[cmucl-ticket] Re: #20: Modular arith bug?

[cmucl]

#20: Modular arith bug?
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  Reporter:  rtoy    |       Owner:  somebody
      Type:  defect  |      Status:  new     
  Priority:  major   |   Milestone:          
 Component:  Core    |     Version:  19e     
Resolution:          |    Keywords:          
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Comment (by rtoy):

 The issue is partly caused by ldb and, perhaps, partly by ash modular
 function optimizer.

 (ldb (byte size posn) x) is source-transformed to (%ldb size posn x),
 which has a deftransform that converts it to (logand (ash x (- posn)) (ash
 (1- (ash 1 vm:word-bits)) (- size vm:word-bits)).

 This expression must confuse logand-defopt-helper.  One workaround is to
 change the %ldb to produce simpler expressions when posn and size are
 known constants.

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Ticket URL: <http://trac.common-lisp.net/cmucl/ticket/20>
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