GHC/HsToCore/Foreign/Call.hs

    {-
    (c) The University of Glasgow 2006
    (c) The AQUA Project, Glasgow University, 1994-1998

    Desugaring foreign calls
    -}

    {-# LANGUAGE CPP #-}

    {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

    module GHC.HsToCore.Foreign.Call
       ( dsCCall
       , mkFCall
       , unboxArg
       , boxResult
       , resultWrapper
       )
    where

    #include "HsVersions.h"

    import GHC.Prelude

    import GHC.Core

    import GHC.HsToCore.Monad
    import GHC.Core.Utils
    import GHC.Core.Make
    import GHC.Types.SourceText
    import GHC.Types.Id.Make
    import GHC.Types.ForeignCall
    import GHC.Core.DataCon
    import GHC.HsToCore.Utils

    import GHC.Tc.Utils.TcType
    import GHC.Core.Type
    import GHC.Core.Multiplicity
    import GHC.Core.Coercion
    import GHC.Builtin.Types.Prim
    import GHC.Core.TyCon
    import GHC.Builtin.Types
    import GHC.Types.Basic
    import GHC.Types.Literal
    import GHC.Builtin.Names
    import GHC.Driver.Session
    import GHC.Utils.Outputable
    import GHC.Utils.Misc
    import GHC.Utils.Panic

    import Data.Maybe

    {-
    Desugaring of @ccall@s consists of adding some state manipulation,
    unboxing any boxed primitive arguments and boxing the result if
    desired.

    The state stuff just consists of adding in
    @PrimIO (\ s -> case s of { State# s# -> ... })@ in an appropriate place.

    The unboxing is straightforward, as all information needed to unbox is
    available from the type.  For each boxed-primitive argument, we
    transform:
    \begin{verbatim}
       _ccall_ foo [ r, t1, ... tm ] e1 ... em
       |
       |
       V
       case e1 of { T1# x1# ->
       ...
       case em of { Tm# xm# -> xm#
       ccall# foo [ r, t1#, ... tm# ] x1# ... xm#
       } ... }
    \end{verbatim}

    The reboxing of a @_ccall_@ result is a bit tricker: the types don't
    contain information about the state-pairing functions so we have to
    keep a list of \tr{(type, s-p-function)} pairs.  We transform as
    follows:
    \begin{verbatim}
       ccall# foo [ r, t1#, ... tm# ] e1# ... em#
       |
       |
       V
       \ s# -> case (ccall# foo [ r, t1#, ... tm# ] s# e1# ... em#) of
              (StateAnd<r># result# state#) -> (R# result#, realWorld#)
    \end{verbatim}
    -}

    dsCCall :: CLabelString -- C routine to invoke
            -> [CoreExpr]   -- Arguments (desugared)
                            -- Precondition: none have levity-polymorphic types
            -> Safety       -- Safety of the call
            -> Type         -- Type of the result: IO t
            -> DsM CoreExpr -- Result, of type ???

    dsCCall lbl args may_gc result_ty
      = do (unboxed_args, arg_wrappers) <- mapAndUnzipM unboxArg args
           (ccall_result_ty, res_wrapper) <- boxResult result_ty
           uniq <- newUnique
           dflags <- getDynFlags
           let
               target = StaticTarget NoSourceText lbl Nothing True
               the_fcall    = CCall (CCallSpec target CCallConv may_gc)
               the_prim_app = mkFCall dflags uniq the_fcall unboxed_args ccall_result_ty
           return (foldr ($) (res_wrapper the_prim_app) arg_wrappers)

