Why is this `unsafe-set-car!` so much slower than `unsafe-set-immutable-car!`?

Questions & Answers
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2

IIUC:

I''m using cs: as a prefix for Chez Scheme functions, when they are different from the racket ones.

unsafe-set-immutable-car! is translated as cs:unsafe-set-car!. I think it has a special case in schemify, like most build-in racket functions.

unsafe-set-car! is translated as a function defined by a cs:lambda in another module, that is not inlined.

Your code is expanded to a linklet that (after a lot of simplifications and lies) is:

(lambda (globals my-unsafe-set-car!)
  (let ([p (cons 1 2)])
    (cs:unsafe-set-car! p 7))
  (let ([p (cons 1 2)])
    ((if (cs:procedure? my-unsafe-set-car!)
        my-unsafe-set-car!
        (slow-extract-function my-unsafe-set-car!))
     p 7))
)

I'm surprised it's not even slower.