[Development] QDateTime is missing shared null optimization

[Robert Knight]

Follow-up question - if a data type is a union of a pointer and some
other representation, can it be stored in a QVariant without
transitioning to the allocated version?

On 3 August 2015 at 03:39, Thiago Macieira <thiago.macieira at intel.com> wrote:
> On Monday 03 August 2015 03:23:44 Mark Langezaal wrote:
>> On 2015-08-01 23:51, Thiago Macieira wrote:
>> > QDateTime API accepts the offset from UTC in seconds, but all existing
>> > timezones are multiples of a quarter hour. With a range from -12 to
>> > +13.5,  we
>> > need ceil(log((13.5 + 12) * 4) / log(2)) = 7 bits for the timezone.
>> > With the
>> > one bit for the tag, we have 48 bits left for the msecs portion on
>> > 64-bit
>> > platforms.
>> >
>> > A 48-bit millisecond field spans 3257812 days. JD 3257812 is
>> > 4207-06-27.
>>
>> Actually you used 1 + 7 + 48 = 56 bits, which leave another 8 bits for
>
> Ooops... Einstein also failed at basic math, you know? :-)
>
>> useful data such as Spec. Or am I missing something obvious here?
>> What is the recommended way to share a d-ptr with other data? By using a
>> union on the d-ptr and a struct containing bit fields?
>
> We don't need more than 7 bits for the timespec. With 56 bits, we get a span
> of 833999930 days. If we used it signed, we get a range up until the year
> 1136995, which is plenty.
>
> To support up until year 9999, we need 49 bits. If we make it signed, that's
> 50 bits. We have 6 bits free then.
>
> In order to do this, yes, you'd put the structure with the bit fields in the
> same union as the d pointer. Please be careful about endianness. The tag bit
> must be the first bit in little-endian and the last bit in big-endian.
>
> And like I'd said below: you cannot call any of the QDateTimePrivate member
> functions when d isn't a real pointer.
>
>> > Anyone want to try this?
>> >
>> > Please note you cannot manipulate the this pointer in a member
>> > function, so
>> > all the QDateTimePrivate member functions need to be made static first.
>
> --
> Thiago Macieira - thiago.macieira (AT) intel.com
>   Software Architect - Intel Open Source Technology Center
>
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