[GHC] #14742: Unboxed sums can treat Int#s as Word#s

30 Jan 2018

      #14742: Unboxed sums can treat Int#s as Word#s
-------------------------------------+-------------------------------------
           Reporter:  duog           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:  UnboxedSums    |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following module:
 {{{
 {-# language MagicHash, UnboxedSums #-}

 {-# options_ghc -ddump-stg -dppr-debug -fprint-explicit-kinds -ddump-to-
 file #-}

 module Bug where
 import GHC.Prim
 import GHC.Types

 mkUnboxedSum :: () -> (# Float# | Int# #)
 mkUnboxedSum _ = (# | 9# #)
 {-# noinline mkUnboxedSum #-}

 foo :: Int
 foo = case mkUnboxedSum () of
   (# | i# #) -> I# i#
   (# f# | #) -> 8
 }}}

 The full .dump-stg is attached. An abbreviation of the case statement in
 foo is:
 {{{
         case (...)
         of
         (...)
         { ghc-prim:GHC.Prim.(#,,#){(w) d 89} ((us_g1h9{v} [lid] :: ghc-
 prim:GHC.Types.Any{(w) tc 35K}
                                                                      (ghc-
 prim:GHC.Prim.TYPE{(w) tc 32Q}
 'ghc-prim:GHC.Types.WordRep{(w) d 63J}))
                                                 :: ghc-
 prim:GHC.Types.Any{(w) tc 35K}
                                                      (ghc-
 prim:GHC.Prim.TYPE{(w) tc 32Q}
                                                         'ghc-
 prim:GHC.Types.WordRep{(w) d 63J}))
                                              ((us_g1ha{v} [lid] :: ghc-
 prim:GHC.Types.Any{(w) tc 35K}
                                                                      (ghc-
 prim:GHC.Prim.TYPE{(w) tc 32Q}
 'ghc-prim:GHC.Types.WordRep{(w) d 63J}))
                                                 :: ghc-
 prim:GHC.Types.Any{(w) tc 35K}
                                                      (ghc-
 prim:GHC.Prim.TYPE{(w) tc 32Q}
                                                         'ghc-
 prim:GHC.Types.WordRep{(w) d 63J}))
                                              ((us_g1hb{v} [lid] :: ghc-
 prim:GHC.Types.Any{(w) tc 35K}
                                                                      (ghc-
 prim:GHC.Prim.TYPE{(w) tc 32Q}
 'ghc-prim:GHC.Types.FloatRep{(w) d 63V}))
                                                 :: ghc-
 prim:GHC.Types.Any{(w) tc 35K}
                                                      (ghc-
 prim:GHC.Prim.TYPE{(w) tc 32Q}
                                                         'ghc-
 prim:GHC.Types.FloatRep{(w) d 63V})) ->
               case
                   (us_g1h9{v} [lid] :: ghc-prim:GHC.Types.Any{(w) tc 35K}
                                          (ghc-prim:GHC.Prim.TYPE{(w) tc
 32Q}
                                             'ghc-
 prim:GHC.Types.WordRep{(w) d 63J})) :: Prim IntRep
               of
               ((tag_g1hc{v} [lid] :: ghc-prim:GHC.Prim.Int#{(w) tc 3s})
                  :: ghc-prim:GHC.Prim.Int#{(w) tc 3s})
               { __DEFAULT -> ghc-prim:GHC.Types.I#{(w) d 6i} [8#];
                 2# ->
                     ghc-prim:GHC.Types.I#{(w) d 6i} [(us_g1ha{v} [lid] ::
 ghc-prim:GHC.Types.Any{(w) tc 35K}
 (ghc-prim:GHC.Prim.TYPE{(w) tc 32Q}
 'ghc-prim:GHC.Types.WordRep{(w) d 63J}))];
               };
 }}}

 Note that:
 * `us_g1h9 :: Any (TYPE WordRep)`;
 * `us_g1ha :: Any (Type WordRep)`;
 * `tag_g1hc :: Int#`;
 * The `2#` alternative passes `us_g1ha` to an `I#` constructor.

 This seems wrong to me.

 It comes about because `slotPrimRep . primRepSlot` (in RepType) is not the
 identity.

 StgLint found this while I was working on ticket:14541

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14742
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler