[GHC] #12656: ghci floats out constant despite -fno-cse, resulting in very unintuitive behaviour

3 Oct 2016


      #12656: ghci floats out constant despite -fno-cse, resulting in very unintuitive
behaviour
-------------------------------------+-------------------------------------
           Reporter:  nh2            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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:
-------------------------------------+-------------------------------------
 Consider this program using `Data.Vector.unsafeThaw` and `-fno-cse`:

 {{{
 {-# OPTIONS_GHC -fno-cse #-}

 import qualified Data.Vector as V
 import qualified Data.Vector.Mutable as VM

 main :: IO ()
 main = do
   foo 100000
   foo 100000

 foo :: Int -> IO ()
 foo n = do
   indexVector <- V.unsafeThaw $ V.generate n id

   x <- VM.read indexVector 5
   VM.write indexVector 5 (x * x)

   print x

 -- In GHCI, we get:
 --
 -- > :set -fno-cse
 --
 -- > foo 100000
 -- 5
 -- > foo 100000
 -- 5
 --
 -- > let f = foo 100000 in f >> f
 -- 5
 -- 25
 }}}

 I would expect that

 {{{
...
let f = foo 100000 in f >> f
 5
 5
 }}}
Shouldn't `-fno-cse` fix this?

 (Note: With `ghc` instead of `ghci`, we don't observe this behaviour.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12656
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