Sorry for re-sending, my previous attempt got ignored by gtk2hs-devel mailing list as I wasn't subscribed. Now I am.

On Wed, Nov 2, 2011 at 3:14 PM, Eugene Kirpichov <> wrote:

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 <> wrote:

On Wed, Nov 2, 2011 at 8:15 AM, Eugene Kirpichov <> 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:

 :: forall a_a3TN b_a3TO.
    (RealFloat a_a3TN, RealFrac b_a3TO) =>
    a_a3TN -> b_a3TO

 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.$wcFloatConv =
 \ (@ a_a3TN)
   (@ b_a3TO)
   (w_s5zg :: RealFloat a_a3TN)
   (ww_s5zj :: RealFrac b_a3TO)
   (w1_s5zA :: a_a3TN) ->
     @ b_a3TO
     ($p2RealFrac @ b_a3TO ww_s5zj)
        @ a_a3TN
           @ a_a3TN ($p1RealFloat @ a_a3TN w_s5zg))

Note that this is basically cFloatConv = fromRational . toRational.

*However*, GHC also compiled a Double -> Double specialization:

 :: Double -> Double

 Unf=Unf{Src=InlineStable, TopLvl=True, Arity=1, Value=True,
        ConLike=True, Cheap=True, Expandable=True,
        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...



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Eugene Kirpichov
Principal Engineer, Mirantis Inc.

Eugene Kirpichov
Principal Engineer, Mirantis Inc.