#14564: CAF isn't floated -------------------------------------+------------------------------------- Reporter: neil.mayhew | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: FloatOut Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => FloatOut Comment: I can think of various ways to improve this * Make the ad-hoc special handling for case-expressions fire only if * The destination level is no top-level (then no extra thunk is created after all) * The scrutinee is lifted (so a separate decision can really be taken for the scrutinee -------------- I note also another problem with `Note [Case MFEs]`. Consider {{{ g y = h y (case (f 3) of Nothing -> True Just x -> expensive s) }}} If we don't float the whole case-expression, we can float `(f 3)` instead. But we won't float `expensive x`; so it will be computed for each call of `g`. Not very clever. Maybe we should just kill off the special case, and put up with a couple of nofib regressions. This needs a bit of careful performance comparison. Any volunteers? I can advise. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14564#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler