libcxx/include/__string/constexpr_c_functions.h

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #ifndef _LIBCPP___STRING_CONSTEXPR_C_FUNCTIONS_H
  10 #define _LIBCPP___STRING_CONSTEXPR_C_FUNCTIONS_H
  11
  12 #include <__config>
  13 #include <__cstddef/size_t.h>
  14 #include <__memory/addressof.h>
  15 #include <__memory/construct_at.h>
  16 #include <__type_traits/datasizeof.h>
  17 #include <__type_traits/enable_if.h>
  18 #include <__type_traits/is_always_bitcastable.h>
  19 #include <__type_traits/is_assignable.h>
  20 #include <__type_traits/is_constant_evaluated.h>
  21 #include <__type_traits/is_constructible.h>
  22 #include <__type_traits/is_equality_comparable.h>
  23 #include <__type_traits/is_integral.h>
  24 #include <__type_traits/is_same.h>
  25 #include <__type_traits/is_trivially_copyable.h>
  26 #include <__type_traits/is_trivially_lexicographically_comparable.h>
  27 #include <__type_traits/remove_cv.h>
  28 #include <__utility/is_pointer_in_range.h>
  29
  30 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  31 #  pragma GCC system_header
  32 #endif
  33
  34 _LIBCPP_BEGIN_NAMESPACE_STD
  35
  36 // Type used to encode that a function takes an integer that represents a number
  37 // of elements as opposed to a number of bytes.
  38 enum class __element_count : size_t {};
  39
  40 template <class _Tp>
  41 inline const bool __is_char_type = false;
  42
  43 template <>
  44 inline const bool __is_char_type<char> = true;
  45
  46 #if _LIBCPP_HAS_CHAR8_T
  47 template <>
  48 inline const bool __is_char_type<char8_t> = true;
  49 #endif
  50
  51 template <class _Tp>
  52 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 size_t __constexpr_strlen(const _Tp* __str) _NOEXCEPT {
  53   static_assert(__is_char_type<_Tp>, "__constexpr_strlen only works with char and char8_t");
  54   // GCC currently doesn't support __builtin_strlen for heap-allocated memory during constant evaluation.
  55   // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70816
  56   if (__libcpp_is_constant_evaluated()) {
  57 #if _LIBCPP_STD_VER >= 17 && defined(_LIBCPP_COMPILER_CLANG_BASED)
  58     if constexpr (is_same_v<_Tp, char>)
  59       return __builtin_strlen(__str);
  60 #endif
  61     size_t __i = 0;
  62     for (; __str[__i] != '\0'; ++__i)
  63       ;
  64     return __i;
  65   }
  66   return __builtin_strlen(reinterpret_cast<const char*>(__str));
  67 }
  68
  69 // Because of __is_trivially_lexicographically_comparable_v we know that comparing the object representations is
  70 // equivalent to a std::memcmp. Since we have multiple objects contiguously in memory, we can call memcmp once instead
  71 // of invoking it on every object individually.
  72 template <class _Tp, class _Up>
  73 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int
  74 __constexpr_memcmp(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
  75   static_assert(__is_trivially_lexicographically_comparable_v<_Tp, _Up>,
  76                 "_Tp and _Up have to be trivially lexicographically comparable");
  77
  78   auto __count = static_cast<size_t>(__n);
  79
  80   if (__libcpp_is_constant_evaluated()) {
  81 #ifdef _LIBCPP_COMPILER_CLANG_BASED
  82     if (sizeof(_Tp) == 1 && !is_same<_Tp, bool>::value)
  83       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
  84 #endif
  85
  86     while (__count != 0) {
  87       if (*__lhs < *__rhs)
  88         return -1;
  89       if (*__rhs < *__lhs)
  90         return 1;
  91
  92       --__count;
  93       ++__lhs;
  94       ++__rhs;
  95     }
  96     return 0;
  97   } else {
  98     return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
  99   }
 100 }
 101
 102 // Because of __libcpp_is_trivially_equality_comparable we know that comparing the object representations is equivalent
 103 // to a std::memcmp(...) == 0. Since we have multiple objects contiguously in memory, we can call memcmp once instead
 104 // of invoking it on every object individually.
