#10148: Optimization causes repeated computation -------------------------------------+------------------------------------- Reporter: akio | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1-rc1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by nomeata): Fun fact: Most changes make the problem go away. * Changing the `data Array' a = ALeaf a` to a `newtype Array' a = ALeaf a` * not calling `go` in `array_adjust` * adding a `trace` to the pattern matching in `adjustA` * removing the pattern match there completely * removing the Bang Pattern in `gstep` * removing the first parameter from `Array` * marking `mkMaschine` `NOINLINE` Here is a cue: Removing the `!` on `t` in `mkMachine` removes *all* trace output. So it seems that (possibly as a result of minimizing the code), `t` and hence `array_adjust` is not actually used. This (function strict in an argument, but argument is unused) might make GHC be a bit too liberal in what it does with the code. (Didn’t look at Core so far, though). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10148#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler