[Haskell-cafe] Data.Complex.magnitude slow?

Lennart Augustsson lennart at augustsson.net
Thu Jul 17 15:46:12 EDT 2008

If scaleFloat and exponent are implemented with bit twiddling they can
be quite fast.
I have a feeling that they involve slow FFI calls in GHC (being the
original author of a lot of the code involved).

On Thu, Jul 17, 2008 at 8:21 PM, stefan kersten <sk at k-hornz.de> wrote:
> On 17.07.2008, at 17:18, Henning Thielemann wrote:
>>> i've attached an example program which seems to indicate that the
>>> magnitude function from Data.Complex is very slow compared to a more naive
>>> implementation (for Complex Float). on my machine (intel core2 duo, osx
>>> 10.4) the CPU time using the library function is about 6-7 times as much as
>>> when using the other function. any ideas what might be going on? any flaws
>>> in my measurement code?
>> Complex.magnitude must prevent overflows, that is, if you just square
>> 1e200::Double you get an overflow, although the end result may be also
>> around 1e200. I guess, that to this end Complex.magnitude will separate
>> mantissa and exponent, but this is done via Integers, I'm afraid.
> very enlightening, thanks! it might be possible to (almost) get the best of
> two worlds (ported from dejagnu's libm):
> c_magnitude4 :: Complex Float -> Float
> c_magnitude4 (x:+y) = if x' < y'
>                        then mag y' x'
>                        else mag x' y'
>    where
>        x'      = abs x
>        y'      = abs y
>        sqr x   = x * x
>        mag a 0 = a
>        mag a b = a * sqrt (1 + sqr (b/a))
> is fast and doesn't overflow intermediate results but accuracy isn't so
> great ...
> <sk>
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