[GHC] #14768: -O1 changes result at runtime, duplicating __DEFAULT case

6 Feb 2018

      #14768: -O1 changes result at runtime, duplicating __DEFAULT case
-------------------------------------+-------------------------------------
           Reporter:  Bodigrim       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.4.1-alpha3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Here is a program, which works as expected in GHC 8.4.1-alpha3 with `-O0`,
 but changes it behaviour with `-O1`.

 {{{#!hs
 {-# LANGUAGE MagicHash #-}

 import qualified Data.Vector.Unboxed as U
 import GHC.Exts

 vec :: U.Vector Moebius
 vec = U.singleton Moebius0

 main :: IO ()
 main = print $ U.head vec == U.head vec

 data Moebius = Moebius0 | Moebius1 | Moebius2
   deriving (Eq)

 fromMoebius :: Moebius -> Int
 fromMoebius Moebius0 = 0
 fromMoebius Moebius1 = 1
 fromMoebius Moebius2 = 2

 toMoebius :: Int -> Moebius
 toMoebius (I# i#) = tagToEnum# i#

 {- ...unboxed vector instances, see file attached... -}
 }}}

 It is expected that this program will print `True`. However, when compiled
 with `-O1` it prints `False`.

 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.4.0.20180204
 $ ghc -O0 Sieve.hs && ./Sieve
 [1 of 1] Compiling Main             ( Sieve.hs, Sieve.o ) [Optimisation
 flags changed]
 Linking Sieve ...
 True
 $ ghc -O1 Sieve.hs && ./Sieve
 [1 of 1] Compiling Main             ( Sieve.hs, Sieve.o ) [Optimisation
 flags changed]
 Linking Sieve ...
 False
 }}}

 It reproduces on OS X and Ubuntu, but worked fine in GHC 8.2.

 I looked into generated Core and found a suspicious function, having two
 `__DEFAULT` cases with different bodies.

 {{{#!hs
 main2 :: String
 main2
   = case vec `cast` Co:3 of
     { Vector ipv_sb7L ipv1_sb7M ipv2_sb7N ->
     case <# 0# ipv1_sb7M of {
       __DEFAULT -> case main3 ipv1_sb7M of wild_00 { };
       1# ->
         case indexIntArray# ipv2_sb7N ipv_sb7L of {
           __DEFAULT -> $fShowBool4;
           __DEFAULT -> $fShowBool2;
           1# -> $fShowBool2;
           2# -> $fShowBool2
         }
     }
     }
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14768
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