[GHC] #11158: Combine exprIsTrivial and cpe_ExprIsTrivial

[GHC]

#11158: Combine exprIsTrivial and cpe_ExprIsTrivial
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  task              |               Status:  infoneeded
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1656
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by nomeata):

 It arose out of `main = return ()`. In CoreTidy, it is
 {{{
 main = return @ IO GHC.Base.$fMonadIO @ () GHC.Tuple.()
 }}}
 and CorePrep turns that (due to my change of `cpe_ExprIsTrivial`!) into
 {{{
 -- RHS size: {terms: 1, types: 0, coercions: 0}
 sat_sNJ :: ()
 [LclId]
 sat_sNJ = GHC.Tuple.()

 -- RHS size: {terms: 3, types: 2, coercions: 0}
 T11158.main :: GHC.Types.IO ()
 [GblId]
 T11158.main =
   GHC.Base.return @ GHC.Types.IO GHC.Base.$fMonadIO @ () sat_sNJ
 }}}
 which yields this STG
 {{{
 sat_sNJ :: ()
 [LclId] =
     NO_CCS ()! [];

 T11158.main :: GHC.Types.IO ()
 [GblId] =
     \u [] GHC.Base.return GHC.Base.$fMonadIO sat_sNJ;
 }}}

 Given that the crash happens in `scavenge_static ()` I think it is likely
 that the SRT table is related to the issue.

 I don’t see anywhere in `cpeArg` (which creates the float) code that would
 set CAF information on the floated argument. According to note `Note
 [CafInfo and floating]` it is safe to have top-level binds with
 `HasCafRef` even when they do not actually reference any CAFs.

 (Trying to set `NoCafInfo` on all the extra floats I am generating here,
 and rebuilding the libraries. Will report back.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11158#comment:18>
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