#14880: GHC panic: updateRole -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.8.1 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 tdammers): Replying to [comment:94 goldfire]:
I have two takeaways from this:
- An accumulator style is the Right Way to go when building up a set. This idea should probably be abstracted over right in the `VarSet` interface. But do we get these gains without eta-expanding everything? (See Note [FV eta expansion] in FV.)
IIRC, I eta-reduced the relevant functions (`tyCoVarsOfXXXX`), but found no significant difference. I only did this for the `VarSet` approach though; I believe the reasoning in that note only holds for FV. Baking accumulator style into `VarSet` is logical, but if we do this, we also need to add the `in_scope` filtering thing found in `FV`, because, as the `nondet-no-in-scope` version shows, not doing this absolutely devastates performance. However, `VarSet` plus accumulator style is exactly `NDFV`, which is `FV` minus the list, which in turn makes no significant difference anyway. So effectively, this takeaway says we should be using `FV`.
- The extra work to track order has no effect. I find this surprising, but not overly so, because we can avoid the extra work via laziness.
Well, when I wrote "no effect", I was actually lying a little bit - getting rid of the list reduces overall allocations by about 500 bytes. Which, I think, is about what you'd expect from removing something that never gets evaluated anyway. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14880#comment:95 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler