[GHC] #9718: Avoid TidyPgm predicting what CorePrep will do

[GHC]

#9718: Avoid TidyPgm predicting what CorePrep will do
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:  CodeGen, CAFs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by osa1):

 > Build an SRT for each closure that lists the CAFFy things it mentions.
 It's not obvious where that is done today -- can you point to it?

 We don't build SRTs until Cmm (`doSRTs` in `CmmBuildInfoTables.hs`). In
 `TidyPgm` we decide on CAFFYness and record it in `IdInfo`s of binders. As
 far as I can see there are no CAF or SRT related computation done in STG.

 Btw, I found this commented-out code in `StgSyn`:

 {{{
 pprStgExpr (StgLet srt (StgNonRec bndr (StgRhsClosure cc bi free_vars
 upd_flag args rhs))
                         expr@(StgLet _ _))
   = ...
 }}}

 It seems like at some point someone tried to decorate `StgLet`s with SRTs.

 I don't understand how the CAFFYness info in Ids used in the code gen, but
 I wonder if even after this work there will still be room for bugs because
 `idCafInfo` of an id (not in binder position) may disagree with the final
 CAFFYness information of the id.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9718#comment:15>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler