Also check out Hacker's Delight :

http://www.hackersdelight.org/

http://www.amazon.com/Hackers-Delight-Henry-S-Warren/dp/0201...

Ah... I have some good memories of adapting some of these for microcontrollers while still in school.

My favorite is Kernighan's method for counting set bits. It's as short and clean as the naive method and not immediately obvious: http://graphics.stanford.edu/~seander/bithacks.html#CountBit...

The http://graphics.stanford.edu/~seander/bithacks.html#ZeroInWo... hack is reminiscent of the strlen discussion the other day: http://news.ycombinator.com/item?id=510326

If the number of CPU cores increases significantly, with the individual cores getting much simpler, I wonder if bit twiddling will be back.

These are good for showing off to other programmers and ensuring your job security/future contracts at your present place of work.

Not if you comment properly. Don't worry, these are safe to use!