[GHC] #14619: Output value of program changes upon compiling with -O optimizations

[GHC]

#14619: Output value of program changes upon compiling with -O optimizations
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        Reporter:  sheaf             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Windows           |         Architecture:  x86_64
 Type of failure:  Incorrect result  |  (amd64)
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AndreasK):

 I shrank the repo case a bit:


 {{{
 module Main where

 import Prelude hiding((*>), (<*))

 type V3 = (Double, Double, Double)


 (<+>) :: V3 -> V3 -> V3
 (<+>) (_, _, z) (_, _, z') = (0,0, z+z')
 (<.>) :: V3 -> V3 -> Double
 (<.>) (x, y, z) (x', y', z') = x*x' +y*y'+z*z'


 {-# NOINLINE sphereIntersection #-}
 sphereIntersection :: V3 -> V3 -> Maybe (V3, Double)
 sphereIntersection orig dir@(_, _, dirz)
   | b < 0  = Nothing
   | t1   > 0  = Just (dir, dirz)
   | t1   < 0  = Just (orig, dirz)
   | otherwise = Nothing
     where b  = orig <.> dir
           sqrtDisc = sqrt b
           t1 = b - sqrtDisc

 main = print . fmap fst $ sphereIntersection (0, 0, 200) (0, 0, 1)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14619#comment:31>
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