libstdc++: Value-initialize std::atomic for C++20 (P0883R2)

This implements the new requirements for C++20 that std::atomic should
initialize the atomic variable in its default constructor.

This patch does not add the deprecated attribute to atomic_init, but
that should be done at some point as it's deprecated in C++20.

The paper also deprecates the ATOMIC_FLAG_INIT macro, although we can't
apply the deprecated attribute to a macro.

	PR libstdc++/58605
	* include/bits/atomic_base.h (__cpp_lib_atomic_value_initialization):
	Define.
	(__atomic_flag_base, __atomic_base, __atomic_base<_PTp*>)
	(__atomic_float): Add default member initializer for C++20.
	* include/std/atomic (atomic): Likewise.
	(atomic::atomic()): Remove noexcept-specifier on default constructor.
	* include/std/version (__cpp_lib_atomic_value_initialization): Define.
	* testsuite/29_atomics/atomic/cons/assign_neg.cc: Adjust dg-error line
	number.
	* testsuite/29_atomics/atomic/cons/copy_neg.cc: Likewise.
	* testsuite/29_atomics/atomic/cons/value_init.cc: New test.
	* testsuite/29_atomics/atomic_flag/cons/value_init.cc: New test.
	* testsuite/29_atomics/atomic_flag/requirements/trivial.cc: Adjust
	expected result for is_trivially_default_constructible.
	* testsuite/29_atomics/atomic_float/requirements.cc: Likewise.
	* testsuite/29_atomics/atomic_float/value_init.cc: New test.
	* testsuite/29_atomics/atomic_integral/cons/assign_neg.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/copy_neg.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/value_init.cc
	* testsuite/29_atomics/atomic_integral/requirements/trivial.cc: Adjust
	expected results for is_trivially_default_constructible.
	* testsuite/util/testsuite_common_types.h (has_trivial_dtor): Add
	new test generator.

libstdc++-v3/ChangeLog

+2020-01-13  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/58605
+	* include/bits/atomic_base.h (__cpp_lib_atomic_value_initialization):
+	Define.
+	(__atomic_flag_base, __atomic_base, __atomic_base<_PTp*>)
+	(__atomic_float): Add default member initializer for C++20.
+	* include/std/atomic (atomic): Likewise.
+	(atomic::atomic()): Remove noexcept-specifier on default constructor.
+	* include/std/version (__cpp_lib_atomic_value_initialization): Define.
+	* testsuite/29_atomics/atomic/cons/assign_neg.cc: Adjust dg-error line
+	number.
+	* testsuite/29_atomics/atomic/cons/copy_neg.cc: Likewise.
+	* testsuite/29_atomics/atomic/cons/value_init.cc: New test.
+	* testsuite/29_atomics/atomic_flag/cons/value_init.cc: New test.
+	* testsuite/29_atomics/atomic_flag/requirements/trivial.cc: Adjust
+	expected result for is_trivially_default_constructible.
+	* testsuite/29_atomics/atomic_float/requirements.cc: Likewise.
+	* testsuite/29_atomics/atomic_float/value_init.cc: New test.
+	* testsuite/29_atomics/atomic_integral/cons/assign_neg.cc: Likewise.
+	* testsuite/29_atomics/atomic_integral/cons/copy_neg.cc: Likewise.
+	* testsuite/29_atomics/atomic_integral/cons/value_init.cc
+	* testsuite/29_atomics/atomic_integral/requirements/trivial.cc: Adjust
+	expected results for is_trivially_default_constructible.
+	* testsuite/util/testsuite_common_types.h (has_trivial_dtor): Add
+	new test generator.
+
 2020-01-10  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/util/testsuite_iterators.h: Improve comment.

libstdc++-v3/include/bits/atomic_base.h

@@ -139,6 +139,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _IntTp>
     struct __atomic_base;
 
+#if __cplusplus <= 201703L
+# define _GLIBCXX20_INIT(I)
+#else
+# define __cpp_lib_atomic_value_initialization 201911L
+# define _GLIBCXX20_INIT(I) = I
+#endif
 
 #define ATOMIC_VAR_INIT(_VI) { _VI }
 
@@ -169,7 +175,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   struct __atomic_flag_base
   {
-    __atomic_flag_data_type _M_i;
+    __atomic_flag_data_type _M_i _GLIBCXX20_INIT({});
   };
 
   _GLIBCXX_END_EXTERN_C
@@ -267,7 +273,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr int _S_alignment =
 	sizeof(_ITp) > alignof(_ITp) ? sizeof(_ITp) : alignof(_ITp);
 
-      alignas(_S_alignment) __int_type _M_i;
+      alignas(_S_alignment) __int_type _M_i _GLIBCXX20_INIT(0);
 
     public:
       __atomic_base() noexcept = default;
@@ -595,7 +601,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     private:
       typedef _PTp* 	__pointer_type;
 
-      __pointer_type 	_M_p;
+      __pointer_type 	_M_p _GLIBCXX20_INIT(nullptr);
 
       // Factored out to facilitate explicit specialization.
       constexpr ptrdiff_t
@@ -1175,8 +1181,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return __atomic_impl::__sub_fetch_flt(&_M_fp, __i); }
 
     private:
-      alignas(_S_alignment) _Fp _M_fp;
+      alignas(_S_alignment) _Fp _M_fp _GLIBCXX20_INIT(0);
     };
+#undef _GLIBCXX20_INIT
 
   template<typename _Tp,
 	   bool = is_integral_v<_Tp>, bool = is_floating_point_v<_Tp>>

libstdc++-v3/include/std/atomic

@@ -165,6 +165,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return _M_base.compare_exchange_strong(__i1, __i2, __m); }
   };
 
+#if __cplusplus <= 201703L
+# define _GLIBCXX20_INIT(I)
+#else
+# define _GLIBCXX20_INIT(I) = I
+#endif
 
   /**
    *  @brief Generic atomic type, primary class template.
@@ -185,7 +190,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr int _S_alignment
         = _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);
 
-      alignas(_S_alignment) _Tp _M_i;
+      alignas(_S_alignment) _Tp _M_i _GLIBCXX20_INIT(_Tp());
 
       static_assert(__is_trivially_copyable(_Tp),
 		    "std::atomic requires a trivially copyable type");
@@ -194,7 +199,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		    "Incomplete or zero-sized types are not supported");
 
     public:
-      atomic() noexcept = default;
+      atomic() = default;
       ~atomic() noexcept = default;
       atomic(const atomic&) = delete;
       atomic& operator=(const atomic&) = delete;
@@ -348,7 +353,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return compare_exchange_strong(__e, __i, __m,
                                        __cmpexch_failure_order(__m)); }
     };
-
+#undef _GLIBCXX20_INIT
 
   /// Partial specialization for pointer types.
   template<typename _Tp>

libstdc++-v3/include/std/version

@@ -164,6 +164,7 @@
 #if __cplusplus > 201703L
 // c++2a
 #define __cpp_lib_atomic_ref 201806L
+#define __cpp_lib_atomic_value_initialization 201911L
 #define __cpp_lib_bitops 201907L
 #define __cpp_lib_bounded_array_traits 201902L
 #if __cpp_concepts

libstdc++-v3/testsuite/29_atomics/atomic/cons/assign_neg.cc

@@ -27,5 +27,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 639 }
+// { dg-error "deleted" "" { target *-*-* } 659 }
 // { dg-prune-output "include" }

libstdc++-v3/testsuite/29_atomics/atomic/cons/copy_neg.cc

@@ -27,5 +27,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 678 }
+// { dg-error "deleted" "" { target *-*-* } 698 }
 // { dg-prune-output "include" }

libstdc++-v3/testsuite/29_atomics/atomic/cons/value_init.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+
+#ifndef __cpp_lib_atomic_value_initialization
+# error "Feature test macro for atomic value-initialization is missing"
+#elif __cpp_lib_atomic_value_initialization < 201911L
+# error "Feature test macro for atomic value-initialization has wrong value"
+#endif
+
+#include <testsuite_hooks.h>
+
+struct A
+{
+  constexpr A() : val(42) { }
+  int val;
+};
+
+constexpr std::atomic<A> a;
+
+void
+test01()
+{
+  VERIFY(a.load().val == 42);
+  static_assert(!std::is_nothrow_default_constructible_v<std::atomic<A>>);
+}
+
+struct B
+{
+  constexpr B() noexcept : val(99) { }
+  int val;
+};
+
+constexpr std::atomic<B> b;
+
+void
+test02()
+{
+  VERIFY(b.load().val == 99);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<B>>);
+}
+
+constexpr std::atomic<int*> c;
+
+void
+test03()
+{
+  VERIFY(c.load() == nullptr);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<int*>>);
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+}

libstdc++-v3/testsuite/29_atomics/atomic_flag/cons/value_init.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::atomic_flag f;
+  VERIFY(!f.test_and_set());
+  VERIFY(f.test_and_set());
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic_flag>);
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/29_atomics/atomic_flag/requirements/trivial.cc

@@ -22,6 +22,10 @@
 
 void test01()
 {
+#if __cplusplus <= 201703L
   __gnu_test::has_trivial_cons_dtor test;
+#else
+  __gnu_test::has_trivial_dtor test;
+#endif
   test.operator()<std::atomic_flag>();
 }

libstdc++-v3/testsuite/29_atomics/atomic_float/requirements.cc

@@ -25,7 +25,7 @@ test01()
 {
   using A = std::atomic<float>;
   static_assert( std::is_standard_layout_v<A> );
-  static_assert( std::is_trivially_default_constructible_v<A> );
+  static_assert( !std::is_trivially_default_constructible_v<A> );
   static_assert( std::is_trivially_destructible_v<A> );
   static_assert( std::is_same_v<A::value_type, float> );
   static_assert( std::is_same_v<A::difference_type, A::value_type> );
@@ -41,7 +41,7 @@ test02()
 {
   using A = std::atomic<double>;
   static_assert( std::is_standard_layout_v<A> );
-  static_assert( std::is_trivially_default_constructible_v<A> );
+  static_assert( !std::is_trivially_default_constructible_v<A> );
   static_assert( std::is_trivially_destructible_v<A> );
   static_assert( std::is_same_v<A::value_type, double> );
   static_assert( std::is_same_v<A::difference_type, A::value_type> );
@@ -57,7 +57,7 @@ test03()
 {
   using A = std::atomic<long double>;
   static_assert( std::is_standard_layout_v<A> );
-  static_assert( std::is_trivially_default_constructible_v<A> );
+  static_assert( !std::is_trivially_default_constructible_v<A> );
   static_assert( std::is_trivially_destructible_v<A> );
   static_assert( std::is_same_v<A::value_type, long double> );
   static_assert( std::is_same_v<A::difference_type, A::value_type> );

libstdc++-v3/testsuite/29_atomics/atomic_float/value_init.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+constexpr std::atomic<double> a;
+
+void
+test01()
+{
+  VERIFY(a.load() == 0);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<double>>);
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/assign_neg.cc

@@ -28,5 +28,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 639 }
+// { dg-error "deleted" "" { target *-*-* } 659 }
 // { dg-prune-output "include" }

libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/copy_neg.cc

@@ -28,5 +28,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 678 }
+// { dg-error "deleted" "" { target *-*-* } 698 }
 // { dg-prune-output "include" }

libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/value_init.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+constexpr std::atomic<int> a;
+
+void
+test01()
+{
+  VERIFY(a.load() == 0);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<int>>);
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/29_atomics/atomic_integral/requirements/trivial.cc

@@ -22,7 +22,11 @@
 
 void test01()
 {
+#if __cplusplus <= 201703L
   __gnu_test::has_trivial_cons_dtor test;
+#else
+  __gnu_test::has_trivial_dtor test;
+#endif
   __gnu_cxx::typelist::apply_generator(test,
 				       __gnu_test::atomic_integrals::type());
 }

libstdc++-v3/testsuite/util/testsuite_common_types.h

@@ -558,7 +558,6 @@ namespace __gnu_test
       }
   };
 
-  // Generator to test standard layout
   struct has_trivial_cons_dtor
   {
     template<typename _Tp>
@@ -582,6 +581,27 @@ namespace __gnu_test
       }
   };
 
+  struct has_trivial_dtor
+  {
+    template<typename _Tp>
+      void
+      operator()()
+      {
+	struct _Concept
+	{
+	  void __constraint()
+	  {
+	    typedef std::is_trivially_destructible<_Tp> dtor_p;
+	    static_assert(dtor_p::value, "destructor not trivial");
+	  }
+	};
+
+	void (_Concept::*__x)() __attribute__((unused))
+	  = &_Concept::__constraint;
+      }
+  };
+
+  // Generator to test standard layout
   struct standard_layout
   {
     template<typename _Tp>