Is there a faster way to multiply by 6 in C?-CodePudding

I am currently writing a program in C which needs to run as fast as possible and am currently looking at improving the speed of some of the calculations performed. I am aware that the slowest operations are multiplication and division, and I have replaced my multiplications/divisions by a power of 2 with left/right shifts.

I was wondering if I could do something similar for multiplication by 6, as it seems that this operation pops up a lot in my program. Regularly it is being multiplied by a number which is 1 or 0, and I feel there should be a good way to speed this up but can't think of any.

CodePudding user response：

Trust your compiler. Do not think too much about microoptimizations. They usually have an adverse effect. All versions compile to the same code:

int imul6(int x)
{
    int y = x   x;
    return y y y;
}


int imoul6_1(int n)
{
    return ((n << 1)   n) << 1;
}

int imul6_2(int x)
{
    return x x x x x x;
}

int imul6_3(int x)
{
    return x*6;
}

https://godbolt.org/z/vMos69PaM

CodePudding user response：

You can multiply number n by 6 to get m doing : m = ((n << 1) n) << 1