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[libc++] Use __datasizeof for __libcpp_datasizeof if available (#72104)

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This avoids the UB and makes things a bit cheaper in terms of
compile-times.
This commit is contained in:
Nikolas Klauser 2023-12-24 08:45:25 +01:00 committed by GitHub
parent 0f1721c480
commit eeeb963841
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 8 additions and 3 deletions
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11
libcxx/include/__type_traits/datasizeof.h
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@ -28,13 +28,17 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <class _Tp>
struct __libcpp_datasizeof {
#if __has_cpp_attribute(__no_unique_address__)
#if __has_extension(datasizeof)
static const size_t value = __datasizeof(_Tp);
#else
// NOLINTNEXTLINE(readability-redundant-preprocessor) This is https://llvm.org/PR64825
# if __has_cpp_attribute(__no_unique_address__)
template <class = char>
struct _FirstPaddingByte {
[[__no_unique_address__]] _Tp __v_;
char __first_padding_byte_;
};
#else
# else
template <bool = __libcpp_is_final<_Tp>::value || !is_class<_Tp>::value>
struct _FirstPaddingByte : _Tp {
char __first_padding_byte_;
@ -45,7 +49,7 @@ struct __libcpp_datasizeof {
_Tp __v_;
char __first_padding_byte_;
};
#endif
# endif // __has_cpp_attribute(__no_unique_address__)
// _FirstPaddingByte<> is sometimes non-standard layout. Using `offsetof` is UB in that case, but GCC and Clang allow
// the use as an extension.
@ -53,6 +57,7 @@ struct __libcpp_datasizeof {
_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
static const size_t value = offsetof(_FirstPaddingByte<>, __first_padding_byte_);
_LIBCPP_DIAGNOSTIC_POP
#endif // __has_extension(datasizeof)
};
_LIBCPP_END_NAMESPACE_STD