[GHC] #13386: Poor compiler performance with type families

7 Mar 2017

      #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
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13386
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