[GHC] #8285: unexpected behavior with encodeFloat on large inputs

[GHC]

#8285: unexpected behavior with encodeFloat on large inputs
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       Reporter:  carter  |             Owner:
           Type:  bug     |            Status:  new
       Priority:  normal  |         Milestone:  7.8.1
      Component:          |           Version:  7.7
  Compiler                |  Operating System:  Unknown/Multiple
       Keywords:          |   Type of failure:  Incorrect result at runtime
   Architecture:          |         Test Case:
  Unknown/Multiple        |          Blocking:
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
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 benjamin scarlet discovered some unexpected behavior in encodeFloat today

 {{{
 <bscarlet>   > map (\x -> encodeFloat 1 (shiftL 1 x)) [30,31]
 [23:47:47] <lambdabot>     [Infinity,0.0]

 [23:49:09] <arkeet>  > encodeFloat 1 60
 [23:49:10] <lambdabot>     1.152921504606847e18

 [23:49:09] <arkeet>  > encodeFloat 1 60
 [23:49:10] <lambdabot>     1.152921504606847e18
 [23:49:18] <arkeet>  er.
 [23:50:21] <bscarlet>    down in the sane range I suspect it's just fine.
 [23:50:43] <bscarlet>    > encodeFloat 1 1000
 [23:50:44] <lambdabot>     1.0715086071862673e301
 [23:50:52] <bscarlet>    > encodeFloat 1 10000
 [23:50:53] <lambdabot>     Infinity
 [23:50:58] <bscarlet>    That's sane.
 [23:51:03] <arkeet>  > encodeFloat 1 (2^31)
 [23:51:04] <lambdabot>     0.0
 [23:51:13] <bscarlet>    That's what I think is weird.
 [23:51:19] <carter>  bscarlet floats are weird
 [23:51:23] <arkeet>  > encodeFloat 1 (2^31-1)
 [23:51:24] <lambdabot>     Infinity

 }}}
 Benjamin did a wee bit of investigation, and the
 problem seems to lie in https://github.com/ghc/packages-integer-
 gmp/blob/master/cbits/float.c#L177
 in integer-gmp, where it deliberately doesn't handle the edge cases
 (probably worth investigating) if integer-simple has similar behavior or
 not

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8285>
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