[GHC] #16406: Array code works with GHC 8.4.4, hangs with GHC 8.6.4

7 Mar 2019

      #16406: Array code works with GHC 8.4.4, hangs with GHC 8.6.4
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           Reporter:  arybczak       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  libraries      |           Version:  8.6.4
  (other)                            |
           Keywords:  array          |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following:

 ```haskell
 import Data.Array.IArray
 import Data.Bits
 import Data.Word

 loop :: Array Int Word32
 loop = accumArray (curry snd) 0 (0,79) $ [(i, xxor i) | i <- [16..79]]
   where
     xxor :: Int -> Word32
     xxor i = loop ! (i-16) `xor` loop ! (i-3) `xor` loop ! (i-8) `xor`
 loop ! (i-14)
 ```

 When loaded into ghci-8.4.4, `loop` works fine:

 ```
 GHCi, version 8.4.4: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/unknown/.ghci
 λ> :l array.hs
 [1 of 1] Compiling Main             ( array.hs, interpreted )
 Ok, one module loaded.
 λ> loop
 array (0,79)
 [(0,0),(1,0),(2,0),(3,0),(4,0),(5,0),(6,0),(7,0),(8,0),(9,0),(10,0),(11,0),(12,0),(13,0),(14,0),(15,0),(16,0),(17,0),(18,0),(19,0),(20,0),(21,0),(22,0),(23,0),(24,0),(25,0),(26,0),(27,0),(28,0),(29,0),(30,0),(31,0),(32,0),(33,0),(34,0),(35,0),(36,0),(37,0),(38,0),(39,0),(40,0),(41,0),(42,0),(43,0),(44,0),(45,0),(46,0),(47,0),(48,0),(49,0),(50,0),(51,0),(52,0),(53,0),(54,0),(55,0),(56,0),(57,0),(58,0),(59,0),(60,0),(61,0),(62,0),(63,0),(64,0),(65,0),(66,0),(67,0),(68,0),(69,0),(70,0),(71,0),(72,0),(73,0),(74,0),(75,0),(76,0),(77,0),(78,0),(79,0)]
 ```

 However, when loaded into ghci-8.6.4, it hangs (with no CPU utilization):
 ```
 GHCi, version 8.6.4: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/unknown/.ghci
 λ> :l array.hs
 [1 of 1] Compiling Main             ( array.hs, interpreted )
 Ok, one module loaded.
 λ> loop
 array
 ```

 The definition of loop is a piece of code taken from Crypto library
 (https://hackage.haskell.org/package/Crypto-4.2.5.1/docs/src/Data-Digest-
 SHA1.html, `oneBlock` function)

 Granted, this library is pretty much dead (last update in 2012), but it's
 a bit suspect that code suddenly stops working in the worst way possible.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16406
GHC http://www.haskell.org/ghc/
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