for-loop mechanism efficiency tips

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

One important suggestion: move as much calculation to the outer loop as possible. Not all compilers can do that automatically. For eample, instead of:

for row = 0 to 999
    for col = 0 to 999
        cell[row*1000+col] = row * 7 + col

use:

for row = 0 to 999
    x = row * 1000
    y = row * 7
    for col = 0 to 999
        cell[x+col] = y + col

Answer 2

Try to make your loops contiguous in memory, this will optimize cache usage. That is, don't do this:

for (int i = 0; i < m; i++)  
    for (j = 0; j < n; j++)  
        s += arr[j][i];

• If processing images, convert two loops to one loop on the pixels with a single index.
• Don't make loops that will run zero times, as the pipeline is optimized to assume a loop will continue rather than end.

Answer 3

Have you measured the overhead? Do you know how much time is spent processing the for loops vs. how much time is spent executing your application code? What is your goal?