#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 dfeuer): Ah... I am not entirely wrong, but I am partly wrong, because types with many constructors get tagged 1 when evaluated. I think we should at least be able to add a rule that recognizes when `dataToTag#` is used with a type with only a few constructors: see the logic in the `cgAlts gc_plan bndr (AlgAlt tycon) alts` case in `compiler/codeGen/StgCmmExpr.hs`. That way we only have to call `getConstrTag` when 1. The type is unknown, 2. The type has too many constructors, or 3. The tag bits are 0. I imagine you can do this quite easily by copying a bit of the `splitTyConApp_maybe` logic from `tagToEnumRule` to `dataToTagRule` in `PrelRules.hs` and adding a `DataToTagUsingTagOp` to supplement `DataToTagOp`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15696#comment:26 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler