__cxx03/__string/constexpr_c_functions.h

0001 //===----------------------------------------------------------------------===//
0002 //
0003 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
0004 // See https://llvm.org/LICENSE.txt for license information.
0005 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
0006 //
0007 //===----------------------------------------------------------------------===//
0008
0009 #ifndef _LIBCPP___CXX03___STRING_CONSTEXPR_C_FUNCTIONS_H
0010 #define _LIBCPP___CXX03___STRING_CONSTEXPR_C_FUNCTIONS_H
0011
0012 #include <__cxx03/__config>
0013 #include <__cxx03/__memory/addressof.h>
0014 #include <__cxx03/__memory/construct_at.h>
0015 #include <__cxx03/__type_traits/datasizeof.h>
0016 #include <__cxx03/__type_traits/is_always_bitcastable.h>
0017 #include <__cxx03/__type_traits/is_assignable.h>
0018 #include <__cxx03/__type_traits/is_constant_evaluated.h>
0019 #include <__cxx03/__type_traits/is_constructible.h>
0020 #include <__cxx03/__type_traits/is_equality_comparable.h>
0021 #include <__cxx03/__type_traits/is_same.h>
0022 #include <__cxx03/__type_traits/is_trivially_copyable.h>
0023 #include <__cxx03/__type_traits/is_trivially_lexicographically_comparable.h>
0024 #include <__cxx03/__type_traits/remove_cv.h>
0025 #include <__cxx03/__utility/is_pointer_in_range.h>
0026 #include <__cxx03/cstddef>
0027
0028 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0029 #  pragma GCC system_header
0030 #endif
0031
0032 _LIBCPP_BEGIN_NAMESPACE_STD
0033
0034 // Type used to encode that a function takes an integer that represents a number
0035 // of elements as opposed to a number of bytes.
0036 enum class __element_count : size_t {};
0037
0038 template <class _Tp>
0039 inline const bool __is_char_type = false;
0040
0041 template <>
0042 inline const bool __is_char_type<char> = true;
0043
0044 #ifndef _LIBCPP_HAS_NO_CHAR8_T
0045 template <>
0046 inline const bool __is_char_type<char8_t> = true;
0047 #endif
0048
0049 template <class _Tp>
0050 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 size_t __constexpr_strlen(const _Tp* __str) _NOEXCEPT {
0051   static_assert(__is_char_type<_Tp>, "__constexpr_strlen only works with char and char8_t");
0052   // GCC currently doesn't support __builtin_strlen for heap-allocated memory during constant evaluation.
0053   // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70816
0054   if (__libcpp_is_constant_evaluated()) {
0055 #if _LIBCPP_STD_VER >= 17 && defined(_LIBCPP_COMPILER_CLANG_BASED)
0056     if constexpr (is_same_v<_Tp, char>)
0057       return __builtin_strlen(__str);
0058 #endif
0059     size_t __i = 0;
0060     for (; __str[__i] != '\0'; ++__i)
0061       ;
0062     return __i;
0063   }
0064   return __builtin_strlen(reinterpret_cast<const char*>(__str));
0065 }
0066
0067 // Because of __libcpp_is_trivially_lexicographically_comparable we know that comparing the object representations is
0068 // equivalent to a std::memcmp. Since we have multiple objects contiguously in memory, we can call memcmp once instead
0069 // of invoking it on every object individually.
0070 template <class _Tp, class _Up>
0071 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int
0072 __constexpr_memcmp(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
0073   static_assert(__libcpp_is_trivially_lexicographically_comparable<_Tp, _Up>::value,
0074                 "_Tp and _Up have to be trivially lexicographically comparable");
0075
0076   auto __count = static_cast<size_t>(__n);
0077
0078   if (__libcpp_is_constant_evaluated()) {
0079 #ifdef _LIBCPP_COMPILER_CLANG_BASED
0080     if (sizeof(_Tp) == 1 && !is_same<_Tp, bool>::value)
0081       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
0082 #endif
0083
0084     while (__count != 0) {
0085       if (*__lhs < *__rhs)
0086         return -1;
0087       if (*__rhs < *__lhs)
0088         return 1;
0089
0090       --__count;
0091       ++__lhs;
0092       ++__rhs;
0093     }
0094     return 0;
0095   } else {
0096     return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
0097   }
0098 }
0099
0100 // Because of __libcpp_is_trivially_equality_comparable we know that comparing the object representations is equivalent
0101 // to a std::memcmp(...) == 0. Since we have multiple objects contiguously in memory, we can call memcmp once instead
0102 // of invoking it on every object individually.
