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

26 Dec 2017

      #14619: Output value of program changes upon compiling with -O optimizations
-------------------------------------+-------------------------------------
           Reporter:  sheaf          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           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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 The optimisation pass seems to affect the behaviour of the attached
 program (a basic ray-sphere intersection test, 40 lines long, no
 dependencies).

 Specifically, compiling the attached source file with -O or -O2, and
 considering the definition of main:

 {{{#!hs
 result = sphereIntersection testRay testSphere

 main :: IO()
 main = do
   print $ result
   print $ sphereIntersection testRay testSphere
 }}}

 I get the troubling output:

 {{{
...
bug.exe
 Just ((0.0,0.0,0.0),0.0)
 Just ((0.0,0.0,100.0),1.0)
 }}}
This is especially troubling because the second component of the output
 should always be 1.0:

 {{{#!hs
 testRay = ((0, 0, 0),(0, 0, 1))

 sphereIntersection (orig, dir@(_, _, dirz)) (c,r)
   | disc < 0  = Nothing
   | t1   > 0  = Just (t1 *> dir <+> orig, dirz)
   | t2   > 0  = Just (t2 *> dir <+> orig, dirz)
   | otherwise = Nothing
     where ...
 }}}

 as we are returning the "dirz" component of the input ray straight back
 out.

 On the other hand, when the program is run without optimisations, I get
 the expected output:

 {{{
...
bug.exe
 Just ((0.0,0.0,100.0),1.0)
 Just ((0.0,0.0,100.0),1.0)
 }}}
This behaviour was present on all versions of GHC that I tested (7.10.1,
 8.0.1, 8.2.1, 8.2.2). I initially ran into this issue when I noticed that
 the profiling builds of my program produced different results, which was
 boiled down to this problem.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14619
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