Is it better to avoid using the mod operator when possible?

My general advice is as follows. Use whichever version you think is easier on the eye, and then profile your entire system. Only optimize those parts of the code that the profiler flags up as bottlenecks. I'll bet my bottom dollar that the modulo operator isn't going to be among them.

As far as the specific example goes, only benchmarking can tell which is faster on your specific architecture using your specific compiler. You are potentially replacing modulo with branching, and it's anything but obvious which would be faster.

Some simple measurement:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    int test = atoi(argv[1]);
    int divisor = atoi(argv[2]);
    int iterations = atoi(argv[3]);

    int a = 0;

    if (test == 0) {
        for (int i = 0; i < iterations; i++)
            a = (a + 1) % divisor;
    } else if (test == 1) {
        for (int i = 0; i < iterations; i++)
            a = a + 1 == divisor ? 0 : a + 1;
    }

    printf("%d\n", a);
}

Compiling with either gcc or clang with -O3, and running time ./a.out 0 42 1000000000 (modulo version) or time ./a.out 1 42 1000000000 (comparison version) results in

• 6.25 seconds user runtime for the modulo version,
• 1.03 seconds for the comparison version.

(using gcc 5.2.1 or clang 3.6.2; Intel Core i5-4690K @ 3.50GHz; 64-bit Linux)

This means that it is probably a good idea to use the comparison version.