Is it faster to get year and month directly from COleDateTime or converting to SYSTEMTIME first?

Question

I want to get the year and month from a COleDateTime object and I want it to be as fast as possible. I have 2 options;

COleDateTime my_date_time;
int year = my_date_time.GetYear();
int month = my_date_time.GetMonth();

or

COleDateTime my_date_time;
SYSTEMTIME mydt_as_systemtime;
my_date_time.GetAsSystemTime(mydt_as_systemtime);
int year = mydt_as_systemtime.wYear;
int month = mydt_as_systemtime.wMonth;

The question is, which would be faster?

COleDateTime stores it's internal date representation as a DATE typedef, and so when you call GetYear() and GetMonth() it has to calculate these each time. In the SYSTEMTIME case, the values of wYear and wMonth are stored as DWORDs so it is just a case of retrieving the values, but there is an overhead in converting a COleDateTime to a SYSTEMTIME.

Thanks,

Sterren

Are you doing this in a context where the speed really matters? If so, benchmark both and come back to tell us the result. — Mark Ransom, May 31 '12 at 17:30
@MarkRansom Yes, speed matters in the code context. If I don't get any definitive explanatory answer here then I will end up benchmarking it. I just hoped that an experienced user may know that the conversion from `COleDateTime` to `SYSTEMTIME` algorith is more efficient than the `DATE` to year and then `DATE` to month or otherwise. — Steztric, May 31 '12 at 18:56
Honestly, you can bench this faster than anyone here can reply. — Mahmoud Al-Qudsi, May 31 '12 at 18:58
One thing that might help, Microsoft provides sources for its classes including COleDateTime. You can look at the source to get an idea of how much work each function does. It's still a crap shoot though to relate that to actual run time. — Mark Ransom, May 31 '12 at 19:02
@MahmoudAl-Qudsi I pitty your negative attitude. I am writing this question from home, where I only have Visual Studio 2010 Express installed. Express edition does not come with ATL support and therefore I cannot benchmark it at this time. — Steztric, May 31 '12 at 19:12
@MarkRansom Thank's Mark. That helps me a lot. I will definitely check it out. — Steztric, May 31 '12 at 19:13
@MahmoudAl-Qudsi I'm sure there are, but without ATL support I do not have `COleDateTime` because this is an ATL class defined in `ATLComTime.h` — Steztric, Jun 01 '12 at 07:59

Steztric · Accepted Answer · 2012-06-04T22:41:11.113

Thanks to the point in the right direction @MarkRansom, I tracked down the source code for COleDateTime. Here are the functions;

ATLCOMTIME_INLINE int COleDateTime::GetYear() const throw()
{
    SYSTEMTIME st;
    return GetAsSystemTime(st) ? st.wYear : error;
}

ATLCOMTIME_INLINE int COleDateTime::GetMonth() const throw()
{
    SYSTEMTIME st;
    return GetAsSystemTime(st) ? st.wMonth : error;
}

So COleDateTime::GetYear() and ::GetMonth() do the conversion to SYSTEMTIME anyway!

Since these are inline functions, these will be put in place at the call site. Since GetAsSystemTime(st) is common between these functions, compiler optimisation should factor out this into a temporary variable and therefore the two code snippets in my qestion are equivalent. Since option 1 is simpler, there is no reason not to go with that.

Update:

Once I got the opportunity, I benchmarked the code. It looks like the compiler optimisation I was talking about does not apply to the aforementioned code. The timings for 1 million operations of either method are as follows;

Direct calls: 154ms
SYSTEMTIME method: 75ms

Well that settles it. Conversion to SYSTEMTIME it is.

Funny thing, optimization. Things you think are simple don't get optimized, and things that are incredibly esoteric do manage it. Glad you got it all figured out! — Mark Ransom, Jun 04 '12 at 22:46
Indeed @MarkRansom. The moral of the story is don't assume anything and rely on the acid test - actual timings - when it comes to optimisation. — Steztric, Jun 12 '12 at 13:10

Is it faster to get year and month directly from COleDateTime or converting to SYSTEMTIME first?

1 Answers1