[GHC] #13077: Worker/wrapper can break the let-app invariant

6 Jan 2017


      #13077: Worker/wrapper can break the let-app invariant
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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 ,
  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 ,
  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=] {
       __DEFAULT ->
         let {
           v_s1IX [Dmd=] :: 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=] {
               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.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13077
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