libcxx/include/__cxx03/__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 <__cxx03/__config>
  13 #include <__cxx03/__memory/addressof.h>
  14 #include <__cxx03/__memory/construct_at.h>
  15 #include <__cxx03/__type_traits/datasizeof.h>
  16 #include <__cxx03/__type_traits/is_always_bitcastable.h>
  17 #include <__cxx03/__type_traits/is_assignable.h>
  18 #include <__cxx03/__type_traits/is_constant_evaluated.h>
  19 #include <__cxx03/__type_traits/is_constructible.h>
  20 #include <__cxx03/__type_traits/is_equality_comparable.h>
  21 #include <__cxx03/__type_traits/is_same.h>
  22 #include <__cxx03/__type_traits/is_trivially_copyable.h>
  23 #include <__cxx03/__type_traits/is_trivially_lexicographically_comparable.h>
  24 #include <__cxx03/__type_traits/remove_cv.h>
  25 #include <__cxx03/__utility/is_pointer_in_range.h>
  26 #include <__cxx03/cstddef>
  27
  28 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  29 #  pragma GCC system_header
  30 #endif
  31
  32 _LIBCPP_BEGIN_NAMESPACE_STD
  33
  34 // Type used to encode that a function takes an integer that represents a number
  35 // of elements as opposed to a number of bytes.
  36 enum class __element_count : size_t {};
  37
  38 template <class _Tp>
  39 inline const bool __is_char_type = false;
  40
  41 template <>
  42 inline const bool __is_char_type<char> = true;
  43
  44 #ifndef _LIBCPP_HAS_NO_CHAR8_T
  45 template <>
  46 inline const bool __is_char_type<char8_t> = true;
  47 #endif
  48
  49 template <class _Tp>
  50 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 size_t __constexpr_strlen(const _Tp* __str) _NOEXCEPT {
  51   static_assert(__is_char_type<_Tp>, "__constexpr_strlen only works with char and char8_t");
  52   // GCC currently doesn't support __builtin_strlen for heap-allocated memory during constant evaluation.
  53   // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70816
  54   if (__libcpp_is_constant_evaluated()) {
  55 #if _LIBCPP_STD_VER >= 17 && defined(_LIBCPP_COMPILER_CLANG_BASED)
  56     if constexpr (is_same_v<_Tp, char>)
  57       return __builtin_strlen(__str);
  58 #endif
  59     size_t __i = 0;
  60     for (; __str[__i] != '\0'; ++__i)
  61       ;
  62     return __i;
  63   }
  64   return __builtin_strlen(reinterpret_cast<const char*>(__str));
  65 }
  66
  67 // Because of __libcpp_is_trivially_lexicographically_comparable we know that comparing the object representations is
  68 // equivalent to a std::memcmp. Since we have multiple objects contiguously in memory, we can call memcmp once instead
  69 // of invoking it on every object individually.
  70 template <class _Tp, class _Up>
  71 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int
  72 __constexpr_memcmp(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
  73   static_assert(__libcpp_is_trivially_lexicographically_comparable<_Tp, _Up>::value,
  74                 "_Tp and _Up have to be trivially lexicographically comparable");
  75
  76   auto __count = static_cast<size_t>(__n);
  77
  78   if (__libcpp_is_constant_evaluated()) {
  79 #ifdef _LIBCPP_COMPILER_CLANG_BASED
  80     if (sizeof(_Tp) == 1 && !is_same<_Tp, bool>::value)
  81       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
  82 #endif
  83
  84     while (__count != 0) {
  85       if (*__lhs < *__rhs)
  86         return -1;
  87       if (*__rhs < *__lhs)
  88         return 1;
  89
  90       --__count;
  91       ++__lhs;
  92       ++__rhs;
  93     }
  94     return 0;
  95   } else {
  96     return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
  97   }
  98 }
  99
 100 // Because of __libcpp_is_trivially_equality_comparable we know that comparing the object representations is equivalent
 101 // to a std::memcmp(...) == 0. Since we have multiple objects contiguously in memory, we can call memcmp once instead
 102 // of invoking it on every object individually.
