How is this Generic-based instance implementation optimized by GHC?

[José Pedro Magalhães]

Hi there,

GHC can often do a pretty good job at optimising generics. I wrote a paper
that looks at that in detail:

José Pedro Magalhães. Optimisation of Generic Programs through Inlining. In
24th Symposium on Implementation and Application of Functional Languages
(IFL'12), 2013.
http://dreixel.net/research/pdf/ogpi.pdf


Cheers,
Pedro

On Sat, Aug 22, 2015 at 11:26 PM, Ömer Sinan Ağacan <omeragacan at gmail.com>
wrote:

> Hi all,
>
> I'm very confused by an optimization GHC is doing. I have this code:
>
>
>     data Tree a = Leaf a | Branch (Tree a) (Tree a)
>       deriving (Generic, Show, NFData)
>
>     data Tree1 a = Leaf1 a | Branch1 (Tree1 a) (Tree1 a)
>       deriving (Show)
>
>     instance NFData a => NFData (Tree1 a) where
>       rnf (Leaf1 a) = rnf a
>       rnf (Branch1 t1 t2) = rnf t1 `seq` rnf t2
>
>
> When I benchmarked rnf calls I realized that they're too close, and I
> looked at
> simplifier outputs. I believe these are relevant parts:
>
>     Rec {
>     Main.$fNFDataTree_$crnf [Occ=LoopBreaker]
>       :: forall a_ab5v. NFData a_ab5v => Tree a_ab5v -> ()
>     Main.$fNFDataTree_$crnf =
>       \ (@ a17_ab5v)
>         ($dNFData_ab5w :: NFData a17_ab5v)
>         (eta_B1 :: Tree a17_ab5v) ->
>         case eta_B1 of _ [Occ=Dead] {
>           Leaf g1_aaHO ->
>             ($dNFData_ab5w
>              `cast` (Control.DeepSeq.NTCo:NFData[0] <a17_ab5v>_N
>                      :: NFData a17_ab5v ~R# (a17_ab5v -> ())))
>               g1_aaHO;
>           Branch g1_aaHP g2_aaHQ ->
>             case Main.$fNFDataTree_$crnf @ a17_ab5v $dNFData_ab5w g1_aaHP
>             of _ [Occ=Dead] { () ->
>             Main.$fNFDataTree_$crnf @ a17_ab5v $dNFData_ab5w g2_aaHQ
>             }
>         }
>     end Rec }
>
>     Rec {
>     Main.$fNFDataTree1_$crnf [Occ=LoopBreaker]
>       :: forall a_abd4. NFData a_abd4 => Tree1 a_abd4 -> ()
>     Main.$fNFDataTree1_$crnf =
>       \ (@ a17_abd4)
>         ($dNFData_abd5 :: NFData a17_abd4)
>         (eta_B1 :: Tree1 a17_abd4) ->
>         case eta_B1 of _ [Occ=Dead] {
>           Leaf1 a18_a4tg ->
>             ($dNFData_abd5
>              `cast` (Control.DeepSeq.NTCo:NFData[0] <a17_abd4>_N
>                      :: NFData a17_abd4 ~R# (a17_abd4 -> ())))
>               a18_a4tg;
>           Branch1 t1_a4th t2_a4ti ->
>             case Main.$fNFDataTree1_$crnf @ a17_abd4 $dNFData_abd5 t1_a4th
>             of _ [Occ=Dead] { () ->
>             Main.$fNFDataTree1_$crnf @ a17_abd4 $dNFData_abd5 t2_a4ti
>             }
>         }
>     end Rec }
>
> First one is generated by GHC and second one is hand-written. If you
> compare,
> you'll see that they're identical. This looks like some serious magic,
> because
> first one is generated from a default method that uses Generic methods and
> types. Does anyone know how is that possible? Which optimization passes are
> involved in this?
>
> Thanks.
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20150823/675dac5b/attachment.html>