    {-# LANGUAGE CPP, NondecreasingIndentation, TupleSections, RecordWildCards #-}
    {-# OPTIONS_GHC -fno-cse #-}
    --
    --  (c) The University of Glasgow 2002-2006
    --

    -- -fno-cse is needed for GLOBAL_VAR's to behave properly

    -- | The dynamic linker for GHCi.
    --
    -- This module deals with the top-level issues of dynamic linking,
    -- calling the object-code linker and the byte-code linker where
    -- necessary.
    module Linker ( getHValue, showLinkerState,
                    linkExpr, linkDecls, unload, withExtendedLinkEnv,
                    extendLinkEnv, deleteFromLinkEnv,
                    extendLoadedPkgs,
                    linkPackages,initDynLinker,linkModule,
                    linkCmdLineLibs,

                    -- Saving/restoring globals
                    PersistentLinkerState, saveLinkerGlobals, restoreLinkerGlobals
            ) where

    #include "HsVersions.h"

    import GHCi
    import GHCi.RemoteTypes
    import LoadIface
    import ByteCodeLink
    import ByteCodeAsm
    import ByteCodeTypes
    import TcRnMonad
    import Packages
    import DriverPhases
    import Finder
    import HscTypes
    import Name
    import NameEnv
    import NameSet
    import UniqFM
    import Module
    import ListSetOps
    import DynFlags
    import BasicTypes
    import Outputable
    import Panic
    import Util
    import ErrUtils
    import SrcLoc
    import qualified Maybes
    import UniqSet
    import FastString
    import Platform
    import SysTools

    -- Standard libraries
    import Control.Monad
    import Control.Applicative((<|>))

    import Data.IORef
    import Data.List
    import Data.Maybe
    import Control.Concurrent.MVar

    import System.FilePath
    import System.Directory

    import Exception

    {- **********************************************************************

                            The Linker's state

      ********************************************************************* -}

    {-
    The persistent linker state *must* match the actual state of the
    C dynamic linker at all times, so we keep it in a private global variable.

    The global IORef used for PersistentLinkerState actually contains another MVar.
    The reason for this is that we want to allow another loaded copy of the GHC
    library to side-effect the PLS and for those changes to be reflected here.

    The PersistentLinkerState maps Names to actual closures (for
    interpreted code only), for use during linking.
    -}

    GLOBAL_VAR_M(v_PersistentLinkerState, newMVar (panic "Dynamic linker not initialised"), MVar PersistentLinkerState)
    GLOBAL_VAR(v_InitLinkerDone, False, Bool) -- Set True when dynamic linker is initialised

    modifyPLS_ :: (PersistentLinkerState -> IO PersistentLinkerState) -> IO ()
    modifyPLS_ f = readIORef v_PersistentLinkerState >>= flip modifyMVar_ f

    modifyPLS :: (PersistentLinkerState -> IO (PersistentLinkerState, a)) -> IO a
    modifyPLS f = readIORef v_PersistentLinkerState >>= flip modifyMVar f

    data PersistentLinkerState
       = PersistentLinkerState {

            -- Current global mapping from Names to their true values
            closure_env :: ClosureEnv,

            -- The current global mapping from RdrNames of DataCons to
            -- info table addresses.
            -- When a new Unlinked is linked into the running image, or an existing
            -- module in the image is replaced, the itbl_env must be updated
            -- appropriately.
            itbl_env    :: !ItblEnv,

            -- The currently loaded interpreted modules (home package)
            bcos_loaded :: ![Linkable],

            -- And the currently-loaded compiled modules (home package)
            objs_loaded :: ![Linkable],

            -- The currently-loaded packages; always object code
            -- Held, as usual, in dependency order; though I am not sure if
            -- that is really important
            pkgs_loaded :: ![UnitId],

            -- we need to remember the name of previous temporary DLL/.so
            -- libraries so we can link them (see #10322)
            temp_sos :: ![(FilePath, String)] }

    emptyPLS :: DynFlags -> PersistentLinkerState
    emptyPLS _ = PersistentLinkerState {
                            closure_env = emptyNameEnv,
                            itbl_env    = emptyNameEnv,
                            pkgs_loaded = init_pkgs,
                            bcos_loaded = [],
                            objs_loaded = [],
                            temp_sos = [] }

      -- Packages that don't need loading, because the compiler
      -- shares them with the interpreted program.
      --
      -- The linker's symbol table is populated with RTS symbols using an
      -- explicit list.  See rts/Linker.c for details.
      where init_pkgs = [rtsUnitId]

    extendLoadedPkgs :: [UnitId] -> IO ()
    extendLoadedPkgs pkgs =
      modifyPLS_ $ \s ->
          return s{ pkgs_loaded = pkgs ++ pkgs_loaded s }

    extendLinkEnv :: [(Name,ForeignHValue)] -> IO ()
    extendLinkEnv new_bindings =
      modifyPLS_ $ \pls -> do
        let ce = closure_env pls
        let new_ce = extendClosureEnv ce new_bindings
        return pls{ closure_env = new_ce }

    deleteFromLinkEnv :: [Name] -> IO ()
    deleteFromLinkEnv to_remove =
      modifyPLS_ $ \pls -> do
        let ce = closure_env pls
        let new_ce = delListFromNameEnv ce to_remove
        return pls{ closure_env = new_ce }

    -- | Get the 'HValue' associated with the given name.
    --
    -- May cause loading the module that contains the name.
    --
    -- Throws a 'ProgramError' if loading fails or the name cannot be found.
    getHValue :: HscEnv -> Name -> IO ForeignHValue
    getHValue hsc_env name = do
      initDynLinker hsc_env
      pls <- modifyPLS $ \pls -> do
               if (isExternalName name) then do
                 (pls', ok) <- linkDependencies hsc_env pls noSrcSpan
                                  [nameModule name]
                 if (failed ok) then throwGhcExceptionIO (ProgramError "")
                                else return (pls', pls')
                else
                 return (pls, pls)
      case lookupNameEnv (closure_env pls) name of
        Just (_,aa) -> return aa
        Nothing
            -> ASSERT2(isExternalName name, ppr name)
               do let sym_to_find = nameToCLabel name "closure"
                  m <- lookupClosure hsc_env (unpackFS sym_to_find)
                  case m of
                    Just hvref -> mkFinalizedHValue hsc_env hvref
                    Nothing -> linkFail "ByteCodeLink.lookupCE"
                                 (unpackFS sym_to_find)

    linkDependencies :: HscEnv -> PersistentLinkerState
                     -> SrcSpan -> [Module]
                     -> IO (PersistentLinkerState, SuccessFlag)
    linkDependencies hsc_env pls span needed_mods = do
    --   initDynLinker (hsc_dflags hsc_env)
       let hpt = hsc_HPT hsc_env
           dflags = hsc_dflags hsc_env
       -- The interpreter and dynamic linker can only handle object code built
       -- the "normal" way, i.e. no non-std ways like profiling or ticky-ticky.
       -- So here we check the build tag: if we're building a non-standard way
       -- then we need to find & link object files built the "normal" way.
       maybe_normal_osuf <- checkNonStdWay dflags span

       -- Find what packages and linkables are required
       (lnks, pkgs) <- getLinkDeps hsc_env hpt pls
                                   maybe_normal_osuf span needed_mods

       -- Link the packages and modules required
       pls1 <- linkPackages' hsc_env pkgs pls
       linkModules hsc_env pls1 lnks

    -- | Temporarily extend the linker state.

    withExtendedLinkEnv :: (ExceptionMonad m) =>
                           [(Name,ForeignHValue)] -> m a -> m a
    withExtendedLinkEnv new_env action
        = gbracket (liftIO $ extendLinkEnv new_env)
                   (\_ -> reset_old_env)
                   (\_ -> action)
        where
            -- Remember that the linker state might be side-effected
            -- during the execution of the IO action, and we don't want to
            -- lose those changes (we might have linked a new module or
            -- package), so the reset action only removes the names we
            -- added earlier.
              reset_old_env = liftIO $ do
                modifyPLS_ $ \pls ->
                    let cur = closure_env pls
                        new = delListFromNameEnv cur (map fst new_env)
                    in return pls{ closure_env = new }

    -- | Display the persistent linker state.
    showLinkerState :: DynFlags -> IO ()
    showLinkerState dflags
      = do pls <- readIORef v_PersistentLinkerState >>= readMVar
           log_action dflags dflags NoReason SevDump noSrcSpan defaultDumpStyle
                     (vcat [text "----- Linker state -----",
                            text "Pkgs:" <+> ppr (pkgs_loaded pls),
                            text "Objs:" <+> ppr (objs_loaded pls),
                            text "BCOs:" <+> ppr (bcos_loaded pls)])

    {- **********************************************************************

                            Initialisation

      ********************************************************************* -}

    -- | Initialise the dynamic linker.  This entails
    --
    --  a) Calling the C initialisation procedure,
    --
    --  b) Loading any packages specified on the command line,
    --
    --  c) Loading any packages specified on the command line, now held in the
    --     @-l@ options in @v_Opt_l@,
    --
    --  d) Loading any @.o\/.dll@ files specified on the command line, now held
    --     in @ldInputs@,
    --
    --  e) Loading any MacOS frameworks.
    --
    -- NOTE: This function is idempotent; if called more than once, it does
    -- nothing.  This is useful in Template Haskell, where we call it before
    -- trying to link.
    --
    initDynLinker :: HscEnv -> IO ()
    initDynLinker hsc_env =
      modifyPLS_ $ \pls0 -> do
        done <- readIORef v_InitLinkerDone
        if done then return pls0
                else do writeIORef v_InitLinkerDone True
                        reallyInitDynLinker hsc_env

    reallyInitDynLinker :: HscEnv -> IO PersistentLinkerState
    reallyInitDynLinker hsc_env = do
      -- Initialise the linker state
      let dflags = hsc_dflags hsc_env
          pls0 = emptyPLS dflags

      -- (a) initialise the C dynamic linker
      initObjLinker hsc_env

      -- (b) Load packages from the command-line (Note [preload packages])
      pls <- linkPackages' hsc_env (preloadPackages (pkgState dflags)) pls0

      -- steps (c), (d) and (e)
      linkCmdLineLibs' hsc_env pls

    linkCmdLineLibs :: HscEnv -> IO ()
    linkCmdLineLibs hsc_env = do
      initDynLinker hsc_env
      modifyPLS_ $ \pls -> do
        linkCmdLineLibs' hsc_env pls

    linkCmdLineLibs' :: HscEnv -> PersistentLinkerState -> IO PersistentLinkerState
    linkCmdLineLibs' hsc_env pls =
      do
          let dflags@(DynFlags { ldInputs = cmdline_ld_inputs
                               , libraryPaths = lib_paths}) = hsc_dflags hsc_env

          -- (c) Link libraries from the command-line
          let minus_ls = [ lib | Option ('-':'l':lib) <- cmdline_ld_inputs ]
          libspecs <- mapM (locateLib hsc_env False lib_paths) minus_ls

          -- (d) Link .o files from the command-line
          classified_ld_inputs <- mapM (classifyLdInput dflags)
                                    [ f | FileOption _ f <- cmdline_ld_inputs ]

          -- (e) Link any MacOS frameworks
          let platform = targetPlatform dflags
          let (framework_paths, frameworks) =
                if platformUsesFrameworks platform
                 then (frameworkPaths dflags, cmdlineFrameworks dflags)
                  else ([],[])

          -- Finally do (c),(d),(e)
          let cmdline_lib_specs = catMaybes classified_ld_inputs
                               ++ libspecs
                               ++ map Framework frameworks
          if null cmdline_lib_specs then return pls
                                    else do

          -- Add directories to library search paths
          let all_paths = let paths = framework_paths
                                   ++ lib_paths
                                   ++ [ takeDirectory dll | DLLPath dll <- libspecs ]
                          in nub $ map normalise paths
          pathCache <- mapM (addLibrarySearchPath hsc_env) all_paths

          pls1 <- foldM (preloadLib hsc_env lib_paths framework_paths) pls
                        cmdline_lib_specs
          maybePutStr dflags "final link ... "
          ok <- resolveObjs hsc_env

          -- DLLs are loaded, reset the search paths
          mapM_ (removeLibrarySearchPath hsc_env) $ reverse pathCache

          if succeeded ok then maybePutStrLn dflags "done"
          else throwGhcExceptionIO (ProgramError "linking extra libraries/objects failed")

          return pls1

    {- Note [preload packages]

    Why do we need to preload packages from the command line?  This is an
    explanation copied from #2437:

    I tried to implement the suggestion from #3560, thinking it would be
    easy, but there are two reasons we link in packages eagerly when they
    are mentioned on the command line:

      * So that you can link in extra object files or libraries that
        depend on the packages. e.g. ghc -package foo -lbar where bar is a
        C library that depends on something in foo. So we could link in
        foo eagerly if and only if there are extra C libs or objects to
        link in, but....

      * Haskell code can depend on a C function exported by a package, and
        the normal dependency tracking that TH uses can't know about these
        dependencies. The test ghcilink004 relies on this, for example.

    I conclude that we need two -package flags: one that says "this is a
    package I want to make available", and one that says "this is a
    package I want to link in eagerly". Would that be too complicated for
    users?
    -}

    classifyLdInput :: DynFlags -> FilePath -> IO (Maybe LibrarySpec)
    classifyLdInput dflags f
      | isObjectFilename platform f = return (Just (Object f))
      | isDynLibFilename platform f = return (Just (DLLPath f))
      | otherwise          = do
            log_action dflags dflags NoReason SevInfo noSrcSpan defaultUserStyle
                (text ("Warning: ignoring unrecognised input `" ++ f ++ "'"))
            return Nothing
        where platform = targetPlatform dflags

    preloadLib
      :: HscEnv -> [String] -> [String] -> PersistentLinkerState
      -> LibrarySpec -> IO PersistentLinkerState
    preloadLib hsc_env lib_paths framework_paths pls lib_spec = do
      maybePutStr dflags ("Loading object " ++ showLS lib_spec ++ " ... ")
      case lib_spec of
        Object static_ish -> do
          (b, pls1) <- preload_static lib_paths static_ish
          maybePutStrLn dflags (if b  then "done" else "not found")
          return pls1

        Archive static_ish -> do
          b <- preload_static_archive lib_paths static_ish
          maybePutStrLn dflags (if b  then "done" else "not found")
          return pls

        DLL dll_unadorned -> do
          maybe_errstr <- loadDLL hsc_env (mkSOName platform dll_unadorned)
          case maybe_errstr of
             Nothing -> maybePutStrLn dflags "done"
             Just mm | platformOS platform /= OSDarwin ->
               preloadFailed mm lib_paths lib_spec
             Just mm | otherwise -> do
               -- As a backup, on Darwin, try to also load a .so file
               -- since (apparently) some things install that way - see
               -- ticket #8770.
               let libfile = ("lib" ++ dll_unadorned) <.> "so"
               err2 <- loadDLL hsc_env libfile
               case err2 of
                 Nothing -> maybePutStrLn dflags "done"
                 Just _  -> preloadFailed mm lib_paths lib_spec
          return pls

        DLLPath dll_path -> do
          do maybe_errstr <- loadDLL hsc_env dll_path
             case maybe_errstr of
                Nothing -> maybePutStrLn dflags "done"
                Just mm -> preloadFailed mm lib_paths lib_spec
             return pls

        Framework framework ->
          if platformUsesFrameworks (targetPlatform dflags)
          then do maybe_errstr <- loadFramework hsc_env framework_paths framework
                  case maybe_errstr of
                     Nothing -> maybePutStrLn dflags "done"
                     Just mm -> preloadFailed mm framework_paths lib_spec
                  return pls
          else panic "preloadLib Framework"

      where
        dflags = hsc_dflags hsc_env

        platform = targetPlatform dflags

        preloadFailed :: String -> [String] -> LibrarySpec -> IO ()
        preloadFailed sys_errmsg paths spec
           = do maybePutStr dflags "failed.\n"
                throwGhcExceptionIO $
                  CmdLineError (
                        "user specified .o/.so/.DLL could not be loaded ("
                        ++ sys_errmsg ++ ")\nWhilst trying to load:  "
                        ++ showLS spec ++ "\nAdditional directories searched:"
                        ++ (if null paths then " (none)" else
                            intercalate "\n" (map ("   "++) paths)))

        -- Not interested in the paths in the static case.
        preload_static _paths name
           = do b <- doesFileExist name
                if not b then return (False, pls)
                         else if dynamicGhc
                                 then  do pls1 <- dynLoadObjs hsc_env pls [name]
                                          return (True, pls1)
                                 else  do loadObj hsc_env name
                                          return (True, pls)

        preload_static_archive _paths name
           = do b <- doesFileExist name
                if not b then return False
                         else do if dynamicGhc
                                     then panic "Loading archives not supported"
                                     else loadArchive hsc_env name
                                 return True

    {- **********************************************************************

                            Link a byte-code expression

      ********************************************************************* -}

    -- | Link a single expression, /including/ first linking packages and
    -- modules that this expression depends on.
    --
    -- Raises an IO exception ('ProgramError') if it can't find a compiled
    -- version of the dependents to link.
    --
    linkExpr :: HscEnv -> SrcSpan -> UnlinkedBCO -> IO ForeignHValue
    linkExpr hsc_env span root_ul_bco
      = do {
         -- Initialise the linker (if it's not been done already)
       ; initDynLinker hsc_env

         -- Take lock for the actual work.
       ; modifyPLS $ \pls0 -> do {

         -- Link the packages and modules required
       ; (pls, ok) <- linkDependencies hsc_env pls0 span needed_mods
       ; if failed ok then
            throwGhcExceptionIO (ProgramError "")
         else do {

         -- Link the expression itself
         let ie = itbl_env pls
             ce = closure_env pls

         -- Link the necessary packages and linkables

       ; let nobreakarray = error "no break array"
             bco_ix = mkNameEnv [(unlinkedBCOName root_ul_bco, 0)]
       ; resolved <- linkBCO hsc_env ie ce bco_ix nobreakarray root_ul_bco
       ; [root_hvref] <- createBCOs hsc_env [resolved]
       ; fhv <- mkFinalizedHValue hsc_env root_hvref
       ; return (pls, fhv)
       }}}
       where
         free_names = nameSetElems (bcoFreeNames root_ul_bco)

         needed_mods :: [Module]
         needed_mods = [ nameModule n | n <- free_names,
                         isExternalName n,      -- Names from other modules
                         not (isWiredInName n)  -- Exclude wired-in names
                       ]                        -- (see note below)
            -- Exclude wired-in names because we may not have read
            -- their interface files, so getLinkDeps will fail
            -- All wired-in names are in the base package, which we link
            -- by default, so we can safely ignore them here.

    dieWith :: DynFlags -> SrcSpan -> MsgDoc -> IO a
    dieWith dflags span msg = throwGhcExceptionIO (ProgramError (showSDoc dflags (mkLocMessage SevFatal span msg)))

    checkNonStdWay :: DynFlags -> SrcSpan -> IO (Maybe FilePath)
    checkNonStdWay dflags srcspan
      | gopt Opt_ExternalInterpreter dflags = return Nothing
        -- with -fexternal-interpreter we load the .o files, whatever way
        -- they were built.  If they were built for a non-std way, then
        -- we will use the appropriate variant of the iserv binary to load them.

      | interpWays == haskellWays = return Nothing
        -- Only if we are compiling with the same ways as GHC is built
        -- with, can we dynamically load those object files. (see #3604)

      | objectSuf dflags == normalObjectSuffix && not (null haskellWays)
      = failNonStd dflags srcspan

      | otherwise = return (Just (interpTag ++ "o"))
      where
        haskellWays = filter (not . wayRTSOnly) (ways dflags)
        interpTag = case mkBuildTag interpWays of
                      "" -> ""
                      tag -> tag ++ "_"

    normalObjectSuffix :: String
    normalObjectSuffix = phaseInputExt StopLn

    failNonStd :: DynFlags -> SrcSpan -> IO (Maybe FilePath)
    failNonStd dflags srcspan = dieWith dflags srcspan $
      text "Cannot load" <+> compWay <+>
         text "objects when GHC is built" <+> ghciWay $$
      text "To fix this, either:" $$
      text "  (1) Use -fexternal-interprter, or" $$
      text "  (2) Build the program twice: once" <+>
                           ghciWay <> text ", and then" $$
      text "      with" <+> compWay <+>
         text "using -osuf to set a different object file suffix."
        where compWay
                | WayDyn `elem` ways dflags = text "-dynamic"
                | WayProf `elem` ways dflags = text "-prof"
                | otherwise = text "normal"
              ghciWay
                | dynamicGhc = text "with -dynamic"
                | rtsIsProfiled = text "with -prof"
                | otherwise = text "the normal way"

    getLinkDeps :: HscEnv -> HomePackageTable
                -> PersistentLinkerState
                -> Maybe FilePath                   -- replace object suffices?
                -> SrcSpan                          -- for error messages
                -> [Module]                         -- If you need these
                -> IO ([Linkable], [UnitId])     -- ... then link these first
    -- Fails with an IO exception if it can't find enough files

    getLinkDeps hsc_env hpt pls replace_osuf span mods
    -- Find all the packages and linkables that a set of modules depends on
     = do {
            -- 1.  Find the dependent home-pkg-modules/packages from each iface
            -- (omitting modules from the interactive package, which is already linked)
          ; (mods_s, pkgs_s) <- follow_deps (filterOut isInteractiveModule mods)
                                            emptyUniqSet emptyUniqSet;

          ; let {
            -- 2.  Exclude ones already linked
            --      Main reason: avoid findModule calls in get_linkable
                mods_needed = mods_s `minusList` linked_mods     ;
                pkgs_needed = pkgs_s `minusList` pkgs_loaded pls ;

                linked_mods = map (moduleName.linkableModule)
                                    (objs_loaded pls ++ bcos_loaded pls)  }

            -- 3.  For each dependent module, find its linkable
            --     This will either be in the HPT or (in the case of one-shot
            --     compilation) we may need to use maybe_getFileLinkable
          ; let { osuf = objectSuf dflags }
          ; lnks_needed <- mapM (get_linkable osuf) mods_needed

          ; return (lnks_needed, pkgs_needed) }
      where
        dflags = hsc_dflags hsc_env
        this_pkg = thisPackage dflags

            -- The ModIface contains the transitive closure of the module dependencies
            -- within the current package, *except* for boot modules: if we encounter
            -- a boot module, we have to find its real interface and discover the
            -- dependencies of that.  Hence we need to traverse the dependency
            -- tree recursively.  See bug #936, testcase ghci/prog007.
        follow_deps :: [Module]             -- modules to follow
                    -> UniqSet ModuleName         -- accum. module dependencies
                    -> UniqSet UnitId          -- accum. package dependencies
                    -> IO ([ModuleName], [UnitId]) -- result
        follow_deps []     acc_mods acc_pkgs
            = return (uniqSetToList acc_mods, uniqSetToList acc_pkgs)
        follow_deps (mod:mods) acc_mods acc_pkgs
            = do
              mb_iface <- initIfaceCheck hsc_env $
                            loadInterface msg mod (ImportByUser False)
              iface <- case mb_iface of
                        Maybes.Failed err      -> throwGhcExceptionIO (ProgramError (showSDoc dflags err))
                        Maybes.Succeeded iface -> return iface

              when (mi_boot iface) $ link_boot_mod_error mod

              let
                pkg = moduleUnitId mod
                deps  = mi_deps iface

                pkg_deps = dep_pkgs deps
                (boot_deps, mod_deps) = partitionWith is_boot (dep_mods deps)
                        where is_boot (m,True)  = Left m
                              is_boot (m,False) = Right m

                boot_deps' = filter (not . (`elementOfUniqSet` acc_mods)) boot_deps
                acc_mods'  = addListToUniqSet acc_mods (moduleName mod : mod_deps)
                acc_pkgs'  = addListToUniqSet acc_pkgs $ map fst pkg_deps
              --
              if pkg /= this_pkg
                 then follow_deps mods acc_mods (addOneToUniqSet acc_pkgs' pkg)
                 else follow_deps (map (mkModule this_pkg) boot_deps' ++ mods)
                                  acc_mods' acc_pkgs'
            where
                msg = text "need to link module" <+> ppr mod <+>
                      text "due to use of Template Haskell"

        link_boot_mod_error mod =
            throwGhcExceptionIO (ProgramError (showSDoc dflags (
                text "module" <+> ppr mod <+>
                text "cannot be linked; it is only available as a boot module")))

        no_obj :: Outputable a => a -> IO b
        no_obj mod = dieWith dflags span $
                         text "cannot find object file for module " <>
                            quotes (ppr mod) $$
                         while_linking_expr

        while_linking_expr = text "while linking an interpreted expression"

            -- This one is a build-system bug

        get_linkable osuf mod_name      -- A home-package module
            | Just mod_info <- lookupUFM hpt mod_name
            = adjust_linkable (Maybes.expectJust "getLinkDeps" (hm_linkable mod_info))
            | otherwise
            = do    -- It's not in the HPT because we are in one shot mode,
                    -- so use the Finder to get a ModLocation...
                 mb_stuff <- findHomeModule hsc_env mod_name
                 case mb_stuff of
                      Found loc mod -> found loc mod
                      _ -> no_obj mod_name
            where
                found loc mod = do {
                    -- ...and then find the linkable for it
                   mb_lnk <- findObjectLinkableMaybe mod loc ;
                   case mb_lnk of {
                      Nothing  -> no_obj mod ;
                      Just lnk -> adjust_linkable lnk
                  }}

                adjust_linkable lnk
                    | Just new_osuf <- replace_osuf = do
                            new_uls <- mapM (adjust_ul new_osuf)
                                            (linkableUnlinked lnk)
                            return lnk{ linkableUnlinked=new_uls }
                    | otherwise =
                            return lnk

                adjust_ul new_osuf (DotO file) = do
                    MASSERT(osuf `isSuffixOf` file)
                    let file_base = fromJust (stripExtension osuf file)
                        new_file = file_base <.> new_osuf
                    ok <- doesFileExist new_file
                    if (not ok)
                       then dieWith dflags span $
                              text "cannot find object file "
                                    <> quotes (text new_file) $$ while_linking_expr
                       else return (DotO new_file)
                adjust_ul _ (DotA fp) = panic ("adjust_ul DotA " ++ show fp)
                adjust_ul _ (DotDLL fp) = panic ("adjust_ul DotDLL " ++ show fp)
                adjust_ul _ l@(BCOs {}) = return l

    {- **********************************************************************

                  Loading a Decls statement

      ********************************************************************* -}

    linkDecls :: HscEnv -> SrcSpan -> CompiledByteCode -> IO ()
    linkDecls hsc_env span cbc@CompiledByteCode{..} = do
        -- Initialise the linker (if it's not been done already)
        initDynLinker hsc_env

        -- Take lock for the actual work.
        modifyPLS $ \pls0 -> do

        -- Link the packages and modules required
        (pls, ok) <- linkDependencies hsc_env pls0 span needed_mods
        if failed ok
          then throwGhcExceptionIO (ProgramError "")
          else do

        -- Link the expression itself
        let ie = plusNameEnv (itbl_env pls) bc_itbls
            ce = closure_env pls

        -- Link the necessary packages and linkables
        new_bindings <- linkSomeBCOs hsc_env ie ce [cbc]
        nms_fhvs <- makeForeignNamedHValueRefs hsc_env new_bindings
        let pls2 = pls { closure_env = extendClosureEnv ce nms_fhvs
                       , itbl_env    = ie }
        return (pls2, ())
      where
        free_names =  concatMap (nameSetElems . bcoFreeNames) bc_bcos

        needed_mods :: [Module]
        needed_mods = [ nameModule n | n <- free_names,
                        isExternalName n,       -- Names from other modules
                        not (isWiredInName n)   -- Exclude wired-in names
                      ]                         -- (see note below)
        -- Exclude wired-in names because we may not have read
        -- their interface files, so getLinkDeps will fail
        -- All wired-in names are in the base package, which we link
        -- by default, so we can safely ignore them here.

