Question

As I am using for-loops on large multi-dim arrays, any saving on the for-loop mechanism itself is meaningful.

Accordingly, I am looking for any tips on how to reduce this overhead.

e.g. : counting down using uint instead of int and != 0 as stop instead of >0 allows the CPU to do less work (heard it once, not sure it is always true)

Answer 1

First, don’t sweat the small stuff. Details like counting up versus counting down are usually completely irrelevant in running time. Humans are notoriously bad at spotting areas in code that need to be sped up. Use a profiler. Pay little or no attention to any part of the loop that is not repeated, unless the profiler says otherwise. Remember that what is written in an inner loop is not necessarily executed in an inner loop, as modern compilers are pretty smart about avoiding unnecessary repetition.

That being said, be very wary of unrolling loops on modern CPUs. The tighter they are, the better they will fit into cache. In a high-performance application I worked on last year, I improved performance significantly by using loops instead of straight-line code, and tightening them up as much as I could. (Yes, I profiled; the function in question took up 80% of the run time. I also benchmarked times over typical input, so I knew the changes helped.)

Moreover, there’s no harm in developing habits that favor efficient code. In C++, you should get in the habit of using pre-increment (++i) rather than post-increment (i++) to increment loop variables. It usually doesn’t matter, but can make a significant difference, it doesn’t make code less readable or writable, and won’t hurt.