id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	difficulty
1620	Some let bindings are not replaced resulting in unnecessary CPS calls	sjamaan		"We've seen in #1604 that this is a performance killer:

This code is fast when compiled with `-O5 -strict-types -fixnum-arithmetic`:

{{{
(define (fib n)
  (if (or (eq? n 0) (eq? n 1))
      n
      (+ (fib (- n 1)) (fib (- n 2)))))

(let loop ((n 0))
  (when (< n 35)
    (print ""n="" n "" => "" (fib n))
    (loop (+ n 1))))
}}}

This code is slow when compiled with the same options, even though the generated intermediate Scheme code is equivalent:

{{{
(define (fib n)
  (if (or (= n 0) (= n 1))
      n
      (+ (fib (- n 1)) (fib (- n 2)))))

(let loop ((n 0))
  (when (< n 35)
    (print ""n="" n "" => "" (fib n))
    (loop (+ n 1))))
}}}

The reason is that in the first case, the `eq?` calls get replaced by `(##core#inline ""eqp"" a b)` while in the second case, the `=` calls get replaced by `(let ((x a) (y b)) (##core_inline ""C_eqp"" x y))` and the `let` is not considered `replacable` even though (I think?) it should be.

That's because `fib`'s arguments are marked by `analyze-expression` in `core.scm` as `captured`.

Fixing this could potentially make a lot of code a lot faster! We definitely should run the benchmarks with and without the fix to find out how dramatic the improvement is."	defect	new	major	5.2	compiler	5.0.0				hard