Re: [boost] [date_time] local_date_time noticeably faster than ptime

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 
BOOST_DATE_TIME_POSIX_TIME_STD_CONFIG.

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.