8015422: Large performance hit when the StringTable is walked twice in Parallel Scavenge

Summary: Combine the calls to StringTable::unlink and StringTable::oops_do in Parallel Scavenge.
Reviewed-by: pliden, coleenp

src/share/vm/classfile/symbolTable.cpp

@@ -737,7 +737,7 @@ oop StringTable::intern(const char* utf8_string, TRAPS) {
   return result;
 }
 
-void StringTable::unlink(BoolObjectClosure* is_alive) {
+void StringTable::unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* f) {
   // Readers of the table are unlocked, so we should only be removing
   // entries at a safepoint.
   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
@@ -748,6 +748,9 @@ void StringTable::unlink(BoolObjectClosure* is_alive) {
       assert(!entry->is_shared(), "CDS not used for the StringTable");
 
       if (is_alive->do_object_b(entry->literal())) {
+        if (f != NULL) {
+          f->do_oop((oop*)entry->literal_addr());
+        }
         p = entry->next_addr();
       } else {
         *p = entry->next();

src/share/vm/classfile/symbolTable.hpp

@@ -272,7 +272,10 @@ public:
 
   // GC support
   //   Delete pointers to otherwise-unreachable objects.
-  static void unlink(BoolObjectClosure* cl);
+  static void unlink_or_oops_do(BoolObjectClosure* cl, OopClosure* f);
+  static void unlink(BoolObjectClosure* cl) {
+    unlink_or_oops_do(cl, NULL);
+  }
 
   // Invoke "f->do_oop" on the locations of all oops in the table.
   static void oops_do(OopClosure* f);

src/share/vm/gc_implementation/parallelScavenge/psScavenge.cpp

@@ -450,11 +450,9 @@ bool PSScavenge::invoke_no_policy() {
       reference_processor()->enqueue_discovered_references(NULL);
     }
 
-      // Unlink any dead interned Strings
-      StringTable::unlink(&_is_alive_closure);
-      // Process the remaining live ones
-      PSScavengeRootsClosure root_closure(promotion_manager);
-      StringTable::oops_do(&root_closure);
+    // Unlink any dead interned Strings and process the remaining live ones.
+    PSScavengeRootsClosure root_closure(promotion_manager);
+    StringTable::unlink_or_oops_do(&_is_alive_closure, &root_closure);
 
     // Finally, flush the promotion_manager's labs, and deallocate its stacks.
     PSPromotionManager::post_scavenge();