[Chicken-janitors] #823: arithmetic-shift fails in exactly one edge case

[Chicken Trac]

#823: arithmetic-shift fails in exactly one edge case
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 Reporter:  sjamaan                                  |       Owner:       
     Type:  defect                                   |      Status:  new  
 Priority:  minor                                    |   Milestone:  4.9.0
Component:  core libraries                           |     Version:  4.7.x
 Keywords:  edge cases, numbers, shifting, overflow  |  
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 I don't know whether it's worth the trouble to fix this, but I'll put it
 on here so at least it's documented.

 {{{(arithmetic-shift -123 most-negative-fixnum) => -123}}}

 This should be {{{-1}}}. It fails because in chicken.h the
 {{{C_i_fixnum_arithmetic_shift}}} function calls
 {{{C_fixnum_shift_right}}} with the negated value of the shift amount.
 This overflows and wraps around to zero.

 Fixing it would be simplest by just inlining the shift and negating
 _after_ unboxing.  This is easy, but there are some other places that call
 {{{C_fixnum_shift_right}}} in this way.

 This code is obviously wrong but I'm not sure it's worth fixing, since
 it's such an extreme edge case (if you subtract 1 from the shift amount
 you get an error that the argument is not a fixnum). On the other hand,
 not fixing it just seems, I dunno... lazy and possibly dangerous. :)

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Ticket URL: <http://bugs.call-cc.org/ticket/823>
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