Here is a cue: Removing the `!` on `t` in `mkMachine` removes *all*
#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 akio): Thank you for looking at this! Replying to [comment:1 nomeata]: 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. Yes, this is indeed a result of minimizing the code. I'll attach a version of the test program that does not rely solely on the bangs. In this version, if I remove the bangs in `mkMachine` (on both `m` and `t`), the amount of trace output grows exponentially (not quadratically) when compiled with `-O1`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10148#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler