c++performanceoptimizationmodulomodular-arithmetic

Built-in mod ('%') vs custom mod function: improve the performance of modulus operation

Recently I came to know that the mod('%') operator is very slow. So I made a function which will work just like a%b. But is it faster than the mod operator?

Here's my function

int mod(int a, int b)
{
    int tmp = a/b;
    return a - (b*tmp);
}
Solution

  • According to Chandler Carruth's benchmarks at CppCon 2015, the fastest modulo operator (on x86, when compiled with Clang) is:

    int fast_mod(const int input, const int ceil) {
    // apply the modulo operator only when needed
    // (i.e. when the input is greater than the ceiling)
    return input >= ceil ? input % ceil : input;
    // NB: the assumption here is that the numbers are positive
}

    I suggest that you watch the whole talk, he goes into more details on why this method is faster than just using % unconditionally.