https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89825
Bug ID: 89825
Summary: Jump table for variant visitation could be shortened
for never empty variants
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
The `__do_cookie` computation in `_Multi_array` seems suboptimal. There are
variant types that are never empty, so they never need the cookie value at all.
`_Variant_storage<true, _Types...>::_M_valid()` already use that knowledge to
always return `true`. The same logic could be used for the `__do_cookie`.
Pseudo-code:
+ template<typename _Variant>
+ struct _Never_empty;
+ template<typename... _Types>
+ struct _Never_empty<variant<_Types...>>
+ {
+ static constexpr bool _S_value = (is_trivially_copyable_v<_Types> && ...);
+ };
template<typename _Ret,
typename _Visitor,
typename... _Variants,
size_t __first, size_t... __rest>
struct _Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>
{
+ static constexpr size_t __index = sizeof...(_Variants) -
sizeof...(__rest) - 1;
+ using _Variant_current = __remove_cvref_t<typename _Nth_type<__index,
_Variants...>::type>;
static constexpr int __do_cookie =
- is_same_v<_Ret, __variant_cookie> ? 1 : 0;
+ is_same_v<_Ret, __variant_cookie> &&
_Never_empty<_Variant_current>::_S_value ? 1 : 0;
using _Tp = _Ret(*)(_Visitor, _Variants...);
template<typename... _Args>
constexpr const _Tp&
_M_access(size_t __first_index, _Args... __rest_indices) const
{ return _M_arr[__first_index +
__do_cookie]._M_access(__rest_indices...); }
_Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
};
template<size_t... __var_indices>
static constexpr void
_S_apply_all_alts(_Array_type& __vtable,
std::index_sequence<__var_indices...>)
{
- if constexpr (is_same_v<_Result_type, __variant_cookie>)
+ if constexpr (is_same_v<_Result_type, __variant_cookie>
+ && !_Never_empty<remove_cv_t<_Next>>::_S_value)
(_S_apply_single_alt<true, __var_indices>(
__vtable._M_arr[__var_indices + 1],
&(__vtable._M_arr[0])), ...);
else
(_S_apply_single_alt<false, __var_indices>(
__vtable._M_arr[__var_indices]), ...);
}
The above patch reduces jump table size on up to 2*sizeof...(_Types) for binary
visitations.
