[GHC] #15164: Slowdown in ghc compile times from GHC 8.0.2 to GHC 8.2.1 when doing Called arity analysis

[GHC]

#15164: Slowdown in ghc compile times from GHC 8.0.2 to GHC 8.2.1 when doing Called
arity analysis
-------------------------------------+-------------------------------------
        Reporter:  flip101           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  Compile-time      |            Test Case:
  performance bug                    |  perf/compiler/T15164
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Joachim Breitner <mail@…>):

 In [changeset:"db6085b84139f4454cebf34f887cb5560a4fbc7b/ghc"
 db6085b8/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="db6085b84139f4454cebf34f887cb5560a4fbc7b"
 Improve performance of CallArity

 the hot path contained a call to

     v `elemUnVarSet` (neighbors g v)

 and creating the set of neighbors just to check if `v` is inside
 accounted for half the allocations of the test case of #15164.

 By introducing a non-allocating function `hasLoopAt` for this we shave
 off half the allocations. This brings the total cost of Call Arity down
 to 20% of time and 23% of allocations, according to a profiled run. Not
 amazing, but still much better.

 Differential Revision: https://phabricator.haskell.org/D4718
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15164#comment:23>
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