Re: [GHC] #15436: Compile-time panic, Prelude.!!: negative index

25 Jul 2018

      #15436: Compile-time panic, Prelude.!!: negative index
-------------------------------------+-------------------------------------
        Reporter:  pbrisbin          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hsyl20):

 Given this simpler code:

 {{{#!hs

 module Bug0 where

 import GHC.Enum

 data XXX = AL | AK | AZ | AR | CA | CO | CT | DE | FL
     deriving (Enum)

 data Z = Y | X XXX

 instance Enum Z where
   fromEnum (X s) = 1 + fromEnum s
   --fromEnum Y = 0
   toEnum 0 = Y
   --toEnum i = X . toEnum $ i - 1
 }}}

 We have the following sequence of transformations for `succ`:

 {{{#!hs
 toEnumZ :: Int -> Z
 toEnumZ 0 = Y
 toEnumZ x = ...

 fromEnumZ :: Z -> Int
 fromEnumZ Y = ...
 fromEnumZ (X x) = 1 + fromEnumX x

 succZ :: Z -> Z
 succZ = toEnumZ . (+1) . fromEnumZ

 ===> {inline toEnumZ}

 succZ z = case (fromEnumZ z) + 1 of
    0 -> Y
    x -> ...

 ===> {case-folding}

 succZ z = case fromEnumZ z of
    -1 -> Y
    x  -> ...
 }}}

 We could stop here: `fromEnumZ` is basically `dataToTag#` and we have a
 negative literal alternative.

 If we continue:

 {{{#!hs
 ===> {inline fromEnumZ}

 succZ z = case (case z of
                   Y -> 0
                   (X x) -> 1 + fromEnumX x) of
    -1 -> Y
    x  -> ...

 ===> {case-of-case}

 succZ z = case z of
    Y -> ...
    X x -> case 1 + fromEnumX x of
       -1 -> Y
       s  -> ...
 }}}

 And this is what we get:

 {{{#!hs
 $csucc_a2vu :: Z -> Z
 $csucc_a2vu
   = \ (x_a2Km :: Z) ->
       case x_a2Km of {
         Y -> case lvl_s2RN of wild_00 { };
         X s_aED ->
           case s_aED of x1_a2IA { __DEFAULT ->
           case GHC.Prim.dataToTag# @ XXX x1_a2IA of a#_aI4 { __DEFAULT ->
           case GHC.Prim.+# 1# a#_aI4 of lwild_s2St {
             __DEFAULT -> lvl_s2R2;
             -1# -> Bug0.Y
           }
           }
           }
       }
 }}}

 When we have fewer data constructors for XXX, `fromEnumX` is inlined as a
 case so there is no `dataToTag#` involved:

 {{{#!hs
 $csucc_a2vh :: Z -> Z
 $csucc_a2vh
   = \ (x_a2K7 :: Z) ->
       case x_a2K7 of {
         Y -> case lvl_s2Ry of wild_00 { };
         X s_aEC ->
           join {
             $j_s2Sa :: GHC.Prim.Int# -> Z
             [LclId[JoinId(1)], Arity=1]
             $j_s2Sa (x1_a2Ka [OS=OneShot] :: GHC.Prim.Int#)
               = case x1_a2Ka of lwild_s2S9 {
                   __DEFAULT -> lvl_s2QN;
                   -1# -> Bug0.Y
                 } } in
           case s_aEC of {
             AL -> jump $j_s2Sa 1#;
             AK -> jump $j_s2Sa 2#;
             AZ -> jump $j_s2Sa 3#;
             AR -> jump $j_s2Sa 4#;
             CA -> jump $j_s2Sa 5#;
             CO -> jump $j_s2Sa 6#;
             CT -> jump $j_s2Sa 7#;
             DE -> jump $j_s2Sa 8#
           }
       }
 }}}

 Perhaps we could simply discard negative literal alternatives when we
 match on `dataToTag#`?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15436#comment:5
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