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