[GHC] #14827: Recognize when inlining would create a join point

[GHC]

#14827: Recognize when inlining would create a join point
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        Reporter:  ersetzen          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Great example.  Can someone explain to me why GHCi is faster?  And what it
 has to do with INLINE pragmas?

 Looking only at the Description, yes loopification should do a good job
 here.  We'd get this
 {{{
   letrec
      f a = case e of
              p1 -> f a'
              p2 -> (# l, r #)
   in case f e2 of (# l, r #) -> e3

 ==>  (loopify)

   let
      f a = joinrec jf a = case e of
                              p1 -> jump jf a'
                              p2 -> (# l, r #)
            in jump jf a
   in case f e2 of (# l, r #) -> e3

 ==> (inline `f`)

   case (let a = e2 in
         joinrec jf a = case e of
                              p1 -> jump jf a'
                              p2 -> (# l, r #)
         in jump jf a) of (# l, r #) -> e3

 ==> (move the outer case inwards, into the RHS of the joinrec)

   let a = e2 in
   joinrec jf a = case e of
                    p1 -> jump jf a'
                    p2 -> case (# l, r #) of (# l, r #) -> e3
   in jump jf a

 ==> (case of known constructor; and inline e2)

   joinrec jf a = case e of
                    p1 -> jump jf a'
                    p2 -> e3
   in jump jf e2
 }}}
 as desired.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14827#comment:5>
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