8375630: G1: Convert G1ConcurrentMark to use Atomic<T>

Reviewed-by: kbarrett, shade

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 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
@@ -67,7 +67,6 @@
 #include "nmt/memTracker.hpp"
 #include "oops/access.inline.hpp"
 #include "oops/oop.inline.hpp"
-#include "runtime/atomicAccess.hpp"
 #include "runtime/globals_extension.hpp"
 #include "runtime/handles.inline.hpp"
 #include "runtime/java.hpp"
@@ -148,25 +147,25 @@ bool G1CMMarkStack::initialize() {
 }
 
 G1CMMarkStack::TaskQueueEntryChunk* G1CMMarkStack::ChunkAllocator::allocate_new_chunk() {
-  if (_size >= _max_capacity) {
+  if (_size.load_relaxed() >= _max_capacity) {
     return nullptr;
   }
 
-  size_t cur_idx = AtomicAccess::fetch_then_add(&_size, 1u);
+  size_t cur_idx = _size.fetch_then_add(1u);
 
   if (cur_idx >= _max_capacity) {
     return nullptr;
   }
 
   size_t bucket = get_bucket(cur_idx);
-  if (AtomicAccess::load_acquire(&_buckets[bucket]) == nullptr) {
+  if (_buckets[bucket].load_acquire() == nullptr) {
     if (!_should_grow) {
       // Prefer to restart the CM.
       return nullptr;
     }
 
     MutexLocker x(G1MarkStackChunkList_lock, Mutex::_no_safepoint_check_flag);
-    if (AtomicAccess::load_acquire(&_buckets[bucket]) == nullptr) {
+    if (_buckets[bucket].load_acquire() == nullptr) {
       size_t desired_capacity = bucket_size(bucket) * 2;
       if (!try_expand_to(desired_capacity)) {
         return nullptr;
@@ -175,7 +174,7 @@ G1CMMarkStack::TaskQueueEntryChunk* G1CMMarkStack::ChunkAllocator::allocate_new_
   }
 
   size_t bucket_idx = get_bucket_index(cur_idx);
-  TaskQueueEntryChunk* result = ::new (&_buckets[bucket][bucket_idx]) TaskQueueEntryChunk;
+  TaskQueueEntryChunk* result = ::new (&_buckets[bucket].load_relaxed()[bucket_idx]) TaskQueueEntryChunk;
   result->next = nullptr;
   return result;
 }
@@ -197,10 +196,10 @@ bool G1CMMarkStack::ChunkAllocator::initialize(size_t initial_capacity, size_t m
   _max_capacity = max_capacity;
   _num_buckets  = get_bucket(_max_capacity) + 1;
 
-  _buckets = NEW_C_HEAP_ARRAY(TaskQueueEntryChunk*, _num_buckets, mtGC);
+  _buckets = NEW_C_HEAP_ARRAY(Atomic<TaskQueueEntryChunk*>, _num_buckets, mtGC);
 
   for (size_t i = 0; i < _num_buckets; i++) {
-    _buckets[i] = nullptr;
+    _buckets[i].store_relaxed(nullptr);
   }
 
   size_t new_capacity = bucket_size(0);
@@ -240,9 +239,9 @@ G1CMMarkStack::ChunkAllocator::~ChunkAllocator() {
   }
 
   for (size_t i = 0; i < _num_buckets; i++) {
-    if (_buckets[i] != nullptr) {
-      MmapArrayAllocator<TaskQueueEntryChunk>::free(_buckets[i],  bucket_size(i));
-      _buckets[i] = nullptr;
+    if (_buckets[i].load_relaxed() != nullptr) {
+      MmapArrayAllocator<TaskQueueEntryChunk>::free(_buckets[i].load_relaxed(),  bucket_size(i));
+      _buckets[i].store_relaxed(nullptr);
     }
   }
 
@@ -259,7 +258,7 @@ bool G1CMMarkStack::ChunkAllocator::reserve(size_t new_capacity) {
   // and the new capacity (new_capacity). This step ensures that there are no gaps in the
   // array and that the capacity accurately reflects the reserved memory.
   for (; i <= highest_bucket; i++) {
-    if (AtomicAccess::load_acquire(&_buckets[i]) != nullptr) {
+    if (_buckets[i].load_acquire() != nullptr) {
       continue; // Skip over already allocated buckets.
     }
 
@@ -279,7 +278,7 @@ bool G1CMMarkStack::ChunkAllocator::reserve(size_t new_capacity) {
       return false;
     }
     _capacity += bucket_capacity;
-    AtomicAccess::release_store(&_buckets[i], bucket_base);
+    _buckets[i].release_store(bucket_base);
   }
   return true;
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 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
@@ -37,6 +37,7 @@
 #include "gc/shared/workerThread.hpp"
 #include "gc/shared/workerUtils.hpp"
 #include "memory/allocation.hpp"
+#include "runtime/atomic.hpp"
 #include "utilities/compilerWarnings.hpp"
 #include "utilities/numberSeq.hpp"
 
@@ -172,9 +173,9 @@ private:
     size_t _capacity;
     size_t _num_buckets;
     bool _should_grow;
-    TaskQueueEntryChunk* volatile* _buckets;
+    Atomic<TaskQueueEntryChunk*>* _buckets;
     char _pad0[DEFAULT_PADDING_SIZE];
-    volatile size_t _size;
+    Atomic<size_t> _size;
     char _pad4[DEFAULT_PADDING_SIZE - sizeof(size_t)];
 
     size_t bucket_size(size_t bucket) {
@@ -212,7 +213,7 @@ private:
     bool initialize(size_t initial_capacity, size_t max_capacity);
 
     void reset() {
-      _size = 0;
+      _size.store_relaxed(0);
       _should_grow = false;
     }