[GHC] #952: Floating point exception building HEAD with 6.6

#952: Floating point exception building HEAD with 6.6
-------------------------------+--------------------------------------------
    Reporter:  simonmar        |       Owner:       
        Type:  bug             |      Status:  new  
    Priority:  normal          |   Milestone:  6.6.1
   Component:  Compiler        |     Version:  6.6  
    Severity:  normal          |    Keywords:       
  Difficulty:  Unknown         |    Testcase:       
Architecture:  x86_64 (amd64)  |          Os:  Linux
-------------------------------+--------------------------------------------
I get a floating point exception building HEAD with 6.6.  I tracked it
 down to `getCommonNodeUFMData` in `UniqFM`:

 {{{
 getCommonNodeUFMData :: NodeUFMData -> NodeUFMData -> NodeUFMData

 getCommonNodeUFMData (NodeUFMData i p) (NodeUFMData i2 p2)
   | p ==# p2    = getCommonNodeUFMData_ p j j2
   | p <# p2     = getCommonNodeUFMData_ p2 (j `quotFastInt` (p2
 `quotFastInt` p)) j2
   | otherwise   = getCommonNodeUFMData_ p j (j2 `quotFastInt` (p
 `quotFastInt` p2))
   where
     l  = (_ILIT(1) :: FastInt)
     j  = i  `quotFastInt` (p  `shiftL_` l)
     j2 = i2 `quotFastInt` (p2 `shiftL_` l)

     getCommonNodeUFMData_ :: FastInt -> FastInt -> FastInt -> NodeUFMData

     getCommonNodeUFMData_ p j j_
       | j ==# j_
       = NodeUFMData (((j `shiftL_` l) +# l) *# p) p
       | otherwise
       = getCommonNodeUFMData_ (p `shiftL_`  l) (j `shiftR_` l) (j_
 `shiftR_` l)
 }}}

 The division `j2` is a divide-by-zero; `p2` is zero.

 Compiling `UniqFM` with `DEBUG` makes it go away.  Compiling with `-fasm`
 has no effect, so it isn't dependent on the back-end.

 This may be architecture-specific; I've only seen it on x86_64 so far.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/952>
GHC <http://www.haskell.org/ghc/>
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