Support the GHCi debugger with -fexternal-interpreter

* All the tests in tests/ghci.debugger now pass with
  -fexternal-interpreter. These tests are now run with the ghci-ext way
  in addition to the normal way so we won't break it in the future.

* I removed all the unsafeCoerce# calls from RtClosureInspect. Yay!

The main changes are:

* New messages: GetClosure and Seq.  GetClosure is a remote interface to
  GHC.Exts.Heap.getClosureData, which required Binary instances for
  various datatypes. Fortunately this wasn't too painful thanks to
  DeriveGeneric.

* No cheating by unsafeCoercing values when printing them. Now we have
  to turn the Closure representation back into the native representation
  when printing Int, Float, Double, Integer and Char. Of these, Integer
  was the most painful - we now have a dependency on integer-gmp due to
  needing access to the representation.

* Fixed a bug in rts/Heap.c - it was bogusly returning stack content as
  pointers for an AP_STACK closure.

Test Plan:
* `cd testsuite/tests/ghci.debugger && make`
* validate

Reviewers: bgamari, patrickdoc, nomeata, angerman, hvr, erikd, goldfire

Subscribers: alpmestan, snowleopard, rwbarton, thomie, carter

GHC Trac Issues: #13184

Differential Revision: https://phabricator.haskell.org/D4955

14 files changed, 241 insertions, 78 deletions

diff --git a/compiler/ghc.cabal.in b/compiler/ghc.cabal.in
index d4a1dc3c6e..5c9d88f8cc 100644
--- a/compiler/ghc.cabal.in
+++ b/compiler/ghc.cabal.in
@@ -45,6 +45,11 @@ Flag terminfo
     Default: True
     Manual: True
 
+Flag integer-gmp
+    Description: Use integer-gmp
+    Manual: True
+    Default: False
+
 Library
     Default-Language: Haskell2010
     Exposed: False
@@ -84,6 +89,11 @@ Library
         CPP-Options: -DGHCI
         Include-Dirs: ../rts/dist/build @FFIIncludeDir@
 
+    -- gmp internals are used by the GHCi debugger if available
+    if flag(integer-gmp)
+        CPP-Options: -DINTEGER_GMP
+        build-depends: integer-gmp >= 1.0.2
+
     Other-Extensions:
         BangPatterns
         CPP
diff --git a/compiler/ghci/Debugger.hs b/compiler/ghci/Debugger.hs
index 0db74cb5cb..5942715c12 100644
--- a/compiler/ghci/Debugger.hs
+++ b/compiler/ghci/Debugger.hs
@@ -44,8 +44,6 @@ import Data.List
 import Data.Maybe
 import Data.IORef
 
-import GHC.Exts
-
 -------------------------------------
 -- | The :print & friends commands
 -------------------------------------
@@ -120,11 +118,10 @@ bindSuspensions t = do
           availNames   = map ((prefix++) . show) [(1::Int)..] \\ alreadyUsedNames
       availNames_var  <- liftIO $ newIORef availNames
       (t', stuff)     <- liftIO $ foldTerm (nameSuspensionsAndGetInfos hsc_env availNames_var) t
-      let (names, tys, hvals) = unzip3 stuff
+      let (names, tys, fhvs) = unzip3 stuff
       let ids = [ mkVanillaGlobal name ty
                 | (name,ty) <- zip names tys]
           new_ic = extendInteractiveContextWithIds ictxt ids
-      fhvs <- liftIO $ mapM (mkFinalizedHValue hsc_env <=< mkRemoteRef) hvals
       liftIO $ extendLinkEnv (zip names fhvs)
       setSession hsc_env {hsc_IC = new_ic }
       return t'
@@ -132,7 +129,7 @@ bindSuspensions t = do
 
 --    Processing suspensions. Give names and recopilate info
         nameSuspensionsAndGetInfos :: HscEnv -> IORef [String]
-                                   -> TermFold (IO (Term, [(Name,Type,HValue)]))
+                                   -> TermFold (IO (Term, [(Name,Type,ForeignHValue)]))
         nameSuspensionsAndGetInfos hsc_env freeNames = TermFold
                       {
                         fSuspension = doSuspension hsc_env freeNames
@@ -163,7 +160,7 @@ showTerm term = do
        then cPprTerm (liftM2 (++) (\_y->[cPprShowable]) cPprTermBase) term
        else cPprTerm cPprTermBase term
  where
-  cPprShowable prec t@Term{ty=ty, val=val} =
+  cPprShowable prec t@Term{ty=ty, val=fhv} =
     if not (isFullyEvaluatedTerm t)
      then return Nothing
      else do
@@ -176,13 +173,14 @@ showTerm term = do
                       -- does this still do what it is intended to do
                       -- with the changed error handling and logging?
            let noop_log _ _ _ _ _ _ = return ()
-               expr = "show " ++ showPpr dflags bname
+               expr = "Prelude.return (Prelude.show " ++
+                         showPpr dflags bname ++
+                      ") :: Prelude.IO Prelude.String"
            _ <- GHC.setSessionDynFlags dflags{log_action=noop_log}
-           fhv <- liftIO $ mkFinalizedHValue hsc_env =<< mkRemoteRef val
            txt_ <- withExtendedLinkEnv [(bname, fhv)]
-                                       (GHC.compileExpr expr)
+                                       (GHC.compileExprRemote expr)
            let myprec = 10 -- application precedence. TODO Infix constructors
-           let txt = unsafeCoerce# txt_ :: [a]
+           txt <- liftIO $ evalString hsc_env txt_
            if not (null txt) then
              return $ Just $ cparen (prec >= myprec && needsParens txt)
                                     (text txt)
diff --git a/compiler/ghci/GHCi.hs b/compiler/ghci/GHCi.hs
index 472f0857cb..579053999f 100644
--- a/compiler/ghci/GHCi.hs
+++ b/compiler/ghci/GHCi.hs
@@ -21,6 +21,8 @@ module GHCi
   , enableBreakpoint
   , breakpointStatus
   , getBreakpointVar
+  , getClosure
+  , seqHValue
 
   -- * The object-code linker
   , initObjLinker
@@ -77,6 +79,7 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString.Lazy as LB
 import Data.IORef
 import Foreign hiding (void)
+import GHC.Exts.Heap
 import GHC.Stack.CCS (CostCentre,CostCentreStack)
 import System.Exit
 import Data.Maybe
@@ -350,6 +353,17 @@ getBreakpointVar hsc_env ref ix =
     mb <- iservCmd hsc_env (GetBreakpointVar apStack ix)
     mapM (mkFinalizedHValue hsc_env) mb
 
+getClosure :: HscEnv -> ForeignHValue -> IO (GenClosure ForeignHValue)
+getClosure hsc_env ref =
+  withForeignRef ref $ \hval -> do
+    mb <- iservCmd hsc_env (GetClosure hval)
+    mapM (mkFinalizedHValue hsc_env) mb
+
+seqHValue :: HscEnv -> ForeignHValue -> IO ()
+seqHValue hsc_env ref =
+  withForeignRef ref $ \hval ->
+    iservCmd hsc_env (Seq hval) >>= fromEvalResult
+
 -- -----------------------------------------------------------------------------
 -- Interface to the object-code linker
 
diff --git a/compiler/ghci/RtClosureInspect.hs b/compiler/ghci/RtClosureInspect.hs
index d540983139..b7614078e6 100644
--- a/compiler/ghci/RtClosureInspect.hs
+++ b/compiler/ghci/RtClosureInspect.hs
@@ -27,6 +27,7 @@ module RtClosureInspect(
 
 import GhcPrelude
 
+import GHCi
 import GHCi.RemoteTypes
 import HscTypes
 
@@ -62,8 +63,12 @@ import GHC.IO ( IO(..) )
 import SMRep ( roundUpTo )
 
 import Control.Monad
+import Data.Array.Base
 import Data.Maybe
 import Data.List
+#if defined(INTEGER_GMP)
+import GHC.Integer.GMP.Internals
+#endif
 import qualified Data.Sequence as Seq
 import Data.Sequence (viewl, ViewL(..))
 import Foreign
@@ -79,7 +84,7 @@ data Term = Term { ty        :: RttiType
                                -- Carries a text representation if the datacon is
                                -- not exported by the .hi file, which is the case
                                -- for private constructors in -O0 compiled libraries
-                 , val       :: HValue
+                 , val       :: ForeignHValue
                  , subTerms  :: [Term] }
 
           | Prim { ty        :: RttiType
@@ -87,7 +92,7 @@ data Term = Term { ty        :: RttiType
 
           | Suspension { ctype    :: ClosureType
                        , ty       :: RttiType
-                       , val      :: HValue
+                       , val      :: ForeignHValue
                        , bound_to :: Maybe Name   -- Useful for printing
                        }
           | NewtypeWrap{       -- At runtime there are no newtypes, and hence no
@@ -126,22 +131,22 @@ isThunk APStackClosure{} = True
 isThunk _             = False
 
 -- Lookup the name in a constructor closure
-constrClosToName :: HscEnv -> Closure -> IO (Either String Name)
+constrClosToName :: HscEnv -> GenClosure a -> IO (Either String Name)
 constrClosToName hsc_env ConstrClosure{pkg=pkg,modl=mod,name=occ} = do
    let occName = mkOccName OccName.dataName occ
        modName = mkModule (stringToUnitId pkg) (mkModuleName mod)
    Right `fmap` lookupOrigIO hsc_env modName occName
 constrClosToName _hsc_env clos =
-   return (Left ("conClosToName: Expected ConstrClosure, got " ++ show clos))
+   return (Left ("conClosToName: Expected ConstrClosure, got " ++ show (fmap (const ()) clos)))
 
 -----------------------------------
 -- * Traversals for Terms
 -----------------------------------
-type TermProcessor a b = RttiType -> Either String DataCon -> HValue -> [a] -> b
+type TermProcessor a b = RttiType -> Either String DataCon -> ForeignHValue -> [a] -> b
 
 data TermFold a = TermFold { fTerm        :: TermProcessor a a
                            , fPrim        :: RttiType -> [Word] -> a
-                           , fSuspension  :: ClosureType -> RttiType -> HValue
+                           , fSuspension  :: ClosureType -> RttiType -> ForeignHValue
                                             -> Maybe Name -> a
                            , fNewtypeWrap :: RttiType -> Either String DataCon
                                             -> a -> a
@@ -152,7 +157,7 @@ data TermFold a = TermFold { fTerm        :: TermProcessor a a
 data TermFoldM m a =
                    TermFoldM {fTermM        :: TermProcessor a (m a)
                             , fPrimM        :: RttiType -> [Word] -> m a
-                            , fSuspensionM  :: ClosureType -> RttiType -> HValue
+                            , fSuspensionM  :: ClosureType -> RttiType -> ForeignHValue
                                              -> Maybe Name -> m a
                             , fNewtypeWrapM :: RttiType -> Either String DataCon
                                             -> a -> m a
@@ -317,19 +322,26 @@ cPprTermBase y =
                                       . subTerms)
   , ifTerm (\t -> isTyCon listTyCon (ty t) && subTerms t `lengthIs` 2)
            ppr_list
-  , ifTerm (isTyCon intTyCon    . ty) ppr_int
-  , ifTerm (isTyCon charTyCon   . ty) ppr_char
-  , ifTerm (isTyCon floatTyCon  . ty) ppr_float
-  , ifTerm (isTyCon doubleTyCon . ty) ppr_double
-  , ifTerm (isIntegerTy         . ty) ppr_integer
+  , ifTerm' (isTyCon intTyCon    . ty) ppr_int
+  , ifTerm' (isTyCon charTyCon   . ty) ppr_char
+  , ifTerm' (isTyCon floatTyCon  . ty) ppr_float
+  , ifTerm' (isTyCon doubleTyCon . ty) ppr_double
+#if defined(INTEGER_GMP)
+  , ifTerm' (isIntegerTy         . ty) ppr_integer
+#endif
   ]
  where
    ifTerm :: (Term -> Bool)
           -> (Precedence -> Term -> m SDoc)
           -> Precedence -> Term -> m (Maybe SDoc)
-   ifTerm pred f prec t@Term{}
-       | pred t    = Just `liftM` f prec t
-   ifTerm _ _ _ _  = return Nothing
+   ifTerm pred f = ifTerm' pred (\prec t -> Just <$> f prec t)
+
+   ifTerm' :: (Term -> Bool)
+          -> (Precedence -> Term -> m (Maybe SDoc))
+          -> Precedence -> Term -> m (Maybe SDoc)
+   ifTerm' pred f prec t@Term{}
+       | pred t    = f prec t
+   ifTerm' _ _ _ _  = return Nothing
 
    isTupleTy ty    = fromMaybe False $ do
      (tc,_) <- tcSplitTyConApp_maybe ty
@@ -343,13 +355,67 @@ cPprTermBase y =
      (tc,_) <- tcSplitTyConApp_maybe ty
      return (tyConName tc == integerTyConName)
 
-   ppr_int, ppr_char, ppr_float, ppr_double, ppr_integer
-      :: Precedence -> Term -> m SDoc
-   ppr_int     _ v = return (Ppr.int     (unsafeCoerce# (val v)))
-   ppr_char    _ v = return (Ppr.char '\'' <> Ppr.char (unsafeCoerce# (val v)) <> Ppr.char '\'')
-   ppr_float   _ v = return (Ppr.float   (unsafeCoerce# (val v)))
-   ppr_double  _ v = return (Ppr.double  (unsafeCoerce# (val v)))
-   ppr_integer _ v = return (Ppr.integer (unsafeCoerce# (val v)))
+   ppr_int, ppr_char, ppr_float, ppr_double
+      :: Precedence -> Term -> m (Maybe SDoc)
+   ppr_int _ Term{subTerms=[Prim{valRaw=[w]}]} =
+      return (Just (Ppr.int (fromIntegral w)))
+   ppr_int _ _ = return Nothing
+
+   ppr_char _ Term{subTerms=[Prim{valRaw=[w]}]} =
+      return (Just (Ppr.pprHsChar (chr (fromIntegral w))))
+   ppr_char _ _ = return Nothing
+
+   ppr_float   _ Term{subTerms=[Prim{valRaw=[w]}]} = do
+      let f = unsafeDupablePerformIO $
+                alloca $ \p -> poke p w >> peek (castPtr p)
+      return (Just (Ppr.float f))
+   ppr_float _ _ = return Nothing
+
+   ppr_double  _ Term{subTerms=[Prim{valRaw=[w]}]} = do
+      let f = unsafeDupablePerformIO $
+                alloca $ \p -> poke p w >> peek (castPtr p)
+      return (Just (Ppr.double f))
+   -- let's assume that if we get two words, we're on a 32-bit
+   -- machine. There's no good way to get a DynFlags to check the word
+   -- size here.
+   ppr_double  _ Term{subTerms=[Prim{valRaw=[w1,w2]}]} = do
+      let f = unsafeDupablePerformIO $
+                alloca $ \p -> do
+                  poke p (fromIntegral w1 :: Word32)
+                  poke (p `plusPtr` 4) (fromIntegral w2 :: Word32)
+                  peek (castPtr p)
+      return (Just (Ppr.double f))
+   ppr_double _ _ = return Nothing
+
+   ppr_integer :: Precedence -> Term -> m (Maybe SDoc)
+#if defined(INTEGER_GMP)
+   -- Reconstructing Integers is a bit of a pain. This depends deeply
+   -- on the integer-gmp representation, so it'll break if that
+   -- changes (but there are several tests in
+   -- tests/ghci.debugger/scripts that will tell us if this is wrong).
+   --
+   --   data Integer
+   --     = S# Int#
+   --     | Jp# {-# UNPACK #-} !BigNat
+   --     | Jn# {-# UNPACK #-} !BigNat
+   --
+   --   data BigNat = BN# ByteArray#
+   --
+   ppr_integer _ Term{subTerms=[Prim{valRaw=[W# w]}]} =
+      return (Just (Ppr.integer (S# (word2Int# w))))
+   ppr_integer _ Term{dc=Right con,
+                      subTerms=[Term{subTerms=[Prim{valRaw=ws}]}]} = do
+      -- We don't need to worry about sizes that are not an integral
+      -- number of words, because luckily GMP uses arrays of words
+      -- (see GMP_LIMB_SHIFT).
+      let
+        !(UArray _ _ _ arr#) = listArray (0,length ws-1) ws
+        constr
+          | "Jp#" <- occNameString (nameOccName (dataConName con)) = Jp#
+          | otherwise = Jn#
+      return (Just (Ppr.integer (constr (BN# arr#))))
+#endif
+   ppr_integer _ _ = return Nothing
 
    --Note pprinting of list terms is not lazy
    ppr_list :: Precedence -> Term -> m SDoc
@@ -357,10 +423,12 @@ cPprTermBase y =
        let elems      = h : getListTerms t
            isConsLast = not (termType (last elems) `eqType` termType h)
            is_string  = all (isCharTy . ty) elems
+           chars = [ chr (fromIntegral w)
+                   | Term{subTerms=[Prim{valRaw=[w]}]} <- elems ]
 
        print_elems <- mapM (y cons_prec) elems
        if is_string
-        then return (Ppr.doubleQuotes (Ppr.text (unsafeCoerce# (map val elems))))
+        then return (Ppr.doubleQuotes (Ppr.text chars))
         else if isConsLast
         then return $ cparen (p >= cons_prec)
                     $ pprDeeperList fsep
@@ -553,7 +621,7 @@ cvObtainTerm
     -> Int      -- ^ How many times to recurse for subterms
     -> Bool     -- ^ Force thunks
     -> RttiType -- ^ Type of the object to reconstruct
-    -> HValue   -- ^ Object to reconstruct
+    -> ForeignHValue   -- ^ Object to reconstruct
     -> IO Term
 cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
   -- we quantify existential tyvars as universal,
@@ -599,7 +667,7 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
             text "Type obtained: " <> ppr (termType term))
    return term
     where
-  go :: Int -> Type -> Type -> HValue -> TcM Term
+  go :: Int -> Type -> Type -> ForeignHValue -> TcM Term
    -- I believe that my_ty should not have any enclosing
    -- foralls, nor any free RuntimeUnk skolems;
    -- that is partly what the quantifyType stuff achieved
@@ -609,29 +677,31 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
   go 0 my_ty _old_ty a = do
     traceTR (text "Gave up reconstructing a term after" <>
                   int max_depth <> text " steps")
-    clos <- trIO $ getClosureData a
+    clos <- trIO $ GHCi.getClosure hsc_env a
     return (Suspension (tipe (info clos)) my_ty a Nothing)
   go !max_depth my_ty old_ty a = do
     let monomorphic = not(isTyVarTy my_ty)
     -- This ^^^ is a convention. The ancestor tests for
     -- monomorphism and passes a type instead of a tv
-    clos <- trIO $ getClosureData a
+    clos <- trIO $ GHCi.getClosure hsc_env a
     case clos of
 -- Thunks we may want to force
-      t | isThunk t && force -> traceTR (text "Forcing a " <> text (show t)) >>
-                                seq a (go (pred max_depth) my_ty old_ty a)
+      t | isThunk t && force -> do
+         traceTR (text "Forcing a " <> text (show (fmap (const ()) t)))
+         liftIO $ GHCi.seqHValue hsc_env a
+         go (pred max_depth) my_ty old_ty a
 -- Blackholes are indirections iff the payload is not TSO or BLOCKING_QUEUE.  So we
 -- treat them like indirections; if the payload is TSO or BLOCKING_QUEUE, we'll end up
 -- showing '_' which is what we want.
       BlackholeClosure{indirectee=ind} -> do
          traceTR (text "Following a BLACKHOLE")
-         (\(Box x) -> go max_depth my_ty old_ty (HValue x)) ind
+         go max_depth my_ty old_ty ind
 -- We always follow indirections
       IndClosure{indirectee=ind} -> do
          traceTR (text "Following an indirection" )
-         (\(Box x) -> go max_depth my_ty old_ty (HValue x)) ind
+         go max_depth my_ty old_ty ind
 -- We also follow references
-      MutVarClosure{}
+      MutVarClosure{var=contents}
          | Just (tycon,[world,contents_ty]) <- tcSplitTyConApp_maybe old_ty
              -> do
                   -- Deal with the MutVar# primitive
@@ -640,7 +710,6 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
                   -- MutVar# :: contents_ty -> MutVar# s contents_ty
          traceTR (text "Following a MutVar")
          contents_tv <- newVar liftedTypeKind
-         contents <- trIO$ IO$ \w -> readMutVar# (unsafeCoerce# a) w
          ASSERT(isUnliftedType my_ty) return ()
          (mutvar_ty,_) <- instScheme $ quantifyType $ mkFunTy
                             contents_ty (mkTyConApp tycon [world,contents_ty])
@@ -649,8 +718,8 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
          return (RefWrap my_ty x)
 
  -- The interesting case
-      ConstrClosure{ptrArgs=pArgs} -> do
-        traceTR (text "entering a constructor " <>
+      ConstrClosure{ptrArgs=pArgs,dataArgs=dArgs} -> do
+        traceTR (text "entering a constructor " <> ppr dArgs <+>
                       if monomorphic
                         then parens (text "already monomorphic: " <> ppr my_ty)
                         else Ppr.empty)
@@ -667,8 +736,8 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
                            tag = showPpr dflags dcname
                        vars     <- replicateM (length pArgs)
                                               (newVar liftedTypeKind)
-                       subTerms <- sequence $ zipWith (\(Box x) tv ->
-                           go (pred max_depth) tv tv (HValue x)) pArgs vars
+                       subTerms <- sequence $ zipWith (\x tv ->
+                           go (pred max_depth) tv tv x) pArgs vars
                        return (Term my_ty (Left ('<' : tag ++ ">")) a subTerms)
           Just dc -> do
             traceTR (text "Is constructor" <+> (ppr dc $$ ppr my_ty))
@@ -676,9 +745,17 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
             subTerms <- extractSubTerms (\ty -> go (pred max_depth) ty ty) clos subTtypes
             return (Term my_ty (Right dc) a subTerms)
 
+      -- This is to support printing of Integers. It's not a general
+      -- mechanism by any means; in particular we lose the size in
+      -- bytes of the array.
+      ArrWordsClosure{bytes=b, arrWords=ws} -> do
+         traceTR (text "ByteArray# closure, size " <> ppr b)
+         return (Term my_ty (Left "ByteArray#") a [Prim my_ty ws])
+
 -- The otherwise case: can be a Thunk,AP,PAP,etc.
       _ -> do
-         traceTR (text "Unknown closure:" <+> text (show clos))
+         traceTR (text "Unknown closure:" <+>
+                  text (show (fmap (const ()) clos)))
          return (Suspension (tipe (info clos)) my_ty a Nothing)
 
   -- insert NewtypeWraps around newtypes
@@ -698,8 +775,8 @@ cvObtainTerm hsc_env max_depth force old_ty hval = runTR hsc_env $ do
       worker ct ty hval n | isFunTy ty = Suspension ct (dictsView ty) hval n
                           | otherwise  = Suspension ct ty hval n
 
-extractSubTerms :: (Type -> HValue -> TcM Term)
-                -> Closure -> [Type] -> TcM [Term]
+extractSubTerms :: (Type -> ForeignHValue -> TcM Term)
+                -> GenClosure ForeignHValue -> [Type] -> TcM [Term]
 extractSubTerms recurse clos = liftM thdOf3 . go 0 0
   where
     array = dataArgs clos
@@ -733,7 +810,7 @@ extractSubTerms recurse clos = liftM thdOf3 . go 0 0
 
     go_rep ptr_i arr_i ty rep
       | isGcPtrRep rep = do
-          t <- (\(Box x) -> recurse ty (HValue x)) $ (ptrArgs clos)!!ptr_i
+          t <- recurse ty $ (ptrArgs clos)!!ptr_i
           return (ptr_i + 1, arr_i, t)
       | otherwise = do
           -- This is a bit involved since we allow packing multiple fields
@@ -805,7 +882,7 @@ cvReconstructType
     :: HscEnv
     -> Int       -- ^ How many times to recurse for subterms
     -> GhciType  -- ^ Type to refine
-    -> HValue    -- ^ Refine the type using this value
+    -> ForeignHValue  -- ^ Refine the type using this value
     -> IO (Maybe Type)
 cvReconstructType hsc_env max_depth old_ty hval = runTR_maybe hsc_env $ do
    traceTR (text "RTTI started with initial type " <> ppr old_ty)
@@ -845,15 +922,14 @@ cvReconstructType hsc_env max_depth old_ty hval = runTR_maybe hsc_env $ do
                   search stop expand (xx `mappend` Seq.fromList new) $! (pred d)
 
    -- returns unification tasks,since we are going to want a breadth-first search
-  go :: Type -> HValue -> TR [(Type, HValue)]
+  go :: Type -> ForeignHValue -> TR [(Type, ForeignHValue)]
   go my_ty a = do
     traceTR (text "go" <+> ppr my_ty)
-    clos <- trIO $ getClosureData a
+    clos <- trIO $ GHCi.getClosure hsc_env a
     case clos of
-      BlackholeClosure{indirectee=ind} -> (\(Box x) -> go my_ty (HValue x)) ind
-      IndClosure{indirectee=ind} -> (\(Box x) -> go my_ty (HValue x)) ind
-      MutVarClosure{} -> do
-         contents <- trIO$ IO$ \w -> readMutVar# (unsafeCoerce# a) w
+      BlackholeClosure{indirectee=ind} -> go my_ty ind
+      IndClosure{indirectee=ind} -> go my_ty ind
+      MutVarClosure{var=contents} -> do
          tv'   <- newVar liftedTypeKind
          world <- newVar liftedTypeKind
          addConstraint my_ty (mkTyConApp mutVarPrimTyCon [world,tv'])
@@ -864,15 +940,15 @@ cvReconstructType hsc_env max_depth old_ty hval = runTR_maybe hsc_env $ do
         (_,mb_dc) <- tryTc (tcLookupDataCon dcname)
         case mb_dc of
           Nothing-> do
-            forM pArgs $ \(Box x) -> do
+            forM pArgs $ \x -> do
               tv <- newVar liftedTypeKind
-              return (tv, HValue x)
+              return (tv, x)
 
           Just dc -> do
             arg_tys <- getDataConArgTys dc my_ty
             (_, itys) <- findPtrTyss 0 arg_tys
             traceTR (text "Constr2" <+> ppr dcname <+> ppr arg_tys)
-            return $ zipWith (\(_,ty) (Box x) -> (ty, HValue x)) itys pArgs
+            return $ zipWith (\(_,ty) x -> (ty, x)) itys pArgs
       _ -> return []
 
 findPtrTys :: Int  -- Current pointer index
diff --git a/compiler/main/InteractiveEval.hs b/compiler/main/InteractiveEval.hs
index 3f2309e7f5..bec52e6001 100644
--- a/compiler/main/InteractiveEval.hs
+++ b/compiler/main/InteractiveEval.hs
@@ -990,20 +990,22 @@ moduleIsBootOrNotObjectLinkable mod_summary = withSession $ \hsc_env ->
 -- RTTI primitives
 
 obtainTermFromVal :: HscEnv -> Int -> Bool -> Type -> a -> IO Term
-obtainTermFromVal hsc_env bound force ty x =
-              cvObtainTerm hsc_env bound force ty (unsafeCoerce# x)
+obtainTermFromVal hsc_env bound force ty x
+  | gopt Opt_ExternalInterpreter (hsc_dflags hsc_env)
+  = throwIO (InstallationError
+      "this operation requires -fno-external-interpreter")
+  | otherwise
+  = cvObtainTerm hsc_env bound force ty (unsafeCoerce# x)
 
 obtainTermFromId :: HscEnv -> Int -> Bool -> Id -> IO Term
 obtainTermFromId hsc_env bound force id =  do
-  let dflags = hsc_dflags hsc_env
-  hv <- Linker.getHValue hsc_env (varName id) >>= wormhole dflags
+  hv <- Linker.getHValue hsc_env (varName id)
   cvObtainTerm hsc_env bound force (idType id) hv
 
 -- Uses RTTI to reconstruct the type of an Id, making it less polymorphic
 reconstructType :: HscEnv -> Int -> Id -> IO (Maybe Type)
 reconstructType hsc_env bound id = do
-  let dflags = hsc_dflags hsc_env
-  hv <- Linker.getHValue hsc_env (varName id) >>= wormhole dflags
+  hv <- Linker.getHValue hsc_env (varName id)
   cvReconstructType hsc_env bound (idType id) hv
 
 mkRuntimeUnkTyVar :: Name -> Kind -> TyVar
diff --git a/ghc.mk b/ghc.mk
index eed172e48f..e0d5837a26 100644
--- a/ghc.mk
+++ b/ghc.mk
@@ -616,6 +616,7 @@ libraries/ghc-prim_dist-install_EXTRA_HADDOCK_SRCS = libraries/ghc-prim/dist-ins
 ifneq "$(CLEANING)" "YES"
 ifeq "$(INTEGER_LIBRARY)" "integer-gmp"
 libraries/base_dist-install_CONFIGURE_OPTS += --flags=integer-gmp
+compiler_stage2_CONFIGURE_OPTS += --flags=integer-gmp
 else ifeq "$(INTEGER_LIBRARY)" "integer-simple"
 libraries/base_dist-install_CONFIGURE_OPTS += --flags=integer-simple
 else
diff --git a/libraries/ghc-heap/GHC/Exts/Heap/ClosureTypes.hs b/libraries/ghc-heap/GHC/Exts/Heap/ClosureTypes.hs
index 7cd85fe99e..677e3b64e7 100644
--- a/libraries/ghc-heap/GHC/Exts/Heap/ClosureTypes.hs
+++ b/libraries/ghc-heap/GHC/Exts/Heap/ClosureTypes.hs
@@ -1,10 +1,13 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE DeriveGeneric #-}
 
 module GHC.Exts.Heap.ClosureTypes
     ( ClosureType(..)
     , closureTypeHeaderSize
     ) where
 
+import GHC.Generics
+
 {- ---------------------------------------------
 -- Enum representing closure types
 -- This is a mirror of:
@@ -77,7 +80,7 @@ data ClosureType
     | SMALL_MUT_ARR_PTRS_FROZEN_CLEAN
     | COMPACT_NFDATA
     | N_CLOSURE_TYPES
- deriving (Enum, Eq, Ord, Show)
+ deriving (Enum, Eq, Ord, Show, Generic)
 
