[Haskell-cafe] Bad interaction of inlinePerformIO and mutable vectors

[Felipe Lessa]

Checking a bit deeper using:

  $ ghc-core --no-cast --no-asm snoyberg.hs -O1 -ddump-simpl \
             -dverbose-core2core -dcore-lint

This is the last time we see the "inlinePerformIO" function call (before
it being optimized away):

lvl_s2R1 =
  \ (vm_a2gL [OS=ProbOneShot]
       :: Data.Vector.Mutable.MVector
            RealWorld Char) ->
    thenIO
      @ ()
      @ ()
      lvl_s2QY
      (let {
         a_s2QN
           :: State# RealWorld
              -> (# State# RealWorld, () #)

         a_s2QN =
           \ (eta_Xm [OS=OneShot] :: State# RealWorld) ->
             ((bindIO
                 @ (Data.Vector.Vector Char)
                 @ ()
                 (Data.Vector.unsafeFreeze
                    @ IO
                    @ Char
                    Control.Monad.Primitive.$fPrimMonadIO
                    (vm_a2gL `cast` ...))
                 lvl_s2QV)
              `cast` ...)
               eta_Xm } in
       thenIO
         @ ()
         @ ()
         (a_s2QN `cast` ...)
         (case $
                 @ (IO ())
                 @ ()
                 (Data.ByteString.Internal.inlinePerformIO @ ())
                 (Data.Vector.Mutable.write
                    @ IO
                    @ Char
                    Control.Monad.Primitive.$fPrimMonadIO
                    (vm_a2gL `cast` ...)
                    lvl_s2QZ
                    lvl_s2R0)
          of _ [Occ=Dead] { () ->
          a_s2QN `cast` ...
          }))

lvl_s2QV is the action that prints the array contents.  The above
snippet prints the contents, writes the new value then prints the
contents again.

But then comes a simplifier phase which lead us to:

    ...
    of _ [Occ=Dead] { (# ipv_X3cp [OS=OneShot], ipv1_X3cr #) ->
    let {
      a_s2QN
        :: State# RealWorld
           -> (# State# RealWorld, () #)
      [LclId,
       Arity=1,

       Unf=Unf{Src=<vanilla>, TopLvl=False, Arity=1, Value=True,
               ConLike=True, WorkFree=True, Expandable=True,
               Guidance=IF_ARGS [0] 91 0}]
      a_s2QN =
        \ (eta_Xm [OS=OneShot] :: State# RealWorld) ->
          case unsafeFreezeArray#
                 @ (Control.Monad.Primitive.PrimState IO)
                 @ Char
                 ipv1_a2NI
                 (eta_Xm `cast` ...)
          of _ [Occ=Dead] { (# ipv_a2Oy [OS=OneShot], ipv1_a2Oz #) ->
          Handle.Text.hPutStr2
            Handle.FD.stdout
            (Data.Vector.$fShowVector_$cshow
               @ Char
               $fShowChar
               (Data.Vector.Vector @ Char 0 1 ipv1_a2Oz))
            True
            (ipv_a2Oy `cast` ...)
          } } in
    case a_s2QN ipv_X3cp
    of _ [Occ=Dead] { (# ipv_X3cv [OS=OneShot], ipv1_X3dt #) ->
    a_s2QN ipv_X3cv
    ...

Note that a_s2QN has changed due to inlining but still performs the same
action.  However!  ipv_X3cp is *not* the same as the big case with our
inlinePerformIO, it's merely the resulting RealWorld from printing
"inlinePerformIO"!  This is the spot.

Now I'm stuck, though.  I have no idea why the simplifier did this.

Cheers!

-- 
Felipe.

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