[GHC] #3207: readMutVar# is inlined/duplicated

[GHC]

#3207: readMutVar# is inlined/duplicated
-------------------------------+--------------------------------------------
  Reporter:  simonmar          |          Owner:                  
      Type:  bug               |         Status:  new             
  Priority:  normal            |      Milestone:  6.12.1          
 Component:  Compiler          |        Version:  6.10.2          
  Severity:  normal            |       Keywords:                  
Difficulty:  Unknown           |       Testcase:                  
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
-------------------------------+--------------------------------------------
 The following code:

 {{{
 module Main where

 import Control.Monad.ST.Lazy
 import Data.STRef.Lazy
 import Data.Array.ST
 import Int
 import Debug.Trace

 data Refs s = Refs
     { memory    :: STArray s Int8 Int8
     , pc        :: STRef s Int8
     }

 main :: IO ()
 main = do
     print $ runST m
   where
     m = do
       m <- newArray_ (0,30)
       p <- newSTRef 0
       let r = Refs m p
       writeArray m 0 0x4
       v <- readSTRef p
       modifySTRef p (+1)
       trace ("v: " ++ show v) $ return ()
       op <- readArray m v
       case trace ("v: " ++ show v) $ op of
           0x4 -> modifySTRef p (+100) -- should run this
           n   -> error ("should never match this: " ++ show n)
 }}}

 generates this output:

 {{{
 > ./st
 v: 0
 v: 1
 st: MArray: undefined array element
 }}}

 That is, the two instances of `trace (show v)` are printing different
 results, for the same v!

 Looking at the Core, the problem seems to be that a call to `readMutVar#`
 has been inlined and duplicated.  The `readMutVar#` primop doesn't have
 the `has_side_effects` flag set, and setting it (in `primops.txt.pp`)
 fixes it, but I'm not completely sure if that's the right fix.  If it is,
 then there are lots of other similar primops that also need that flag set.

 Bug report originally from a haskell-cafe post:
 [http://www.haskell.org/pipermail/haskell-cafe/2009-May/061010.html]

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3207>
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