Refactoring module dependencies

* Make mkDependencies pure
* Use Sets instead of sorted lists

Notable perf changes:

      MultiLayerModules(normal) ghc/alloc  4130851520.0  2981473072.0 -27.8%
                 T13719(normal) ghc/alloc  4313296052.0  4151647512.0  -3.7%

Metric Decrease:
    MultiLayerModules
    T13719

3 files changed, 22 insertions, 6 deletions

diff --git a/compiler/GHC/Unit/Module/Deps.hs b/compiler/GHC/Unit/Module/Deps.hs
index 9832383d8a..ebdd4b351f 100644
--- a/compiler/GHC/Unit/Module/Deps.hs
+++ b/compiler/GHC/Unit/Module/Deps.hs
@@ -16,6 +16,9 @@ import GHC.Unit.Module
 import GHC.Utils.Fingerprint
 import GHC.Utils.Binary
 
+import Data.Set (Set)
+import qualified Data.Set as Set
+
 -- | Dependency information about ALL modules and packages below this one
 -- in the import hierarchy. This is the serialisable version of `ImportAvails`.
 --
@@ -25,10 +28,10 @@ import GHC.Utils.Binary
 --
 -- See Note [Transitive Information in Dependencies]
 data Dependencies = Deps
-   { dep_direct_mods :: [ModuleNameWithIsBoot]
+   { dep_direct_mods :: Set ModuleNameWithIsBoot
       -- ^ All home-package modules which are directly imported by this one.
 
-   , dep_direct_pkgs :: [UnitId]
+   , dep_direct_pkgs :: Set UnitId
       -- ^ All packages directly imported by this module
       -- I.e. packages to which this module's direct imports belong.
       --
@@ -38,12 +41,12 @@ data Dependencies = Deps
     -- ^ Transitive closure of hsig files in the home package
 
 
-   , dep_trusted_pkgs :: [UnitId]
+   , dep_trusted_pkgs :: Set UnitId
       -- Packages which we are required to trust
       -- when the module is imported as a safe import
       -- (Safe Haskell). See Note [Tracking Trust Transitively] in GHC.Rename.Names
 
-   , dep_boot_mods :: [ModuleNameWithIsBoot]
+   , dep_boot_mods :: Set ModuleNameWithIsBoot
       -- ^ All modules which have boot files below this one, and whether we
       -- should use the boot file or not.
       -- This information is only used to populate the eps_is_boot field.
@@ -96,7 +99,15 @@ instance Binary Dependencies where
                                dep_finsts = fis })
 
 noDependencies :: Dependencies
-noDependencies = Deps [] [] [] [] [] [] []
+noDependencies = Deps
+  { dep_direct_mods  = Set.empty
+  , dep_direct_pkgs  = Set.empty
+  , dep_sig_mods     = []
+  , dep_boot_mods    = Set.empty
+  , dep_trusted_pkgs = Set.empty
+  , dep_orphs        = []
+  , dep_finsts       = []
+  }
 
 -- | Records modules for which changes may force recompilation of this module
 -- See wiki: https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/recompilation-avoidance
diff --git a/compiler/GHC/Unit/Module/ModGuts.hs b/compiler/GHC/Unit/Module/ModGuts.hs
index e799ebf2a1..1558e5944a 100644
--- a/compiler/GHC/Unit/Module/ModGuts.hs
+++ b/compiler/GHC/Unit/Module/ModGuts.hs
@@ -36,6 +36,8 @@ import GHC.Types.SourceFile ( HscSource(..), hscSourceToIsBoot )
 import GHC.Types.SrcLoc
 import GHC.Types.CostCentre
 
+import Data.Set (Set)
+
 
 -- | A ModGuts is carried through the compiler, accumulating stuff as it goes
 -- There is only one ModGuts at any time, the one for the module
@@ -135,7 +137,7 @@ data CgGuts
         cg_ccs       :: [CostCentre], -- List of cost centres used in bindings and rules
         cg_foreign   :: !ForeignStubs,   -- ^ Foreign export stubs
         cg_foreign_files :: ![(ForeignSrcLang, FilePath)],
-        cg_dep_pkgs  :: ![UnitId], -- ^ Dependent packages, used to
+        cg_dep_pkgs  :: !(Set UnitId),      -- ^ Dependent packages, used to
                                             -- generate #includes for C code gen
         cg_hpc_info  :: !HpcInfo,           -- ^ Program coverage tick box information
         cg_modBreaks :: !(Maybe ModBreaks), -- ^ Module breakpoints
diff --git a/compiler/GHC/Unit/Types.hs b/compiler/GHC/Unit/Types.hs
index 5dca26a90f..890e92b008 100644
--- a/compiler/GHC/Unit/Types.hs
+++ b/compiler/GHC/Unit/Types.hs
@@ -660,6 +660,9 @@ data GenWithIsBoot mod = GWIB
   } deriving ( Eq, Ord, Show
              , Functor, Foldable, Traversable
              )
+  -- the Ord instance must ensure that we first sort by Module and then by
+  -- IsBootInterface: this is assumed to perform filtering of non-boot modules,
+  -- e.g. in GHC.Driver.Env.hptModulesBelow
 
 type ModuleNameWithIsBoot = GenWithIsBoot ModuleName