[GHC] #9020: Massive blowup of code size on trivial program

[GHC]

#9020: Massive blowup of code size on trivial program
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       Reporter:  simonpj           |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 The test `simplCore/should_compile/simpl015` was leading to massive
 compile times. This is weird: it looks like:
 {{{
 main = do { return ()
           ; return ()
           ... hundreds more times ...
           ; return () }
 }}}
 It turns out that the cause was over-eager eta-expansion.  Each `return
 ()` gives a top-level PAP thus
 {{{
 lvl17 :: IO ()
 lvl17 = returnIO ()
 }}}
 But these definitions are then eta-expanded, thus:
 {{{
 lvl17 = ((\eta. returnIO () |> sym g) eta) |> g

     where   g :: State# RealWorld -> (# State# RealWorld, () #)
                  ~
                  IO ()
 }}}
 Now in general it makes sense to eta-expand through newtypes (as is done
 here), because it exposes more lambadas.  But it really doesn't make sense
 to eta-expand a PAP like this.

 Fortunately the fix is easy: use `exprArity` rather than `manifestArity`
 in `SimplUtils.tryEtaExpandRhs`.

 The effect on `simpl015` is dramatic: compiler allocation drops from 6.6G
 to 812M; and residency drops from 1.8G to 45M.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9020>
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