    {- **********************************************************************

                  Loading a single module

      ********************************************************************* -}

    linkModule :: HscEnv -> Module -> IO ()
    linkModule hsc_env mod = do
      initDynLinker hsc_env
      modifyPLS_ $ \pls -> do
        (pls', ok) <- linkDependencies hsc_env pls noSrcSpan [mod]
        if (failed ok) then throwGhcExceptionIO (ProgramError "could not link module")
          else return pls'

    {- **********************************************************************

                    Link some linkables
            The linkables may consist of a mixture of
            byte-code modules and object modules

      ********************************************************************* -}

    linkModules :: HscEnv -> PersistentLinkerState -> [Linkable]
                -> IO (PersistentLinkerState, SuccessFlag)
    linkModules hsc_env pls linkables
      = mask_ $ do  -- don't want to be interrupted by ^C in here

            let (objs, bcos) = partition isObjectLinkable
                                  (concatMap partitionLinkable linkables)

                    -- Load objects first; they can't depend on BCOs
            (pls1, ok_flag) <- dynLinkObjs hsc_env pls objs

            if failed ok_flag then
                    return (pls1, Failed)
              else do
                    pls2 <- dynLinkBCOs hsc_env pls1 bcos
                    return (pls2, Succeeded)

    -- HACK to support f-x-dynamic in the interpreter; no other purpose
    partitionLinkable :: Linkable -> [Linkable]
    partitionLinkable li
       = let li_uls = linkableUnlinked li
             li_uls_obj = filter isObject li_uls
             li_uls_bco = filter isInterpretable li_uls
         in
             case (li_uls_obj, li_uls_bco) of
                (_:_, _:_) -> [li {linkableUnlinked=li_uls_obj},
                               li {linkableUnlinked=li_uls_bco}]
                _ -> [li]

    findModuleLinkable_maybe :: [Linkable] -> Module -> Maybe Linkable
    findModuleLinkable_maybe lis mod
       = case [LM time nm us | LM time nm us <- lis, nm == mod] of
            []   -> Nothing
            [li] -> Just li
            _    -> pprPanic "findModuleLinkable" (ppr mod)

    linkableInSet :: Linkable -> [Linkable] -> Bool
    linkableInSet l objs_loaded =
      case findModuleLinkable_maybe objs_loaded (linkableModule l) of
            Nothing -> False
            Just m  -> linkableTime l == linkableTime m

    {- **********************************************************************

                    The object-code linker

      ********************************************************************* -}

    dynLinkObjs :: HscEnv -> PersistentLinkerState -> [Linkable]
                -> IO (PersistentLinkerState, SuccessFlag)
    dynLinkObjs hsc_env pls objs = do
            -- Load the object files and link them
            let (objs_loaded', new_objs) = rmDupLinkables (objs_loaded pls) objs
                pls1                     = pls { objs_loaded = objs_loaded' }
                unlinkeds                = concatMap linkableUnlinked new_objs
                wanted_objs              = map nameOfObject unlinkeds

            if interpreterDynamic (hsc_dflags hsc_env)
                then do pls2 <- dynLoadObjs hsc_env pls1 wanted_objs
                        return (pls2, Succeeded)
                else do mapM_ (loadObj hsc_env) wanted_objs

                        -- Link them all together
                        ok <- resolveObjs hsc_env

                        -- If resolving failed, unload all our
                        -- object modules and carry on
                        if succeeded ok then do
                                return (pls1, Succeeded)
                          else do
                                pls2 <- unload_wkr hsc_env [] pls1
                                return (pls2, Failed)

    dynLoadObjs :: HscEnv -> PersistentLinkerState -> [FilePath]
                -> IO PersistentLinkerState
    dynLoadObjs _       pls []   = return pls
    dynLoadObjs hsc_env pls objs = do
        let dflags = hsc_dflags hsc_env
        let platform = targetPlatform dflags
        let minus_ls = [ lib | Option ('-':'l':lib) <- ldInputs dflags ]
        let minus_big_ls = [ lib | Option ('-':'L':lib) <- ldInputs dflags ]
        (soFile, libPath , libName) <- newTempLibName dflags (soExt platform)
        let
            dflags2 = dflags {
                          -- We don't want the original ldInputs in
                          -- (they're already linked in), but we do want
                          -- to link against previous dynLoadObjs
                          -- libraries if there were any, so that the linker
                          -- can resolve dependencies when it loads this
                          -- library.
                          ldInputs =
                            concatMap
                                (\(lp, l) ->
                                     [ Option ("-L" ++ lp)
                                     , Option ("-Wl,-rpath")
                                     , Option ("-Wl," ++ lp)
                                     , Option ("-l" ++  l)
                                     ])
                                (temp_sos pls)
                            ++ concatMap
                                 (\lp ->
                                     [ Option ("-L" ++ lp)
                                     , Option ("-Wl,-rpath")
                                     , Option ("-Wl," ++ lp)
                                     ])
                                 minus_big_ls
                            ++ map (\l -> Option ("-l" ++ l)) minus_ls,
                          -- Add -l options and -L options from dflags.
                          --
                          -- When running TH for a non-dynamic way, we still
                          -- need to make -l flags to link against the dynamic
                          -- libraries, so we need to add WayDyn to ways.
                          --
                          -- Even if we're e.g. profiling, we still want
                          -- the vanilla dynamic libraries, so we set the
                          -- ways / build tag to be just WayDyn.
                          ways = [WayDyn],
                          buildTag = mkBuildTag [WayDyn],
                          outputFile = Just soFile
                      }
        -- link all "loaded packages" so symbols in those can be resolved
        -- Note: We are loading packages with local scope, so to see the
        -- symbols in this link we must link all loaded packages again.
        linkDynLib dflags2 objs (pkgs_loaded pls)
        consIORef (filesToNotIntermediateClean dflags) soFile
        m <- loadDLL hsc_env soFile
        case m of
            Nothing -> return pls { temp_sos = (libPath, libName) : temp_sos pls }
            Just err -> panic ("Loading temp shared object failed: " ++ err)

    rmDupLinkables :: [Linkable]    -- Already loaded
                   -> [Linkable]    -- New linkables
                   -> ([Linkable],  -- New loaded set (including new ones)
                       [Linkable])  -- New linkables (excluding dups)
    rmDupLinkables already ls
      = go already [] ls
      where
        go already extras [] = (already, extras)
        go already extras (l:ls)
            | linkableInSet l already = go already     extras     ls
            | otherwise               = go (l:already) (l:extras) ls

    {- **********************************************************************

                    The byte-code linker

      ********************************************************************* -}

    dynLinkBCOs :: HscEnv -> PersistentLinkerState -> [Linkable]
                -> IO PersistentLinkerState
    dynLinkBCOs hsc_env pls bcos = do

            let (bcos_loaded', new_bcos) = rmDupLinkables (bcos_loaded pls) bcos
                pls1                     = pls { bcos_loaded = bcos_loaded' }
                unlinkeds :: [Unlinked]
                unlinkeds                = concatMap linkableUnlinked new_bcos

                cbcs :: [CompiledByteCode]
                cbcs      = map byteCodeOfObject unlinkeds

                ies        = map bc_itbls cbcs
                gce       = closure_env pls
                final_ie  = foldr plusNameEnv (itbl_env pls) ies

            names_and_refs <- linkSomeBCOs hsc_env final_ie gce cbcs

            -- We only want to add the external ones to the ClosureEnv
            let (to_add, to_drop) = partition (isExternalName.fst) names_and_refs

