[GHC] #7233: Int64 division buggy on a 32-bit installation

[GHC]

#7233: Int64 division buggy on a 32-bit installation
------------------------------+---------------------------------------------
 Reporter:  lerkok            |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.6.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
------------------------------+---------------------------------------------

Comment(by lerkok):

 Some more information. It appears that the expression:
 {{{
     x `div` y
 }}}
 will be incorrectly computed for ```Int64``` values when the internal word
 size is 32 bits,  in the following cases:
 {{{
    1.  If x > 0 && y < 0, then whenever ((x-y)-1) overflows/underflows the
 Int64 range
    2.  if x < 0 && y > 0, then whenever ((x-y)+1) overflows/underflows the
 Int64 range
 }}}

 So, this will impact whenever the difference between ```x``` and ```y```
 are rather large; but errors will pop-up regularly through the data-space,
 not just at end points. (That is, for every valid value of `x`, there's a
 value of `y` that will exhibit the bug; and vice versa.)

 I think the culprit is here:
 http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/GHC-
 Int.html#divInt64. The code in there implements ```div``` in terms of
 ```quot```, computing``(x-y)-1``` and ```(x-y)+1``` without worrying about
 overflow/underflow.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7233#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler