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

[GHC]

#15938: Hadrian's recompilation check is extremely slow
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        Reporter:  bgamari           |                Owner:  alpmestan
            Type:  bug               |               Status:  new
        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):
       Wiki Page:                    |
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Comment (by NeilMitchell):

 I actually managed to eliminate the ''O(n^2^)'' much more easily than
 expected. Now both builds take the same 0.31s. Having many rules is still
 going to be expensive for every file you match, but the cost is now not
 prohibitive. I'd still encourage the refactoring Alp is doing. I'll
 release a new Shake later today.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15938#comment:34>
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