#16122: `round :: Double -> Int64` much slower than `fromIntegral @Int @Int64 . round` -------------------------------------+------------------------------------- Reporter: Fuuzetsu | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Fuuzetsu): Replying to [comment:4 simonpj]:
The reason things go wrong is that in the RHS of hte rule in `GHC.Int` you really want to apply the rule `"round/Double->Int"` in `GHC.Float`, but perhaps it is not imported by `GHC.Int`?
Anyway, probably the best solution here is to rewrite the rule in `GHC.Int` to {{{ "round/Double->Int64" round = (fromIntegral :: Int -> Int64) . GHC.Float.roundDoubleInt }}} (And maybe the same for `fromIntegral`?)
It seems wrong to me to manually expand rules that already exist. Seems very unprincipled and easy to miss other existing cases: I'm confident {{{Int64}}} isn't the only offender, there are similar rules all around. I'm happy to submit a patch as proposed but I wonder if there isn't a better way to ensure the relevant rules are in scope together... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16122#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler