root/compiler/main/GhcMake.hs

{-# LANGUAGE ScopedTypeVariables #-}

-- -----------------------------------------------------------------------------
--
-- (c) The University of Glasgow, 2011
--
-- This module implements multi-module compilation, and is used
-- by --make and GHCi.
--
-- -----------------------------------------------------------------------------
module GhcMake( 
        depanal, 
        load, LoadHowMuch(..),

        topSortModuleGraph, 

        noModError, cyclicModuleErr
    ) where

#include "HsVersions.h"

#ifdef GHCI
import qualified Linker         ( unload )
#endif

import DriverPhases
import DriverPipeline
import DynFlags
import ErrUtils
import Finder
import GhcMonad
import HeaderInfo
import HsSyn
import HscTypes
import Module
import RdrName          ( RdrName )
import TcIface          ( typecheckIface )
import TcRnMonad        ( initIfaceCheck )

import Bag              ( listToBag )
import BasicTypes
import Digraph
import Exception        ( evaluate, tryIO )
import FastString
import Maybes           ( expectJust, mapCatMaybes )
import Outputable
import Panic
import SrcLoc
import StringBuffer
import SysTools
import UniqFM
import Util

import qualified Data.Map as Map
import qualified FiniteMap as Map ( insertListWith )

import Control.Monad
import Data.List
import qualified Data.List as List
import Data.Maybe
import Data.Time
import System.Directory
import System.FilePath
import System.IO        ( fixIO )
import System.IO.Error  ( isDoesNotExistError )

-- -----------------------------------------------------------------------------
-- Loading the program

-- | Perform a dependency analysis starting from the current targets
-- and update the session with the new module graph.
--
-- Dependency analysis entails parsing the @import@ directives and may
-- therefore require running certain preprocessors.
--
-- Note that each 'ModSummary' in the module graph caches its 'DynFlags'.
-- These 'DynFlags' are determined by the /current/ session 'DynFlags' and the
-- @OPTIONS@ and @LANGUAGE@ pragmas of the parsed module.  Thus if you want to
-- changes to the 'DynFlags' to take effect you need to call this function
-- again.
--
depanal :: GhcMonad m =>
           [ModuleName]  -- ^ excluded modules
        -> Bool          -- ^ allow duplicate roots
        -> m ModuleGraph
depanal excluded_mods allow_dup_roots = do
  hsc_env <- getSession
  let
         dflags  = hsc_dflags hsc_env
         targets = hsc_targets hsc_env
         old_graph = hsc_mod_graph hsc_env
        
  liftIO $ showPass dflags "Chasing dependencies"
  liftIO $ debugTraceMsg dflags 2 (hcat [
             text "Chasing modules from: ",
             hcat (punctuate comma (map pprTarget targets))])

  mod_graph <- liftIO $ downsweep hsc_env old_graph excluded_mods allow_dup_roots
  modifySession $ \_ -> hsc_env { hsc_mod_graph = mod_graph }
  return mod_graph

-- | Describes which modules of the module graph need to be loaded.
data LoadHowMuch
   = LoadAllTargets
     -- ^ Load all targets and its dependencies.
   | LoadUpTo ModuleName
     -- ^ Load only the given module and its dependencies.
   | LoadDependenciesOf ModuleName
     -- ^ Load only the dependencies of the given module, but not the module
     -- itself.

-- | Try to load the program.  See 'LoadHowMuch' for the different modes.
--
-- This function implements the core of GHC's @--make@ mode.  It preprocesses,
-- compiles and loads the specified modules, avoiding re-compilation wherever
-- possible.  Depending on the target (see 'DynFlags.hscTarget') compilating
-- and loading may result in files being created on disk.
--
-- Calls the 'reportModuleCompilationResult' callback after each compiling
-- each module, whether successful or not.
--
-- Throw a 'SourceError' if errors are encountered before the actual
-- compilation starts (e.g., during dependency analysis).  All other errors
-- are reported using the callback.
--
load :: GhcMonad m => LoadHowMuch -> m SuccessFlag
load how_much = do
    mod_graph <- depanal [] False
    guessOutputFile
    hsc_env <- getSession

    let hpt1   = hsc_HPT hsc_env
    let dflags = hsc_dflags hsc_env

    -- The "bad" boot modules are the ones for which we have
    -- B.hs-boot in the module graph, but no B.hs
    -- The downsweep should have ensured this does not happen
    -- (see msDeps)
    let all_home_mods = [ms_mod_name s 
                        | s <- mod_graph, not (isBootSummary s)]
        bad_boot_mods = [s        | s <- mod_graph, isBootSummary s,
                                    not (ms_mod_name s `elem` all_home_mods)]
    ASSERT( null bad_boot_mods ) return ()

    -- check that the module given in HowMuch actually exists, otherwise
    -- topSortModuleGraph will bomb later.
    let checkHowMuch (LoadUpTo m)           = checkMod m
        checkHowMuch (LoadDependenciesOf m) = checkMod m
        checkHowMuch _ = id

        checkMod m and_then
            | m `elem` all_home_mods = and_then
            | otherwise = do 
                    liftIO $ errorMsg dflags (text "no such module:" <+>
                                     quotes (ppr m))
                    return Failed

    checkHowMuch how_much $ do

    -- mg2_with_srcimps drops the hi-boot nodes, returning a 
    -- graph with cycles.  Among other things, it is used for
    -- backing out partially complete cycles following a failed
    -- upsweep, and for removing from hpt all the modules
    -- not in strict downwards closure, during calls to compile.
    let mg2_with_srcimps :: [SCC ModSummary]
        mg2_with_srcimps = topSortModuleGraph True mod_graph Nothing

    -- If we can determine that any of the {-# SOURCE #-} imports
    -- are definitely unnecessary, then emit a warning.
    warnUnnecessarySourceImports mg2_with_srcimps

    let
        -- check the stability property for each module.
        stable_mods@(stable_obj,stable_bco)
            = checkStability hpt1 mg2_with_srcimps all_home_mods

        -- prune bits of the HPT which are definitely redundant now,
        -- to save space.
        pruned_hpt = pruneHomePackageTable hpt1 
                            (flattenSCCs mg2_with_srcimps)
                            stable_mods

    _ <- liftIO $ evaluate pruned_hpt

    -- before we unload anything, make sure we don't leave an old
    -- interactive context around pointing to dead bindings.  Also,
    -- write the pruned HPT to allow the old HPT to be GC'd.
    modifySession $ \_ -> discardIC $ hsc_env { hsc_HPT = pruned_hpt }

    liftIO $ debugTraceMsg dflags 2 (text "Stable obj:" <+> ppr stable_obj $$
                            text "Stable BCO:" <+> ppr stable_bco)

    -- Unload any modules which are going to be re-linked this time around.
    let stable_linkables = [ linkable
                           | m <- stable_obj++stable_bco,
                             Just hmi <- [lookupUFM pruned_hpt m],
                             Just linkable <- [hm_linkable hmi] ]
    liftIO $ unload 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
    -- to let upsweep_mods do this, so at least some useful work gets
    -- done before the upsweep is abandoned.
    --hPutStrLn stderr "after tsort:\n"
    --hPutStrLn stderr (showSDoc (vcat (map ppr mg2)))

    -- Now do the upsweep, calling compile for each module in
    -- turn.  Final result is version 3 of everything.

    -- Topologically sort the module graph, this time including hi-boot
    -- nodes, and possibly just including the portion of the graph
    -- reachable from the module specified in the 2nd argument to load.
    -- 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 :: [SCC ModSummary]
        full_mg    = topSortModuleGraph False mod_graph Nothing

        maybe_top_mod = case how_much of
                            LoadUpTo m           -> Just m
                            LoadDependenciesOf m -> Just m
                            _                    -> Nothing

        partial_mg0 :: [SCC ModSummary]
        partial_mg0 = topSortModuleGraph False mod_graph maybe_top_mod

        -- LoadDependenciesOf m: we want the upsweep to stop just
        -- short of the specified module (unless the specified module
        -- is stable).
        partial_mg
            | LoadDependenciesOf _mod <- how_much
            = ASSERT( case last partial_mg0 of 
                        AcyclicSCC ms -> ms_mod_name ms == _mod; _ -> False )
              List.init partial_mg0
            | otherwise
            = partial_mg0

        stable_mg = 
            [ AcyclicSCC ms
            | AcyclicSCC ms <- full_mg,
              ms_mod_name ms `elem` stable_obj++stable_bco,
              ms_mod_name ms `notElem` [ ms_mod_name ms' | 
                                            AcyclicSCC ms' <- partial_mg ] ]

        mg = stable_mg ++ partial_mg

    -- clean up between compilations
    let cleanup hsc_env = intermediateCleanTempFiles dflags
                              (flattenSCCs mg2_with_srcimps)
                              hsc_env

    liftIO $ debugTraceMsg dflags 2 (hang (text "Ready for upsweep")
                               2 (ppr mg))

    setSession hsc_env{ hsc_HPT = emptyHomePackageTable }
    (upsweep_ok, modsUpswept)
       <- upsweep pruned_hpt stable_mods cleanup mg

    -- Make modsDone be the summaries for each home module now
    -- available; this should equal the domain of hpt3.
    -- Get in in a roughly top .. bottom order (hence reverse).

    let modsDone = reverse modsUpswept

    -- Try and do linking in some form, depending on whether the
    -- upsweep was completely or only partially successful.

    if succeeded upsweep_ok

     then 
       -- Easy; just relink it all.
       do liftIO $ debugTraceMsg dflags 2 (text "Upsweep completely successful.")

          -- Clean up after ourselves
          hsc_env1 <- getSession
          liftIO $ intermediateCleanTempFiles dflags modsDone hsc_env1

          -- Issue a warning for the confusing case where the user
          -- said '-o foo' but we're not going to do any linking.
          -- We attempt linking if either (a) one of the modules is
          -- called Main, or (b) the user said -no-hs-main, indicating
          -- that main() is going to come from somewhere else.
          --
          let ofile = outputFile dflags
          let no_hs_main = dopt Opt_NoHsMain dflags
          let 
            main_mod = mainModIs dflags
            a_root_is_Main = any ((==main_mod).ms_mod) mod_graph
            do_linking = a_root_is_Main || no_hs_main || ghcLink dflags == LinkDynLib

          when (ghcLink dflags == LinkBinary 
                && isJust ofile && not do_linking) $
            liftIO $ debugTraceMsg dflags 1 $
                text ("Warning: output was redirected with -o, " ++
                      "but no output will be generated\n" ++
                      "because there is no " ++ 
                      moduleNameString (moduleName main_mod) ++ " module.")

          -- link everything together
          linkresult <- liftIO $ link (ghcLink dflags) dflags do_linking (hsc_HPT hsc_env1)

          loadFinish Succeeded linkresult

     else 
       -- Tricky.  We need to back out the effects of compiling any
       -- half-done cycles, both so as to clean up the top level envs
       -- and to avoid telling the interactive linker to link them.
       do liftIO $ debugTraceMsg dflags 2 (text "Upsweep partially successful.")

          let modsDone_names
                 = map ms_mod modsDone
          let mods_to_zap_names 
                 = findPartiallyCompletedCycles modsDone_names 
                      mg2_with_srcimps
          let mods_to_keep
                 = filter ((`notElem` mods_to_zap_names).ms_mod) 
                      modsDone

          hsc_env1 <- getSession
          let hpt4 = retainInTopLevelEnvs (map ms_mod_name mods_to_keep) 
                                          (hsc_HPT hsc_env1)

          -- Clean up after ourselves
          liftIO $ intermediateCleanTempFiles dflags mods_to_keep hsc_env1

          -- there should be no Nothings where linkables should be, now
          ASSERT(all (isJust.hm_linkable) 
                    (eltsUFM (hsc_HPT hsc_env))) do
    
          -- Link everything together
          linkresult <- liftIO $ link (ghcLink dflags) dflags False hpt4

          modifySession $ \hsc_env -> hsc_env{ hsc_HPT = hpt4 }
          loadFinish Failed linkresult


-- | Finish up after a load.
loadFinish :: GhcMonad m => SuccessFlag -> SuccessFlag -> m SuccessFlag

-- If the link failed, unload everything and return.
loadFinish _all_ok Failed
  = do hsc_env <- getSession
       liftIO $ unload hsc_env []
       modifySession discardProg
       return Failed

-- Empty the interactive context and set the module context to the topmost
-- newly loaded module, or the Prelude if none were loaded.
loadFinish all_ok Succeeded
  = do modifySession discardIC
       return all_ok


-- | Forget the current program, but retain the persistent info in HscEnv
discardProg :: HscEnv -> HscEnv
discardProg hsc_env
  = discardIC $ hsc_env { hsc_mod_graph = emptyMG
                        , hsc_HPT = emptyHomePackageTable }

-- | Discard the contents of the InteractiveContext, but keep the DynFlags
discardIC :: HscEnv -> HscEnv
discardIC hsc_env
  = hsc_env { hsc_IC = emptyInteractiveContext (ic_dflags (hsc_IC hsc_env)) }

intermediateCleanTempFiles :: DynFlags -> [ModSummary] -> HscEnv -> IO ()
intermediateCleanTempFiles dflags summaries hsc_env
 = cleanTempFilesExcept dflags except
  where
    except =
          -- Save preprocessed files. The preprocessed file *might* be
          -- the same as the source file, but that doesn't do any
          -- harm.
          map ms_hspp_file summaries ++
          -- Save object files for loaded modules.  The point of this
          -- is that we might have generated and compiled a stub C
          -- file, and in the case of GHCi the object file will be a
          -- temporary file which we must not remove because we need
          -- to load/link it later.
          hptObjs (hsc_HPT hsc_env)

-- | If there is no -o option, guess the name of target executable
-- by using top-level source file name as a base.
guessOutputFile :: GhcMonad m => m ()
guessOutputFile = modifySession $ \env ->
    let dflags = hsc_dflags env
        mod_graph = hsc_mod_graph env
        mainModuleSrcPath :: Maybe String
        mainModuleSrcPath = do
            let isMain = (== mainModIs dflags) . ms_mod
            [ms] <- return (filter isMain mod_graph)
            ml_hs_file (ms_location ms)
        name = fmap dropExtension mainModuleSrcPath

#if defined(mingw32_HOST_OS)
        -- we must add the .exe extention unconditionally here, otherwise
        -- when name has an extension of its own, the .exe extension will
        -- not be added by DriverPipeline.exeFileName.  See #2248
        name_exe = fmap (<.> "exe") name
#else
        name_exe = name
#endif
    in
    case outputFile dflags of
        Just _ -> env
        Nothing -> env { hsc_dflags = dflags { outputFile = name_exe } }

-- -----------------------------------------------------------------------------
--
-- | Prune the HomePackageTable
--
-- Before doing an upsweep, we can throw away:
--
--   - For non-stable modules:
--      - all ModDetails, all linked code
--   - all unlinked code that is out of date with respect to
--     the source file
--
-- This is VERY IMPORTANT otherwise we'll end up requiring 2x the
-- space at the end of the upsweep, because the topmost ModDetails of the
-- old HPT holds on to the entire type environment from the previous
-- compilation.
pruneHomePackageTable :: HomePackageTable
                      -> [ModSummary]
                      -> ([ModuleName],[ModuleName])
                      -> HomePackageTable
pruneHomePackageTable hpt summ (stable_obj, stable_bco)
  = mapUFM prune hpt
  where prune hmi
          | is_stable modl = hmi'
          | otherwise      = hmi'{ hm_details = emptyModDetails }
          where
           modl = moduleName (mi_module (hm_iface hmi))
           hmi' | Just l <- hm_linkable hmi, linkableTime l < ms_hs_date ms
                = hmi{ hm_linkable = Nothing }
                | otherwise
                = hmi
                where ms = expectJust "prune" (lookupUFM ms_map modl)

        ms_map = listToUFM [(ms_mod_name ms, ms) | ms <- summ]

        is_stable m = m `elem` stable_obj || m `elem` stable_bco

-- -----------------------------------------------------------------------------
--
-- | Return (names of) all those in modsDone who are part of a cycle as defined
-- by theGraph.
findPartiallyCompletedCycles :: [Module] -> [SCC ModSummary] -> [Module]
findPartiallyCompletedCycles modsDone theGraph
   = chew theGraph
     where
        chew [] = []
        chew ((AcyclicSCC _):rest) = chew rest    -- acyclic?  not interesting.
        chew ((CyclicSCC vs):rest)
           = let names_in_this_cycle = nub (map ms_mod vs)
                 mods_in_this_cycle  
                    = nub ([done | done <- modsDone, 
                                   done `elem` names_in_this_cycle])
                 chewed_rest = chew rest
             in 
             if   notNull mods_in_this_cycle
                  && length mods_in_this_cycle < length names_in_this_cycle
             then mods_in_this_cycle ++ chewed_rest
             else chewed_rest


-- ---------------------------------------------------------------------------
--
-- | Unloading
unload :: HscEnv -> [Linkable] -> IO ()
unload hsc_env stable_linkables -- Unload everthing *except* 'stable_linkables'
  = case ghcLink (hsc_dflags hsc_env) of
#ifdef GHCI
        LinkInMemory -> Linker.unload (hsc_dflags hsc_env) stable_linkables
#else
        LinkInMemory -> panic "unload: no interpreter"
                                -- urgh.  avoid warnings:
                                hsc_env stable_linkables
#endif
        _other -> return ()

-- -----------------------------------------------------------------------------
{- |

  Stability tells us which modules definitely do not need to be recompiled.
  There are two main reasons for having stability:
  
   - avoid doing a complete upsweep of the module graph in GHCi when
     modules near the bottom of the tree have not changed.

   - to tell GHCi when it can load object code: we can only load object code
     for a module when we also load object code fo  all of the imports of the
     module.  So we need to know that we will definitely not be recompiling
     any of these modules, and we can use the object code.

  The stability check is as follows.  Both stableObject and
  stableBCO are used during the upsweep phase later.

@
  stable m = stableObject m || stableBCO m

  stableObject m = 
        all stableObject (imports m)
        && old linkable does not exist, or is == on-disk .o
        && date(on-disk .o) > date(.hs)

  stableBCO m =
        all stable (imports m)
        && date(BCO) > date(.hs)
@

  These properties embody the following ideas:

    - if a module is stable, then:

        - if it has been compiled in a previous pass (present in HPT)
          then it does not need to be compiled or re-linked.

        - if it has not been compiled in a previous pass,
          then we only need to read its .hi file from disk and
          link it to produce a 'ModDetails'.

    - if a modules is not stable, we will definitely be at least
      re-linking, and possibly re-compiling it during the 'upsweep'.
      All non-stable modules can (and should) therefore be unlinked
      before the 'upsweep'.

    - Note that objects are only considered stable if they only depend
      on other objects.  We can't link object code against byte code.
-}
checkStability
        :: HomePackageTable   -- HPT from last compilation
        -> [SCC ModSummary]   -- current module graph (cyclic)
        -> [ModuleName]       -- all home modules
        -> ([ModuleName],     -- stableObject
            [ModuleName])     -- stableBCO

checkStability hpt sccs all_home_mods = foldl checkSCC ([],[]) sccs
  where
   checkSCC (stable_obj, stable_bco) scc0
     | stableObjects = (scc_mods ++ stable_obj, stable_bco)
     | stableBCOs    = (stable_obj, scc_mods ++ stable_bco)
     | otherwise     = (stable_obj, stable_bco)
     where
        scc = flattenSCC scc0
        scc_mods = map ms_mod_name scc
        home_module m   = m `elem` 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 (`elem` stable_obj) scc_allimps
        stable_bco_imps = map (`elem` stable_bco) scc_allimps

        stableObjects = 
           and stable_obj_imps
           && all object_ok scc

        stableBCOs = 
           and (zipWith (||) stable_obj_imps stable_bco_imps)
           && all bco_ok scc

        object_ok ms
          | dopt Opt_ForceRecomp (ms_hspp_opts ms) = False
          | Just t <- ms_obj_date ms  =  t >= ms_hs_date ms
                                         && same_as_prev t
          | otherwise = False
          where
             same_as_prev t = case lookupUFM hpt (ms_mod_name ms) of
                                Just hmi  | Just l <- hm_linkable hmi
                                 -> isObjectLinkable l && t == linkableTime l
                                _other  -> True
                -- why '>=' rather than '>' above?  If the filesystem stores
                -- times to the nearset second, we may occasionally find that
                -- the object & source have the same modification time, 
                -- especially if the source was automatically generated
                -- and compiled.  Using >= is slightly unsafe, but it matches
                -- make's behaviour.
                --
                -- But see #5527, where someone ran into this and it caused
                -- a problem.

        bco_ok ms
          | dopt Opt_ForceRecomp (ms_hspp_opts ms) = False
          | otherwise = case lookupUFM hpt (ms_mod_name ms) of
                Just hmi  | Just l <- hm_linkable hmi ->
                        not (isObjectLinkable l) && 
                        linkableTime l >= ms_hs_date ms
                _other  -> False

