Optimise nested comparisons

GHC isn't capable of this optimisation:

  case (x ># y) of             ==>    case (x ># y) of
    True -> ...(x ==# y)...               True -> ...False...
    False -> ...                          False -> ...

That is, knowing that (x>y) we know that the two are not equal.

Also, consider this:

  case (x ># y) of               ==>   case (x >=# y) of
     True -> e                           True -> e
     False -> case (x ==# y) of          False -> e'
                True -> e
                False -> e'

Again this needs special knowlege about comparison operators. However, it does arise. Consider this:

  data T = MkT Int deriving( Eq, Ord )

The derived (>) operation looks like this:

Foo.$dm> =
  \ (eta_a8q :: Foo.T) (eta1_a8r :: Foo.T) ->
    case eta_a8q of wild_B1 { Foo.MkT a1_a60 ->
    case eta1_a8r of wild1_XO { Foo.MkT b1_a62 ->
    case a1_a60 of wild2_a9I { GHC.Base.I# x#_a9K ->
    case b1_a62 of wild11_a9M { GHC.Base.I# y#_a9O ->
    case GHC.Prim.<# x#_a9K y#_a9O of wild3_a9W {
      GHC.Base.False ->
        case GHC.Prim.==# x#_a9K y#_a9O of wild12_a9Z {
          GHC.Base.False -> GHC.Base.True; GHC.Base.True -> GHC.Base.False
        };
      GHC.Base.True -> GHC.Base.False

See also #2130 (closed)

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown