#11095: -O0 -g slows GHC down on list literals (compared to -O0 without -g) -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): phab:D3001 Wiki Page: | -------------------------------------+------------------------------------- Comment (by scpmw): Well, turns out the "elegant" way does actually not work: The intermediate source notes get floated up, and we still end up with `O(n^3)` complexity at intermediate `wrapTicks` stages. Plus the top-level source note had actually been in the list of floats anyway - just at first position, which gets applied by `foldr` last. So we were applying all sorts of fine- grained ticks on `n` expressions just to get rid of all of them in the end. phab:D3037 fixes this behaviour of `wrapTicks`. It's rather straightforward, really: Instead of using a `foldr` to apply ticks from the right, we start at the left and drop ticks immediately when we see that they are redundant. This should be `O(n)` for list literals now. Sorry for that, rather silly mistake :/ -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11095#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler