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[libc++] Improve code gen for string's operator== #100926

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Aug 1, 2024
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[libc++] Improve code gen for string's operator== #100926

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16 changes: 11 additions & 5 deletions libcxx/include/string
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Original file line number Diff line number Diff line change
Expand Up @@ -2246,6 +2246,10 @@ private:
friend constexpr basic_string operator+ <>(const basic_string&, type_identity_t<__self_view>);
friend constexpr basic_string operator+ <>(type_identity_t<__self_view>, const basic_string&);
#endif

template <class _CharT2, class _Traits2, class _Allocator2>
friend inline _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI bool
operator==(const basic_string<_CharT2, _Traits2, _Allocator2>&, const _CharT2*) _NOEXCEPT;
Comment on lines +2250 to +2252
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Why not define the operator here instead?

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Because we wouldn't be conforming then. This operator== isn't defined as a hidden friend in the standard.

};

// These declarations must appear before any functions are implicitly used
Expand Down Expand Up @@ -3855,16 +3859,18 @@ operator==(const _CharT* __lhs, const basic_string<_CharT, _Traits, _Allocator>&
template <class _CharT, class _Traits, class _Allocator>
inline _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI bool
operator==(const basic_string<_CharT, _Traits, _Allocator>& __lhs, const _CharT* __rhs) _NOEXCEPT {
#if _LIBCPP_STD_VER >= 20
return basic_string_view<_CharT, _Traits>(__lhs) == basic_string_view<_CharT, _Traits>(__rhs);
#else
Comment on lines -3858 to -3860
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What was the purpose of this #if?

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AFAICT it only exists to match the standards wording.

typedef basic_string<_CharT, _Traits, _Allocator> _String;
_LIBCPP_ASSERT_NON_NULL(__rhs != nullptr, "operator==(basic_string, char*): received nullptr");

using _String = basic_string<_CharT, _Traits, _Allocator>;

size_t __rhs_len = _Traits::length(__rhs);
if (__builtin_constant_p(__rhs_len) && !_String::__fits_in_sso(__rhs_len)) {
if (!__lhs.__is_long())
return false;
}
if (__rhs_len != __lhs.size())
return false;
return __lhs.compare(0, _String::npos, __rhs, __rhs_len) == 0;
#endif
}

#if _LIBCPP_STD_VER >= 20
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