8376328

Hi all,

  please review this change to convert the `PLABStats` class to use `Atomic<T>`.

Testing: gha

Thanks,
  Thomas

src/hotspot/share/gc/g1/g1EvacStats.cpp

@@ -48,11 +48,11 @@ void G1EvacStats::log_plab_allocation() {
                       "used: %zuB, "
                       "undo waste: %zuB, ",
                       _description,
-                      _allocated * HeapWordSize,
-                      _wasted * HeapWordSize,
-                      _unused * HeapWordSize,
+                      allocated() * HeapWordSize,
+                      wasted() * HeapWordSize,
+                      unused() * HeapWordSize,
                       used() * HeapWordSize,
-                      _undo_wasted * HeapWordSize);
+                      undo_wasted() * HeapWordSize);
   log_debug(gc, plab)("%s other allocation: "
                       "region end waste: %zuB, "
                       "regions filled: %u, "
@@ -157,13 +157,13 @@ void G1EvacStats::adjust_desired_plab_size() {
     assert(is_object_aligned(max_size()) && min_size() <= max_size(),
            "PLAB clipping computation may be incorrect");
 
-    assert(_allocated != 0 || _unused == 0,
+    assert(allocated() != 0 || unused() == 0,
            "Inconsistency in PLAB stats: "
            "_allocated: %zu, "
            "_wasted: %zu, "
            "_unused: %zu, "
            "_undo_wasted: %zu",
-           _allocated, _wasted, _unused, _undo_wasted);
+           allocated(), wasted(), unused(), undo_wasted());
 
     size_t plab_size = compute_desired_plab_size();
     // Take historical weighted average

src/hotspot/share/gc/shared/plab.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2023, 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
@@ -27,6 +27,7 @@
 
 #include "gc/shared/collectedHeap.hpp"
 #include "memory/allocation.hpp"
+#include "runtime/atomic.hpp"
 #include "utilities/globalDefinitions.hpp"
 
 // Forward declarations.
@@ -149,16 +150,16 @@ class PLABStats : public CHeapObj<mtGC> {
 protected:
   const char* _description;   // Identifying string.
 
-  size_t _allocated;          // Total allocated
-  size_t _wasted;             // of which wasted (internal fragmentation)
-  size_t _undo_wasted;        // of which wasted on undo (is not used for calculation of PLAB size)
-  size_t _unused;             // Unused in last buffer
+  Atomic<size_t> _allocated;          // Total allocated
+  Atomic<size_t> _wasted;             // of which wasted (internal fragmentation)
+  Atomic<size_t> _undo_wasted;        // of which wasted on undo (is not used for calculation of PLAB size)
+  Atomic<size_t> _unused;             // Unused in last buffer
 
   virtual void reset() {
-    _allocated   = 0;
-    _wasted      = 0;
-    _undo_wasted = 0;
-    _unused      = 0;
+    _allocated.store_relaxed(0);
+    _wasted.store_relaxed(0);
+    _undo_wasted.store_relaxed(0);
+    _unused.store_relaxed(0);
   }
 
 public:
@@ -172,11 +173,11 @@ public:
 
   virtual ~PLABStats() { }
 
-  size_t allocated() const { return _allocated; }
-  size_t wasted() const { return _wasted; }
-  size_t unused() const { return _unused; }
+  size_t allocated() const { return _allocated.load_relaxed(); }
+  size_t wasted() const { return _wasted.load_relaxed(); }
+  size_t undo_wasted() const { return _undo_wasted.load_relaxed(); }
+  size_t unused() const { return _unused.load_relaxed(); }
   size_t used() const { return allocated() - (wasted() + unused()); }
-  size_t undo_wasted() const { return _undo_wasted; }
 
   static size_t min_size() {
     return PLAB::min_size();

src/hotspot/share/gc/shared/plab.inline.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 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
@@ -29,22 +29,21 @@
 
 #include "gc/shared/collectedHeap.inline.hpp"
 #include "memory/allocation.inline.hpp"
-#include "runtime/atomicAccess.hpp"
 
 void PLABStats::add_allocated(size_t v) {
-  AtomicAccess::add(&_allocated, v);
+  _allocated.add_then_fetch(v);
 }
 
 void PLABStats::add_unused(size_t v) {
-  AtomicAccess::add(&_unused, v);
+  _unused.add_then_fetch(v);
 }
 
 void PLABStats::add_wasted(size_t v) {
-  AtomicAccess::add(&_wasted, v);
+  _wasted.add_then_fetch(v);
 }
 
 void PLABStats::add_undo_wasted(size_t v) {
-  AtomicAccess::add(&_undo_wasted, v);
+  _undo_wasted.add_then_fetch(v);
 }
 
 #endif // SHARE_GC_SHARED_PLAB_INLINE_HPP