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re PR libstdc++/89825 (Jump table for variant visitation could be shortened for never empty variants)

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PR libstdc++/89825

Fix based on a suggestion by Antony Polukhin.
* include/std/variant (_Extra_visit_slot_needed): New.
(_Multi_array): Use it.
(_S_apply_all_alts): Likewise.

From-SVN: r269947
This commit is contained in:
Ville Voutilainen 2019-03-26 17:00:05 +02:00 • committed by Ville Voutilainen
parent cf66c831c1
commit da97b98ad3
2 changed files with 36 additions and 3 deletions
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10
libstdc++-v3/ChangeLog
View file

@ -1,3 +1,13 @@
2019-03-26 Ville Voutilainen <ville.voutilainen@gmail.com>
PR libstdc++/89825
Fix based on a suggestion by Antony Polukhin.
* include/std/variant (__never_valueless): New.
(_M_valid): Use it.
(_Extra_visit_slot_needed): New.
(_Multi_array): Use it.
(_S_apply_all_alts): Likewise.
2019-03-26 Ville Voutilainen <ville.voutilainen@gmail.com>
PR libstdc++/89824

29
libstdc++-v3/include/std/variant
View file

@ -323,6 +323,12 @@ namespace __variant
_Variadic_union<_Rest...> _M_rest;
};
template <typename... _Types>
constexpr bool __never_valueless()
{
return (is_trivially_copyable_v<_Types> && ...);
}
// Defines index and the dtor, possibly trivial.
template<bool __trivially_destructible, typename... _Types>
struct _Variant_storage;
@ -408,7 +414,7 @@ namespace __variant
constexpr bool
_M_valid() const noexcept
{
if constexpr ((is_trivially_copyable_v<_Types> && ...))
if constexpr (__never_valueless<_Types...>())
return true;
return this->_M_index != __index_type(variant_npos);
}
@ -747,6 +753,20 @@ namespace __variant
void* __get_storage(_Variant&& __v)
{ return __v._M_storage(); }
template <typename _Maybe_variant_cookie, typename _Variant>
struct _Extra_visit_slot_needed
{
template <typename> struct _Variant_never_valueless;
template <typename... _Types>
struct _Variant_never_valueless<variant<_Types...>>
: bool_constant<__never_valueless<_Types...>()> {};
static constexpr bool value =
is_same_v<_Maybe_variant_cookie, __variant_cookie>
&& !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
};
// Used for storing multi-dimensional vtable.
template<typename _Tp, size_t... _Dimensions>
struct _Multi_array
@ -764,8 +784,11 @@ namespace __variant
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 = typename _Nth_type<__index, _Variants...>::type;
static constexpr int __do_cookie =
is_same_v<_Ret, __variant_cookie> ? 1 : 0;
_Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
using _Tp = _Ret(*)(_Visitor, _Variants...);
template<typename... _Args>
constexpr const _Tp&
@ -832,7 +855,7 @@ namespace __variant
_S_apply_all_alts(_Array_type& __vtable,
std::index_sequence<__var_indices...>)
{
if constexpr (is_same_v<_Result_type, __variant_cookie>)
if constexpr (_Extra_visit_slot_needed<_Result_type, _Next>::value)
(_S_apply_single_alt<true, __var_indices>(
__vtable._M_arr[__var_indices + 1],
&(__vtable._M_arr[0])), ...);