[GHC] #13615: Wrong results from what supposed to be pure function with presense of parallel evaluation

25 Apr 2017

      #13615: Wrong results from what supposed to be pure function with presense of
parallel evaluation
-------------------------------------+-------------------------------------
           Reporter:  pacak          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Mostly self contained sample - only depends on base, deepseq and locally
 provided hashable and unordered-containers. No unsafePtrEq involved, but
 unsafeThaw/Freese  is - affected function uses `foldl'` to insert stuff
 into hashmap - I don't think there's a way to obtain multiple references
 to the same map. Making arguments strict or rnf'ing them doesn't help.

 Issue is reproduceable with HEAD with -O0 but not with O2. in ghc 8.0.2
 and 7.10.1 it's reproduceable with -O2 as well.

 {{{
 sum (map snd xs) = map snd (toList $ fromListWith (+) xs)
 }}}

 ^ this statement fails to hold when xs contains stuff with memocombinators
 and parallel evaluation from evaluation strategies

 I found which inplace update needs to be replaced with copy and update to
 make  things work - bug is fixed after changing unsafeInsertWith,
 BitmapIndexed branch:

 {{{
 -            A.unsafeUpdateM ary i st'
 -            return t
 +            let  ary' = A.update ary i st'
 +            return $ BitmapIndexed b ary'
 }}}

 Steps:

 clone this:

 {{{
 https://github.com/pacak/cuddly-bassoon
 }}}

 {{{
 stack build
 }}}

 {{{
  ./.stack-work/install/x86_64-linux/ghc-8.0.2/8.0.2/bin/gamebooksolver-
 solvebook02
 }}}

 Last command will either finish successfully printing `"solving\n0.0"` or
 will die with `error` in `src/Solver.hs` `regroup` function.

 To reproduce the problem computer must have at least 3 CPUs, running with
 +RTS -N1 or -N2 "fixes" the problem.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13615
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler