#4867: ghci displays negative floats incorrectly (was: Incorrect result from
trig
functions)
-------------------------------+--------------------------------------------
Reporter: gwright | Owner: gwright
Type: bug | Status: new
Priority: high | Milestone: 7.0.2
Component: GHCi | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: MacOS X | Blocking:
Architecture: x86_64 (amd64) | Failure: Incorrect result at runtime
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Comment(by gwright):
Replying to [comment:26 altaic]:
> Not sure if you had already discovered this, but it appears the bug is
limited to `Double`, while `Float` appears to be fine:
> {{{
> Prelude> -1.0 :: Float
> -1.0
> Prelude> -1.0 :: Double
> -3.666940035476786e76
> }}}
>
> Oddly, the transform that's producing the garbage value is reversible
(guessing there'd be loss of bits at extremes):
> {{{
> Prelude> -1.0 :: Double
> -3.666940035476786e76
> Prelude> -3.666940035476786e76 :: Double
> -1.0
> }}}
I don't think I noticed the `Float`/`Double` difference earlier but with
the latest linker patch everything seems OK. I'll check again to verify
this.
The failed relocations caused garbage values to be printed by `ghci`, but
the intermediate values in the calcuations are OK. The error went like
this: when a negative `Double` is printed, a '-' sign is printed, then
the negative of the value is printed. IEEE 754 doubles are negated by
`xor`-ing them with a fixed bitmask, which complements the sign bit. The
problem was that the relocation to the bitmask was wrong, so the double
was `xor`-ed with a constant, incorrect mask.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4867#comment:27>
GHC <http://www.haskell.org/ghc/>
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