 -- | Return the size of the closures header in words
 closureTypeHeaderSize :: ClosureType -> Int
diff --git a/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs b/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs
index 09a94a0f3f..bdfac8bf8b 100644
--- a/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs
+++ b/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs
@@ -4,6 +4,8 @@
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE UnliftedFFITypes #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveTraversable #-}
 
 module GHC.Exts.Heap.Closures (
     -- * Closures
@@ -35,6 +37,7 @@ import Data.Bits
 import Data.Int
 import Data.Word
 import GHC.Exts
+import GHC.Generics
 import Numeric
 
 ------------------------------------------------------------------------
@@ -222,7 +225,7 @@ data GenClosure b
     -- | A @MutVar#@
   | MutVarClosure
         { info       :: !StgInfoTable
-        , var        :: !b              -- ^ Pointer to closure
+        , var        :: !b              -- ^ Pointer to contents
         }
 
     -- | An STM blocking queue.
@@ -285,7 +288,7 @@ data GenClosure b
   | UnsupportedClosure
         { info       :: !StgInfoTable
         }
-  deriving (Show)
+  deriving (Show, Generic, Functor, Foldable, Traversable)
 
 
 data PrimType
@@ -296,7 +299,7 @@ data PrimType
   | PAddr
   | PFloat
   | PDouble
-  deriving (Eq, Show)
+  deriving (Eq, Show, Generic)
 
 -- | For generic code, this function returns all referenced closures.
 allClosures :: GenClosure b -> [b]
diff --git a/libraries/ghc-heap/GHC/Exts/Heap/InfoTable/Types.hsc b/libraries/ghc-heap/GHC/Exts/Heap/InfoTable/Types.hsc
index d8666d6b1d..0ba535d039 100644
--- a/libraries/ghc-heap/GHC/Exts/Heap/InfoTable/Types.hsc
+++ b/libraries/ghc-heap/GHC/Exts/Heap/InfoTable/Types.hsc
@@ -1,3 +1,4 @@
+{-# LANGUAGE DeriveGeneric #-}
 module GHC.Exts.Heap.InfoTable.Types
     ( StgInfoTable(..)
     , EntryFunPtr
@@ -7,6 +8,7 @@ module GHC.Exts.Heap.InfoTable.Types
 
 #include "Rts.h"
 
+import GHC.Generics
 import GHC.Exts.Heap.ClosureTypes
 import Foreign
 
@@ -34,4 +36,4 @@ data StgInfoTable = StgInfoTable {
    tipe   :: ClosureType,
    srtlen :: HalfWord,
    code   :: Maybe ItblCodes -- Just <=> ghciTablesNextToCode
-  } deriving (Show)
+  } deriving (Show, Generic)
diff --git a/libraries/ghci/GHCi/Message.hs b/libraries/ghci/GHCi/Message.hs
index 3f0bad9888..9b6740cc51 100644
--- a/libraries/ghci/GHCi/Message.hs
+++ b/libraries/ghci/GHCi/Message.hs
@@ -43,6 +43,7 @@ import Data.Dynamic
 import Data.Typeable (TypeRep)
 import Data.IORef
 import Data.Map (Map)
+import Foreign
 import GHC.Generics
 import GHC.Stack.CCS
 import qualified Language.Haskell.TH        as TH
@@ -202,6 +203,18 @@ data Message a where
                    -> [RemoteRef (TH.Q ())]
                    -> Message (QResult ())
 
+  -- | Remote interface to GHC.Exts.Heap.getClosureData. This is used by
+  -- the GHCi debugger to inspect values in the heap for :print and
+  -- type reconstruction.
+  GetClosure
+    :: HValueRef
+    -> Message (GenClosure HValueRef)
+
+  -- | Evaluate something. This is used to support :force in GHCi.
+  Seq
+    :: HValueRef
+    -> Message (EvalResult ())
+
 deriving instance Show (Message a)
 
 
@@ -410,6 +423,22 @@ data QState = QState
   }
 instance Show QState where show _ = "<QState>"
 
+-- Orphan instances of Binary for Ptr / FunPtr by conversion to Word64.
+-- This is to support Binary StgInfoTable which includes these.
+instance Binary (Ptr a) where
+  put p = put (fromIntegral (ptrToWordPtr p) :: Word64)
+  get = (wordPtrToPtr . fromIntegral) <$> (get :: Get Word64)
+
+instance Binary (FunPtr a) where
+  put = put . castFunPtrToPtr
+  get = castPtrToFunPtr <$> get
+
+-- Binary instances to support the GetClosure message
+instance Binary StgInfoTable
+instance Binary ClosureType
+instance Binary PrimType
+instance Binary a => Binary (GenClosure a)
+
 data Msg = forall a . (Binary a, Show a) => Msg (Message a)
 
 getMessage :: Get Msg
@@ -450,7 +479,9 @@ getMessage = do
       31 -> Msg <$> return StartTH
       32 -> Msg <$> (RunModFinalizers <$> get <*> get)
       33 -> Msg <$> (AddSptEntry <$> get <*> get)
-      _  -> Msg <$> (RunTH <$> get <*> get <*> get <*> get)
+      34 -> Msg <$> (RunTH <$> get <*> get <*> get <*> get)
+      35 -> Msg <$> (GetClosure <$> get)
+      _  -> Msg <$> (Seq <$> get)
 
 putMessage :: Message a -> Put
 putMessage m = case m of
@@ -489,6 +520,8 @@ putMessage m = case m of
   RunModFinalizers a b        -> putWord8 32 >> put a >> put b
   AddSptEntry a b             -> putWord8 33 >> put a >> put b
   RunTH st q loc ty           -> putWord8 34 >> put st >> put q >> put loc >> put ty
+  GetClosure a                -> putWord8 35 >> put a
+  Seq a                       -> putWord8 36 >> put a
 
