[GHC] #15560: Full laziness destroys opportunities for join points

23 Aug 2018

      #15560: Full laziness destroys opportunities for join points
-------------------------------------+-------------------------------------
           Reporter:  AndreasK       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
  (CodeGen)                          |
           Keywords:  JoinPoints     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #14287
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Even if we already know a binding is a join point we STILL float it to the
 top and turn it into a function.

 The simple example below results in a join point after the first
 simplifier run. Then we run the float out pass immediately undoing this by
 making it a top level binding.

 It then stays at the top till we are done resulting in the core I've put
 in the comments.

 {{{
 #!haskell
 data T = A | B | C | D | E | F | G

 {-# NOINLINE n #-}
 n :: T -> T
 n A = B
 n B = C
 n _ = A

 f :: Int -> T -> T -> T
 f sel x y =
     -- function large enough to avoid being simply inlined
     let j z = n . n . n . n . n . n $ z
     in case sel of
         -- j is always tailcalled
         0   -> j x
         _   -> j y

 -- j is floated to top level instead of ending up as joinpoint.
 -- T.f_j
 --   = \ (eta_B1 [OS=OneShot] :: T) -> n (n (n (n (n (n eta_B1)))))

 -- -- RHS size: {terms: 14, types: 6, coercions: 0, joins: 0/0}
 -- f :: Int -> T -> T -> T
 -- f = \ (sel_aYP :: Int) (x_aYQ :: T) (y_aYR :: T) ->
 --       case sel_aYP of { GHC.Types.I# ds_d2fL ->
 --       case ds_d2fL of {
 --         __DEFAULT -> T.f_j y_aYR;
 --         0# -> T.f_j x_aYQ
 --       }
 --       }
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15560
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