[Haskell-cafe] fast Eucl. dist. - Haskell vs C
claus.reinke at talk21.com
Mon May 18 09:28:25 EDT 2009
> My current best try uses the uvector package, has two 'vectors' of type
> (UArr Double) as input, and relies on the sumU and zipWithU functions
> which use streaming to compute the result:
> dist_fast :: UArr Double -> UArr Double -> Double
> dist_fast p1 p2 = sumDs `seq` sqrt sumDs
> sumDs = sumU ds
> ds = zipWithU euclidean p1 p2
> euclidean x y = d*d
> d = x-y
You'll probably want to make sure that 'euclidian' is specialized to
the types you need (here 'Double'), not used overloaded for 'Num a=>a'
(check -ddump-tc, or -ddump-simpl output).
After that, unrolling the fused fold loop (uvector internal) might help
a bit, but isn't there yet:
And even if that gets implemented, it doesn't apply directly to your
case, where the loop is in a library, but you might want to control its
unrolling in your client code. Having the loop unrolled by a default
factor (8x or so) should help for loops like this, with little computation.
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