Increase type sharingwip/T20541

Fixes #20541 by making mkTyConApp do more sharing of types.
In particular, replace

* BoxedRep Lifted    ==>  LiftedRep
* BoxedRep Unlifted  ==>  UnliftedRep
* TupleRep '[]       ==>  ZeroBitRep
* TYPE ZeroBitRep    ==>  ZeroBitType

In each case, the thing on the right is a type synonym
for the thing on the left, declared in ghc-prim:GHC.Types.
See Note [Using synonyms to compress types] in GHC.Core.Type.

The synonyms for ZeroBitRep and ZeroBitType are new, but absolutely
in the same spirit as the other ones.   (These synonyms are mainly
for internal use, though the programmer can use them too.)

I also renamed GHC.Core.Ty.Rep.isVoidTy to isZeroBitTy, to be
compatible with the "zero-bit" nomenclature above.  See discussion
on !6806.

There is a tricky wrinkle: see GHC.Core.Types
  Note [Care using synonyms to compress types]

Compiler allocation decreases by up to 0.8%.

1 files changed, 4 insertions, 4 deletions

diff --git a/compiler/GHC/StgToCmm/Closure.hs b/compiler/GHC/StgToCmm/Closure.hs
index 3ff745a719..8b9e4f044b 100644
--- a/compiler/GHC/StgToCmm/Closure.hs
+++ b/compiler/GHC/StgToCmm/Closure.hs
@@ -151,23 +151,23 @@ instance (Outputable a) => Outputable (NonVoid a) where
   ppr (NonVoid a) = ppr a
 
 nonVoidIds :: [Id] -> [NonVoid Id]
-nonVoidIds ids = [NonVoid id | id <- ids, not (isVoidTy (idType id))]
+nonVoidIds ids = [NonVoid id | id <- ids, not (isZeroBitTy (idType id))]
 
 -- | Used in places where some invariant ensures that all these Ids are
 -- non-void; e.g. constructor field binders in case expressions.
 -- See Note [Post-unarisation invariants] in "GHC.Stg.Unarise".
 assertNonVoidIds :: [Id] -> [NonVoid Id]
-assertNonVoidIds ids = assert (not (any (isVoidTy . idType) ids)) $
+assertNonVoidIds ids = assert (not (any (isZeroBitTy . idType) ids)) $
                        coerce ids
 
 nonVoidStgArgs :: [StgArg] -> [NonVoid StgArg]
-nonVoidStgArgs args = [NonVoid arg | arg <- args, not (isVoidTy (stgArgType arg))]
+nonVoidStgArgs args = [NonVoid arg | arg <- args, not (isZeroBitTy (stgArgType arg))]
 
 -- | Used in places where some invariant ensures that all these arguments are
 -- non-void; e.g. constructor arguments.
 -- See Note [Post-unarisation invariants] in "GHC.Stg.Unarise".
 assertNonVoidStgArgs :: [StgArg] -> [NonVoid StgArg]
-assertNonVoidStgArgs args = assert (not (any (isVoidTy . stgArgType) args)) $
+assertNonVoidStgArgs args = assert (not (any (isZeroBitTy . stgArgType) args)) $
                             coerce args