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Speed up compilation of common uses of std::visit() by ~8x #164196

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Speed up compilation of common uses of std::visit() by ~8x #164196

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43 changes: 39 additions & 4 deletions libcxx/include/variant
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Original file line number Diff line number Diff line change
Expand Up @@ -1325,6 +1325,10 @@ private:
friend struct __variant_detail::__visitation::__variant;
};

template <class... _Types>
variant<_Types...> __upcast_to_variant(const volatile variant<_Types...>*);
void __upcast_to_variant(...);

template <size_t _Ip, class... _Types>
_LIBCPP_HIDE_FROM_ABI constexpr bool __holds_alternative(const variant<_Types...>& __v) noexcept {
return __v.index() == _Ip;
Expand Down Expand Up @@ -1578,11 +1582,42 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __throw_if_valueless(_Vs&&... __vs) {
}
}

template < class _Visitor, class... _Vs, typename>
template <class _Visitor, class... _Vs, typename>
_LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(_Visitor&& __visitor, _Vs&&... __vs) {
using __variant_detail::__visitation::__variant;
std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
return __variant::__visit_value(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
# define _XDispatchIndex(_I) \
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@frederick-vs-ja frederick-vs-ja Oct 20, 2025 •
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Looks like that we can use the same technique for the visit<R> overload added in C++20.

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@higher-performance higher-performance Oct 20, 2025

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Yeah, I deliberately avoided doing more work to make it easier to review and get feedback first. It adds edge cases to testing so I'm not sure how folks feel about it.

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@higher-performance higher-performance Oct 23, 2025

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@philnik777 is this approach something you'd be okay with?

case _I: \
if constexpr (__variant_size::value > _I) { \
return std::forward<_Visitor>(__visitor)(__variant::__get_alt<_I>(std::forward<_Vs>(__vs)...).__value); \
} \
[[__fallthrough__]]
# define _XDispatchCount 8 // Speed up compilation for the common cases
using __variant_size = variant_size<decltype(std::__upcast_to_variant(std::declval<remove_reference_t<_Vs>*>()...))>;
if constexpr (sizeof...(_Vs) == 1 &&
conditional_t<is_same_v<variant_size<void>, __variant_size>,
std::integral_constant<size_t, variant_npos>,
__variant_size>::value < _XDispatchCount) {
using __variant_detail::__access::__variant;
const size_t __indexes[] = {
__vs.decltype(std::__upcast_to_variant(std::declval<remove_reference_t<_Vs>*>()))::index()...};
switch (__indexes[0]) {
_XDispatchIndex(_XDispatchCount - 8);
_XDispatchIndex(_XDispatchCount - 7);
_XDispatchIndex(_XDispatchCount - 6);
_XDispatchIndex(_XDispatchCount - 5);
_XDispatchIndex(_XDispatchCount - 4);
_XDispatchIndex(_XDispatchCount - 3);
_XDispatchIndex(_XDispatchCount - 2);
_XDispatchIndex(_XDispatchCount - 1);
default:
__throw_bad_variant_access();
}
} else {
using __variant_detail::__visitation::__variant;
std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
return __variant::__visit_value(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
}
# undef _XDispatchCount
# undef _XDispatchIndex
}

# if _LIBCPP_STD_VER >= 20
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