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

[Patrick Palka <ppalka@redhat.com>]

On Sun, 7 Jan 2024, Patrick Palka wrote:

> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> 
> -- >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.

Perhaps we should also test this change with by setting -ftemplate-depth
to something between 256 and 512:

 libstdc++-v3/testsuite/20_util/variant/87619.cc | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libstdc++-v3/testsuite/20_util/variant/87619.cc b/libstdc++-v3/testsuite/20_util/variant/87619.cc
index 45418e16ca8..b7cfc20858a 100644
--- a/libstdc++-v3/testsuite/20_util/variant/87619.cc
+++ b/libstdc++-v3/testsuite/20_util/variant/87619.cc
@@ -16,6 +16,7 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-do compile { target c++17 } }
+// { dg-additional-options "-ftemplate-depth=300" }
 
 #include <variant>
 #include <utility>

> 
> 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
> 
>