#9157: cmm common block not eliminated -------------------------------------+------------------------------------ Reporter: wojteknar | Owner: jstolarek Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: Other | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonmar): Yes, I think Simon PJ's reason above is exactly why hashes don't include labels: so that the hashes remain stable under elimination of common blocks. After eliminating some blocks, more opportunities for elimination emerge, so the algorithm keeps iterating until there are no more blocks to eliminate. Originally CBE was there to catch a common case of common blocks that occurred in the code we generated for case expressions. But that pattern is handled directly by the code generator (we don't generate the common blocks anymore), and hence CBE is only enabled with -O. As with most optimisations, CBE interacts with other passes, so there may be more (or fewer) opportunities for CBE at different stages during the pipeline. Whether it's worthwhile (a) having another CBE pass (maybe with -O2 only) or (b) moving the existing CBE pass later in the pipeline, is a matter for measurement. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9157#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler