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

[GHC]

#14827: Recognize when inlining would create a join point
-------------------------------------+-------------------------------------
           Reporter:  ersetzen       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           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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 [https://www.reddit.com/r/haskell/comments/7yh8ar/i_wrote_a_program_that_runs_about_10_times_faster/
 This discussion] revolved around a program that runs 10x faster under
 ghci.

 One way to solve this is to remove a superfluous inline pragma which
 allows the following transformation to happen:

 {{{
 letrec {
 f a = case e of {
     p1 -> f a';
     p2 -> (# l, r #);
 }} in case f e2 of { (# l, r #) -> e3; }
 }}}

 into

 {{{
 joinrec {
 f a = case e of {
     p1 -> jump f a';
     p2 -> e3;
 }} in jump f e2
 }}}

 More generally a recursive let binding that is called exactly once from
 the outside. If all recursive calls are tail calls and the outside one
 isn't then we could safely replace the call with the binding and end up
 with join points. In this case it means a 10x speedup so it might be worth
 doing generally.


 {{{
 letrec { fi = ei; } in ... (fj e) ... => ... (joinrec { fi = ei; } in fj
 e) ...
 }}}

 [https://gist.github.com/AndreasPK/8e6f0cbf253f0930f4cda81e685ac136  Self
 contained example]

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