[GHC] #12566: Memory leak

3 Sep 2016

      #12566: Memory leak
-------------------------------------+-------------------------------------
           Reporter:  igloo          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 With the attached write.hs (which generates a file called "data") and
 read.hs (which consumes that file), if I run

 {{{
 ghc -Wall -Werror -O write.hs -o write
 ghc -Wall -Werror -O -prof -auto-all read.hs -o read1
 ghc -Wall -Werror -O -prof -auto-all read.hs -o read2 -DLEAK
 ./write
 ./read1 +RTS -h
 ./read2 +RTS -h
 }}}

 then read2's heap profile shows that it is retaining a lot of extra data.
 Perhaps I am missing something, but I can't see why this needs to be
 retained. I would expect the two heap profiles to look the same.

 Sources and heap profiles (using GHC 8.0.1) attached. I've copied the
 sources below for convenience:

 write.hs:
 {{{#!hs
 module Main (main) where

 import qualified Data.ByteString.Lazy.Char8 as L

 main :: IO ()
 main = L.writeFile "data" $ L.concat $ map mkByteString [1..100000]

 mkByteString :: Int -> L.ByteString
 mkByteString i = L.concat (L.pack ("#" ++ show i ++ "\n")
                : replicate 100 (L.pack "Something else\n"))
 }}}

 read.hs:
 {{{#!hs
 {-# LANGUAGE BangPatterns, CPP #-}

 module Main (main) where

 import Data.List
 import Data.Set (Set)
 import qualified Data.Set as Set

 import qualified Data.ByteString.Char8 as S
 import qualified Data.ByteString.Lazy.Char8 as L

 main :: IO ()
 main = do bs <- L.readFile "data"
           let stats = getMaybes bs
               s = mkSet stats
           print $ Set.size s

 mkSet :: [Maybe S.ByteString] -> Set S.ByteString
 mkSet ms = foldl' f Set.empty ms
     where f s (Just l) = Set.insert l s
           f s _ = s

 getMaybes :: L.ByteString -> [Maybe S.ByteString]
 getMaybes bs = if L.null bs then []
                else case getMaybe bs of
                     (stat, bs') ->
                         stat : getMaybes bs'

 getMaybe :: L.ByteString -> (Maybe S.ByteString, L.ByteString)
 getMaybe bs = case L.uncons bs of
               Just ('#', bs') ->
                   case L.break ('\n' ==) bs' of
                   (l, bs'') ->
                       let !l' = copy l
                       in (Just l', bs'')
               _ ->
                   case L.break ('\n' ==) bs of
                   (_x, bs') ->
 #ifdef LEAK
                       copy _x `seq`
 #endif
                       (Nothing, L.tail bs')

 copy :: L.ByteString -> S.ByteString
 copy bs = S.copy $ L.toStrict bs
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12566
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