Remove concept of stable module

33 files changed, 314 insertions, 392 deletions

diff --git a/compiler/GHC.hs b/compiler/GHC.hs
index 9d3e5235f1..f5c1d89b95 100644
--- a/compiler/GHC.hs
+++ b/compiler/GHC.hs
@@ -56,7 +56,7 @@ module GHC (
         SuccessFlag(..), succeeded, failed,
         defaultWarnErrLogger, WarnErrLogger,
         workingDirectoryChanged,
-        parseModule, typecheckModule, desugarModule, loadModule,
+        parseModule, typecheckModule, desugarModule,
         ParsedModule(..), TypecheckedModule(..), DesugaredModule(..),
         TypecheckedSource, ParsedSource, RenamedSource,   -- ditto
         TypecheckedMod, ParsedMod,
@@ -315,7 +315,6 @@ import GHC.Driver.Config
 import GHC.Driver.Main
 import GHC.Driver.Make
 import GHC.Driver.Hooks
-import GHC.Driver.Pipeline   ( compileOne', doesIfaceHashMatch )
 import GHC.Driver.Monad
 import GHC.Driver.Ppr
 
@@ -390,7 +389,6 @@ import GHC.Types.TyThing
 import GHC.Types.Name.Env
 import GHC.Types.Name.Ppr
 import GHC.Types.TypeEnv
-import GHC.Types.SourceFile
 import GHC.Types.Error ( DiagnosticMessage )
 
 import GHC.Unit
@@ -1188,47 +1186,6 @@ desugarModule tcm = do
        dm_core_module        = guts
      }
 
--- | Load a module.  Input doesn't need to be desugared.
---
--- A module must be loaded before dependent modules can be typechecked.  This
--- always includes generating a 'ModIface' and, depending on the
--- @DynFlags@\'s 'GHC.Driver.Session.backend', may also include code generation.
---
--- This function will always cause recompilation and will always overwrite
--- previous compilation results (potentially files on disk).
---
-loadModule :: (TypecheckedMod mod, GhcMonad m) => mod -> m mod
-loadModule tcm = do
-   let ms = modSummary tcm
-   let mod = ms_mod_name ms
-   let loc = ms_location ms
-   let (tcg, _details) = tm_internals tcm
-
-   hsc_env <- getSession
-   mb_linkable <-
-      case (ms_iface_date ms, ms_obj_date ms) of
-          -- See Note [When source is considered modified]
-          (Just hi_date, Just obj_date) | obj_date >= hi_date -> liftIO $ do
-              prev_hash_matches <- doesIfaceHashMatch hsc_env ms
-              if prev_hash_matches
-                  then fmap Just $ findObjectLinkable
-                                          (ms_mod ms)
-                                          (ml_obj_file loc)
-                                          obj_date
-                  else return Nothing
-          _ -> return Nothing
-
-   let source_modified | isNothing mb_linkable = SourceModified
-                       | otherwise             = SourceUnmodified
-                       -- we can't determine stability here
-
-   -- compile doesn't change the session
-   mod_info <- liftIO $ compileOne' (Just tcg) Nothing
-                                    hsc_env ms 1 1 Nothing mb_linkable
-                                    source_modified
-
-   modifySession $ hscUpdateHPT (\hpt -> addToHpt hpt mod mod_info)
-   return tcm
 
 
 -- %************************************************************************
diff --git a/compiler/GHC/Core/Opt/Specialise.hs b/compiler/GHC/Core/Opt/Specialise.hs
index fc62f5fa8a..9f121c9c3f 100644
--- a/compiler/GHC/Core/Opt/Specialise.hs
+++ b/compiler/GHC/Core/Opt/Specialise.hs
@@ -2560,7 +2560,7 @@ data CallInfoSet = CIS Id (Bag CallInfo)
 data CallInfo
   = CI { ci_key  :: [SpecArg]   -- All arguments
        , ci_fvs  :: IdSet       -- Free Ids of the ci_key call
-                                -- *not* including the main id itself, of course
+                                -- _not_ including the main id itself, of course
                                 -- NB: excluding tyvars:
                                 --     See Note [Specialising polymorphic dictionaries]
     }
diff --git a/compiler/GHC/Driver/Main.hs b/compiler/GHC/Driver/Main.hs
index 7d43d6b336..3d5916cc05 100644
--- a/compiler/GHC/Driver/Main.hs
+++ b/compiler/GHC/Driver/Main.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE NondecreasingIndentation #-}
+{-# LANGUAGE TupleSections #-}
 
 {-# OPTIONS_GHC -fprof-auto-top #-}
 
@@ -86,6 +87,7 @@ module GHC.Driver.Main
     , ioMsgMaybe
     , showModuleIndex
     , hscAddSptEntries
+    , writeInterfaceOnlyMode
     ) where
 
 import GHC.Prelude
@@ -691,13 +693,14 @@ hscIncrementalFrontend :: Bool -- always do basic recompilation check?
                        -> Maybe Messager
                        -> ModSummary
                        -> SourceModified
+                       -> Maybe Linkable
                        -> Maybe ModIface  -- Old interface, if available
                        -> (Int,Int)       -- (i,n) = module i of n (for msgs)
-                       -> Hsc (Either ModIface (FrontendResult, Maybe Fingerprint))
+                       -> Hsc (Either (ModIface, Maybe Linkable) (FrontendResult, Maybe Fingerprint))
 
 hscIncrementalFrontend
   always_do_basic_recompilation_check m_tc_result
-  mHscMessage mod_summary source_modified mb_old_iface mod_index
+  mHscMessage mod_summary source_modified old_linkable mb_old_iface mod_index
     = do
     hsc_env <- getHscEnv
 
@@ -717,19 +720,42 @@ hscIncrementalFrontend
               Just h  -> h mod_summary
             return $ Right (tc_result, mb_old_hash)
 
-        stable = case source_modified of
-                     SourceUnmodifiedAndStable -> True
-                     _                         -> False
 
     case m_tc_result of
+         -- This case only happens from loadModule, which is not used
+         -- anywhere
          Just tc_result
           | not always_do_basic_recompilation_check ->
              return $ Right (FrontendTypecheck tc_result, Nothing)
+
          _ -> do
-            (recomp_reqd, mb_checked_iface)
+            -- First check to see if the interface file agrees with the
+            -- source file.
+            (recomp_iface_reqd, mb_checked_iface)
                 <- {-# SCC "checkOldIface" #-}
                    liftIO $ checkOldIface hsc_env mod_summary
                                 source_modified mb_old_iface
+
+            -- Check to see whether the expected build products already exist.
+            -- If they don't exists then we trigger recompilation.
+            let lcl_dflags = ms_hspp_opts mod_summary
+            (recomp_obj_reqd, mb_linkable) <-
+              case () of
+                -- No need for a linkable, we're good to go
+                _ | writeInterfaceOnlyMode lcl_dflags -> return (UpToDate, Nothing)
+                  -- Interpreter can use either already loaded bytecode or loaded object code
+                  | not (backendProducesObject (backend lcl_dflags)) -> do
+                      res <- liftIO $ checkByteCode old_linkable
+                      case res of
+                        (_, Just{}) -> return res
+                        _ -> liftIO $ checkObjects old_linkable mod_summary
+                  -- Need object files for making object files
+                  | backendProducesObject (backend lcl_dflags) -> liftIO $ checkObjects old_linkable mod_summary
+                  | otherwise -> pprPanic "hscIncrementalFrontend" (text $ show $ backend lcl_dflags)
+
+
+            let recomp_reqd = recomp_iface_reqd `mappend` recomp_obj_reqd
+
             -- save the interface that comes back from checkOldIface.
             -- In one-shot mode we don't have the old iface until this
             -- point, when checkOldIface reads it from the disk.
@@ -737,31 +763,45 @@ hscIncrementalFrontend
 
             case mb_checked_iface of
                 Just iface | not (recompileRequired recomp_reqd) ->
-                    -- If the module used TH splices when it was last
-                    -- compiled, then the recompilation check is not
-                    -- accurate enough (#481) and we must ignore
-                    -- it.  However, if the module is stable (none of
-                    -- the modules it depends on, directly or
-                    -- indirectly, changed), then we *can* skip
-                    -- recompilation. This is why the SourceModified
-                    -- type contains SourceUnmodifiedAndStable, and
-                    -- it's pretty important: otherwise ghc --make
-                    -- would always recompile TH modules, even if
-                    -- nothing at all has changed. Stability is just
-                    -- the same check that make is doing for us in
-                    -- one-shot mode.
-                    case m_tc_result of
-                    Nothing
-                     | mi_used_th iface && not stable ->
-                        compile mb_old_hash (RecompBecause "TH")
-                    _ ->
-                        skip iface
+                  skip (iface, mb_linkable)
                 _ ->
-                    case m_tc_result of
+                  case m_tc_result of
                     Nothing -> compile mb_old_hash recomp_reqd
                     Just tc_result ->
                         return $ Right (FrontendTypecheck tc_result, mb_old_hash)
 
+-- | Check that the .o files produced by compilation are already up-to-date
+-- or not.
+checkObjects :: Maybe Linkable -> ModSummary -> IO (RecompileRequired, Maybe Linkable)
+checkObjects mb_old_linkable summary =
+  let
+    this_mod    = ms_mod summary
+    mb_obj_date = ms_obj_date summary
+    mb_if_date  = ms_iface_date summary
+    obj_fn      = ml_obj_file (ms_location summary)
+  in do
+    case (,) <$> mb_obj_date <*> mb_if_date of
+      Just (obj_date, if_date)
+        | obj_date >= if_date ->
+            case mb_old_linkable of
+              Just old_linkable
+                | isObjectLinkable old_linkable, linkableTime old_linkable == obj_date
+                -> return $ (UpToDate, Just old_linkable)
+              _ -> (UpToDate,) . Just <$> findObjectLinkable this_mod obj_fn obj_date
+      _ -> return (MustCompile, Nothing)
+
+-- | Check to see if we can reuse the old linkable, by this point we will
+-- have just checked that the old interface matches up with the source hash, so
+-- no need to check that again here
+checkByteCode ::  Maybe Linkable -> IO (RecompileRequired, Maybe Linkable)
+checkByteCode mb_old_linkable =
+  case mb_old_linkable of
+    Just old_linkable
+      | not (isObjectLinkable old_linkable)
+      -> return $ (UpToDate, Just old_linkable)
+    _ -> return $ (MustCompile, Nothing)
+
+
 --------------------------------------------------------------
 -- Compilers
 --------------------------------------------------------------
@@ -777,11 +817,12 @@ hscIncrementalCompile :: Bool
                       -> HscEnv
                       -> ModSummary
                       -> SourceModified
+                      -> Maybe Linkable
                       -> Maybe ModIface
                       -> (Int,Int)
                       -> IO (HscStatus, HscEnv)
 hscIncrementalCompile always_do_basic_recompilation_check m_tc_result
-    mHscMessage hsc_env' mod_summary source_modified mb_old_iface mod_index
+    mHscMessage hsc_env' mod_summary source_modified old_linkable mb_old_iface mod_index
   = do
     hsc_env'' <- initializePlugins hsc_env'
 
@@ -801,13 +842,13 @@ hscIncrementalCompile always_do_basic_recompilation_check m_tc_result
     -- because the desugarer runs ioMsgMaybe.)
     runHsc hsc_env $ do
     e <- hscIncrementalFrontend always_do_basic_recompilation_check m_tc_result mHscMessage
-            mod_summary source_modified mb_old_iface mod_index
+            mod_summary source_modified old_linkable mb_old_iface mod_index
     case e of
         -- We didn't need to do any typechecking; the old interface
         -- file on disk was good enough.
-        Left iface -> do
+        Left (iface, linkable) -> do
             details <- liftIO $ initModDetails hsc_env mod_summary iface
-            return (HscUpToDate iface details, hsc_env')
+            return (HscUpToDate (HomeModInfo iface details linkable), hsc_env')
         -- We finished type checking.  (mb_old_hash is the hash of
         -- the interface that existed on disk; it's possible we had
         -- to retypecheck but the resulting interface is exactly
@@ -940,7 +981,7 @@ finish summary tc_result mb_old_hash = do
         liftIO $ hscMaybeWriteIface logger dflags True iface mb_old_iface_hash (ms_location summary)
 
         return $ case bcknd of
-          NoBackend -> HscNotGeneratingCode iface details
+          NoBackend -> HscNotGeneratingCode (HomeModInfo iface details Nothing)
           _         -> case hsc_src of
                         HsBootFile -> HscUpdateBoot iface details
                         HsigFile   -> HscUpdateSig iface details
@@ -2226,3 +2267,8 @@ showModuleIndex (i,n) = text "[" <> pad <> int i <> text " of " <> int n <> text
     -- compute the length of x > 0 in base 10
     len x = ceiling (logBase 10 (fromIntegral x+1) :: Float)
     pad = text (replicate (len n - len i) ' ') -- TODO: use GHC.Utils.Ppr.RStr
+
+writeInterfaceOnlyMode :: DynFlags -> Bool
+writeInterfaceOnlyMode dflags =
+ gopt Opt_WriteInterface dflags &&
+ NoBackend == backend dflags
diff --git a/compiler/GHC/Driver/Make.hs b/compiler/GHC/Driver/Make.hs
index bff2e770cd..e9e152ef7f 100644
--- a/compiler/GHC/Driver/Make.hs
+++ b/compiler/GHC/Driver/Make.hs
@@ -125,7 +125,7 @@ import qualified Control.Monad.Catch as MC
 import Data.IORef
 import Data.List (nub, sort, sortBy, partition)
 import qualified Data.List as List
-import Data.Foldable (toList, foldlM)
+import Data.Foldable (toList)
 import Data.Maybe
 import Data.Ord ( comparing )
 import Data.Time
@@ -477,8 +477,7 @@ load' how_much mHscMessage mod_graph = do
     warnUnnecessarySourceImports mg2_with_srcimps
 
     -- check the stability property for each module.
-    stable_mods@(stable_obj,stable_bco) <-
-        liftIO $ checkStability hsc_env hpt1 mg2_with_srcimps all_home_mods
+    let stable_mods = (emptyUniqSet, emptyUniqSet)
 
     let
         -- prune bits of the HPT which are definitely redundant now,
@@ -494,19 +493,9 @@ load' how_much mHscMessage mod_graph = do
     -- write the pruned HPT to allow the old HPT to be GC'd.
     setSession $ discardIC $ hscUpdateHPT (const pruned_hpt) hsc_env
 
-    liftIO $ debugTraceMsg logger dflags 2 (text "Stable obj:" <+> ppr stable_obj $$
-                            text "Stable BCO:" <+> ppr stable_bco)
+    -- Unload everything
+    liftIO $ unload interp hsc_env []
 
-    -- Unload any modules which are going to be re-linked this time around.
-    let stable_linkables = [ linkable
-                           | m <- nonDetEltsUniqSet stable_obj ++
-                                  nonDetEltsUniqSet stable_bco,
-                             -- It's OK to use nonDetEltsUniqSet here
-                             -- because it only affects linking. Besides
-                             -- this list only serves as a poor man's set.
-                             Just hmi <- [lookupHpt pruned_hpt m],
-                             Just linkable <- [hm_linkable hmi] ]
-    liftIO $ unload interp hsc_env stable_linkables
 
     -- We could at this point detect cycles which aren't broken by
     -- a source-import, and complain immediately, but it seems better
@@ -524,9 +513,7 @@ load' how_much mHscMessage mod_graph = do
     -- This graph should be cycle-free.
     -- If we're restricting the upsweep to a portion of the graph, we
     -- also want to retain everything that is still stable.
-    let full_mg, partial_mg0, partial_mg, unstable_mg :: [SCC ModuleGraphNode]
-        stable_mg :: [SCC ExtendedModSummary]
-        full_mg    = topSortModuleGraph False mod_graph Nothing
+    let partial_mg0, partial_mg:: [SCC ModuleGraphNode]
 
         maybe_top_mod = case how_much of
                             LoadUpTo m           -> Just m
@@ -546,27 +533,7 @@ load' how_much mHscMessage mod_graph = do
             | otherwise
             = partial_mg0
 
-        stable_mg =
-            [ AcyclicSCC ems
-            | AcyclicSCC (ModuleNode ems@(ExtendedModSummary ms _)) <- full_mg
-            , stable_mod_summary ms
-            ]
-
-        stable_mod_summary ms =
-          ms_mod_name ms `elementOfUniqSet` stable_obj ||
-          ms_mod_name ms `elementOfUniqSet` stable_bco
-
-        -- the modules from partial_mg that are not also stable
-        -- NB. also keep cycles, we need to emit an error message later
-        unstable_mg = filter not_stable partial_mg
-          where not_stable (CyclicSCC _) = True
-                not_stable (AcyclicSCC (InstantiationNode _)) = True
-                not_stable (AcyclicSCC (ModuleNode (ExtendedModSummary ms _)))
-                   = not $ stable_mod_summary ms
-
-        -- Load all the stable modules first, before attempting to load
-        -- an unstable module (#7231).
-        mg = fmap (fmap ModuleNode) stable_mg ++ unstable_mg
+        mg = partial_mg
 
     liftIO $ debugTraceMsg logger dflags 2 (hang (text "Ready for upsweep")
                                2 (ppr mg))
@@ -579,7 +546,7 @@ load' how_much mHscMessage mod_graph = do
 
     setSession $ hscUpdateHPT (const emptyHomePackageTable) hsc_env
     (upsweep_ok, modsUpswept) <- withDeferredDiagnostics $
-      upsweep_fn mHscMessage pruned_hpt stable_mods mg
+      upsweep_fn mHscMessage pruned_hpt mg
 
     -- Make modsDone be the summaries for each home module now
     -- available; this should equal the domain of hpt3.
@@ -913,76 +880,6 @@ type StableModules =
   )
 
 
-checkStability
-        :: HscEnv
-        -> HomePackageTable   -- HPT from last compilation
-        -> [SCC ModSummary]   -- current module graph (cyclic)
-        -> UniqSet ModuleName -- all home modules
-        -> IO StableModules
-
-checkStability hsc_env hpt sccs all_home_mods =
-  foldlM checkSCC (emptyUniqSet, emptyUniqSet) sccs
-  where
-   checkSCC :: StableModules -> SCC ModSummary -> IO StableModules
-   checkSCC (!stable_obj, !stable_bco) scc0 = do
-       stableObjects <- checkStableObjects
-       return $ case () of
-        _ | stableObjects -> (addListToUniqSet stable_obj scc_mods, stable_bco)
-          | stableBCOs    -> (stable_obj, addListToUniqSet stable_bco scc_mods)
-          | otherwise     -> (stable_obj, stable_bco)
-     where
-        scc = flattenSCC scc0
-        scc_mods = map ms_mod_name scc
-        home_module m =
-          m `elementOfUniqSet` all_home_mods && m `notElem` scc_mods
-
-        scc_allimps = nub (filter home_module (concatMap ms_home_allimps scc))
-            -- all imports outside the current SCC, but in the home pkg
-
-        stable_obj_imps = map (`elementOfUniqSet` stable_obj) scc_allimps
-        stable_bco_imps = map (`elementOfUniqSet` stable_bco) scc_allimps
-
-        checkStableObjects = do
-           if and stable_obj_imps
-              then allM object_ok scc
-              else return False
-
-        stableBCOs =
-           and (zipWith (||) stable_obj_imps stable_bco_imps)
-           && all bco_ok scc
-
-        object_ok ms
-          | gopt Opt_ForceRecomp (ms_hspp_opts ms) = return False
-          | Just obj_date <- ms_obj_date ms
-          , Just hi_date <- ms_iface_date ms
-          , obj_date >= hi_date = do
-                mb_hi_hash <- readIfaceSourceHash' hsc_env ms
-                case mb_hi_hash of
-                    Nothing -> return False
-                    Just hi_hash -> return $
-                        hi_hash == ms_hs_hash ms &&
-                        same_as_prev obj_date
-          | otherwise = return False
-          where
-             same_as_prev t = case lookupHpt hpt (ms_mod_name ms) of
-                Just hmi  | Just l <- hm_linkable hmi ->
-                              isObjectLinkable l &&
-                              t == linkableTime l
-                _other  -> True
-
-        bco_ok ms
-          | gopt Opt_ForceRecomp (ms_hspp_opts ms) = False
-          | otherwise = case lookupHpt hpt (ms_mod_name ms) of
-                Just hmi  | Just l <- hm_linkable hmi ->
-                              not (isObjectLinkable l) &&
-                              -- We check the hash from the HomeModInfo here
-                              -- instead of the linkableTime, because if we got
-                              -- here then the linkable doesn't represent a
-                              -- file on disk and the time is therefore mostly
-                              -- meaningless
-                              mi_src_hash (hm_iface hmi) == ms_hs_hash ms
-                _other  -> False
-
 {- Parallel Upsweep
  -
  - The parallel upsweep attempts to concurrently compile the modules in the
@@ -1040,13 +937,6 @@ buildCompGraph (scc:sccs) = case scc of
 data BuildModule = BuildModule_Unit {-# UNPACK #-} !InstantiatedUnit | BuildModule_Module {-# UNPACK #-} !ModuleWithIsBoot
   deriving (Eq, Ord)
 
--- | Tests if an 'HscSource' is a boot file, primarily for constructing elements
--- of 'BuildModule'. We conflate signatures and modules because they are bound
--- in the same namespace; only boot interfaces can be disambiguated with
--- `import {-# SOURCE #-}`.
-hscSourceToIsBoot :: HscSource -> IsBootInterface
-hscSourceToIsBoot HsBootFile = IsBoot
-hscSourceToIsBoot _ = NotBoot
 
 mkBuildModule :: ModuleGraphNode -> BuildModule
 mkBuildModule = \case
@@ -1079,11 +969,10 @@ parUpsweep
     -- ^ The number of workers we wish to run in parallel
     -> Maybe Messager
     -> HomePackageTable
-    -> StableModules
     -> [SCC ModuleGraphNode]
     -> m (SuccessFlag,
           [ModuleGraphNode])
-parUpsweep n_jobs mHscMessage old_hpt stable_mods sccs = do
+parUpsweep n_jobs mHscMessage old_hpt sccs = do
     hsc_env <- getSession
     let dflags = hsc_dflags hsc_env
     let logger = hsc_logger hsc_env
@@ -1208,7 +1097,7 @@ parUpsweep n_jobs mHscMessage old_hpt stable_mods sccs = do
                                      lcl_logger lcl_tmpfs dflags (hsc_home_unit hsc_env)
                                      mHscMessage
                                      par_sem hsc_env_var old_hpt_var
-                                     stable_mods mod_idx (length sccs)
+                                     mod_idx (length sccs)
 
                 res <- case m_res of
                     Right flag -> return flag
@@ -1317,8 +1206,6 @@ parUpsweep_one
     -- ^ The MVar that synchronizes updates to the global HscEnv
     -> IORef HomePackageTable
     -- ^ The old HPT
-    -> StableModules
-    -- ^ Sets of stable objects and BCOs
     -> Int
     -- ^ The index of this module
     -> Int
@@ -1326,7 +1213,7 @@ parUpsweep_one
     -> IO SuccessFlag
     -- ^ The result of this compile
 parUpsweep_one mod home_mod_map comp_graph_loops lcl_logger lcl_tmpfs lcl_dflags home_unit mHscMessage par_sem
-               hsc_env_var old_hpt_var stable_mods mod_index num_mods = do
+               hsc_env_var old_hpt_var mod_index num_mods = do
 
     let this_build_mod = mkBuildModule0 mod
 
@@ -1456,7 +1343,7 @@ parUpsweep_one mod home_mod_map comp_graph_loops lcl_logger lcl_tmpfs lcl_dflags
                                  map (moduleName . gwib_mod) loop
 
                 -- Compile the module.
-                mod_info <- upsweep_mod lcl_hsc_env'' mHscMessage old_hpt stable_mods
+                mod_info <- upsweep_mod lcl_hsc_env'' mHscMessage old_hpt
                                         mod mod_index num_mods
                 return (Just mod_info)
 
@@ -1510,7 +1397,6 @@ upsweep
     .  GhcMonad m
     => Maybe Messager
     -> HomePackageTable            -- ^ HPT from last time round (pruned)
-    -> StableModules               -- ^ stable modules (see checkStability)
     -> [SCC ModuleGraphNode]       -- ^ Mods to do (the worklist)
     -> m (SuccessFlag,
           [ModuleGraphNode])
@@ -1520,7 +1406,7 @@ upsweep
        --  2. The 'HscEnv' in the monad has an updated HPT
        --  3. A list of modules which succeeded loading.
 
-upsweep mHscMessage old_hpt stable_mods sccs = do
+upsweep mHscMessage old_hpt sccs = do
    (res, done) <- upsweep' old_hpt emptyMG sccs 1 (length sccs)
    return (res, reverse $ mgModSummaries' done)
  where
@@ -1609,7 +1495,7 @@ upsweep mHscMessage old_hpt stable_mods sccs = do
         mb_mod_info
             <- handleSourceError
                    (\err -> do logg mod (Just err); return Nothing) $ do
-                 mod_info <- liftIO $ upsweep_mod hsc_env2 mHscMessage old_hpt stable_mods
+                 mod_info <- liftIO $ upsweep_mod hsc_env2 mHscMessage old_hpt
                                                   mod mod_index nmods
                  logg mod Nothing -- log warnings
                  return (Just mod_info)
@@ -1680,21 +1566,13 @@ upsweep_inst hsc_env mHscMessage mod_index nmods iuid = do
 upsweep_mod :: HscEnv
             -> Maybe Messager
             -> HomePackageTable
-            -> StableModules
             -> ModSummary
             -> Int  -- index of module
             -> Int  -- total number of modules
             -> IO HomeModInfo
-upsweep_mod hsc_env mHscMessage old_hpt (stable_obj, stable_bco) summary mod_index nmods
+upsweep_mod hsc_env mHscMessage old_hpt summary mod_index nmods
    =    let
             this_mod_name = ms_mod_name summary
-            this_mod    = ms_mod summary
-            mb_obj_date = ms_obj_date summary
-            mb_if_date  = ms_iface_date summary
-            obj_fn      = ml_obj_file (ms_location summary)
-
-            is_stable_obj = this_mod_name `elementOfUniqSet` stable_obj
-            is_stable_bco = this_mod_name `elementOfUniqSet` stable_bco
 
             old_hmi = lookupHpt old_hpt this_mod_name
 
@@ -1739,111 +1617,13 @@ upsweep_mod hsc_env mHscMessage old_hpt (stable_obj, stable_bco) summary mod_ind
                                    where
                                      iface = hm_iface hm_info
 
-            compile_it :: Maybe Linkable -> SourceModified -> IO HomeModInfo
-            compile_it  mb_linkable src_modified =
-                  compileOne' Nothing mHscMessage hsc_env summary' mod_index nmods
-                             mb_old_iface mb_linkable src_modified
-
-            compile_it_discard_iface :: Maybe Linkable -> SourceModified
-                                     -> IO HomeModInfo
-            compile_it_discard_iface mb_linkable  src_modified =
+            compile_it :: Maybe Linkable -> IO HomeModInfo
+            compile_it  mb_linkable =
                   compileOne' Nothing mHscMessage hsc_env summary' mod_index nmods
-                             Nothing mb_linkable src_modified
-
-            -- With NoBackend we create empty linkables to avoid recompilation.
-            -- We have to detect these to recompile anyway if the backend changed
-            -- since the last compile.
-            is_fake_linkable
-               | Just hmi <- old_hmi, Just l <- hm_linkable hmi =
-                  null (linkableUnlinked l)
-               | otherwise =
-                   -- we have no linkable, so it cannot be fake
-                   False
-
-            implies False _ = True
-            implies True x  = x
-
-            debug_trace n t = liftIO $ debugTraceMsg (hsc_logger hsc_env) (hsc_dflags hsc_env) n t
+                             mb_old_iface mb_linkable
 
         in
-        case () of
-         _
-                -- Regardless of whether we're generating object code or
-                -- byte code, we can always use an existing object file
-                -- if it is *stable* (see checkStability).
-          | is_stable_obj, Just hmi <- old_hmi -> do
-                debug_trace 5 (text "skipping stable obj mod:" <+> ppr this_mod_name)
-                return hmi
-                -- object is stable, and we have an entry in the
-                -- old HPT: nothing to do
-
-          | is_stable_obj, isNothing old_hmi -> do
-                debug_trace 5 (text "compiling stable on-disk mod:" <+> ppr this_mod_name)
-                linkable <- findObjectLinkable this_mod obj_fn
-                                (expectJust "upsweep1" mb_obj_date)
-                compile_it (Just linkable) SourceUnmodifiedAndStable
-                -- object is stable, but we need to load the interface
-                -- off disk to make a HMI.
-
-          | not (backendProducesObject bcknd), is_stable_bco,
-            (bcknd /= NoBackend) `implies` not is_fake_linkable ->
-                ASSERT(isJust old_hmi) -- must be in the old_hpt
-                let Just hmi = old_hmi in do
-                debug_trace 5 (text "skipping stable BCO mod:" <+> ppr this_mod_name)
-                return hmi
-                -- BCO is stable: nothing to do
-
-          | not (backendProducesObject bcknd),
-            Just hmi <- old_hmi,
-            Just l <- hm_linkable hmi,
-            not (isObjectLinkable l),
-            (bcknd /= NoBackend) `implies` not is_fake_linkable,
-            mi_src_hash (hm_iface hmi) == ms_hs_hash summary -> do
-                debug_trace 5 (text "compiling non-stable BCO mod:" <+> ppr this_mod_name)
-                compile_it (Just l) SourceUnmodified
-                -- we have an old BCO that is up to date with respect
-                -- to the source: do a recompilation check as normal.
-
-          -- When generating object code, if there's an up-to-date
-          -- object file on the disk, then we can use it.
-          -- However, if the object file is new (compared to any
-          -- linkable we had from a previous compilation), then we
-          -- must discard any in-memory interface, because this
-          -- means the user has compiled the source file
-          -- separately and generated a new interface, that we must
-          -- read from the disk. See Note [When source is considered modified]
-          | backendProducesObject bcknd,
-            Just obj_date <- mb_obj_date,
-            Just if_date <- mb_if_date,
-            obj_date >= if_date -> do
-                prev_hash_matches <- doesIfaceHashMatch hsc_env summary
-                if prev_hash_matches
-                    then case old_hmi of
-                        Just hmi
-                          | Just l <- hm_linkable hmi,
-                            isObjectLinkable l && linkableTime l == obj_date -> do
-                                debug_trace 5 (text "compiling mod with new on-disk obj:" <+> ppr this_mod_name)
-                                compile_it (Just l) SourceUnmodified
-                        _otherwise -> do
-                                debug_trace 5 (text "compiling mod with new on-disk obj2:" <+> ppr this_mod_name)
-                                linkable <- findObjectLinkable this_mod obj_fn obj_date
-                                compile_it_discard_iface (Just linkable) SourceUnmodified
-                    else compile_it Nothing SourceModified
-
-          -- See Note [When source is considered modified]
-          | writeInterfaceOnlyMode lcl_dflags -> do
-                prev_hash_matches <- doesIfaceHashMatch hsc_env summary
-                if prev_hash_matches
-                    then do
-                        debug_trace 5 (text "skipping tc'd mod:" <+> ppr this_mod_name)
-                        compile_it Nothing SourceUnmodified
-                    else do
-                        debug_trace 5 (text "re-tc'ing mod with new on-disk source:" <+> ppr this_mod_name)
-                        compile_it Nothing SourceModified
-
-         _otherwise -> do
-                debug_trace 5 (text "compiling mod:" <+> ppr this_mod_name)
-                compile_it Nothing SourceModified
+          compile_it (old_hmi >>= hm_linkable)
 
 
 {- Note [-fno-code mode]
@@ -2865,11 +2645,15 @@ makeNewModSummary hsc_env MakeNewModSummary{..} = do
     , emsInstantiatedUnits = inst_deps
     }
 
