id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	difficulty
1236	equal? can break at random moments	sjamaan		"This is really bad.  C_equal is defined to run inline, so it can't clear up stack space.  This is really really bad, because it basically means that you're playing russian roulette every time you call {{{equal?}}}; it can break or not, which depends on the state of the stack, over which you have very little control. It is also very hard to predict when exactly it will fail.

I found this out [[http://salmonella-linux-x86-64.call-cc.org/master-debugbuild/gcc/linux/x86-64/2015/12/22/salmonella-report/test/numbers.html|the hard way]].

Here's a standalone version that boils down to the code in the numbers tests:

{{{
#!scm

;; Creates lists like (((1))) and ((((1)))) for 3 and 4, respectively.
(define-syntax recompose
  (er-macro-transformer
   (lambda (e r c)
     (let ((n (cadr e))
           (op (caddr e))
           (arg (cadddr e)))
       (define (recompose-1 n)
         (if (= n 1)
             `(,op ,arg)
             `(,op ,(recompose-1 (- n 1)))))
       (recompose-1 n)))))

(let lp ((i 0))
  (print i "" "" (equal? (recompose 1000 list '(1))
                       (recompose 1000 list (list 1))))
  (lp (add1 i)))
}}}

You'll see this blows up after only 16 or so iterations.  I had to up the list nesting to 1000 because 100 only seems to crash once every blue moon.

A fix could involve dynamically allocating (with {{{malloc}}} or so) a ""working list"" stack and use that to convert the recursion to a loop.  Alternatively, we could ""de-inline"" it so it can reclaim memory.  On the plus side, this allows us to convert it to Scheme and taking the CPS hit.

Originally I thought it might be possible to use the scratch space in CHICKEN 5, but I'm not sure that's suitable in this case because there's nothing in the heap or nursery that points directly to it.  We could create one and make an actual Scheme list of ""todo"" objects, but it would be such a long chain that it would become pretty slow I think."	defect	new	critical	4.11.0	core libraries	4.10.x