[GHC] #8852: 7.8.1 uses a lot of memory when compiling attoparsec programs using <|>

6 Mar 2014

      #8852: 7.8.1 uses a lot of memory when compiling attoparsec programs using <|>
-------------------------+-------------------------------------------------
       Reporter:         |             Owner:
  joelteon               |            Status:  new
           Type:  bug    |         Milestone:
       Priority:         |           Version:  7.8.1-rc2
  normal                 |  Operating System:  Unknown/Multiple
      Component:         |   Type of failure:  Compile-time performance bug
  Compiler               |         Test Case:
       Keywords:         |          Blocking:
   Architecture:         |
  Unknown/Multiple       |
     Difficulty:         |
  Unknown                |
     Blocked By:         |
Related Tickets:         |
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 To reproduce, install a pre-`0.11.2.1` version of attoparsec. This bug was
 worked around in 0.11.2.1 by removing the `INLINE` on `plus` in
 attoparsec.

 With this test program:

 {{{#!haskell
 {-# LANGUAGE OverloadedStrings #-}

 import Control.Applicative
 import Data.Attoparsec.Text
 import Data.Text (Text)

 parser :: Parser Text
 parser = string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"
      <|> string "a" <|> string "a"

 main :: IO ()
 main = parseTest parser "a"
 }}}

 and using GHC 7.8.1.rc2:

 Compiling using `-O2`, GHC tops out at ~1GB of RAM and takes 25s.

 Using `-O0`, GHC takes 0.47s and uses <150MB of RAM.

 Compare this with GHC 7.6.3:

 Compiling using `-O2`, GHC uses <150MB and takes 3.7s. Memory usage is
 similar with `-O0` although compile time goes down to 0.36s.

 An extreme version of this bug can be found in the `thyme` package here:
 https://github.com/liyang/thyme/blob/master/src/Data/Thyme/Format.hs#L589-L6....
 Compiling that module with an unfixed attoparsec makes GHC use all
 available memory and stall out, forcing kill -9. Replacing the function
 body with `undefined` makes the package compile as expected.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8852
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