8375643: G1: Convert G1RegionMarkStatsCache to use Atomic<T>

Reviewed-by: shade, kbarrett

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

@@ -557,14 +557,14 @@ public:
   // mark_in_bitmap call. Updates various statistics data.
   void add_to_liveness(uint worker_id, oop const obj, size_t size);
   // Did the last marking find a live object between bottom and TAMS?
-  bool contains_live_object(uint region) const { return _region_mark_stats[region]._live_words != 0; }
+  bool contains_live_object(uint region) const { return _region_mark_stats[region].live_words() != 0; }
   // Live bytes in the given region as determined by concurrent marking, i.e. the amount of
   // live bytes between bottom and TAMS.
-  size_t live_bytes(uint region) const { return _region_mark_stats[region]._live_words * HeapWordSize; }
+  size_t live_bytes(uint region) const { return _region_mark_stats[region].live_words() * HeapWordSize; }
   // Set live bytes for concurrent marking.
-  void set_live_bytes(uint region, size_t live_bytes) { _region_mark_stats[region]._live_words = live_bytes / HeapWordSize; }
+  void set_live_bytes(uint region, size_t live_bytes) { _region_mark_stats[region]._live_words.store_relaxed(live_bytes / HeapWordSize); }
   // Approximate number of incoming references found during marking.
-  size_t incoming_refs(uint region) const { return _region_mark_stats[region]._incoming_refs; }
+  size_t incoming_refs(uint region) const { return _region_mark_stats[region].incoming_refs(); }
 
   // Update the TAMS for the given region to the current top.
   inline void update_top_at_mark_start(G1HeapRegion* r);

src/hotspot/share/gc/g1/g1FullCollector.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2017, 2026, 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
@@ -122,7 +122,7 @@ public:
   ReferenceProcessor*      reference_processor();
   size_t live_words(uint region_index) const {
     assert(region_index < _heap->max_num_regions(), "sanity");
-    return _live_stats[region_index]._live_words;
+    return _live_stats[region_index].live_words();
   }
 
   void before_marking_update_attribute_table(G1HeapRegion* hr);

src/hotspot/share/gc/g1/g1RegionMarkStatsCache.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2026, 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
@@ -27,6 +27,7 @@
 
 #include "memory/allocation.hpp"
 #include "oops/oop.hpp"
+#include "runtime/atomic.hpp"
 #include "utilities/debug.hpp"
 #include "utilities/globalDefinitions.hpp"
 #include "utilities/pair.hpp"
@@ -40,20 +41,23 @@
 // * the number of incoming references found during marking. This is an approximate
 //   value because we do not mark through all objects.
 struct G1RegionMarkStats {
-  size_t _live_words;
-  size_t _incoming_refs;
+  Atomic<size_t> _live_words;
+  Atomic<size_t> _incoming_refs;
 
   // Clear all members.
   void clear() {
-    _live_words = 0;
-    _incoming_refs = 0;
+    _live_words.store_relaxed(0);
+    _incoming_refs.store_relaxed(0);
   }
   // Clear all members after a marking overflow. Only needs to clear the number of
   // incoming references as all objects will be rescanned, while the live words are
   // gathered whenever a thread can mark an object, which is synchronized.
   void clear_during_overflow() {
-    _incoming_refs = 0;
+    _incoming_refs.store_relaxed(0);
   }
+
+  size_t live_words() const { return _live_words.load_relaxed(); }
+  size_t incoming_refs() const { return _incoming_refs.load_relaxed(); }
 };
 
 // Per-marking thread cache for the region mark statistics.
@@ -112,12 +116,16 @@ public:
   void add_live_words(oop obj);
   void add_live_words(uint region_idx, size_t live_words) {
     G1RegionMarkStatsCacheEntry* const cur = find_for_add(region_idx);
-    cur->_stats._live_words += live_words;
+    // This method is only ever called single-threaded, so we do not need atomic
+    // update here.
+    cur->_stats._live_words.store_relaxed(cur->_stats.live_words() + live_words);
   }
 
   void inc_incoming_refs(uint region_idx) {
     G1RegionMarkStatsCacheEntry* const cur = find_for_add(region_idx);
-    cur->_stats._incoming_refs++;
+    // This method is only ever called single-threaded, so we do not need atomic
+    // update here.
+    cur->_stats._incoming_refs.store_relaxed(cur->_stats.incoming_refs() + 1u);
   }
 
   void reset(uint region_idx) {

src/hotspot/share/gc/g1/g1RegionMarkStatsCache.inline.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2026, 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
@@ -27,8 +27,6 @@
 
 #include "gc/g1/g1RegionMarkStatsCache.hpp"
 
-#include "runtime/atomicAccess.hpp"
-
 inline G1RegionMarkStatsCache::G1RegionMarkStatsCacheEntry* G1RegionMarkStatsCache::find_for_add(uint region_idx) {
   uint const cache_idx = hash(region_idx);
 
@@ -46,12 +44,12 @@ inline G1RegionMarkStatsCache::G1RegionMarkStatsCacheEntry* G1RegionMarkStatsCac
 
 inline void G1RegionMarkStatsCache::evict(uint idx) {
   G1RegionMarkStatsCacheEntry* cur = &_cache[idx];
-  if (cur->_stats._live_words != 0) {
-    AtomicAccess::add(&_target[cur->_region_idx]._live_words, cur->_stats._live_words);
+  if (cur->_stats.live_words() != 0) {
+    _target[cur->_region_idx]._live_words.add_then_fetch(cur->_stats.live_words());
   }
 
-  if (cur->_stats._incoming_refs != 0) {
-    AtomicAccess::add(&_target[cur->_region_idx]._incoming_refs, cur->_stats._incoming_refs);
+  if (cur->_stats.incoming_refs() != 0) {
+    _target[cur->_region_idx]._incoming_refs.add_then_fetch(cur->_stats.incoming_refs());
   }
 
   cur->clear();