Alternative to using % operator and / Operator in C++

Question

It is told that modulo operator "%" and divide operator "/" are very inefficient in embedded C++.

How can I alternatively achieve the following expression:

a = b % c;

I understand that this can be achieved using the following logic:

a = b - c;
while (a >= c) {
  a = a - c;
}

But my question is, is this code involving while loops efficient enough, compared to % operator?

Thanks, Kirti

Answer 1

Division and modulus are indeed costly hardware operations, whatever you do (this is more related to hardware architecture than to languages or compilers), perhaps ten times slower than addition.

^{However, on current laptops or servers, and on high-end microcontrollers, cache misses are often much slower than divisions!}

The GCC compiler is often able to optimize them, when the divisor is a constant.

Your naive loop is usually much more slower than using the hardware division instruction (or the library routine doing it, if not provided by hardware). I believe you are wrong in avoiding the division & replacing it with your loop.

You might tune your algorithms -e.g. by having power of twos- but I don't recommend using your code. Remember that premature optimization is evil so first try to get your program correct, then profile it to find the trouble spots.