[GHC] #13536: Program which terminated in GHC 8.0.2 loops with 8.2.1

[GHC]

#13536: Program which terminated in GHC 8.0.2 loops with 8.2.1
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 More progress. Here is a program which //deterministically// exhibits the
 issue (i.e., it doesn't rely on system-generated pseudorandomness):

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Main (main) where

 import System.Random.TF.Gen (seedTFGen)
 import Test.QuickCheck (arbitrary)
 import Test.QuickCheck.Gen (Gen(..))
 import Test.QuickCheck.Random (QCGen(..))
 import Text.Show.Functions ()

 main :: IO ()
 main = do
   let tfGen = seedTFGen ( 543863073959529591
                         , 14453565003432405558
                         , 3036645681517334938
                         , 17781306407512891751 )
       qcGen = QCGen tfGen
       (f, (i, b), v) = case arbitrary of
                          MkGen g -> g qcGen 30
   print $ foldlTest f (i, b) v

 type FoldlTest a = (a -> a -> a) -> a -> [a] -> Bool

 foldlTest :: FoldlTest (Int, Int)
 foldlTest f (i, b) v =
   foldl f (i, b) v == foldl (\x -> f (unmodel x)) (i, b) v

 class TestData a where
   type Model a
   unmodel :: Model a -> a

 instance TestData Int where
   type Model Int = Int
   unmodel = id

 instance (Eq a, Eq b, TestData a, TestData b) => TestData (a,b) where
   type Model (a,b) = (Model a, Model b)
   unmodel (a,b) = (unmodel a, unmodel b)
 }}}

 I'd also like to retract my claim that this program is looping forever. I
 timed this program when compiled with `-O2` on GHC 8.2.1:

 {{{
 137.01user 0.47system 2:17.43elapsed 100%CPU (0avgtext+0avgdata
 7476maxresident)k
 0inputs+0outputs (0major+936minor)pagefaults 0swaps
 }}}

 As opposed to GHC 8.0.2:

 {{{
 0.00user 0.00system 0:00.00elapsed 0%CPU (0avgtext+0avgdata
 4768maxresident)k
 0inputs+0outputs (0major+289minor)pagefaults 0swaps
 }}}

 So there's still a bug, but I don't think it's infinite in nature.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13536#comment:3>
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