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Marge Bot pushed to branch wip/marge_bot_batch_merge_job at Glasgow Haskell Compiler / GHC

Commits:

15faa2de

by Rodrigo Mesquita at 2026-08-14T17:22:05-04:00

loopImports: Don't dup ms_uid in summary imports

We were writing the ms_unitid of the mod summary with every single
import of that module

That complicated the code (as though the UnitId in that list could ever
be something else) and also allocates unnecessarily per every mod
import. Very slight allocation decrease measured locally in a few tests:
(MultiComponentModulesRecomp: -0.06%; MultiComponentModulesRecomp100: -0.05%)

Purely a clean up.

5207a07f

by Rodrigo Mesquita at 2026-08-14T17:22:05-04:00

downsweep: make control flow simpler and cache correct

This refactor extracts the control flow of downsweep into a single
function `dfsBuild`, which takes care of iteratively expanding and
traversing all nodes of the in-construction module graph necessary to
build a full `ModuleGraph`.

There are three levels of caching going on, all of which are necessary
to make sure we don't do repeated work (notably, NEVER summarise the
same module twice).

1. `dfsBuild` accumulates the final module graph and never revisits the
   same node of the module graph. Cache is keyed by the final
   `ModuleGraph`s `NodeKey`s.

2. For Module A in home-unit u1, each import in the list of imports
   needs to be *found* (call to `findImportedModuleWithIsBoot`): at this
   point, we only have the `ModuleName` of the import, not the `Module`.
   This *finding* is somewhat expensive, so we cache it as well
   (`ImportsCache`). The cache key is the home-unit to which the module
   belongs~[1], the import package qualifier, and the ModuleName.

   [1] Different home-units will have different package flags, which means
   potentially different `Module` resolution for the same `ModuleName`.

3. The most expensive operation we want to avoid is summarising a
   `Module` into a `ModSummary`, which notably involves parsing the
   module header from scratch.
   The third cache, in essence, maps a `Module` to its `ModSummary`
   (named `ModSummaryCache`). This cache upholds the invariant: we NEVER
   summarise the same module twice. In practice, the cache key is the
   Module's UnitId and the Source path; the reason is we need to
   distinguish between `.hs` and `.hs-boot` files, as their summaries
   will differ.

   Note that (2) can't guarantee this alone: Two ModuleName imports in
   separate units can (and likely do) map to the same `Module`.

Note that the previous implementation failed to achieve the
no-duplicate-work summarisation invariant, and we ended up doing a
quadratic amount of processing in scenarios like test
`MultiComponentModules100`.

See also Note [Downsweep Control Flow and Caching]

Fixes #27461

Perf changes:
         MultiComponentModules(normal) ghc/alloc  2,097,389,264  1,992,186,736  -5.0% GOOD
      MultiComponentModules100(normal) ghc/alloc 24,310,173,770 21,293,867,360 -12.4% GOOD
   MultiComponentModulesRecomp(normal) ghc/alloc    602,761,394    498,543,984 -17.3% GOOD
MultiComponentModulesRecomp100(normal) ghc/alloc 11,885,968,240  8,895,404,864 -25.2% GOOD

-------------------------
Metric Decrease:
    MultiComponentModules
    MultiComponentModules100
    MultiComponentModulesRecomp
    MultiComponentModulesRecomp100
-------------------------

d82a6733

by Rodrigo Mesquita at 2026-08-14T17:22:05-04:00

implicitRequirementsShallow can never reach HoleUnit

findImportedModule will never return `HoleUnit` for a `ModuleName`
(a `HoleUnit` can only be found as a signature instantiation, never as a
directly *imported* thing)

Therefore, we can drop `[ModuleName]` returned by
`implicitRequirementsShallow`, which makes many things dead code.
Namely, the call to `implicitRequirementsShallow` from
GHC.Driver.Downsweep which was a performance bottleneck (for doing lots
of duplicate work in findImportedModule) is now entirely gone.

Fixes #27053

In an MR with this patch and the downsweep refactor (previous commit), CI says:

         MultiComponentModules(normal) ghc/alloc  2,097,396,728  1,943,662,304  -7.3% GOOD
      MultiComponentModules100(normal) ghc/alloc 24,310,182,136 17,227,574,440 -29.1% GOOD
   MultiComponentModulesRecomp(normal) ghc/alloc    602,769,518    449,973,656 -25.3% GOOD
MultiComponentModulesRecomp100(normal) ghc/alloc 11,885,976,408  4,828,894,160 -59.4% GOOD

-------------------------
Metric Decrease:
    MultiComponentModules
    MultiComponentModules100
    MultiComponentModulesRecomp
    MultiComponentModulesRecomp100
-------------------------

ccd7d978

by Rodrigo Mesquita at 2026-08-14T17:22:05-04:00

downsweep: Cache negative results

When traversing a module graph structure, a uniquely identified node
should always expand to the same thing.

I don't see how visiting the same node which failed to be expanded a
first time would ever successfully expand the second time we try to
expand it (eg. when coming from a different edge to it -- it is still
the same node!). The node expansion is local, based just based on the
node itself, not on the path to get there.

Therefore, this patch removes the weird behavior and commentary of
`dfsBuild` wrt to `Nothing` not being cached and being potentially
expanded a second time around to something different, which was
misleading and, ultimately, incorrect.

Now, we have a `MGRes`, which is more explicit about a node being
Skipped just being a node that is ignored whenever it is found (and that
skip is cached) -- and we may want to do this due to failures or due to
just trying nodes which might not work on purpose, like hs-boots.

We uniformly cache positive and negative results and remove the
assumption that there might be an ordering in which the same node
visited at a later time might be expanded differently.

This makes it possible to traverse the module nodes in parallel without
a change in behavior, since there's no longer a hidden ordering
requirement.

bbd87119

by Rodrigo Mesquita at 2026-08-14T17:22:05-04:00

Organize and clean-up GHC.Driver.Downsweep

Simply some cosmetic changes, moving definitions around to structure the
module better into its relevant sections

(In go (ns ++ ss), it's not a problem to use ++ because it's a good
producer and we won't have to append fully before processing the next
item in go)

05aee047

by mangoiv at 2026-08-14T17:22:07-04:00

hadrian: set the executable bit for hie-bios.bat

21 changed files:

+ changelog.d/downsweep-refactor
compiler/GHC/Driver/Backpack.hs
compiler/GHC/Driver/Downsweep.hs
compiler/GHC/Driver/Env.hs
compiler/GHC/Tc/Utils/Backpack.hs
compiler/GHC/Unit/Env.hs
hadrian/hie-bios.bat
+ testsuite/tests/driver/T27461/Main1.hs
+ testsuite/tests/driver/T27461/Main2.hs
+ testsuite/tests/driver/T27461/Makefile
+ testsuite/tests/driver/T27461/T27461a.stderr
+ testsuite/tests/driver/T27461/T27461b.script
+ testsuite/tests/driver/T27461/T27461b.stderr
+ testsuite/tests/driver/T27461/T27461b.stdout
+ testsuite/tests/driver/T27461/all.T
+ testsuite/tests/driver/T27461/src/Bar.hs
testsuite/tests/ghc-api/fixed-nodes/FixedNodes.hs
testsuite/tests/ghc-api/fixed-nodes/InterfaceModuleGraph.hs
testsuite/tests/ghc-api/fixed-nodes/ModuleGraphInvariants.hs
testsuite/tests/splice-imports/SI35.hs
utils/check-ppr/Main.hs

Changes:

changelog.d/downsweep-refactor

 +section: compiler
 +synopsis: Significantly improve the performance of downsweep
 +issues: #27461
 +mrs: !16330
 +description: {
 +    Rewrite the downsweep pass to make the control flow clearer and fix the
 +    caching strategy. Allocations during downsweep in multi-home-unit-heavy and
 +    module-heavy tests are reduced by -30% to -60%
 +}

compiler/GHC/Driver/Backpack.hs

@@ -896,7 +896,7 @@ hsModuleToModSummary home_keys pn hsc_src modname
      extra_sig_imports <- liftIO $ findExtraSigImports hsc_env hsc_src modname
 -    (implicit_sigs, inst_deps) <- liftIO $ implicitRequirementsShallow hsc_env textual_imports
 +    inst_deps <- liftIO $ implicitRequirementsShallow hsc_env textual_imports
      -- So that Finder can find it, even though it doesn't exist...
      this_mod <- liftIO $ do
@@ -916,8 +916,7 @@ hsModuleToModSummary home_keys pn hsc_src modname
                             -- We have to do something special here:
                             -- due to merging, requirements may end up with
                             -- extra imports
 -                           ++ (generatedImport FromBackpackSig . noLoc <$> extra_sig_imports)
 -                           ++ (generatedImport FromBackpackSig . noLoc <$> implicit_sigs),
 +                           ++ (generatedImport FromBackpackSig . noLoc <$> extra_sig_imports),
              -- This is our hack to get the parse tree to the right spot
              ms_parsed_mod = Just (HsParsedModule {
                      hpm_module = hsmod,

