#14626: No need to enter a scrutinised value -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: heisenbug Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: performance Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13861 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Puzzled by comment:2 I tried it myself. Sure enough, the evaluted-ness flags, so carefully attached by `CoreTidy` were being lost in `CorePrep`. This turned out to date back at least 7 years. * See `Note [dataToTag magic]` in `CorePrep`. It relies evaluated-ness flags. * But those flags were being killed off in `cpCloneBndr`, for reasons described in `Note [Dead code in CorePrep]`. * This was just a bug: it means that the `dataToTag magic` doesn't work properly. Sure enough we get a redundant case (after `CorePrep`) with this program {{{ data T = MkT !Bool f v = case v of MkT y -> dataToTag# y }}} After `CorePrep` we end up with {{{ f v = case v of MkT y -> case y of z -> dataToTag# z }}} which is silly. I'll fix all this and add suitable comments -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14626#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler