#4381: scaleFloat does not handle overflow correctly.
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Reporter: draconx | Owner: daniel.is.fischer
Type: bug | Status: new
Priority: normal | Milestone:
Component: libraries/base | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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Changes (by daniel.is.fischer):
* owner: => daniel.is.fischer
Comment:
I can't reproduce precisely that, so I believe it's a 64-bit overflow,
`encodeFloat` using 32-bit C-ints:
{{{
Prelude> decodeFloat (1 :: Double)
(4503599627370496,-52)
Prelude Data.Int> fromIntegral ((maxBound :: Int64) - 52) :: Int
-53
}}}
However,
{{{
Prelude Data.Int> scaleFloat minBound 1
Infinity
}}}
and I suspect that would evaluate to 1 on a 64-bit platform.
Easy enough to fix, though it'll cost a little speed. Just clamp the
scaling factor between say -3000 and +3000.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4381#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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