libstdc++: Add std::bit_cast for C++20 [PR 93121]

Thanks to Jakub's addition of the built-in, we can add this to the
library now. The compiler tests for the built-in are quite extensive,
including verifying the constraints, so this only adds minimal tests to
the library testsuite.

This doesn't add a new _GLIBCXX_HAVE_BUILTIN_BIT_CAST because using
__has_builtin(__builtin_bit_cast) works for GCC and versions of Clang
that provide the built-in.

libstdc++-v3/ChangeLog:

	PR libstdc++/93121
	* include/std/bit (__cpp_lib_bit_cast, bit_cast): Define.
	* include/std/version (__cpp_lib_bit_cast): Define.
	* testsuite/26_numerics/bit/bit.cast/bit_cast.cc: New test.
	* testsuite/26_numerics/bit/bit.cast/version.cc: New test.

libstdc++-v3/include/std/bit

@@ -49,6 +49,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    * @{
    */
 
+#if __cplusplus > 201703l && __has_builtin(__builtin_bit_cast)
+#define __cpp_lib_bit_cast 201806L
+
+  /// Create a value of type `To` from the bits of `from`.
+  template<typename _To, typename _From>
+    constexpr _To
+    bit_cast(const _From& __from) noexcept
+    {
+      return __builtin_bit_cast(_To, __from);
+    }
+#endif
+
   /// @cond undoc
 
   template<typename _Tp>

libstdc++-v3/include/std/version

@@ -201,6 +201,9 @@
 # define __cpp_lib_atomic_wait 201907L
 #endif
 #define __cpp_lib_bind_front 201907L
+#if __has_builtin(__builtin_bit_cast)
+# define __cpp_lib_bit_cast 201806L
+#endif
 // FIXME: #define __cpp_lib_execution 201902L
 #define __cpp_lib_integer_comparison_functions 202002L
 #define __cpp_lib_constexpr_algorithms 201806L

libstdc++-v3/testsuite/26_numerics/bit/bit.cast/bit_cast.cc

@@ -0,0 +1,81 @@
+// 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 compile { target c++2a } }
+
+#include <bit>
+
+#ifndef __cpp_lib_bit_cast
+# error "Feature-test macro for bit_cast wait missing in <bit>"
+#elif __cpp_lib_bit_cast != 201806L
+# error "Feature-test macro for bit_cast wait has wrong value in <bit>"
+#endif
+
+#include <cstdint>
+#include <cstring>
+#include <testsuite_hooks.h>
+
+template<typename To, typename From>
+constexpr bool
+check(const From& from)
+{
+  return std::bit_cast<From>(std::bit_cast<To>(from)) == from;
+}
+
+void
+test01()
+{
+  static_assert( std::bit_cast<int>(123) == 123 );
+  static_assert( std::bit_cast<int>(123u) == 123 );
+  static_assert( std::bit_cast<int>(~0u) == ~0 );
+
+  if constexpr (sizeof(int) == sizeof(float))
+    static_assert( check<int>(12.34f) );
+  if constexpr (sizeof(unsigned long long) == sizeof(double))
+    static_assert( check<unsigned long long>(123.456) );
+  if constexpr (sizeof(std::intptr_t) == sizeof(void(*)()))
+    VERIFY( check<std::intptr_t>(&test01) );
+}
+
+void
+test02()
+{
+  struct S
+  {
+    int i;
+
+    bool operator==(const char* s) const
+    { return std::memcmp(&i, s, sizeof(i)) == 0; }
+  };
+
+  char arr[sizeof(int)];
+  char arr2[sizeof(int)];
+  for (int i = 0; i < sizeof(int); ++i)
+  {
+    arr[i] = i + 1;
+    arr2[i] = (i + 1) * -(i % 2);
+  }
+  VERIFY( std::bit_cast<S>(arr) == arr );
+  VERIFY( std::bit_cast<S>(arr2) == arr2 );
+}
+
+int main()
+{
+  test01();
+  test02();
+}

libstdc++-v3/testsuite/26_numerics/bit/bit.cast/version.cc

@@ -0,0 +1,27 @@
+// 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 compile { target c++2a } }
+
+#include <version>
+
+#ifndef __cpp_lib_bit_cast
+# error "Feature-test macro for bit_cast wait missing in <version>"
+#elif __cpp_lib_bit_cast != 201806L
+# error "Feature-test macro for bit_cast wait has wrong value in <version>"
+#endif