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

[Felipe Almeida Lessa]

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.