compiler/GHC/Driver/Downsweep.hs

@@ -5,6 +5,8 @@
  {-# LANGUAGE RecordWildCards #-}
  {-# LANGUAGE BlockArguments #-}
  {-# LANGUAGE ViewPatterns #-}
++
 +-- | See Note [The ModuleGraph]
  module GHC.Driver.Downsweep
    ( downsweep
    , downsweepThunk
@@ -90,7 +92,7 @@ import GHC.Unit.Module.Deps
  import qualified GHC.Unit.Home.Graph as HUG
  import GHC.Unit.Module.Stage
 -import Data.Either ( rights, partitionEithers, lefts )
 +import Data.Either ( partitionEithers, lefts )
  import qualified Data.Map as Map
  import qualified Data.Set as Set
@@ -110,19 +112,39 @@ import Control.Monad.Trans.Reader
  import qualified Data.Map.Strict as M
  import Control.Monad.Trans.Class
  import System.IO.Unsafe (unsafeInterleaveIO)
 +import Data.IORef
 +import qualified Data.List.NonEmpty as NE
  {-
 -Note [Downsweep and the ModuleGraph]
 -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 +Note [The ModuleGraph]
 +~~~~~~~~~~~~~~~~~~~~~~
 +The 'ModuleGraph' stores the relationship between all the modules, units, and
 +instantiations in the current session, allowing e.g. to answer questions about
 +the transitive closure of the imports.
++
 +* A /node/ of the `ModuleGraph`, of type `ModuleGraphNode`, corresponds
 +  1-1 with a home-package module of source code, N.hs or N.hs-boot.
 +  See the haddocks of `ModuleGraphNode`.
++
 +  The `ModuleNodeInfo` field of the `ModuleGraphNode` contains a `ModSummary`
 +  that in turn describes where the source file is (its `ModLocation`), when it
 +  was read, its contents etc. See Note [Module Types in the ModuleGraph].
 -The ModuleGraph stores the relationship between all the modules, units, and
 -instantiations in the current session.
 +  Each node has a distinct `NodeKey` (an instance of Ord); the function
 +        mkNodeKey :: ModuleGraphNode -> NodeKey
 +  get the `NodeKey` of a node
 -When we do downsweep, we build up a new ModuleGraph, starting from the root
 -modules. By following all the dependencies we construct a graph which allows
 -us to answer questions about the transitive closure of the imports.
 +* An /edge/ of the `ModuleGraph` from N1 to N2 typically corresponds to a
 +  direct import of module N2 in module N1: one edge for each import.
 +  Imports of modules from non-home-packages are featured in the `ModuleGraph`
 +  as `UnitNode`s, or `InstantiationNodes` when backpack is involved.
 -The module graph is accessible in the HscEnv.
 +  Each node contains a list of all its out-edges or, more precisely, of the
 +  `NodeKey`s of its direct dependencies.
++
 +Because a node in the `ModuleGraph` describes the precise dependencies of the module, each node has its
 +own `UnitId`.  Remember, a single module can be compiled against many different versions of a library; but
 +once we fix its dependencies we can compile it, and give it a `UnitId`.  See Note [About units] in GHC.Unit.
  When is this graph constructed?
@@ -139,17 +161,54 @@ When is this graph constructed?
  The result is having a uniform graph available for the whole compilation pipeline.
 --}
 +See Note [Downsweep Control Flow and Caching] for implementation details of
 +the algorithm and caching.
++
 +Note [Downsweep: building and maintaining the module graph]
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 +The module graph can be built from scratch by starting from a set of /root nodes/
 +and exploring their dependencies. This is done by `GHC.Driver.Downsweep.downsweep`.
++
 +Another scenario is when we already /have/ a `ModuleGraph` and want to update
 +it (e.g. to reflect any file-system changes that have taken place since the
 +last invocation of `downsweep`) or augment it by exploring new roots (e.g. for
 +incrementally constructing a ModuleGraph using the GHC API; See #27054). So
 +`downsweep` takes a `Maybe ModuleGraph` as one of its arguments.
 --- This caches the answer to the question, if we are in this unit, what does
 --- an import of this module mean.
 -type DownsweepCache = M.Map (UnitId, PkgQual, ModuleNameWithIsBoot) [Either DriverMessages ModuleNodeInfo]
 +Downsweep iteratively *expands* each so-called 'DownsweepNode' into a list of
 +its dependencies, and recursively traverses all reachable nodes in a
 +depth-first order using 'dfsBuild'. A 'DownsweepNode' is *expanded* by 'dsNodeExpand':
 -moduleGraphNodeMap :: ModuleGraph -> M.Map NodeKey ModuleGraphNode
 -moduleGraphNodeMap graph
 -    = M.fromList [(mkNodeKey node, node) | node <- mgModSummaries' graph]
 +  dsNodeExpand :: DownsweepNode -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
++
 +Most notably:
++
 +  - 'DSMod' (Module-based) nodes can be expanded by preprocessing and
 +  parsing the module header, then listing the imports (direct and SOURCE imports)
 +  (see 'expandModuleSummary' and 'expandFixedModuleNode')
++
 +  - 'DSUnit' is expanded by finding the unit dependencies of that unit by id
 +  (see 'expandUnitNode').
++
 +Besides its dependencies, expanding a 'DownsweepNode' produces a
 +'ModuleGraphNode'. The final 'ModuleGraph' is constructed from the list of
 +'ModuleGraphNode's accumulated by expanding all reachable 'DownsweepNode's.
++
 +A 'ModuleGraphNode' is essentially the resolved version of 'DownsweepNode':
 +it records the payload (e.g. a Module) *and* its dependencies, unlike
 +'DownsweepNode' which has the just the payload that is used as a seed (and
 +potentially some context information, like the current home-unit)
++
 +TL;DR: We recursively traverse 'DownsweepNodes' to discover and build the 'ModuleGraph'.
++
 +See also Note [Downsweep Control Flow and Caching] for implementation details.
 +See Note [The ModuleGraph] for an overview when we do downsweep.
 +-}
  -----------------------------------------------------------------------------
 +-- * Top-level entry to downsweep
 +-----------------------------------------------------------------------------
++
  --
  -- | Downsweep (dependency analysis) for --make mode
  --
@@ -161,7 +220,7 @@ moduleGraphNodeMap graph
  -- cache to avoid recalculating a module summary if the source is
  -- unchanged.
  --
 --- Downsweeping can start from scratch for from a given module graph. In the
 +-- Downsweeping can start from scratch or from a given module graph. In the
  -- latter case, the given graph is fully included in the resulting graph, even
  -- if parts of it are not reachable from any of the given roots. When an import
  -- is processed, the source of the imported module is not consulted if this
@@ -177,6 +236,8 @@ moduleGraphNodeMap graph
  --
  -- It will also turn on code generation for any modules that need it by calling
  -- 'enableCodeGenForTH'.
 +--
 +-- See also Note [The ModuleGraph]
  downsweep :: HscEnv
            -> (GhcMessage -> AnyGhcDiagnostic)
            -> Maybe Messager
@@ -194,8 +255,11 @@ downsweep :: HscEnv
                  -- (Modules, IsBoot) identifiers, unless the Bool is true in
                  -- which case there can be repeats
  downsweep hsc_env diag_wrapper msg old_summaries maybe_base_graph excl_mods allow_dup_roots = do
 -  n_jobs <- mkWorkerLimit (hsc_dflags hsc_env)
 -  (root_errs, root_summaries) <- rootSummariesParallel n_jobs hsc_env diag_wrapper msg summary
 +  n_jobs     <- mkWorkerLimit (hsc_dflags hsc_env)
 +  summ_cache <- newIORef (mkModSummaryCache (zip old_summaries (repeat SummOld)))
 +  imps_cache <- newIORef Map.empty
 +  (root_errs, root_summaries) <- rootSummariesParallel n_jobs hsc_env diag_wrapper msg
 +                                   (getRootSummary excl_mods summ_cache imps_cache)
    let closure_errs = checkHomeUnitsClosed unit_env
        unit_env = hsc_unit_env hsc_env
@@ -203,9 +267,13 @@ downsweep hsc_env diag_wrapper msg old_summaries maybe_base_graph excl_mods allo
    case all_errs of
      [] -> do
 -       (downsweep_errs, downsweep_nodes) <- downsweepFromRootNodes hsc_env old_summary_map maybe_base_graph excl_mods allow_dup_roots DownsweepUseCompile (map ModuleNodeCompile root_summaries) []
 +       (downsweep_errs, downsweep_nodes) <-
 +          downsweepFromRootNodes hsc_env summ_cache imps_cache maybe_base_graph
 +            excl_mods allow_dup_roots DownsweepUseCompile (map ModuleNodeCompile root_summaries) []
 -       let (other_errs, unit_nodes) = partitionEithers $ HUG.unitEnv_foldWithKey (\nodes uid hue -> nodes ++ unitModuleNodes downsweep_nodes uid hue) [] (hsc_HUG hsc_env)
 +       let (other_errs, unit_nodes) = partitionEithers $
 +              HUG.unitEnv_foldWithKey (\nodes uid hue -> nodes ++ unitModuleNodes downsweep_nodes uid hue) []
 +                                      (hsc_HUG hsc_env)
         let all_nodes = downsweep_nodes ++ unit_nodes
         let all_errs = downsweep_errs ++ other_errs
@@ -221,22 +289,40 @@ downsweep hsc_env diag_wrapper msg old_summaries maybe_base_graph excl_mods allo
         return (all_errs, th_configured_nodes)
      _  -> return (all_errs, emptyMG)
    where
 -    summary = getRootSummary excl_mods old_summary_map
+-
 -    -- A cache from file paths to the already summarised modules. The same file
 -    -- can be used in multiple units so the map is also keyed by which unit the
 -    -- file was used in.
 -    -- Reuse these if we can because the most expensive part of downsweep is
 -    -- reading the headers.
 -    old_summary_map :: M.Map (UnitId, OsPath) ModSummary
 -    old_summary_map =
 -      M.fromList [((ms_unitid ms, msHsFileOsPath ms), ms) | ms <- old_summaries]
+-
      -- Dependencies arising on a unit (backpack and module linking deps)
      unitModuleNodes :: [ModuleGraphNode] -> UnitId -> HomeUnitEnv -> [Either (Messages DriverMessage) ModuleGraphNode]
      unitModuleNodes summaries uid hue =
        maybeToList (linkNodes summaries uid hue)
 +    -- The linking plan for each module. If we need to do linking for a home unit
 +    -- then this function returns a graph node which depends on all the modules in the home unit.
++
 +    -- At the moment nothing can depend on these LinkNodes.
 +    linkNodes :: [ModuleGraphNode] -> UnitId -> HomeUnitEnv -> Maybe (Either (Messages DriverMessage) ModuleGraphNode)
 +    linkNodes summaries uid hue =
 +      let dflags = homeUnitEnv_dflags hue
 +          ofile = outputFile_ dflags
++
 +          unit_nodes :: [NodeKey]
 +          unit_nodes = map mkNodeKey (filter ((== uid) . mgNodeUnitId) summaries)
 +      -- 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.
 +      --
 +          no_hs_main = gopt Opt_NoHsMain dflags
++
 +          main_sum = any (== NodeKey_Module (ModNodeKeyWithUid (GWIB (mainModuleNameIs dflags) NotBoot) uid)) unit_nodes
++
 +          do_linking =  main_sum || no_hs_main || ghcLink dflags == LinkDynLib || ghcLink dflags == LinkStaticLib || ghcLink dflags == LinkBytecodeLib
++
 +      in if | isExecutableLink (ghcLink dflags) && isJust ofile && not do_linking ->
 +                Just (Left $ singleMessage $ mkPlainErrorMsgEnvelope noSrcSpan (DriverRedirectedNoMain $ mainModuleNameIs dflags))
 +            -- This should be an error, not a warning (#10895).
 +            | ghcLink dflags /= NoLink, do_linking -> Just (Right (LinkNode unit_nodes uid))
 +            | otherwise  -> Nothing
++
  -- | Calculate the module graph starting from a single ModSummary. The result is a
  -- thunk, which when forced will perform the downsweep. This is useful in oneshot
  -- mode where the module graph may never be needed.
@@ -244,7 +330,9 @@ downsweep hsc_env diag_wrapper msg old_summaries maybe_base_graph excl_mods allo
  downsweepThunk :: HscEnv -> ModSummary -> IO ModuleGraph
  downsweepThunk hsc_env mod_summary = unsafeInterleaveIO $ do
    debugTraceMsg (hsc_logger hsc_env) 3 $ text "Computing Module Graph thunk..."
 -  ~(errs, mg) <- downsweepFromRootNodes hsc_env mempty Nothing [] True DownsweepUseFixed [ModuleNodeCompile mod_summary] []
 +  summs <- newIORef (mkModSummaryCache [(mod_summary,SummOld)])
 +  imps  <- newIORef mempty
 +  ~(errs, mg) <- downsweepFromRootNodes hsc_env summs imps Nothing [] True DownsweepUseFixed [ModuleNodeCompile mod_summary] []
    let dflags = hsc_dflags hsc_env
    liftIO $ printOrThrowDiagnostics (hsc_logger hsc_env)
                                     (initPrintConfig dflags)
@@ -268,80 +356,19 @@ downsweepInteractiveImports hsc_env ic = unsafeInterleaveIO $ do
    debugTraceMsg (hsc_logger hsc_env) 3 $ (text "Computing Interactive Module Graph thunk...")
    let imps = ic_imports (hsc_IC hsc_env)
 -  let interactive_mn = icInteractiveModule ic
 -  -- No sensible value for ModLocation.. if you hit this panic then you probably
 -  -- need to add proper support for modules without any source files to the driver.
 -  let ml = pprPanic "modLocation" (ppr interactive_mn <+> ppr imps)
 -  let key = moduleToMnk interactive_mn NotBoot
 -  let node_type = ModuleNodeFixed key ml
 +      interactive_mn = icInteractiveModule ic
    -- The existing nodes in the module graph. This will be populated when GHCi runs
    -- :load. Any home package modules need to already be in here.
 -  let cached_nodes = Map.fromList [ (mkNodeKey n, n) | n <- mg_mss (hsc_mod_graph hsc_env) ]
+-
 -  (module_edges, graph) <- loopFromInteractive hsc_env (map mkEdge imps) cached_nodes
 -  let interactive_node = ModuleNode module_edges node_type
+-
 -  let all_nodes  = M.elems graph
 -  return $ mkModuleGraph (interactive_node : all_nodes)
+-
 -  where
 - --
 -    mkEdge :: InteractiveImport -> Either ModuleNodeEdge (UnitId, UnresolvedImport PkgQual)
 -    -- A simple edge to a module from the same home unit
 -    mkEdge (IIModule n) =
 -      let
 -        mod_node_key = ModNodeKeyWithUid
 -          { mnkModuleName = GWIB (moduleName n) NotBoot
 -          , mnkUnitId =
 -              -- 'toUnitId' is safe here, as we can't import modules that
 -              -- don't have a 'UnitId'.
 -              toUnitId (moduleUnit n)
 -          }
 -        mod_node_edge =
 -          ModuleNodeEdge NormalLevel (NodeKey_Module mod_node_key)
 -      in Left mod_node_edge
 -    -- A complete import statement
 -    mkEdge (IIDecl i) =
 -      let unitId = homeUnitId $ hsc_home_unit hsc_env
 -          imp = rnUnresolvedImportPkgQual (renameRawPkgQual (hsc_unit_env hsc_env))
 -                                          (mkUnresolvedImport i)
 -      in Right (unitId, imp)
+-
 -loopFromInteractive :: HscEnv
 -                    -> [Either ModuleNodeEdge (UnitId, UnresolvedImport PkgQual)]
 -                    -> M.Map NodeKey ModuleGraphNode
 -                    -> IO ([ModuleNodeEdge],M.Map NodeKey ModuleGraphNode)
 -loopFromInteractive _ [] cached_nodes = return ([], cached_nodes)
 -loopFromInteractive hsc_env (edge:edges) cached_nodes =
 -  case edge of
 -    Left edge -> do
 -        (edges, cached_nodes') <- loopFromInteractive hsc_env edges cached_nodes
 -        return (edge : edges, cached_nodes')
 -    Right (unitId, imp@(UnresolvedImport { ui_level = lvl, ui_boot = is_boot })) -> do
 -      let home_unit = ue_unitHomeUnit unitId (hsc_unit_env hsc_env)
 -      let k _ loc mod =
 -            let key = moduleToMnk mod is_boot
 -            in return $ FoundHome (ModuleNodeFixed key loc)
 -      found <- liftIO $ summariseModuleDispatch k hsc_env home_unit imp []
 -      case found of
 -        -- Case 1: Home modules have to already be in the cache.
 -        FoundHome (ModuleNodeFixed mod _) -> do
 -          let edge = ModuleNodeEdge lvl (NodeKey_Module mod)
 -          -- Note: Does not perform any further downsweep as the module must already be in the cache.
 -          (edges, cached_nodes') <- loopFromInteractive hsc_env edges cached_nodes
 -          return (edge : edges, cached_nodes')
 -        -- Case 2: External units may not be in the cache, if we haven't already initialised the
 -        -- module graph. We can construct the module graph for those here by calling loopUnit.
 -        External uid -> do
 -          let hsc_env' = hscSetActiveHomeUnit home_unit hsc_env
 -              cached_nodes' = loopUnit hsc_env' cached_nodes [uid]
 -              edge = ModuleNodeEdge lvl (NodeKey_ExternalUnit uid)
 -          (edges, cached_nodes') <- loopFromInteractive hsc_env edges cached_nodes'
 -          return (edge : edges, cached_nodes')
 -        -- And if it's not found.. just carry on and hope.
 -        _ -> loopFromInteractive hsc_env edges cached_nodes
 +  let cached_nodes = Map.fromList [ (mkNodeKey n, NSuccess n) | n <- mg_mss (hsc_mod_graph hsc_env) ]
 +  summ_cache <- newIORef mempty
 +  imps_cache <- newIORef mempty
 +  let env = DownsweepEnv hsc_env DownsweepUseFixed{-or DownsweepUseCompile?-} summ_cache imps_cache []
 +  graph <- runDownsweepM env do
 +    loopFromInteractive cached_nodes interactive_mn imps
 +  let all_nodes  = [s | NSuccess s <- M.elems graph ]
 +  return $ mkModuleGraph all_nodes
  -- | Create a module graph from a list of installed modules.
  -- This is used by the loader when we need to load modules but there
@@ -370,7 +397,9 @@ downsweepInstalledModules hsc_env mods = do
              _ -> throwGhcException $ ProgramError $ showSDoc (hsc_dflags hsc_env) $ text "downsweepInstalledModules: Could not find installed module" <+> ppr i
      nodes <- mapM process installed_mods
 -    (errs, mg) <- downsweepFromRootNodes hsc_env mempty Nothing [] True DownsweepUseFixed nodes external_uids
 +    summs <- newIORef mempty
 +    imps  <- newIORef mempty
 +    (errs, mg) <- downsweepFromRootNodes hsc_env summs imps Nothing [] True DownsweepUseFixed nodes external_uids
      -- Similarly here, we should really not get any errors, but print them out if we do.
      let dflags = hsc_dflags hsc_env
@@ -381,7 +410,35 @@ downsweepInstalledModules hsc_env mods = do
      return (mkModuleGraph mg)
 +-----------------------------------------------------------------------------
 +-- * Orchestrator: downsweepFromRootNodes
 +-----------------------------------------------------------------------------
++
 +type ModSummaryCache = IORef ModSummaryCacheMap
 +type ImportsCache    = IORef ImportsCacheMap
 +-- | A cache from file paths to the already summarised modules. The same file
 +-- can be used in multiple units so the map is actually also keyed by which
 +-- unit the file was used in.
 +--
 +-- We want to reuse ModSummaries as far as possible because the most expensive
 +-- part of downsweep is reading and parsing the headers.
 +--
 +-- See Note [Downsweep Control Flow and Caching]
 +type ModSummaryCacheMap
 +      -- The cache can't be keyed by 'Module' because that isn't sufficient to
 +      -- distinguish .hs from .hs-boot files. Use path+unit instead.
 +      = ( M.Map (UnitId, OsPath) (Either DriverMessages (ModSummary, SummProvenance)) )
++
 +-- | A 'ModSummary's provenance during downsweep: an old previously constructed
 +-- ModSummary, that might be potentially outdated, or a freshly constructed one
 +-- during this downsweep which is certainly up to date?
 +data SummProvenance
 +  -- | Constructed during this downsweep: trivially up to date
 +  = SummFresh
 +  -- | Carried over from a previous run: may be stale, must be hash-checked
 +  -- (and considered by -fforce-recomp)
 +  | SummOld
  -- | Whether downsweep should use compiler or fixed nodes. Compile nodes are used
  -- by --make mode, and fixed nodes by oneshot mode.
@@ -394,7 +451,8 @@ data DownsweepMode = DownsweepUseCompile | DownsweepUseFixed
  -- This function will start at the given roots, and traverse downwards to find
  -- all the dependencies, all the way to the leaf units.
  downsweepFromRootNodes :: HscEnv
 -                  -> M.Map (UnitId, OsPath) ModSummary
 +                  -> ModSummaryCache
 +                  -> ImportsCache
                    -> Maybe ModuleGraph
                    -> [ModuleName]
                    -> Bool
@@ -402,278 +460,368 @@ downsweepFromRootNodes :: HscEnv
                    -> [ModuleNodeInfo] -- ^ The starting ModuleNodeInfo
                    -> [UnitId] -- ^ The starting units
                    -> IO ([DriverMessages], [ModuleGraphNode])
 -downsweepFromRootNodes hsc_env old_summaries maybe_base_graph excl_mods allow_dup_roots mode root_nodes root_uids
 -   = do
 -       let root_map = mkRootMap root_nodes
 -       checkDuplicates root_map
 -       let env = DownsweepEnv hsc_env mode old_summaries excl_mods
 -       (deps', map0) <- runDownsweepM env  $ do
 -                    let base_nodes = maybe M.empty moduleGraphNodeMap maybe_base_graph
 -                    (module_deps, map0) <- loopModuleNodeInfos root_nodes (base_nodes, root_map)
 -                    let all_deps = loopUnit hsc_env module_deps root_uids
 -                    let all_instantiations =  getHomeUnitInstantiations hsc_env
 -                    deps' <- loopInstantiations all_instantiations all_deps
 -                    return (deps', map0)
+-
+-
 -       let downsweep_errs = lefts $ concat $ M.elems map0
 -           downsweep_nodes = M.elems deps'
+-
 -       return (downsweep_errs, downsweep_nodes)
 -     where
 -        getHomeUnitInstantiations :: HscEnv -> [(UnitId, InstantiatedUnit)]
 -        getHomeUnitInstantiations hsc_env = HUG.unitEnv_foldWithKey (\nodes uid hue -> nodes ++  instantiationNodes uid (homeUnitEnv_units hue)) [] (hsc_HUG hsc_env)
+-
 -        -- 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
 -          :: DownsweepCache
 -          -> IO ()
 -        checkDuplicates root_map
 -           | not allow_dup_roots
 -           , dup_root:_ <- dup_roots = liftIO $ multiRootsErr sec dup_root
 -           | otherwise = pure ()
 -           where
 -             sec = initSourceErrorContext (hsc_dflags hsc_env)
 -             dup_roots :: [[ModuleNodeInfo]]        -- Each at least of length 2
 -             dup_roots = filterOut isSingleton $ map rights (M.elems root_map)
+-
+-
 -calcDeps :: ModSummary -> [(UnitId, UnresolvedImport PkgQual)]
 -calcDeps ms =
 -  -- Add a dependency on the HsBoot file if it exists
 -  -- This gets passed to the loopImports function which just ignores it if it
 -  -- can't be found.
 -  [ (ms_unitid ms, self_boot) | NotBoot <- [isBootSummary ms] ] ++
 -  [ (ms_unitid ms, e) | e <- ms_imps ms ]
 +downsweepFromRootNodes hsc_env summ_cache imps_cache maybe_base_graph excl_mods allow_dup_roots mode root_nodes root_uids = do
 +     when (not allow_dup_roots) $
 +       case root_duplicates of
 +         []           -> return ()
 +         (dup_root:_) -> multiRootsErr sec dup_root
 +     modifyImpsCache imps_cache (`M.union` mkRootMap root_nodes) -- add root nodes to imports cache
 +     let env = DownsweepEnv hsc_env mode summ_cache imps_cache excl_mods
 +     deps' <- runDownsweepM env  $ do
 +        let base_nodes = maybe M.empty moduleGraphNodeMap maybe_base_graph
 +        module_deps <- loopModuleNodeInfos base_nodes root_nodes
 +        all_deps    <- loopUnits module_deps (hscActiveUnitId hsc_env) root_uids
 +        deps'       <- loopInstantiations all_deps (getHomeUnitInstantiations hsc_env)
 +        return deps'
 +     f_cache <- readIORef summ_cache
 +     let downsweep_errs = lefts (M.elems f_cache)
 +         downsweep_nodes = [ s | NSuccess s <- M.elems deps' ]
++
 +     return (downsweep_errs, downsweep_nodes)
    where
 -    self_boot = (generatedImport FromSelfBoot (noLoc (ms_mod_name ms)))
 -                  { ui_boot = IsBoot }
+-
 +    getHomeUnitInstantiations :: HscEnv -> [(UnitId, InstantiatedUnit)]
 +    getHomeUnitInstantiations hsc_env = HUG.unitEnv_foldWithKey
 +      (\nodes uid hue -> nodes ++  instantiationNodes uid (homeUnitEnv_units hue)) [] (hsc_HUG hsc_env)
++
 +    -- 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).
 +    root_duplicates :: [NE.NonEmpty ModuleNodeInfo]
 +    root_duplicates = mapMaybe takes2 (M.elems root_map)
 +       where
 +         takes2 (a:as@(_:_)) = Just (a NE.:| as) -- Each at least of length 2
 +         takes2 _            = Nothing
++
 +         root_map = Map.fromListWith (flip (++))
 +           [ ((moduleNodeInfoUnitId s, moduleNodeInfoMnwib s), [s])
 +           | s <- root_nodes ]
++
 +    moduleGraphNodeMap :: ModuleGraph -> M.Map NodeKey (NodeRes ModuleGraphNode)
 +    moduleGraphNodeMap graph
 +        = M.fromList [(mkNodeKey node, NSuccess node) | node <- mgModSummaries' graph]
++
 +    sec = initSourceErrorContext (hsc_dflags hsc_env)
++
 +--------------------------------------------------------------------------------
 +-- ** 'DownsweepM'
 +--------------------------------------------------------------------------------
  type DownsweepM a = ReaderT DownsweepEnv IO a
  data DownsweepEnv = DownsweepEnv {
        downsweep_hsc_env :: HscEnv
      , _downsweep_mode :: DownsweepMode
 -    , _downsweep_old_summaries :: M.Map (UnitId, OsPath) ModSummary
 +    , _downsweep_summaries_cache :: ModSummaryCache
 +    , downsweep_imports_cache :: ImportsCache
      , _downsweep_excl_mods :: [ModuleName]
+ }
 +mkModSummaryCache :: [(ModSummary, SummProvenance)] -> ModSummaryCacheMap
 +mkModSummaryCache summs = foldl' (flip (uncurry addModSummaryCache)) M.empty summs
++
 +addModSummaryCache :: ModSummary -> SummProvenance -> ModSummaryCacheMap -> ModSummaryCacheMap
 +addModSummaryCache ms pr fe = upd_fe fe
 +  where
 +    upd_fe fe
 +      | Just src_fn_os <- ml_hs_file_ospath (ms_location ms)
 +      = M.insert (ms_unitid ms, src_fn_os) (Right (ms, pr)) fe
 +      | otherwise = fe
++
 +modifySummCache :: ModSummaryCache -> (ModSummaryCacheMap -> ModSummaryCacheMap) -> IO ()
 +modifyImpsCache :: ImportsCache    -> (ImportsCacheMap    -> ImportsCacheMap)    -> IO ()
 +modifySummCache r f = atomicModifyIORef' r (\c -> (f c, ()))
 +modifyImpsCache r f = atomicModifyIORef' r (\c -> (f c, ()))
++
 +-- | A cache from a module import (in given home unit context, with a package
 +-- qualifier, and the imported module name (with or without SOURCE)) to the
 +-- result of summarising that import (see 'summariseModuleDispatch').
 +--
 +-- See Note [Downsweep Control Flow and Caching]
 +type ImportsCacheMap
 +      = M.Map (UnitId, PkgQual, ModuleNameWithIsBoot) SummariseResult
++
 +-- | Populate the 'ImportsCacheMap' with the root modules.
 +mkRootMap :: [ModuleNodeInfo] -> ImportsCacheMap
 +mkRootMap summaries = Map.fromList
 +  [ ((moduleNodeInfoUnitId s, NoPkgQual, moduleNodeInfoMnwib s), FoundHome s) | s <- summaries ]
++
  runDownsweepM :: DownsweepEnv -> DownsweepM a -> IO a
  runDownsweepM env act = runReaderT act env
+-
 -loopInstantiations :: [(UnitId, InstantiatedUnit)]
 -                   -> M.Map NodeKey ModuleGraphNode
 -                   -> DownsweepM (M.Map NodeKey ModuleGraphNode)
 -loopInstantiations [] done = pure done
 -loopInstantiations ((home_uid, iud) :xs) done = do
 -  hsc_env <- asks downsweep_hsc_env
 -  let home_unit = ue_unitHomeUnit home_uid (hsc_unit_env hsc_env)
 -  let hsc_env' = hscSetActiveHomeUnit home_unit hsc_env
 -      done' = loopUnit hsc_env' done [instUnitInstanceOf iud]
 -      payload = InstantiationNode home_uid iud
 -  loopInstantiations xs (M.insert (mkNodeKey payload) payload done')
+-
+-
 --- This loops over all the mod summaries in the dependency graph, accumulates the actual dependencies for each module/unit
 -loopSummaries :: [ModSummary]
 -      -> (M.Map NodeKey ModuleGraphNode,
 -            DownsweepCache)
 -      -> DownsweepM ((M.Map NodeKey ModuleGraphNode), DownsweepCache)
 -loopSummaries [] done = pure done
 -loopSummaries (ms:next) (done, summarised)
 -  | Just {} <- M.lookup k done
 -  = loopSummaries next (done, summarised)
 -  -- Didn't work out what the imports mean yet, now do that.
 -  | otherwise = do
 -     (final_deps, done', summarised') <- loopImports (calcDeps ms) done summarised
 -     -- This has the effect of finding a .hs file if we are looking at the .hs-boot file.
 -     (_, done'', summarised'') <- loopImports (maybeToList hs_file_for_boot) done' summarised'
 -     loopSummaries next (M.insert k (ModuleNode final_deps (ModuleNodeCompile ms)) done'', summarised'')
 +loopDownsweepNodes  :: M.Map NodeKey (NodeRes ModuleGraphNode) -> [DownsweepNode]               -> DownsweepM (M.Map NodeKey (NodeRes ModuleGraphNode))
 +loopModuleNodeInfos :: M.Map NodeKey (NodeRes ModuleGraphNode) -> [ModuleNodeInfo]              -> DownsweepM (M.Map NodeKey (NodeRes ModuleGraphNode))
 +loopUnits           :: M.Map NodeKey (NodeRes ModuleGraphNode) -> UnitId -> [UnitId]            -> DownsweepM (M.Map NodeKey (NodeRes ModuleGraphNode))
 +loopInstantiations  :: M.Map NodeKey (NodeRes ModuleGraphNode) -> [(UnitId, InstantiatedUnit)]  -> DownsweepM (M.Map NodeKey (NodeRes ModuleGraphNode))
 +loopFromInteractive :: M.Map NodeKey (NodeRes ModuleGraphNode) -> Module -> [InteractiveImport] -> DownsweepM (M.Map NodeKey (NodeRes ModuleGraphNode))
 +loopDownsweepNodes  base_map nodes = dfsBuild (Just base_map) nodes dsNodeInfoKey dsNodeExpand
 +loopModuleNodeInfos base_map       = loopDownsweepNodes base_map . map DSMod
 +loopUnits           base_map homud = loopDownsweepNodes base_map . map (DSUnit homud)
 +loopInstantiations  base_map       = loopDownsweepNodes base_map . map (uncurry DSInst)
 +loopFromInteractive base_map m     = loopDownsweepNodes base_map . (:[]) . DSInteractive m
++
 +--------------------------------------------------------------------------------
 +-- * Expanding 'DownsweepNode's into payload and node dependencies
 +--------------------------------------------------------------------------------
++
 +-- | A 'DownsweepNode' is the basic block of the downsweep algorithm which
 +-- encompasses the types of nodes we can iteratively expand to construct the
 +-- full module graph. See 'loopDownsweepNodes'.
 +--
 +-- See Note [Downsweep Control Flow and Caching]
 +data DownsweepNode
 +  -- | A module node to expand
 +  = DSMod ModuleNodeInfo
 +  -- | A unit node to expand
 +  | DSUnit
 +  { home_context_uid :: UnitId
 +  -- ^ The home unit which introduced the dependency on this 'node_uid'. This
 +  -- 'node_uid' can only be expanded in the context ('HscEnv') where
 +  -- 'home_context_uid' is the active home unit, to make sure the package flags
 +  -- are the ones attributed to the home package that introduced this node.
 +  , node_uid         :: UnitId
 +  -- ^ The unit node to expand
 +  }
 +  -- | FIXME: document the meaning of 'DSInst'
 +  | DSInst
 +  { home_context_uid :: UnitId
 +  , instantiated_ud  :: InstantiatedUnit
 +  }
 +  -- | A group of interactive imports from this interactive Module
 +  | DSInteractive Module [InteractiveImport]
++
 +instance Outputable DownsweepNode where
 +  ppr = \case
 +    DSMod (ModuleNodeCompile ms) -> text "DSModC" <+> ppr (ms_mod_name ms)
 +    DSMod (ModuleNodeFixed key _) -> text "DSModF" <+> ppr key
 +    DSUnit{node_uid} -> text "DSUnit" <+> ppr node_uid
 +    DSInst{instantiated_ud} -> text "DSInst" <+> ppr instantiated_ud
 +    DSInteractive mod ii    -> text "DSInteractive" <+> ppr mod <+> ppr ii
++
 +-- | They key by which to cache previously visited 'DownsweepNode's
 +dsNodeInfoKey :: DownsweepNode -> NodeKey
 +dsNodeInfoKey = \case
 +  DSMod (ModuleNodeCompile ms)  -> NodeKey_Module (msKey ms)
 +  DSMod (ModuleNodeFixed mod _) -> NodeKey_Module mod
 +  DSUnit{node_uid}              -> NodeKey_ExternalUnit node_uid
 +  DSInst{instantiated_ud}       -> NodeKey_Unit instantiated_ud
 +  DSInteractive mod _imps       -> NodeKey_Module $ moduleToMnk mod NotBoot
++
 +dsNodeExpand :: DownsweepNode -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
 +dsNodeExpand = \case
 +  DSMod (ModuleNodeCompile ms)         -> expandModuleSummary ms
 +  DSMod (ModuleNodeFixed key loc)      -> expandFixedModuleNode key loc
 +  DSUnit{ node_uid, home_context_uid } -> expandUnitNode node_uid home_context_uid
 +  DSInst{ instantiated_ud
 +        , home_context_uid }           -> expandInstantiatedUnit instantiated_ud home_context_uid
 +  DSInteractive imod iis               -> expandInteractiveImports imod iis
++
 +expandModuleSummary :: ModSummary -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
 +expandModuleSummary ms = do -- Didn't work out what the imports mean yet, now do that.
 +    hsc_env <- asks downsweep_hsc_env
 +    let home_uid  = ms_unitid ms
 +        home_unit = ue_unitHomeUnit home_uid (hsc_unit_env hsc_env)
 +    (final_deps, todo) <- unzip <$> mapM (expandModImport home_uid home_unit) (calcDeps ms)
++
 +    -- This has the effect of finding a .hs file if we are looking at the .hs-boot file.
 +    boot_todo <-
 +      if | HsBootFile <- ms_hsc_src ms
 +         -> do
 +            r <- downsweepSummarise home_unit (generatedImport FromSelfBoot (noLoc (ms_mod_name ms))) Nothing
 +            case r of
 +              FoundHome s -> pure [DSMod s]
 +              _           -> pure []
 +         | otherwise      -> pure []
++
 +    return $ NSuccess
 +      ( ModuleNode (catMaybes final_deps) (ModuleNodeCompile ms)
 +      , boot_todo ++ concat todo
 +      )
    where
 -    k = NodeKey_Module (msKey ms)
+-
 -    hs_file_for_boot
 -      | HsBootFile <- ms_hsc_src ms
 -      = Just ( ms_unitid ms
 -             , generatedImport FromSelfBoot (noLoc (ms_mod_name ms)) )
 -      | otherwise
 -      = Nothing
+-
 -loopModuleNodeInfos :: [ModuleNodeInfo] -> (M.Map NodeKey ModuleGraphNode, DownsweepCache) -> DownsweepM (M.Map NodeKey ModuleGraphNode, DownsweepCache)
 -loopModuleNodeInfos is cache = foldM (flip loopModuleNodeInfo) cache is
+-
 -loopModuleNodeInfo :: ModuleNodeInfo -> (M.Map NodeKey ModuleGraphNode, DownsweepCache) -> DownsweepM (M.Map NodeKey ModuleGraphNode, DownsweepCache)
 -loopModuleNodeInfo mod_node_info (done, summarised) = do
 -  case mod_node_info of
 -    ModuleNodeCompile ms -> do
 -      loopSummaries [ms] (done, summarised)
 -    ModuleNodeFixed mod ml -> do
 -      done' <- loopFixedModule mod ml done
 -      return (done', summarised)
+-
 --- NB: loopFixedModule does not take a downsweep cache, because if you
 --- ever reach a Fixed node, everything under that also must be fixed.
 -loopFixedModule :: ModNodeKeyWithUid -> ModLocation
 -                -> M.Map NodeKey ModuleGraphNode
 -                -> DownsweepM (M.Map NodeKey ModuleGraphNode)
 -loopFixedModule key loc done = do
 -  let nk = NodeKey_Module key
 -  hsc_env <- asks downsweep_hsc_env
 -  case M.lookup nk done of
 -    Just {} -> return done
 -    Nothing -> do
 -      -- MP: TODO, we should just read the dependency info from the interface rather than either
 -      -- a. Loading the whole thing into the EPS (this might never nececssary and causes lots of things to be permanently loaded into memory)
 -      -- b. Loading the whole interface into a buffer before discarding it. (wasted allocation and deserialisation)
 -      read_result <- liftIO $
 -        -- 1. Check if the interface is already loaded into the EPS by some other
 -        -- part of the compiler.
 -        lookupIfaceByModuleHsc hsc_env (mnkToModule key) >>= \case
 -          Just iface -> return (M.Succeeded iface)
 -          Nothing -> readIface (hsc_hooks hsc_env) (hsc_logger hsc_env) (hsc_dflags hsc_env) (hsc_NC hsc_env) (mnkToModule key) (ml_hi_file loc)
 -      case read_result of
 -        M.Succeeded iface -> do
 -          -- Computer information about this node
 -          let node_deps = ifaceDeps (mi_deps iface)
 -              edges = map mkFixedEdge node_deps
 -              node = ModuleNode edges (ModuleNodeFixed key loc)
 -          foldM (loopFixedNodeKey (mnkUnitId key)) (M.insert nk node done) (bimap snd snd <$> node_deps)
 -        -- Ignore any failure, we might try to read a .hi-boot file for
 -        -- example, even if there is not one.
 -        M.Failed {} ->
 -          return done
+-
 -loopFixedNodeKey :: UnitId -> M.Map NodeKey ModuleGraphNode -> Either ModNodeKeyWithUid UnitId -> DownsweepM  (M.Map NodeKey ModuleGraphNode)
 -loopFixedNodeKey _ done (Left key) = do
 -  loopFixedImports [key] done
 -loopFixedNodeKey home_uid done (Right uid) = do
 -  -- Set active unit so that looking loopUnit finds the correct
 -  -- -package flags in the unit state.
 -  hsc_env <- asks downsweep_hsc_env
 -  let hsc_env' = hscSetActiveUnitId home_uid hsc_env
 -  return $ loopUnit hsc_env' done [uid]
+-
 -mkFixedEdge :: Either (ImportLevel, ModNodeKeyWithUid) (ImportLevel, UnitId) -> ModuleNodeEdge
 -mkFixedEdge (Left (lvl, key)) = mkModuleEdge lvl (NodeKey_Module key)
 -mkFixedEdge (Right (lvl, uid)) = mkModuleEdge lvl (NodeKey_ExternalUnit uid)
+-
 -ifaceDeps :: Dependencies -> [Either (ImportLevel, ModNodeKeyWithUid) (ImportLevel, UnitId)]
 -ifaceDeps deps =
 -  [ Left (tcImportLevel lvl, ModNodeKeyWithUid dep uid)
 -  | (lvl, uid, dep) <- Set.toList (dep_direct_mods deps)
 -  ] ++
 -  [ Right (tcImportLevel lvl, uid)
 -  | (lvl, uid) <- Set.toList (dep_direct_pkgs deps)
 -  ]
+-
 --- Like loopImports, but we already know exactly which module we are looking for.
 -loopFixedImports :: [ModNodeKeyWithUid]
 -                 -> M.Map NodeKey ModuleGraphNode
 -                 -> DownsweepM (M.Map NodeKey ModuleGraphNode)
 -loopFixedImports [] done = pure done
 -loopFixedImports (key:keys) done = do
 -  let nk = NodeKey_Module key
 -  hsc_env <- asks downsweep_hsc_env
 -  case M.lookup nk done of
 -    Just {} -> loopFixedImports keys done
 -    Nothing -> do
 +    expandModImport home_uid home_unit imp = do
 +      let UnresolvedImport { ui_level = lvl } = imp
 +      mb_s <- downsweepSummarise home_unit imp Nothing
 +      case mb_s of
 +        NotThere -> return
 +          ( Nothing, [] )
 +        External uid -> return
 +          ( Just $ mkModuleEdge lvl (NodeKey_ExternalUnit uid)
 +          -- Specify home unit, as each unit might have a different visible package database.
 +          , [DSUnit{node_uid = uid, home_context_uid = home_uid}] )
 +        FoundInstantiation iud -> return
 +          ( Just (mkModuleEdge lvl (NodeKey_Unit iud)), [] )
 +        FoundHomeWithError (_uid, _e) -> return
 +          ( Nothing, [] )
 +          -- the error @e@ is already stored in the summarisation cache,
 +          -- (the IORef in DownsweepM) and will get reported at the end.
 +        FoundHome s -> return
 +          -- MP: This assumes that we can only instantiate non home units, which is probably fair enough for now.
 +          ( Just $ mkModuleEdge lvl (NodeKey_Module (mnKey s))
 +          , [DSMod s] )
++
 +    calcDeps :: ModSummary -> [UnresolvedImport PkgQual]
 +    calcDeps ms =
 +      -- Add a dependency on the HsBoot file if it exists
 +      -- This gets passed to the loopImports function which just ignores it if it
 +      -- can't be found.
 +      [ self_boot | NotBoot <- [isBootSummary ms] ] ++
 +      [ e | e <- ms_imps ms ]
 +      where
 +        self_boot = (generatedImport FromSelfBoot (noLoc (ms_mod_name ms)))
 +                      { ui_boot = IsBoot }
++
 +-- | Expand a 'ModuleNodeFixed' node
 +-- NB: If you ever reach a Fixed node, everything under that also must be fixed.
 +expandFixedModuleNode :: ModNodeKeyWithUid -> ModLocation -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
 +expandFixedModuleNode key loc = do
 +    hsc_env <- asks downsweep_hsc_env
 +    -- MP: TODO, we should just read the dependency info from the interface rather than either
 +    -- a. Loading the whole thing into the EPS (this might never nececssary and causes lots of things to be permanently loaded into memory)
 +    -- b. Loading the whole interface into a buffer before discarding it. (wasted allocation and deserialisation)
 +    read_result <- liftIO $
 +      -- 1. Check if the interface is already loaded into the EPS by some other
 +      -- part of the compiler.
 +      lookupIfaceByModuleHsc hsc_env (mnkToModule key) >>= \case
 +        Just iface -> return (M.Succeeded iface)
 +        Nothing -> readIface (hsc_hooks hsc_env) (hsc_logger hsc_env) (hsc_dflags hsc_env) (hsc_NC hsc_env) (mnkToModule key) (ml_hi_file loc)
 +    case read_result of
 +      M.Succeeded iface -> do
 +        -- Computer information about this node
 +        let node_deps = ifaceDeps (mi_deps iface)
 +            edges = map mkFixedEdge node_deps
 +            node = ModuleNode edges (ModuleNodeFixed key loc)
 +        deps' <- catMaybes <$> mapM (mk_dep hsc_env) (bimap snd snd <$> node_deps)
 +        pure $ NSuccess (node, deps')
++
 +      -- Skip any failure, we might try to read a .hi-boot file for
 +      -- example, even if there is not one.
 +      M.Failed {} ->
 +        pure NSkip
 +  where
 +    mk_dep hsc_env (Left key) = do
 +      -- Like expandImports, but we already know exactly which module we are looking for.
        read_result <- liftIO $ findExactModule hsc_env (mnkToInstalledModule key) (mnkIsBoot key)
        case read_result of
          InstalledFound loc -> do
 -          done' <- loopFixedModule key loc done
 -          loopFixedImports keys done'
 +          pure $ Just $ DSMod (ModuleNodeFixed key loc)
          _otherwise ->
            -- If the finder fails, just keep going, there will be another
 -          -- error later.
 -          loopFixedImports keys done
 +          -- error later when we try to expand this dependency.
 +          pure Nothing
 +    mk_dep _ (Right uid_dep) = do
 +      -- Set active unit so that looking loopUnit finds the correct
 +      -- -package flags in the unit state.
 +      let home_uid = mnkUnitId key
 +      pure (Just DSUnit{node_uid=uid_dep, home_context_uid=home_uid})
++
 +    mkFixedEdge :: Either (ImportLevel, ModNodeKeyWithUid) (ImportLevel, UnitId) -> ModuleNodeEdge
 +    mkFixedEdge (Left (lvl, key))  = mkModuleEdge lvl (NodeKey_Module key)
 +    mkFixedEdge (Right (lvl, uid)) = mkModuleEdge lvl (NodeKey_ExternalUnit uid)
++
 +    ifaceDeps :: Dependencies -> [Either (ImportLevel, ModNodeKeyWithUid) (ImportLevel, UnitId)]
 +    ifaceDeps deps =
 +      [ Left (tcImportLevel lvl, ModNodeKeyWithUid dep uid)
 +      | (lvl, uid, dep) <- Set.toList (dep_direct_mods deps)
 +      ] ++
 +      [ Right (tcImportLevel lvl, uid)
 +      | (lvl, uid) <- Set.toList (dep_direct_pkgs deps)
 +      ]
++
 +-- | Expand a unit id under the context of a certain home unit
 +expandUnitNode :: UnitId {-^ @node_uid@ -} -> UnitId {-^ Home unit from where @node_uid@ was introduced -}
 +               -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
 +expandUnitNode node_uid home_context_uid = do
 +    -- Set active unit so that looking loopUnit finds the correct
 +    -- -package flags in the unit state.
 +    hsc_env <- asks downsweep_hsc_env
 +    let lcl_hsc_env = hscSetActiveUnitId home_context_uid hsc_env
 +    case unitDepends <$> lookupUnitId (hsc_units lcl_hsc_env) node_uid of
 +      Just us -> pure $ NSuccess ((UnitNode us node_uid), map (\u -> DSUnit{node_uid=u, home_context_uid{-inherit-}}) us)
 +      Nothing -> pprPanic "loopUnit" (text "Malformed package database, missing " <+> ppr node_uid)
++
 +expandInstantiatedUnit :: InstantiatedUnit -> UnitId {-^ Home unit -} -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
 +expandInstantiatedUnit iud home_uid = pure $ NSuccess
 +  ( InstantiationNode home_uid iud
 +  , [DSUnit{node_uid=instUnitInstanceOf iud, home_context_uid=home_uid}] )
++
 +expandInteractiveImports :: Module -> [InteractiveImport] -> DownsweepM (NodeRes (ModuleGraphNode, [DownsweepNode]))
 +expandInteractiveImports imod imps = do
 +  hsc_env    <- asks downsweep_hsc_env
 +  imps_cache <- asks downsweep_imports_cache
++
 +  let
 +    -- A simple edge to a module from the same home unit
 +    mkEdge (IIModule n) = return $
 +      let
 +        mod_node_key = ModNodeKeyWithUid
 +          { mnkModuleName = GWIB (moduleName n) NotBoot
 +          , mnkUnitId =
 +              -- 'toUnitId' is safe here, as we can't import modules that
 +              -- don't have a 'UnitId'.
 +              toUnitId (moduleUnit n)
 +          }
 +       in (Just $ ModuleNodeEdge NormalLevel (NodeKey_Module mod_node_key), [])
++
 +    -- A complete import statement
 +    mkEdge (IIDecl i) =
 +      let unitId = homeUnitId $ hsc_home_unit hsc_env
 +          imp = rnUnresolvedImportPkgQual (renameRawPkgQual (hsc_unit_env hsc_env))
 +                                          (mkUnresolvedImport i)
 +          UnresolvedImport { ui_level = lvl, ui_boot = is_boot } = imp
 +      in do
 +        let home_unit = ue_unitHomeUnit unitId (hsc_unit_env hsc_env)
 +        let k _ loc mod =
 +              let key = moduleToMnk mod is_boot
 +              in return $ FoundHome (ModuleNodeFixed key loc)
++
 +        found <- liftIO $ summariseModuleDispatch k hsc_env imps_cache home_unit imp []
 +        case found of
 +          -- Case 1: Home modules have to already be in the cache.
 +          FoundHome (ModuleNodeFixed mod _) -> do
 +            let edge = ModuleNodeEdge lvl (NodeKey_Module mod)
 +            -- Note: Does not perform any further downsweep as the module must already be in the cache.
 +            return (Just edge, [])
 +          -- Case 2: External units may not be in the cache, if we haven't already initialised the
 +          -- module graph. We can construct the module graph for those here by calling loopUnit.
 +          External uid -> do
 +            let edge = ModuleNodeEdge lvl (NodeKey_ExternalUnit uid)
 +            return (Just edge, [DSUnit{node_uid=uid, home_context_uid=homeUnitId home_unit}])
 +          -- And if it's not found.. just carry on and hope.
 +          _ -> return (Nothing, [])
++
 +  (module_edges, todo) <- unzip <$> mapM mkEdge imps
 +  pure $ NSuccess
 +    ( ModuleNode (catMaybes module_edges) node_type, concat todo )
 +  where
 +    -- No sensible value for ModLocation.. if you hit this panic then you probably
 +    -- need to add proper support for modules without any source files to the driver.
 +    ml = pprPanic "modLocation" (ppr imod <+> ppr imps)
 +    key = moduleToMnk imod NotBoot
 +    node_type = ModuleNodeFixed key ml
++
 +--------------------------------------------------------------------------------
 +-- * Constructing Module Summaries
 +--------------------------------------------------------------------------------
  downsweepSummarise :: HomeUnit
                     -> UnresolvedImport PkgQual
                     -> Maybe (StringBuffer, UTCTime)
                     -> DownsweepM SummariseResult
  downsweepSummarise home_unit imp maybe_buf = do
 -  DownsweepEnv hsc_env mode old_summaries excl_mods <- ask
 -  case mode of
 -    DownsweepUseCompile -> liftIO $ summariseModule hsc_env home_unit old_summaries imp maybe_buf excl_mods
 -    DownsweepUseFixed -> liftIO $ summariseModuleInterface hsc_env home_unit imp excl_mods
+-
+-
 --- This loops over each import in each summary. It is mutually recursive with loopSummaries if we discover
 --- a new module by doing this.
 -loopImports :: [(UnitId, UnresolvedImport PkgQual)]
 -                -- Work list: process these modules
 -     -> M.Map NodeKey ModuleGraphNode
 -     -> DownsweepCache
 -                -- Visited set; the range is a list because
 -                -- the roots can have the same module names
 -                -- if allow_dup_roots is True
 -     -> DownsweepM ([ModuleNodeEdge],
 -          M.Map NodeKey ModuleGraphNode, DownsweepCache)
 -                -- The result is the completed NodeMap
 -loopImports [] done summarised = return ([], done, summarised)
 -loopImports ((home_uid, imp) : ss) done summarised
 -  | Just summs <- M.lookup cache_key summarised
 -  = case summs of
 -      [Right ms] -> do
 -        let nk = mkModuleEdge lvl (NodeKey_Module (mnKey ms))
 -        (rest, summarised', done') <- loopImports ss done summarised
 -        return (nk: rest, summarised', done')
 -      [Left _err] ->
 -        loopImports ss done summarised
 -      _errs ->  do
 -        loopImports ss done summarised
 -  | otherwise
 -  = do
 -       hsc_env <- asks downsweep_hsc_env
 -       let home_unit = ue_unitHomeUnit home_uid (hsc_unit_env hsc_env)
 -       mb_s <- downsweepSummarise home_unit imp Nothing
 -       case mb_s of
 -           NotThere -> loopImports ss done summarised
 -           External uid -> do
 -            -- Pass an updated hsc_env to loopUnit, as each unit might
 -            -- have a different visible package database.
 -            let hsc_env' = hscSetActiveHomeUnit home_unit hsc_env
 -            let done' = loopUnit hsc_env' done [uid]
 -            (other_deps, done'', summarised') <- loopImports ss done' summarised
 -            return (mkModuleEdge lvl (NodeKey_ExternalUnit uid) : other_deps, done'', summarised')
 -           FoundInstantiation iud -> do
 -            (other_deps, done', summarised') <- loopImports ss done summarised
 -            return (mkModuleEdge lvl (NodeKey_Unit iud) : other_deps, done', summarised')
 -           FoundHomeWithError (_uid, e) ->  loopImports ss done (Map.insert cache_key [(Left e)] summarised)
 -           FoundHome s -> do
 -             (done', summarised') <-
 -               loopModuleNodeInfo s (done, Map.insert cache_key [Right s] summarised)
 -             (other_deps, final_done, final_summarised) <- loopImports ss done' summarised'
+-
 -             -- MP: This assumes that we can only instantiate non home units, which is probably fair enough for now.
 -             return (mkModuleEdge lvl (NodeKey_Module (mnKey s)) : other_deps, final_done, final_summarised)
 -  where
 -    UnresolvedImport { ui_level = lvl, ui_pkg_qual = mb_pkg
 -                     , ui_boot = is_boot, ui_mod_name = wanted_mod } = imp
 -    cache_key = (home_uid, mb_pkg, GWIB (unLoc wanted_mod) is_boot)
+-
 -loopUnit :: HscEnv -> Map.Map NodeKey ModuleGraphNode -> [UnitId] -> Map.Map NodeKey ModuleGraphNode
 -loopUnit _ cache [] = cache
 -loopUnit lcl_hsc_env cache (u:uxs) = do
 -   let nk = (NodeKey_ExternalUnit u)
 -   case Map.lookup nk cache of
 -     Just {} -> loopUnit lcl_hsc_env cache uxs
 -     Nothing -> case unitDepends <$> lookupUnitId (hsc_units lcl_hsc_env) u of
 -                 Just us -> loopUnit lcl_hsc_env (loopUnit lcl_hsc_env (Map.insert nk (UnitNode us u) cache) us) uxs
 -                 Nothing -> pprPanic "loopUnit" (text "Malformed package database, missing " <+> ppr u)
+-
 -multiRootsErr :: SourceErrorContext -> [ModuleNodeInfo] -> IO ()
 -multiRootsErr _ [] = panic "multiRootsErr"
 -multiRootsErr sec summs@(summ1:_)
 +  DownsweepEnv hsc_env mode summaries_cache_ref imports_cache_ref excl_mods <- ask
 +  liftIO $ case mode of
 +    DownsweepUseCompile ->
 +      summariseModule hsc_env home_unit summaries_cache_ref imports_cache_ref
 +                      imp maybe_buf excl_mods
 +    DownsweepUseFixed ->
 +      summariseModuleInterface hsc_env home_unit imports_cache_ref imp excl_mods
++
 +multiRootsErr :: SourceErrorContext -> NE.NonEmpty ModuleNodeInfo -> IO ()
 +multiRootsErr sec (summ1 NE.:| summs)
    = throwOneError sec $ fmap GhcDriverMessage $
      mkPlainErrorMsgEnvelope noSrcSpan $ DriverDuplicatedModuleDeclaration mod files
    where
      mod = moduleNodeInfoModule summ1
 -    files = mapMaybe (ml_hs_file . moduleNodeInfoLocation) summs
 +    files = mapMaybe (ml_hs_file . moduleNodeInfoLocation) (summ1:summs)
  moduleNotFoundErr :: UnitId -> ModuleName -> DriverMessages
  moduleNotFoundErr uid mod = singleMessage $ mkPlainErrorMsgEnvelope noSrcSpan (DriverModuleNotFound uid mod)
@@ -696,48 +844,20 @@ instantiationNodes uid unit_state = map (uid,) iuids_to_check
          , recur <- (indef :) $ goUnitId $ moduleUnit $ snd inst
+         ]
 --- The linking plan for each module. If we need to do linking for a home unit
 --- then this function returns a graph node which depends on all the modules in the home unit.
+-
 --- At the moment nothing can depend on these LinkNodes.
 -linkNodes :: [ModuleGraphNode] -> UnitId -> HomeUnitEnv -> Maybe (Either (Messages DriverMessage) ModuleGraphNode)
 -linkNodes summaries uid hue =
 -  let dflags = homeUnitEnv_dflags hue
 -      ofile = outputFile_ dflags
+-
 -      unit_nodes :: [NodeKey]
 -      unit_nodes = map mkNodeKey (filter ((== uid) . mgNodeUnitId) summaries)
 -  -- 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.
 -  --
 -      no_hs_main = gopt Opt_NoHsMain dflags
+-
 -      main_sum = any (== NodeKey_Module (ModNodeKeyWithUid (GWIB (mainModuleNameIs dflags) NotBoot) uid)) unit_nodes
+-
 -      do_linking =  main_sum || no_hs_main || ghcLink dflags == LinkDynLib || ghcLink dflags == LinkStaticLib || ghcLink dflags == LinkBytecodeLib
+-
 -  in if | isExecutableLink (ghcLink dflags) && isJust ofile && not do_linking ->
 -            Just (Left $ singleMessage $ mkPlainErrorMsgEnvelope noSrcSpan (DriverRedirectedNoMain $ mainModuleNameIs dflags))
 -        -- This should be an error, not a warning (#10895).
 -        | ghcLink dflags /= NoLink, do_linking -> Just (Right (LinkNode unit_nodes uid))
 -        | otherwise  -> Nothing
+-
  getRootSummary ::
    [ModuleName] ->
 -  M.Map (UnitId, OsPath) ModSummary ->
 +  ModSummaryCache ->
 +  ImportsCache ->
    HscEnv ->
    Target ->
    IO (Either DriverMessages ModSummary)
 -getRootSummary excl_mods old_summary_map hsc_env target
 +getRootSummary excl_mods summ_cache imports_cache hsc_env target
    | TargetFile file mb_phase <- targetId
    = do
      let offset_file = augmentByWorkingDirectory dflags file
      exists <- liftIO $ doesFileExist offset_file
      if exists || isJust maybe_buf
 -    then summariseFile hsc_env home_unit old_summary_map offset_file mb_phase
 +    then summariseFile hsc_env home_unit summ_cache offset_file mb_phase
           maybe_buf
      else
        return $ Left $ singleMessage $
@@ -746,7 +866,7 @@ getRootSummary excl_mods old_summary_map hsc_env target
    = do
      let root_imp = (generatedImport FromTarget (L rootLoc modl))
                       { ui_pkg_qual = ThisPkg (homeUnitId home_unit) }
 -    maybe_summary <- summariseModule hsc_env home_unit old_summary_map root_imp
 +    maybe_summary <- summariseModule hsc_env home_unit summ_cache imports_cache root_imp
                       maybe_buf excl_mods
      pure case maybe_summary of
        FoundHome (ModuleNodeCompile s)  -> Right s
@@ -809,6 +929,10 @@ rootSummariesParallel n_jobs hsc_env diag_wrapper msg get_summary = do
                throwIO e
              a -> pure a
 +--------------------------------------------------------------------------------
 +-- * Check/validate properties and error out
 +--------------------------------------------------------------------------------
++
  -- | This function checks then important property that if both p and q are home units
  -- then any dependency of p, which transitively depends on q is also a home unit.
  --
@@ -856,6 +980,10 @@ checkHomeUnitsClosed ue
                        let todo'' = (depends Set.\\ done) `Set.union` todo'
                        in DigraphNode uid uid (Set.toList depends) : go (Set.insert uid done) todo''
 +--------------------------------------------------------------------------------
 +-- * Enable Code Gen for Template Haskell
 +--------------------------------------------------------------------------------
++
  -- | Update the every ModSummary that is depended on
  -- by a module that needs template haskell. We enable codegen to
  -- the specified target, disable optimization and change the .hi
@@ -1173,15 +1301,9 @@ Potential TODOS:
    generating temporary ones.
  -}
 --- | Populate the Downsweep cache with the root modules.
 -mkRootMap
 -  :: [ModuleNodeInfo]
 -  -> DownsweepCache
 -mkRootMap summaries = Map.fromListWith (flip (++))
 -  [ ((moduleNodeInfoUnitId s, NoPkgQual, moduleNodeInfoMnwib s), [Right s]) | s <- summaries ]
+-
  -----------------------------------------------------------------------------
 --- Summarising modules
 +-- * Pre-processing and Summarising and modules
 +-----------------------------------------------------------------------------
  -- We have two types of summarisation:
  --
@@ -1196,33 +1318,39 @@ mkRootMap summaries = Map.fromListWith (flip (++))
  summariseFile
          :: HscEnv
          -> HomeUnit
 -        -> M.Map (UnitId, OsPath) ModSummary    -- old summaries
 +        -> ModSummaryCache
          -> FilePath                     -- source file name
          -> Maybe Phase                  -- start phase
          -> Maybe (StringBuffer,UTCTime)
          -> IO (Either DriverMessages ModSummary)
 -summariseFile hsc_env' home_unit old_summaries src_fn mb_phase maybe_buf
 -        -- we can use a cached summary if one is available and the
 -        -- source file hasn't changed,
 -   | Just old_summary <- M.lookup (homeUnitId home_unit, src_fn_os) old_summaries
 -   = do
 -        let location = ms_location $ old_summary
+-
 -        src_hash <- get_src_hash
 -                -- The file exists; we checked in getRootSummary above.
 -                -- If it gets removed subsequently, then this
 -                -- getFileHash may fail, but that's the right
 -                -- behaviour.
+-
 -                -- return the cached summary if the source didn't change
 -        checkSummaryHash
 -            hsc_env (new_summary src_fn)
 -            old_summary location src_hash
+-
 -   | otherwise
 -   = do src_hash <- get_src_hash
 -        new_summary src_fn src_hash
 +summariseFile hsc_env' home_unit summ_cache_ref src_fn mb_phase maybe_buf
 +   = do file_summ_cache <- readIORef summ_cache_ref
 +        case M.lookup (homeUnitId home_unit, src_fn_os) file_summ_cache of
 +          Just (Right (chd_summary, SummFresh)) ->
 +            -- Fresh: use it straight away
 +            pure (Right chd_summary)
 +          Just (Right (old_summary, SummOld)) -> do
 +            -- we can use a cached summary if one is available and the
 +            -- source file hasn't changed,
 +            let location = ms_location $ old_summary
++
 +            src_hash <- get_src_hash
 +                    -- The file exists; we checked in getRootSummary above.
 +                    -- If it gets removed subsequently, then this
 +                    -- getFileHash may fail, but that's the right
 +                    -- behaviour.
++
 +                    -- return the cached summary if the source didn't change
 +            res <- checkSummaryHash
 +                hsc_env (new_summary src_fn)
 +                old_summary location src_hash
 +            case res of
 +              Right ms -> modifySummCache summ_cache_ref (addModSummaryCache ms SummFresh)
 +              Left _   -> pure ()
 +            return res
 +          _ -> do src_hash <- get_src_hash
 +                  new_summary src_fn src_hash
    where
      -- change the main active unit so all operations happen relative to the given unit
      hsc_env = hscSetActiveHomeUnit home_unit hsc_env'
@@ -1233,7 +1361,8 @@ summariseFile hsc_env' home_unit old_summaries src_fn mb_phase maybe_buf
                        Just (buf,_) -> return $ fingerprintStringBuffer buf
                        Nothing -> liftIO $ getFileHash src_fn
 -    new_summary src_fn src_hash = runExceptT $ do
 +    new_summary src_fn src_hash = do
 +      res <- runExceptT $ do
          preimps@PreprocessedImports {..}
              <- getPreprocessedImports hsc_env src_fn mb_phase maybe_buf
@@ -1264,6 +1393,10 @@ summariseFile hsc_env' home_unit old_summaries src_fn mb_phase maybe_buf
              , nms_mod = mod
              , nms_preimps = preimps
+             }
 +      modifySummCache summ_cache_ref $ case res of
 +        Left e   -> M.insert (homeUnitId home_unit, src_fn_os) (Left e)
 +        Right ms -> addModSummaryCache ms SummFresh
 +      return res
  checkSummaryHash
      :: HscEnv
@@ -1316,13 +1449,14 @@ data SummariseResult =
  -- --make mode.
  summariseModule :: HscEnv
                  -> HomeUnit
 -                -> M.Map (UnitId, OsPath) ModSummary
 +                -> ModSummaryCache
 +                -> ImportsCache
                  -> UnresolvedImport PkgQual -- ^ The import being summarised
                  -> Maybe (StringBuffer, UTCTime)
                  -> [ModuleName]
                  -> IO SummariseResult
 -summariseModule hsc_env home_unit old_summaries imp maybe_buf excl_mods =
 -  summariseModuleDispatch k hsc_env home_unit imp excl_mods
 +summariseModule hsc_env home_unit old_summaries imps_cache imp maybe_buf excl_mods =
 +  summariseModuleDispatch k hsc_env imps_cache home_unit imp excl_mods
    where
      k = summariseModuleWithSource home_unit old_summaries (ui_boot imp) maybe_buf
@@ -1331,11 +1465,12 @@ summariseModule hsc_env home_unit old_summaries imp maybe_buf excl_mods =
  -- This version always returns a ModuleNodeFixed node.
  summariseModuleInterface :: HscEnv
                          -> HomeUnit
 +                        -> ImportsCache
                          -> UnresolvedImport PkgQual -- ^ The import being summarised
                          -> [ModuleName]
                          -> IO SummariseResult
 -summariseModuleInterface hsc_env home_unit imp excl_mods =
 -  summariseModuleDispatch k hsc_env home_unit imp excl_mods
 +summariseModuleInterface hsc_env home_unit imps_cache imp excl_mods =
 +  summariseModuleDispatch k hsc_env imps_cache home_unit imp excl_mods
    where
      k _hsc_env loc mod = do
        -- The finder will return a path to the .hi-boot even if it doesn't actually
@@ -1352,129 +1487,167 @@ summariseModuleInterface hsc_env home_unit imp excl_mods =
  summariseModuleDispatch
            :: (HscEnv -> ModLocation -> Module -> IO SummariseResult) -- ^ Continuation about how to summarise a home module.
            -> HscEnv
 +          -> ImportsCache
            -> HomeUnit
            -> UnresolvedImport PkgQual -- ^ The import being summarised
            -> [ModuleName]       -- Modules to exclude
            -> IO SummariseResult
 -summariseModuleDispatch k hsc_env' home_unit imp excl_mods
 -  | wanted_mod `elem` excl_mods
 +summariseModuleDispatch k hsc_env' imps_cache_ref home_unit imp excl_mods
 +  | unLoc wanted_mod `elem` excl_mods
    = return NotThere
    | otherwise  = find_it
    where
 -    wanted_mod = unLoc (ui_mod_name imp)
+-
      -- Temporarily change the currently active home unit so all operations
      -- happen relative to it
      hsc_env   = hscSetActiveHomeUnit home_unit hsc_env'
      find_it :: IO SummariseResult
      find_it = do
 -        found <- resolveImport hsc_env imp
 -        case found of
 -             Found location mod
 -                | moduleUnitId mod `Set.member` hsc_all_home_unit_ids hsc_env ->
 -                        -- Home package
 -                         k hsc_env location mod
 -                | VirtUnit iud <- moduleUnit mod
 -                , not (isHomeModule home_unit mod)
 -                  -> return $ FoundInstantiation iud
 -                | otherwise -> return $ External (moduleUnitId mod)
 -             _ -> return NotThere
 -                        -- Not found
 -                        -- (If it is TRULY not found at all, we'll
 -                        -- error when we actually try to compile)
+-
 +      imps_cache <- readIORef imps_cache_ref
 +      case M.lookup cache_key imps_cache of
 +        Just result -> return result
 +        Nothing -> do
 +          found <- resolveImport hsc_env imp
 +          r <- case found of
 +               Found location mod
 +                  | moduleUnitId mod `Set.member` hsc_all_home_unit_ids hsc_env ->
 +                          -- Home package
 +                           k hsc_env location mod
 +                  | VirtUnit iud <- moduleUnit mod
 +                  , not (isHomeModule home_unit mod)
 +                    -> return $ FoundInstantiation iud
 +                  | otherwise -> return $ External (moduleUnitId mod)
 +               _ -> return NotThere
 +                          -- Not found
 +                          -- (If it is TRULY not found at all, we'll
 +                          -- error when we actually try to compile)
 +          modifyImpsCache imps_cache_ref (M.insert cache_key r)
 +          return r
++
 +    UnresolvedImport { ui_pkg_qual = mb_pkg, ui_boot = is_boot
 +                     , ui_mod_name = wanted_mod } = imp
 +    cache_key = ( homeUnitId home_unit, mb_pkg
 +                , GWIB{ gwib_mod = unLoc wanted_mod, gwib_isBoot = is_boot })
  -- | The continuation to summarise a home module if we want to find the source file
  -- for it and potentially compile it.
  summariseModuleWithSource
            :: HomeUnit
 -          -> M.Map (UnitId, OsPath) ModSummary
 -          -- ^ Map of old summaries
 +          -> ModSummaryCache
 +          -- ^ Cache of constructed summaries
            -> IsBootInterface    -- True <=> a {-# SOURCE #-} import
            -> Maybe (StringBuffer, UTCTime)
            -> HscEnv
            -> ModLocation
            -> Module
            -> IO SummariseResult
 -summariseModuleWithSource home_unit old_summary_map is_boot maybe_buf hsc_env location mod = do
 -        -- Adjust location to point to the hs-boot source file,
 -        -- hi file, object file, when is_boot says so
 -        let src_fn = expectJust (ml_hs_file location)
+-
 -                -- Check that it exists
 -                -- It might have been deleted since the Finder last found it
 +summariseModuleWithSource home_unit summ_cache_ref is_boot maybe_buf hsc_env location mod = do
 +    -- Adjust location to point to the hs-boot source file,
 +    -- hi file, object file, when is_boot says so
 +    let src_fn = expectJust (ml_hs_file location)
 +    summ_cache <- readIORef summ_cache_ref
++
 +    -- Reject the cache result if the module name doesn't match the inferred
 +    -- module name based on the file name.
 +    -- See (W1) in Note [Downsweep Control Flow and Caching]
 +    let cached = do
 +          p   <- ml_hs_file_ospath location
 +          res <- M.lookup (moduleUnitId mod, p) summ_cache
 +          case res of
 +            Right (ms, _) | msKey ms /= moduleToMnk mod is_boot ->
 +              -- Module name doesn't match the file path name.
 +              -- We fall through to @new_summary@, where this will be
 +              -- discovered and the correct error message will be thrown.
 +              Nothing
 +            _ -> Just res
++
 +    case cached of
 +      Just (Right (chd_summary, SummFresh)) ->
 +        -- Fresh! just return it
 +        pure $ FoundHome (ModuleNodeCompile chd_summary)
++
 +      Just (Left err) ->
 +        -- Failure, don't try to summarise it again
 +        pure $ FoundHomeWithError (moduleUnitId mod, err)
++
 +      mb_old -> do
 +        -- Either Nothing or a potentially old summary, must check.
++
 +        -- Check that it exists
 +        -- It might have been deleted since the Finder last found it
          maybe_h <- fileHashIfExists src_fn
          case maybe_h of
            -- This situation can also happen if we have found the .hs file but the
            -- .hs-boot file doesn't exist.
            Nothing -> return NotThere
            Just h  -> do
 -            fresult <- new_summary_cache_check location mod src_fn h
 +            fresult <- case mb_old of
 +              Just (Right (old_summary, SummOld)) ->
 +                -- check the hash on the source file, and return the cached
 +                -- summary if it hasn't changed. If the file has changed then
 +                -- need to resummarise.
 +                case maybe_buf of
 +                  Just (buf,_) ->
 +                      checkSummaryHash hsc_env (new_summary location mod src_fn) old_summary location (fingerprintStringBuffer buf)
 +                  Nothing    ->
 +                      checkSummaryHash hsc_env (new_summary location mod src_fn) old_summary location h
 +              Nothing ->
 +                new_summary location mod src_fn h
              return $ case fresult of
