#11029: Performance loss due to eta expansion -------------------------------------+------------------------------------- Reporter: NeilMitchell | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): JFTR, the relevant code is this bit in `CoreArity.lhs`: {{{#!hs arityType env (Case scrut _ _ alts) | exprIsBottom scrut || null alts = ABot 0 -- Do not eta expand -- See Note [Dealing with bottom (1)] | otherwise = case alts_type of ABot n | n>0 -> ATop [] -- Don't eta expand | otherwise -> ABot 0 -- if RHS is bottomming -- See Note [Dealing with bottom (2)] ATop as | not (ae_ped_bot env) -- See Note [Dealing with bottom (3)] , ae_cheap_fn env scrut Nothing -> ATop as | exprOkForSpeculation scrut -> ATop as | otherwise -> ATop (takeWhile isOneShotInfo as) where alts_type = foldr1 andArityType [arityType env rhs | (_,_,rhs) <- alts] }}} and you can prevent this behaviour with `-fpedantic-bottoms`. And it looks that I made this eta-expansion more aggressive in changeset:2931d19e90d2366f2ce308d65a36333336ca6059 when trying to fix #2915. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11029#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler