Re: [GHC] #12545: Compilation time/space regression in GHC 8.0/8.1 (search in type-level lists and -O)

#12545: Compilation time/space regression in GHC 8.0/8.1 (search in type-level lists and -O) -------------------------------------+------------------------------------- Reporter: | Owner: (none) mikhail.vorozhtsov | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Inlining Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Well hot dang. It turns out that commit 2b74bd9d8b4c6b20f3e8d9ada12e7db645cc3c19 (Stop the specialiser generating loopy code) is once again responsible for fixing the problem! {{{ Commit 92a4f908f2599150bec0530d688997f03780646e (Spelling typos) <> real 0m59.150s user 0m59.020s sys 0m0.224s Commit 2b74bd9d8b4c6b20f3e8d9ada12e7db645cc3c19 (Stop the specialiser generating loopy code) <> real 0m4.549s user 0m4.440s sys 0m0.096s }}} For reference, the time for GHC 7.10.3 was: {{{ <> real 0m13.477s user 0m13.128s sys 0m0.104s }}} So now it's even //better// than it used to be! That's pretty cool. I don't have any particular insight as to why that commit made things so much better, but sure enough, `TypeList.hs` contains some classes with significant superclasses: {{{#!hs class (FindElem 'HeadElem a l ~ 'Elem p) => ElemAt p (a :: k) (l :: [k]) where elemWitness :: Proxy a -> Proxy l -> ElemWitness p a l class IsElem t (ElemsOf a) => ElemOf a t where }}} So it's entirely possible that their corresponding dictionaries are just getting optimized better. It would be great to have someone verify this at the Core level. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12545#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler