#5302: Unused arguments in join points -------------------------------------+------------------------------------- Reporter: reinerp | Owner: simonpj Type: bug | Status: new Priority: low | Milestone: 8.4.1 Component: Compiler | Version: 7.0.3 Resolution: | Keywords: JoinPoints 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: | -------------------------------------+------------------------------------- Comment (by simonpj): Still reproducible in HEAD. * The reason the late demand-analysis doesn't catch it is because we don't do w/w. (Reason: see `Note [Final Demand Analyser run]` in `DmdAnal`.) * The absent argument is actually a case binder, passed as a result of `Note [Case binders and join points]` in `Simplify`. But in this case it turns out that not only is the case-binder argument eventually unused, but (even worse) every `juump` to that join point looks like `jump j x (I# x)`. So we box it, and then immediately discard it. Urk. Just recording breadcrumbs for now. My thought: never pass the case binder to a join point; instead re-box it. It's much the same trade-off as in other places where we worry about re-boxing. And it would be a lot simpler; in particular, we can stop having unfoldings in lambda-binders, which is pretty dodgy anyway. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5302#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler