#14226: Common Block Elimination pass doesn't eliminate common blocks -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 (CodeGen) | 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): OK, got it. Yes, totally. CBE should be insensitive to alpha-renaming of local register. One way to do this would be for the hashing function to be sensitive to binding sites. Concretely, suppose that we had {{{ hashInstrs :: HashEnv -> [Instruction] -> HashCode type HashEnv = (Map LocalReg HashCode, Int) }}} Now we could have {{{ hashInstrs (env, next) (Load reg expr : instrs) = hashInstrs (extendEnv env reg next, next+1) instrs `hashPlus` hashExpr env expr ... hashExpr env (LocalReg r) | Just hc <- lookupEnv env r = hc | otherwise = hashLocalReg r -- as now }}} The mapping tells what hashcode to use for a local register. We deBruijn- number all binding sites as we come across them, mapping them to 1, 2, 3, etc. Now we'll be insensitive to what name is chosen. I think we already have a function that, for an instruction, says what local regs it assigns to. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14226#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler