Worker/wrapper can break the let-app invariant

Consider this

{-# LANGUAGE MagicHash #-}
module Bar where
import GHC.Exts

data X = A | B | C

data T = MkT !X Int# Int#

f (MkT x 0# _) = True
f (MkT x n  _)  = let v = case x of
                         A -> 1#
                         B -> 2#
                         C -> n
                  in f (MkT x v v)

Compile with -O and (with GHC 8) you'll get

*** Core Lint errors : in result of Simplifier ***
Bar.hs:10:23: Warning:
    [RHS of v_s1IX :: Int#]
    The type of this binder is primitive: v_s1IX
    Binder's type: Int#
*** Offending Program ***
Rec {
f [InlPrag=INLINE[0]] :: T -> Bool
[LclIdX,
 Arity=1,
 Str=DmdType <S(SSL),1*U(U,1*U,A)>,
 Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True,
         Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=False)
         Tmpl= \ (w_s1Jq [Occ=Once!] :: T) ->
                 case w_s1Jq
                 of _ [Occ=Dead]
                 { MkT ww_s1Jt [Occ=Once] ww_s1Ju [Occ=Once] _ [Occ=Dead] ->
                 $wf_s1Jx ww_s1Jt ww_s1Ju
                 }}]
f =
  \ (w_s1Jq :: T) ->
    case w_s1Jq of _ [Occ=Dead] { MkT ww_s1Jt ww_s1Ju ww_s1Jv ->
    $wf_s1Jx ww_s1Jt ww_s1Ju
    }

$wf_s1Jx [InlPrag=[0], Occ=LoopBreaker] :: X -> Int# -> Bool
[LclId,
 Arity=2,
 Str=DmdType <S,U><S,1*U>,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True, Guidance=IF_ARGS [40 30] 80 10}]
$wf_s1Jx =
  \ (ww_s1Jt :: X) (ww_s1Ju :: Int#) ->
    case ww_s1Ju of ds_X1IO [Dmd=<L,1*U>] {
      __DEFAULT ->
        let {
          v_s1IX [Dmd=<S,U>] :: Int#
          [LclId,
           Str=DmdType,
           Unf=Unf{Src=<vanilla>, TopLvl=False, Value=False, ConLike=False,
                   WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 30 0}]
          v_s1IX =
            case ww_s1Jt of _ [Occ=Dead, Dmd=<L,A>] {
              A -> 1;
              B -> 2;
              C -> ds_X1IO
            } } in
        $wf_s1Jx ww_s1Jt v_s1IX;
      0 -> True
    }
end Rec }

Reason: in the worker, the lambda-bound arguments don't say they are evaluated, so the previously ok-for-speculation RHS of the 'let' is no longer so.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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