path: root/compiler/GHC/StgToCmm/Foreign.hs

-----------------------------------------------------------------------------
--
-- Code generation for foreign calls.
--
-- (c) The University of Glasgow 2004-2006
--
-----------------------------------------------------------------------------

module GHC.StgToCmm.Foreign (
  cgForeignCall,
  emitPrimCall, emitCCall,
  emitForeignCall,
  emitSaveThreadState,
  saveThreadState,
  emitLoadThreadState,
  loadThreadState,
  emitOpenNursery,
  emitCloseNursery,
 ) where

import GhcPrelude hiding( succ, (<*>) )

import GHC.Stg.Syntax
import GHC.StgToCmm.Prof (storeCurCCS, ccsType)
import GHC.StgToCmm.Env
import GHC.StgToCmm.Monad
import GHC.StgToCmm.Utils
import GHC.StgToCmm.Closure
import GHC.StgToCmm.Layout

import GHC.Cmm.BlockId (newBlockId)
import GHC.Cmm
import GHC.Cmm.Utils
import GHC.Cmm.Graph
import GHC.Core.Type
import GHC.Types.RepType
import GHC.Cmm.CLabel
import GHC.Runtime.Heap.Layout
import ForeignCall
import GHC.Driver.Session
import GHC.Platform
import Maybes
import Outputable
import UniqSupply
import BasicTypes

import GHC.Core.TyCo.Rep
import TysPrim
import Util (zipEqual)

import Control.Monad

-----------------------------------------------------------------------------
-- Code generation for Foreign Calls
-----------------------------------------------------------------------------

-- | Emit code for a foreign call, and return the results to the sequel.
-- Precondition: the length of the arguments list is the same as the
-- arity of the foreign function.
cgForeignCall :: ForeignCall            -- the op
              -> Type                   -- type of foreign function
              -> [StgArg]               -- x,y    arguments
              -> Type                   -- result type
              -> FCode ReturnKind

cgForeignCall (CCall (CCallSpec target cconv safety)) typ stg_args res_ty
  = do  { platform <- getPlatform
        ; let -- in the stdcall calling convention, the symbol needs @size appended
              -- to it, where size is the total number of bytes of arguments.  We
              -- attach this info to the CLabel here, and the CLabel pretty printer
              -- will generate the suffix when the label is printed.
            call_size args
              | StdCallConv <- cconv = Just (sum (map arg_size args))
              | otherwise            = Nothing

              -- ToDo: this might not be correct for 64-bit API
            arg_size (arg, _) = max (widthInBytes $ typeWidth $ cmmExprType platform arg)
                                     (platformWordSizeInBytes platform)
        ; cmm_args <- getFCallArgs stg_args typ
        ; (res_regs, res_hints) <- newUnboxedTupleRegs res_ty
        ; let ((call_args, arg_hints), cmm_target)
                = case target of
                   StaticTarget _ _   _      False ->
                       panic "cgForeignCall: unexpected FFI value import"
                   StaticTarget _ lbl mPkgId True
                     -> let labelSource
                                = case mPkgId of
                                        Nothing         -> ForeignLabelInThisPackage
                                        Just pkgId      -> ForeignLabelInPackage pkgId
                            size = call_size cmm_args
                        in  ( unzip cmm_args
                            , CmmLit (CmmLabel
                                        (mkForeignLabel lbl size labelSource IsFunction)))

                   DynamicTarget    ->  case cmm_args of
                                           (fn,_):rest -> (unzip rest, fn)
                                           [] -> panic "cgForeignCall []"
              fc = ForeignConvention cconv arg_hints res_hints CmmMayReturn
              call_target = ForeignTarget cmm_target fc

        -- we want to emit code for the call, and then emitReturn.
        -- However, if the sequel is AssignTo, we shortcut a little
        -- and generate a foreign call that assigns the results
        -- directly.  Otherwise we end up generating a bunch of
        -- useless "r = r" assignments, which are not merely annoying:
        -- they prevent the common block elimination from working correctly
        -- in the case of a safe foreign call.
        -- See Note [safe foreign call convention]
        --
        ; sequel <- getSequel
        ; case sequel of
            AssignTo assign_to_these _ ->
                emitForeignCall safety assign_to_these call_target call_args

            _something_else ->
                do { _ <- emitForeignCall safety res_regs call_target call_args
                   ; emitReturn (map (CmmReg . CmmLocal) res_regs)
                   }
         }

{- Note [safe foreign call convention]

The simple thing to do for a safe foreign call would be the same as an
unsafe one: just

    emitForeignCall ...
    emitReturn ...

but consider what happens in this case

   case foo x y z of
     (# s, r #) -> ...

The sequel is AssignTo [r].  The call to newUnboxedTupleRegs picks [r]
as the result reg, and we generate

  r = foo(x,y,z) returns to L1  -- emitForeignCall
 L1:
  r = r  -- emitReturn
  goto L2
L2:
  ...

Now L1 is a proc point (by definition, it is the continuation of the
safe foreign call).  If L2 does a heap check, then L2 will also be a
proc point.

Furthermore, the stack layout algorithm has to arrange to save r
somewhere between the call and the jump to L1, which is annoying: we
would have to treat r differently from the other live variables, which
have to be saved *before* the call.

So we adopt a special convention for safe foreign calls: the results
are copied out according to the NativeReturn convention by the call,
and the continuation of the call should copyIn the results.  (The
copyOut code is actually inserted when the safe foreign call is
lowered later).  The result regs attached to the safe foreign call are
only used temporarily to hold the results before they are copied out.

We will now generate this:

  r = foo(x,y,z) returns to L1
 L1:
  r = R1  -- copyIn, inserted by mkSafeCall
  goto L2
 L2:
  ... r ...

And when the safe foreign call is lowered later (see Note [lower safe
foreign calls]) we get this:

  suspendThread()
  r = foo(x,y,z)
  resumeThread()
  R1 = r  -- copyOut, inserted by lowerSafeForeignCall
  jump L1
 L1:
  r = R1  -- copyIn, inserted by mkSafeCall
  goto L2
 L2:
  ... r ...

Now consider what happens if L2 does a heap check: the Adams
optimisation kicks in and commons up L1 with the heap-check
continuation, resulting in just one proc point instead of two. Yay!
-}


emitCCall :: [(CmmFormal,ForeignHint)]
          -> CmmExpr
          -> [(CmmActual,ForeignHint)]
          -> FCode ()
emitCCall hinted_results fn hinted_args
  = void $ emitForeignCall PlayRisky results target args
  where
    (args, arg_hints) = unzip hinted_args
    (results, result_hints) = unzip hinted_results
    target = ForeignTarget fn fc
    fc = ForeignConvention CCallConv arg_hints result_hints CmmMayReturn


emitPrimCall :: [CmmFormal] -> CallishMachOp -> [CmmActual] -> FCode ()
emitPrimCall res op args
  = void $ emitForeignCall PlayRisky res (PrimTarget op) args

-- alternative entry point, used by GHC.Cmm.Parser
emitForeignCall
        :: Safety
        -> [CmmFormal]          -- where to put the results
        -> ForeignTarget        -- the op
        -> [CmmActual]          -- arguments
        -> FCode ReturnKind
emitForeignCall safety results target args
  | not (playSafe safety) = do
    dflags <- getDynFlags
    let (caller_save, caller_load) = callerSaveVolatileRegs dflags
    emit caller_save
    target' <- load_target_into_temp target
    args' <- mapM maybe_assign_temp args
    emit $ mkUnsafeCall target' results args'
    emit caller_load
    return AssignedDirectly

  | otherwise = do
    dflags <- getDynFlags
    platform <- getPlatform
    updfr_off <- getUpdFrameOff
    target' <- load_target_into_temp target
    args' <- mapM maybe_assign_temp args
    k <- newBlockId
    let (off, _, copyout) = copyInOflow dflags NativeReturn (Young k) results []
       -- see Note [safe foreign call convention]
    tscope <- getTickScope
    emit $
           (    mkStore (CmmStackSlot (Young k) (widthInBytes (wordWidth platform)))
                        (CmmLit (CmmBlock k))
            <*> mkLast (CmmForeignCall { tgt  = target'
                                       , res  = results
                                       , args = args'
                                       , succ = k
                                       , ret_args = off
                                       , ret_off = updfr_off
                                       , intrbl = playInterruptible safety })
            <*> mkLabel k tscope
            <*> copyout
           )
    return (ReturnedTo k off)

load_target_into_temp :: ForeignTarget -> FCode ForeignTarget
load_target_into_temp (ForeignTarget expr conv) = do
  tmp <- maybe_assign_temp expr
  return (ForeignTarget tmp conv)
load_target_into_temp other_target@(PrimTarget _) =
  return other_target

-- What we want to do here is create a new temporary for the foreign
-- call argument if it is not safe to use the expression directly,
-- because the expression mentions caller-saves GlobalRegs (see
-- Note [Register Parameter Passing]).
--
-- However, we can't pattern-match on the expression here, because
-- this is used in a loop by GHC.Cmm.Parser, and testing the expression
-- results in a black hole.  So we always create a temporary, and rely
-- on GHC.Cmm.Sink to clean it up later.  (Yuck, ToDo).  The generated code
-- ends up being the same, at least for the RTS .cmm code.
--
maybe_assign_temp :: CmmExpr -> FCode CmmExpr
maybe_assign_temp e = do
  platform <- getPlatform
  reg <- newTemp (cmmExprType platform e)
  emitAssign (CmmLocal reg) e
  return (CmmReg (CmmLocal reg))

-- -----------------------------------------------------------------------------
-- Save/restore the thread state in the TSO

-- This stuff can't be done in suspendThread/resumeThread, because it
-- refers to global registers which aren't available in the C world.

emitSaveThreadState :: FCode ()
emitSaveThreadState = do
  dflags <- getDynFlags
  code <- saveThreadState dflags
  emit code

-- | Produce code to save the current thread state to @CurrentTSO@
saveThreadState :: MonadUnique m => DynFlags -> m CmmAGraph
saveThreadState dflags = do
  let platform = targetPlatform dflags
  tso <- newTemp (gcWord platform)
  close_nursery <- closeNursery dflags tso
  pure $ catAGraphs [
    -- tso = CurrentTSO;
    mkAssign (CmmLocal tso) currentTSOExpr,
    -- tso->stackobj->sp = Sp;
    mkStore (cmmOffset platform
                       (CmmLoad (cmmOffset platform
                                           (CmmReg (CmmLocal tso))
                                           (tso_stackobj dflags))
                                (bWord platform))
                       (stack_SP dflags))
            spExpr,
    close_nursery,
    -- and save the current cost centre stack in the TSO when profiling:
    if gopt Opt_SccProfilingOn dflags then
        mkStore (cmmOffset platform (CmmReg (CmmLocal tso)) (tso_CCCS dflags)) cccsExpr
      else mkNop
    ]

emitCloseNursery :: FCode ()
emitCloseNursery = do
  dflags <- getDynFlags
  platform <- getPlatform
  tso <- newTemp (bWord platform)
  code <- closeNursery dflags tso
  emit $ mkAssign (CmmLocal tso) currentTSOExpr <*> code

{- |
@closeNursery dflags tso@ produces code to close the nursery.
A local register holding the value of @CurrentTSO@ is expected for
efficiency.

Closing the nursery corresponds to the following code:

@
  tso = CurrentTSO;
  cn = CurrentNuresry;

  // Update the allocation limit for the current thread.  We don't
  // check to see whether it has overflowed at this point, that check is
  // made when we run out of space in the current heap block (stg_gc_noregs)
  // and in the scheduler when context switching (schedulePostRunThread).
  tso->alloc_limit -= Hp + WDS(1) - cn->start;

  // Set cn->free to the next unoccupied word in the block
  cn->free = Hp + WDS(1);
@
-}
closeNursery :: MonadUnique m => DynFlags -> LocalReg -> m CmmAGraph
closeNursery df tso = do
  let tsoreg  = CmmLocal tso
      platform = targetPlatform df
  cnreg      <- CmmLocal <$> newTemp (bWord platform)
  pure $ catAGraphs [
    mkAssign cnreg currentNurseryExpr,

    -- CurrentNursery->free = Hp+1;
    mkStore (nursery_bdescr_free df cnreg) (cmmOffsetW platform hpExpr 1),

    let alloc =
           CmmMachOp (mo_wordSub platform)
              [ cmmOffsetW platform hpExpr 1
              , CmmLoad (nursery_bdescr_start df cnreg) (bWord platform)
              ]

        alloc_limit = cmmOffset platform (CmmReg tsoreg) (tso_alloc_limit df)
    in

    -- tso->alloc_limit += alloc
    mkStore alloc_limit (CmmMachOp (MO_Sub W64)
                               [ CmmLoad alloc_limit b64
                               , CmmMachOp (mo_WordTo64 platform) [alloc] ])
   ]

emitLoadThreadState :: FCode ()
emitLoadThreadState = do
  dflags <- getDynFlags
  code <- loadThreadState dflags
  emit code

-- | Produce code to load the current thread state from @CurrentTSO@
loadThreadState :: MonadUnique m => DynFlags -> m CmmAGraph
loadThreadState dflags = do
  let platform = targetPlatform dflags
  tso <- newTemp (gcWord platform)
  stack <- newTemp (gcWord platform)
  open_nursery <- openNursery dflags tso
  pure $ catAGraphs [
    -- tso = CurrentTSO;
    mkAssign (CmmLocal tso) currentTSOExpr,
    -- stack = tso->stackobj;
    mkAssign (CmmLocal stack) (CmmLoad (cmmOffset platform (CmmReg (CmmLocal tso)) (tso_stackobj dflags)) (bWord platform)),
    -- Sp = stack->sp;
    mkAssign spReg (CmmLoad (cmmOffset platform (CmmReg (CmmLocal stack)) (stack_SP dflags)) (bWord platform)),
    -- SpLim = stack->stack + RESERVED_STACK_WORDS;
    mkAssign spLimReg (cmmOffsetW platform (cmmOffset platform (CmmReg (CmmLocal stack)) (stack_STACK dflags))
                                (rESERVED_STACK_WORDS dflags)),
    -- HpAlloc = 0;
    --   HpAlloc is assumed to be set to non-zero only by a failed
    --   a heap check, see HeapStackCheck.cmm:GC_GENERIC
    mkAssign hpAllocReg (zeroExpr platform),
    open_nursery,
    -- and load the current cost centre stack from the TSO when profiling:
    if gopt Opt_SccProfilingOn dflags
       then storeCurCCS
              (CmmLoad (cmmOffset platform (CmmReg (CmmLocal tso))
                 (tso_CCCS dflags)) (ccsType platform))
       else mkNop
   ]


emitOpenNursery :: FCode ()
emitOpenNursery = do
  dflags <- getDynFlags
  platform <- getPlatform
  tso <- newTemp (bWord platform)
  code <- openNursery dflags tso
  emit $ mkAssign (CmmLocal tso) currentTSOExpr <*> code

{- |
@openNursery dflags tso@ produces code to open the nursery. A local register
holding the value of @CurrentTSO@ is expected for efficiency.

Opening the nursery corresponds to the following code:

@
   tso = CurrentTSO;
   cn = CurrentNursery;
   bdfree = CurrentNursery->free;
   bdstart = CurrentNursery->start;

   // We *add* the currently occupied portion of the nursery block to
   // the allocation limit, because we will subtract it again in
   // closeNursery.
   tso->alloc_limit += bdfree - bdstart;

   // Set Hp to the last occupied word of the heap block.  Why not the
   // next unocupied word?  Doing it this way means that we get to use
   // an offset of zero more often, which might lead to slightly smaller
   // code on some architectures.
   Hp = bdfree - WDS(1);

   // Set HpLim to the end of the current nursery block (note that this block
   // might be a block group, consisting of several adjacent blocks.
   HpLim = bdstart + CurrentNursery->blocks*BLOCK_SIZE_W - 1;
@
-}
openNursery :: MonadUnique m => DynFlags -> LocalReg -> m CmmAGraph
openNursery dflags tso = do
  let tsoreg =  CmmLocal tso
      platform = targetPlatform dflags
  cnreg      <- CmmLocal <$> newTemp (bWord platform)
  bdfreereg  <- CmmLocal <$> newTemp (bWord platform)
  bdstartreg <- CmmLocal <$> newTemp (bWord platform)

  -- These assignments are carefully ordered to reduce register
  -- pressure and generate not completely awful code on x86.  To see
  -- what code we generate, look at the assembly for
  -- stg_returnToStackTop in rts/StgStartup.cmm.
  pure $ catAGraphs [
     mkAssign cnreg currentNurseryExpr,
     mkAssign bdfreereg  (CmmLoad (nursery_bdescr_free dflags cnreg)  (bWord platform)),

     -- Hp = CurrentNursery->free - 1;
     mkAssign hpReg (cmmOffsetW platform (CmmReg bdfreereg) (-1)),

     mkAssign bdstartreg (CmmLoad (nursery_bdescr_start dflags cnreg) (bWord platform)),

     -- HpLim = CurrentNursery->start +
     --              CurrentNursery->blocks*BLOCK_SIZE_W - 1;
     mkAssign hpLimReg
         (cmmOffsetExpr platform
             (CmmReg bdstartreg)
             (cmmOffset platform
               (CmmMachOp (mo_wordMul platform) [
                 CmmMachOp (MO_SS_Conv W32 (wordWidth platform))
                   [CmmLoad (nursery_bdescr_blocks dflags cnreg) b32],
                 mkIntExpr platform (bLOCK_SIZE dflags)
                ])
               (-1)
             )
         ),

     -- alloc = bd->free - bd->start
     let alloc =
           CmmMachOp (mo_wordSub platform) [CmmReg bdfreereg, CmmReg bdstartreg]

         alloc_limit = cmmOffset platform (CmmReg tsoreg) (tso_alloc_limit dflags)
     in

     -- tso->alloc_limit += alloc
     mkStore alloc_limit (CmmMachOp (MO_Add W64)
                               [ CmmLoad alloc_limit b64
                               , CmmMachOp (mo_WordTo64 platform) [alloc] ])

   ]

nursery_bdescr_free, nursery_bdescr_start, nursery_bdescr_blocks
  :: DynFlags -> CmmReg -> CmmExpr
nursery_bdescr_free   dflags cn =
  cmmOffset (targetPlatform dflags) (CmmReg cn) (oFFSET_bdescr_free dflags)
nursery_bdescr_start  dflags cn =
  cmmOffset (targetPlatform dflags) (CmmReg cn) (oFFSET_bdescr_start dflags)
nursery_bdescr_blocks dflags cn =
  cmmOffset (targetPlatform dflags) (CmmReg cn) (oFFSET_bdescr_blocks dflags)

tso_stackobj, tso_CCCS, tso_alloc_limit, stack_STACK, stack_SP :: DynFlags -> ByteOff
tso_stackobj dflags = closureField dflags (oFFSET_StgTSO_stackobj dflags)
tso_alloc_limit dflags = closureField dflags (oFFSET_StgTSO_alloc_limit dflags)
tso_CCCS     dflags = closureField dflags (oFFSET_StgTSO_cccs dflags)
stack_STACK  dflags = closureField dflags (oFFSET_StgStack_stack dflags)
stack_SP     dflags = closureField dflags (oFFSET_StgStack_sp dflags)


closureField :: DynFlags -> ByteOff -> ByteOff
closureField dflags off = off + fixedHdrSize dflags

-- Note [Unlifted boxed arguments to foreign calls]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
-- For certain types passed to foreign calls, we adjust the actual
-- value passed to the call.  For ByteArray#, Array#, SmallArray#,
-- and ArrayArray#, we pass the address of the array's payload, not
-- the address of the heap object. For example, consider
--   foreign import "c_foo" foo :: ByteArray# -> Int# -> IO ()
-- At a Haskell call like `foo x y`, we'll generate a C call that
-- is more like
--   c_foo( x+8, y )
-- where the "+8" takes the heap pointer (x :: ByteArray#) and moves
-- it past the header words of the ByteArray object to point directly
-- to the data inside the ByteArray#. (The exact offset depends
-- on the target architecture and on profiling) By contrast, (y :: Int#)
-- requires no such adjustment.
--
-- This adjustment is performed by 'add_shim'. The size of the
-- adjustment depends on the type of heap object. But
-- how can we determine that type? There are two available options.
-- We could use the types of the actual values that the foreign call
-- has been applied to, or we could use the types present in the
-- foreign function's type. Prior to GHC 8.10, we used the former
-- strategy since it's a little more simple. However, in issue #16650
-- and more compellingly in the comments of
-- https://gitlab.haskell.org/ghc/ghc/merge_requests/939, it was
-- demonstrated that this leads to bad behavior in the presence
-- of unsafeCoerce#. Returning to the above example, suppose the
-- Haskell call looked like
--   foo (unsafeCoerce# p)
-- where the types of expressions comprising the arguments are
--   p :: (Any :: TYPE 'UnliftedRep)
--   i :: Int#
-- so that the unsafe-coerce is between Any and ByteArray#.
-- These two types have the same kind (they are both represented by
-- a heap pointer) so no GC errors will occur if we do this unsafe coerce.
-- By the time this gets to the code generator the cast has been
-- discarded so we have
--   foo p y
-- But we *must* adjust the pointer to p by a ByteArray# shim,
-- *not* by an Any shim (the Any shim involves no offset at all).
--
-- To avoid this bad behavior, we adopt the second strategy: use
-- the types present in the foreign function's type.
-- In collectStgFArgTypes, we convert the foreign function's
-- type to a list of StgFArgType. Then, in add_shim, we interpret
-- these as numeric offsets.

getFCallArgs ::
     [StgArg]
  -> Type -- the type of the foreign function
  -> FCode [(CmmExpr, ForeignHint)]
-- (a) Drop void args
-- (b) Add foreign-call shim code
-- It's (b) that makes this differ from getNonVoidArgAmodes
-- Precondition: args and typs have the same length
-- See Note [Unlifted boxed arguments to foreign calls]
getFCallArgs args typ
  = do  { mb_cmms <- mapM get (zipEqual "getFCallArgs" args (collectStgFArgTypes typ))
        ; return (catMaybes mb_cmms) }
  where
    get (arg,typ)
      | null arg_reps
      = return Nothing
      | otherwise
      = do { cmm <- getArgAmode (NonVoid arg)
           ; dflags <- getDynFlags
           ; return (Just (add_shim dflags typ cmm, hint)) }
      where
        arg_ty   = stgArgType arg
        arg_reps = typePrimRep arg_ty
        hint     = typeForeignHint arg_ty

-- The minimum amount of information needed to determine
-- the offset to apply to an argument to a foreign call.
-- See Note [Unlifted boxed arguments to foreign calls]
data StgFArgType
  = StgPlainType
  | StgArrayType
  | StgSmallArrayType
  | StgByteArrayType

-- See Note [Unlifted boxed arguments to foreign calls]
add_shim :: DynFlags -> StgFArgType -> CmmExpr -> CmmExpr
add_shim dflags ty expr = case ty of
  StgPlainType -> expr
  StgArrayType -> cmmOffsetB platform expr (arrPtrsHdrSize dflags)
  StgSmallArrayType -> cmmOffsetB platform expr (smallArrPtrsHdrSize dflags)
  StgByteArrayType -> cmmOffsetB platform expr (arrWordsHdrSize dflags)
  where
    platform = targetPlatform dflags

-- From a function, extract information needed to determine
-- the offset of each argument when used as a C FFI argument.
-- See Note [Unlifted boxed arguments to foreign calls]
collectStgFArgTypes :: Type -> [StgFArgType]
collectStgFArgTypes = go []
  where
    -- Skip foralls
    go bs (ForAllTy _ res) = go bs res
    go bs (AppTy{}) = reverse bs
    go bs (TyConApp{}) = reverse bs
    go bs (LitTy{}) = reverse bs
    go bs (TyVarTy{}) = reverse bs
    go  _ (CastTy{}) = panic "myCollectTypeArgs: CastTy"
    go  _ (CoercionTy{}) = panic "myCollectTypeArgs: CoercionTy"
    go bs (FunTy {ft_arg = arg, ft_res=res}) =
      go (typeToStgFArgType arg:bs) res

-- Choose the offset based on the type. For anything other
-- than an unlifted boxed type, there is no offset.
-- See Note [Unlifted boxed arguments to foreign calls]
typeToStgFArgType :: Type -> StgFArgType
typeToStgFArgType typ
  | tycon == arrayPrimTyCon = StgArrayType
  | tycon == mutableArrayPrimTyCon = StgArrayType
  | tycon == arrayArrayPrimTyCon = StgArrayType
  | tycon == mutableArrayArrayPrimTyCon = StgArrayType
  | tycon == smallArrayPrimTyCon = StgSmallArrayType
  | tycon == smallMutableArrayPrimTyCon = StgSmallArrayType
  | tycon == byteArrayPrimTyCon = StgByteArrayType
  | tycon == mutableByteArrayPrimTyCon = StgByteArrayType
  | otherwise = StgPlainType
  where
  -- Should be a tycon app, since this is a foreign call. We look
  -- through newtypes so the offset does not change if a user replaces
  -- a type in a foreign function signature with a representationally
  -- equivalent newtype.
  tycon = tyConAppTyCon (unwrapType typ)