8297487: G1 Remark: no need to keep alive oop constants of nmethods on stack

Reviewed-by: tschatzl, ayang, eosterlund

Author: reinrich

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp

@@ -1779,30 +1779,17 @@ class G1CMSATBBufferClosure : public SATBBufferClosure {
 
 class G1RemarkThreadsClosure : public ThreadClosure {
   G1SATBMarkQueueSet& _qset;
-  G1CMOopClosure _cm_cl;
-  MarkingCodeBlobClosure _code_cl;
   uintx _claim_token;
 
  public:
   G1RemarkThreadsClosure(G1CollectedHeap* g1h, G1CMTask* task) :
     _qset(G1BarrierSet::satb_mark_queue_set()),
-    _cm_cl(g1h, task),
-    _code_cl(&_cm_cl, !CodeBlobToOopClosure::FixRelocations, true /* keepalive nmethods */),
     _claim_token(Threads::thread_claim_token()) {}
 
   void do_thread(Thread* thread) {
     if (thread->claim_threads_do(true, _claim_token)) {
       // Transfer any partial buffer to the qset for completed buffer processing.
       _qset.flush_queue(G1ThreadLocalData::satb_mark_queue(thread));
-      if (thread->is_Java_thread()) {
-        // In theory it should not be necessary to explicitly walk the nmethods to find roots for concurrent marking
-        // however the liveness of oops reachable from nmethods have very complex lifecycles:
-        // * Alive if on the stack of an executing method
-        // * Weakly reachable otherwise
-        // Some objects reachable from nmethods, such as the class loader (or klass_holder) of the receiver should be
-        // live by the SATB invariant but other oops recorded in nmethods may behave differently.
-        JavaThread::cast(thread)->nmethods_do(&_code_cl);
-      }
     }
   }
 };

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

@@ -57,8 +57,9 @@ static BarrierSetNMethod* select_barrier_set_nmethod(BarrierSetNMethod* barrier_
     // The GC needs nmethod entry barriers to do concurrent GC
     return barrier_set_nmethod;
   } else {
-    // The GC needs nmethod entry barriers to deal with continuations
-    // and code cache unloading
+    // The GC needs nmethod entry barriers for code cache unloading (recently
+    // used heuristics) and, if it's a SATB GC, to keep alive constant objects
+    // of nmethods because they are weakly referenced.
     return new BarrierSetNMethod();
   }
 }