[cmucl-ticket] #19: Modular arith bug 2

[cmucl]

#19: Modular arith bug 2
--------------------+-------------------------------------------------------
 Reporter:  rtoy    |       Owner:  somebody
     Type:  defect  |      Status:  new     
 Priority:  major   |   Milestone:          
Component:  Core    |     Version:  19b     
 Keywords:          |  
--------------------+-------------------------------------------------------
 This was reported by Martin Cracauer to cmucl-imp, 2005-07-20.

 {{{
 (defun fixnum-hasher (a)
   (declare
    (optimize (speed 3) (safety 0) #+nil(debug 1)) ;; BUG HERE, only in
 fast mode
    (type fixnum a)) ;; bug equally apears if you leave this out
   (logand
    (the fixnum ;; BUG HERE.  Drop this declaration and the bug disappears
      (logxor (ash a -20) -482305527))
    1023))
 }}}

 Repeatedly calling {{{(fixnum-hasher 27)}}} returns random values!  The
 problem is caused by modular arithmetic because if it is turned off (by
 {{{setf c::*enable-modular-arithmetic* nil)}}} the bug doesn't happen.
 The answer should be 521.

 A related bug is

 {{{
 (defun foo (a b)
   (locally (declare (optimize (speed 3) (safety 0) (debug 1)))
     (the fixnum
       (logand (the fixnum
                 (logxor (the fixnum a)
                         (the fixnum b)))
               (the fixnum (1- (ash 1 18)))))))
 }}}

 This bug is mentioned in the same thread, but is from sbcl-devel, 2005-04-
 29.

 {{{(foo -1234 1234)}}} returns random values.  The correct answer is
 262140.

 In both cases, there is a call to vm::logxor-mod32 with fixnum args,
 returning an unsigned 32-bit result.  This is not right.

-- 
Ticket URL: <http://trac.common-lisp.net/cmucl/ticket/19>
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