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

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:

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@gmail.com> wrote:

...

+gtk2hs-devel

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:

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/

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