It wasn't. For a given architecture you can enable/disable extensions via +sse,-mmx,+avx,-avx512 etc. Certain enables or disables imply enabling/disabling others. How that was being computed was accidentally quadratic, and further could be memoized.
You should send that to Accidentally Quadratic (https://accidentallyquadratic.tumblr.com/). There have been no new stories there in a year, but perhaps they just need submissions.
