong tag = entry->tag();
         guarantee(tag != 0, "checking");
@@ -3384,7 +3384,7 @@
         ++freed;
       } else {
         f->do_oop(entry->object_addr());
-        oop new_oop = entry->object();
+        oop new_oop = entry->object_peek();
 
         // if the object has moved then re-hash it and move its
         // entry to its new location.
@@ -3418,7 +3418,7 @@
   // Re-add all the entries which were kept aside
   while (delayed_add != NULL) {
     JvmtiTagHashmapEntry* next = delayed_add->next();
-    unsigned int pos = JvmtiTagHashmap::hash(delayed_add->object(), size);
+    unsigned int pos = JvmtiTagHashmap::hash(delayed_add->object_peek(), size);
     delayed_add->set_next(table[pos]);
     table[pos] = delayed_add;
     delayed_add = next;