#8662: GHC does not inline cheap inner loop when used in two places --------------------------------------------+------------------------------ Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by simonpj): GHC decides when to inline like this: * It computes the "size" of the function * At a call site, it takes the size of the function, subtracts a call- site "discount", and if the result is less than the "unfolding threshold" it does the inlining. * The discount is increased if both (a) an actual argument has some structure (eg is a constructor application) and (b) that argument is scrutinised by a case expression in the function body. * There is also a "result discount" if (a) the function call is consumed by a `case`, and (b) the function body returns a constructor application All this is computed in module `CoreUnfold`, so it is nicely separated from the rest of the compiler. Do by all means have a go at changing the computation of size or discount. Then measure (a) the change in code size and complication time vs (b) the change in allocation and/or run-time. If you can reduce (b) without increasing (a) you are winning! But it's easy to mess up. I have spent many happy hours fiddling with these heuristics. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8662#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler