[GHC] #13426: compile-time memory-usage regression for DynFlags between GHC 8.0 and GHC 8.2

[GHC]

#13426: compile-time memory-usage regression for DynFlags between GHC 8.0 and GHC
8.2
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  rwbarton
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3399,
       Wiki Page:                    |  Phab:D3400
-------------------------------------+-------------------------------------

Comment (by simonpj):

 About forcing the type before passing to `mkBoringStop`.  Often the type
 passed into `mkBoringStop` is discarded unused, so I'd prefer not to force
 it when building a `SimplCont`.  I'd prefer to force it when getting it
 out of the `SimplCont`.

 Notably, `contResultType` does that.  So, possible alternative plan: when
 extracting the `contResultType` can we just force it then?  E.g.
 {{{
         ; let alts_ty' = contResultType dup_cont
         ; case_expr <- mkCase dflags scrut' case_bndr' alts_ty' alts'
 }}}
 Aha  -- you have already added a `seq` there!
 Here's another (line 1622 of `Simplify`)
 {{{
 missingAlt env case_bndr _ cont
   = WARN( True, text "missingAlt" <+> ppr case_bndr )
     return (env, mkImpossibleExpr (contResultType cont))
 }}}
 Here want to force that type.  Ditto here (line 1656)
 {{{
   | not (contIsTrivial cont)     -- Only do this if there is a non-trivial
   = return (env, castBottomExpr res cont_ty)  -- continuation to discard,
 else we do it
   where                                       -- again and again!
     res     = argInfoExpr fun rev_args
     cont_ty = contResultType cont
 }}}


 But not here (line 1591)
 {{{
     trim_cont 0 cont
       = mkBoringStop (contResultType cont)
 }}
 because we are just building another `SimplCont`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13426#comment:19>
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