[GHC] #11029: Performance loss due to eta expansion

[GHC]

#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