[GHC] #9304: Floating point woes; Different behavior on Mac vs Linux

[GHC]

#9304: Floating point woes; Different behavior on Mac vs Linux
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       Reporter:  lerkok            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  high              |         Milestone:
      Component:  Compiler          |           Version:  7.8.3
       Keywords:  floating point    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I've the following snippet:

 {{{
 x, y, r  :: Double
 x = -4.4
 y = 2.4999999999999956
 r = x * y
 }}}

 Using GHC 7.8.3, on a Mac, I get the following response from ghci:

 {{{
 *Main> decodeFloat r
 (-6192449487634421,-49)
 }}}

 Using GHC 7.8.3, on a Linux machine, I get the following response:

 {{{
 *Main> decodeFloat r
 (-6192449487634422,-49)
 }}}

 Note that off-by-one difference in the first component of the output.

 I'm not 100% sure as to which one is actually correct; but the point is
 that these are IEEE floating point numbers running on the same
 architecture (Intel X86), and thus should decode in precisely the same
 way.

 While I observed this with 7.8.3; I don't think this is a new regression;
 I suspect it'll show up in older versions as well.

 Also, for full disclosure: I ran the Mac version naively; but the Linux
 version on top of a VirtualBox image. I'd very much doubt that would make
 a difference, but there might be 32/64-bit concerns. So, if someone can
 validate the Linux output on a true 64-bit Linux machine, that would
 really help track down the issue further.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9304>
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