            -- Immediately release any HValueRefs we're not going to add
            freeHValueRefs hsc_env (map snd to_drop)
            -- Wrap finalizers on the ones we want to keep
            new_binds <- makeForeignNamedHValueRefs hsc_env to_add

            return pls1 { closure_env = extendClosureEnv gce new_binds,
                          itbl_env    = final_ie }

    -- Link a bunch of BCOs and return references to their values
    linkSomeBCOs :: HscEnv
                 -> ItblEnv
                 -> ClosureEnv
                 -> [CompiledByteCode]
                 -> IO [(Name,HValueRef)]
                            -- The returned HValueRefs are associated 1-1 with
                            -- the incoming unlinked BCOs.  Each gives the
                            -- value of the corresponding unlinked BCO

    linkSomeBCOs hsc_env ie ce mods = foldr fun do_link mods []
     where
      fun CompiledByteCode{..} inner accum =
        case bc_breaks of
          Nothing -> inner ((panic "linkSomeBCOs: no break array", bc_bcos) : accum)
          Just mb -> withForeignRef (modBreaks_flags mb) $ \breakarray ->
                       inner ((breakarray, bc_bcos) : accum)

      do_link [] = return []
      do_link mods = do
        let flat = [ (breakarray, bco) | (breakarray, bcos) <- mods, bco <- bcos ]
            names = map (unlinkedBCOName . snd) flat
            bco_ix = mkNameEnv (zip names [0..])
        resolved <- sequence [ linkBCO hsc_env ie ce bco_ix breakarray bco
                             | (breakarray, bco) <- flat ]
        hvrefs <- createBCOs hsc_env resolved
        return (zip names hvrefs)

    -- | Useful to apply to the result of 'linkSomeBCOs'
    makeForeignNamedHValueRefs
      :: HscEnv -> [(Name,HValueRef)] -> IO [(Name,ForeignHValue)]
    makeForeignNamedHValueRefs hsc_env bindings =
      mapM (\(n, hvref) -> (n,) <$> mkFinalizedHValue hsc_env hvref) bindings

    {- **********************************************************************

                    Unload some object modules

      ********************************************************************* -}

    -- ---------------------------------------------------------------------------
    -- | Unloading old objects ready for a new compilation sweep.
    --
    -- The compilation manager provides us with a list of linkables that it
    -- considers \"stable\", i.e. won't be recompiled this time around.  For
    -- each of the modules current linked in memory,
    --
    --   * if the linkable is stable (and it's the same one -- the user may have
    --     recompiled the module on the side), we keep it,
    --
    --   * otherwise, we unload it.
    --
    --   * we also implicitly unload all temporary bindings at this point.
    --
    unload :: HscEnv
           -> [Linkable] -- ^ The linkables to *keep*.
           -> IO ()
    unload hsc_env linkables
      = mask_ $ do -- mask, so we're safe from Ctrl-C in here

            -- Initialise the linker (if it's not been done already)
            initDynLinker hsc_env

            new_pls
                <- modifyPLS $ \pls -> do
                     pls1 <- unload_wkr hsc_env linkables pls
                     return (pls1, pls1)

            let dflags = hsc_dflags hsc_env
            debugTraceMsg dflags 3 $
              text "unload: retaining objs" <+> ppr (objs_loaded new_pls)
            debugTraceMsg dflags 3 $
              text "unload: retaining bcos" <+> ppr (bcos_loaded new_pls)
            return ()

    unload_wkr :: HscEnv
               -> [Linkable]                -- stable linkables
               -> PersistentLinkerState
               -> IO PersistentLinkerState
    -- Does the core unload business
    -- (the wrapper blocks exceptions and deals with the PLS get and put)

    unload_wkr hsc_env keep_linkables pls = do
      let (objs_to_keep, bcos_to_keep) = partition isObjectLinkable keep_linkables

          discard keep l = not (linkableInSet l keep)

          (objs_to_unload, remaining_objs_loaded) =
             partition (discard objs_to_keep) (objs_loaded pls)
          (bcos_to_unload, remaining_bcos_loaded) =
             partition (discard bcos_to_keep) (bcos_loaded pls)

      mapM_ unloadObjs objs_to_unload
      mapM_ unloadObjs bcos_to_unload

      -- If we unloaded any object files at all, we need to purge the cache
      -- of lookupSymbol results.
      when (not (null (objs_to_unload ++
                       filter (not . null . linkableObjs) bcos_to_unload))) $
        purgeLookupSymbolCache hsc_env

      let bcos_retained = map linkableModule remaining_bcos_loaded

          -- Note that we want to remove all *local*
          -- (i.e. non-isExternal) names too (these are the
          -- temporary bindings from the command line).
          keep_name (n,_) = isExternalName n &&
                            nameModule n `elem` bcos_retained

          itbl_env'     = filterNameEnv keep_name (itbl_env pls)
          closure_env'  = filterNameEnv keep_name (closure_env pls)

          new_pls = pls { itbl_env = itbl_env',
                          closure_env = closure_env',
                          bcos_loaded = remaining_bcos_loaded,
                          objs_loaded = remaining_objs_loaded }

      return new_pls
      where
        unloadObjs :: Linkable -> IO ()
        unloadObjs lnk
          | dynamicGhc = return ()
            -- We don't do any cleanup when linking objects with the
            -- dynamic linker.  Doing so introduces extra complexity for
            -- not much benefit.
          | otherwise
          = mapM_ (unloadObj hsc_env) [f | DotO f <- linkableUnlinked lnk]
                    -- The components of a BCO linkable may contain
                    -- dot-o files.  Which is very confusing.
                    --
                    -- But the BCO parts can be unlinked just by
                    -- letting go of them (plus of course depopulating
                    -- the symbol table which is done in the main body)

    {- **********************************************************************

                    Loading packages

      ********************************************************************* -}

    data LibrarySpec
       = Object FilePath    -- Full path name of a .o file, including trailing .o
                            -- For dynamic objects only, try to find the object
                            -- file in all the directories specified in
                            -- v_Library_paths before giving up.

       | Archive FilePath   -- Full path name of a .a file, including trailing .a

       | DLL String         -- "Unadorned" name of a .DLL/.so
                            --  e.g.    On unix     "qt"  denotes "libqt.so"
                            --          On Windows  "burble"  denotes "burble.DLL" or "libburble.dll"
                            --  loadDLL is platform-specific and adds the lib/.so/.DLL
                            --  suffixes platform-dependently

       | DLLPath FilePath   -- Absolute or relative pathname to a dynamic library
                            -- (ends with .dll or .so).

       | Framework String   -- Only used for darwin, but does no harm

    -- If this package is already part of the GHCi binary, we'll already
    -- have the right DLLs for this package loaded, so don't try to
    -- load them again.
    --
    -- But on Win32 we must load them 'again'; doing so is a harmless no-op
    -- as far as the loader is concerned, but it does initialise the list
    -- of DLL handles that rts/Linker.c maintains, and that in turn is
    -- used by lookupSymbol.  So we must call addDLL for each library
    -- just to get the DLL handle into the list.
    partOfGHCi :: [PackageName]
    partOfGHCi
     | isWindowsHost || isDarwinHost = []
     | otherwise = map (PackageName . mkFastString)
                       ["base", "template-haskell", "editline"]

    showLS :: LibrarySpec -> String
    showLS (Object nm)    = "(static) " ++ nm
    showLS (Archive nm)   = "(static archive) " ++ nm
    showLS (DLL nm)       = "(dynamic) " ++ nm
    showLS (DLLPath nm)   = "(dynamic) " ++ nm
    showLS (Framework nm) = "(framework) " ++ nm

    -- | Link exactly the specified packages, and their dependents (unless of
    -- course they are already linked).  The dependents are linked
    -- automatically, and it doesn't matter what order you specify the input
    -- packages.
    --
    linkPackages :: HscEnv -> [UnitId] -> IO ()
    -- NOTE: in fact, since each module tracks all the packages it depends on,
    --       we don't really need to use the package-config dependencies.
    --
    -- However we do need the package-config stuff (to find aux libs etc),
    -- and following them lets us load libraries in the right order, which
    -- perhaps makes the error message a bit more localised if we get a link
    -- failure.  So the dependency walking code is still here.

    linkPackages hsc_env new_pkgs = do
      -- It's probably not safe to try to load packages concurrently, so we take
      -- a lock.
      initDynLinker hsc_env
      modifyPLS_ $ \pls -> do
        linkPackages' hsc_env new_pkgs pls

    linkPackages' :: HscEnv -> [UnitId] -> PersistentLinkerState
                 -> IO PersistentLinkerState
    linkPackages' hsc_env new_pks pls = do
        pkgs' <- link (pkgs_loaded pls) new_pks
        return $! pls { pkgs_loaded = pkgs' }
      where
         dflags = hsc_dflags hsc_env

         link :: [UnitId] -> [UnitId] -> IO [UnitId]
         link pkgs new_pkgs =
             foldM link_one pkgs new_pkgs

         link_one pkgs new_pkg
            | new_pkg `elem` pkgs   -- Already linked
            = return pkgs

            | Just pkg_cfg <- lookupPackage dflags new_pkg
            = do {  -- Link dependents first
                   pkgs' <- link pkgs (depends pkg_cfg)
                    -- Now link the package itself
                 ; linkPackage hsc_env pkg_cfg
                 ; return (new_pkg : pkgs') }

            | otherwise
            = throwGhcExceptionIO (CmdLineError ("unknown package: " ++ unitIdString new_pkg))

    linkPackage :: HscEnv -> PackageConfig -> IO ()
    linkPackage hsc_env pkg
       = do
            let dflags    = hsc_dflags hsc_env
                platform  = targetPlatform dflags
                dirs | interpreterDynamic dflags = Packages.libraryDynDirs pkg
                     | otherwise                 = Packages.libraryDirs pkg

            let hs_libs   =  Packages.hsLibraries pkg
                -- The FFI GHCi import lib isn't needed as
                -- compiler/ghci/Linker.hs + rts/Linker.c link the
                -- interpreted references to FFI to the compiled FFI.
                -- We therefore filter it out so that we don't get
                -- duplicate symbol errors.
                hs_libs'  =  filter ("HSffi" /=) hs_libs

            -- Because of slight differences between the GHC dynamic linker and
            -- the native system linker some packages have to link with a
            -- different list of libraries when using GHCi. Examples include: libs
            -- that are actually gnu ld scripts, and the possibility that the .a
            -- libs do not exactly match the .so/.dll equivalents. So if the
            -- package file provides an "extra-ghci-libraries" field then we use
            -- that instead of the "extra-libraries" field.
                extra_libs =
                          (if null (Packages.extraGHCiLibraries pkg)
                                then Packages.extraLibraries pkg
                                else Packages.extraGHCiLibraries pkg)
                          ++ [ lib | '-':'l':lib <- Packages.ldOptions pkg ]

            hs_classifieds    <- mapM (locateLib hsc_env True  dirs) hs_libs'
            extra_classifieds <- mapM (locateLib hsc_env False dirs) extra_libs
            let classifieds = hs_classifieds ++ extra_classifieds

            -- Complication: all the .so's must be loaded before any of the .o's.
            let known_dlls = [ dll  | DLLPath dll    <- classifieds ]
                dlls       = [ dll  | DLL dll        <- classifieds ]
                objs       = [ obj  | Object obj     <- classifieds ]
                archs      = [ arch | Archive arch   <- classifieds ]

            -- Add directories to library search paths
            let dll_paths  = map takeDirectory known_dlls
                all_paths  = nub $ map normalise $ dll_paths ++ dirs
            pathCache <- mapM (addLibrarySearchPath hsc_env) all_paths

            maybePutStr dflags
                ("Loading package " ++ sourcePackageIdString pkg ++ " ... ")

            -- See comments with partOfGHCi
            when (packageName pkg `notElem` partOfGHCi) $ do
                loadFrameworks hsc_env platform pkg
                mapM_ (load_dyn hsc_env)
                  (known_dlls ++ map (mkSOName platform) dlls)

            -- After loading all the DLLs, we can load the static objects.
            -- Ordering isn't important here, because we do one final link
            -- step to resolve everything.
            mapM_ (loadObj hsc_env) objs
            mapM_ (loadArchive hsc_env) archs

            maybePutStr dflags "linking ... "
            ok <- resolveObjs hsc_env

            -- DLLs are loaded, reset the search paths
            -- Import libraries will be loaded via loadArchive so only
            -- reset the DLL search path after all archives are loaded
            -- as well.
            mapM_ (removeLibrarySearchPath hsc_env) $ reverse pathCache

            if succeeded ok
               then maybePutStrLn dflags "done."
               else let errmsg = "unable to load package `"
                                 ++ sourcePackageIdString pkg ++ "'"
                     in throwGhcExceptionIO (InstallationError errmsg)

    -- we have already searched the filesystem; the strings passed to load_dyn
    -- can be passed directly to loadDLL.  They are either fully-qualified
    -- ("/usr/lib/libfoo.so"), or unqualified ("libfoo.so").  In the latter case,
    -- loadDLL is going to search the system paths to find the library.
    --
    load_dyn :: HscEnv -> FilePath -> IO ()
    load_dyn hsc_env dll = do
      r <- loadDLL hsc_env dll
      case r of
        Nothing  -> return ()
        Just err -> throwGhcExceptionIO (CmdLineError ("can't load .so/.DLL for: "
                                                    ++ dll ++ " (" ++ err ++ ")" ))

    loadFrameworks :: HscEnv -> Platform -> PackageConfig -> IO ()
    loadFrameworks hsc_env platform pkg
        = when (platformUsesFrameworks platform) $ mapM_ load frameworks
      where
        fw_dirs    = Packages.frameworkDirs pkg
        frameworks = Packages.frameworks pkg

        load fw = do  r <- loadFramework hsc_env fw_dirs fw
                      case r of
                        Nothing  -> return ()
                        Just err -> throwGhcExceptionIO (CmdLineError ("can't load framework: "
                                                            ++ fw ++ " (" ++ err ++ ")" ))

    -- Try to find an object file for a given library in the given paths.
    -- If it isn't present, we assume that addDLL in the RTS can find it,
    -- which generally means that it should be a dynamic library in the
    -- standard system search path.
    -- For GHCi we tend to prefer dynamic libraries over static ones as
    -- they are easier to load and manage, have less overhead.
    locateLib :: HscEnv -> Bool -> [FilePath] -> String -> IO LibrarySpec
    locateLib hsc_env is_hs dirs lib
      | not is_hs
        -- For non-Haskell libraries (e.g. gmp, iconv):
        --   first look in library-dirs for a dynamic library (libfoo.so)
        --   then  look in library-dirs for a static library (libfoo.a)
        --   then look in library-dirs and inplace GCC for a dynamic library (libfoo.so)
        --   then  check for system dynamic libraries (e.g. kernel32.dll on windows)
        --   then  try looking for import libraries on Windows (.dll.a, .lib)
        --   then  try "gcc --print-file-name" to search gcc's search path
        --   then  look in library-dirs and inplace GCC for a static library (libfoo.a)
        --       for a dynamic library (#5289)
        --   otherwise, assume loadDLL can find it
        --
      = findDll     `orElse`
        findSysDll  `orElse`
        tryImpLib   `orElse`
        tryGcc      `orElse`
        findArchive `orElse`
        assumeDll

      | loading_dynamic_hs_libs -- search for .so libraries first.
      = findHSDll     `orElse`
        findDynObject `orElse`
        assumeDll

      | loading_profiled_hs_libs -- only a libHSfoo_p.a archive will do.
      = findArchive `orElse`
        assumeDll

      | otherwise
        -- HSfoo.o is the best, but only works for the normal way
        -- libHSfoo.a is the backup option.
      = findObject  `orElse`
        findArchive `orElse`
        assumeDll

       where
         dflags = hsc_dflags hsc_env

         obj_file     = lib <.> "o"
         dyn_obj_file = lib <.> "dyn_o"
         arch_file = "lib" ++ lib ++ lib_tag <.> "a"
         lib_tag = if is_hs && loading_profiled_hs_libs then "_p" else ""

         loading_profiled_hs_libs = interpreterProfiled dflags
         loading_dynamic_hs_libs  = interpreterDynamic dflags

         import_libs  = [lib <.> "lib", "lib" ++ lib <.> "lib", "lib" ++ lib <.> "dll.a"]

         hs_dyn_lib_name = lib ++ '-':programName dflags ++ projectVersion dflags
         hs_dyn_lib_file = mkHsSOName platform hs_dyn_lib_name

         so_name     = mkSOName platform lib
         lib_so_name = "lib" ++ so_name
         dyn_lib_file = case (arch, os) of
                                 (ArchX86_64, OSSolaris2) -> "64" </> so_name
                                 _ -> so_name

         findObject    = liftM (fmap Object)  $ findFile dirs obj_file
         findDynObject = liftM (fmap Object)  $ findFile dirs dyn_obj_file
         findArchive   = let local  = liftM (fmap Archive) $ findFile dirs arch_file
                             linked = liftM (fmap Archive) $ searchForLibUsingGcc dflags arch_file dirs
                         in liftM2 (<|>) local linked
         findHSDll     = liftM (fmap DLLPath) $ findFile dirs hs_dyn_lib_file
         findDll       = liftM (fmap DLLPath) $ findFile dirs dyn_lib_file
         findSysDll    = fmap (fmap $ DLL . takeFileName) $ findSystemLibrary hsc_env so_name
         tryGcc        = let short = liftM (fmap DLLPath) $ searchForLibUsingGcc dflags so_name     dirs
                             full  = liftM (fmap DLLPath) $ searchForLibUsingGcc dflags lib_so_name dirs
                         in liftM2 (<|>) short full
         tryImpLib     = case os of
                           OSMinGW32 -> let check name = liftM (fmap Archive) $ searchForLibUsingGcc dflags name dirs
                                        in apply (map check import_libs)
                           _         -> return Nothing

         assumeDll   = return (DLL lib)
         infixr `orElse`
         f `orElse` g = f >>= maybe g return

         apply []     = return Nothing
         apply (x:xs) = do x' <- x
                           if isJust x'
                              then return x'
                              else apply xs

         platform = targetPlatform dflags
         arch = platformArch platform
         os = platformOS platform

    searchForLibUsingGcc :: DynFlags -> String -> [FilePath] -> IO (Maybe FilePath)
    searchForLibUsingGcc dflags so dirs = do
       -- GCC does not seem to extend the library search path (using -L) when using
       -- --print-file-name. So instead pass it a new base location.
       str <- askCc dflags (map (FileOption "-B") dirs
                              ++ [Option "--print-file-name", Option so])
       let file = case lines str of
                    []  -> ""
                    l:_ -> l
       if (file == so)
          then return Nothing
          else return (Just file)

    -- ----------------------------------------------------------------------------
    -- Loading a dynamic library (dlopen()-ish on Unix, LoadLibrary-ish on Win32)

    -- Darwin / MacOS X only: load a framework
    -- a framework is a dynamic library packaged inside a directory of the same
    -- name. They are searched for in different paths than normal libraries.
    loadFramework :: HscEnv -> [FilePath] -> FilePath -> IO (Maybe String)
    loadFramework hsc_env extraPaths rootname
       = do { either_dir <- tryIO getHomeDirectory
            ; let homeFrameworkPath = case either_dir of
                                      Left _ -> []
                                      Right dir -> [dir </> "Library/Frameworks"]
                  ps = extraPaths ++ homeFrameworkPath ++ defaultFrameworkPaths
            ; mb_fwk <- findFile ps fwk_file
            ; case mb_fwk of
                Just fwk_path -> loadDLL hsc_env fwk_path
                Nothing       -> return (Just "not found") }
                    -- Tried all our known library paths, but dlopen()
                    -- has no built-in paths for frameworks: give up
       where
         fwk_file = rootname <.> "framework" </> rootname
            -- sorry for the hardcoded paths, I hope they won't change anytime soon:
         defaultFrameworkPaths = ["/Library/Frameworks", "/System/Library/Frameworks"]

    {- **********************************************************************

                    Helper functions

      ********************************************************************* -}

    maybePutStr :: DynFlags -> String -> IO ()
    maybePutStr dflags s
        = when (verbosity dflags > 1) $
              do let act = log_action dflags
                 act dflags
                     NoReason
                     SevInteractive
                     noSrcSpan
                     defaultUserStyle
                     (text s)

    maybePutStrLn :: DynFlags -> String -> IO ()
    maybePutStrLn dflags s = maybePutStr dflags (s ++ "\n")

    {- **********************************************************************

            Tunneling global variables into new instance of GHC library

      ********************************************************************* -}

    saveLinkerGlobals :: IO (MVar PersistentLinkerState, Bool)
    saveLinkerGlobals = liftM2 (,) (readIORef v_PersistentLinkerState) (readIORef v_InitLinkerDone)

    restoreLinkerGlobals :: (MVar PersistentLinkerState, Bool) -> IO ()
    restoreLinkerGlobals (pls, ild) = do
        writeIORef v_PersistentLinkerState pls
        writeIORef v_InitLinkerDone ild