Re: [GHC] #14068: Loopification using join points (was: Turn tail-recursive functions into recursive jointpoints)

1 Aug 2017

      #14068: Loopification using join points
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  mpickering
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13966 #14067     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by simonpj):

 * keywords:   => JoinPoints

Old description:
...
The function
{{{
let f x y = case y of
              A -> f x' y'
              B -> e2
              C -> e3
in g f
}}}
is not turned into a recursive join point, because the call to `f` is not
in tail call position. But the recursive calls are, and these matter
performance-wise! Hence, it would be beneficial to turn this into
{{{
let f x y = joinrec $j x y = case x y of
                              A -> $j x' y'
                              B -> e2
                              C -> e3
            in $j x y
in g f
}}}
This has the additional effect that now `f` is no longer recursive and
may get inlined.
This came up in #13966, and might go well with #14067.
New description:

 The function
 {{{
 let f x y = case y of
               A -> f x' y'
               B -> e2
               C -> e3
 in g f
 }}}
 is not turned into a recursive join point, because the call to `f` is not
 in tail call position. But the recursive calls are, and these matter
 performance-wise! Hence, it would be beneficial to turn this into
 {{{
 let f x y = joinrec $j x y = case x y of
                               A -> $j x' y'
                               B -> e2
                               C -> e3
             in $j x y
 in g f
 }}}

 This has the additional effect that now `f` is no longer recursive and may
 get inlined.

 The idea is described under "New idea: use join points" in
 [wiki:Commentary/Compiler/Loopification].

 It came up in #13966, and might go well with #14067.

 It might work well with #13051, which Thomas Jakway is still thinking
 about.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14068#comment:3
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