[GHC] #12656: ghc floats out constant despite -fno-cse

[GHC]

#12656: ghc floats out constant despite -fno-cse
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        Reporter:  nh2               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 bitonic):

 Actually I spoke too soon: this doesn't work even with `NOINLINE`!

 {{{
 $ cat weird-float.hs
 import qualified Data.Vector as V
 import qualified Data.Vector.Mutable as VM

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

   let f = foo 100000 in f >> f

 {-# NOINLINE foo #-}
 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
 $ stack exec --resolver lts-7 --package vector -- ghc -fforce-recomp
 -package vector -O0 -fno-full-laziness -fno-cse weird-float.hs -o weird-
 float-lts-7
 Run from outside a project, using implicit global project config
 Using resolver: lts-7 specified on command line
 Selected resolver: lts-7.2
 [1 of 1] Compiling Main             ( weird-float.hs, weird-float.o )
 Linking weird-float-lts-7 ...
 $ ./weird-float-lts-7
 5
 5
 5
 25
 $ stack exec --resolver lts-7 --package vector -- ghc --version
 Run from outside a project, using implicit global project config
 Using resolver: lts-7 specified on command line
 Selected resolver: lts-7.2
 The Glorious Glasgow Haskell Compilation System, version 8.0.1
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12656#comment:9>
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