Re: [GHC] #13535: vector test suite uses excessive memory on GHC 8.2

14 Jul 2018

      #13535: vector test suite uses excessive memory on GHC 8.2
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10800            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by harendra):

 It looks like I am seeing this same bug on GHC 8.2.2. This is again in a
 streaming library with stream fusion like vector and I am seeing the
 problem specifically with the "filter" code in the library, though it does
 not always happen, only in some cases. I have two branches in my repo that
 reproduce the problem:

 * See branch https://github.com/composewell/streamly/tree/ghc-8.2.2-bug .
 The last commit on this branch
 https://github.com/composewell/streamly/commit/8f08248eba6702159f7bc3fe99e0c...
 disables the culprit code.

 * The second branch is
 https://github.com/composewell/streamly/tree/ghc-8.2.2-bug2 . The last
 commit on this branch disables the culprit code.

 The "filter" API code that is being used in these cases is defined in
 https://github.com/composewell/streamly/blob/ghc-8.2.2-bug2/src/Streamly/Str...
 . The code goes like this:

 {{{
 {-# INLINE_NORMAL filterM #-}
 filterM :: Monad m => (a -> m Bool) -> Stream m a -> Stream m a
 filterM f (Stream step state) = Stream step' state
   where
     {-# INLINE_LATE step' #-}
     step' gst st = do
         r <- step (rstState gst) st
         case r of
             Yield x s -> do
                 b <- f x
                 if b
                 then return $ Yield x s
                 else step' gst s
             Stop -> return $ Stop

 {-# INLINE filter #-}
 filter :: Monad m => (a -> Bool) -> Stream m a -> Stream m a
 filter f = filterM (return . f)
 }}}

 This is very much like the vector code, except that there is no Skip
 constructor. I was originally thinking that it may have something to do
 with the join point optimization. I hope this will shed some light on the
 issue. I am not seeing the issue with GHC-8.4.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13535#comment:48
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