 105 template <class _Tp, class _Up>
 106 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
 107 __constexpr_memcmp_equal(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
 108   static_assert(__libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
 109                 "_Tp and _Up have to be trivially equality comparable");
 110
 111   auto __count = static_cast<size_t>(__n);
 112
 113   if (__libcpp_is_constant_evaluated()) {
 114 #ifdef _LIBCPP_COMPILER_CLANG_BASED
 115     if (sizeof(_Tp) == 1 && is_integral<_Tp>::value && !is_same<_Tp, bool>::value)
 116       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
 117 #endif
 118     while (__count != 0) {
 119       if (*__lhs != *__rhs)
 120         return false;
 121
 122       --__count;
 123       ++__lhs;
 124       ++__rhs;
 125     }
 126     return true;
 127   } else {
 128     return ::__builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
 129   }
 130 }
 131
 132 template <class _Tp, class _Up>
 133 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_memchr(_Tp* __str, _Up __value, size_t __count) {
 134   static_assert(sizeof(_Tp) == 1 && __libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
 135                 "Calling memchr on non-trivially equality comparable types is unsafe.");
 136
 137   if (__libcpp_is_constant_evaluated()) {
 138 // use __builtin_char_memchr to optimize constexpr evaluation if we can
 139 #if _LIBCPP_STD_VER >= 17 && __has_builtin(__builtin_char_memchr)
 140     if constexpr (is_same_v<remove_cv_t<_Tp>, char> && is_same_v<remove_cv_t<_Up>, char>)
 141       return __builtin_char_memchr(__str, __value, __count);
 142 #endif
 143
 144     for (; __count; --__count) {
 145       if (*__str == __value)
 146         return __str;
 147       ++__str;
 148     }
 149     return nullptr;
 150   } else {
 151     char __value_buffer = 0;
 152     __builtin_memcpy(&__value_buffer, &__value, sizeof(char));
 153     return static_cast<_Tp*>(__builtin_memchr(__str, __value_buffer, __count));
 154   }
 155 }
 156
 157 // This function performs an assignment to an existing, already alive TriviallyCopyable object
 158 // from another TriviallyCopyable object.
 159 //
 160 // It basically works around the fact that TriviallyCopyable objects are not required to be
 161 // syntactically copy/move constructible or copy/move assignable. Technically, only one of the
 162 // four operations is required to be syntactically valid -- but at least one definitely has to
 163 // be valid.
 164 //
 165 // This is necessary in order to implement __constexpr_memmove below in a way that mirrors as
 166 // closely as possible what the compiler's __builtin_memmove is able to do.
 167 template <class _Tp, class _Up, __enable_if_t<is_assignable<_Tp&, _Up const&>::value, int> = 0>
 168 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
 169   __dest = __src;
 170   return __dest;
 171 }
 172
 173 // clang-format off
 174 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
 175                                                is_assignable<_Tp&, _Up&&>::value, int> = 0>
 176 // clang-format on
 177 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
 178   __dest =
 179       static_cast<_Up&&>(__src); // this is safe, we're not actually moving anything since the assignment is trivial
 180   return __dest;
 181 }
 182
 183 // clang-format off
 184 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
 185                                               !is_assignable<_Tp&, _Up&&>::value &&
 186                                                is_constructible<_Tp, _Up const&>::value, int> = 0>
 187 // clang-format on
 188 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
 189   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
 190   // that was there previously
 191   std::__construct_at(std::addressof(__dest), __src);
 192   return __dest;
 193 }
 194
 195 // clang-format off
 196 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
 197                                               !is_assignable<_Tp&, _Up&&>::value &&
 198                                               !is_constructible<_Tp, _Up const&>::value &&
 199                                                is_constructible<_Tp, _Up&&>::value, int> = 0>
 200 // clang-format on
 201 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
 202   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
 203   // that was there previously
 204   std::__construct_at(
 205       std::addressof(__dest),
 206       static_cast<_Up&&>(__src)); // this is safe, we're not actually moving anything since the constructor is trivial
 207   return __dest;
 208 }
 209
 210 template <class _Tp, class _Up, __enable_if_t<__is_always_bitcastable<_Up, _Tp>::value, int> = 0>
 211 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp*
 212 __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n) {
 213   size_t __count = static_cast<size_t>(__n);
 214   if (__libcpp_is_constant_evaluated()) {
 215 #ifdef _LIBCPP_COMPILER_CLANG_BASED
 216     if (is_same<__remove_cv_t<_Tp>, __remove_cv_t<_Up> >::value) {
 217       ::__builtin_memmove(__dest, __src, __count * sizeof(_Tp));
 218       return __dest;
 219     }
 220 #endif
 221     if (std::__is_pointer_in_range(__src, __src + __count, __dest)) {
 222       for (; __count > 0; --__count)
 223         std::__assign_trivially_copyable(__dest[__count - 1], __src[__count - 1]);
 224     } else {
 225       for (size_t __i = 0; __i != __count; ++__i)
 226         std::__assign_trivially_copyable(__dest[__i], __src[__i]);
 227     }
 228   } else if (__count > 0) {
 229     ::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __datasizeof_v<_Tp>);
 230   }
 231   return __dest;
 232 }
 233
 234 _LIBCPP_END_NAMESPACE_STD
 235
 236 #endif // _LIBCPP___STRING_CONSTEXPR_C_FUNCTIONS_H