v1/__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___STRING_CONSTEXPR_C_FUNCTIONS_H
0010 #define _LIBCPP___STRING_CONSTEXPR_C_FUNCTIONS_H
0011
0012 #include <__config>
0013 #include <__cstddef/size_t.h>
0014 #include <__memory/addressof.h>
0015 #include <__memory/construct_at.h>
0016 #include <__type_traits/datasizeof.h>
0017 #include <__type_traits/enable_if.h>
0018 #include <__type_traits/is_always_bitcastable.h>
0019 #include <__type_traits/is_assignable.h>
0020 #include <__type_traits/is_constant_evaluated.h>
0021 #include <__type_traits/is_constructible.h>
0022 #include <__type_traits/is_equality_comparable.h>
0023 #include <__type_traits/is_integral.h>
0024 #include <__type_traits/is_same.h>
0025 #include <__type_traits/is_trivially_copyable.h>
0026 #include <__type_traits/is_trivially_lexicographically_comparable.h>
0027 #include <__type_traits/remove_cv.h>
0028 #include <__utility/element_count.h>
0029 #include <__utility/is_pointer_in_range.h>
0030
0031 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0032 #  pragma GCC system_header
0033 #endif
0034
0035 _LIBCPP_BEGIN_NAMESPACE_STD
0036
0037 template <class _Tp>
0038 inline const bool __is_char_type = false;
0039
0040 template <>
0041 inline const bool __is_char_type<char> = true;
0042
0043 #if _LIBCPP_HAS_CHAR8_T
0044 template <>
0045 inline const bool __is_char_type<char8_t> = true;
0046 #endif
0047
0048 template <class _Tp>
0049 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 size_t __constexpr_strlen(const _Tp* __str) _NOEXCEPT {
0050   static_assert(__is_char_type<_Tp>, "__constexpr_strlen only works with char and char8_t");
0051   // GCC currently doesn't support __builtin_strlen for heap-allocated memory during constant evaluation.
0052   // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70816
0053   if (__libcpp_is_constant_evaluated()) {
0054 #if _LIBCPP_STD_VER >= 17 && defined(_LIBCPP_COMPILER_CLANG_BASED)
0055     if constexpr (is_same_v<_Tp, char>)
0056       return __builtin_strlen(__str);
0057 #endif
0058     size_t __i = 0;
0059     for (; __str[__i] != '\0'; ++__i)
0060       ;
0061     return __i;
0062   }
0063   return __builtin_strlen(reinterpret_cast<const char*>(__str));
0064 }
0065
0066 // Because of __is_trivially_lexicographically_comparable_v we know that comparing the object representations is
0067 // equivalent to a std::memcmp. Since we have multiple objects contiguously in memory, we can call memcmp once instead
0068 // of invoking it on every object individually.
0069 template <class _Tp, class _Up>
0070 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int
0071 __constexpr_memcmp(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
0072   static_assert(__is_trivially_lexicographically_comparable_v<_Tp, _Up>,
0073                 "_Tp and _Up have to be trivially lexicographically comparable");
0074
0075   auto __count = static_cast<size_t>(__n);
0076
0077   if (__libcpp_is_constant_evaluated()) {
0078 #ifdef _LIBCPP_COMPILER_CLANG_BASED
0079     if (sizeof(_Tp) == 1 && !is_same<_Tp, bool>::value)
0080       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
0081 #endif
0082
0083     while (__count != 0) {
0084       if (*__lhs < *__rhs)
0085         return -1;
0086       if (*__rhs < *__lhs)
0087         return 1;
0088
0089       --__count;
0090       ++__lhs;
0091       ++__rhs;
0092     }
0093     return 0;
0094   } else {
0095     return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp));
0096   }
0097 }
0098
0099 // Because of __libcpp_is_trivially_equality_comparable we know that comparing the object representations is equivalent
0100 // to a std::memcmp(...) == 0. Since we have multiple objects contiguously in memory, we can call memcmp once instead
0101 // of invoking it on every object individually.
0102 template <class _Tp, class _Up>
0103 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
0104 __constexpr_memcmp_equal(const _Tp* __lhs, const _Up* __rhs, __element_count __n) {
0105   static_assert(__libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
0106                 "_Tp and _Up have to be trivially equality comparable");
0107
0108   auto __count = static_cast<size_t>(__n);
0109
0110   if (__libcpp_is_constant_evaluated()) {
0111 #ifdef _LIBCPP_COMPILER_CLANG_BASED
0112     if (sizeof(_Tp) == 1 && is_integral<_Tp>::value && !is_same<_Tp, bool>::value)
0113       return __builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
0114 #endif
0115     while (__count != 0) {
0116       if (*__lhs != *__rhs)
0117         return false;
0118
0119       --__count;
0120       ++__lhs;
0121       ++__rhs;
0122     }
0123     return true;
0124   } else {
0125     return ::__builtin_memcmp(__lhs, __rhs, __count * sizeof(_Tp)) == 0;
0126   }
0127 }
0128
0129 template <class _Tp, class _Up>
0130 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_memchr(_Tp* __str, _Up __value, size_t __count) {
0131   static_assert(sizeof(_Tp) == 1 && __libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
0132                 "Calling memchr on non-trivially equality comparable types is unsafe.");
0133
0134   if (__libcpp_is_constant_evaluated()) {
0135 // use __builtin_char_memchr to optimize constexpr evaluation if we can
0136 #if _LIBCPP_STD_VER >= 17 && __has_builtin(__builtin_char_memchr)
0137     if constexpr (is_same_v<remove_cv_t<_Tp>, char> && is_same_v<remove_cv_t<_Up>, char>)
0138       return __builtin_char_memchr(__str, __value, __count);
0139 #endif
0140
0141     for (; __count; --__count) {
0142       if (*__str == __value)
0143         return __str;
0144       ++__str;
0145     }
0146     return nullptr;
0147   } else {
0148     char __value_buffer = 0;
0149     __builtin_memcpy(&__value_buffer, &__value, sizeof(char));
0150     return static_cast<_Tp*>(__builtin_memchr(__str, __value_buffer, __count));
0151   }
0152 }
0153
0154 // This function performs an assignment to an existing, already alive TriviallyCopyable object
0155 // from another TriviallyCopyable object.
0156 //
0157 // It basically works around the fact that TriviallyCopyable objects are not required to be
0158 // syntactically copy/move constructible or copy/move assignable. Technically, only one of the
0159 // four operations is required to be syntactically valid -- but at least one definitely has to
0160 // be valid.
0161 //
0162 // This is necessary in order to implement __constexpr_memmove below in a way that mirrors as
0163 // closely as possible what the compiler's __builtin_memmove is able to do.
0164 template <class _Tp, class _Up, __enable_if_t<is_assignable<_Tp&, _Up const&>::value, int> = 0>
0165 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
0166   __dest = __src;
0167   return __dest;
0168 }
0169
0170 // clang-format off
0171 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
0172                                                is_assignable<_Tp&, _Up&&>::value, int> = 0>
0173 // clang-format on
0174 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
0175   __dest =
0176       static_cast<_Up&&>(__src); // this is safe, we're not actually moving anything since the assignment is trivial
0177   return __dest;
0178 }
0179
0180 // clang-format off
0181 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
0182                                               !is_assignable<_Tp&, _Up&&>::value &&
0183                                                is_constructible<_Tp, _Up const&>::value, int> = 0>
0184 // clang-format on
0185 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up const& __src) {
0186   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
0187   // that was there previously
0188   std::__construct_at(std::addressof(__dest), __src);
0189   return __dest;
0190 }
0191
0192 // clang-format off
0193 template <class _Tp, class _Up, __enable_if_t<!is_assignable<_Tp&, _Up const&>::value &&
0194                                               !is_assignable<_Tp&, _Up&&>::value &&
0195                                               !is_constructible<_Tp, _Up const&>::value &&
0196                                                is_constructible<_Tp, _Up&&>::value, int> = 0>
0197 // clang-format on
0198 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& __assign_trivially_copyable(_Tp& __dest, _Up& __src) {
0199   // _Tp is trivially destructible, so we don't need to call its destructor to end the lifetime of the object
0200   // that was there previously
0201   std::__construct_at(
0202       std::addressof(__dest),
0203       static_cast<_Up&&>(__src)); // this is safe, we're not actually moving anything since the constructor is trivial
0204   return __dest;
0205 }
0206
0207 template <class _Tp, class _Up, __enable_if_t<__is_always_bitcastable<_Up, _Tp>::value, int> = 0>
0208 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp*
0209 __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n) {
0210   size_t __count = static_cast<size_t>(__n);
0211   if (__libcpp_is_constant_evaluated()) {
0212 #ifdef _LIBCPP_COMPILER_CLANG_BASED
0213     if (is_same<__remove_cv_t<_Tp>, __remove_cv_t<_Up> >::value) {
0214       ::__builtin_memmove(__dest, __src, __count * sizeof(_Tp));
0215       return __dest;
0216     }
0217 #endif
0218     if (std::__is_pointer_in_range(__src, __src + __count, __dest)) {
0219       for (; __count > 0; --__count)
0220         std::__assign_trivially_copyable(__dest[__count - 1], __src[__count - 1]);
0221     } else {
0222       for (size_t __i = 0; __i != __count; ++__i)
0223         std::__assign_trivially_copyable(__dest[__i], __src[__i]);
0224     }
0225   } else if (__count > 0) {
0226     ::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __datasizeof_v<_Tp>);
0227   }
0228   return __dest;
0229 }
0230
0231 _LIBCPP_END_NAMESPACE_STD
0232
0233 #endif // _LIBCPP___STRING_CONSTEXPR_C_FUNCTIONS_H