On Oct 16, 2:04 pm, Erik Wikström <Erik-wikst...@telia.com> wrote:
> On 2007-10-16 07:25, lali....@gmail.com wrote:
>
> > Somewhere in a tutorial i read that if statement has performance
> > overheads as code within the if statement cannot take benefit of
> > pipeling of microprocessor and also that the compiler cannot
> > agressively optimize that code.
>
> This is totally off topic:
>
> That is only partially true one a modern PC processor (as opposed to
> embedded processors which I have little knowledge about) since they all
> have pretty good branch prediction these days. This is best demonstrated
> by a simple loop:
>
> for (int i = 0; i < 10; ++i)
> {
> // do stuff
> }
> // do other stuff
>
> The processor recognises a loop when it sees one, and it will assume
> that you will perform the iterations, so the loop can be optimised very
> well. The problem comes when the last iteration is done, since the
> processor wrongly assumes that you will iterate you get a small
> performance hit when it discovers that you do not.
>
> Similarly the processor can optimise if statements and other control
> statements. Even better, they can learn, so if you have an if statement
> and you time after time go to the else clause the processor will
> remember this and will start executing the else clause when reaching the
> if statement before the comparison is complete. Again, should it happen
> that the assumption is wrong you get a performance hit.
>
> Notice though that this performance hit is less noticeable on modern
> hardware than is was on a P4, since the processors of today are not as
> deeply pipelined.
>
> >>From that day onwards i have been trying to avoid if statement withing
> > my functions as much as possible and also try to have minimum code
> > withing if block.
>
> The best way to optimise an if statement is to write the code that is
> most likely to be executed in the if clause and the least likely in the
> else clause, since that will save the processor a jump in most cases.
>
> > However, i am bit skeptic about this.
>
> Rightly you should be, you should be sceptic about any optimisation that
> is not at the algorithmic level.
>
> > I need some guidance. Performance is always the key issue for me when
> > it comes to writing programs.
>
> Select the best algorithms and data structures for the task and then use
> a good profiler.
>
> --
> Erik Wikström

Thank you very much for your response.

lali