[colechristensen]

Inverse square root is for normalizing vectors particularly in computer graphics calculations, it needs to be run a whole lot very fast.

https://en.m.wikipedia.org/wiki/Fast_inverse_square_root#Mot...

[hinkley]

Famously the magic constant in the Quake engine that nobody remembers inventing.

That article does say there’s an SSE instruction rsqrtss that is better.