[GHC] #13623: join points produce bad code for stream fusion

[GHC]

#13623: join points produce bad code for stream fusion
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        Reporter:  choenerzs         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.2.1-rc1
      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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"9e47dc451788cce20acb6a8208c56a7e4dbe246b/ghc"
 9e47dc45/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="9e47dc451788cce20acb6a8208c56a7e4dbe246b"
 Fix loss-of-SpecConstr bug

 This bug, reported in Trac #13623 has been present since

   commit b8b3e30a6eedf9f213b8a718573c4827cfa230ba
   Author: Edward Z. Yang <ezyang at cs.stanford.edu>
   Date:   Fri Jun 24 11:03:47 2016 -0700

       Axe RecFlag on TyCons.

 SpecConstr tries not to specialise indefinitely, and had a
 limit (see Note [Limit recursive specialisation]) that made
 use of info about whether or not a data constructor was
 "recursive".  This info vanished in the above commit, making
 the limit fire much more often -- and indeed it fired in this
 test case, in a situation where specialisation is /highly/
 desirable.

 I refactored the test, to look instead at the number of
 iterations of the loop of "and now specialise calls that
 arise from the specialisation".  Actually less code, and
 more robust.

 I also added record field names to a couple of constructors,
 and renamed RuleInfo to SpecInfo.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13623#comment:10>
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