add bit_cast and tests (#583)

* add bit_cast and tests
Mirror MSVC-PR-228495

stl/inc/bit

@@ -25,6 +25,16 @@ _STL_DISABLE_CLANG_WARNINGS
 _STD_BEGIN
 enum class endian { little = 0, big = 1, native = little };
 
+#ifdef __cpp_lib_bit_cast // TRANSITION, VSO-1041044
+template <class _To, class _From,
+    enable_if_t<conjunction_v<bool_constant<sizeof(_To) == sizeof(_From)>, is_trivially_copyable<_To>,
+                    is_trivially_copyable<_From>>,
+        int> = 0>
+_NODISCARD constexpr _To bit_cast(const _From& _Val) noexcept {
+    return __builtin_bit_cast(_To, _Val);
+}
+#endif // TRANSITION, VSO-1041044
+
 #ifdef __cpp_lib_bitops // TRANSITION, VSO-1020212
 template <class _Ty>
 inline constexpr bool _Is_standard_unsigned_integer =

stl/inc/yvals_core.h

@@ -140,6 +140,7 @@
 // P0457R2 starts_with()/ends_with() For basic_string/basic_string_view
 // P0458R2 contains() For Ordered And Unordered Associative Containers
 // P0463R1 endian
+// P0476R2 <bit> bit_cast
 // P0482R6 Library Support For char8_t
 //     (mbrtoc8 and c8rtomb not yet implemented)
 // P0487R1 Fixing operator>>(basic_istream&, CharT*)
@@ -1078,6 +1079,10 @@
 #define __cpp_lib_atomic_float 201711L
 #define __cpp_lib_bind_front   201907L
 
+#ifndef __EDG__ // TRANSITION, VSO-1041044
+#define __cpp_lib_bit_cast 201806L
+#endif // __EDG__
+
 #if defined(__clang__) || defined(__EDG__)
 #define __cpp_lib_bitops 201907L
 #else // ^^^ Clang and EDG / MSVC vvv

tests/std/test.lst

@@ -210,6 +210,7 @@ tests\P0357R3_supporting_incomplete_types_in_reference_wrapper
 tests\P0414R2_shared_ptr_for_arrays
 tests\P0426R1_constexpr_char_traits
 tests\P0433R2_deduction_guides
+tests\P0476R2_bit_cast
 tests\P0487R1_fixing_operator_shl_basic_istream_char_pointer
 tests\P0513R0_poisoning_the_hash
 tests\P0553R4_bit_rotating_and_counting_functions

tests/std/tests/P0476R2_bit_cast/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/P0476R2_bit_cast/test.cpp

