Incorrect value computed, nested ifs with <=, >=

[Paul F. Dietz]

(defparameter *form235*
   '(let ((v6
           (boole boole-xor
            (boole boole-nor 254 -8060811023048695627)
            (boole boole-nor 254
             (if a
                 0
                 -8060811023048695627)))))
     (let ((v2 (f235a (boole boole-andc1 524283 c))))
       (list v2 v6
             (if (>= v2 v6)
                 (if (<= v2 v6)
                     0
                     1)
                 2)))))

(declaim (notinline f235a))
(defun f235a (x)
  (logand #xffff x))

(defun bug235 ()
  (let* ((form *form235*)
         (lam1 `(lambda (a c)
                  ,form))
         (lam2 `(lambda (a c)
                  (declare (notinline <=))
                  ,form))
         (vals '(nil 2840466901390780041)))
    (values (apply (compile nil lam1) vals)
            (apply (compile nil lam2) vals))))

(bug235) ==> (0 0 1), (0 0 0)

x86-64, "2.3.8.144-d6ae98654"

[Stas Boukarev (stassats)]

Tricky to reproduce, seems indeed that something is incorrectly cached.

[Paul F. Dietz (paul-f-dietz)]

It reproduces for me on a freshly started sbcl.

[Stas Boukarev (stassats)]

It depends on random bits beyond an allocated bignum.

[Paul F. Dietz (paul-f-dietz)]

> It depends on random bits beyond an allocated bignum.

Very interesting bug! Is there a way to reliably generate bignums with trash in those bits, to test for such bugs?

[Stas Boukarev (stassats)]

Look at the test included with the fix.