[GHC] #14895: STG CSE makes dead binders undead

#14895: STG CSE makes dead binders undead -------------------------------------+------------------------------------- Reporter: hsyl20 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Debugging Unknown/Multiple | information is incorrect Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following example: {{{#!hs go :: (a -> b) -> Either String a -> Either String b go f (Right a) = Right (f a) go _ (Left e) = Left e }}} GHC with `-O2` converts it into the following STG: {{{#!hs TestUndead.go :: forall a b. (a -> b) -> Data.Either.Either GHC.Base.String a -> Data.Either.Either GHC.Base.String b [GblId, Arity=2, Caf=NoCafRefs, Str=, Unf=OtherCon []] = \r [f_s17n ds_s17o] case ds_s17o of { Data.Either.Left e_s17q [Occ=Once] -> wild_s17p; Data.Either.Right a1_s17r [Occ=Once] -> let { sat_s17s [Occ=Once] :: b_aVN [LclId] = \u [] f_s17n a1_s17r; } in Data.Either.Right [sat_s17s]; }; }}} Notice that the dead binder `wild_s17p` is now alive (in the first alternative) but it isn't shown in `case ds_s17o of {` because the pretty- printer still assumes it is dead. I think that in `stgCseExpr .. (StgCase ...)` (simplStg/StgCse.hs) we should check if the new binder is alive in the new alternatives, just like we do in `coreToStgExpr (Case ...)` (stgSyn/CoreToStg.hs), and use `setIdOccInfo (ManyOccs NoTailCallInfo)` on the new binder if necessary. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14895 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler