#13386: Poor compiler performance with type families -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 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): | Wiki Page: -------------------------------------+------------------------------------- Consider: {{{#!hs {-# LANGUAGE DataKinds, TypeApplications, TypeFamilies, TypeOperators, UndecidableInstances #-} {-# OPTIONS_GHC -freduction-depth=0 #-} module TypeFamilyPerf where import GHC.TypeLits type DivisibleBy x y = Help x y 0 (CmpNat x 0) type family Help x y z b where Help x y z EQ = True Help x y z LT = False Help x y z GT = Help x y (z+y) (CmpNat x z) foo :: DivisibleBy y 3 ~ True => proxy y -> () foo _ = () type N = 1002 k = foo @N undefined }}} On my machine `ghc-8.0.2 -O0` takes 4-5s to compile this module, with the time increasing as `N` increases (but note that it must be a multiple of 3, otherwise typechecking fails quickly). The problem seems to be that desugaring creates an enormous number of coercions in the representation of `k`. Perhaps this isn't terribly surprising, but I think we can do better. It should be possible to represent a proof of `DivisibleBy 1002 3 ~ True` compactly: the only information required should be the LHS and the number of reduction steps to take (and perhaps caching the RHS might be worthwhile). {{{ [1 of 1] Compiling TypeFamilyPerf ( TypeFamilyPerf.hs, TypeFamilyPerf.o ) *** Parser [TypeFamilyPerf]: !!! Parser [TypeFamilyPerf]: finished in 0.50 milliseconds, allocated 0.655 megabytes *** Renamer/typechecker [TypeFamilyPerf]: !!! Renamer/typechecker [TypeFamilyPerf]: finished in 91.70 milliseconds, allocated 58.617 megabytes *** Desugar [TypeFamilyPerf]: Result size of Desugar (after optimization) = {terms: 44, types: 70, coercions: 6,058} !!! Desugar [TypeFamilyPerf]: finished in 3781.83 milliseconds, allocated 8775.375 megabytes *** Simplifier [TypeFamilyPerf]: Result size of Simplifier iteration=1 = {terms: 27, types: 62, coercions: 6,060} Result size of Simplifier = {terms: 27, types: 62, coercions: 6,053} !!! Simplifier [TypeFamilyPerf]: finished in 42.93 milliseconds, allocated 68.321 megabytes *** CoreTidy [TypeFamilyPerf]: Result size of Tidy Core = {terms: 27, types: 62, coercions: 6,053} !!! CoreTidy [TypeFamilyPerf]: finished in 0.52 milliseconds, allocated 0.766 megabytes Created temporary directory: /tmp/ghc5526_0 *** CorePrep [TypeFamilyPerf]: Result size of CorePrep = {terms: 32, types: 74, coercions: 6,053} !!! CorePrep [TypeFamilyPerf]: finished in 0.37 milliseconds, allocated 0.160 megabytes *** Stg2Stg: *** CodeGen [TypeFamilyPerf]: !!! CodeGen [TypeFamilyPerf]: finished in 0.00 milliseconds, allocated 1.310 megabytes }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13386 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler