#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: | Phab:D5201 -------------------------------------+------------------------------------- Comment (by simonpj):
Simon, what do you think about marking dataToTag# as can't fail?
It makes sense to me to do so -- but it's an unforced change. It's not so marked today, primops.txt.pp even says why {{{ {- Note [dataToTag#] ~~~~~~~~~~~~~~~~~~~~ The dataToTag# primop should always be applied to an evaluated argument. The way to ensure this is to invoke it via the 'getTag' wrapper in GHC.Base: getTag :: a -> Int# getTag !x = dataToTag# x But now consider \z. case x of y -> let v = dataToTag# y in ... To improve floating, the FloatOut pass (deliberately) does a binder-swap on the case, to give \z. case x of y -> let v = dataToTag# x in ... Now FloatOut might float that v-binding outside the \z. But that is bad because that might mean x gets evaluated much too early! (CorePrep adds an eval to a dataToTag# call, to ensure that the argument really is evaluated; see CorePrep Note [dataToTag magic].) Solution: make DataToTag into a can_fail primop. That will stop it floating (see Note [PrimOp can_fail and has_side_effects] in PrimOp). It's a bit of a hack but never mind. }}} Are we good to go if you don't make this change? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15696#comment:50 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler