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Subject: Re: [boost] [date_time] local_date_time noticeably faster than ptime
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2009-01-30 17:53:49

oneill1979 wrote:
> We noticed recently that using a local_date_time in tight loops was several
> times faster than using a ptime object. As these are both based largerly on
> the same template code this was a surprise. It appears this is caused by the
> implementation of the operater+=/-= and was wondering if there was any
> justification for the difference or if this was just an oversight.
> .\boost\date_time\local_time\local_date_time.hpp
> //! Local_date_time += time_duration
> local_date_time_base operator+=(const time_duration_type& td)
> {
> this->time_ = time_system_type::add_time_duration(this->time_,td);
> return *this;
> }
> .\boost\date_time\time.hpp
> time_type operator+=(const time_duration_type& td)
> {
> time_ = (time_system::get_time_rep(date(), time_of_day() + td));
> return time_type(time_);
> }
> It looks like the ptime implementation is doing extra work to split the
> ptime into date and time_duration components and if it is changed to follow
> a similar pattern to that used in the local time the performance is then
> indistinguishable.
> time_type operator+=(const time_duration_type& td)
> {
> this->time_ = time_system::add_time_duration(this->time_, td);
> return time_type(time_);
> }
> I can only think that maybe this was done to work around an issue and in
> which case should the same then be applied to the local time implementation?

The library can store ptime in one of the two ways: as a single 64-bit
time duration or a pair of date and time duration (thus yielding a
96-bit tandem). The latter is available by defining

I'm guessing, the code in time_base implementation is oriented to the
extended 96-bit mode, while local_date_time does not make that
assumption. So, the time_base code may perform better if
BOOST_DATE_TIME_POSIX_TIME_STD_CONFIG is defined. I may be wrong, though.

Anyway, I think it's better create a Trac ticket for this issue.

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