8344593: GenShen: Review of ReduceInitialCardMarks

Reviewed-by: shade, cslucas

src/hotspot/share/gc/shenandoah/c2/shenandoahBarrierSetC2.cpp

@@ -465,11 +465,17 @@ void ShenandoahBarrierSetC2::post_barrier(GraphKit* kit,
   }
 
   if (ReduceInitialCardMarks && obj == kit->just_allocated_object(kit->control())) {
-    // We can skip marks on a freshly-allocated object in Eden.
-    // Keep this code in sync with new_deferred_store_barrier() in runtime.cpp.
-    // That routine informs GC to take appropriate compensating steps,
-    // upon a slow-path allocation, so as to make this card-mark
-    // elision safe.
+    // We use card marks to track old to young references in Generational Shenandoah;
+    // see flag ShenandoahCardBarrier above.
+    // Objects are always allocated in the young generation and initialized
+    // before they are promoted. There's always a safepoint (e.g. at final mark)
+    // before an object is promoted from young to old. Promotion entails dirtying of
+    // the cards backing promoted objects, so they will be guaranteed to be scanned
+    // at the next remembered set scan of the old generation.
+    // Thus, we can safely skip card-marking of initializing stores on a
+    // freshly-allocated object. If any of the assumptions above change in
+    // the future, this code will need to be re-examined; see check in
+    // ShenandoahCardBarrier::on_slowpath_allocation_exit().
     return;
   }
 

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

@@ -88,6 +88,14 @@ bool ShenandoahBarrierSet::need_keep_alive_barrier(DecoratorSet decorators, Basi
   return (on_weak_ref || unknown) && keep_alive;
 }
 
+void ShenandoahBarrierSet::on_slowpath_allocation_exit(JavaThread* thread, oop new_obj) {
+#if COMPILER2_OR_JVMCI
+  assert(!ReduceInitialCardMarks || !ShenandoahCardBarrier || ShenandoahGenerationalHeap::heap()->is_in_young(new_obj),
+         "Error: losing card mark on initialzing store to old gen");
+#endif // COMPILER2_OR_JVMCI
+  assert(thread->deferred_card_mark().is_empty(), "We don't use this");
+}
+
 void ShenandoahBarrierSet::on_thread_create(Thread* thread) {
   // Create thread local data
   ShenandoahThreadLocalData::create(thread);

src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.hpp

@@ -77,17 +77,21 @@ class ShenandoahBarrierSet: public BarrierSet {
     return (decorators & IN_NATIVE) != 0;
   }
 
-  void print_on(outputStream* st) const;
+  void print_on(outputStream* st) const override;
 
   template <class T>
   inline void arraycopy_barrier(T* src, T* dst, size_t count);
   inline void clone_barrier(oop src);
   void clone_barrier_runtime(oop src);
 
-  virtual void on_thread_create(Thread* thread);
-  virtual void on_thread_destroy(Thread* thread);
-  virtual void on_thread_attach(Thread* thread);
-  virtual void on_thread_detach(Thread* thread);
+  // Support for optimizing compilers to call the barrier set on slow path allocations
+  // that did not enter a TLAB. Used for e.g. ReduceInitialCardMarks to take any
+  // compensating actions to restore card-marks that might otherwise be incorrectly elided.
+  void on_slowpath_allocation_exit(JavaThread* thread, oop new_obj) override;
+  void on_thread_create(Thread* thread) override;
+  void on_thread_destroy(Thread* thread) override;
+  void on_thread_attach(Thread* thread) override;
+  void on_thread_detach(Thread* thread) override;
 
   static inline oop resolve_forwarded_not_null(oop p);
   static inline oop resolve_forwarded_not_null_mutator(oop p);