#14880: GHC panic: updateRole -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: #15076 | Differential Rev(s): Phab:D4769 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I have a new hypothesis, provoked by this merge popping up in the statistics. * Previously, `tyCoVarsOfTypes` used `tyCoFVsOfTypes`, which in turn uses `unionFV` (from `utils/FV.hs`). This `FV` thing is relatively elaborate, maintaining a `VarSet` alongside a list. Plus an "interesting" predicate which, in the case of `tyCoVarsOfType` is `const True`. * In the patch, `tyCoVarsOfType` switches to using `unionVarSet`. This "obviously" shold be more efficient, because it just maintains a `VarSet`, with no list. * BUT `tyCoVarsOfType` does not use an accumulator; it just straightforwardly unions the free-vars in a tree-like way. In contrast, `unionFV` does use accumulator style, behind the scenes. * Bottom line: in the old version we make many calls to `extendVarSet`; in the new one we make many calls to `unionVarSet`. It should jolly well be the case that inserting 100 things one by one into a set should be no faster than unioning a balanced tree of the same 100 things. But looking at `Data.Map.Internal.insert`, and comparing with `Data.Map.Internal.mergeWithKey'`, I bet the former is faster. This hypothesis is easy to test: just change the new `tcvs_of_type` to take an accumulator. If this turns out to make a difference, I think we should file a bug on `containers`. Users should not have to worry about this! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14880#comment:48 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler