[GHC] #12191: 7% allocation regression in Haddock performance tests

[GHC]

#12191: 7% allocation regression in Haddock performance tests
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        Reporter:  bgamari           |                Owner:  niteria
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10482            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 When typechecking a particular module M, `checkFamInstConsistency` does
 indeed need to check that the imported family instances are consistent;
 and that requires checking any instance for `F` imported from `I1` with
 all instances of `F` imported from some other module `I2`.

 (Multiple instances all imported from `I1` have already been checked, when
 compiling `I1`.)

 I don't understand Haddock's use-case to know why the algorithm is
 behaving so badly, but I bet there is some low-hanging fruit.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12191#comment:11>
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