Question

The requirement is like following:

/* length must be >= 18 */

int calcActualLength(int length) {
    int remainder = (length - 18) % 8;
    if (remainder == 0)
        return length;
    return length + 8 - remainder;
}

using bit-wise operator, I could refactor the 1st line

int remainder = (length - 2) & 7;

Can it be further optimized?

Answer 1

((length+5)&~7)+2

int calcActualLength(int length) {
    int remainder = (length - 18) % 8;
    if (remainder == 0)
        return length;
    return length + 8 - remainder;
}
==>
int HELPER_calcActualLength(int length) {
    int remainder = length % 8;
    if (remainder == 0)
        return length;
    return length + 8 - remainder;
}
int calcActualLength(int length) {
    return 18 + HELPER_calcActualLength(length - 18);
}

And HELPER_calcActualLength() equals to ROUNDUP_8() in the semantics when the argument >= 0

And more simpler ROUNDUP_8() can be:

#define ROUNDUP_8(x) (((x)+7)&~7)

int calcActualLength(int length) {
    return 18 + ROUNDUP_8(length - 18);
}
==>    2 + ROUNDUP_8(length - 18 + 16);
==>    2 + ROUNDUP_8(length - 2);
==>    2 + (((length - 2)+7)&~7)
==>    ((length+5)&~7)+2