On Sun, Jan 7, 2024 at 3:33 PM Patrick Palka <ppa...@redhat.com> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
Ping.
>
> -- >8 --
>
> The recursively defined constraints on _Variadic_union's user-defined
> destructor (necessary for maintaining trivial destructibility of the
> variant iff all of its alternatives are) effectively require a template
> instantiation depth of 3x the number of variants, with the instantiation
> stack looking like
>
> ...
> _Variadic_union<B, C, ...>
> std::is_trivially_destructible_v<_Variadic_union<B, C, ...>>
> _Variadic_union<A, B, C, ...>::~_Variadic_union()
> _Variadic_union<A, B, C, ...>
> ...
>
> Ideally the template depth should be ~equal to the number of variants
> (plus a constant). Luckily it seems we don't need to compute trivial
> destructibility of the alternatives at all from _Variadic_union, since
> its only user _Variant_storage already has that information. To that
> end this patch removes these recursive constraints and instead passes
> this information down from _Variant_storage. After this patch, the
> template instantiation depth for 87619.cc is ~270 instead of ~780.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/variant (__detail::__variant::_Variadic_union):
> Add bool __trivially_destructible template parameter.
> (__detail::__variant::_Variadic_union::~_Variadic_union):
> Use __trivially_destructible in constraints instead.
> (_Variant_storage): Pass __trivially_destructible value to
> _Variadic_union.
> ---
> libstdc++-v3/include/std/variant | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/variant
> b/libstdc++-v3/include/std/variant
> index 20a76c8aa87..4b9002e0917 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -392,7 +392,7 @@ namespace __variant
> };
>
> // Defines members and ctors.
> - template<typename... _Types>
> + template<bool __trivially_destructible, typename... _Types>
> union _Variadic_union
> {
> _Variadic_union() = default;
> @@ -401,8 +401,8 @@ namespace __variant
> _Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
> };
>
> - template<typename _First, typename... _Rest>
> - union _Variadic_union<_First, _Rest...>
> + template<bool __trivially_destructible, typename _First, typename... _Rest>
> + union _Variadic_union<__trivially_destructible, _First, _Rest...>
> {
> constexpr _Variadic_union() : _M_rest() { }
>
> @@ -427,13 +427,12 @@ namespace __variant
> ~_Variadic_union() = default;
>
> constexpr ~_Variadic_union()
> - requires (!is_trivially_destructible_v<_First>)
> - || (!is_trivially_destructible_v<_Variadic_union<_Rest...>>)
> + requires (!__trivially_destructible)
> { }
> #endif
>
> _Uninitialized<_First> _M_first;
> - _Variadic_union<_Rest...> _M_rest;
> + _Variadic_union<__trivially_destructible, _Rest...> _M_rest;
> };
>
> // _Never_valueless_alt is true for variant alternatives that can
> @@ -514,7 +513,7 @@ namespace __variant
> return this->_M_index != __index_type(variant_npos);
> }
>
> - _Variadic_union<_Types...> _M_u;
> + _Variadic_union<false, _Types...> _M_u;
> using __index_type = __select_index<_Types...>;
> __index_type _M_index;
> };
> @@ -552,7 +551,7 @@ namespace __variant
> return this->_M_index != static_cast<__index_type>(variant_npos);
> }
>
> - _Variadic_union<_Types...> _M_u;
> + _Variadic_union<true, _Types...> _M_u;
> using __index_type = __select_index<_Types...>;
> __index_type _M_index;
> };
> --
> 2.43.0.254.ga26002b628
>
