8242847: G1 should not clear mark bitmaps with no marks

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

@@ -581,29 +581,65 @@ class G1ClearBitMapTask : public AbstractGangTask {
   static size_t chunk_size() { return M; }
 
 private:
-  // Heap region closure used for clearing the given mark bitmap.
+  // Heap region closure used for clearing the _next_mark_bitmap.
   class G1ClearBitmapHRClosure : public HeapRegionClosure {
   private:
-    G1CMBitMap* _bitmap;
     G1ConcurrentMark* _cm;
-  public:
-    G1ClearBitmapHRClosure(G1CMBitMap* bitmap, G1ConcurrentMark* cm) : HeapRegionClosure(), _bitmap(bitmap), _cm(cm) {
+    G1CMBitMap* _bitmap;
+    bool _suspendible; // If suspendible, do yield checks.
+
+    bool suspendible() {
+      return _suspendible;
+    }
+
+    bool is_clear_concurrent_undo() {
+      return suspendible() && _cm->cm_thread()->in_undo_mark();
+    }
+
+    bool has_aborted() {
+      if (suspendible()) {
+        _cm->do_yield_check();
+        return _cm->has_aborted();
+      }
+      return false;
+    }
+
+    HeapWord* region_clear_limit(HeapRegion* r) {
+      // During a Concurrent Undo Mark cycle, the _next_mark_bitmap is  cleared
+      // without swapping with the _prev_mark_bitmap. Therefore, the per region
+      // next_top_at_mark_start and live_words data are current wrt
+      // _next_mark_bitmap. We use this information to only clear ranges of the
+      // bitmap that require clearing.
+      if (is_clear_concurrent_undo()) {
+        // No need to clear bitmaps for empty regions.
+        if (_cm->live_words(r->hrm_index()) == 0) {
+          assert(_bitmap->get_next_marked_addr(r->bottom(), r->end()) == r->end(), "Should not have marked bits");
+          return r->bottom();
+        }
+        assert(_bitmap->get_next_marked_addr(r->next_top_at_mark_start(), r->end()) == r->end(), "Should not have marked bits above ntams");
+      }
+      return r->end();
     }
 
+  public:
+    G1ClearBitmapHRClosure(G1ConcurrentMark* cm, bool suspendible) :
+      HeapRegionClosure(),
+      _cm(cm),
+      _bitmap(cm->next_mark_bitmap()),
+      _suspendible(suspendible)
+    { }
+
     virtual bool do_heap_region(HeapRegion* r) {
-      size_t const chunk_size_in_words = G1ClearBitMapTask::chunk_size() / HeapWordSize;
+      if (has_aborted()) {
+        return true;
+      }
 
       HeapWord* cur = r->bottom();
-      HeapWord* const end = r->end();
+      HeapWord* const end = region_clear_limit(r);
+
+      size_t const chunk_size_in_words = G1ClearBitMapTask::chunk_size() / HeapWordSize;
 
       while (cur < end) {
-        // Abort iteration if necessary.
-        if (_cm != NULL) {
-          _cm->do_yield_check();
-          if (_cm->has_aborted()) {
-            return true;
-          }
-        }
 
         MemRegion mr(cur, MIN2(cur + chunk_size_in_words, end));
         _bitmap->clear_range(mr);
@@ -614,10 +650,15 @@ class G1ClearBitMapTask : public AbstractGangTask {
         // as asserts here to minimize their overhead on the product. However, we
         // will have them as guarantees at the beginning / end of the bitmap
         // clearing to get some checking in the product.
-        assert(_cm == NULL || _cm->cm_thread()->in_progress(), "invariant");
-        assert(_cm == NULL || !G1CollectedHeap::heap()->collector_state()->mark_or_rebuild_in_progress(), "invariant");
+        assert(!suspendible() || _cm->cm_thread()->in_progress(), "invariant");
+        assert(!suspendible() || !G1CollectedHeap::heap()->collector_state()->mark_or_rebuild_in_progress(), "invariant");
+
+        // Abort iteration if necessary.
+        if (has_aborted()) {
+          return true;
+        }
       }
-      assert(cur == end, "Must have completed iteration over the bitmap for region %u.", r->hrm_index());
+      assert(cur >= end, "Must have completed iteration over the bitmap for region %u.", r->hrm_index());
 
       return false;
     }
@@ -628,9 +669,9 @@ class G1ClearBitMapTask : public AbstractGangTask {
   bool _suspendible; // If the task is suspendible, workers must join the STS.
 
 public:
-  G1ClearBitMapTask(G1CMBitMap* bitmap, G1ConcurrentMark* cm, uint n_workers, bool suspendible) :
+  G1ClearBitMapTask(G1ConcurrentMark* cm, uint n_workers, bool suspendible) :
     AbstractGangTask("G1 Clear Bitmap"),
-    _cl(bitmap, suspendible ? cm : NULL),
+    _cl(cm, suspendible),
     _hr_claimer(n_workers),
     _suspendible(suspendible)
   { }
@@ -645,15 +686,15 @@ class G1ClearBitMapTask : public AbstractGangTask {
   }
 };
 
-void G1ConcurrentMark::clear_bitmap(G1CMBitMap* bitmap, WorkGang* workers, bool may_yield) {
+void G1ConcurrentMark::clear_next_bitmap(WorkGang* workers, bool may_yield) {
   assert(may_yield || SafepointSynchronize::is_at_safepoint(), "Non-yielding bitmap clear only allowed at safepoint.");
 
   size_t const num_bytes_to_clear = (HeapRegion::GrainBytes * _g1h->num_regions()) / G1CMBitMap::heap_map_factor();
   size_t const num_chunks = align_up(num_bytes_to_clear, G1ClearBitMapTask::chunk_size()) / G1ClearBitMapTask::chunk_size();
 
   uint const num_workers = (uint)MIN2(num_chunks, (size_t)workers->active_workers());
 
-  G1ClearBitMapTask cl(bitmap, this, num_workers, may_yield);
+  G1ClearBitMapTask cl(this, num_workers, may_yield);
 
   log_debug(gc, ergo)("Running %s with %u workers for " SIZE_FORMAT " work units.", cl.name(), num_workers, num_chunks);
   workers->run_task(&cl, num_workers);
@@ -671,7 +712,7 @@ void G1ConcurrentMark::cleanup_for_next_mark() {
   // is the case.
   guarantee(!_g1h->collector_state()->mark_or_rebuild_in_progress(), "invariant");
 
-  clear_bitmap(_next_mark_bitmap, _concurrent_workers, true);
+  clear_next_bitmap(_concurrent_workers, true);
 
   // Repeat the asserts from above.
   guarantee(cm_thread()->in_progress(), "invariant");
@@ -685,7 +726,7 @@ void G1ConcurrentMark::clear_next_bitmap(WorkGang* workers) {
   // as efficiently as possible the number of active workers are temporarily
   // increased to include all currently created workers.
   WithUpdatedActiveWorkers update(workers, workers->created_workers());
-  clear_bitmap(_next_mark_bitmap, workers, false);
+  clear_next_bitmap(workers, false);
 }
 
 class NoteStartOfMarkHRClosure : public HeapRegionClosure {

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

@@ -444,9 +444,9 @@ class G1ConcurrentMark : public CHeapObj<mtGC> {
   void enter_first_sync_barrier(uint worker_id);
   void enter_second_sync_barrier(uint worker_id);
 
-  // Clear the given bitmap in parallel using the given WorkGang. If may_yield is
+  // Clear the next marking bitmap in parallel using the given WorkGang. If may_yield is
   // true, periodically insert checks to see if this method should exit prematurely.
-  void clear_bitmap(G1CMBitMap* bitmap, WorkGang* workers, bool may_yield);
+  void clear_next_bitmap(WorkGang* workers, bool may_yield);
 
   // Region statistics gathered during marking.
   G1RegionMarkStats* _region_mark_stats;

src/hotspot/share/gc/g1/g1ConcurrentMarkThread.cpp

@@ -324,6 +324,8 @@ void G1ConcurrentMarkThread::concurrent_undo_cycle_do() {
   // some reason.
   if (_cm->has_aborted()) { return; }
 
+  _cm->flush_all_task_caches();
+
   // Phase 1: Clear bitmap for next mark.
   phase_clear_bitmap_for_next_mark();
 }

src/hotspot/share/gc/g1/g1ConcurrentMarkThread.hpp

@@ -108,6 +108,8 @@ class G1ConcurrentMarkThread: public ConcurrentGCThread {
   // marking bitmap has been cleared and in_progress() is
   // cleared).
   bool in_progress() const;
+
+  bool in_undo_mark() const;
 };
 
 #endif // SHARE_GC_G1_G1CONCURRENTMARKTHREAD_HPP

src/hotspot/share/gc/g1/g1ConcurrentMarkThread.inline.hpp

@@ -60,4 +60,8 @@ inline bool G1ConcurrentMarkThread::in_progress() const {
   return !idle();
 }
 
+inline bool G1ConcurrentMarkThread::in_undo_mark() const {
+  return _state == UndoMark;
+}
+
 #endif // SHARE_GC_G1_G1CONCURRENTMARKTHREAD_INLINE_HPP