path: root/compiler/GHC/Stg/Lint.hs

{- |
(c) The GRASP/AQUA Project, Glasgow University, 1993-1998

A lint pass to check basic STG invariants:

- Variables should be defined before used.

- Let bindings should not have unboxed types (unboxed bindings should only
  appear in case), except when they're join points (see Note [Core let-can-float
  invariant] and #14117).

- If linting after unarisation, invariants listed in Note [Post-unarisation
  invariants].

Because we don't have types and coercions in STG we can't really check types
here.

Some history:

StgLint used to check types, but it never worked and so it was disabled in 2000
with this note:

    WARNING:
    ~~~~~~~~

    This module has suffered bit-rot; it is likely to yield lint errors
    for Stg code that is currently perfectly acceptable for code
    generation.  Solution: don't use it!  (KSW 2000-05).

Since then there were some attempts at enabling it again, as summarised in #14787.
It's finally decided that we remove all type checking and only look for
basic properties listed above.

Note [Linting StgApp]
~~~~~~~~~~~~~~~~~~~~~
To lint an application of the form `f a_1 ... a_n`, we check that
the representations of the arguments `a_1`, ..., `a_n` match those
that the function expects.

More precisely, suppose the types in the application `f a_1 ... a_n`
are as follows:

  f :: t_1 -> ... -> t_n -> res
  a_1 :: s_1, ..., a_n :: s_n

  t_1 :: TYPE r_1, ..., t_n :: TYPE r_n
  s_1 :: TYPE p_1, ..., a_n :: TYPE p_n

Then we must check that each r_i is compatible with s_i. Compatibility
is weaker than on-the-nose equality: for example, IntRep and WordRep are
compatible. See Note [Bad unsafe coercion] in GHC.Core.Lint.

Wrinkle: it can sometimes happen that an argument type in the type of
the function does not have a fixed runtime representation, i.e.
there is an r_i such that runtimeRepPrimRep r_i crashes.
See https://gitlab.haskell.org/ghc/ghc/-/issues/21399 for an example.
Fixing this issue would require significant changes to the type system
of STG, so for now we simply skip the Lint check when we detect such
representation-polymorphic situations.

Note [Typing the STG language]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In Core, programs must be /well-typed/.  So if f :: ty1 -> ty2,
then in the application (f e), we must have  e :: ty1

STG is still a statically typed language, but the type system
is much coarser. In particular, STG programs must be /well-kinded/.
More precisely, if f :: ty1 -> ty2, then in the application (f e)
where e :: ty1', we must have kind(ty1) = kind(ty1').

So the STG type system does not distinguish beteen Int and Bool,
but it /does/ distinguish beteen Int and Int#, because they have
different kinds.  Actually, since all terms have kind (TYPE rep),
we might say that the STG language is well-runtime-rep'd.

This coarser type system makes fewer distinctions, and that allows
many nonsensical programs (such as ('x' && "foo")) -- but all type
systems accept buggy programs!  But the coarseness also permits
some optimisations that are ill-typed in Core.  For example, see
the module STG.CSE, which is all about doing CSE in STG that would
be ill-typed in Core.  But it must still be well-kinded!

-}

{-# LANGUAGE ScopedTypeVariables, FlexibleContexts, TypeFamilies,
  DeriveFunctor #-}

module GHC.Stg.Lint ( lintStgTopBindings ) where

import GHC.Prelude

import GHC.Stg.Syntax
import GHC.Stg.Utils

import GHC.Builtin.PrimOps  ( primOpArgForms )
import GHC.Core.Lint        ( interactiveInScope )
import GHC.Core.DataCon
import GHC.Core             ( AltCon(..) )
import GHC.Core.Type

import GHC.Types.Basic      ( TopLevelFlag(..), isTopLevel, OpArgForm(..), isMarkedCbv )
import GHC.Types.CostCentre ( isCurrentCCS )
import GHC.Types.Error      ( DiagnosticReason(WarningWithoutFlag) )
import GHC.Types.Id
import GHC.Types.Var.Set
import GHC.Types.Name       ( getSrcLoc, nameIsLocalOrFrom )
import GHC.Types.RepType
import GHC.Types.SrcLoc

import GHC.Utils.Logger
import GHC.Utils.Outputable
import GHC.Utils.Error      ( mkLocMessage, DiagOpts )
import qualified GHC.Utils.Error as Err

import GHC.Unit.Module            ( Module )
import GHC.Runtime.Context        ( InteractiveContext )

import GHC.Data.Bag         ( Bag, emptyBag, isEmptyBag, snocBag, bagToList )

import Control.Monad
import Data.Maybe
import GHC.Utils.Misc
import GHC.Core.Multiplicity (scaledThing)
import GHC.Settings (Platform)
import GHC.Core.TyCon (primRepCompatible)
import GHC.Utils.Panic.Plain (panic)

lintStgTopBindings :: forall a . (OutputablePass a, BinderP a ~ Id)
                   => Platform
                   -> Logger
                   -> DiagOpts
                   -> StgPprOpts
                   -> InteractiveContext
                   -> Module -- ^ module being compiled
                   -> Bool   -- ^ have we run Unarise yet?
                   -> String -- ^ who produced the STG?
                   -> [GenStgTopBinding a]
                   -> IO ()

lintStgTopBindings platform logger diag_opts opts ictxt this_mod unarised whodunnit binds
  = {-# SCC "StgLint" #-}
    case initL platform diag_opts this_mod unarised opts top_level_binds (lint_binds binds) of
      Nothing  ->
        return ()
      Just msg -> do
        logMsg logger Err.MCDump noSrcSpan
          $ withPprStyle defaultDumpStyle
          (vcat [ text "*** Stg Lint ErrMsgs: in" <+>
                        text whodunnit <+> text "***",
                  msg,
                  text "*** Offending Program ***",
                  pprGenStgTopBindings opts binds,
                  text "*** End of Offense ***"])
        Err.ghcExit logger 1
  where
    -- Bring all top-level binds into scope because CoreToStg does not generate
    -- bindings in dependency order (so we may see a use before its definition).
    top_level_binds = extendVarSetList (mkVarSet (bindersOfTopBinds binds))
                                       (interactiveInScope ictxt)

    lint_binds :: [GenStgTopBinding a] -> LintM ()

    lint_binds [] = return ()
    lint_binds (bind:binds) = do
        binders <- lint_bind bind
        addInScopeVars binders $
            lint_binds binds

    lint_bind (StgTopLifted bind) = lintStgBinds TopLevel bind
    lint_bind (StgTopStringLit v _) = return [v]

lintStgArg :: StgArg -> LintM ()
lintStgArg (StgLitArg _) = return ()
lintStgArg (StgVarArg v) = lintStgVar v

lintStgOpArg
    :: (OutputablePass a, BinderP a ~ Id)
    => OpArgForm -> GenStgOpArg a -> LintM ()
lintStgOpArg _          (StgValueArg arg) =
    lintStgArg arg
lintStgOpArg OpArgCont  (StgContArg bndr body) =
    addInScopeVars [bndr] $ lintStgExpr body
lintStgOpArg form arg = addErrL doc
  where
    doc = text "mismatched op argument form:"
       $$ text "expected:" <+> ppr form
       $$ text "found:" <+> ppr arg

lintStgVar :: Id -> LintM ()
lintStgVar id = checkInScope id

lintStgBinds
    :: (OutputablePass a, BinderP a ~ Id)
    => TopLevelFlag -> GenStgBinding a -> LintM [Id] -- Returns the binders
lintStgBinds top_lvl (StgNonRec binder rhs) = do
    lint_binds_help top_lvl (binder,rhs)
    return [binder]

lintStgBinds top_lvl (StgRec pairs)
  = addInScopeVars binders $ do
        mapM_ (lint_binds_help top_lvl) pairs
        return binders
  where
    binders = [b | (b,_) <- pairs]

lint_binds_help
    :: (OutputablePass a, BinderP a ~ Id)
    => TopLevelFlag
    -> (Id, GenStgRhs a)
    -> LintM ()
lint_binds_help top_lvl (binder, rhs)
  = addLoc (RhsOf binder) $ do
        when (isTopLevel top_lvl) (checkNoCurrentCCS rhs)
        lintStgRhs rhs
        opts <- getStgPprOpts
        -- Check binder doesn't have unlifted type or it's a join point
        checkL ( isJoinId binder
              || not (isUnliftedType (idType binder))
              || isDataConWorkId binder || isDataConWrapId binder) -- until #17521 is fixed
          (mkUnliftedTyMsg opts binder rhs)

-- | Top-level bindings can't inherit the cost centre stack from their
-- (static) allocation site.
checkNoCurrentCCS
    :: (OutputablePass a, BinderP a ~ Id)
    => GenStgRhs a
    -> LintM ()
checkNoCurrentCCS rhs = do
   opts <- getStgPprOpts
   let rhs' = pprStgRhs opts rhs
   case rhs of
      StgRhsClosure _ ccs _ _ _
         | isCurrentCCS ccs
         -> addErrL (text "Top-level StgRhsClosure with CurrentCCS" $$ rhs')
      StgRhsCon ccs _ _ _ _
         | isCurrentCCS ccs
         -> addErrL (text "Top-level StgRhsCon with CurrentCCS" $$ rhs')
      _ -> return ()

lintStgRhs :: (OutputablePass a, BinderP a ~ Id) => GenStgRhs a -> LintM ()

lintStgRhs (StgRhsClosure _ _ _ [] expr)
  = lintStgExpr expr

lintStgRhs (StgRhsClosure _ _ _ binders expr)
  = addLoc (LambdaBodyOf binders) $
      addInScopeVars binders $
        lintStgExpr expr

lintStgRhs rhs@(StgRhsCon _ con _ _ args) = do
    opts <- getStgPprOpts
    when (isUnboxedTupleDataCon con || isUnboxedSumDataCon con) $ do
      addErrL (text "StgRhsCon is an unboxed tuple or sum application" $$
               pprStgRhs opts rhs)

    lintConApp con args (pprStgRhs opts rhs)

    mapM_ lintStgArg args
    mapM_ checkPostUnariseConArg args

lintStgExpr :: (OutputablePass a, BinderP a ~ Id) => GenStgExpr a -> LintM ()

lintStgExpr (StgLit _) = return ()

lintStgExpr e@(StgApp fun args) = do
  lintStgVar fun
  mapM_ lintStgArg args
  lintAppCbvMarks e
  lintStgAppReps fun args



lintStgExpr app@(StgConApp con _n args _arg_tys) = do
    -- unboxed sums should vanish during unarise
    lf <- getLintFlags
    let !unarised = lf_unarised lf
    when (unarised && isUnboxedSumDataCon con) $ do
      opts <- getStgPprOpts
      addErrL (text "Unboxed sum after unarise:" $$
               pprStgExpr opts app)

    opts <- getStgPprOpts
    lintConApp con args (pprStgExpr opts app)

    mapM_ lintStgArg args
    mapM_ checkPostUnariseConArg args

lintStgExpr (StgOpApp op args _) =
    mapM_ (uncurry lintStgOpArg)
      $ zipEqual "lintStgExpr(StgOpApp)" arg_forms args
  where
    arg_forms
      | StgPrimOp primop <- op = primOpArgForms primop
      | otherwise = repeat OpArgValue

lintStgExpr (StgLet _ binds body) = do
    binders <- lintStgBinds NotTopLevel binds
    addLoc (BodyOfLetRec binders) $
      addInScopeVars binders $
        lintStgExpr body

lintStgExpr (StgLetNoEscape _ binds body) = do