-- -----------------------------------------------------------------------------
--
-- | The upsweep
--
-- This is where we compile each module in the module graph, in a pass
-- from the bottom to the top of the graph.
--
-- There better had not be any cyclic groups here -- we check for them.
upsweep
    :: GhcMonad m
    => HomePackageTable            -- ^ HPT from last time round (pruned)
    -> ([ModuleName],[ModuleName]) -- ^ stable modules (see checkStability)
    -> (HscEnv -> IO ())           -- ^ How to clean up unwanted tmp files
    -> [SCC ModSummary]            -- ^ Mods to do (the worklist)
    -> m (SuccessFlag,
          [ModSummary])
       -- ^ Returns:
       --
       --  1. A flag whether the complete upsweep was successful.
       --  2. The 'HscEnv' in the monad has an updated HPT
       --  3. A list of modules which succeeded loading.

upsweep old_hpt stable_mods cleanup sccs = do
   (res, done) <- upsweep' old_hpt [] sccs 1 (length sccs)
   return (res, reverse done)
 where

  upsweep' _old_hpt done
     [] _ _
   = return (Succeeded, done)

  upsweep' _old_hpt done
     (CyclicSCC ms:_) _ _
   = do dflags <- getSessionDynFlags
        liftIO $ fatalErrorMsg dflags (cyclicModuleErr ms)
        return (Failed, done)

  upsweep' old_hpt done
     (AcyclicSCC mod:mods) mod_index nmods
   = do -- putStrLn ("UPSWEEP_MOD: hpt = " ++ 
        --           show (map (moduleUserString.moduleName.mi_module.hm_iface) 
        --                     (moduleEnvElts (hsc_HPT hsc_env)))
        let logger _mod = defaultWarnErrLogger

        hsc_env <- getSession

        -- Remove unwanted tmp files between compilations
        liftIO (cleanup hsc_env)

        mb_mod_info
            <- handleSourceError
                   (\err -> do logger mod (Just err); return Nothing) $ do
                 mod_info <- liftIO $ upsweep_mod hsc_env old_hpt stable_mods
                                                  mod mod_index nmods
                 logger mod Nothing -- log warnings
                 return (Just mod_info)

        case mb_mod_info of
          Nothing -> return (Failed, done)
          Just mod_info -> do
                let this_mod = ms_mod_name mod

                        -- Add new info to hsc_env
                    hpt1     = addToUFM (hsc_HPT hsc_env) this_mod mod_info
                    hsc_env1 = hsc_env { hsc_HPT = hpt1 }

                        -- Space-saving: delete the old HPT entry
                        -- for mod BUT if mod is a hs-boot
                        -- node, don't delete it.  For the
                        -- interface, the HPT entry is probaby for the
                        -- main Haskell source file.  Deleting it
                        -- would force the real module to be recompiled
                        -- every time.
                    old_hpt1 | isBootSummary mod = old_hpt
                             | otherwise = delFromUFM old_hpt this_mod

                    done' = mod:done

                        -- fixup our HomePackageTable after we've finished compiling
                        -- a mutually-recursive loop.  See reTypecheckLoop, below.
                hsc_env2 <- liftIO $ reTypecheckLoop hsc_env1 mod done'
                setSession hsc_env2

                upsweep' old_hpt1 done' mods (mod_index+1) nmods

-- | Compile a single module.  Always produce a Linkable for it if
-- successful.  If no compilation happened, return the old Linkable.
upsweep_mod :: HscEnv
            -> HomePackageTable
            -> ([ModuleName],[ModuleName])
            -> ModSummary
            -> Int  -- index of module
            -> Int  -- total number of modules
            -> IO HomeModInfo
upsweep_mod hsc_env old_hpt (stable_obj, stable_bco) summary mod_index nmods
   =    let 
            this_mod_name = ms_mod_name summary
            this_mod    = ms_mod summary
            mb_obj_date = ms_obj_date summary
            obj_fn      = ml_obj_file (ms_location summary)
            hs_date     = ms_hs_date summary

            is_stable_obj = this_mod_name `elem` stable_obj
            is_stable_bco = this_mod_name `elem` stable_bco

            old_hmi = lookupUFM old_hpt this_mod_name

            -- We're using the dflags for this module now, obtained by
            -- applying any options in its LANGUAGE & OPTIONS_GHC pragmas.
            dflags = ms_hspp_opts summary
            prevailing_target = hscTarget (hsc_dflags hsc_env)
            local_target      = hscTarget dflags

            -- If OPTIONS_GHC contains -fasm or -fvia-C, be careful that
            -- we don't do anything dodgy: these should only work to change
            -- from -fvia-C to -fasm and vice-versa, otherwise we could 
            -- end up trying to link object code to byte code.
            target = if prevailing_target /= local_target
                        && (not (isObjectTarget prevailing_target)
                            || not (isObjectTarget local_target))
                        then prevailing_target
                        else local_target 

            -- store the corrected hscTarget into the summary
            summary' = summary{ ms_hspp_opts = dflags { hscTarget = target } }

            -- The old interface is ok if
            --  a) we're compiling a source file, and the old HPT
            --     entry is for a source file
            --  b) we're compiling a hs-boot file
            -- Case (b) allows an hs-boot file to get the interface of its
            -- real source file on the second iteration of the compilation
            -- manager, but that does no harm.  Otherwise the hs-boot file
            -- will always be recompiled
            
            mb_old_iface 
                = case old_hmi of
                     Nothing                              -> Nothing
                     Just hm_info | isBootSummary summary -> Just iface
                                  | not (mi_boot iface)   -> Just iface
                                  | otherwise             -> Nothing
                                   where 
                                     iface = hm_iface hm_info

            compile_it :: Maybe Linkable -> SourceModified -> IO HomeModInfo
            compile_it  mb_linkable src_modified =
                  compile 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 hsc_env summary' mod_index nmods
                          Nothing mb_linkable src_modified

            -- With the HscNothing target we create empty linkables to avoid
            -- recompilation.  We have to detect these to recompile anyway if
            -- the target 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

        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
                liftIO $ debugTraceMsg (hsc_dflags hsc_env) 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
                liftIO $ debugTraceMsg (hsc_dflags hsc_env) 5
                           (text "compiling stable on-disk mod:" <+> ppr this_mod_name)
                linkable <- liftIO $ 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 (isObjectTarget target), is_stable_bco,
            (target /= HscNothing) `implies` not is_fake_linkable ->
                ASSERT(isJust old_hmi) -- must be in the old_hpt
                let Just hmi = old_hmi in do
                liftIO $ debugTraceMsg (hsc_dflags hsc_env) 5
                           (text "skipping stable BCO mod:" <+> ppr this_mod_name)
                return hmi
                -- BCO is stable: nothing to do

          | not (isObjectTarget target),
            Just hmi <- old_hmi,
            Just l <- hm_linkable hmi,
            not (isObjectLinkable l),
            (target /= HscNothing) `implies` not is_fake_linkable,
            linkableTime l >= ms_hs_date summary -> do
                liftIO $ debugTraceMsg (hsc_dflags hsc_env) 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.
          --
          | isObjectTarget target,
            Just obj_date <- mb_obj_date,
            obj_date >= hs_date -> do
                case old_hmi of
                  Just hmi
                    | Just l <- hm_linkable hmi,
                      isObjectLinkable l && linkableTime l == obj_date -> do
                          liftIO $ debugTraceMsg (hsc_dflags hsc_env) 5
                                     (text "compiling mod with new on-disk obj:" <+> ppr this_mod_name)
                          compile_it (Just l) SourceUnmodified
                  _otherwise -> do
                          liftIO $ debugTraceMsg (hsc_dflags hsc_env) 5
                                     (text "compiling mod with new on-disk obj2:" <+> ppr this_mod_name)
                          linkable <- liftIO $ findObjectLinkable this_mod obj_fn obj_date
                          compile_it_discard_iface (Just linkable) SourceUnmodified

         _otherwise -> do
                liftIO $ debugTraceMsg (hsc_dflags hsc_env) 5
                           (text "compiling mod:" <+> ppr this_mod_name)
                compile_it Nothing SourceModified



-- Filter modules in the HPT
retainInTopLevelEnvs :: [ModuleName] -> HomePackageTable -> HomePackageTable
retainInTopLevelEnvs keep_these hpt
   = listToUFM   [ (mod, expectJust "retain" mb_mod_info)
                 | mod <- keep_these
                 , let mb_mod_info = lookupUFM hpt mod
                 , isJust mb_mod_info ]

-- ---------------------------------------------------------------------------
-- Typecheck module loops
{-
See bug #930.  This code fixes a long-standing bug in --make.  The
problem is that when compiling the modules *inside* a loop, a data
type that is only defined at the top of the loop looks opaque; but
after the loop is done, the structure of the data type becomes
apparent.

The difficulty is then that two different bits of code have
different notions of what the data type looks like.

The idea is that after we compile a module which also has an .hs-boot
file, we re-generate the ModDetails for each of the modules that
depends on the .hs-boot file, so that everyone points to the proper
TyCons, Ids etc. defined by the real module, not the boot module.
Fortunately re-generating a ModDetails from a ModIface is easy: the
function TcIface.typecheckIface does exactly that.

Picking the modules to re-typecheck is slightly tricky.  Starting from
the module graph consisting of the modules that have already been
compiled, we reverse the edges (so they point from the imported module
to the importing module), and depth-first-search from the .hs-boot
node.  This gives us all the modules that depend transitively on the
.hs-boot module, and those are exactly the modules that we need to
re-typecheck.

Following this fix, GHC can compile itself with --make -O2.
-}
reTypecheckLoop :: HscEnv -> ModSummary -> ModuleGraph -> IO HscEnv
reTypecheckLoop hsc_env ms graph
  | not (isBootSummary ms) && 
    any (\m -> ms_mod m == this_mod && isBootSummary m) graph
  = do
        let mss = reachableBackwards (ms_mod_name ms) graph
            non_boot = filter (not.isBootSummary) mss
        debugTraceMsg (hsc_dflags hsc_env) 2 $
           text "Re-typechecking loop: " <> ppr (map ms_mod_name non_boot)
        typecheckLoop hsc_env (map ms_mod_name non_boot)
  | otherwise
  = return hsc_env
 where
  this_mod = ms_mod ms

typecheckLoop :: HscEnv -> [ModuleName] -> IO HscEnv
typecheckLoop hsc_env mods = do
  new_hpt <-
    fixIO $ \new_hpt -> do
      let new_hsc_env = hsc_env{ hsc_HPT = new_hpt }
      mds <- initIfaceCheck new_hsc_env $ 
                mapM (typecheckIface . hm_iface) hmis
      let new_hpt = addListToUFM old_hpt 
                        (zip mods [ hmi{ hm_details = details }
                                  | (hmi,details) <- zip hmis mds ])
      return new_hpt
  return hsc_env{ hsc_HPT = new_hpt }
  where
    old_hpt = hsc_HPT hsc_env
    hmis    = map (expectJust "typecheckLoop" . lookupUFM old_hpt) mods

reachableBackwards :: ModuleName -> [ModSummary] -> [ModSummary]
reachableBackwards mod summaries
  = [ ms | (ms,_,_) <- reachableG (transposeG graph) root ]
  where -- the rest just sets up the graph:
        (graph, lookup_node) = moduleGraphNodes False summaries
        root  = expectJust "reachableBackwards" (lookup_node HsBootFile mod)

-- ---------------------------------------------------------------------------
--
-- | Topological sort of the module graph
topSortModuleGraph
          :: Bool
          -- ^ Drop hi-boot nodes? (see below)
          -> [ModSummary]
          -> Maybe ModuleName
             -- ^ Root module name.  If @Nothing@, use the full graph.
          -> [SCC ModSummary]
-- ^ Calculate SCCs of the module graph, possibly dropping the hi-boot nodes
-- The resulting list of strongly-connected-components is in topologically
-- sorted order, starting with the module(s) at the bottom of the
-- dependency graph (ie compile them first) and ending with the ones at
-- the top.
--
-- Drop hi-boot nodes (first boolean arg)? 
--
-- - @False@:   treat the hi-boot summaries as nodes of the graph,
--              so the graph must be acyclic
--
-- - @True@:    eliminate the hi-boot nodes, and instead pretend
--              the a source-import of Foo is an import of Foo
--              The resulting graph has no hi-boot nodes, but can be cyclic

topSortModuleGraph drop_hs_boot_nodes summaries mb_root_mod
  = map (fmap summaryNodeSummary) $ stronglyConnCompG initial_graph
  where
    (graph, lookup_node) = moduleGraphNodes drop_hs_boot_nodes summaries
    
    initial_graph = case mb_root_mod of
        Nothing -> graph
        Just root_mod ->
            -- restrict the graph to just those modules reachable from
            -- the specified module.  We do this by building a graph with
            -- the full set of nodes, and determining the reachable set from
            -- the specified node.
            let root | Just node <- lookup_node HsSrcFile root_mod, graph `hasVertexG` node = node
                     | otherwise = ghcError (ProgramError "module does not exist")
            in graphFromEdgedVertices (seq root (reachableG graph root))

type SummaryNode = (ModSummary, Int, [Int])

summaryNodeKey :: SummaryNode -> Int
summaryNodeKey (_, k, _) = k

summaryNodeSummary :: SummaryNode -> ModSummary
summaryNodeSummary (s, _, _) = s

moduleGraphNodes :: Bool -> [ModSummary]
  -> (Graph SummaryNode, HscSource -> ModuleName -> Maybe SummaryNode)
moduleGraphNodes drop_hs_boot_nodes summaries = (graphFromEdgedVertices nodes, lookup_node)
  where
    numbered_summaries = zip summaries [1..]

    lookup_node :: HscSource -> ModuleName -> Maybe SummaryNode
    lookup_node hs_src mod = Map.lookup (mod, hs_src) node_map

    lookup_key :: HscSource -> ModuleName -> Maybe Int
    lookup_key hs_src mod = fmap summaryNodeKey (lookup_node hs_src mod)

    node_map :: NodeMap SummaryNode
    node_map = Map.fromList [ ((moduleName (ms_mod s), ms_hsc_src s), node)
                            | node@(s, _, _) <- nodes ]

    -- We use integers as the keys for the SCC algorithm
    nodes :: [SummaryNode]
    nodes = [ (s, key, out_keys)
            | (s, key) <- numbered_summaries
             -- Drop the hi-boot ones if told to do so
            , not (isBootSummary s && drop_hs_boot_nodes)
            , let out_keys = out_edge_keys hs_boot_key (map unLoc (ms_home_srcimps s)) ++
                             out_edge_keys HsSrcFile   (map unLoc (ms_home_imps s)) ++
                             (-- see [boot-edges] below
                              if drop_hs_boot_nodes || ms_hsc_src s == HsBootFile 
                              then [] 
                              else case lookup_key HsBootFile (ms_mod_name s) of
                                    Nothing -> []
                                    Just k  -> [k]) ]

    -- [boot-edges] if this is a .hs and there is an equivalent
    -- .hs-boot, add a link from the former to the latter.  This
    -- has the effect of detecting bogus cases where the .hs-boot
    -- depends on the .hs, by introducing a cycle.  Additionally,
    -- it ensures that we will always process the .hs-boot before
    -- the .hs, and so the HomePackageTable will always have the
    -- most up to date information.

    -- Drop hs-boot nodes by using HsSrcFile as the key
    hs_boot_key | drop_hs_boot_nodes = HsSrcFile
                | otherwise          = HsBootFile

    out_edge_keys :: HscSource -> [ModuleName] -> [Int]
    out_edge_keys hi_boot ms = mapCatMaybes (lookup_key hi_boot) ms
        -- If we want keep_hi_boot_nodes, then we do lookup_key with
        -- the IsBootInterface parameter True; else False


type NodeKey   = (ModuleName, HscSource)  -- The nodes of the graph are 
type NodeMap a = Map.Map NodeKey a        -- keyed by (mod, src_file_type) pairs

msKey :: ModSummary -> NodeKey
msKey (ModSummary { ms_mod = mod, ms_hsc_src = boot }) = (moduleName mod,boot)

mkNodeMap :: [ModSummary] -> NodeMap ModSummary
mkNodeMap summaries = Map.fromList [ (msKey s, s) | s <- summaries]
        
nodeMapElts :: NodeMap a -> [a]
nodeMapElts = Map.elems

-- | If there are {-# SOURCE #-} imports between strongly connected
-- components in the topological sort, then those imports can
-- definitely be replaced by ordinary non-SOURCE imports: if SOURCE
-- were necessary, then the edge would be part of a cycle.
warnUnnecessarySourceImports :: GhcMonad m => [SCC ModSummary] -> m ()
warnUnnecessarySourceImports sccs = do
  logWarnings (listToBag (concatMap (check.flattenSCC) sccs))
  where check ms =
           let mods_in_this_cycle = map ms_mod_name ms in
           [ warn i | m <- ms, i <- ms_home_srcimps m,
                      unLoc i `notElem`  mods_in_this_cycle ]

        warn :: Located ModuleName -> WarnMsg
        warn (L loc mod) = 
           mkPlainErrMsg loc
                (ptext (sLit "Warning: {-# SOURCE #-} unnecessary in import of ")
                 <+> quotes (ppr mod))

-----------------------------------------------------------------------------
--
-- | Downsweep (dependency analysis)
--
-- Chase downwards from the specified root set, returning summaries
-- for all home modules encountered.  Only follow source-import
-- links.
--
-- We pass in the previous collection of summaries, which is used as a
-- cache to avoid recalculating a module summary if the source is
-- unchanged.
--
-- The returned list of [ModSummary] nodes has one node for each home-package
-- module, plus one for any hs-boot files.  The imports of these nodes 
-- are all there, including the imports of non-home-package modules.
downsweep :: HscEnv
          -> [ModSummary]       -- Old summaries
          -> [ModuleName]       -- Ignore dependencies on these; treat
                                -- them as if they were package modules
          -> Bool               -- True <=> allow multiple targets to have 
                                --          the same module name; this is 
                                --          very useful for ghc -M
          -> IO [ModSummary]
                -- The elts of [ModSummary] all have distinct
                -- (Modules, IsBoot) identifiers, unless the Bool is true
                -- in which case there can be repeats
downsweep hsc_env old_summaries excl_mods allow_dup_roots
   = do
       rootSummaries <- mapM getRootSummary roots
       let root_map = mkRootMap rootSummaries
       checkDuplicates root_map
       summs <- loop (concatMap msDeps rootSummaries) root_map
       return summs
     where
        roots = hsc_targets hsc_env

        old_summary_map :: NodeMap ModSummary
        old_summary_map = mkNodeMap old_summaries

        getRootSummary :: Target -> IO ModSummary
        getRootSummary (Target (TargetFile file mb_phase) obj_allowed maybe_buf)
           = do exists <- liftIO $ doesFileExist file
                if exists 
                    then summariseFile hsc_env old_summaries file mb_phase 
                                       obj_allowed maybe_buf
                    else throwOneError $ mkPlainErrMsg noSrcSpan $
                           text "can't find file:" <+> text file
        getRootSummary (Target (TargetModule modl) obj_allowed maybe_buf)
           = do maybe_summary <- summariseModule hsc_env old_summary_map False 
                                           (L rootLoc modl) obj_allowed 
                                           maybe_buf excl_mods
                case maybe_summary of
                   Nothing -> packageModErr modl
                   Just s  -> return s

        rootLoc = mkGeneralSrcSpan (fsLit "<command line>")

        -- In a root module, the filename is allowed to diverge from the module
        -- name, so we have to check that there aren't multiple root files
        -- defining the same module (otherwise the duplicates will be silently
        -- ignored, leading to confusing behaviour).
        checkDuplicates :: NodeMap [ModSummary] -> IO ()
        checkDuplicates root_map 
           | allow_dup_roots = return ()
           | null dup_roots  = return ()
           | otherwise       = liftIO $ multiRootsErr (head dup_roots)
           where
             dup_roots :: [[ModSummary]]        -- Each at least of length 2
             dup_roots = filterOut isSingleton (nodeMapElts root_map)

        loop :: [(Located ModuleName,IsBootInterface)]
                        -- Work list: process these modules
             -> NodeMap [ModSummary]
                        -- Visited set; the range is a list because
                        -- the roots can have the same module names
                        -- if allow_dup_roots is True
             -> IO [ModSummary]
                        -- The result includes the worklist, except
                        -- for those mentioned in the visited set
        loop [] done      = return (concat (nodeMapElts done))
        loop ((wanted_mod, is_boot) : ss) done 
          | Just summs <- Map.lookup key done
          = if isSingleton summs then
                loop ss done
            else
                do { multiRootsErr summs; return [] }
          | otherwise
          = do mb_s <- summariseModule hsc_env old_summary_map 
                                       is_boot wanted_mod True
                                       Nothing excl_mods
               case mb_s of
                   Nothing -> loop ss done
                   Just s  -> loop (msDeps s ++ ss) (Map.insert key [s] done)
          where
            key = (unLoc wanted_mod, if is_boot then HsBootFile else HsSrcFile)

mkRootMap :: [ModSummary] -> NodeMap [ModSummary]
mkRootMap summaries = Map.insertListWith (flip (++))
                                         [ (msKey s, [s]) | s <- summaries ]
                                         Map.empty

-- | Returns the dependencies of the ModSummary s.
-- A wrinkle is that for a {-# SOURCE #-} import we return
--      *both* the hs-boot file
--      *and* the source file
-- as "dependencies".  That ensures that the list of all relevant
-- modules always contains B.hs if it contains B.hs-boot.
-- Remember, this pass isn't doing the topological sort.  It's
-- just gathering the list of all relevant ModSummaries
msDeps :: ModSummary -> [(Located ModuleName, IsBootInterface)]
msDeps s = 
    concat [ [(m,True), (m,False)] | m <- ms_home_srcimps s ] 
         ++ [ (m,False) | m <- ms_home_imps s ] 

home_imps :: [Located (ImportDecl RdrName)] -> [Located ModuleName]
home_imps imps = [ ideclName i |  L _ i <- imps, isLocal (ideclPkgQual i) ]
  where isLocal Nothing = True
        isLocal (Just pkg) | pkg == fsLit "this" = True -- "this" is special
        isLocal _ = False

ms_home_allimps :: ModSummary -> [ModuleName]
ms_home_allimps ms = map unLoc (ms_home_srcimps ms ++ ms_home_imps ms)

ms_home_srcimps :: ModSummary -> [Located ModuleName]
ms_home_srcimps = home_imps . ms_srcimps

ms_home_imps :: ModSummary -> [Located ModuleName]
ms_home_imps = home_imps . ms_imps

-----------------------------------------------------------------------------
-- Summarising modules

-- We have two types of summarisation:
--
--    * Summarise a file.  This is used for the root module(s) passed to
--      cmLoadModules.  The file is read, and used to determine the root
--      module name.  The module name may differ from the filename.
--
--    * Summarise a module.  We are given a module name, and must provide
--      a summary.  The finder is used to locate the file in which the module
--      resides.

summariseFile
        :: HscEnv
        -> [ModSummary]                 -- old summaries
        -> FilePath                     -- source file name
        -> Maybe Phase                  -- start phase
        -> Bool                         -- object code allowed?
        -> Maybe (StringBuffer,UTCTime)
        -> IO ModSummary

summariseFile hsc_env old_summaries file mb_phase obj_allowed maybe_buf
        -- we can use a cached summary if one is available and the
        -- source file hasn't changed,  But we have to look up the summary
        -- by source file, rather than module name as we do in summarise.
   | Just old_summary <- findSummaryBySourceFile old_summaries file
   = do
        let location = ms_location old_summary

        src_timestamp <- get_src_timestamp
                -- The file exists; we checked in getRootSummary above.
                -- If it gets removed subsequently, then this 
                -- getModificationUTCTime may fail, but that's the right
                -- behaviour.

                -- return the cached summary if the source didn't change
        if ms_hs_date old_summary == src_timestamp
           then do -- update the object-file timestamp
                  obj_timestamp <-
                    if isObjectTarget (hscTarget (hsc_dflags hsc_env)) 
                        || obj_allowed -- bug #1205
                        then liftIO $ getObjTimestamp location False
                        else return Nothing
                  return old_summary{ ms_obj_date = obj_timestamp }
           else
                new_summary src_timestamp

   | otherwise
   = do src_timestamp <- get_src_timestamp
        new_summary src_timestamp
  where
    get_src_timestamp = case maybe_buf of
                           Just (_,t) -> return t
                           Nothing    -> liftIO $ getModificationUTCTime file
                        -- getMofificationUTCTime may fail

    new_summary src_timestamp = do
        let dflags = hsc_dflags hsc_env

        (dflags', hspp_fn, buf)
            <- preprocessFile hsc_env file mb_phase maybe_buf

        (srcimps,the_imps, L _ mod_name) <- getImports dflags' buf hspp_fn file

        -- Make a ModLocation for this file
        location <- liftIO $ mkHomeModLocation dflags mod_name file

        -- Tell the Finder cache where it is, so that subsequent calls
        -- to findModule will find it, even if it's not on any search path
        mod <- liftIO $ addHomeModuleToFinder hsc_env mod_name location

        -- when the user asks to load a source file by name, we only
        -- use an object file if -fobject-code is on.  See #1205.
        obj_timestamp <-
            if isObjectTarget (hscTarget (hsc_dflags hsc_env)) 
               || obj_allowed -- bug #1205
                then liftIO $ modificationTimeIfExists (ml_obj_file location)
                else return Nothing

        return (ModSummary { ms_mod = mod, ms_hsc_src = HsSrcFile,
                             ms_location = location,
                             ms_hspp_file = hspp_fn,
                             ms_hspp_opts = dflags',
                             ms_hspp_buf  = Just buf,
                             ms_srcimps = srcimps, ms_textual_imps = the_imps,
                             ms_hs_date = src_timestamp,
                             ms_obj_date = obj_timestamp })

findSummaryBySourceFile :: [ModSummary] -> FilePath -> Maybe ModSummary
findSummaryBySourceFile summaries file
  = case [ ms | ms <- summaries, HsSrcFile <- [ms_hsc_src ms],
                                 expectJust "findSummaryBySourceFile" (ml_hs_file (ms_location ms)) == file ] of
        [] -> Nothing
        (x:_) -> Just x

-- Summarise a module, and pick up source and timestamp.
summariseModule
          :: HscEnv
          -> NodeMap ModSummary -- Map of old summaries
          -> IsBootInterface    -- True <=> a {-# SOURCE #-} import
          -> Located ModuleName -- Imported module to be summarised
          -> Bool               -- object code allowed?
          -> Maybe (StringBuffer, UTCTime)
          -> [ModuleName]               -- Modules to exclude
          -> IO (Maybe ModSummary)      -- Its new summary

summariseModule hsc_env old_summary_map is_boot (L loc wanted_mod) 
                obj_allowed maybe_buf excl_mods
  | wanted_mod `elem` excl_mods
  = return Nothing

  | Just old_summary <- Map.lookup (wanted_mod, hsc_src) old_summary_map
  = do          -- Find its new timestamp; all the 
                -- ModSummaries in the old map have valid ml_hs_files
        let location = ms_location old_summary
            src_fn = expectJust "summariseModule" (ml_hs_file location)

                -- check the modification time on the source file, and
                -- return the cached summary if it hasn't changed.  If the
                -- file has disappeared, we need to call the Finder again.
        case maybe_buf of
           Just (_,t) -> check_timestamp old_summary location src_fn t
           Nothing    -> do
                m <- tryIO (getModificationUTCTime src_fn)
                case m of
                   Right t -> check_timestamp old_summary location src_fn t
                   Left e | isDoesNotExistError e -> find_it
                          | otherwise             -> ioError e

  | otherwise  = find_it
  where
    dflags = hsc_dflags hsc_env

    hsc_src = if is_boot then HsBootFile else HsSrcFile

    check_timestamp old_summary location src_fn src_timestamp
        | ms_hs_date old_summary == src_timestamp = do
                -- update the object-file timestamp
                obj_timestamp <- 
                    if isObjectTarget (hscTarget (hsc_dflags hsc_env))
                       || obj_allowed -- bug #1205
                       then getObjTimestamp location is_boot
                       else return Nothing
                return (Just old_summary{ ms_obj_date = obj_timestamp })
        | otherwise = 
                -- source changed: re-summarise.
                new_summary location (ms_mod old_summary) src_fn src_timestamp

    find_it = do
        -- Don't use the Finder's cache this time.  If the module was
        -- previously a package module, it may have now appeared on the
        -- search path, so we want to consider it to be a home module.  If
        -- the module was previously a home module, it may have moved.
        uncacheModule hsc_env wanted_mod
        found <- findImportedModule hsc_env wanted_mod Nothing
        case found of
             Found location mod 
                | isJust (ml_hs_file location) ->
                        -- Home package
                         just_found location mod
                | otherwise -> 
                        -- Drop external-pkg
                        ASSERT(modulePackageId mod /= thisPackage dflags)
                        return Nothing
                        
             err -> noModError dflags loc wanted_mod err
                        -- Not found

    just_found location mod = do
                -- Adjust location to point to the hs-boot source file, 
                -- hi file, object file, when is_boot says so
        let location' | is_boot   = addBootSuffixLocn location
                      | otherwise = location
            src_fn = expectJust "summarise2" (ml_hs_file location')

                -- Check that it exists
                -- It might have been deleted since the Finder last found it
        maybe_t <- modificationTimeIfExists src_fn
        case maybe_t of
          Nothing -> noHsFileErr loc src_fn
          Just t  -> new_summary location' mod src_fn t


    new_summary location mod src_fn src_timestamp
      = do
        -- Preprocess the source file and get its imports
        -- The dflags' contains the OPTIONS pragmas
        (dflags', hspp_fn, buf) <- preprocessFile hsc_env src_fn Nothing maybe_buf
        (srcimps, the_imps, L mod_loc mod_name) <- getImports dflags' buf hspp_fn src_fn

        when (mod_name /= wanted_mod) $
                throwOneError $ mkPlainErrMsg mod_loc $ 
                              text "File name does not match module name:" 
                              $$ text "Saw:" <+> quotes (ppr mod_name)
                              $$ text "Expected:" <+> quotes (ppr wanted_mod)

                -- Find the object timestamp, and return the summary
        obj_timestamp <-
           if isObjectTarget (hscTarget (hsc_dflags hsc_env))
              || obj_allowed -- bug #1205
              then getObjTimestamp location is_boot
              else return Nothing

        return (Just (ModSummary { ms_mod       = mod,
                              ms_hsc_src   = hsc_src,
                              ms_location  = location,
                              ms_hspp_file = hspp_fn,
                              ms_hspp_opts = dflags',
                              ms_hspp_buf  = Just buf,
                              ms_srcimps      = srcimps,
                              ms_textual_imps = the_imps,
                              ms_hs_date   = src_timestamp,
                              ms_obj_date  = obj_timestamp }))


getObjTimestamp :: ModLocation -> Bool -> IO (Maybe UTCTime)
getObjTimestamp location is_boot
  = if is_boot then return Nothing
               else modificationTimeIfExists (ml_obj_file location)


preprocessFile :: HscEnv
               -> FilePath
               -> Maybe Phase -- ^ Starting phase
               -> Maybe (StringBuffer,UTCTime)
               -> IO (DynFlags, FilePath, StringBuffer)
preprocessFile hsc_env src_fn mb_phase Nothing
  = do
        (dflags', hspp_fn) <- preprocess hsc_env (src_fn, mb_phase)
        buf <- hGetStringBuffer hspp_fn
        return (dflags', hspp_fn, buf)

preprocessFile hsc_env src_fn mb_phase (Just (buf, _time))
  = do
        let dflags = hsc_dflags hsc_env
        let local_opts = getOptions dflags buf src_fn

        (dflags', leftovers, warns)
            <- parseDynamicFilePragma dflags local_opts
        checkProcessArgsResult leftovers
        handleFlagWarnings dflags' warns

        let needs_preprocessing
                | Just (Unlit _) <- mb_phase    = True
                | Nothing <- mb_phase, Unlit _ <- startPhase src_fn  = True
                  -- note: local_opts is only required if there's no Unlit phase
                | xopt Opt_Cpp dflags'          = True
                | dopt Opt_Pp  dflags'          = True
                | otherwise                     = False

        when needs_preprocessing $
           ghcError (ProgramError "buffer needs preprocesing; interactive check disabled")

        return (dflags', src_fn, buf)


-----------------------------------------------------------------------------
--                      Error messages
-----------------------------------------------------------------------------

noModError :: DynFlags -> SrcSpan -> ModuleName -> FindResult -> IO ab
-- ToDo: we don't have a proper line number for this error
noModError dflags loc wanted_mod err
  = throwOneError $ mkPlainErrMsg loc $ cannotFindModule dflags wanted_mod err
                                
noHsFileErr :: SrcSpan -> String -> IO a
noHsFileErr loc path
  = throwOneError $ mkPlainErrMsg loc $ text "Can't find" <+> text path
 
packageModErr :: ModuleName -> IO a
packageModErr mod
  = throwOneError $ mkPlainErrMsg noSrcSpan $
        text "module" <+> quotes (ppr mod) <+> text "is a package module"

multiRootsErr :: [ModSummary] -> IO ()
multiRootsErr [] = panic "multiRootsErr"
multiRootsErr summs@(summ1:_)
  = throwOneError $ mkPlainErrMsg noSrcSpan $
        text "module" <+> quotes (ppr mod) <+> 
        text "is defined in multiple files:" <+>
        sep (map text files)
  where
    mod = ms_mod summ1
    files = map (expectJust "checkDup" . ml_hs_file . ms_location) summs

cyclicModuleErr :: [ModSummary] -> SDoc
-- From a strongly connected component we find 
-- a single cycle to report
cyclicModuleErr mss
  = ASSERT( not (null mss) )
    case findCycle graph of
       Nothing   -> ptext (sLit "Unexpected non-cycle") <+> ppr mss
       Just path -> vcat [ ptext (sLit "Module imports form a cycle:")
                         , nest 2 (show_path path) ]
  where
    graph :: [Node NodeKey ModSummary]
    graph = [(ms, msKey ms, get_deps ms) | ms <- mss]

    get_deps :: ModSummary -> [NodeKey]
    get_deps ms = ([ (unLoc m, HsBootFile) | m <- ms_home_srcimps ms ] ++
                   [ (unLoc m, HsSrcFile)  | m <- ms_home_imps    ms ])

    show_path []         = panic "show_path"
    show_path [m]        = ptext (sLit "module") <+> ppr_ms m
                           <+> ptext (sLit "imports itself")
    show_path (m1:m2:ms) = vcat ( nest 7 (ptext (sLit "module") <+> ppr_ms m1)
                                : nest 6 (ptext (sLit "imports") <+> ppr_ms m2)
                                : go ms )
       where
         go []     = [ptext (sLit "which imports") <+> ppr_ms m1]
         go (m:ms) = (ptext (sLit "which imports") <+> ppr_ms m) : go ms
       

    ppr_ms :: ModSummary -> SDoc
    ppr_ms ms = quotes (ppr (moduleName (ms_mod ms))) <+> 
                (parens (text (msHsFilePath ms)))