@@ -0,0 +1,266 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifdef __EDG__ // TRANSITION, VSO-1041044
+int main() {}
+#else // __EDG__ ^^^ / vvv !__EDG__
+
+#include <assert.h>
+#include <bit>
+#include <cmath>
+#include <limits>
+#include <stddef.h>
+#include <string.h>
+#include <type_traits>
+#include <utility>
+
+// structure sizing depends on a packing of 8, which is the default
+
+union test_union_1 {
+    char a;
+    int b;
+};
+union test_union_2 {
+    char a;
+    float b;
+};
+
+struct middle_class_1 {
+    double d;
+    virtual void a_member_function_1() {}
+};
+
+struct middle_class_2 {
+    int a;
+    virtual void a_member_function_2() {}
+};
+
+struct derived_class : middle_class_1, middle_class_2 {
+    virtual void a_member_function_2() override {}
+};
+
+struct test_struct_1 {
+    char a;
+    // char[3]
+    int b;
+    short c;
+    // char[6]
+    double d;
+
+    void a_member_function() {}
+};
+
+struct test_struct_2 {
+    short a;
+    // char[2]
+    float b;
+    float c;
+    int d;
+    unsigned int e;
+    char f;
+    // char[1]
+    short g;
+
+    void a_member_function() {}
+};
+
+struct test_struct_3_member_fn_pointer {
+    char a;
+    // char[3]
+    int b;
+    short c;
+    char d[2];
+    void (test_struct_1::*fn)();
+};
+
+struct test_struct_4_large_member_fn_pointer {
+    char a;
+    // char[3]
+    int b;
+    double c;
+    double d;
+    void (derived_class::*fn)(); // "large" member fn pointers are aligned to 8 on both x64 and x86
+};
+
+struct test_struct_5_struct {
+    char a;
+    // char[3]
+    int b;
+    double c;
+    double d;
+    void* e;
+    size_t f;
+};
+
+struct test_struct_6 {
+    char a;
+    // char[3]
+    int b;
+    int c;
+    // char[4] on x64
+    void* v;
+};
+
+struct test_struct_7_member_fn_pointer {
+    char a;
+    // char[3]
+    int b;
+    int c;
+    void (test_struct_1::*fn)();
+};
+
+struct test_struct_1_not_trivially_copyable {
+    char a;
+    // char[3]
+    int b;
+    short c;
+    // char[2]
+    double d;
+
+    test_struct_1_not_trivially_copyable& operator=(const test_struct_1_not_trivially_copyable&) {
+        return *this;
+    }
+};
+
+#pragma pack(push, 1)
+struct test_struct_1_packed {
+    char a;
+    int b;
+    short c;
+    double d;
+};
+struct test_struct_2_packed {
+    short a;
+    float b;
+    float c;
+    int d;
+    unsigned int e;
+    char f;
+    short g;
+};
+#pragma pack(pop)
+
+static_assert(sizeof(test_struct_1) == sizeof(test_struct_2));
+
+template <typename To, typename From, typename = void>
+constexpr bool bit_cast_invocable = false;
+
+template <typename To, typename From>
+constexpr bool bit_cast_invocable<To, From, std::void_t<decltype(std::bit_cast<To>(std::declval<From>()))>> = true;
+
+template <int zero = 0, int = ((void) std::bit_cast<test_union_1>(test_union_2{}), zero)>
+constexpr bool bit_cast_is_constexpr_union(int) {
+    return true;
+}
+
+constexpr bool bit_cast_is_constexpr_union(long) {
+    return false;
+}
+
+template <int zero = 0, int = ((void) std::bit_cast<float*>(nullptr), zero)>
+constexpr bool bit_cast_is_constexpr_pointer(int) {
+    return true;
+}
+
+constexpr bool bit_cast_is_constexpr_pointer(long) {
+    return false;
+}
+
+template <int zero = 0,
+    int            = ((void) std::bit_cast<void (test_struct_1::*)()>(&test_struct_2::a_member_function), zero)>
+constexpr bool bit_cast_is_constexpr_member_fn_pointer(int) {
+    return true;
+}
+
+constexpr bool bit_cast_is_constexpr_member_fn_pointer(long) {
+    return false;
+}
+
+template <int zero = 0, int = ((void) std::bit_cast<test_struct_6>(test_struct_3_member_fn_pointer{}), zero)>
+constexpr bool bit_cast_is_constexpr_pmf_datamember(int) {
+    return true;
+}
+
+constexpr bool bit_cast_is_constexpr_pmf_datamember(long) {
+    return false;
+}
+
+template <int zero = 0,
+    int            = ((void) std::bit_cast<test_struct_5_struct>(test_struct_4_large_member_fn_pointer{}), zero)>
+constexpr bool bit_cast_is_constexpr_large_member_fn_pointer(int) {
+    return true;
+}
+
+constexpr bool bit_cast_is_constexpr_large_member_fn_pointer(long) {
+    return false;
+}
+
+template <typename To, typename From>
+void zero_initialized_round_trip() {
+    From before{};
+    To middle = std::bit_cast<To>(before);
+    assert(memcmp(&before, &middle, sizeof(From)) == 0);
+    From after = std::bit_cast<From>(middle);
+    assert(memcmp(&before, &after, sizeof(From)) == 0);
+}
+
+constexpr bool test_float() {
+    unsigned int as_int = std::bit_cast<unsigned int>(0x0.000002p-126f);
+    assert(as_int == 1);
+    assert(std::bit_cast<float>(as_int) == 0x0.000002p-126f);
+    as_int = std::bit_cast<unsigned int>(0x1.1p1f);
+    assert(as_int == 0x40080000);
+    assert(std::bit_cast<float>(as_int) == 0x1.1p1f);
+    as_int = std::bit_cast<unsigned int>(0x0.0p0f);
+    assert(as_int == 0);
+    assert(std::bit_cast<float>(as_int) == 0x0.0p0f);
+    if (!std::is_constant_evaluated()) {
+        assert(std::signbit(std::bit_cast<float>(as_int)) == false);
+    }
+    as_int = std::bit_cast<unsigned int>(-0x0.0p0f);
+    assert(as_int == 0x80000000);
+    assert(std::bit_cast<float>(as_int) == -0x0.0p0f);
+    if (!std::is_constant_evaluated()) {
+        assert(std::signbit(std::bit_cast<float>(as_int)) == true);
+    }
+    // signaling nan
+    as_int     = 0x7fc00001;
+    float snan = std::bit_cast<float>(as_int);
+    assert(as_int == std::bit_cast<unsigned int>(snan));
+    as_int = std::bit_cast<unsigned int>(std::numeric_limits<float>::infinity());
+    assert(as_int == 0x7f800000);
+    assert(std::bit_cast<float>(as_int) == std::numeric_limits<float>::infinity());
+    return true;
+}
+
+int main() {
+    static_assert(!bit_cast_invocable<test_struct_2_packed, test_struct_1_packed>);
+    static_assert(!bit_cast_invocable<test_struct_1_not_trivially_copyable, test_struct_1>);
+    static_assert(!bit_cast_invocable<test_struct_1, test_struct_1_not_trivially_copyable>);
+    static_assert(!bit_cast_invocable<test_struct_1_not_trivially_copyable, test_struct_1_not_trivially_copyable>);
+    static_assert(bit_cast_invocable<test_union_1, test_union_2>);
+    static_assert(bit_cast_invocable<ptrdiff_t, void (test_struct_1::*)()>);
+    static_assert(bit_cast_invocable<ptrdiff_t, void (*)()>);
+    static_assert(bit_cast_invocable<test_struct_1, test_struct_2>);
+    static_assert(bit_cast_invocable<test_struct_4_large_member_fn_pointer, test_struct_5_struct>);
+
+    // tests for conditions on constexprness
+    static_assert(!bit_cast_is_constexpr_union(0));
+    static_assert(!bit_cast_is_constexpr_pointer(0));
+    static_assert(!bit_cast_is_constexpr_member_fn_pointer(0));
+    static_assert(!bit_cast_is_constexpr_pmf_datamember(0));
+    static_assert(!bit_cast_is_constexpr_large_member_fn_pointer(0));
+
+    zero_initialized_round_trip<test_struct_1, test_struct_2>();
+    zero_initialized_round_trip<test_struct_3_member_fn_pointer, test_struct_3_member_fn_pointer>();
+    zero_initialized_round_trip<test_struct_3_member_fn_pointer, test_struct_7_member_fn_pointer>();
+    zero_initialized_round_trip<test_struct_4_large_member_fn_pointer, test_struct_5_struct>();
+    zero_initialized_round_trip<test_struct_6, test_struct_3_member_fn_pointer>();
+    zero_initialized_round_trip<float, int>();
+    zero_initialized_round_trip<double, long long>();
+    zero_initialized_round_trip<unsigned int, float>();
+
+    assert(test_float());
+    static_assert(test_float());
+}
+#endif // __EDG__

tests/std/tests/VSO_0157762_feature_test_macros/test.cpp

@@ -1513,6 +1513,20 @@ STATIC_ASSERT(__cpp_lib_bind_front == 201907L);
 #endif
 #endif
 
+#if CXX20_MODE && !defined(__EDG__) // TRANSITION, VSO-1041044
+#ifndef __cpp_lib_bit_cast
+#error BOOM
+#elif __cpp_lib_bit_cast != 201806L
+#error BOOM
+#else
+STATIC_ASSERT(__cpp_lib_bit_cast == 201806L);
+#endif
+#else
+#ifdef __cpp_lib_bit_cast
+#error BOOM
+#endif
+#endif
+
 #if CXX20_MODE && (defined(__clang__) || defined(__EDG__)) // TRANSITION, VSO-1020212
 #ifndef __cpp_lib_bitops
 #error BOOM