libstdc++: Implement std::byteswap for C++23

This patch attempts to implement P1272R4 (except for the std::bit_cast
changes in there which seem quite unrelated to this and will need to be
fixed on the compiler side).
While at least for GCC __builtin_bswap{16,32,64,128} should work fine
in constant expressions, I wonder about other compilers, so I'm using
a fallback implementation for constexpr evaluation always.
If you think that is unnecessary, I can drop the
__cpp_if_consteval >= 202106L &&
if !consteval
  {
and
  }
and reformat.
The fallback implementation is an attempt to make it work even for integral
types that don't have number of bytes divisible by 2 or when __CHAR_BIT__
is e.g. 16.

2021-11-28  Jakub Jelinek  <jakub@redhat.com>

	* include/std/bit (__cpp_lib_byteswap, byteswap): Define.
	* include/std/version (__cpp_lib_byteswap): Define.
	* testsuite/26_numerics/bit/bit.byteswap/byteswap.cc: New test.
	* testsuite/26_numerics/bit/bit.byteswap/version.cc: New test.

4 files changed, 189 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit
index c5aae8bab03..18ce5ca220c 100644
--- a/libstdc++-v3/include/std/bit
+++ b/libstdc++-v3/include/std/bit
@@ -78,6 +78,55 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 #endif
 
+#if __cplusplus > 202002L
+#define __cpp_lib_byteswap 202110L
+
+  /// Reverse order of bytes in the object representation of `value`.
+  template<typename _Tp>
+    constexpr enable_if_t<is_integral<_Tp>::value, _Tp>
+    byteswap(_Tp __value) noexcept
+    {
+      if constexpr (sizeof(_Tp) == 1)
+	return __value;
+#if __cpp_if_consteval >= 202106L && __CHAR_BIT__ == 8
+      if !consteval
+	{
+	  if constexpr (sizeof(_Tp) == 2)
+	    return __builtin_bswap16(__value);
+	  if constexpr (sizeof(_Tp) == 4)
+	    return __builtin_bswap32(__value);
+	  if constexpr (sizeof(_Tp) == 8)
+	    return __builtin_bswap64(__value);
+	  if constexpr (sizeof(_Tp) == 16)
+#if __has_builtin(__builtin_bswap128)
+	    return __builtin_bswap128(__value);
+#else
+	    return (__builtin_bswap64(__value >> 64)
+		    | (static_cast<_Tp>(__builtin_bswap64(__value)) << 64));
+#endif
+	}
+#endif
+
+      // Fallback implementation that handles even __int24 etc.
+      using _Up = typename __make_unsigned<__remove_cv_t<_Tp>>::__type;
+      size_t __diff = __CHAR_BIT__ * (sizeof(_Tp) - 1);
+      _Up __mask1 = static_cast<unsigned char>(~0);
+      _Up __mask2 = __mask1 << __diff;
+      _Up __val = __value;
+      for (size_t __i = 0; __i < sizeof(_Tp) / 2; ++__i)
+	{
+	  _Up __byte1 = __val & __mask1;
+	  _Up __byte2 = __val & __mask2;
+	  __val = (__val ^ __byte1 ^ __byte2
+		   ^ (__byte1 << __diff) ^ (__byte2 >> __diff));
+	  __mask1 <<= __CHAR_BIT__;
+	  __mask2 >>= __CHAR_BIT__;
+	  __diff -= 2 * __CHAR_BIT__;
+	}
+      return __val;
+    }
+#endif
+
   /// @cond undoc
 
