[GHC] #15938: Hadrian's recompilation check is extremely slow

[GHC]

#15938: Hadrian's recompilation check is extremely slow
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  alpmestan
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.8.1
       Component:  Build System      |              Version:  8.6.2
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5412,
       Wiki Page:                    |  Phab:D5422
-------------------------------------+-------------------------------------

Comment (by snowleopard):

 Many thanks, Alp! I agree that rewriting `Rules.Generate` is most likely
 unnecessary.

 I think the worst-case startup time of 5 seconds is acceptable, although
 we might come back to optimising Hadrian further at some point. This
 probably deserves a separate, lower priority ticket.

 @bgamari: Could you check Alp's results on your setup?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15938#comment:41>
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