[Haskell-cafe] Desired behaviour of rounding etc.

[Lennart Augustsson]

That code is incorrect.  You can't assume that the base for floating
point numbers is 2, that's something you have to check.
(POWER6 and z9 has hardware support for base 10 floating point.)

  -- Lennart

On Fri, Oct 8, 2010 at 2:08 PM, Daniel Fischer <daniel.is.fischer at web.de> wrote:
> The methods of the RealFrac class produce garbage when the value lies
> outside the range of the target type, e.g.
>
> Prelude GHC.Float> truncate 1.234e11 :: Int  -- 32-bits
> -1154051584
>
> and, in the case of truncate, different garbage when the rewrite rule
> fires:
>
> Prelude GHC.Float> double2Int 1.234e11
> -2147483648
>
> I'm currently working on faster implementations of properFraction,
> truncate, round, ceiling and floor for Float and Double, so I'd like to
> know
>
> - does it matter at all what garbage is returned in the above case?
> - if it does, what is the desired behaviour (at least for Int, I can't
> cater for all possibilities)?
>
>
> On a related note, in my benchmarks,
>
> truncFloatGen :: Integral a => Float -> a
> truncFloatGen = fromInteger . truncFloatInteger
>
> truncFloatInteger :: Float -> Integer
> truncFloatInteger x =
>  case decodeFloat x of
>    (m,e) | e == 0  -> m
>          | e < 0   ->
>            let s = -e
>            in if m < 0
>                  then - ((-m) `shiftR` s)
>                  else m `shiftR` s
>          | otherwise -> m `shiftL` e
>
> is more than twice as fast as GHC.Float.float2Int, the corresponding for
> Double almost twice as fast as double2Int.
>
> Can anybody confirm that the above is faster than float2Int on other
> machines/architectures?
>
> Cheers,
> Daniel
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