Re: [GHC] #12996: Memory leak in recursion when switching from -O1 to -O2

17 Dec 2016

      #12996: Memory leak in recursion when switching from -O1 to -O2
-------------------------------------+-------------------------------------
        Reporter:  AndreasK          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  Memory leak,
                                     |  optimization
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:                    |
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Comment (by osa1):

 I analyzed this a little bit, here's my findings:

 - The function that causes this slowness is generated by `SpecConstr`. If
 you
   compile with `-O2 -fno-spec-constr` it works fine.

 - When `appLoop` function is defined like this:

   {{{#!haskell
 appLoop :: AppState -> IO ()
 appLoop s = do
   let AppState state = s
   print state
   appLoop $ AppState (cycleState state)
   }}}

   Demand analyzer thinks `appLoop` is not strict on `s`. If we define it
 like this:

   {{{#!haskell
 appLoop :: AppState -> IO ()
 appLoop (AppState state) = do
   print state
   appLoop $ AppState (cycleState state)
   }}}

   Worker/wrapper runs instead of SpecConstr and this version compiles to
 fast code.

 I don't understand why `-O2` version is getting slower in each iteration,
 but I
 suspect it has do to with Cmm rather than STG.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12996#comment:3
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