                Left err -> FoundHomeWithError (moduleUnitId mod, err)
                Right ms -> FoundHome (ModuleNodeCompile ms)
+-
    where
      dflags    = hsc_dflags hsc_env
 -    new_summary_cache_check loc mod src_fn h
 -      | Just old_summary <- Map.lookup ((toUnitId (moduleUnit mod), src_fn_os)) old_summary_map =
+-
 -         -- check the hash on the source file, and
 -         -- return the cached summary if it hasn't changed.  If the
 -         -- file has changed then need to resummarise.
 -        case maybe_buf of
 -           Just (buf,_) ->
 -               checkSummaryHash hsc_env (new_summary loc mod src_fn) old_summary loc (fingerprintStringBuffer buf)
 -           Nothing    ->
 -               checkSummaryHash hsc_env (new_summary loc mod src_fn) old_summary loc h
 -      | otherwise = new_summary loc mod src_fn h
 -      where
 -        src_fn_os = unsafeEncodeUtf src_fn
+-
      new_summary :: ModLocation
                    -> Module
                    -> FilePath
                    -> Fingerprint
                    -> IO (Either DriverMessages ModSummary)
      new_summary location mod src_fn src_hash
 -      = runExceptT $ do
 -        preimps@PreprocessedImports {..}
 -            -- Remember to set the active unit here, otherwise the wrong include paths are passed to CPP
 -            -- See multiHomeUnits_cpp2 test
 -            <- getPreprocessedImports (hscSetActiveUnitId (moduleUnitId mod) hsc_env) src_fn Nothing maybe_buf
+-
 -        -- NB: Despite the fact that is_boot is a top-level parameter, we
 -        -- don't actually know coming into this function what the HscSource
 -        -- of the module in question is.  This is because we may be processing
 -        -- this module because another module in the graph imported it: in this
 -        -- case, we know if it's a boot or not because of the {-# SOURCE #-}
 -        -- annotation, but we don't know if it's a signature or a regular
 -        -- module until we actually look it up on the filesystem.
 -        let hsc_src
 -              | is_boot == IsBoot           = HsBootFile
 -              | isHaskellSigFilename src_fn = HsigFile
 -              | otherwise                   = HsSrcFile
+-
 -        when (pi_mod_name /= moduleName mod) $
 -                throwE $ singleMessage $ mkPlainErrorMsgEnvelope pi_mod_name_loc
 -                       $ DriverFileModuleNameMismatch pi_mod_name (moduleName mod)
+-
 -        let instantiations = homeUnitInstantiations home_unit
 -        when (hsc_src == HsigFile && isNothing (lookup pi_mod_name instantiations)) $
 -            throwE $ singleMessage $ mkPlainErrorMsgEnvelope pi_mod_name_loc
 -                   $ DriverUnexpectedSignature pi_mod_name (checkBuildingCabalPackage dflags) instantiations
+-
 -        liftIO $ makeNewModSummary hsc_env $ MakeNewModSummary
 -            { nms_src_fn = src_fn
 -            , nms_src_hash = src_hash
 -            , nms_hsc_src = hsc_src
 -            , nms_location = location
 -            , nms_mod = mod
 -            , nms_preimps = preimps
 -            }
 +      = do
 +        res <- runExceptT $ do
 +          preimps@PreprocessedImports {..}
 +              -- Remember to set the active unit here, otherwise the wrong include paths are passed to CPP
 +              -- See multiHomeUnits_cpp2 test
 +              <- getPreprocessedImports (hscSetActiveUnitId (moduleUnitId mod) hsc_env) src_fn Nothing maybe_buf
++
 +          -- NB: Despite the fact that is_boot is a top-level parameter, we
 +          -- don't actually know coming into this function what the HscSource
 +          -- of the module in question is.  This is because we may be processing
 +          -- this module because another module in the graph imported it: in this
 +          -- case, we know if it's a boot or not because of the {-# SOURCE #-}
 +          -- annotation, but we don't know if it's a signature or a regular
 +          -- module until we actually look it up on the filesystem.
 +          let hsc_src
 +                | is_boot == IsBoot           = HsBootFile
 +                | isHaskellSigFilename src_fn = HsigFile
 +                | otherwise                   = HsSrcFile
++
 +          when (pi_mod_name /= moduleName mod) $
 +                  throwE $ singleMessage $ mkPlainErrorMsgEnvelope pi_mod_name_loc
 +                         $ DriverFileModuleNameMismatch pi_mod_name (moduleName mod)
++
 +          let instantiations = homeUnitInstantiations home_unit
 +          when (hsc_src == HsigFile && isNothing (lookup pi_mod_name instantiations)) $
 +              throwE $ singleMessage $ mkPlainErrorMsgEnvelope pi_mod_name_loc
 +                     $ DriverUnexpectedSignature pi_mod_name (checkBuildingCabalPackage dflags) instantiations
++
 +          liftIO $ makeNewModSummary hsc_env $ MakeNewModSummary
 +              { nms_src_fn = src_fn
 +              , nms_src_hash = src_hash
 +              , nms_hsc_src = hsc_src
 +              , nms_location = location
 +              , nms_mod = mod
 +              , nms_preimps = preimps
 +              }
 +        modifySummCache summ_cache_ref $ case res of
 +          Left e -> case ml_hs_file_ospath location of
 +            Just p  -> M.insert (moduleUnitId mod, p) (Left e)
 +            Nothing -> id
 +          Right ms -> addModSummaryCache ms SummFresh
 +        return res
  -- | Convenience named arguments for 'makeNewModSummary' only used to make
  -- code more readable, not exported.
@@ -1497,7 +1670,6 @@ makeNewModSummary hsc_env MakeNewModSummary{..} = do
    hie_timestamp <- modificationTimeIfExists (ml_hie_file_ospath nms_location)
    bytecode_timestamp <- modificationTimeIfExists (ml_bytecode_file_ospath nms_location)
    extra_sig_imports <- findExtraSigImports hsc_env nms_hsc_src pi_mod_name
 -  (implicit_sigs, _inst_deps) <- implicitRequirementsShallow (hscSetActiveUnitId (moduleUnitId nms_mod) hsc_env) pi_imps
    return $
          ModSummary
@@ -1510,7 +1682,6 @@ makeNewModSummary hsc_env MakeNewModSummary{..} = do
          , ms_parsed_mod = Nothing
          , ms_textual_imps =
              (generatedImport FromBackpackSig . noLoc <$> extra_sig_imports) ++
 -            (generatedImport FromBackpackSig . noLoc <$> implicit_sigs) ++
              pi_imps
          , ms_hs_hash = nms_src_hash
          , ms_iface_date = hi_timestamp
@@ -1549,3 +1720,160 @@ getPreprocessedImports hsc_env src_fn mb_phase maybe_buf = do
            return (first (mkMessages . fmap mkDriverPsHeaderMessage . getMessages) mimps)
    let pi_imps = map (rnUnresolvedImportPkgQual (renameRawPkgQual (hsc_unit_env hsc_env))) pi_imps'
    return PreprocessedImports {..}
++
 +--------------------------------------------------------------------------------
 +-- * Generic traversal of iteratively-built graph: dfsBuild
 +--------------------------------------------------------------------------------
++
 +-- | The result of expanding a node in 'dfsBuild'.
 +data NodeRes v
 +  -- | Computed the node payload successfully
 +  = NSuccess v
 +  -- | Skip a node! This means this node doesn't produce a payload and we can
 +  -- just ignore it if we ever come across it.
 +  --
 +  -- In practice, this might happen because of an error or maybe from an
 +  -- attempt to expand e.g. an hs-boot node just to see if it sticks, but we
 +  -- don't distinguish these uses. Skip just means ignore this node and don't
 +  -- abort.
 +  | NSkip
++
 +-- | In a depth-first order, and starting from the given roots, traverse a
 +-- graph by iteratively expanding a node into a payload and a list of children
 +-- nodes to visit next.
 +--
 +-- A node is NEVER visited/expanded more than once, as long as the node key
 +-- @k@, computed from the node @n@, uniquely identifies that node.
 +--
 +-- The first argument @base_map@ is the starting set of already visited nodes
 +-- (these nodes won't be expanded again!).
 +--
 +-- The result is a mapping from the key of every node transitively reachable
 +-- from the root nodes (inclusively) to the payload returned by expanding that
 +-- node. The result includes the previously visited nodes given in @base_map@,
 +-- s.t. @dfsBuild base_map [] _ _ == base_map@.
 +--
 +-- The @expand@ function returns an 'NResult'. See the 'NResult' documentation
 +-- for more information about each result type.
 +--
 +-- Error handling and exiting early can be achieved by selecting a @Monad m@
 +-- accordingly, such as @Control.Monad.Except.Except@
 +--
 +-- Example usage: @n@ is instanced to @DownsweepNode@, @k@ is @NodeKey@, and @v@ is @ModuleNodeEdge@.
 +--
 +-- See also Note [Downsweep Control Flow and Caching]
 +dfsBuild :: (Ord k, Monad m)
 +         => Maybe (Map.Map k (NodeRes v))
 +         -- ^ Base map, existing results. We won't re-expand any of the nodes
 +         -- already present in this map.
 +         -> [n]
 +         -- ^ The root nodes from where to start traversal
 +         -> (n -> k)
 +         -- ^ Compute the key which uniquely identifies this node
 +         -> (n -> m (NodeRes (v,[n])))
 +         -- ^ Expand this node into its payload result and into the list of
 +         -- children nodes to visit next.
 +         -> m (Map.Map k (NodeRes v))
 +         -- ^ The result accumulates the payload of expanding the root nodes
 +         -- and all nodes transitively reachable from those roots.
 +dfsBuild base_map roots key expand = go roots (fromMaybe Map.empty base_map)
 +  where
 +    go []     visited = pure visited
 +    go (s:ss) visited
 +      | k `Map.member` visited
 +      = go ss visited
 +      | otherwise
 +      = do r <- expand s
 +           case r of
 +             NSkip ->
 +               go ss
 +                  (Map.insert k NSkip        visited) -- Skip!
 +             NSuccess (v,ns) ->
 +               go (ns ++ ss)
 +                  (Map.insert k (NSuccess v) visited)
 +      where
 +        k = key s
++
 +{-
 +Note [Downsweep Control Flow and Caching]
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 +The control flow of downsweep is extracted into a single function `dfsBuild`,
 +which takes care of iteratively expanding and traversing all nodes of the
 +in-construction module graph necessary to build a full `ModuleGraph` at the
 +end.
++
 +There are three levels of caching going on, all of which are necessary to make
 +sure we don't do repeated work (notably, we NEVER summarise the same module
 +twice).
++
 +1. `dfsBuild` accumulates the final module graph and never revisits the
 +   same node of the module graph. Cache is keyed by the final
 +   `ModuleGraph`s `NodeKey`s.
++
 +    For example, suppose
++
 +       A imports B and C
 +       B imports D
 +       C imports D
++
 +    Then, starting from A we will expand A and push B and C to the worklist;
 +    then, going back to B, we expand B which pushes D to the worklist. After
 +    processing D, we go to C, which imports D, but we have already visited that
 +    module so we can just use the already-constructed `ModuleGraphNode` for D.
++
 +2. For Module A in home-unit u1, each import in the list of imports
 +   needs to be *found* (call to `findImportedModuleWithIsBoot`): at this
 +   point, we only have the `ModuleName` of the import, not the `Module`.
 +   This *finding* is somewhat expensive, so we cache it as well
 +   (`ImportsCache`). The cache key is the home-unit to which the module
 +   belongs~[1], the import package qualifier, and the ModuleName.
++
 +   Same example, suppose
++
 +      A imports B and C
 +      B imports D
 +      C imports D
++
 +   When expanding B, we will findImportedModule "import D".
 +   When expanding C, we would findImportedModule "import D", but we can just
 +   look it up in the cache
++
 +   [1] Different home-units will have different package flags, which means
 +   potentially different `Module` resolution for the same `ModuleName`.
++
 +3. The most expensive operation we want to avoid is summarising a
 +   `Module` into a `ModSummary`, which notably involves parsing the
 +   module header from scratch.
 +   The third cache, in essence, maps a `Module` to its `ModSummary`
 +   (named `ModSummaryCache`). This cache upholds the invariant: we NEVER
 +   summarise the same module twice. In practice, the cache key is the
 +   Module's UnitId and the Source path; the reason is we need to
 +   distinguish between `.hs` and `.hs-boot` files, as their summaries
 +   will differ.
++
 +   Note that this covers more than just (1), because we summarise all imports
 +   of a single module when expanding it (see 'expandModuleSummary'), before
 +   returning from the expansion function.
++
 +   Note that (2) can't guarantee this alone: Two ModuleName imports in
 +   separate units can (and likely do) map to the same `Module`.
++
 +(W1)
 +   In `summariseModuleWithSource`, on a cache hit, we must check if the module
 +   name matches the file name, because the cache might have been populated by
 +   `summariseFile`:
++
 +   - `summariseFile` is used for summarising file targets, where
 +     the file name needn't match the module name: e.g., the `Main` module is
 +     sometimes not defined in a file named `Main.hs`.
++
 +   - `summariseModuleWithSource` is used for summarising module targets, like
 +     an `import Bar`, where `Bar.hs` must contain `module Bar where`
 +     specifically (since we will later look for .hi files based on the module
 +     name).
++
 +   See tests T27461a and T27461b.
++
 +See also Note [Downsweep: building and maintaining the module graph] and
 +Note [The ModuleGraph].
 +-}

compiler/GHC/Driver/Env.hs

@@ -286,7 +286,7 @@ hugSomeThingsBelowUs :: (HomeModInfo -> [a]) -> Bool -> HscEnv -> UnitId -> Modu
  -- These things are currently stored in the EPS for home packages. (See #25795 for
  -- progress in removing these kind of checks; and making these functions of
  -- `UnitEnv` rather than `HscEnv`)
 --- See Note [Downsweep and the ModuleGraph]
 +-- See Note [The ModuleGraph]
  hugSomeThingsBelowUs _ _ hsc_env _ _ | isOneShot (ghcMode (hsc_dflags hsc_env)) = return []
  hugSomeThingsBelowUs extract include_hi_boot hsc_env uid mn
    = let hug = hsc_HUG hsc_env

compiler/GHC/Tc/Utils/Backpack.hs

@@ -292,28 +292,28 @@ implicitRequirements hsc_env normal_imports
    where
      mhome_unit = hsc_home_unit_maybe hsc_env
 --- | Like @implicitRequirements'@, but returns either the module name, if it is
 --- a free hole, or the instantiated unit the imported module is from, so that
 --- that instantiated unit can be processed and via the batch mod graph (rather
 --- than a transitive closure done here) all the free holes are still reachable.
 +-- | Like @implicitRequirements'@, but returns the instantiated unit the
 +-- imported module is from, so that that instantiated unit can be processed and
 +-- via the batch mod graph (rather than a transitive closure done here) all the
 +-- free holes are still reachable.
  implicitRequirementsShallow
    :: HscEnv
    -> [UnresolvedImport PkgQual]
 -  -> IO ([ModuleName], [InstantiatedUnit])
 -implicitRequirementsShallow hsc_env normal_imports = go ([], []) normal_imports
 +  -> IO [InstantiatedUnit]
 +implicitRequirementsShallow hsc_env normal_imports = go [] normal_imports
   where
    mhome_unit = hsc_home_unit_maybe hsc_env
    go acc [] = pure acc
 -  go (accL, accR) (e:imports) = do
 +  go accR (e:imports) = do
      found <- resolveImport hsc_env e
      let acc' = case found of
            Found _ mod | notHomeModuleMaybe mhome_unit mod ->
                case moduleUnit mod of
 -                  HoleUnit -> (moduleName mod : accL, accR)
 -                  RealUnit _ -> (accL, accR)
 -                  VirtUnit u -> (accL, u:accR)
 -          _ -> (accL, accR)
 +                  HoleUnit -> panic "implicitRequirementsShallow: HoleUnit is unreachable through findImportedModule!"
 +                  RealUnit _ -> accR
 +                  VirtUnit u -> u:accR
 +          _ -> accR
      go acc' imports
  -- | Given a 'Unit', make sure it is well typed.  This is because

compiler/GHC/Unit/Env.hs

@@ -167,7 +167,7 @@ data UnitEnv = UnitEnv
      , ue_module_graph    :: ModuleGraph
          -- ^ The module graph of the current session
 -        -- See Note [Downsweep and the ModuleGraph] for when this is constructed.
 +        -- See Note [The ModuleGraph] for when this is constructed.
      , ue_home_unit_graph :: !HomeUnitGraph
          -- See Note [Multiple Home Units]

hadrian/hie-bios.bat

testsuite/tests/driver/T27461/Main1.hs

 +module Main where
++
 +import Bar () -- resolves to src/Bar.hs, which declares module Foo
++
 +main :: IO ()
 +main = return ()

testsuite/tests/driver/T27461/Main2.hs

 +module Main where
++
 +main :: IO ()
 +main = return ()

testsuite/tests/driver/T27461/Makefile

 +TOP=../../..
 +include $(TOP)/mk/boilerplate.mk
 +include $(TOP)/mk/test.mk
++
 +# src/Bar.hs declares module Foo, which is fine for a file target, but Main's
 +# `import Bar` resolves to that same file and must be rejected.
 +T27461a :
 +	cp Main1.hs src/Main.hs
 +	! '$(TEST_HC)' $(TEST_HC_OPTS) --make -fno-code -v0 -isrc src/Main.hs src/Bar.hs

testsuite/tests/driver/T27461/T27461a.stderr

 +src/Bar.hs:1:8: error: [GHC-28623]
 +    File name does not match module name:
 +    Saw     : ‘Foo’
 +    Expected: ‘Bar’

testsuite/tests/driver/T27461/T27461b.script

 +"-- Successfully load modules if file target is not imported"
 +:! cp Main2.hs src/Main.hs
 +:load src/Main.hs src/Bar.hs
 +main
 +:! cp Main1.hs src/Main.hs
 +"-- Crash on reload as we import a file target that has the wrong module name"
 +:reload

testsuite/tests/driver/T27461/T27461b.stderr

 +src/Bar.hs:1:8: error: [GHC-28623]
 +    File name does not match module name:
 +    Saw     : ‘Foo’
 +    Expected: ‘Bar’
++

testsuite/tests/driver/T27461/T27461b.stdout

	1	+"-- Successfully load modules if file target is not imported"
	2	+"-- Crash on reload as we import a file target that has the wrong module name"

testsuite/tests/driver/T27461/all.T

 +test('T27461a', extra_files(['src/', 'Main1.hs']), makefile_test, [])
 +test('T27461b', [extra_files(['src/', 'Main1.hs', 'Main2.hs']), extra_hc_opts('-isrc')],
 +     ghci_script, ['T27461b.script'])

testsuite/tests/driver/T27461/src/Bar.hs

 +module Foo where
 +-- Named Bar.hs but declares module Foo: allowed for a file target.
++
 +foo :: Int
 +foo = 1

testsuite/tests/ghc-api/fixed-nodes/FixedNodes.hs

@@ -24,6 +24,7 @@ import Control.Monad.Catch (handle, throwM)
  import Control.Exception.Context
  import GHC.Driver.MakeFile
  import GHC.Utils.Outputable
 +import Data.IORef (newIORef)
  -- | Convert a ModuleNodeCompile to a ModuleNodeFixed
  convertToFixed :: ModuleNodeInfo -> ModuleNodeInfo
  convertToFixed (ModuleNodeCompile ms) =
@@ -151,5 +152,6 @@ main = do
          getModSummaryFromTarget :: FilePath -> Ghc ModSummary
          getModSummaryFromTarget file = do
            hsc_env <- getSession
 -          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) mempty file Nothing Nothing
 +          summ_cache <- liftIO $ newIORef mempty
 +          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) summ_cache file Nothing Nothing
            return ms

testsuite/tests/ghc-api/fixed-nodes/InterfaceModuleGraph.hs

@@ -16,6 +16,7 @@ import GHC.Types.SourceFile
  import System.Environment
  import Control.Monad (void, when)
  import Data.Maybe (fromJust)
 +import Data.IORef (newIORef)
  import Control.Exception (ExceptionWithContext(..), SomeException)
  import Control.Monad.Catch (handle, throwM)
  import Control.Exception.Context
@@ -67,7 +68,9 @@ main = do
            keyC = msKey msC
        let mkGraph s = do
 -            ([], nodes) <- downsweepFromRootNodes hsc_env mempty Nothing [] True DownsweepUseFixed s []
 +            summ_cache <- newIORef mempty
 +            imps_cache <- newIORef mempty
 +            ([], nodes) <- downsweepFromRootNodes hsc_env summ_cache imps_cache Nothing [] True DownsweepUseFixed s []
              return $ mkModuleGraph nodes
        graph <- liftIO $ mkGraph [ModuleNodeCompile msC]
@@ -98,5 +101,6 @@ main = do
          getModSummaryFromTarget :: FilePath -> Ghc ModSummary
          getModSummaryFromTarget file = do
            hsc_env <- getSession
 -          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) mempty file Nothing Nothing
 +          summ_cache <- liftIO $ newIORef mempty
 +          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) summ_cache file Nothing Nothing
            return ms

testsuite/tests/ghc-api/fixed-nodes/ModuleGraphInvariants.hs

@@ -23,6 +23,7 @@ import Control.Monad.Catch (handle, throwM)
  import Control.Exception.Context
  import GHC.Utils.Outputable
  import Data.List
 +import Data.IORef (newIORef)
  -- | Convert a ModuleNodeCompile to a ModuleNodeFixed
  convertToFixed :: ModuleNodeInfo -> ModuleNodeInfo
@@ -132,5 +133,6 @@ main = do
          getModSummaryFromTarget :: FilePath -> Ghc ModSummary
          getModSummaryFromTarget file = do
            hsc_env <- getSession
 -          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) mempty file Nothing Nothing
 +          summ_cache <- liftIO $ newIORef mempty
 +          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) summ_cache file Nothing Nothing
            return ms

testsuite/tests/splice-imports/SI35.hs

@@ -28,6 +28,7 @@ import GHC.Unit.Module.Stage
  import GHC.Data.Graph.Directed.Reachability
  import GHC.Utils.Trace
  import GHC.Unit.Module.Graph
 +import Data.IORef (newIORef)
  main :: IO ()
  main = do
@@ -75,5 +76,6 @@ main = do
          getModSummaryFromTarget :: FilePath -> Ghc ModSummary
          getModSummaryFromTarget file = do
            hsc_env <- getSession
 -          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) mempty file Nothing Nothing
 +          summ_cache <- liftIO $ newIORef mempty
 +          Right ms <- liftIO $ summariseFile hsc_env (DefiniteHomeUnit mainUnitId Nothing) summ_cache file Nothing Nothing
            return ms
 \ No newline at end of file

utils/check-ppr/Main.hs

@@ -18,6 +18,7 @@ import System.Environment( getArgs )
  import System.Exit
  import System.FilePath
  import System.IO
 +import Data.IORef
  usage :: String
  usage = unlines
@@ -85,7 +86,8 @@ parseOneFile libdir fileName = do
           let dflags2 = dflags `gopt_set` Opt_KeepRawTokenStream
           _ <- setSessionDynFlags dflags2
           hsc_env <- getSession
 -         mms <- liftIO $ summariseFile hsc_env (hsc_home_unit hsc_env) mempty fileName Nothing Nothing
 +         cache <- liftIO $ newIORef mempty
 +         mms <- liftIO $ summariseFile hsc_env (hsc_home_unit hsc_env) cache fileName Nothing Nothing
           case mms of
             Left _err -> error "parseOneFile"
             Right ms -> parseModule ms