#9688: Improve the interaction between CSE and the join point transformation -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: CSE Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:1 simonpj]:
Or, to put it another way, if the user wrote the above `Core`, complete with the local function `$j_s2BC`, would you expect it to be optimised?
Probably so, but I don't really know. I think the basic concept is that we should let more information flow from the analysis of the main body into the analysis of `j`. I think these two steps would probably be a good start, although I imagine they could be refined. Of course, you may well know of seven fatal flaws in this approach. 1. The simple one: If the value of a certain expression is always available when calling `j`, check if that expression is used in `j`; if so, pass it in. This is really just CSE, broadened to cross the boundary between the local function and the main body. 2. The less simple one: If the scrutinee of a `case` in `j` has always been scrutinized before `j` is called, "trim" the branches to only the ones that are still possible, and then consider splitting `j` into its remaining branches. In the examples described above, one relevant `case` will be the outermost one in `j`, and that should pretty much always be merged upwards. Doing the same thing a few times will take care of the rest. I think this basically does what would have happened had `j` been inlined, but we actually ''know'' that we're getting something out of it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9688#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler