Optimiziation questions (c++)

[NightStrike]

On 12/6/07, NightStrike <nightstrike@gmail.com> wrote:
> (1)
> I am curious about a certain aspect of how the g++ optimizer works at
> -O3.  If I have a for-loop, there are times when it is beneficial to
> have a function call in the while-condition.  For instance:
>
> vector<int> v;
> ...
> for ( int i=1; i < v.size(); ++i)
>
> Now, I if v is not changing inside of that for-loop, then this would
> appear at first glance to be more efficient:
>
> int max = v.size();
> for ( int i=1; i < max; ++i)
>
> By using 'max' instead of v.size(), I am eliminating a function call
> from every iteration of the for loop.  If the loop is iterated a large
> number of times, this could have an advantage.  However, is it all the
> same when you are optimizing?  Does g++ remove the need for the second
> method?
>
>
> Before I end this email, I am sure that at least someone is thinking,
> "Wait, you're using a vector, and using an old style for loop to
> traverse it instead of iterators!"  Well, yes, that's true.  But I'm
> doing that for two reasons -- 1) I'm interested in the question as it
> stands, so I asked it the way I did, and 2) My next question will
> already cover that, as it involves a comparison between traversing an
> array as I do above and using iterators.
>
>
> (2)
> Is there a big difference in speed between using a standard for-loop
> for vector traversal and using iterators?  I already verified that I
> can get a significant speed improvement (with very large N) by
> accessing the vector directory with v[i] instead of using the at()
> method.  But what about iterators?  If I write it with iterators
> instead, will that be optimized out at -O3 to reduce to essentially
> the same thing?
>


I received feedback on #1.  Any ideas on #2 from this email?