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] _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 ->
($dNFData_abd5
`cast` (Control.DeepSeq.NTCo:NFData[0] _N
:: NFData a17_abd4 ~R# (a17_abd4 -> ())))
a18_a4tg;
Branch1 t1_a4th t2_a4ti ->
case Main.$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.
