From 84f0448c56b4a8b37e34fc1ae6faca20c7c1729c Mon Sep 17 00:00:00 2001
From: Sebastian Graf <sebastian.graf@kit.edu>
Date: Wed, 8 Apr 2020 17:26:55 +0200
Subject: Special case `isConstraintKindCon` on `AlgTyCon`

Previously, the `tyConUnique` record selector would unfold into a huge
case expression that would be inlined in all call sites, such as the
`INLINE`-annotated `coreView`, see #18026. `constraintKindTyConKey` only
occurs as the `Unique` of an `AlgTyCon` anyway, so we can make the code
a lot more compact, but have to move it to GHC.Core.TyCon.

Metric Decrease:
    T12150
    T12234
---
 compiler/GHC/Core/TyCon.hs     | 11 ++++++++++-
 compiler/GHC/Core/Type.hs      |  3 ---
 compiler/prelude/TysWiredIn.hs |  1 +
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs
index 64782e02b4..d28d8b0f0c 100644
--- a/compiler/GHC/Core/TyCon.hs
+++ b/compiler/GHC/Core/TyCon.hs
@@ -45,7 +45,7 @@ module GHC.Core.TyCon(
         noTcTyConScopedTyVars,
 
         -- ** Predicates on TyCons
-        isAlgTyCon, isVanillaAlgTyCon,
+        isAlgTyCon, isVanillaAlgTyCon, isConstraintKindCon,
         isClassTyCon, isFamInstTyCon,
         isFunTyCon,
         isPrimTyCon,
@@ -1868,6 +1868,15 @@ isVanillaAlgTyCon :: TyCon -> Bool
 isVanillaAlgTyCon (AlgTyCon { algTcParent = VanillaAlgTyCon _ }) = True
 isVanillaAlgTyCon _                                              = False
 
+-- | Returns @True@ for the 'TyCon' of the 'Constraint' kind.
+isConstraintKindCon :: TyCon -> Bool
+-- NB: We intentionally match on AlgTyCon, because 'constraintKindTyCon' is
+-- always an AlgTyCon (see 'pcTyCon' in TysWiredIn) and the record selector
+-- for 'tyConUnique' would generate unreachable code for every other data
+-- constructor of TyCon (see #18026).
+isConstraintKindCon AlgTyCon { tyConUnique = u } = u == constraintKindTyConKey
+isConstraintKindCon _                            = False
+
 isDataTyCon :: TyCon -> Bool
 -- ^ Returns @True@ for data types that are /definitely/ represented by
 -- heap-allocated constructors.  These are scrutinised by Core-level
diff --git a/compiler/GHC/Core/Type.hs b/compiler/GHC/Core/Type.hs
index 7e7a72fe94..a218e7c7b5 100644
--- a/compiler/GHC/Core/Type.hs
+++ b/compiler/GHC/Core/Type.hs
@@ -2924,9 +2924,6 @@ distinct uniques, they are treated as equal at all times except
 during type inference.
 -}
 
-isConstraintKindCon :: TyCon -> Bool
-isConstraintKindCon tc = tyConUnique tc == constraintKindTyConKey
-
 -- | Tests whether the given kind (which should look like @TYPE x@)
 -- is something other than a constructor tree (that is, constructors at every node).
 -- E.g.  True of   TYPE k, TYPE (F Int)
diff --git a/compiler/prelude/TysWiredIn.hs b/compiler/prelude/TysWiredIn.hs
index 4bf7ad6642..682c9d7d8a 100644
--- a/compiler/prelude/TysWiredIn.hs
+++ b/compiler/prelude/TysWiredIn.hs
@@ -639,6 +639,7 @@ typeNatKind    = mkTyConTy typeNatKindCon
 typeSymbolKind = mkTyConTy typeSymbolKindCon
 
 constraintKindTyCon :: TyCon
+-- 'TyCon.isConstraintKindCon' assumes that this is an AlgTyCon!
 constraintKindTyCon = pcTyCon constraintKindTyConName Nothing [] []
 
 liftedTypeKind, typeToTypeKind, constraintKind :: Kind
-- 
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