 -- -----------------------------------------------------------------------------
 -- Reading/writing messages
diff --git a/libraries/ghci/GHCi/Run.hs b/libraries/ghci/GHCi/Run.hs
index 2988ec202a..8ec7659abe 100644
--- a/libraries/ghci/GHCi/Run.hs
+++ b/libraries/ghci/GHCi/Run.hs
@@ -31,8 +31,9 @@ import Data.Binary.Get
 import Data.ByteString (ByteString)
 import qualified Data.ByteString.Unsafe as B
 import GHC.Exts
+import GHC.Exts.Heap
 import GHC.Stack
-import Foreign
+import Foreign hiding (void)
 import Foreign.C
 import GHC.Conc.Sync
 import GHC.IO hiding ( bracket )
@@ -86,6 +87,10 @@ run m = case m of
   MkConInfoTable ptrs nptrs tag ptrtag desc ->
     toRemotePtr <$> mkConInfoTable ptrs nptrs tag ptrtag desc
   StartTH -> startTH
+  GetClosure ref -> do
+    clos <- getClosureData =<< localRef ref
+    mapM (\(Box x) -> mkRemoteRef (HValue x)) clos
+  Seq ref -> tryEval (void $ evaluate =<< localRef ref)
   _other -> error "GHCi.Run.run"
 
 evalStmt :: EvalOpts -> EvalExpr HValueRef -> IO (EvalStatus [HValueRef])
diff --git a/libraries/integer-gmp/src/GHC/Integer/Type.hs b/libraries/integer-gmp/src/GHC/Integer/Type.hs
index 9636b9f443..3434df29c4 100644
--- a/libraries/integer-gmp/src/GHC/Integer/Type.hs
+++ b/libraries/integer-gmp/src/GHC/Integer/Type.hs
@@ -150,6 +150,11 @@ data Integer  = S#                !Int#
               | Jn# {-# UNPACK #-} !BigNat
                 -- ^ iff value in @]-inf, minBound::'Int'[@ range
 
+-- NOTE: the above representation is baked into the GHCi debugger in
+-- compiler/ghci/RtClosureInspect.hs. If you change it here, fixes
+-- will be required over there too. Tests for this are in
+-- testsuite/tests/ghci.debugger.
+
 -- TODO: experiment with different constructor-ordering
 
 instance Eq Integer where
diff --git a/rts/Heap.c b/rts/Heap.c
index 7ab628d2da..dfd32aff0c 100644
--- a/rts/Heap.c
+++ b/rts/Heap.c
@@ -162,9 +162,14 @@ StgMutArrPtrs *heap_view_closurePtrs(Capability *cap, StgClosure *closure) {
 
         case AP_STACK:
             ptrs[nptrs++] = ((StgAP_STACK *)closure)->fun;
-            for (i = 0; i < ((StgAP_STACK *)closure)->size; ++i) {
-                ptrs[nptrs++] = ((StgAP_STACK *)closure)->payload[i];
-            }
+            /*
+              The payload is a stack, which consists of a mixture of pointers
+              and non-pointers.  We can't simply pretend it's all pointers,
+              because that will cause crashes in the GC later.  We could
+              traverse the stack and extract pointers and non-pointers, but that
+              would be complicated, so let's just ignore the payload for now.
+              See #15375.
+            */
             break;
 
         case BCO:
diff --git a/testsuite/tests/ghci.debugger/scripts/all.T b/testsuite/tests/ghci.debugger/scripts/all.T
index f2e2658d49..496c637fc6 100644
--- a/testsuite/tests/ghci.debugger/scripts/all.T
+++ b/testsuite/tests/ghci.debugger/scripts/all.T
@@ -1,4 +1,5 @@
 setTestOpts([extra_run_opts('-ignore-dot-ghci'),
+             extra_ways(['ghci-ext']), # test with -fexternal-interpreter
              normalise_slashes])
 
 test('print001', normal, ghci_script, ['print001.script'])
@@ -19,7 +20,12 @@ test('print016', extra_files(['../Test.hs']), ghci_script, ['print016.script'])
 test('print017', extra_files(['../Test.hs']), ghci_script, ['print017.script'])
 test('print018', extra_files(['../Test.hs']), ghci_script, ['print018.script'])
 test('print019', extra_files(['../Test.hs']), ghci_script, ['print019.script'])
-test('print020', extra_files(['../HappyTest.hs']), ghci_script, ['print020.script'])
+
+# The ghci-ext way emits messages in a slightly different order due to
+# printing from two processes, so let's just skip it.
+test('print020', [extra_files(['../HappyTest.hs']),
+                  omit_ways(['ghci-ext'])], ghci_script, ['print020.script'])
+
 test('print021', normal, ghci_script, ['print021.script'])
 test('print022',
      [when(arch('powerpc64'), expect_broken(14455))],