[GHC] #16335: Make CPR Analysis more aggressive for inductive cases

[GHC]

#16335: Make CPR Analysis more aggressive for inductive cases
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        Reporter:  sgraf             |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:  CPRAnalysis
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 sgraf):

 To expand on the "Return pattern specialisation" thing:

 ||=SpecConstr=||= CPR+WW =||
 ||Multiple specialisations||One specialisation||
 ||Looks at how much the definition scrutinises its arguments in ''any'' of
 its case expressions||Looks at how deep the constructed products are in
 ''all'' its return branches (mostly since we only do one specialisation)||
 ||Compares that to how the function is called and only allows matching
 specialisations (unless `-fspec-constr-keen`||Doesn't look at calls, but
 only provides the most conservative specialisation anyway||

 Put another way: If we allowed multiple specialisations, we could have the
 following two specialisations, no reboxing happening (no promises made on
 correctness):

 {{{
 Foo.$weval
   = \ (w_s1cn :: Expr) ->
       case w_s1cn of {
         Lit dt_d11b -> case dt_d11b { I# ds_abcd -> ds_abcd };
         Plus a_aXf b_aXg ->
           case Foo.$weval a_aXf of ww_s1cq { __DEFAULT ->
           case Foo.$weval b_aXg of ww1_X1dh { __DEFAULT ->
           GHC.Prim.+# ww_s1cq ww1_X1dh
           }
           }
       }

 eval
   = \ (ds_d112 :: Expr) ->
       case ds_d112 of {
         Lit n_aXf -> n_aXf;
         Plus a_aXg b_aXh ->
           case $weval a_aXg of { __DEFAULT ->
           case $weval b_aXh of { __DEFAULT ->
           GHC.Types.I# (GHC.Prim.+# a_aXg b_aXh)
           }
           }
       }
 }}}

 Note that we were able to rewrite both inductive calls in terms of
 `$weval`, but still have the vanilla version around if we ever need the
 boxed result of `eval`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16335#comment:2>
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