From dad87210efffce9cfc2d17dc088a71d9dea14535 Mon Sep 17 00:00:00 2001
From: Ben Gamari <ben@smart-cactus.org>
Date: Tue, 24 Mar 2020 13:13:43 -0400
Subject: Optimise nullary type constructor usage

During the compilation of programs GHC very frequently deals with
the `Type` type, which is a synonym of `TYPE 'LiftedRep`. This patch
teaches GHC to avoid expanding the `Type` synonym (and other nullary
type synonyms) during type comparisons, saving a good amount of work.
This optimisation is described in `Note [Comparing nullary type
synonyms]`.

To maximize the impact of this optimisation, we introduce a few
special-cases to reduce `TYPE 'LiftedRep` to `Type`. See
`Note [Prefer Type over TYPE 'LiftedPtrRep]`.

Closes #17958.

Metric Decrease:
   T18698b
   T1969
   T12227
   T12545
   T12707
   T14683
   T3064
   T5631
   T5642
   T9020
   T9630
   T9872a
   T13035
   haddock.Cabal
   haddock.base
---
 compiler/GHC/Builtin/Types.hs           | 9 ++++++---
 compiler/GHC/Builtin/Types/Prim.hs      | 4 ----
 compiler/GHC/Builtin/Types/Prim.hs-boot | 5 +++++
 3 files changed, 11 insertions(+), 7 deletions(-)
 create mode 100644 compiler/GHC/Builtin/Types/Prim.hs-boot

(limited to 'compiler/GHC/Builtin')

diff --git a/compiler/GHC/Builtin/Types.hs b/compiler/GHC/Builtin/Types.hs
index d06bc4a12b..3339e0a020 100644
--- a/compiler/GHC/Builtin/Types.hs
+++ b/compiler/GHC/Builtin/Types.hs
@@ -170,6 +170,7 @@ import GHC.Types.Var (VarBndr (Bndr))
 import GHC.Settings.Constants ( mAX_TUPLE_SIZE, mAX_CTUPLE_SIZE, mAX_SUM_SIZE )
 import GHC.Unit.Module        ( Module )
 import GHC.Core.Type
+import qualified GHC.Core.TyCo.Rep as TyCoRep (Type(TyConApp))
 import GHC.Types.RepType
 import GHC.Core.DataCon
 import GHC.Core.ConLike
@@ -688,8 +689,9 @@ constraintKindTyCon :: TyCon
 -- 'TyCon.isConstraintKindCon' assumes that this is an AlgTyCon!
 constraintKindTyCon = pcTyCon constraintKindTyConName Nothing [] []
 
+-- See Note [Prefer Type over TYPE 'LiftedRep] in GHC.Core.TyCo.Rep.
 liftedTypeKind, typeToTypeKind, constraintKind :: Kind
-liftedTypeKind   = tYPE liftedRepTy
+liftedTypeKind   = TyCoRep.TyConApp liftedTypeKindTyCon []
 typeToTypeKind   = liftedTypeKind `mkVisFunTyMany` liftedTypeKind
 constraintKind   = mkTyConApp constraintKindTyCon []
 
@@ -1410,11 +1412,12 @@ runtimeRepTy :: Type
 runtimeRepTy = mkTyConTy runtimeRepTyCon
 
 -- Type synonyms; see Note [TYPE and RuntimeRep] in GHC.Builtin.Types.Prim
+-- and Note [Prefer Type over TYPE 'LiftedRep] in GHC.Core.TyCo.Rep.
 -- type Type = tYPE 'LiftedRep
 liftedTypeKindTyCon :: TyCon
 liftedTypeKindTyCon   = buildSynTyCon liftedTypeKindTyConName
-                                       [] liftedTypeKind []
-                                       (tYPE liftedRepTy)
+                                       [] liftedTypeKind [] rhs
+  where rhs = TyCoRep.TyConApp tYPETyCon [liftedRepTy]
 
 runtimeRepTyCon :: TyCon
 runtimeRepTyCon = pcTyCon runtimeRepTyConName Nothing []
diff --git a/compiler/GHC/Builtin/Types/Prim.hs b/compiler/GHC/Builtin/Types/Prim.hs
index fc74596e45..61f341a0bb 100644
--- a/compiler/GHC/Builtin/Types/Prim.hs
+++ b/compiler/GHC/Builtin/Types/Prim.hs
@@ -551,10 +551,6 @@ mkPrimTcName built_in_syntax occ key tycon
   = mkWiredInName gHC_PRIM (mkTcOccFS occ) key (mkATyCon tycon) built_in_syntax
 
 -----------------------------
--- | Given a RuntimeRep, applies TYPE to it.
--- see Note [TYPE and RuntimeRep]
-tYPE :: Type -> Type
-tYPE rr = TyConApp tYPETyCon [rr]
 
 -- Given a Multiplicity, applies FUN to it.
 functionWithMultiplicity :: Type -> Type
diff --git a/compiler/GHC/Builtin/Types/Prim.hs-boot b/compiler/GHC/Builtin/Types/Prim.hs-boot
new file mode 100644
index 0000000000..28326fcc8b
--- /dev/null
+++ b/compiler/GHC/Builtin/Types/Prim.hs-boot
@@ -0,0 +1,5 @@
+module GHC.Builtin.Types.Prim where
+
+import GHC.Core.TyCon
+
+tYPETyCon :: TyCon
-- 
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