Various performance improvements

This implements several general performance improvements to GHC,
to offset the effect of the linear types change.

General optimisations:
- Add a `coreFullView` function which iterates `coreView` on the
  head. This avoids making function recursive solely because the
  iterate `coreView` themselves. As a consequence, this functions can
  be inlined, and trigger case-of-known constructor (_e.g._
  `kindRep_maybe`, `isLiftedRuntimeRep`, `isMultiplicityTy`,
  `getTyVar_maybe`, `splitAppTy_maybe`, `splitFunType_maybe`,
  `tyConAppTyCon_maybe`). The common pattern about all these functions
  is that they are almost always used as views, and immediately
  consumed by a case expression. This commit also mark them asx `INLINE`.
- In `subst_ty` add a special case for nullary `TyConApp`, which avoid
  allocations altogether.
- Use `mkTyConApp` in `subst_ty` for the general `TyConApp`. This
  required quite a bit of module shuffling.
  case. `myTyConApp` enforces crucial sharing, which was lost during
  substitution. See also !2952 .
- Make `subst_ty` stricter.
- In `eqType` (specifically, in `nonDetCmpType`), add a special case,
  tested first, for the very common case of nullary `TyConApp`.
  `nonDetCmpType` has been made `INLINE` otherwise it is actually a
  regression. This is similar to the optimisations in !2952.

Linear-type specific optimisations:
- Use `tyConAppTyCon_maybe` instead of the more complex `eqType` in
  the definition of the pattern synonyms `One` and `Many`.
- Break the `hs-boot` cycles between `Multiplicity.hs` and `Type.hs`:
  `Multiplicity` now import `Type` normally, rather than from the
  `hs-boot`. This way `tyConAppTyCon_maybe` can inline properly in the
  `One` and `Many` pattern synonyms.
- Make `updateIdTypeAndMult` strict in its type and multiplicity
- The `scaleIdBy` gets a specialised definition rather than being an
  alias to `scaleVarBy`
- `splitFunTy_maybe` is given the type `Type -> Maybe (Mult, Type,
  Type)` instead of `Type -> Maybe (Scaled Type, Type)`
- Remove the `MultMul` pattern synonym in favour of a view `isMultMul`
  because pattern synonyms appear not to inline well.
- in `eqType`, in a `FunTy`, compare multiplicities last: they are
  almost always both `Many`, so it helps failing faster.
- Cache `manyDataConTy` in `mkTyConApp`, to make sure that all the
  instances of `TyConApp ManyDataConTy []` are physically the same.

This commit has been authored by
* Richard Eisenberg
* Krzysztof Gogolewski
* Arnaud Spiwack

Metric Decrease:
    haddock.base
    T12227
    T12545
    T12990
    T1969
    T3064
    T5030
    T9872b

Metric Increase:
    haddock.base
    haddock.Cabal
    haddock.compiler
    T12150
    T12234
    T12425
    T12707
    T13035
    T13056
    T15164
    T16190
    T18304
    T1969
    T3064
    T3294
    T5631
    T5642
    T5837
    T6048
    T9020
    T9233
    T9675
    T9872a
    T9961
    WWRec

2 files changed, 5 insertions, 6 deletions

diff --git a/testsuite/tests/codeGen/should_fail/T13233.hs b/testsuite/tests/codeGen/should_fail/T13233.hs
index 42a30522f2..f24fc03bfb 100644
--- a/testsuite/tests/codeGen/should_fail/T13233.hs
+++ b/testsuite/tests/codeGen/should_fail/T13233.hs
@@ -3,20 +3,19 @@
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE MagicHash #-}
-{-# LANGUAGE LinearTypes #-}
 module Bug where
 
 import GHC.Exts (TYPE, RuntimeRep, Weak#, State#, RealWorld, mkWeak# )
 
 class Foo (a :: TYPE rep) where
-  bar :: forall rep2 (b :: TYPE rep2). (a #-> a #-> b) -> a #-> a #-> b
+  bar :: forall rep2 (b :: TYPE rep2). (a -> a -> b) -> a -> a -> b
 
-baz :: forall rep (a :: TYPE rep). Foo a => a #-> a #-> (# a, a #)
+baz :: forall rep (a :: TYPE rep). Foo a => a -> a -> (# a, a #)
 baz = bar (#,#)
 
 obscure :: (forall (rep1 :: RuntimeRep) (rep2 :: RuntimeRep)
                    (a :: TYPE rep1) (b :: TYPE rep2).
-                   a #-> b #-> (# a, b #)) -> ()
+                   a -> b -> (# a, b #)) -> ()
 obscure _ = ()
 
 quux :: ()
diff --git a/testsuite/tests/codeGen/should_fail/T13233.stderr b/testsuite/tests/codeGen/should_fail/T13233.stderr
index 1bbe161967..6f1636e544 100644
--- a/testsuite/tests/codeGen/should_fail/T13233.stderr
+++ b/testsuite/tests/codeGen/should_fail/T13233.stderr
@@ -1,11 +1,11 @@
 
-T13233.hs:23:16: error:
+T13233.hs:22:16: error:
     A levity-polymorphic type is not allowed here:
       Type: a
       Kind: TYPE rep1
     When trying to create a variable of type: a
 
-T13233.hs:28:10: error:
+T13233.hs:27:10: error:
     Cannot use function with levity-polymorphic arguments:
       mkWeak# :: a
                  -> b