#11735: Optimize coercionKind -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Yes, I know changing `NthCo` caching is outstanding. For the `simplCast` changes: you're right -- I ignored your suggestions opting for something simpler. In HEAD, `pushTyCoArg` (along with friends) goes to some trouble to create a type-correct `Refl` coercion. But this coercion soon ends up being used in a cast, where `Refl`s are simply discarded. So all that work in building the coercion is wasted. My change is to return `Maybe Coercion`, preferring `Nothing` over `Just (Refl ...)`. This avoids the inefficient one-variable substitution that we were worrying about. It should be a clear win; we're doing strictly less work. The small complication is that there are these `Maybe`s floating about, but I think it's worth it. I'm hoping the comments on the `push` functions are clear enough, but perhaps I didn't include any information on ''why'' `Maybe Coercion` is better than `Coercion` here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11735#comment:74 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler