On Fri, 28 Jun 2024 at 07:53, Maciej Cencora <m.cenc...@gmail.com> wrote:
>
> But constexpr-ness of bit_cast has additional limitations and e.g. providing
> an union as _Tp would be a hard-error. So we have two options:
> - before bitcasting check if type can be bitcast-ed at compile-time,
> - change the 'if constexpr' to regular 'if'.
>
> If we go with the second solution then we will include classes with pointers,
> and unions.
I don't think we want to add runtime comparisons, the point is to
optimize the code not do more work :-)
> Additionally we could also include types with padding by passing
> zero-initialized object (like a class-scope static constexpr or global) into
> bit_cast... but then such a variable would be ODR-used and most-likely won't
> be optimized out.
>
> I guess the best option would be to introduce in C++ language a new
> compiler-backed type trait like:
> std::zero_initialized_object_has_all_zeros_object_representation<T>.
Yes, I think a new built-in is the only approach that will work for
class types. I'll just limit the optimization to scalars (excluding
member pointers).
>
> Regards,
> Maciej
>
> pt., 28 cze 2024 o 00:25 Jonathan Wakely <jwak...@redhat.com> napisał(a):
>>
>> On Thu, 27 Jun 2024 at 14:27, Maciej Cencora <m.cenc...@gmail.com> wrote:
>> >
>> > I think going the bit_cast way would be the best because it enables the
>> > optimization for many more classes including common wrappers like
>> > optional, variant, pair, tuple and std::array.
>>
>> This isn't tested but seems to work on simple cases. But for large
>> objects the loop hits the constexpr iteration limit and compilation
>> fails, so it needs a sizeof(_Tp) < 64 or something.
>>
>> using _ValueType
>> = typename iterator_traits<_ForwardIterator>::value_type;
>> using _Tp = remove_all_extents_t<_ValueType>;
>> // Need value-init to be equivalent to zero-init.
>> if constexpr (is_member_pointer<_Tp>::value)
>> return nullptr;
>> else if constexpr (!is_scalar<_Tp>::value)
>> {
>> using __trivial
>> = __and_<is_trivial<_ValueType>,
>> is_trivially_constructible<_ValueType>>;
>> if constexpr (__trivial::value)
>> {
>> struct _Bytes
>> {
>> unsigned char __b[sizeof(_Tp)];
>>
>> #if __cpp_constexpr >= 201304
>> constexpr bool _M_nonzero() const
>> {
>> for (auto __c : __b)
>> if (__c)
>> return true;
>> return false;
>> }
>> #else
>> constexpr bool _M_nonzero(size_t __n = 0) const
>> {
>> return __n < sizeof(_Tp)
>> && (__b[__n] || _M_nonzero(__n + 1));
>> }
>> #endif
>> };
>> if constexpr (__builtin_bit_cast(_Bytes, _Tp())._M_nonzero())
>> return nullptr;
>> }
>> }
>> using _Ptr = decltype(std::__to_address(__first));
>> // Cannot use memset if _Ptr is cv-qualified.
>> if constexpr (is_convertible<_Ptr, void*>::value)
>> return std::__to_address(__first);
>>
