#14684: combineIdenticalAlts is only partially implemented -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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 simonpj): In `CoreUtils`, `Note [Combine identical alternatives]` acknowledges this problem, saying {{{ To avoid an expensive test, we just merge branches equal to the *first* alternative; this picks up the common cases a) all branches equal b) some branches equal to the DEFAULT (which occurs first) }}} And indeed you are right that it'd be better to combine {{{ foo x = case x of ==> foo x = case x of A -> 0 DEFAULT -> 2 B -> 1 A -> 0 C -> 2 B -> 1 D -> 2 }}} (Reminder: in Core the DEFAULT alternative always comes first, if it exists.) Fortunately, we now have `TrieMap.CoreMap`, an efficient finite map whose keys are `CoreExprs`. Using this I think we can efficiently ask (as we iterate oover the alternatives) "have I seen this RHS in an earlier alternative?". More advanced: find the RHS that occurs most often. Take care that in the common case the RHSs are (a) large and (b) different, then the test does not exhaustively traverse the RHSs; just looks far enough to establish they are different. This a nice well-contained task, if someone would like to have a go. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14684#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler