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[Chicken-janitors] #1236: equal? can break at random moments
From: |
Chicken Trac |
Subject: |
[Chicken-janitors] #1236: equal? can break at random moments |
Date: |
Tue, 22 Dec 2015 14:37:34 -0000 |
#1236: equal? can break at random moments
----------------------------+--------------------
Reporter: sjamaan | Owner:
Type: defect | Status: new
Priority: critical | Milestone: 4.11.0
Component: core libraries | Version: 4.10.x
Keywords: |
----------------------------+--------------------
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.
--
Ticket URL: <http://bugs.call-cc.org/ticket/1236>
CHICKEN Scheme <http://www.call-cc.org/>
CHICKEN Scheme is a compiler for the Scheme programming language.
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