 103 template <class _Tp, class _Up>
 104 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
 105 __constexpr_memcmp_equal(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
 106   static_assert(__libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
 107                 "_Tp and _Up have to be trivially equality comparable");
 108
 109   auto __count = static_cast<size_t>(__n);
 110
 111   if (__libcpp_is_constant_evaluated()) {
 112 #ifdef _LIBCPP_COMPILER_CLANG_BASED
 113     if (sizeof(_Tp) == 1 && is_integral<_Tp>::value && !is_same<_Tp, bool>::value)
 114       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
 115 #endif
 116     while (__count != 0) {
 117       if (*__lhs != *__rhs)
 118         return false;
 119
 120       --__count;
 121       ++__lhs;
 122       ++__rhs;
 123     }
 124     return true;
 125   } else {
 126     return ::__builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
 127   }
 128 }
 129
 130 template <class _Tp, class _Up>
 131 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_memchr(_Tp* __str, _Up __value, size_t __count) {
 132   static_assert(sizeof(_Tp) == 1 && __libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
 133                 "Calling memchr on non-trivially equality comparable types is unsafe.");
 134
 135   if (__libcpp_is_constant_evaluated()) {
 136 // use __builtin_char_memchr to optimize constexpr evaluation if we can
 137 #if _LIBCPP_STD_VER >= 17 && __has_builtin(__builtin_char_memchr)
 138     if constexpr (is_same_v<remove_cv_t<_Tp>, char> && is_same_v<remove_cv_t<_Up>, char>)
 139       return __builtin_char_memchr(__str, __value, __count);
 140 #endif
 141
 142     for (; __count; --__count) {
 143       if (*__str == __value)
 144         return __str;
 145       ++__str;
 146     }
 147     return nullptr;
 148   } else {
 149     char __value_buffer = 0;
 150     __builtin_memcpy(&__value_buffer, &__value, sizeof(char));
 151     return static_cast<_Tp*>(__builtin_memchr(__str, __value_buffer, __count));
 152   }
 153 }
 154
 155 // This function performs an assignment to an existing, already alive TriviallyCopyable object
 156 // from another TriviallyCopyable object.
 157 //
 158 // It basically works around the fact that TriviallyCopyable objects are not required to be
 159 // syntactically copy/move constructible or copy/move assignable. Technically, only one of the
 160 // four operations is required to be syntactically valid -- but at least one definitely has to
 161 // be valid.
 162 //
 163 // This is necessary in order to implement __constexpr_memmove below in a way that mirrors as
 164 // closely as possible what the compiler's __builtin_memmove is able to do.
 165 template <class _Tp, class _Up, __enable_if_t<is_assignable<_Tp&, _Up const&>::value, int> = 0>
 166 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
 167   __dest = __src;
 168   return __dest;
 169 }
 170
 171 // clang-format off
 172 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
 173                                                is_assignable<_Tp&, _Up&&>::value, int> = 0>
 174 // clang-format on
 175 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
 176   __dest =
 177       static_cast<_Up&&>(__src); // this is safe, we're not actually moving anything since the assignment is trivial
 178   return __dest;
 179 }
 180
 181 // clang-format off
 182 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
 183                                               !is_assignable<_Tp&, _Up&&>::value &&
 184                                                is_constructible<_Tp, _Up const&>::value, int> = 0>
 185 // clang-format on
 186 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
 187   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
 188   // that was there previously
 189   std::__construct_at(std::addressof(__dest), __src);
 190   return __dest;
 191 }
 192
 193 // clang-format off
 194 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
 195                                               !is_assignable<_Tp&, _Up&&>::value &&
 196                                               !is_constructible<_Tp, _Up const&>::value &&
 197                                                is_constructible<_Tp, _Up&&>::value, int> = 0>
 198 // clang-format on
 199 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
 200   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
 201   // that was there previously
 202   std::__construct_at(
 203       std::addressof(__dest),
 204       static_cast<_Up&&>(__src)); // this is safe, we're not actually moving anything since the constructor is trivial
 205   return __dest;
 206 }
 207
 208 template <class _Tp, class _Up, __enable_if_t<__is_always_bitcastable<_Up, _Tp>::value, int> = 0>
 209 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp*
 210 __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n) {
 211   size_t __count = static_cast<size_t>(__n);
 212   if (__libcpp_is_constant_evaluated()) {
 213 #ifdef _LIBCPP_COMPILER_CLANG_BASED
 214     if (is_same<__remove_cv_t<_Tp>, __remove_cv_t<_Up> >::value) {
 215       ::__builtin_memmove(__dest, __src, __count * sizeof(_Tp));
 216       return __dest;
 217     }
 218 #endif
 219     if (std::__is_pointer_in_range(__src, __src + __count, __dest)) {
 220       for (; __count > 0; --__count)
 221         std::__assign_trivially_copyable(__dest[__count - 1], __src[__count - 1]);
 222     } else {
 223       for (size_t __i = 0; __i != __count; ++__i)
 224         std::__assign_trivially_copyable(__dest[__i], __src[__i]);
 225     }
 226   } else if (__count > 0) {
 227     ::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __datasizeof_v<_Tp>);
 228   }
 229   return __dest;
 230 }
 231
 232 _LIBCPP_END_NAMESPACE_STD
 233
 234 #endif // _LIBCPP___STRING_CONSTEXPR_C_FUNCTIONS_H