   template<typename _Tp>
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index fd7e1258543..2d4e9bf4eee 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -294,6 +294,7 @@
 #if __cplusplus > 202002L
 // c++2b
 #define __cpp_lib_adaptor_iterator_pair_constructor 202106L
+#define __cpp_lib_byteswap 202110L
 #define __cpp_lib_invoke_r 202106L
 #define __cpp_lib_is_scoped_enum 202011L
 #if __cpp_lib_concepts
diff --git a/libstdc++-v3/testsuite/26_numerics/bit/bit.byteswap/byteswap.cc b/libstdc++-v3/testsuite/26_numerics/bit/bit.byteswap/byteswap.cc
new file mode 100644
index 00000000000..f447af020e6
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/bit/bit.byteswap/byteswap.cc
@@ -0,0 +1,112 @@
+// Copyright (C) 2021 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++2b" }
+// { dg-do compile { target c++23 } }
+
+#include <bit>
+
+#ifndef __cpp_lib_byteswap
+# error "Feature-test macro for byteswap missing in <bit>"
+#elif __cpp_lib_byteswap != 202110L
+# error "Feature-test macro for byteswap has wrong value in <bit>"
+#endif
+
+#include <cstdint>
+#include <cstring>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  static_assert( std::byteswap<int8_t>(0x12) == 0x12 );
+  static_assert( std::byteswap<int16_t>(0x1234) == 0x3412 );
+  static_assert( std::byteswap<int32_t>(0x12345678) == 0x78563412 );
+  static_assert( std::byteswap<int64_t>(0x123456789abcdef0)
+		 == static_cast<int64_t>(0xf0debc9a78563412) );
+  static_assert( std::byteswap<uint8_t>(0x21) == 0x21 );
+  static_assert( std::byteswap<uint16_t>(0x4321) == 0x2143 );
+  static_assert( std::byteswap<uint32_t>(0x87654321) == 0x21436587 );
+  static_assert( std::byteswap<uint64_t>(0xfedcba9876543210)
+		 == static_cast<uint64_t>(0x1032547698badcfe) );
+#if !defined(__STRICT_ANSI__) && defined __SIZEOF_INT128__
+  constexpr __int128_t c1 = (static_cast<__int128_t>(0x0102030405060708) << 64
+			     | 0x090a0b0c0d0e0f10);
+  constexpr __int128_t c2 = (static_cast<__int128_t>(0x100f0e0d0c0b0a09) << 64
+			     | 0x0807060504030201);
+  constexpr __int128_t c3 = (static_cast<__int128_t>(0xf1e2d3c4b5a69788) << 64
+			     | 0x796a5b4c3d2e1f10);
+  constexpr __int128_t c4 = (static_cast<__int128_t>(0x101f2e3d4c5b6a79) << 64
+			     | 0x8897a6b5c4d3e2f1);
+  static_assert( std::byteswap(c1) == c2 );
+  static_assert( std::byteswap(static_cast<__uint128_t>(c1))
+		 == static_cast<__uint128_t>(c2) );
+  static_assert( std::byteswap(c3) == c4 );
+#endif
+  static_assert( std::byteswap<const uint32_t>(0xdeadbeef) == 0xefbeadde );
+  static_assert( std::byteswap<volatile uint32_t>(0xdeadbeef) == 0xefbeadde );
+  static_assert( std::byteswap<int32_t>(0xdeadbeef)
+		 == static_cast<int32_t>(0xefbeadde) );
+}
+
+void
+test02()
+{
+  volatile int8_t a = 0x12;
+  volatile int16_t b = 0x1234;
+  volatile int32_t c = 0x12345678;
+  volatile int64_t d = 0x123456789abcdef0;
+  volatile uint8_t e = 0x21;
+  volatile uint16_t f = 0x4321;
+  volatile uint32_t g = 0x87654321;
+  volatile uint64_t h = 0xfedcba9876543210;
+  VERIFY ( std::byteswap<int8_t>(a) == 0x12 );
+  VERIFY ( std::byteswap<int16_t>(b) == 0x3412 );
+  VERIFY ( std::byteswap(c) == 0x78563412 );
+  VERIFY ( std::byteswap(d) == 0xf0debc9a78563412 );
+  VERIFY ( std::byteswap<uint8_t>(e) == 0x21 );
+  VERIFY ( std::byteswap<uint16_t>(f) == 0x2143 );
+  VERIFY ( std::byteswap(g) == 0x21436587 );
+  VERIFY ( std::byteswap(h) == 0x1032547698badcfe );
+  VERIFY ( std::byteswap(std::byteswap<int8_t>(a)) == a );
+  VERIFY ( std::byteswap(std::byteswap<int16_t>(b)) == b );
+  VERIFY ( std::byteswap(std::byteswap(c)) == c );
+  VERIFY ( std::byteswap(std::byteswap(d)) == d );
+  VERIFY ( std::byteswap(std::byteswap<uint8_t>(e)) == e );
+  VERIFY ( std::byteswap(std::byteswap<uint16_t>(f)) == f );
+  VERIFY ( std::byteswap(std::byteswap(g)) == g );
+  VERIFY ( std::byteswap(std::byteswap(h)) == h );
+#if !defined(__STRICT_ANSI__) && defined __SIZEOF_INT128__
+  volatile __int128_t c1 = (static_cast<__int128_t>(0x0102030405060708) << 64
+			    | 0x090a0b0c0d0e0f10);
+  volatile __int128_t c2 = (static_cast<__int128_t>(0x100f0e0d0c0b0a09) << 64
+			    | 0x0807060504030201);
+  VERIFY ( std::byteswap(c1) == c2 );
+  VERIFY ( std::byteswap<__uint128_t>(c1) == static_cast<__uint128_t>(c2) );
+  VERIFY ( std::byteswap(std::byteswap(c1)) == c1 );
+  VERIFY ( std::byteswap(std::byteswap<__uint128_t>(c2))
+	   == static_cast<__uint128_t>(c2) );
+#endif
+  VERIFY ( std::byteswap<const uint32_t>(0xdeadbeef) == 0xefbeadde );
+  VERIFY ( std::byteswap<volatile uint32_t>(0xdeadbeef) == 0xefbeadde );
+}
+
+int main()
+{
+  test01();
+  test02();
+}
diff --git a/libstdc++-v3/testsuite/26_numerics/bit/bit.byteswap/version.cc b/libstdc++-v3/testsuite/26_numerics/bit/bit.byteswap/version.cc
new file mode 100644
index 00000000000..e380d7baf45
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/bit/bit.byteswap/version.cc
@@ -0,0 +1,27 @@
+// Copyright (C) 2021 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++2b" }
+// { dg-do compile { target c++23 } }
+
+#include <version>
+
+#ifndef __cpp_lib_byteswap
+# error "Feature-test macro for bit_cast missing in <version>"
+#elif __cpp_lib_byteswap != 202110L
+# error "Feature-test macro for byteswap has wrong value in <version>"
+#endif