#15164: Slowdown in ghc compile times from GHC 8.0.2 to GHC 8.2.1 when doing Called arity analysis -------------------------------------+------------------------------------- Reporter: flip101 | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.6.1 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Compile-time | (amd64) performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Great detective work. I see what is happening. First, a workaround is to use `-fno-solve-constant-dicts`. Second, if you don't actually need the contexts on your instances then yes, try omitting them. What's happening is this. The "short cut solver" `TcInteract.shortCutSolver` tries to solve goals using top-level evidence only (see `Note [Shortcut solving]`). But in doing so, it doesn't have enough sharing. To solve G it may need G1 and G2. To solve G1 it may need X; and to solve G2 it may need X. But it doesn't spot that it only needs to solve X once. So it does the "solve X" work (successfully) twice. Then the same thing happens when solving X. And the structure of this particular example is that we get successive doubling. Solution is simple: just be a bit more intelligent about sharing. I can do that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15164#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler