8264818: G1: Improve liveness check for empty pinned regions after full gc marking

Reviewed-by: sjohanss, ayang, mli

Author: Thomas Schatzl

src/hotspot/share/gc/g1/g1FullGCPrepareTask.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2017, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -54,18 +54,18 @@ bool G1FullGCPrepareTask::G1CalculatePointersClosure::do_heap_region(HeapRegion*
         free_humongous_region(hr);
       }
     } else if (hr->is_open_archive()) {
-      bool is_empty = _bitmap->get_next_marked_addr(hr->bottom(), hr->top()) >= hr->top();
+      bool is_empty = _collector->live_words(hr->hrm_index()) == 0;
       if (is_empty) {
         free_open_archive_region(hr);
       }
     } else if (hr->is_closed_archive()) {
       // nothing to do with closed archive region
     } else {
       assert(MarkSweepDeadRatio > 0,
-                "it should not trigger skipping compaction, when MarkSweepDeadRatio == 0");
+             "only skip compaction for other regions when MarkSweepDeadRatio > 0");
 
-      // Force the high live ration region pinned,
-      // as we need skip these regions in the later compact step.
+      // Force the high live ratio region as compacting to skip these regions in the
+      // later compaction step.
       force_not_compacted = true;
       log_debug(gc, phases)("Phase 2: skip compaction region index: %u, live words: " SIZE_FORMAT,
                             hr->hrm_index(), _collector->live_words(hr->hrm_index()));