Re: [GHC] #13615: Nondeterminism in ‘pure’ function w/ parallel evaluation & memo combinators

26 Apr 2017

      #13615: Nondeterminism in ‘pure’ function w/ parallel evaluation & memo combinators
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        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:                    |
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Comment (by bgamari):

 While playing around with the repro, I noticed that the issue is
 reproducible if the `unsafeUpdateM` is performed, even if the resulting
 `HashMap` doesn't refer to the old array. e.g.,
 {{{#!hs
     go h k x s t@(BitmapIndexed b ary)
         | b .&. m == 0 = do
             let ary' = A.insert ary i $! leaf h k x
             return $! bitmapIndexedOrFull (b .|. m) ary'
         | otherwise = do
             st <- A.indexM ary i
             st' <- rnf x `seq` go h k x (s+bitsPerSubkey) st
             let !ary' = A.update ary i $! st'
             A.unsafeUpdateM ary i st'
             return $ BitmapIndexed b ary'
       where m = mask h s
             i = sparseIndex b m
 }}}

 I'm not yet sure whether this is a meaninfgul observation, but it is an
 observation nonetheless.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13615#comment:10
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