[GHC] #14079: Failure to do CPR in the presence of a local letrec

2 Aug 2017

      #14079: Failure to do CPR in the presence of a local letrec
-------------------------------------+-------------------------------------
           Reporter:  nomeata        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           Keywords:  JoinPoints     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider this code:
 {{{
 {-# LANGUAGE BangPatterns #-}
 module NoCPR (e) where
 e :: (Int, Int) -> Int -> Int -> (Int, Int)
 e x y n = je x y
  where je !x y | y > 0 = x
                | otherwise = je x (y + n)
 }}}
 (which is adapted from #5949).

 We get this Core:
 {{{
 -- RHS size: {terms: 38, types: 27, coercions: 0, joins: 1/1}
 e :: (Int, Int) -> Int -> Int -> (Int, Int)
 [GblId,
  Arity=3,
  Caf=NoCafRefs,
  Str=<S,1*U(U,U)><S(S),1*U(U)><L,U(U)>m,
  Unf=OtherCon []]
 e = \ (x [Occ=Once!] :: (Int, Int))
       (y [Occ=Once!] :: Int)
       (n [Occ=OnceL!] :: Int) ->
       case x of { (ww1 [Occ=Once], ww2 [Occ=Once]) ->
       case y of { I# ww4 [Occ=Once] ->
       joinrec {
         $wje [InlPrag=[0], Occ=LoopBreakerT[3]]
           :: Int -> Int -> Int# -> (Int, Int)
         [LclId[JoinId(3)], Arity=3, Str=<L,U><L,U><S,U>m, Unf=OtherCon []]
         $wje (ww5 [Occ=Once*] :: Int)
              (ww6 [Occ=Once*] :: Int)
              (ww7 :: Int#)
           = case ># ww7 0# of {
               __DEFAULT ->
                 case n of { I# y1 [Occ=Once] ->
                 case +# ww7 y1 of sat { __DEFAULT -> jump $wje ww5 ww6 sat
 }
                 };
               1# -> (ww5, ww6)
             }; } in
       jump $wje ww1 ww2 ww4
       }
       }
 }}}

 Why is there no CPR happening for `e`? In fact, why is there no unboxing
 happening – it was for the following similar code:
 {{{
 e :: (Int, Int) -> Int -> (Int, Int)
 e x y = x `seq` if y > 10
         then x
         else e x (y + 1)
 }}}

 (This is a spin-off of the dicussion at
 https://phabricator.haskell.org/D3811#107708).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14079>
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