0103 template <class _Tp, class _Up>
0104 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
0105 __constexpr_memcmp_equal(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
0106   static_assert(__libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
0107                 "_Tp and _Up have to be trivially equality comparable");
0108
0109   auto __count = static_cast<size_t>(__n);
0110
0111   if (__libcpp_is_constant_evaluated()) {
0112 #ifdef _LIBCPP_COMPILER_CLANG_BASED
0113     if (sizeof(_Tp) == 1 && is_integral<_Tp>::value && !is_same<_Tp, bool>::value)
0114       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
0115 #endif
0116     while (__count != 0) {
0117       if (*__lhs != *__rhs)
0118         return false;
0119
0120       --__count;
0121       ++__lhs;
0122       ++__rhs;
0123     }
0124     return true;
0125   } else {
0126     return ::__builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
0127   }
0128 }
0129
0130 template <class _Tp, class _Up>
0131 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_memchr(_Tp* __str, _Up __value, size_t __count) {
0132   static_assert(sizeof(_Tp) == 1 && __libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
0133                 "Calling memchr on non-trivially equality comparable types is unsafe.");
0134
0135   if (__libcpp_is_constant_evaluated()) {
0136 // use __builtin_char_memchr to optimize constexpr evaluation if we can
0137 #if _LIBCPP_STD_VER >= 17 && __has_builtin(__builtin_char_memchr)
0138     if constexpr (is_same_v<remove_cv_t<_Tp>, char> && is_same_v<remove_cv_t<_Up>, char>)
0139       return __builtin_char_memchr(__str, __value, __count);
0140 #endif
0141
0142     for (; __count; --__count) {
0143       if (*__str == __value)
0144         return __str;
0145       ++__str;
0146     }
0147     return nullptr;
0148   } else {
0149     char __value_buffer = 0;
0150     __builtin_memcpy(&__value_buffer, &__value, sizeof(char));
0151     return static_cast<_Tp*>(__builtin_memchr(__str, __value_buffer, __count));
0152   }
0153 }
0154
0155 // This function performs an assignment to an existing, already alive TriviallyCopyable object
0156 // from another TriviallyCopyable object.
0157 //
0158 // It basically works around the fact that TriviallyCopyable objects are not required to be
0159 // syntactically copy/move constructible or copy/move assignable. Technically, only one of the
0160 // four operations is required to be syntactically valid -- but at least one definitely has to
0161 // be valid.
0162 //
0163 // This is necessary in order to implement __constexpr_memmove below in a way that mirrors as
0164 // closely as possible what the compiler's __builtin_memmove is able to do.
0165 template <class _Tp, class _Up, __enable_if_t<is_assignable<_Tp&, _Up const&>::value, int> = 0>
0166 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
0167   __dest = __src;
0168   return __dest;
0169 }
0170
0171 // clang-format off
0172 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
0173                                                is_assignable<_Tp&, _Up&&>::value, int> = 0>
0174 // clang-format on
0175 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
0176   __dest =
0177       static_cast<_Up&&>(__src); // this is safe, we're not actually moving anything since the assignment is trivial
0178   return __dest;
0179 }
0180
0181 // clang-format off
0182 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
0183                                               !is_assignable<_Tp&, _Up&&>::value &&
0184                                                is_constructible<_Tp, _Up const&>::value, int> = 0>
0185 // clang-format on
0186 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
0187   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
0188   // that was there previously
0189   std::__construct_at(std::addressof(__dest), __src);
0190   return __dest;
0191 }
0192
0193 // clang-format off
0194 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
0195                                               !is_assignable<_Tp&, _Up&&>::value &&
0196                                               !is_constructible<_Tp, _Up const&>::value &&
0197                                                is_constructible<_Tp, _Up&&>::value, int> = 0>
0198 // clang-format on
0199 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
0200   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
0201   // that was there previously
0202   std::__construct_at(
0203       std::addressof(__dest),
0204       static_cast<_Up&&>(__src)); // this is safe, we're not actually moving anything since the constructor is trivial
0205   return __dest;
0206 }
0207
0208 template <class _Tp, class _Up, __enable_if_t<__is_always_bitcastable<_Up, _Tp>::value, int> = 0>
0209 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp*
0210 __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n) {
0211   size_t __count = static_cast<size_t>(__n);
0212   if (__libcpp_is_constant_evaluated()) {
0213 #ifdef _LIBCPP_COMPILER_CLANG_BASED
0214     if (is_same<__remove_cv_t<_Tp>, __remove_cv_t<_Up> >::value) {
0215       ::__builtin_memmove(__dest, __src, __count * sizeof(_Tp));
0216       return __dest;
0217     }
0218 #endif
0219     if (std::__is_pointer_in_range(__src, __src + __count, __dest)) {
0220       for (; __count > 0; --__count)
0221         std::__assign_trivially_copyable(__dest[__count - 1], __src[__count - 1]);
0222     } else {
0223       for (size_t __i = 0; __i != __count; ++__i)
0224         std::__assign_trivially_copyable(__dest[__i], __src[__i]);
0225     }
0226   } else if (__count > 0) {
0227     ::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __datasizeof_v<_Tp>);
0228   }
0229   return __dest;
0230 }
0231
0232 _LIBCPP_END_NAMESPACE_STD
0233
0234 #endif // _LIBCPP___CXX03___STRING_CONSTEXPR_C_FUNCTIONS_H