#12150: Compile time performance degradation on code that uses undefined/error with CallStacks -------------------------------------+------------------------------------- Reporter: thomie | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- GHC 8 has a lot of trouble compiling the following program: {{{#!hs module Serialize where data Result a = Success a | Error String {- 80 guards ghc-7.10.3 -O : 0.3s ghc-8.0.1 -O : 1.8s -} instance Functor Result where {-# INLINE fmap #-} fmap | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f where bool = undefined f = undefined }}} Here are some timing results, depending on the number of `| bool = f` clauses: {{{ * ghc-8.0.1 N clauses : time (s) 10 : 0.61 20 : 0.78 40 : 1.03 80 : 1.64 160 : 2.83 320 : 5.16 640 : 10.37 1280 : 21.16 * ghc-7.10.3 N clauses : time (s) 10 : 0.33 20 : 0.29 40 : 0.34 80 : 0.30 160 : 0.32 320 : 0.35 640 : 0.48 1280 : 0.80 }}} I think this compile time difference is caused by the `CallStack` changes introduced in GHC 8.0. When I use a version of `undefined` that doesn't have a CallStack, there is no difference in compile time when using GHC 7.10 or GHC 8.0. This is my implementation of `undefined` without `CallStack`: {{{ import GHC.Exception (errorCallException) import GHC.Prim (raise#) import Prelude (Char) error :: [Char] -> a error s = raise# (errorCallException s) undefined :: a undefined = error "undefined without callstack" }}} This is the quick and dirty Python script I used to generate those timing results (ghc version is hardcoded): {{{#!py import os import tempfile import time import subprocess def src(n): return ''' module Test where data Result a = Success a | Error String instance Functor Result where {{-# INLINE fmap #-}} fmap {0} where bool = undefined f = undefined '''.format('\n | bool = f' * n) tempfile = tempfile.mktemp('.hs') print('tempfile = {0}'.format(tempfile)) print('N clauses : time (s)') for i in range(8): n = 10 * 2 ** i with open(tempfile, 'w') as f: f.write(src(n)) f.flush() t0 = time.time() subprocess.call(['ghc-8.0.1', '-v0', '-O', tempfile]) t1 = time.time() print(str(n).ljust(10) + ': %.2f' % (t1 - t0)) os.remove(tempfile) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12150 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler