[GHC] #11731: Simplifier: Inlining trivial let can lose sharing

[GHC]

#11731: Simplifier: Inlining trivial let can lose sharing
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        Reporter:  nomeata           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 Hmm. Not so easy. Given that problematic code, i.e.

 {{{
 $s$wwwMe_s49K :: Int -> Int -> GHC.Prim.Int# -> (# Int, Int #)
 [LclId, Arity=3, Str=DmdType <L,U(U)><L,1*U(U)><S,1*U>]
 $s$wwwMe_s49K =
   \ (sc_s49I :: Int) (sc_s49J :: Int) (sc_s49H :: GHC.Prim.Int#) ->
     case sc_s49H of ds_X1WV {
       __DEFAULT -> $wwwMe_s48A (GHC.Prim.-# ds_X1WV 1#) lvl_s1Z6;
       0# ->
         case foo @ Int @ Int (sc_s49I, sc_s49J)
         of _ [Occ=Dead] { GHC.Types.I# ipv_s1XV [Dmd=<L,A>] ->
         let {
           b_s1Z1 [Dmd=<L,U(U)>] :: Int
           [LclId, Str=DmdType]
           b_s1Z1 = sc_s49J } in
         let {
           a_s1Z0 [Dmd=<L,U(U)>] :: Int
           [LclId, Str=DmdType]
           a_s1Z0 = sc_s49I } in
         (# GHC.Num.$fNumInt_$c+ b_s1Z1 a_s1Z0,
            GHC.Num.$fNumInt_$c+ a_s1Z0 b_s1Z1 #)
         }
     }
 }}}

 it seems that the `used-once` information is not attached to the
 occurrence of `sc_s49J`, but only to the function `$s$wwwMe_s49J` that
 binds it. This is confirmed by a `pprTrace` of the `idDemandInfo` in
 `exprIsTrivial`:
 {{{
 exprIsTrivial sc_s49J <L,U>
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11731#comment:7>
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