This isn't a universally effective optimization. It's an exquisite optimization for when a little imprecision is tolerable. Micro-optimizations like this are

This isn't a universally effective optimization. It's an exquisite optimization for when a little imprecision is tolerable. Micro-optimizations like this are rarely worth the increase in cognitive complexity of the code, but Quake had to do a lot of math in very little time on 1996 hardware, so I'll trust Carmack (whose programming skills are legendary if you aren't familiar with him).