Superb; on how to avoid boxing.
On Tue, Jun 14, 2011 at 7:51 AM, Daniel Fischer
On Tuesday 14 June 2011, 15:51:57, Daniel Fischer wrote:
On Tuesday 14 June 2011, 14:35:19, Johannes Waldmann wrote:
With id, main1 jumps to foldb_cap, which contains a lot of code for the (cap > 1)-branch, and - that's what causes the slowdown - a worker loop
$s$wfoldlM'_loop_s3EE [Occ=LoopBreaker]
:: GHC.Prim.Int#
-> (GHC.Types.Int, GHC.Types.Int) -> (# GHC.Types.Int, GHC.Types.Int #)
which uses the passed functions (thus you have no inlining of eff and h1, and a boxed tuple of boxed Int's in your worker).
I am sure GHC headquarters will look at this when they find the time but perhaps there's some additional knowledge on this mailing list that might help.
Note that you get good behaviour when you help GHC a bit, in particular a static argument transformation for the function parameters of foldb_cap allows them to be inlined and (in this case) you get the nice loop on unboxed Int#s again:
foldb_cap :: ( V.Unbox a, V.Unbox b ) => Int -> b -> ( a -> b ) -> ( b -> b -> b ) -> Vector a -> b foldb_cap cp strt f g xs = work cp strt xs where work cap e s = if cap <= 1 then V.foldl' g e $ V.map f s -- replace "id" by "undefined" in the following, -- and notice a drastic decrease in runtime - -- although this branch is never executed: else id $ case V.length s of 0 -> e 1 -> f $! V.head s n -> let splitAt k v = ( V.take k v, V.drop k v ) ( s1, s2 ) = splitAt ( div n 2 ) s cap' = div cap 2 v1 = work cap' e s1 v2 = work cap' e s2 v = g v1 v2 in par v1 $ pseq v2 $ v
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-- -- Regards, KC