8376810: Make Atomic<T> default constructor non-explicit

Reviewed-by: kbarrett, aboldtch, azafari, tschatzl

src/hotspot/share/runtime/atomic.hpp

@@ -58,9 +58,10 @@
 //     ValueType -> T
 //
 //   special functions:
-//     explicit constructor(T)
+//     constexpr constructor()             // See (2) below
+//     explicit constexpr constructor(T)
 //     noncopyable
-//     destructor
+//     destructor                          // Trivial
 //
 //   static member functions:
 //     value_offset_in_bytes() -> int   // constexpr
@@ -343,7 +344,8 @@ class AtomicImpl::Atomic<T, AtomicImpl::Category::Integer>
   : public SupportsArithmetic<T>
 {
 public:
-  explicit constexpr Atomic(T value = 0) : SupportsArithmetic<T>(value) {}
+  constexpr Atomic() : Atomic(0) {}
+  explicit constexpr Atomic(T value) : SupportsArithmetic<T>(value) {}
 
   NONCOPYABLE(Atomic);
 
@@ -383,7 +385,8 @@ class AtomicImpl::Atomic<T, AtomicImpl::Category::Byte>
   : public CommonCore<T>
 {
 public:
-  explicit constexpr Atomic(T value = 0) : CommonCore<T>(value) {}
+  constexpr Atomic() : Atomic(0) {}
+  explicit constexpr Atomic(T value) : CommonCore<T>(value) {}
 
   NONCOPYABLE(Atomic);
 
@@ -399,7 +402,8 @@ class AtomicImpl::Atomic<T, AtomicImpl::Category::Pointer>
   : public SupportsArithmetic<T>
 {
 public:
-  explicit constexpr Atomic(T value = nullptr) : SupportsArithmetic<T>(value) {}
+  constexpr Atomic() : Atomic(nullptr) {}
+  explicit constexpr Atomic(T value) : SupportsArithmetic<T>(value) {}
 
   NONCOPYABLE(Atomic);
 

test/hotspot/gtest/runtime/test_atomic.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2025, 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
@@ -22,15 +22,100 @@
  *
  */
 
+#include "cppstdlib/new.hpp"
 #include "cppstdlib/type_traits.hpp"
 #include "metaprogramming/primitiveConversions.hpp"
 #include "runtime/atomic.hpp"
+#include "runtime/os.hpp"
 
 #include "unittest.hpp"
 
 // These tests of Atomic<T> only verify functionality.  They don't verify
 // atomicity.
 
+template<typename T>
+struct AtomicInitializationTestSupport {
+  struct Holder {
+    Atomic<T> _explicitly_initialized;
+    Atomic<T> _default_initialized;
+    Atomic<T> _value_initialized;
+
+    Holder()
+      : _explicitly_initialized(T()),
+        /* _default_initialized */
+        _value_initialized{}
+    {}
+  };
+
+  struct HolderNoConstructor {
+    Atomic<T> _default_initialized;
+  };
+
+  void test() {
+    T t = T();
+
+    {
+      Holder h;
+
+      EXPECT_EQ(t, h._explicitly_initialized.load_relaxed());
+      EXPECT_EQ(t, h._default_initialized.load_relaxed());
+      EXPECT_EQ(t, h._value_initialized.load_relaxed());
+    }
+
+    {
+      Holder h{};
+
+      EXPECT_EQ(t, h._explicitly_initialized.load_relaxed());
+      EXPECT_EQ(t, h._default_initialized.load_relaxed());
+      EXPECT_EQ(t, h._value_initialized.load_relaxed());
+    }
+
+    {
+      alignas(Holder) char mem[sizeof(Holder)];
+      memset(mem, 0xFF, sizeof(Holder));
+      Holder* h = new (mem) Holder();
+
+      EXPECT_EQ(t, h->_explicitly_initialized.load_relaxed());
+      EXPECT_EQ(t, h->_default_initialized.load_relaxed());
+      EXPECT_EQ(t, h->_value_initialized.load_relaxed());
+    }
+
+    // No-constructor variant
+
+    {
+      HolderNoConstructor h;
+
+      EXPECT_EQ(t, h._default_initialized.load_relaxed());
+    }
+
+    {
+      HolderNoConstructor h{};
+
+      EXPECT_EQ(t, h._default_initialized.load_relaxed());
+    }
+
+    {
+      alignas(HolderNoConstructor) char mem[sizeof(HolderNoConstructor)];
+      memset(mem, 0xFF, sizeof(HolderNoConstructor));
+      HolderNoConstructor* h = new (mem) HolderNoConstructor();
+
+      EXPECT_EQ(t, h->_default_initialized.load_relaxed());
+    }
+  }
+};
+
+TEST_VM(AtomicInitializationTest, byte) {
+  AtomicInitializationTestSupport<char>().test();
+}
+
+TEST_VM(AtomicInitializationTest, integer) {
+  AtomicInitializationTestSupport<int32_t>().test();
+}
+
+TEST_VM(AtomicInitializationTest, pointer) {
+  AtomicInitializationTestSupport<void*>().test();
+}
+
 template<typename T>
 struct AtomicIntegerArithmeticTestSupport {
   Atomic<T> _test_value;