new check

src/hotspot/share/gc/g1/g1ParScanThreadState.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/systemDictionary.hpp"
 #include "gc/g1/g1Allocator.inline.hpp"
 #include "gc/g1/g1CollectedHeap.inline.hpp"
 #include "gc/g1/g1CollectionSet.hpp"
@@ -75,10 +76,17 @@ G1ParScanThreadState::G1ParScanThreadState(G1CollectedHeap* g1h,
     _old_gen_is_full(false),
     _partial_objarray_chunk_size(ParGCArrayScanChunk),
     _partial_array_stepper(n_workers),
+    _string_klass_or_null(G1StringDedup::is_enabled()
+                          ? SystemDictionary::String_klass()
+                          : nullptr),
     _num_optional_regions(optional_cset_length),
     _numa(g1h->numa()),
     _obj_alloc_stat(NULL)
 {
+  // Verify klass comparison with _string_klass_or_null is sufficient
+  // to determine whether dedup is enabled and the object is a String.
+  assert(SystemDictionary::String_klass()->is_final(), "precondition");
+
   // We allocate number of young gen regions in the collection set plus one
   // entries, since entry 0 keeps track of surviving bytes for non-young regions.
   // We also add a few elements at the beginning and at the end in
@@ -510,7 +518,7 @@ oop G1ParScanThreadState::do_copy_to_survivor_space(G1HeapRegionAttr const regio
       return obj;
     }
 
-    if (G1StringDedup::is_enabled()) {
+    if (klass == _string_klass_or_null) {
       const bool is_from_young = region_attr.is_young();
       const bool is_to_young = dest_attr.is_young();
       assert(is_from_young == from_region->is_young(),

src/hotspot/share/gc/g1/g1ParScanThreadState.hpp

@@ -42,6 +42,7 @@ class G1OopStarChunkedList;
 class G1PLABAllocator;
 class G1EvacuationRootClosures;
 class HeapRegion;
+class Klass;
 class outputStream;
 
 class G1ParScanThreadState : public CHeapObj<mtGC> {
@@ -83,6 +84,8 @@ class G1ParScanThreadState : public CHeapObj<mtGC> {
   // Size (in elements) of a partial objArray task chunk.
   int _partial_objarray_chunk_size;
   PartialArrayTaskStepper _partial_array_stepper;
+  // Used to check whether string dedup should be applied to an object.
+  Klass* _string_klass_or_null;
 
   G1RedirtyCardsQueue& redirty_cards_queue()     { return _rdcq; }
   G1CardTable* ct()                              { return _ct; }

src/hotspot/share/gc/g1/g1StringDedup.cpp

@@ -63,7 +63,7 @@ void G1StringDedup::enqueue_from_mark(oop java_string, uint worker_id) {
 }
 
 bool G1StringDedup::is_candidate_from_evacuation(bool from_young, bool to_young, oop obj) {
-  if (from_young && java_lang_String::is_instance_inlined(obj)) {
+  if (from_young) {
     if (to_young && obj->age() == StringDeduplicationAgeThreshold) {
       // Candidate found. String is being evacuated from young to young and just
       // reached the deduplication age threshold.
@@ -83,6 +83,7 @@ bool G1StringDedup::is_candidate_from_evacuation(bool from_young, bool to_young,
 
 void G1StringDedup::enqueue_from_evacuation(bool from_young, bool to_young, uint worker_id, oop java_string) {
   assert(is_enabled(), "String deduplication not enabled");
+  assert(java_lang_String::is_instance(java_string), "not a String");
   if (is_candidate_from_evacuation(from_young, to_young, java_string)) {
     G1StringDedupQueue::push(worker_id, java_string);
   }

src/hotspot/share/gc/g1/g1StringDedup.hpp

@@ -75,7 +75,8 @@ class G1StringDedup : public StringDedup {
   // Enqueues a deduplication candidate for later processing by the deduplication
   // thread. Before enqueuing, these functions apply the appropriate candidate
   // selection policy to filters out non-candidates.
-  static void enqueue_from_mark(oop java_string, uint worker_id);
+  static void enqueue_from_mark(oop obj, uint worker_id);
+  // precondition: java_string is a String.
   static void enqueue_from_evacuation(bool from_young, bool to_young,
                                       unsigned int queue, oop java_string);
 };