#13615: Wrong results from what supposed to be pure function with presense of parallel evaluation -------------------------------------+------------------------------------- Reporter: pacak | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler | Version: 8.2.1-rc2 Resolution: | 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: | -------------------------------------+------------------------------------- Description changed by pacak: @@ -0,0 +1,6 @@ + The program linked below uses modified (mostly to trim down on + dependencies) copies of parallel and unordered-containers. Its behavior is + nondeterministic, even though all the operations it uses are supposed to + be pure + + @@ -4,2 +10,9 @@ - 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. + into hashmap. + + + unsafeThaw/Freeze are used to perform implace updates on a HashMap inside + fromListWith function. Updates are sequential and performed with `foldl'`, + I don't see obvious ways of obtaining references to intermediate versions + of HashMap so inplace updates seems to be valid. + + Making arguments to unsafeInsertWith strict or rnf'ing them doesn't help. @@ -11,1 +24,1 @@ - sum (map snd xs) = map snd (toList $ fromListWith (+) xs) + sum (map snd xs) = sum (map snd (toList $ fromListWith (+) xs)) New description: The program linked below uses modified (mostly to trim down on dependencies) copies of parallel and unordered-containers. Its behavior is nondeterministic, even though all the operations it uses are supposed to be pure 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. unsafeThaw/Freeze are used to perform implace updates on a HashMap inside fromListWith function. Updates are sequential and performed with `foldl'`, I don't see obvious ways of obtaining references to intermediate versions of HashMap so inplace updates seems to be valid. Making arguments to unsafeInsertWith 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) = sum (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#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler