[GHC] #13867: Silly definitions remain after SpecConstr

23 Jun 2017

      #13867: Silly definitions remain after SpecConstr
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this
 {{{
 f x xs = let g y = case y of
                   [] -> 0
                   (a:as) -> g as
          in
          g (x:xs)
 }}}
 Compile with -O2 and we get this
 {{{
 f =
   \ (@ t_axv)
     (@ a_axw)
     ($dNum_axx :: Num a_axw)
     (x_amR :: t_axv)
     (xs_amS :: [t_axv]) ->
     let {
       lvl_sxV [Dmd=] :: a_axw
       [LclId, Str=DmdType]
       lvl_sxV = fromInteger @ a_axw $dNum_axx Foo.f1 } in
     letrec {
       $sg_syr [Occ=LoopBreaker] :: t_axv -> [t_axv] -> a_axw
       [LclId, Arity=2, Str=DmdType ]
       $sg_syr = \ _ [Occ=Dead] (sc1_syq :: [t_axv]) -> g_sxS sc1_syq;
       g_sxS [Occ=LoopBreaker] :: [t_axv] -> a_axw
       [LclId, Arity=1, Str=DmdType ]
       g_sxS =
         \ (y_amU :: [t_axv]) ->
           case y_amU of _ [Occ=Dead] {
             [] -> lvl_sxV;
             : a1_amV as_amW -> g_sxS as_amW
           }; } in
     $sg_syr x_amR xs_amS
 }}}
 Look at that stupid `$sg_syr` function.  It should jolly well be inlined
 at its only call site.  But it isn't because it's a loop breaker?  Why is
 it a loop breaker? Because earlier there was a RULE for `g` that mentioned
 `$sg`.

 This is stupid.  It's not killing us but it is obviously wrong.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13867
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