Function calling - which is better, and why? ...

This is a discussion on Function calling - which is better, and why? ... within the C++ Programming forums, part of the General Programming Boards category; OK ... which of these two is better, and why? Example 1 Code: void CHighResolutionTimer::SecondsElapsed(double *ElapsedTime) { QueryPerformanceCounter((LARGE_INTEGER*) &m_i64_ElapsedTime); *ElapsedTime ...

  1. #1
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    Post Function calling - which is better, and why? ...

    OK ... which of these two is better, and why?

    Example 1

    Code:
    void CHighResolutionTimer::SecondsElapsed(double *ElapsedTime)
    {
    	QueryPerformanceCounter((LARGE_INTEGER*) &m_i64_ElapsedTime);
    	*ElapsedTime = double(m_i64_ElapsedTime - m_i64_StartTime) / double(m_i64_Frequency);
    }
    Example 2

    Code:
    double CHighResolutionTimer::SecondsElapsed(void)
    {
    	QueryPerformanceCounter((LARGE_INTEGER*) &m_i64_ElapsedTime);
    	return double(m_i64_ElapsedTime - m_i64_StartTime) / double(m_i64_Frequency);
    }

    I could then call then like ...


    CBlah.SecondsElapsed(&var);
    or
    var = CBlah.SecondsElapsed();


    I'm interested in the performance rather than the readability.

    Regards
    Last edited by Shag; 07-01-2002 at 07:33 PM.

  2. #2
    Registered User quagsire's Avatar
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    In example 2 a temporary variable is created to return, while in example 1 there is no temporary variable created. Example 1 should therefore have a slight performance benefit.

  3. #3
    geek SilentStrike's Avatar
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    But you don't have the syntactic benefit of being able to nest calls.

    double movement = velocity * timer.getSecondsElapsed();

    is impossible with the pass a pointer and fill it notation.
    Prove you can code in C++ or C# at TopCoder, referrer rrenaud
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  4. #4
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    I'm pretty sure that 2 should run faster. 2 has to do an extra copy, but 1 has to access memory through a pointer, which can be relatively expensive.
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  5. #5
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    I did a little test (I used sqrt() versus QueryPerformanceCounter()), and Example 2 was indeed just a tad bit faster.

  6. #6
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    Just thought about it a little more... #2 should definately be faster with optimizations since the single double could probably be handled at register level while pass through reference... well, that's memory no matter how you cut it.
    Callou collei we'll code the way
    Of prime numbers and pings!

  7. #7
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    I finally got a machine to acturately test the two returning methods:

    VC++6 ... No optomisation, no inlining

    Worst case - the two were identical
    Best case - version two about 60% faster.

    Thanks for all the input

    Cheers all

  8. #8
    Banned borko_b's Avatar
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    The Second is faster
    because:

    the varriable is returned in the Proccessor register eax

    while the first uses the stack memory with push and pop
    therefore is slower...

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