'deriving Eq' on recursive datatype makes ghc eat a lot of CPU and RAM
The example is slimmed down unit test of Annotations-0.2.1 hackage package.
If we try to compile Bug.hs with -O0 it compiles quickly. Trying it with -O1 makes GHC-8.0.1 takes a minute to finish.
-- Bug1.hs:
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}
module Bug () where
import Prelude (Eq)
data ExprF rT = ExprF rT rT deriving Eq
newtype Expr = Expr (Fix ExprF) deriving Eq
newtype Fix fT = In (fT (Fix fT))
deriving instance Eq (f (Fix f)) => Eq (Fix f)
$ time ghc-8.0.1 -c -O0 Bug1.hs -fforce-recomp
real 0m0.611s
user 0m0.549s
sys 0m0.053s
$ time ghc-8.0.1 -c -O1 Bug1.hs -fforce-recomp
real 1m2.199s
user 1m1.676s
sys 0m0.465s
- 10.2 for comparison is very quick in both O0/O1:
$ time ghc-7.10.2 -c -O0 Bug1.hs -fforce-recomp
real 0m0.220s
user 0m0.183s
sys 0m0.036s
$ time ghc-7.10.2 -c -O1 Bug1.hs -fforce-recomp
real 0m0.237s
user 0m0.213s
sys 0m0.023s
The real ExprF datatype uses more constructors and instances:
data ExprF rT
= Add rT rT
| Sub rT rT
| Mul rT rT
| Div rT rT
| Num Int
deriving (Eq, Show)
That requires a lot of time and space to finish (Bug2.hs in attach). I've stopped it after 5 minutes (took ~8GB RAM).