[GHC] #9810: encodeFloat 1 2047 = -1024.0

[GHC]

#9810: encodeFloat 1 2047 = -1024.0
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              Reporter:  luite       |            Owner:  hvr
                  Type:  bug         |           Status:  patch
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.9
            Resolution:              |         Keywords:  ieee
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #9811
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D486   |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"e2af452cd533778c5447719c59429d72bb1fe00d/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e2af452cd533778c5447719c59429d72bb1fe00d"
 Restore exact old semantics of `decodeFloat`

 `integer-gmp2` uses the new 64bit-based IEEE deconstructing primop
 introduced in b62bd5ecf3be421778e4835010b6b334e95c5a56.

 However, the returned values differ for exceptional IEEE values:

 Previous (expected) semantics:

   > decodeFloat (-1/0)
   (-4503599627370496,972)

   > decodeFloat (1/0)
   (4503599627370496,972)

   > decodeFloat (0/0)
   (-6755399441055744,972)

 Currently (broken) semantics:

   > decodeFloat (-1/0 :: Double)
   (-9223372036854775808,-53)

   > decodeFloat (1/0 :: Double)
   (-9223372036854775808,-53)

   > decodeFloat (0/0 :: Double)
   (-9223372036854775808,-53)

 This patch reverts to the old expected semantics.

 I plan to revisit the implementation during GHC 7.11 development.

 This should address #9810

 Reviewed By: austin, ekmett, luite

 Differential Revision: https://phabricator.haskell.org/D486
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9810#comment:4>
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