#9279: Optimisation bug -------------------------------------+------------------------------------- Reporter: simonmar | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Differential Revisions: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- Changes (by simonpj): * cc: nicolas.frisby@… (added) Comment: OK I can see what is going on; it is more or less as you say. We have something like {{{ let worker = \x. BIG in let {-# INLINE wrapper #-} wrapper = \y. ...worker.... in ...map wrapper xs.... }}} (Technically, the wrapper has a "stable inlining" which I have not written above, which is a copy of the body of the wrapper.) Now, GHC is (in general rightly) unhappy about inlining `worker` in `wrapper`'s rhs, even though there is currently just one textual occurrence of `worker`, because there might (now or in the future) be many occurrences of `wrapper`, and the INLINE pragma means we'll make a copy of `wrapper`'s rhs at each occurrence. However, in this case, `wrapper` is not inlined because it's not applied to anything, so nothing is gained. But something is lost. I can see various possible solutions: * Lambda-lift local function definitions, late in the pipeline. This is something Nicolas Frisby was working on, with promising results, but he sadly he never completed the work. I'm copying him on this ticket. * Towards the end of the pipeline, forget that wrappers are special by discarding their INLINE pragmas. This is a bit delicate; I think you would ''not'' want to do this for things marked INLINE by the programmer. And certainly not for top-level things either (else you'd defeat the entire w/w idea). I wonder how often this happens. Might not be hard to find out; how often does a local function definition with an INLINE pragma appear in the final Core? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9279#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler