[Patches] Add variant constexpr support for visit, comparisons and get
Tim Shen
timshen@google.com
Sat Dec 3 03:15:00 GMT 2016
On Wed, Nov 30, 2016 at 8:27 AM, Jonathan Wakely wrote:
> On 26/11/16 21:38 -0800, Tim Shen wrote:
>>
>> This 4-patch series contains the following in order:
>>
>> a.diff: Remove uses-allocator ctors. They are going away, and removing
>> it reduces the maintenance burden from now on.
>
>
> Yay! less code.
Yay! Also removed the unused #include.
>
>> - template<typename _Type, bool __is_literal =
>> std::is_literal_type_v<_Type>>
>> + // _Uninitialized nullify the destructor calls.
>
>
> This wording confused me slightly. How about:
>
> "_Uninitialized makes destructors trivial"
Change this section of comment to the discussed content.
>
>> + // This is necessary, since we define _Variadic_union as a recursive
>> union,
>> + // and we don't want to inspect the union members one by one in its
>> dtor,
>> + // it's slow.
>
>
> Please change "it's slow" to "that's slow".
N/A.
>
>> + template<typename _Type, bool = std::is_literal_type_v<_Type>>
>> struct _Uninitialized;
>
>
> I'm still unsure that is_literal_type is the right trait here. If it's
> definitely right then we should probably *not* deprecate it in C++17!
Already discussed.
>
>> template<typename _Type>
>> struct _Uninitialized<_Type, false>
>> {
>> - constexpr _Uninitialized() = default;
>> -
>> template<typename... _Args>
>> constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
>> { ::new (&_M_storage) _Type(std::forward<_Args>(__args)...); }
>>
>> + const _Type& _M_get() const &
>> + {
>> + return *static_cast<const _Type*>(
>> + static_cast<const void*>(&_M_storage));
>> + }
>> +
>> + _Type& _M_get() &
>> + { return *static_cast<_Type*>(static_cast<void*>(&_M_storage)); }
>> +
>> + const _Type&& _M_get() const &&
>> + {
>> + return std::move(*static_cast<const _Type*>(
>> + static_cast<const void*>(&_M_storage)));
>> + }
>> +
>> + _Type&& _M_get() &&
>> + {
>> + return
>> std::move(*static_cast<_Type*>(static_cast<void*>(&_M_storage)));
>> + }
>> +
>> typename std::aligned_storage<sizeof(_Type), alignof(_Type)>::type
>> _M_storage;
>
>
> I think this could use __aligned_membuf, which would reduce the
> alignment requirements for some types (e.g. long long on x86-32).
Done.
>
> That would also mean you get the _M_ptr() member so don't need all the
> casts.
>
>> + ~_Variant_storage()
>> + { _M_destroy_impl(std::make_index_sequence<sizeof...(_Types)>{}); }
>
>
> You can use index_sequence_for<_Types...> here.
Done
>
>> @@ -598,9 +645,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> _S_apply_all_alts(_Array_type& __vtable,
>> index_sequence<__indices...>)
>> { (_S_apply_single_alt<__indices>(__vtable._M_arr[__indices]),
>> ...); }
>>
>> - template<size_t __index>
>> + template<size_t __index, typename T>
>
>
> This needs to be _Tp not T
Done.
>
>> + return __lhs._M_equal_to(__rhs,
>> +
>> std::make_index_sequence<sizeof...(_Types)>{});
>
>
> Another one that could use index_sequence_for<_Types...>
Done.
>
>> + return __lhs._M_less_than(__rhs,
>> +
>> std::make_index_sequence<sizeof...(_Types)>{});
>
>
> Same again.
Same again. ;)
>
>
>> * include/bits/enable_special_members.h: Make
>> _Enable_default_constructor constexpr.
>> * include/std/variant (variant::emplace, variant::swap,
>> std::swap,
>> std::hash): Sfinae on emplace and std::swap; handle
>> __poison_hash bases
>> of duplicated types.
>>
>> diff --git a/libstdc++-v3/include/bits/enable_special_members.h
>> b/libstdc++-v3/include/bits/enable_special_members.h
>> index 07c6c99..4f4477b 100644
>> --- a/libstdc++-v3/include/bits/enable_special_members.h
>> +++ b/libstdc++-v3/include/bits/enable_special_members.h
>> @@ -118,7 +118,8 @@ template<typename _Tag>
>> operator=(_Enable_default_constructor&&) noexcept = default;
>>
>> // Can be used in other ctors.
>> - explicit _Enable_default_constructor(_Enable_default_constructor_tag)
>> { }
>> + constexpr explicit
>> + _Enable_default_constructor(_Enable_default_constructor_tag) { }
>> };
>>
>> + void _M_reset()
>> + {
>> + _M_reset_impl(std::make_index_sequence<sizeof...(_Types)>{});
>> + _M_index = variant_npos;
>> + }
>> +
>> ~_Variant_storage()
>> - { _M_destroy_impl(std::make_index_sequence<sizeof...(_Types)>{}); }
>> + { _M_reset(); }
>
>
> These can also use index_sequence_for<_Types...>
Done.
>
>> @@ -1253,14 +1285,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>
>> template<typename... _Types>
>> struct hash<variant<_Types...>>
>> - : private __poison_hash<remove_const_t<_Types>>...
>> + : private __detail::__variant::_Variant_hash_base<
>> + variant<_Types...>, std::make_index_sequence<sizeof...(_Types)>>
>
>
> And again.
And again.
>
>> {
>> using result_type = size_t;
>> using argument_type = variant<_Types...>;
>>
>> size_t
>> operator()(const variant<_Types...>& __t) const
>> - noexcept((... &&
>> noexcept(hash<decay_t<_Types>>{}(std::declval<_Types>()))))
>> + noexcept((noexcept(hash<decay_t<_Types>>{}(std::declval<_Types>()))
>> + && ...))
>
>
> This could be
> __and_<is_nothrow_callable<hash<decay_t<_Types>>(_Types)>...>
> but I'm not sure it would be an improvement. The is_callable check is
> expensive, but maybe we need it anyway to correctly disable this
> function if the hash specialization should be posisoned?
Done. I just realized that is_nothrow_callable also handles crazy
member pointer cases.
Used fold expression instead of __and_ for consistency.
>
>
>> @@ -1270,17 +1239,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> }
>>
>> template<typename _Visitor, typename... _Variants>
>> - decltype(auto)
>> + constexpr decltype(auto)
>> visit(_Visitor&& __visitor, _Variants&&... __variants)
>> {
>> + if ((__variants.valueless_by_exception() || ...))
>> + __throw_bad_variant_access("Unexpected index");
>> +
>> using _Result_type =
>>
>> decltype(std::forward<_Visitor>(__visitor)(get<0>(__variants)...));
>> - static constexpr auto _S_vtable =
>> + constexpr auto _S_vtable =
>
>
> If this isn't static now it could be called simply __vtable, the _S_
> prefix is misleading. How many of these _S_vtable variables actually
> need to be static? If they're all trivial types and constexpr then it
> probably doesn't matter either way, there shouldn't be any difference.
Ah that's an oversight. Moved the static variable out of visit().
_S_vtable needs to be static, otherwise runtime O(n) assignment will
happen, where n is the size of _S_vtable.
--
Regards,
Tim Shen
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