[Haskell-cafe] Fwd: Ridiculously slow FFI, or cairo binding?

[Eugene Kirpichov]

Yay!!!

I made a small change in Types.chs and got my original cairo-binding-based
program to be just as blazing fast. The only problem I have with this is
that I used multiparameter type classes.

Dear gtk2hs team! Is it possible to incorporate my changes? I'm pretty sure
people will be happy by an order-of-magnitude speedup. Probably the stuff
could be wrapped in #define's for those who aren't using GHC and can't use
multiparameter type classes?

I am pretty sure I could have done the same with rewrite rules, but I tried
for a while and to no avail.

FAILED SOLUTION: rewrite rules
cFloatConv :: (RealFloat a, RealFloat b) => a -> b
cFloatConv  = realToFrac
{-# NOINLINE cFloatConv #-}
{-# RULES "cFloatConv/float2Double" cFloatConv = float2Double #-}
{-# RULES "cFloatConv/double2Float" cFloatConv = double2Float #-}
{-# RULES "cFloatConv/self"         cFloatConv = id           #-}

For some reason, the rules don't fire. Anyone got an idea why?

SUCCEEDED SOLUTION: multiparameter type classes

I rewrote cFloatConv like this:

import GHC.Float
class (RealFloat a, RealFloat b) => CFloatConv a b where
  cFloatConv :: a -> b
  cFloatConv = realToFrac

instance CFloatConv Double Double where cFloatConv = id
instance CFloatConv Double CDouble
instance CFloatConv CDouble Double
instance CFloatConv Float Float where cFloatConv = id
instance CFloatConv Float Double where cFloatConv = float2Double
instance CFloatConv Double Float where cFloatConv = double2Float

and replaced a couple of constraints in functions below by usage of
CFloatConv.


On Wed, Nov 2, 2011 at 2:25 PM, Felipe Almeida Lessa <felipe.lessa at gmail.com
> wrote:

> +gtk2hs-devel
>
> On Wed, Nov 2, 2011 at 8:15 AM, Eugene Kirpichov <ekirpichov at gmail.com>
> wrote:
> > Any idea how to debug why all the GMP calls?
> > I'm looking at even the auto-generated source for cairo bindings, but I
> > don't see anything at all that could lead to *thousands* of them.
>
> Found them.  Look at the Types module and you'll see
>
>  cFloatConv :: (RealFloat a, RealFloat b) => a -> b
>  cFloatConv  = realToFrac
>
> This function (or its cousins peekFloatConv, withFloatConv...) are
> used *everywhere*.
>
> Looking at this module with ghc-core we see that GHC compiled a
> generic version of cFloatConv:
>
> Graphics.Rendering.Cairo.Types.$wcFloatConv
>  :: forall a_a3TN b_a3TO.
>     (RealFloat a_a3TN, RealFrac b_a3TO) =>
>     a_a3TN -> b_a3TO
> [GblId,
>  Arity=3,
>
>  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=3, Value=True,
>         ConLike=True, Cheap=True, Expandable=True,
>         Guidance=IF_ARGS [3 3 0] 12 0}]
> Graphics.Rendering.Cairo.Types.$wcFloatConv =
>  \ (@ a_a3TN)
>    (@ b_a3TO)
>    (w_s5zg :: RealFloat a_a3TN)
>    (ww_s5zj :: RealFrac b_a3TO)
>    (w1_s5zA :: a_a3TN) ->
>    fromRational
>      @ b_a3TO
>      ($p2RealFrac @ b_a3TO ww_s5zj)
>      (toRational
>         @ a_a3TN
>         ($p1RealFrac
>            @ a_a3TN ($p1RealFloat @ a_a3TN w_s5zg))
>         w1_s5zA)
>
> Note that this is basically cFloatConv = fromRational . toRational.
>
> *However*, GHC also compiled a Double -> Double specialization:
>
> Graphics.Rendering.Cairo.Types.cFloatConv1
>  :: Double -> Double
> [GblId,
>  Arity=1,
>
>  Unf=Unf{Src=InlineStable, TopLvl=True, Arity=1, Value=True,
>         ConLike=True, Cheap=True, Expandable=True,
>         Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=False)
>         Tmpl= \ (eta_B1 [Occ=Once!] :: Double) ->
>                 case eta_B1 of _ { D# ww_a5v3 [Occ=Once] ->
>                 case $w$ctoRational ww_a5v3
>                 of _ { (# ww2_a5v8 [Occ=Once], ww3_a5v9 [Occ=Once] #) ->
>                 $wfromRat ww2_a5v8 ww3_a5v9
>                 }
>                 }}]
> Graphics.Rendering.Cairo.Types.cFloatConv1 =
>  \ (eta_B1 :: Double) ->
>    case eta_B1 of _ { D# ww_a5v3 ->
>    case $w$ctoRational ww_a5v3
>    of _ { (# ww2_a5v8, ww3_a5v9 #) ->
>    $wfromRat ww2_a5v8 ww3_a5v9
>    }
>    }
>
> ...which is also equivalent to fromRational . toRational however with
> the type class inlined!  Oh, god...
>
> Cheers,
>
> --
> Felipe.
>
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>



-- 
Eugene Kirpichov
Principal Engineer, Mirantis Inc. http://www.mirantis.com/
Editor, http://fprog.ru/
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