#13289: Terrible loss of optimisation in presence of ticks -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: Profiling | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider {{{ case (error "blah") of BIG }}} You'd expect that to simplify to `error "blah"`, or more precisely to `case error "blah" of {}`. And usually it does. But if there are ticks around it, it does not, and we retain {{{ case (tick x (error "blah")) of BIG }}} The problem is in `Simpify.simplTick`. It calls `splitCont` which implements semantics I do not understand. The Right Thing is surely this. * Always push the tick onto the continuation {{{ simplTick env tickish expr cont = simplExprF env expr (mkTickIt tickish cont) }}} * Implement a suitable `mkTickIt` that pushes a tick past a continuation where possible. For example I think {{{ mkTickIt t (ApplyTo arg cont) = ApplyTo arg (mkTickIt t cont) }}} That is `(tick t e) a` becomes `tick t (e a)`. It's probably conditioned on what sort of tick, but that's fine. * The comments from `7bb0447d` (simonmar) say {{{ -- XXX: we cannot do this, because the simplifier assumes that -- the context can be pushed into a case with a single branch. e.g. -- scc<f> case expensive of p -> e -- becomes -- case expensive of p -> scc<f> e -- -- So I'm disabling this for now. It just means we will do more }}} But that's wrong. All we need to do is to ensure that `prepareCaseCont` always stops at the kind of tick we don't want to push inside. * Finally, the bottom case of `rebuildCall` should retain ticks from the continuation. Not too hard, but requires understanding of the semantics of ticks, which I do not have. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13289 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler