[PATCH] libstdc++: reduce std::variant template instantiation depth

[Jonathan Wakely]

On Mon, 15 Jan 2024 at 19:32, Patrick Palka <ppalka@redhat.com> wrote:
>
> On Sun, Jan 7, 2024 at 3:33 PM Patrick Palka <ppalka@redhat.com> wrote:
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> Ping.

Huh, I thought I'd already approved this ... sorry.

OK for trunk, with the -ftemplate-depth test change too.


>
> >
> > -- >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
> >
>