P1272R4 `byteswap()` (#2235)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/bit

@@ -233,6 +233,34 @@ _NODISCARD int _Checked_arm_arm64_countl_zero(const _Ty _Val) noexcept {
 }
 #endif // defined(_M_ARM) || defined(_M_ARM64)
 
+#if _HAS_CXX23
+_NODISCARD constexpr unsigned short _Byteswap_ushort(const unsigned short _Val) noexcept {
+    if (_STD is_constant_evaluated()) {
+        return static_cast<unsigned short>((_Val << 8) | (_Val >> 8));
+    } else {
+        return _byteswap_ushort(_Val);
+    }
+}
+
+_NODISCARD constexpr unsigned long _Byteswap_ulong(const unsigned long _Val) noexcept {
+    if (_STD is_constant_evaluated()) {
+        return (_Val << 24) | ((_Val << 8) & 0x00FF'0000) | ((_Val >> 8) & 0x0000'FF00) | (_Val >> 24);
+    } else {
+        return _byteswap_ulong(_Val);
+    }
+}
+
+_NODISCARD constexpr unsigned long long _Byteswap_uint64(const unsigned long long _Val) noexcept {
+    if (_STD is_constant_evaluated()) {
+        return (_Val << 56) | ((_Val << 40) & 0x00FF'0000'0000'0000) | ((_Val << 24) & 0x0000'FF00'0000'0000)
+             | ((_Val << 8) & 0x0000'00FF'0000'0000) | ((_Val >> 8) & 0x0000'0000'FF00'0000)
+             | ((_Val >> 24) & 0x0000'0000'00FF'0000) | ((_Val >> 40) & 0x0000'0000'0000'FF00) | (_Val >> 56);
+    } else {
+        return _byteswap_uint64(_Val);
+    }
+}
+#endif // _HAS_CXX23
+
 template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> _Enabled>
 _NODISCARD constexpr int countl_zero(const _Ty _Val) noexcept {
 #if defined(_M_IX86) || (defined(_M_X64) && !defined(_M_ARM64EC))
@@ -268,6 +296,23 @@ _NODISCARD constexpr int popcount(const _Ty _Val) noexcept {
     return _Popcount(_Val);
 }
 
+#if _HAS_CXX23
+template <class _Ty, enable_if_t<is_integral_v<_Ty>, int> = 0>
+_NODISCARD constexpr _Ty byteswap(const _Ty _Val) noexcept {
+    if constexpr (sizeof(_Ty) == 1) {
+        return _Val;
+    } else if constexpr (sizeof(_Ty) == 2) {
+        return static_cast<_Ty>(_Byteswap_ushort(static_cast<unsigned short>(_Val)));
+    } else if constexpr (sizeof(_Ty) == 4) {
+        return static_cast<_Ty>(_Byteswap_ulong(static_cast<unsigned long>(_Val)));
+    } else if constexpr (sizeof(_Ty) == 8) {
+        return static_cast<_Ty>(_Byteswap_uint64(static_cast<unsigned long long>(_Val)));
+    } else {
+        static_assert(_Always_false<_Ty>, "Unexpected integer size");
+    }
+}
+#endif // _HAS_CXX23
+
 enum class endian { little = 0, big = 1, native = little };
 
 _STD_END

stl/inc/yvals_core.h

@@ -275,6 +275,7 @@
 // P0943R6 Supporting C Atomics In C++
 // P1048R1 is_scoped_enum
 // P1132R7 out_ptr(), inout_ptr()
+// P1272R4 byteswap()
 // P1425R4 Iterator Pair Constructors For stack And queue
 // P1659R3 ranges::starts_with, ranges::ends_with
 // P1679R3 contains() For basic_string/basic_string_view
@@ -1363,6 +1364,7 @@
 #define __cpp_lib_allocate_at_least 202106L
 #endif // __cpp_lib_concepts
 
+#define __cpp_lib_byteswap       202110L
 #define __cpp_lib_invoke_r       202106L
 #define __cpp_lib_is_scoped_enum 202011L
 

tests/std/test.lst

@@ -435,6 +435,7 @@ tests\P1135R6_latch
 tests\P1135R6_semaphore
 tests\P1165R1_consistently_propagating_stateful_allocators
 tests\P1208R6_source_location
+tests\P1272R4_byteswap
 tests\P1423R3_char8_t_remediation
 tests\P1425R4_queue_stack_constructors
 tests\P1502R1_standard_library_header_units

tests/std/tests/P1272R4_byteswap/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst

tests/std/tests/P1272R4_byteswap/test.cpp

@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <bit>
+#include <cassert>
+
+using namespace std;
+
+template <typename T>
+constexpr bool test_byteswap(const auto src, const auto result) {
+    static_assert(noexcept(byteswap(T{})));
+    return byteswap(static_cast<T>(src)) == static_cast<T>(result);
+}
+
+constexpr bool test_byteswap_all_types() {
+    assert(test_byteswap<bool>(true, true));
+
+    assert(test_byteswap<char>(0x13, 0x13));
+    assert(test_byteswap<signed char>(0x13, 0x13));
+    assert(test_byteswap<unsigned char>(0x13, 0x13));
+#ifdef __cpp_char8_t
+    assert(test_byteswap<char8_t>(0x13, 0x13));
+#endif
+
+    assert(test_byteswap<short>(0xAC34, 0x34AC));
+    assert(test_byteswap<unsigned short>(0xAC34, 0x34AC));
+    assert(test_byteswap<char16_t>(0xAC34, 0x34AC));
+    assert(test_byteswap<wchar_t>(0xAC34, 0x34AC));
+
+    assert(test_byteswap<int>(0x1234ABCD, 0xCDAB3412));
+    assert(test_byteswap<unsigned int>(0x1234ABCD, 0xCDAB3412));
+    assert(test_byteswap<long>(0x1234ABCD, 0xCDAB3412));
+    assert(test_byteswap<unsigned long>(0x1234ABCD, 0xCDAB3412));
+    assert(test_byteswap<char32_t>(0x1234ABCD, 0xCDAB3412));
+
+    assert(test_byteswap<long long>(0x1234567890ABCDEF, 0xEFCDAB9078563412));
+    assert(test_byteswap<unsigned long long>(0x1234567890ABCDEF, 0xEFCDAB9078563412));
+
+    return true;
+}
+
+static_assert(test_byteswap_all_types());
+
+int main() {
+    test_byteswap_all_types();
+}

tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp

@@ -336,6 +336,20 @@ STATIC_ASSERT(__cpp_lib_byte == 201603L);
 #endif
 #endif
 
+#if _HAS_CXX23
+#ifndef __cpp_lib_byteswap
+#error __cpp_lib_byteswap is not defined
+#elif __cpp_lib_byteswap != 202110L
+#error __cpp_lib_byteswap is not 202110L
+#else
+STATIC_ASSERT(__cpp_lib_byteswap == 202110L);
+#endif
+#else
+#ifdef __cpp_lib_byteswap
+#error __cpp_lib_byteswap is defined
+#endif
+#endif
+
 #if _HAS_CXX20 && defined(__cpp_char8_t)
 #ifndef __cpp_lib_char8_t
 #error __cpp_lib_char8_t is not defined