    binders <- lintStgBinds NotTopLevel binds
    addLoc (BodyOfLetRec binders) $
      addInScopeVars binders $
        lintStgExpr body

lintStgExpr (StgTick _ expr) = lintStgExpr expr

lintStgExpr (StgCase scrut bndr alts_type alts) = do
    lintStgExpr scrut

    lf <- getLintFlags
    let in_scope = stgCaseBndrInScope alts_type (lf_unarised lf)

    addInScopeVars [bndr | in_scope] (mapM_ lintAlt alts)

lintAlt
    :: (OutputablePass a, BinderP a ~ Id)
    => GenStgAlt a -> LintM ()

lintAlt GenStgAlt{ alt_con   = DEFAULT
                 , alt_bndrs = _
                 , alt_rhs   = rhs} = lintStgExpr rhs

lintAlt GenStgAlt{ alt_con   = LitAlt _
                 , alt_bndrs = _
                 , alt_rhs   = rhs} = lintStgExpr rhs

lintAlt GenStgAlt{ alt_con   = DataAlt _
                 , alt_bndrs = bndrs
                 , alt_rhs   = rhs} =
  do
    mapM_ checkPostUnariseBndr bndrs
    addInScopeVars bndrs (lintStgExpr rhs)

-- Post unarise check we apply constructors to the right number of args.
-- This can be violated by invalid use of unsafeCoerce as showcased by test
-- T9208
lintConApp :: Foldable t => DataCon -> t a -> SDoc -> LintM ()
lintConApp con args app = do
    unarised <- lf_unarised <$> getLintFlags
    when (unarised &&
          not (isUnboxedTupleDataCon con) &&
          length (dataConRuntimeRepStrictness con) /= length args) $ do
      addErrL (text "Constructor applied to incorrect number of arguments:" $$
               text "Application:" <> app)

-- See Note [Linting StgApp]
-- See Note [Typing the STG language]
lintStgAppReps :: Id -> [StgArg] -> LintM ()
lintStgAppReps _fun [] = return ()
lintStgAppReps fun args = do
  lf <- getLintFlags
  let platform = lf_platform lf
      (fun_arg_tys, _res) = splitFunTys (idType fun)
      fun_arg_tys' = map (scaledThing ) fun_arg_tys :: [Type]
      fun_arg_tys_reps, actual_arg_reps :: [Maybe [PrimRep]]
      fun_arg_tys_reps = map typePrimRep_maybe fun_arg_tys'
      actual_arg_reps = map (typePrimRep_maybe . stgArgType) args

      match_args :: [Maybe [PrimRep]] -> [Maybe [PrimRep]] -> LintM ()
      -- Might be wrongly typed as polymorphic. See #21399
      match_args (Nothing:_) _   = return ()
      match_args (_) (Nothing:_) = return ()
      match_args (Just actual_rep:actual_reps_left) (Just expected_rep:expected_reps_left)
        -- Common case, reps are exactly the same
        | actual_rep == expected_rep
        = match_args actual_reps_left expected_reps_left

        -- Check for void rep which can be either an empty list *or* [VoidRep]
        | isVoidRep actual_rep && isVoidRep expected_rep
        = match_args actual_reps_left expected_reps_left

        -- Some reps are compatible *even* if they are not the same. E.g. IntRep and WordRep.
        -- We check for that here with primRepCompatible
        | and $ zipWith (primRepCompatible platform) actual_rep expected_rep
        = match_args actual_reps_left expected_reps_left

        | otherwise = addErrL $ hang (text "Function type reps and function argument reps missmatched") 2 $
            (text "In application " <> ppr fun <+> ppr args $$
              text "argument rep:" <> ppr actual_rep $$
              text "expected rep:" <> ppr expected_rep $$
              -- text "expected reps:" <> ppr arg_ty_reps $$
              text "unarised?:" <> ppr (lf_unarised lf))
        where
          isVoidRep [] = True
          isVoidRep [VoidRep] = True
          isVoidRep _ = False

          -- n_arg_ty_reps = length arg_ty_reps

      match_args _ _ = return () -- Functions are allowed to be over/under applied.

  match_args actual_arg_reps fun_arg_tys_reps

lintAppCbvMarks :: OutputablePass pass
                => GenStgExpr pass -> LintM ()
lintAppCbvMarks e@(StgApp fun args) = do
  lf <- getLintFlags
  when (lf_unarised lf) $ do
    -- A function which expects a unlifted argument as n'th argument
    -- always needs to be applied to n arguments.
    -- See Note [CBV Function Ids].
    let marks = fromMaybe [] $ idCbvMarks_maybe fun
    when (length (dropWhileEndLE (not . isMarkedCbv) marks) > length args) $ do
      addErrL $ hang (text "Undersatured cbv marked ID in App" <+> ppr e ) 2 $
        (text "marks" <> ppr marks $$
        text "args" <> ppr args $$
        text "arity" <> ppr (idArity fun) $$
        text "join_arity" <> ppr (isJoinId_maybe fun))
lintAppCbvMarks _ = panic "impossible - lintAppCbvMarks"

{-
************************************************************************
*                                                                      *
The Lint monad
*                                                                      *
************************************************************************
-}

newtype LintM a = LintM
    { unLintM :: Module
              -> LintFlags
              -> DiagOpts          -- Diagnostic options
              -> StgPprOpts        -- Pretty-printing options
              -> [LintLocInfo]     -- Locations
              -> IdSet             -- Local vars in scope
              -> Bag SDoc        -- Error messages so far
              -> (a, Bag SDoc)   -- Result and error messages (if any)
    }
    deriving (Functor)

data LintFlags = LintFlags { lf_unarised :: !Bool
                           , lf_platform :: !Platform
                             -- ^ have we run the unariser yet?
                           }

data LintLocInfo
  = RhsOf Id            -- The variable bound
  | LambdaBodyOf [Id]   -- The lambda-binder
  | BodyOfLetRec [Id]   -- One of the binders

dumpLoc :: LintLocInfo -> (SrcSpan, SDoc)
dumpLoc (RhsOf v) =
  (srcLocSpan (getSrcLoc v), text " [RHS of " <> pp_binders [v] <> char ']' )
dumpLoc (LambdaBodyOf bs) =
  (srcLocSpan (getSrcLoc (head bs)), text " [in body of lambda with binders " <> pp_binders bs <> char ']' )

dumpLoc (BodyOfLetRec bs) =
  (srcLocSpan (getSrcLoc (head bs)), text " [in body of letrec with binders " <> pp_binders bs <> char ']' )


pp_binders :: [Id] -> SDoc
pp_binders bs
  = sep (punctuate comma (map pp_binder bs))
  where
    pp_binder b
      = hsep [ppr b, dcolon, ppr (idType b)]

initL :: Platform -> DiagOpts -> Module -> Bool -> StgPprOpts -> IdSet -> LintM a -> Maybe SDoc
initL platform diag_opts this_mod unarised opts locals (LintM m) = do
  let (_, errs) = m this_mod (LintFlags unarised platform) diag_opts opts [] locals emptyBag
  if isEmptyBag errs then
      Nothing
  else
      Just (vcat (punctuate blankLine (bagToList errs)))

instance Applicative LintM where
      pure a = LintM $ \_mod _lf _df _opts _loc _scope errs -> (a, errs)
      (<*>) = ap
      (*>)  = thenL_

instance Monad LintM where
    (>>=) = thenL
    (>>)  = (*>)

thenL :: LintM a -> (a -> LintM b) -> LintM b
thenL m k = LintM $ \mod lf diag_opts opts loc scope errs
  -> case unLintM m mod lf diag_opts opts loc scope errs of
      (r, errs') -> unLintM (k r) mod lf diag_opts opts loc scope errs'

thenL_ :: LintM a -> LintM b -> LintM b
thenL_ m k = LintM $ \mod lf diag_opts opts loc scope errs
  -> case unLintM m mod lf diag_opts opts loc scope errs of
      (_, errs') -> unLintM k mod lf diag_opts opts loc scope errs'

checkL :: Bool -> SDoc -> LintM ()
checkL True  _   = return ()
checkL False msg = addErrL msg

-- Case alts shouldn't have unboxed sum, unboxed tuple, or void binders.
checkPostUnariseBndr :: Id -> LintM ()
checkPostUnariseBndr bndr = do
    lf <- getLintFlags
    when (lf_unarised lf) $
      forM_ (checkPostUnariseId bndr) $ \unexpected ->
        addErrL $
          text "After unarisation, binder " <>
          ppr bndr <> text " has " <> text unexpected <> text " type " <>
          ppr (idType bndr)

-- Arguments shouldn't have sum, tuple, or void types.
checkPostUnariseConArg :: StgArg -> LintM ()
checkPostUnariseConArg arg = case arg of
    StgLitArg _ ->
      return ()
    StgVarArg id -> do
      lf <- getLintFlags
      when (lf_unarised lf) $
        forM_ (checkPostUnariseId id) $ \unexpected ->
          addErrL $
            text "After unarisation, arg " <>
            ppr id <> text " has " <> text unexpected <> text " type " <>
            ppr (idType id)

-- Post-unarisation args and case alt binders should not have unboxed tuple,
-- unboxed sum, or void types. Return what the binder is if it is one of these.
checkPostUnariseId :: Id -> Maybe String
checkPostUnariseId id
  | isUnboxedSumType id_ty   = Just "unboxed sum"
  | isUnboxedTupleType id_ty = Just "unboxed tuple"
  | isZeroBitTy id_ty        = Just "void"
  | otherwise                = Nothing
  where
    id_ty = idType id

addErrL :: SDoc -> LintM ()
addErrL msg = LintM $ \_mod _lf df _opts loc _scope errs -> ((), addErr df errs msg loc)

addErr :: DiagOpts -> Bag SDoc -> SDoc -> [LintLocInfo] -> Bag SDoc
addErr diag_opts errs_so_far msg locs
  = errs_so_far `snocBag` mk_msg locs
  where
    mk_msg (loc:_) = let (l,hdr) = dumpLoc loc
                     in  mkLocMessage (Err.mkMCDiagnostic diag_opts WarningWithoutFlag)
                                      l (hdr $$ msg)
    mk_msg []      = msg

addLoc :: LintLocInfo -> LintM a -> LintM a
addLoc extra_loc m = LintM $ \mod lf diag_opts opts loc scope errs
   -> unLintM m mod lf diag_opts opts (extra_loc:loc) scope errs

addInScopeVars :: [Id] -> LintM a -> LintM a
addInScopeVars ids m = LintM $ \mod lf diag_opts opts loc scope errs
 -> let
        new_set = mkVarSet ids
    in unLintM m mod lf diag_opts opts loc (scope `unionVarSet` new_set) errs

getLintFlags :: LintM LintFlags
getLintFlags = LintM $ \_mod lf _df _opts _loc _scope errs -> (lf, errs)

getStgPprOpts :: LintM StgPprOpts
getStgPprOpts = LintM $ \_mod _lf _df opts _loc _scope errs -> (opts, errs)

checkInScope :: Id -> LintM ()
checkInScope id = LintM $ \mod _lf diag_opts _opts loc scope errs
 -> if nameIsLocalOrFrom mod (idName id) && not (id `elemVarSet` scope) then
        ((), addErr diag_opts errs (hsep [ppr id, dcolon, ppr (idType id),
                                    text "is out of scope"]) loc)
    else
        ((), errs)

mkUnliftedTyMsg :: OutputablePass a => StgPprOpts -> Id -> GenStgRhs a -> SDoc
mkUnliftedTyMsg opts binder rhs
  = (text "Let(rec) binder" <+> quotes (ppr binder) <+>
     text "has unlifted type" <+> quotes (ppr (idType binder)))
    $$
    (text "RHS:" <+> pprStgRhs opts rhs)