+
+-- This function used to return Nothing for hs-boot.. not sure why..
+-- 19519dc35bad5649226a9f7015eaabb154722e54
+-- This causes hs-boot files to always be recompiled, they should obey the
+-- same recompilation discipline as normal source files.
 getObjTimestamp :: ModLocation -> IsBootInterface -> IO (Maybe UTCTime)
 getObjTimestamp location is_boot
   = case is_boot of
-      IsBoot -> return Nothing
-      NotBoot -> modificationTimeIfExists (ml_obj_file location)
+      _ -> modificationTimeIfExists (ml_obj_file location)
 
 data PreprocessedImports
   = PreprocessedImports
diff --git a/compiler/GHC/Driver/Pipeline.hs b/compiler/GHC/Driver/Pipeline.hs
index 3c189a8883..c8d2d528c9 100644
--- a/compiler/GHC/Driver/Pipeline.hs
+++ b/compiler/GHC/Driver/Pipeline.hs
@@ -31,7 +31,6 @@ module GHC.Driver.Pipeline (
    phaseOutputFilename, getOutputFilename, getPipeState, getPipeEnv,
    hscPostBackendPhase, getLocation, setModLocation, setDynFlags,
    runPhase,
-   readIfaceSourceHash', doesIfaceHashMatch,
    doCpp,
    linkingNeeded, checkLinkInfo, writeInterfaceOnlyMode
   ) where
@@ -89,7 +88,7 @@ import GHC.Data.StringBuffer   ( hGetStringBuffer, hPutStringBuffer )
 import GHC.Data.Maybe          ( expectJust )
 
 import GHC.Iface.Make          ( mkFullIface )
-import GHC.Iface.Load
+
 
 import GHC.Types.Basic       ( SuccessFlag(..) )
 import GHC.Types.Target
@@ -175,7 +174,6 @@ compileOne :: HscEnv
            -> Int             -- ^ ... of M
            -> Maybe ModIface  -- ^ old interface, if we have one
            -> Maybe Linkable  -- ^ old linkable, if we have one
-           -> SourceModified
            -> IO HomeModInfo   -- ^ the complete HomeModInfo, if successful
 
 compileOne = compileOne' Nothing (Just batchMsg)
@@ -188,12 +186,10 @@ compileOne' :: Maybe TcGblEnv
             -> Int             -- ^ ... of M
             -> Maybe ModIface  -- ^ old interface, if we have one
             -> Maybe Linkable  -- ^ old linkable, if we have one
-            -> SourceModified
             -> IO HomeModInfo   -- ^ the complete HomeModInfo, if successful
 
 compileOne' m_tc_result mHscMessage
             hsc_env0 summary mod_index nmods mb_old_iface mb_old_linkable
-            source_modified0
  = do
 
    let logger = hsc_logger hsc_env0
@@ -204,7 +200,7 @@ compileOne' m_tc_result mHscMessage
    (status, plugin_hsc_env) <- hscIncrementalCompile
                         always_do_basic_recompilation_check
                         m_tc_result mHscMessage
-                        hsc_env summary source_modified mb_old_iface (mod_index, nmods)
+                        hsc_env summary source_modified mb_old_linkable mb_old_iface (mod_index, nmods)
    -- Use an HscEnv updated with the plugin info
    let hsc_env' = plugin_hsc_env
 
@@ -217,17 +213,17 @@ compileOne' m_tc_result mHscMessage
                    [ml_obj_file $ ms_location summary]
 
    case (status, bcknd) of
-        (HscUpToDate iface hmi_details, _) ->
+        (HscUpToDate hmi, _) ->
             -- TODO recomp014 triggers this assert. What's going on?!
             -- ASSERT( isJust mb_old_linkable || isNoLink (ghcLink dflags) )
-            return $! HomeModInfo iface hmi_details mb_old_linkable
-        (HscNotGeneratingCode iface hmi_details, NoBackend) -> do
-            unlinked_time <- getCurrentTime
-            let mb_linkable = if isHsBootOrSig src_flavour
-                                then Nothing
-                                else Just (LM unlinked_time this_mod [])
-            return $! HomeModInfo iface hmi_details mb_linkable
-        (HscNotGeneratingCode _ _, _) -> panic "compileOne HscNotGeneratingCode"
+            return $! hmi
+        (HscNotGeneratingCode hmi, NoBackend) -> do
+--            unlinked_time <- getCurrentTime
+--            let mb_linkable = if isHsBootOrSig src_flavour
+--                                then Nothing
+--                                else Just (LM unlinked_time this_mod [])
+            return $! hmi
+        (HscNotGeneratingCode _, _) -> panic "compileOne HscNotGeneratingCode"
         (_, NoBackend) -> panic "compileOne NoBackend"
         (HscUpdateBoot iface hmi_details, Interpreter) ->
             return $! HomeModInfo iface hmi_details Nothing
@@ -367,7 +363,7 @@ compileOne' m_tc_result mHscMessage
          -- if available. So, if the "*" prefix was used, force recompilation
          -- to make sure byte-code is loaded.
          | force_recomp || loadAsByteCode = SourceModified
-         | otherwise = source_modified0
+         | otherwise = SourceUnmodified
 
        always_do_basic_recompilation_check = case bcknd of
                                              Interpreter -> True
@@ -1298,6 +1294,8 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
           let fc        = hsc_FC hsc_env'
           addHomeModuleToFinder fc home_unit mod_name location
 
+
+        o_mod <- liftIO $ getModTime o_file
   -- Make the ModSummary to hand to hscMain
         let
             mod_summary = ModSummary {  ms_mod       = mod,
@@ -1307,7 +1305,7 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
                                         ms_hspp_buf  = hspp_buf,
                                         ms_location  = location,
                                         ms_hs_hash   = src_hash,
-                                        ms_obj_date  = Nothing,
+                                        ms_obj_date  = o_mod,
                                         ms_parsed_mod   = Nothing,
                                         ms_iface_date   = hi_date,
                                         ms_hie_date     = Nothing,
@@ -1323,8 +1321,8 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
           -- Note [When source is considered modified]
           ; if isNothing hi_date then return SourceModified else do {
           ; hi_timestamp <- getModificationUTCTime hi_file
-          ; prev_hash_matches <- doesIfaceHashMatch hsc_env' mod_summary
-          ; if not prev_hash_matches then return SourceModified else do {
+--          ; prev_hash_matches <- doesIfaceHashMatch hsc_env' mod_summary
+--          ; if not prev_hash_matches then return SourceModified else do {
           ; o_file_mod <- if writeInterfaceOnlyMode dflags
                             then return False
                             else sourceModified o_file hi_timestamp
@@ -1336,13 +1334,13 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
                               else pure False
           ; if hie_file_mod then return SourceModified else do {
           ; return SourceUnmodified
-          }}}}}}}
+          }}}}}}
 
   -- run the compiler!
         let msg hsc_env _ what _ = oneShotMsg hsc_env what
         (result, plugin_hsc_env) <-
           liftIO $ hscIncrementalCompile True Nothing (Just msg) hsc_env'
-                            mod_summary source_unchanged Nothing (1,1)
+                            mod_summary source_unchanged Nothing Nothing (1,1)
 
         -- In the rest of the pipeline use the loaded plugins
         setPlugins (hsc_plugins        plugin_hsc_env)
@@ -1363,10 +1361,10 @@ runPhase (HscOut src_flavour mod_name result) _ = do
             next_phase = hscPostBackendPhase src_flavour (backend dflags)
 
         case result of
-            HscNotGeneratingCode _ _ ->
+            HscNotGeneratingCode _ ->
                 return (RealPhase StopLn,
                         panic "No output filename from Hsc when no-code")
-            HscUpToDate _ _ ->
+            HscUpToDate _ ->
                 do liftIO $ touchObjectFile logger dflags o_file
                    -- The .o file must have a later modification date
                    -- than the source file (else we wouldn't get Nothing)
@@ -1796,24 +1794,6 @@ runPhase (RealPhase MergeForeign) input_fn = do
 runPhase (RealPhase other) _input_fn =
    panic ("runPhase: don't know how to run phase " ++ show other)
 
--- | Read the previously recorded hash from a module's iface file, if any.
-readIfaceSourceHash' :: HscEnv -> ModSummary -> IO (Maybe Fingerprint)
-readIfaceSourceHash' hsc_env ms =
-    readIfaceSourceHash
-        (hsc_dflags hsc_env)
-        (hsc_NC hsc_env)
-        (ml_hi_file (ms_location ms))
-
--- | Check whether a module's current hash matches the previously recorded hash
--- in its .hi file, if any. If this function returns False then the module will
--- need recompiling.
-doesIfaceHashMatch :: HscEnv -> ModSummary -> IO Bool
-doesIfaceHashMatch hsc_env ms = do
-    mb_iface_hash <- readIfaceSourceHash' hsc_env ms
-    case mb_iface_hash of
-        Just hash -> return $ hash == ms_hs_hash ms
-        Nothing -> return False
-
 maybeMergeForeign :: CompPipeline Phase
 maybeMergeForeign
  = do
@@ -2130,10 +2110,6 @@ joinObjectFiles logger tmpfs dflags o_files output_fn = do
 -- -----------------------------------------------------------------------------
 -- Misc.
 
-writeInterfaceOnlyMode :: DynFlags -> Bool
-writeInterfaceOnlyMode dflags =
- gopt Opt_WriteInterface dflags &&
- NoBackend == backend dflags
 
 -- | Figure out if the .hi file was modified after some other output file
 -- corresponding to that source file (or if we anyways need to consider the
@@ -2152,6 +2128,15 @@ sourceModified dest_file hi_timestamp = do
      else do t2 <- getModificationUTCTime dest_file
              return (t2 < hi_timestamp)
 
+getModTime :: FilePath -> IO (Maybe UTCTime)
+getModTime dest_file = do
+  dest_file_exists <- doesFileExist dest_file
+  if not dest_file_exists
+    then return Nothing
+     else do t2 <- getModificationUTCTime dest_file
+             return (Just t2)
+
+
 -- | What phase to run after one of the backend code generators has run
 hscPostBackendPhase :: HscSource -> Backend -> Phase
 hscPostBackendPhase HsBootFile _    =  StopLn
diff --git a/compiler/GHC/Iface/Load.hs b/compiler/GHC/Iface/Load.hs
index bd413ea08f..642f3b45b8 100644
--- a/compiler/GHC/Iface/Load.hs
+++ b/compiler/GHC/Iface/Load.hs
@@ -25,7 +25,7 @@ module GHC.Iface.Load (
         -- IfM functions
         loadInterface,
         loadSysInterface, loadUserInterface, loadPluginInterface,
-        findAndReadIface, readIface, readIfaceSourceHash, writeIface,
+        findAndReadIface, readIface, writeIface,
         moduleFreeHolesPrecise,
         needWiredInHomeIface, loadWiredInHomeIface,
 
@@ -65,7 +65,6 @@ import GHC.Utils.Outputable as Outputable
 import GHC.Utils.Panic
 import GHC.Utils.Misc
 import GHC.Utils.Logger
-import GHC.Utils.Fingerprint
 
 import GHC.Settings.Constants
 
@@ -993,22 +992,6 @@ readIface dflags name_cache wanted_mod file_path = do
 
     Left exn    -> return (Failed (text (showException exn)))
 
--- | Like @readIface@, but just get the source file hash out of it if it
--- exists, and don't bother returning the error otherwise.
-readIfaceSourceHash
-  :: DynFlags
-  -> NameCache
-  -> FilePath
-  -> IO (Maybe Fingerprint)
-readIfaceSourceHash dflags name_cache file_path = do
-    let profile = targetProfile dflags
-    res <- tryMost $ readBinIfaceHeader profile name_cache CheckHiWay QuietBinIFace file_path
-    case res of
-      Right (src_hash, _) ->
-        return $ Just src_hash
-      Left _ ->
-        return Nothing
-
 {-
 *********************************************************
 *                                                       *
diff --git a/compiler/GHC/Iface/Make.hs b/compiler/GHC/Iface/Make.hs
index be3d3001bc..10488c75ad 100644
--- a/compiler/GHC/Iface/Make.hs
+++ b/compiler/GHC/Iface/Make.hs
@@ -99,6 +99,8 @@ import Data.List ( findIndex, mapAccumL, sortBy )
 import Data.Ord
 import Data.IORef
 
+import GHC.Fingerprint
+
 {-
 ************************************************************************
 *                                                                      *
@@ -286,6 +288,7 @@ mkIface_ hsc_env
         trust_info  = setSafeMode safe_mode
         annotations = map mkIfaceAnnotation anns
         icomplete_matches = map mkIfaceCompleteMatch complete_matches
+        used_th_hash = if used_th then Just fingerprint0 else Nothing
 
     ModIface {
           mi_module      = this_mod,
@@ -309,7 +312,7 @@ mkIface_ hsc_env
           mi_warns       = warns,
           mi_anns        = annotations,
           mi_globals     = maybeGlobalRdrEnv rdr_env,
-          mi_used_th     = used_th,
+          mi_used_th     = used_th_hash,
           mi_decls       = decls,
           mi_hpc         = isHpcUsed hpc_info,
           mi_trust       = trust_info,
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index b006610084..8463c36cae 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -61,6 +61,7 @@ import GHC.Unit.Module.ModIface
 import GHC.Unit.Module.ModSummary
 import GHC.Unit.Module.Warnings
 import GHC.Unit.Module.Deps
+import GHC.Unit.Home.ModInfo
 
 import Control.Monad
 import Data.Function
@@ -113,7 +114,7 @@ data RecompileRequired
   | RecompBecause String
        -- ^ The .o/.hi files are up to date, but something else has changed
        -- to force recompilation; the String says what (one-line summary)
-   deriving Eq
+   deriving (Eq, Show)
 
 instance Semigroup RecompileRequired where
   UpToDate <> r = r
@@ -237,6 +238,8 @@ checkVersions hsc_env mod_summary iface
        -- but we ALSO must make sure the instantiation matches up.  See
        -- test case bkpcabal04!
        ; hsc_env <- getTopEnv
+       ; if mi_src_hash iface /= ms_hs_hash mod_summary
+            then return (MustCompile, Nothing) else do {
        ; if not (isHomeModule home_unit (mi_module iface))
             then return (RecompBecause "-this-unit-id changed", Nothing) else do {
        ; recomp <- liftIO $ checkFlagHash hsc_env iface
@@ -255,6 +258,8 @@ checkVersions hsc_env mod_summary iface
        ; if recompileRequired recomp then return (recomp, Just iface) else do {
        ; recomp <- checkPlugins hsc_env iface
        ; if recompileRequired recomp then return (recomp, Nothing) else do {
+       ; recomp <- checkSources hsc_env iface
+       ; if recompileRequired recomp then return (recomp, Nothing) else do {
 
 
        -- Source code unchanged and no errors yet... carry on
@@ -274,7 +279,7 @@ checkVersions hsc_env mod_summary iface
        ; recomp <- checkList [checkModUsage (homeUnitAsUnit home_unit) u
                              | u <- mi_usages iface]
        ; return (recomp, Just iface)
-    }}}}}}}}}}
+    }}}}}}}}}}}}
   where
     logger = hsc_logger hsc_env
     dflags = hsc_dflags hsc_env
@@ -283,6 +288,7 @@ checkVersions hsc_env mod_summary iface
     mod_deps :: ModuleNameEnv ModuleNameWithIsBoot
     mod_deps = mkModDeps (dep_mods (mi_deps iface))
 
+
 -- | Check if any plugins are requesting recompilation
 checkPlugins :: HscEnv -> ModIface -> IfG RecompileRequired
 checkPlugins hsc_env iface = liftIO $ do
@@ -1105,6 +1111,8 @@ addFingerprints hsc_env iface0
 
    plugin_hash <- fingerprintPlugins hsc_env
 
+   let th_hash = fingerprintSources hsc_env (mi_deps iface0) <$ mi_used_th iface0
+
    -- the ABI hash depends on:
    --   - decls
    --   - export list
@@ -1150,7 +1158,7 @@ addFingerprints hsc_env iface0
       , mi_fix_fn      = fix_fn
       , mi_hash_fn     = lookupOccEnv local_env
       }
-    final_iface = iface0 { mi_decls = sorted_decls, mi_final_exts = final_iface_exts }
+    final_iface = iface0 { mi_used_th = th_hash, mi_decls = sorted_decls, mi_final_exts = final_iface_exts }
    --
    return final_iface
 
@@ -1163,6 +1171,39 @@ addFingerprints hsc_env iface0
     (non_orph_fis,   orph_fis)   = mkOrphMap ifFamInstOrph (mi_fam_insts iface0)
     ann_fn = mkIfaceAnnCache (mi_anns iface0)
 
+
+-- | Combine source fingerprints of all transitively imported modules
+-- Used for the stability check for modules which use TH
+fingerprintSources :: HscEnv -> Dependencies -> Fingerprint
+fingerprintSources hsc_env deps =
+  let mod_hashes = map (mi_src_hash . hm_iface) $ mapMaybe (lookupHpt (hsc_HPT hsc_env). gwib_mod) (dep_mods deps)
+  in fingerprintFingerprints mod_hashes
+
+
+                    -- If the module used TH splices when it was last
+                    -- compiled, then the recompilation check is not
+                    -- accurate enough (#481) and we must ignore
+                    -- it.  However, if the module is stable (none of
+                    -- the modules it depends on, directly or
+                    -- indirectly, changed), then we *can* skip
+                    -- recompilation. This is why the SourceModified
+                    -- type contains SourceUnmodifiedAndStable, and
+                    -- it's pretty important: otherwise ghc --make
+                    -- would always recompile TH modules, even if
+                    -- nothing at all has changed. Stability is just
+                    -- the same check that make is doing for us in
+                    -- one-shot mode.
+
+-- | Check if the source hash is still accurate
+checkSources :: HscEnv -> ModIface -> IfG RecompileRequired
+checkSources hsc_env iface = do
+  case mi_used_th iface of
+    Just old_fingerprint -> do
+      if old_fingerprint == fingerprintSources hsc_env (mi_deps iface)
+        then return UpToDate
+        else return (RecompBecause "TH")
+    Nothing -> return UpToDate
+
 -- | Retrieve the orphan hashes 'mi_orphan_hash' for a list of modules
 -- (in particular, the orphan modules which are transitively imported by the
 -- current module).
diff --git a/compiler/GHC/Rename/Module.hs b/compiler/GHC/Rename/Module.hs
index 8de0c4a34f..8c84a71378 100644
--- a/compiler/GHC/Rename/Module.hs
+++ b/compiler/GHC/Rename/Module.hs
@@ -761,10 +761,10 @@ rnFamEqn doc atfi extra_kvars
          -- See Note [Renaming associated types].
          -- Per that Note, the LHS type variables consist of:
          --
-         -- * The variables mentioned in the instance's type patterns
+         -- - The variables mentioned in the instance's type patterns
          --   (pat_fvs), and
          --
-         -- * The variables mentioned in an outermost kind signature on the
+         -- - The variables mentioned in an outermost kind signature on the
          --   RHS. This is a subset of `rhs_fvs`. To compute it, we look up
          --   each RdrName in `extra_kvars` to find its corresponding Name in
          --   the LocalRdrEnv.
diff --git a/compiler/GHC/Types/SourceFile.hs b/compiler/GHC/Types/SourceFile.hs
index e8faec7a58..a1e2c63db1 100644
--- a/compiler/GHC/Types/SourceFile.hs
+++ b/compiler/GHC/Types/SourceFile.hs
@@ -1,6 +1,7 @@
 module GHC.Types.SourceFile
    ( HscSource(..)
    , SourceModified (..)
+   , hscSourceToIsBoot
    , isHsBootOrSig
    , isHsigFile
    , hscSourceString
@@ -9,6 +10,7 @@ where
 
 import GHC.Prelude
 import GHC.Utils.Binary
+import GHC.Unit.Types
 
 -- Note [HscSource types]
 -- ~~~~~~~~~~~~~~~~~~~~~~
@@ -49,6 +51,14 @@ data HscSource
    | HsigFile   -- ^ .hsig file
    deriving (Eq, Ord, Show)
 
+-- | Tests if an 'HscSource' is a boot file, primarily for constructing elements
+-- of 'BuildModule'. We conflate signatures and modules because they are bound
+-- in the same namespace; only boot interfaces can be disambiguated with
+-- `import {-# SOURCE #-}`.
+hscSourceToIsBoot :: HscSource -> IsBootInterface
+hscSourceToIsBoot HsBootFile = IsBoot
+hscSourceToIsBoot _ = NotBoot
+
 instance Binary HscSource where
     put_ bh HsSrcFile = putByte bh 0
     put_ bh HsBootFile = putByte bh 1
diff --git a/compiler/GHC/Unit/Module/ModIface.hs b/compiler/GHC/Unit/Module/ModIface.hs
index fb8fb422ae..4e563ff9e9 100644
--- a/compiler/GHC/Unit/Module/ModIface.hs
+++ b/compiler/GHC/Unit/Module/ModIface.hs
@@ -169,7 +169,7 @@ data ModIface_ (phase :: ModIfacePhase)
                 -- exported by this module, and the 'OccName's of those things
 
 
-        mi_used_th  :: !Bool,
+        mi_used_th  :: !(Maybe Fingerprint),
                 -- ^ Module required TH splices when it was compiled.
                 -- This disables recompilation avoidance (see #481).
 
@@ -464,7 +464,7 @@ emptyPartialModIface mod
                mi_deps        = noDependencies,
                mi_usages      = [],
                mi_exports     = [],
-               mi_used_th     = False,
+               mi_used_th     = Nothing,
                mi_fixities    = [],
                mi_warns       = NoWarnings,
                mi_anns        = [],
diff --git a/compiler/GHC/Unit/Module/Status.hs b/compiler/GHC/Unit/Module/Status.hs
index 52938154b4..2adb9d1793 100644
--- a/compiler/GHC/Unit/Module/Status.hs
+++ b/compiler/GHC/Unit/Module/Status.hs
@@ -9,15 +9,16 @@ import GHC.Unit
 import GHC.Unit.Module.ModGuts
 import GHC.Unit.Module.ModIface
 import GHC.Unit.Module.ModDetails
+import GHC.Unit.Home.ModInfo
 
 import GHC.Utils.Fingerprint
 
 -- | Status of a module compilation to machine code
 data HscStatus
     -- | Nothing to do.
-    = HscNotGeneratingCode ModIface ModDetails
+    = HscNotGeneratingCode HomeModInfo
     -- | Nothing to do because code already exists.
-    | HscUpToDate ModIface ModDetails
+    | HscUpToDate HomeModInfo
     -- | Update boot file result.
     | HscUpdateBoot ModIface ModDetails
     -- | Generate signature file (backpack)
diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index c53f6771b5..c10f6051ea 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -480,6 +480,10 @@ interactiveUI config srcs maybe_exprs = do
                       LangExt.MonomorphismRestriction xopt_unset)
                $ dflags
    GHC.setInteractiveDynFlags dflags'
+   GHC.setSessionDynFlags
+               -- Set Opt_KeepGoing so that :reload loads as much as
+               -- possible
+               (gopt_set dflags Opt_KeepGoing)
 
    -- Update the LogAction. Ensure we don't override the user's log action lest
    -- we break -ddump-json (#14078)
diff --git a/testsuite/tests/driver/recomp019/recomp019.stdout b/testsuite/tests/driver/recomp019/recomp019.stdout
index f1e4cd4d73..52bb3608a4 100644
--- a/testsuite/tests/driver/recomp019/recomp019.stdout
+++ b/testsuite/tests/driver/recomp019/recomp019.stdout
@@ -6,6 +6,6 @@ Linking Main ...
 5
 [1 of 1] Compiling B                ( B.hs, nothing )
 second run
-[2 of 3] Compiling B                ( B.hs, B.o )
+[1 of 3] Compiling B                ( B.hs, B.o )
 Linking Main ...
 15
diff --git a/testsuite/tests/driver/recompHash/A.hs b/testsuite/tests/driver/recompHash/A.hs
new file mode 100644
index 0000000000..905110c8cf
--- /dev/null
+++ b/testsuite/tests/driver/recompHash/A.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TemplateHaskell #-}
+module A where
+
+import B
+
+main = print 0
diff --git a/testsuite/tests/driver/recompHash/B.hs b/testsuite/tests/driver/recompHash/B.hs
new file mode 100644
index 0000000000..7b1456b488
--- /dev/null
+++ b/testsuite/tests/driver/recompHash/B.hs
@@ -0,0 +1,3 @@
+module B where
+
+c = print 0
diff --git a/testsuite/tests/driver/recompHash/Makefile b/testsuite/tests/driver/recompHash/Makefile
new file mode 100644
index 0000000000..b0e578a05d
--- /dev/null
+++ b/testsuite/tests/driver/recompHash/Makefile
@@ -0,0 +1,17 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+# Recompilation tests
+
+clean:
+	rm -f *.o*
+	rm -f *.dyn_o*
+	rm -f *.hi*
+
+# Touching a file should not cause recompilation
+
+recompHash: clean
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make A.hs
+	touch B.hs
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make A.hs
diff --git a/testsuite/tests/driver/recompHash/all.T b/testsuite/tests/driver/recompHash/all.T
new file mode 100644
index 0000000000..ab0814b62b
--- /dev/null
+++ b/testsuite/tests/driver/recompHash/all.T
@@ -0,0 +1,3 @@
+test('recompHash', [extra_files(['A.hs', 'B.hs']),
+                   when(fast(), skip)],
+     makefile_test, [])
diff --git a/testsuite/tests/driver/recompHash/recompHash.stdout b/testsuite/tests/driver/recompHash/recompHash.stdout
new file mode 100644
index 0000000000..50dd203d39
--- /dev/null
+++ b/testsuite/tests/driver/recompHash/recompHash.stdout
@@ -0,0 +1,2 @@
+[1 of 2] Compiling B                ( B.hs, B.o, B.dyn_o )
+[2 of 2] Compiling A                ( A.hs, A.o, A.dyn_o )
diff --git a/testsuite/tests/driver/recompNoTH/A.hs b/testsuite/tests/driver/recompNoTH/A.hs
new file mode 100644
index 0000000000..905110c8cf
--- /dev/null
+++ b/testsuite/tests/driver/recompNoTH/A.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TemplateHaskell #-}
+module A where
+
+import B
+
+main = print 0
diff --git a/testsuite/tests/driver/recompNoTH/B1.hs b/testsuite/tests/driver/recompNoTH/B1.hs
new file mode 100644
index 0000000000..7b1456b488
--- /dev/null
+++ b/testsuite/tests/driver/recompNoTH/B1.hs
@@ -0,0 +1,3 @@
+module B where
+
+c = print 0
diff --git a/testsuite/tests/driver/recompNoTH/B2.hs b/testsuite/tests/driver/recompNoTH/B2.hs
new file mode 100644
index 0000000000..c7f392c91a
--- /dev/null
+++ b/testsuite/tests/driver/recompNoTH/B2.hs
@@ -0,0 +1,3 @@
+module B where
+
+c = print 1
diff --git a/testsuite/tests/driver/recompNoTH/Makefile b/testsuite/tests/driver/recompNoTH/Makefile
new file mode 100644
index 0000000000..89f0f26738
--- /dev/null
+++ b/testsuite/tests/driver/recompNoTH/Makefile
@@ -0,0 +1,19 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+# Recompilation tests
+
+clean:
+	rm -f *.o*
+	rm -f *.dyn_o*
+	rm -f *.hi*
+
+# If the source changes, but not the ABI, then only B should be recompiled.
+
+recompNoTH: clean
+	'$(CP)' B1.hs B.hs
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make A.hs
+	'$(CP)' B2.hs B.hs
+	# Should print that only B has been recompiled.
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make A.hs
diff --git a/testsuite/tests/driver/recompNoTH/all.T b/testsuite/tests/driver/recompNoTH/all.T
new file mode 100644
index 0000000000..db66a0af41
--- /dev/null
+++ b/testsuite/tests/driver/recompNoTH/all.T
@@ -0,0 +1,3 @@
+test('recompNoTH', [extra_files(['A.hs', 'B1.hs', 'B2.hs' ]),
+                   when(fast(), skip)],
+     makefile_test, [])
diff --git a/testsuite/tests/driver/recompNoTH/recompNoTH.stdout b/testsuite/tests/driver/recompNoTH/recompNoTH.stdout
new file mode 100644
index 0000000000..310ab29692
--- /dev/null
+++ b/testsuite/tests/driver/recompNoTH/recompNoTH.stdout
@@ -0,0 +1,3 @@
+[1 of 2] Compiling B                ( B.hs, B.o, B.dyn_o )
+[2 of 2] Compiling A                ( A.hs, A.o, A.dyn_o )
+[1 of 2] Compiling B                ( B.hs, B.o, B.dyn_o )
diff --git a/testsuite/tests/driver/recompTH/A.hs b/testsuite/tests/driver/recompTH/A.hs
new file mode 100644
index 0000000000..53ba525e01
--- /dev/null
+++ b/testsuite/tests/driver/recompTH/A.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TemplateHaskell #-}
+module A where
+
+import B
+
+main = $([| print () |])
diff --git a/testsuite/tests/driver/recompTH/B1.hs b/testsuite/tests/driver/recompTH/B1.hs
new file mode 100644
index 0000000000..7b1456b488
--- /dev/null
+++ b/testsuite/tests/driver/recompTH/B1.hs
@@ -0,0 +1,3 @@
+module B where
+
+c = print 0
diff --git a/testsuite/tests/driver/recompTH/B2.hs b/testsuite/tests/driver/recompTH/B2.hs
new file mode 100644
index 0000000000..c7f392c91a
--- /dev/null
+++ b/testsuite/tests/driver/recompTH/B2.hs
@@ -0,0 +1,3 @@
+module B where
+
+c = print 1
diff --git a/testsuite/tests/driver/recompTH/Makefile b/testsuite/tests/driver/recompTH/Makefile
new file mode 100644
index 0000000000..758dd9a37f
--- /dev/null
+++ b/testsuite/tests/driver/recompTH/Makefile
@@ -0,0 +1,20 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+# Recompilation tests
+
+clean:
+	rm -f *.o*
+	rm -f *.dyn_o*
+	rm -f *.hi*
+
+# Test that using a TH splice in a module causes recompilation when the *source*
+# of a dependency changes.
+
+recompTH: clean
+	'$(CP)' B1.hs B.hs
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make A.hs
+	'$(CP)' B2.hs B.hs
+	# Should print that A has been recompiled.
+	'$(TEST_HC)' $(TEST_HC_OPTS) --make A.hs
diff --git a/testsuite/tests/driver/recompTH/all.T b/testsuite/tests/driver/recompTH/all.T
new file mode 100644
index 0000000000..f6d447d397
--- /dev/null
+++ b/testsuite/tests/driver/recompTH/all.T
@@ -0,0 +1,3 @@
+test('recompTH', [extra_files(['A.hs', 'B1.hs', 'B2.hs' ]),
+                   when(fast(), skip)],
+     makefile_test, [])
diff --git a/testsuite/tests/driver/recompTH/recompTH.stdout b/testsuite/tests/driver/recompTH/recompTH.stdout
new file mode 100644
index 0000000000..1af356d229
--- /dev/null
+++ b/testsuite/tests/driver/recompTH/recompTH.stdout
@@ -0,0 +1,4 @@
+[1 of 2] Compiling B                ( B.hs, B.o, B.dyn_o )
+[2 of 2] Compiling A                ( A.hs, A.o, A.dyn_o )
+[1 of 2] Compiling B                ( B.hs, B.o, B.dyn_o )
+[2 of 2] Compiling A                ( A.hs, A.o, A.dyn_o ) [TH]
diff --git a/testsuite/tests/ghc-api/T6145.hs b/testsuite/tests/ghc-api/T6145.hs
index 3d929c8c9d..392c318768 100644
--- a/testsuite/tests/ghc-api/T6145.hs
+++ b/testsuite/tests/ghc-api/T6145.hs
@@ -30,8 +30,7 @@ main = do
                         p <- parseModule modSum
                         t <- typecheckModule p
                         d <- desugarModule t
-                        l <- loadModule d
-                        let ts=typecheckedSource l
+                        let ts=typecheckedSource d
 --                        liftIO (putStr (showSDocDebug (ppr ts)))
                         let fs=filterBag isDataCon ts
                         return $ not $ isEmptyBag fs
diff --git a/testsuite/tests/ghci/prog012/prog012.stderr b/testsuite/tests/ghci/prog012/prog012.stderr
index 71d2bd385a..2145bfbbfb 100644
--- a/testsuite/tests/ghci/prog012/prog012.stderr
+++ b/testsuite/tests/ghci/prog012/prog012.stderr
@@ -1,2 +1,4 @@
 
 Bar.hs:3:7: error: Variable not in scope: nonexistent
+-fkeep-going in use, removing the following dependencies and continuing:
+      FooBar