#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: |
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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>
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