8256516: Simplify clearing References

Provide and use explicit referent clearing instead of set to null.

Reviewed-by: rkennke, shade, pliden, mchung

Author: Kim Barrett

src/hotspot/share/classfile/javaClasses.hpp

@@ -875,7 +875,7 @@ class java_lang_ref_Reference: AllStatic {
   static inline oop weak_referent_no_keepalive(oop ref);
   static inline oop phantom_referent_no_keepalive(oop ref);
   static inline oop unknown_referent_no_keepalive(oop ref);
-  static inline void set_referent_raw(oop ref, oop value);
+  static inline void clear_referent(oop ref);
   static inline HeapWord* referent_addr_raw(oop ref);
   static inline oop next(oop ref);
   static inline void set_next(oop ref, oop value);

src/hotspot/share/classfile/javaClasses.inline.hpp

@@ -113,8 +113,8 @@ oop java_lang_ref_Reference::unknown_referent_no_keepalive(oop ref) {
   return ref->obj_field_access<ON_UNKNOWN_OOP_REF | AS_NO_KEEPALIVE>(_referent_offset);
 }
 
-void java_lang_ref_Reference::set_referent_raw(oop ref, oop value) {
-  ref->obj_field_put_raw(_referent_offset, value);
+void java_lang_ref_Reference::clear_referent(oop ref) {
+  ref->obj_field_put_raw(_referent_offset, nullptr);
 }
 
 HeapWord* java_lang_ref_Reference::referent_addr_raw(oop ref) {

src/hotspot/share/gc/shared/referenceProcessor.cpp

@@ -260,8 +260,6 @@ void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
   assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
   _next_discovered = discovered;
-
-  _referent_addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
   _referent = java_lang_ref_Reference::unknown_referent_no_keepalive(_current_discovered);
   assert(Universe::heap()->is_in_or_null(_referent),
          "Wrong oop found in java.lang.Reference object");
@@ -295,8 +293,17 @@ void DiscoveredListIterator::remove() {
   _refs_list.dec_length(1);
 }
 
+void DiscoveredListIterator::make_referent_alive() {
+  HeapWord* addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
+  if (UseCompressedOops) {
+    _keep_alive->do_oop((narrowOop*)addr);
+  } else {
+    _keep_alive->do_oop((oop*)addr);
+  }
+}
+
 void DiscoveredListIterator::clear_referent() {
-  RawAccess<>::oop_store(_referent_addr, oop(NULL));
+  java_lang_ref_Reference::clear_referent(_current_discovered);
 }
 
 void DiscoveredListIterator::enqueue() {

src/hotspot/share/gc/shared/referenceProcessor.hpp

@@ -71,7 +71,6 @@ class DiscoveredListIterator {
   HeapWord*          _current_discovered_addr;
   oop                _next_discovered;
 
-  HeapWord*          _referent_addr;
   oop                _referent;
 
   OopClosure*        _keep_alive;
@@ -120,14 +119,8 @@ class DiscoveredListIterator {
   // Remove the current reference from the list
   void remove();
 
-  // Make the referent alive.
-  inline void make_referent_alive() {
-    if (UseCompressedOops) {
-      _keep_alive->do_oop((narrowOop*)_referent_addr);
-    } else {
-      _keep_alive->do_oop((oop*)_referent_addr);
-    }
-  }
+  // Apply the keep_alive function to the referent address.
+  void make_referent_alive();
 
   // Do enqueuing work, i.e. notifying the GC about the changed discovered pointers.
   void enqueue();

src/hotspot/share/gc/shared/referenceProcessor.inline.hpp

@@ -61,7 +61,6 @@ DiscoveredListIterator::DiscoveredListIterator(DiscoveredList&    refs_list,
   _current_discovered(refs_list.head()),
   _current_discovered_addr(NULL),
   _next_discovered(NULL),
-  _referent_addr(NULL),
   _referent(NULL),
   _keep_alive(keep_alive),
   _is_alive(is_alive),

src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp

@@ -88,8 +88,8 @@ static oop reference_referent(oop reference) {
   return CompressedOops::decode(heap_oop);
 }
 
-static void reference_set_referent(oop reference, oop referent) {
-  java_lang_ref_Reference::set_referent_raw(reference, referent);
+static void reference_clear_referent(oop reference) {
+  java_lang_ref_Reference::clear_referent(reference);
 }
 
 template <typename T>
@@ -316,7 +316,7 @@ void ShenandoahReferenceProcessor::make_inactive(oop reference, ReferenceType ty
     reference_set_next(reference, reference);
   } else {
     // Clear referent
-    reference_set_referent(reference, NULL);
+    reference_clear_referent(reference);
   }
 }
 
@@ -590,4 +590,4 @@ void ShenandoahReferenceProcessor::collect_statistics() {
                    discovered[REF_SOFT], discovered[REF_WEAK], discovered[REF_FINAL], discovered[REF_PHANTOM]);
   log_info(gc,ref)("Enqueued    references: Soft: " SIZE_FORMAT ", Weak: " SIZE_FORMAT ", Final: " SIZE_FORMAT ", Phantom: " SIZE_FORMAT,
                    enqueued[REF_SOFT], enqueued[REF_WEAK], enqueued[REF_FINAL], enqueued[REF_PHANTOM]);
-}
+}

src/hotspot/share/gc/z/zReferenceProcessor.cpp

@@ -72,8 +72,8 @@ static oop reference_referent(oop reference) {
   return Atomic::load(reference_referent_addr(reference));
 }
 
-static void reference_set_referent(oop reference, oop referent) {
-  java_lang_ref_Reference::set_referent_raw(reference, referent);
+static void reference_clear_referent(oop reference) {
+  java_lang_ref_Reference::clear_referent(reference);
 }
 
 static oop* reference_discovered_addr(oop reference) {
@@ -226,7 +226,7 @@ void ZReferenceProcessor::make_inactive(oop reference, ReferenceType type) const
     reference_set_next(reference, reference);
   } else {
     // Clear referent
-    reference_set_referent(reference, NULL);
+    reference_clear_referent(reference);
   }
 }