[GHC] #14657: Quadratic constructor tag allocation

[GHC]

#14657: Quadratic constructor tag allocation
-------------------------------------+-------------------------------------
        Reporter:  niteria           |                Owner:  niteria
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  performance bug                    |  genManyConstructors
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  phab:D4289
       Wiki Page:                    |
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Comment (by Bartosz Nitka <niteria@…>):

 In [changeset:"dbdf77d92c9cd0bbb269137de0bf8754573cdc1e/ghc" dbdf77d/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="dbdf77d92c9cd0bbb269137de0bf8754573cdc1e"
 Lift constructor tag allocation out of a loop

 Before this change, for each constructor that we want
 to allocate a tag for we would traverse a list of all
 the constructors in a datatype to determine which tag
 a constructor should get.

 This is obviously quadratic and for datatypes with 10k
 constructors it actually makes a big difference.

 This change implements the plan outlined by @simonpj in
 https://mail.haskell.org/pipermail/ghc-devs/2017-October/014974.html
 which is basically about using a map and constructing it outside the
 loop.

 One place where things got a bit awkward was TysWiredIn.hs,
 it would have been possible to just assign the tags by hand, but
 that seemed error-prone to me, so I decided to go through a map
 there as well.

 Test Plan:
 ./validate
 On a file with 10k constructors
 Before:
    8,130,522,344 bytes allocated in the heap
   Total   time    3.682s  (  3.920s elapsed)
 After:
    4,133,478,744 bytes allocated in the heap
   Total   time    2.509s  (  2.750s elapsed)

 Reviewers: simonpj, bgamari

 Reviewed By: simonpj

 Subscribers: goldfire, rwbarton, thomie, simonmar, carter, simonpj

 GHC Trac Issues: #14657

 Differential Revision: https://phabricator.haskell.org/D4289
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14657#comment:2>
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