Cute Tricks in C

This is a collection of short cute tricks that can be done in C (and probably in most other languages) to make the code smaller, cleaner, or faster.

Powers of Two

int PowerOfTwo (unsigned i)
{
    /*
     * Returns TRUE if "i" is a power of 2 (has only one bit on).
     * Doesn't work for i == 0 (returns TRUE).
     */

    return ((i - 1) & i) == 0;
}

Unrolling Small Loops

    switch (length) {
	case 16:	*to++ = *from++;
	case 15:	*to++ = *from++;
	case 14:	*to++ = *from++;
	case 13:	*to++ = *from++;
	case 12:	*to++ = *from++;
	case 11:	*to++ = *from++;
	case 10:	*to++ = *from++;
	case 9:		*to++ = *from++;
	case 8:		*to++ = *from++;
	case 7:		*to++ = *from++;
	case 6:		*to++ = *from++;
	case 5:		*to++ = *from++;
	case 4:		*to++ = *from++;
	case 3:		*to++ = *from++;
	case 2:		*to++ = *from++;
	case 1:		*to++ = *from++;
	default:;
    }

If there's a chance that length will be greater than 16, then put the default case at the top and loop until length <= 16. Add a no-op "case 0" at the end.

Unrolling Bigger Loops

This is called Duff's Device, after Tom Duff. Note that it doesn't work when length == 0.

    loopcount = (length + 15) / 16;
    switch (length % 16) {
	case 0: do {	*to++ = *from++;
	case 15:	*to++ = *from++;
	case 14:	*to++ = *from++;
	case 13:	*to++ = *from++;
	case 12:	*to++ = *from++;
	case 11:	*to++ = *from++;
	case 10:	*to++ = *from++;
	case 9:		*to++ = *from++;
	case 8:		*to++ = *from++;
	case 7:		*to++ = *from++;
	case 6:		*to++ = *from++;
	case 5:		*to++ = *from++;
	case 4:		*to++ = *from++;
	case 3:		*to++ = *from++;
	case 2:		*to++ = *from++;
	case 1:		*to++ = *from++;
		} while (--loopcount);
    }

Counting Bits

This routine counts the number of set bits in time proportional to the number of set bits:

    count = 0;
    while (testnum) {
	testnum &= testnum - 1;
	count++;
    }