[GHC] #14844: SpecConstr also non-recursive function

[GHC]

#14844: SpecConstr also non-recursive function
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:  SpecConstr
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 nomeata):

 I distilled this example to show the effect
 {{{
 module T14844Example (bar1, bar2) where

 large x = x
 {-# NOINLINE large #-}


 foo :: Int -> (a -> b -> Bool) -> (a,b) -> Bool
 foo 0 _ _ = False
 foo s f t = l s' t
    where
      l 0 t = False
      l 1 t = case t of (x,y) -> f x y
      l n (x,y) = l (n-1) (x,y)
      s' = -- To prevent inlining
        large $ large $ large $ large $ large $ large $ large $ large $
        large $ large $ large $ large $ large $ large $ large $ large $
        s

 bar1 :: Int -> (a -> b -> Bool) -> a -> b -> Bool
 bar1 s f x y = foo s f (x,y)

 bar2 :: Int ->  (a -> b -> Bool) -> a -> b -> Bool
 bar2 s f x y = foo (s + 1) f (x,y)
 }}}
 This needs to rounds of !SpecConstr to specialize `foo` for the `(x,y)`
 call pattern; in the first round, `l` is specialized, then the (then non-
 recursive `l`) gets inlined into `foo`, then `foo` gets specialized.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14844#comment:7>
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