[GHC] #9291: Don't reconstruct sum types if the type subtly changes

[GHC]

#9291: Don't reconstruct sum types if the type subtly changes
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        Reporter:  schyler           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.2
      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 rwbarton):

 Replying to [comment:36 nomeata]:
 > > Is this related with typed-ness of the IR?
 >
 > Yes! The type system of Core ensures that we cannot get the ST stuff
 wrong (to some extent. It still requires us to also not inline `runRW#`,
 for example).

 Is this really true? Say I write
 {{{#!hs
 f :: Bool -> IORef Int -> IO ()
 f b v = v `seq` when b (writeIORef v 3)
 }}}
 GHC produces
 {{{
 f1 =
   \ b_auQ v_auR eta_B1 ->
     case v_auR `cast` ... of _ { STRef ipv_sPi ->
     case b_auQ of _ {
       False -> (# eta_B1, () #);
       True ->
         case writeMutVar# ipv_sPi f2 eta_B1 of s2#_aQd { __DEFAULT ->
         (# s2#_aQd, () #)
         }
     }
     }
 }}}
 What stops me from rewriting this via case-of-case to the inequivalent
 {{{
 f1 =
   \ b_auQ v_auR eta_B1 ->
     case v_auR `cast` ... of _ { STRef ipv_sPi ->
     case writeMutVar# ipv_sPi f2 eta_B1 of s2#_aQd { __DEFAULT ->
       case b_auQ of _ {
         False -> (# eta_B1, () #);   -- Haha, don't actually use s2#_aQd
 here.
         True -> (# s2#_aQd, () #)
       }
     }
     }
 }}}
 I think it is just the flag `has_side_effects = True` on `writeMutVar#`,
 and doesn't have to do with types.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:37>
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