    mkFCall :: DynFlags -> Unique -> ForeignCall
            -> [CoreExpr]     -- Args
            -> Type           -- Result type
            -> CoreExpr
    -- Construct the ccall.  The only tricky bit is that the ccall Id should have
    -- no free vars, so if any of the arg tys do we must give it a polymorphic type.
    --      [I forget *why* it should have no free vars!]
    -- For example:
    --      mkCCall ... [s::StablePtr (a->b), x::Addr, c::Char]
    --
    -- Here we build a ccall thus
    --      (ccallid::(forall a b.  StablePtr (a -> b) -> Addr -> Char -> IO Addr))
    --                      a b s x c
    mkFCall dflags uniq the_fcall val_args res_ty
      = ASSERT( all isTyVar tyvars )  -- this must be true because the type is top-level
        mkApps (mkVarApps (Var the_fcall_id) tyvars) val_args
      where
        arg_tys = map exprType val_args
        body_ty = (mkVisFunTysMany arg_tys res_ty)
        tyvars  = tyCoVarsOfTypeWellScoped body_ty
        ty      = mkInfForAllTys tyvars body_ty
        the_fcall_id = mkFCallId dflags uniq the_fcall ty

    unboxArg :: CoreExpr                    -- The supplied argument, not levity-polymorphic
             -> DsM (CoreExpr,              -- To pass as the actual argument
                     CoreExpr -> CoreExpr   -- Wrapper to unbox the arg
                    )
    -- Example: if the arg is e::Int, unboxArg will return
    --      (x#::Int#, \W. case x of I# x# -> W)
    -- where W is a CoreExpr that probably mentions x#

    -- always returns a non-levity-polymorphic expression

    unboxArg arg
      -- Primitive types: nothing to unbox
      | isPrimitiveType arg_ty
      = return (arg, \body -> body)

      -- Recursive newtypes
      | Just(co, _rep_ty) <- topNormaliseNewType_maybe arg_ty
      = unboxArg (mkCastDs arg co)

      -- Booleans
      | Just tc <- tyConAppTyCon_maybe arg_ty,
        tc `hasKey` boolTyConKey
      = do dflags <- getDynFlags
           let platform = targetPlatform dflags
           prim_arg <- newSysLocalDs Many intPrimTy
           return (Var prim_arg,
                  \ body -> Case (mkIfThenElse arg (mkIntLit platform 1) (mkIntLit platform 0))
                                 prim_arg
                                 (exprType body)
                                 [Alt DEFAULT [] body])

      -- Data types with a single constructor, which has a single, primitive-typed arg
      -- This deals with Int, Float etc; also Ptr, ForeignPtr
      | is_product_type && data_con_arity == 1
      = ASSERT2(isUnliftedType data_con_arg_ty1, pprType arg_ty)
                            -- Typechecker ensures this
        do case_bndr <- newSysLocalDs Many arg_ty
           prim_arg <- newSysLocalDs Many data_con_arg_ty1
           return (Var prim_arg,
                   \ body -> Case arg case_bndr (exprType body) [Alt (DataAlt data_con) [prim_arg] body]
                  )

      -- Byte-arrays, both mutable and otherwise; hack warning
      -- We're looking for values of type ByteArray, MutableByteArray
      --    data ByteArray          ix = ByteArray        ix ix ByteArray#
      --    data MutableByteArray s ix = MutableByteArray ix ix (MutableByteArray# s)
      | is_product_type &&
        data_con_arity == 3 &&
        isJust maybe_arg3_tycon &&
        (arg3_tycon ==  byteArrayPrimTyCon ||
         arg3_tycon ==  mutableByteArrayPrimTyCon)
      = do case_bndr <- newSysLocalDs Many arg_ty
           vars@[_l_var, _r_var, arr_cts_var] <- newSysLocalsDs (map unrestricted data_con_arg_tys)
           return (Var arr_cts_var,
                   \ body -> Case arg case_bndr (exprType body) [Alt (DataAlt data_con) vars body]
                  )

      | otherwise
      = do l <- getSrcSpanDs
           pprPanic "unboxArg: " (ppr l <+> ppr arg_ty)
      where
        arg_ty                                      = exprType arg
        maybe_product_type                          = splitDataProductType_maybe arg_ty
        is_product_type                             = isJust maybe_product_type
        Just (_, _, data_con, scaled_data_con_arg_tys) = maybe_product_type
        data_con_arg_tys                            = map scaledThing scaled_data_con_arg_tys
        data_con_arity                              = dataConSourceArity data_con
        (data_con_arg_ty1 : _)                      = data_con_arg_tys

        (_ : _ : data_con_arg_ty3 : _) = data_con_arg_tys
        maybe_arg3_tycon               = tyConAppTyCon_maybe data_con_arg_ty3
        Just arg3_tycon                = maybe_arg3_tycon

    boxResult :: Type
              -> DsM (Type, CoreExpr -> CoreExpr)

    -- Takes the result of the user-level ccall:
    --      either (IO t),
    --      or maybe just t for a side-effect-free call
    -- Returns a wrapper for the primitive ccall itself, along with the
    -- type of the result of the primitive ccall.  This result type
    -- will be of the form
    --      State# RealWorld -> (# State# RealWorld, t' #)
    -- where t' is the unwrapped form of t.  If t is simply (), then
    -- the result type will be
    --      State# RealWorld -> (# State# RealWorld #)

    boxResult result_ty
      | Just (io_tycon, io_res_ty) <- tcSplitIOType_maybe result_ty
            -- isIOType_maybe handles the case where the type is a
            -- simple wrapping of IO.  E.g.
            --      newtype Wrap a = W (IO a)
            -- No coercion necessary because its a non-recursive newtype
            -- (If we wanted to handle a *recursive* newtype too, we'd need
            -- another case, and a coercion.)
            -- The result is IO t, so wrap the result in an IO constructor
      = do  { res <- resultWrapper io_res_ty
            ; let extra_result_tys
                    = case res of
                         (Just ty,_)
                           | isUnboxedTupleType ty
                           -> let Just ls = tyConAppArgs_maybe ty in tail ls
                         _ -> []

                  return_result state anss
                    = mkCoreUbxTup
                        (realWorldStatePrimTy : io_res_ty : extra_result_tys)
                        (state : anss)

            ; (ccall_res_ty, the_alt) <- mk_alt return_result res

            ; state_id <- newSysLocalDs Many realWorldStatePrimTy
            ; let io_data_con = head (tyConDataCons io_tycon)
                  toIOCon     = dataConWrapId io_data_con

                  wrap the_call =
                                  mkApps (Var toIOCon)
                                         [ Type io_res_ty,
                                           Lam state_id $
                                           mkWildCase (App the_call (Var state_id))
                                                 (unrestricted ccall_res_ty)
                                                 (coreAltType the_alt)
                                                 [the_alt]
                                         ]

            ; return (realWorldStatePrimTy `mkVisFunTyMany` ccall_res_ty, wrap) }

    boxResult result_ty
      = do -- It isn't IO, so do unsafePerformIO
           -- It's not conveniently available, so we inline it
           res <- resultWrapper result_ty
           (ccall_res_ty, the_alt) <- mk_alt return_result res
           let
               wrap = \ the_call -> mkWildCase (App the_call (Var realWorldPrimId))
                                               (unrestricted ccall_res_ty)
                                               (coreAltType the_alt)
                                               [the_alt]
           return (realWorldStatePrimTy `mkVisFunTyMany` ccall_res_ty, wrap)
      where
        return_result _ [ans] = ans
        return_result _ _     = panic "return_result: expected single result"

    mk_alt :: (Expr Var -> [Expr Var] -> Expr Var)
           -> (Maybe Type, Expr Var -> Expr Var)
           -> DsM (Type, CoreAlt)
    mk_alt return_result (Nothing, wrap_result)
      = do -- The ccall returns ()
           state_id <- newSysLocalDs Many realWorldStatePrimTy
           let
                 the_rhs = return_result (Var state_id)
                                         [wrap_result (panic "boxResult")]

                 ccall_res_ty = mkTupleTy Unboxed [realWorldStatePrimTy]
                 the_alt      = Alt (DataAlt (tupleDataCon Unboxed 1)) [state_id] the_rhs

           return (ccall_res_ty, the_alt)

    mk_alt return_result (Just prim_res_ty, wrap_result)
      = -- The ccall returns a non-() value
        ASSERT2( isPrimitiveType prim_res_ty, ppr prim_res_ty )
                 -- True because resultWrapper ensures it is so
        do { result_id <- newSysLocalDs Many prim_res_ty
           ; state_id <- newSysLocalDs Many realWorldStatePrimTy
           ; let the_rhs = return_result (Var state_id)
                                    [wrap_result (Var result_id)]
                 ccall_res_ty = mkTupleTy Unboxed [realWorldStatePrimTy, prim_res_ty]
                 the_alt      = Alt (DataAlt (tupleDataCon Unboxed 2)) [state_id, result_id] the_rhs
           ; return (ccall_res_ty, the_alt) }

    resultWrapper :: Type
                  -> DsM (Maybe Type,               -- Type of the expected result, if any
                          CoreExpr -> CoreExpr)     -- Wrapper for the result
    -- resultWrapper deals with the result *value*
    -- E.g. foreign import foo :: Int -> IO T
    -- Then resultWrapper deals with marshalling the 'T' part
    -- So if    resultWrapper ty = (Just ty_rep, marshal)
    --  then      marshal (e :: ty_rep) :: ty
    -- That is, 'marshal' wrape the result returned by the foreign call,
    -- of type ty_rep, into the value Haskell expected, of type 'ty'
    --
    -- Invariant: ty_rep is always a primitive type
    --            i.e. (isPrimitiveType ty_rep) is True

    resultWrapper result_ty
      -- Base case 1: primitive types
      | isPrimitiveType result_ty
      = return (Just result_ty, \e -> e)

      -- Base case 2: the unit type ()
      | Just (tc,_) <- maybe_tc_app
      , tc `hasKey` unitTyConKey
      = return (Nothing, \_ -> unitExpr)

      -- Base case 3: the boolean type
      | Just (tc,_) <- maybe_tc_app
      , tc `hasKey` boolTyConKey
      = do { dflags <- getDynFlags
           ; let platform = targetPlatform dflags
           ; let marshal_bool e
                   = mkWildCase e (unrestricted intPrimTy) boolTy
                       [ Alt DEFAULT                        [] (Var trueDataConId )
                       , Alt (LitAlt (mkLitInt platform 0)) [] (Var falseDataConId)]
           ; return (Just intPrimTy, marshal_bool) }

      -- Newtypes
      | Just (co, rep_ty) <- topNormaliseNewType_maybe result_ty
      = do { (maybe_ty, wrapper) <- resultWrapper rep_ty
           ; return (maybe_ty, \e -> mkCastDs (wrapper e) (mkSymCo co)) }

      -- The type might contain foralls (eg. for dummy type arguments,
      -- referring to 'Ptr a' is legal).
      | Just (tyvar, rest) <- splitForAllTyCoVar_maybe result_ty
      = do { (maybe_ty, wrapper) <- resultWrapper rest
           ; return (maybe_ty, \e -> Lam tyvar (wrapper e)) }

      -- Data types with a single constructor, which has a single arg
      -- This includes types like Ptr and ForeignPtr
      | Just (tycon, tycon_arg_tys) <- maybe_tc_app
      , Just data_con <- tyConSingleAlgDataCon_maybe tycon  -- One constructor
      , null (dataConExTyCoVars data_con)                   -- no existentials
      , [Scaled _ unwrapped_res_ty] <- dataConInstOrigArgTys data_con tycon_arg_tys  -- One argument
      = do { (maybe_ty, wrapper) <- resultWrapper unwrapped_res_ty
           ; let marshal_con e  = Var (dataConWrapId data_con)
                                  `mkTyApps` tycon_arg_tys
                                  `App` wrapper e
           ; return (maybe_ty, marshal_con) }

      | otherwise
      = pprPanic "resultWrapper" (ppr result_ty)
      where
        maybe_tc_app = splitTyConApp_maybe result_ty