Quake 2 was the one with the clever approximate inverse square root code, right? I wonder (especially since there’s an instruction nowadays to draw inspiration from), can you implement it “in hardware,” so to speak?

In x86 that would be the SIMD rsqrtps [0]. So absolutely possible to do at the hardware level.

[0] https://www.felixcloutier.com/x86/rsqrtps

No that was Q3 Arena.