#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 Resolution: | Keywords: Profiling Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonmar): `splitCont` looks like it separates out any outer type applications and coercions from the continuation, so in `(a,b) = splitCont c`, `a` is a continuation that consists only of the outer type applications and coercions from `c`, and `b` is the rest of the continuation. In order to transform `(tick t e) a` into `tick t (e a)`, the conditions should be * Either `a` is a type, or * `tickishScopesLike t NoScope` the second condition says that we must not float things in or out of the tick. (This `tickishScopesLike` thing is Peter's, I'm not all that familiar with it but I think this is right) The other thing that the current code is doing is pushing coercions inside the tick, which is always safe (and probably a good idea, given that I made it do this before). But it's slightly strange that this code appears to be lifting ticks out of type applications, whereas `CoreUtils.mkTick` is doing exactly the opposite! I have no idea why. The semantics wouldn't tell us; both are safe to do (like, say, floating a `let` outside an application), it's just a question of which one gives the best results. There are a couple of things we can do here: - Run the profiling tests in `testsuite/tests/profiling` - Run nofib with profiling on So I suggest making the change you propose, put it up on Phab, and also run the tests and nofib to check whether the changes don't introduce obvious regressions. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13289#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler