#15696: Derived Ord instance for enumerations with more than 8 elements seems to be incorrect -------------------------------------+------------------------------------- Reporter: mrkkrp | Owner: (none) Type: bug | Status: patch Priority: highest | Milestone: 8.6.2 Component: Compiler | Version: 8.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D5196 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I agree with comment:38. Yes, the code generator is the right place to do it. Yes, we could ooptimise for `LFCon`, but the simplifier will have done that already. The only case I think we could reliably optimise, that would not be done already, is {{{ case x of y A -> blah DEFAULT -> ...(dataToTag# y)... }}} In this case we really do know that `y` points to the value. It would not be hard to let the code gen spot this; but I doubt it would happen much.
Right, agreed that looking at tag bits would work for small types.
This should happen automatically if we use the code for `cgCase`. It already has a fast-path for the case when the scrutinee is evaluated. But you point is perhaps that for small types we don't need to index the info table: the tag is in the bits. Yes, that's a good idea. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15696#comment:41 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler