[GHC] #14085: powModInteger sometimes ignores sign of argument

3 Aug 2017


      #14085: powModInteger sometimes ignores sign of argument
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           Reporter:  ocheron        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Core           |           Version:  8.2.1
  Libraries                          |
           Keywords:  integer-gmp    |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Function `intToSBigNat#` returns a `PosBN` result when input is < -1, but
 it should be `NegBN` instead.

 As a consequence functions like `powModInteger` give incorrect result for
 small negative numbers:
 {{{
 $ ghci
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
...
:m GHC.Integer.GMP.Internals
powModInteger (-2) 3 123
 8
(-2)^3 `mod` 123
 115
 }}}
-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14085
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