#8326: Place heap checks common in case alternatives before the case -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: jstolarek Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: #1498 Test Case: | Blocking: 8317 | Differential Revisions: Phab:D343 | -------------------------------------+------------------------------------- Comment (by jstolarek): Thanks for detailed explanation. I was a bit concerned about all that extra stuff like saving cost centres in the general case. Don't we need to worry that any of these will impact performance of the compiled code?
The difficulty here is that it's hard to find out whether the branches allocate without running the code generator on them
Yes, I believe this was our conclusion when we discussed that during my internship. I don't see how to use `StgCmmMonad.getHeapUsage` to implement the solution. We compile all the alternatives in one go by calling `cgAlts (gc_plan,ret_kind) (NonVoid bndr) alt_type alts`, whereas to apply any of the criteria that you proposed we would need to compile the alternatives one by one and analyse their heap usage. My understanding is that if we simply call `cgAlts` in a lambda passed to `getHeapUsage` we will only know how much heap total is used by the compiled alternatives but we won't have any detailed knowledge about each of the alternatives. Is that correct?
But before doing that, the first thing is to decide what the criteria should be.
It's a bit hard to tell without doing the actual implementation and measuring the results. I guess there will be some programs that work better with the first criterion and some that work better with the second criterion. That being said, I'd go with the first one. Slightly larger binaries are a small price to pay for possibility of having better performance. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8326#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler