[GHC] #14880: GHC panic: updateRole

[GHC]

#14880: GHC panic: updateRole
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15076            |  Differential Rev(s):  Phab:D4769
       Wiki Page:                    |
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Comment (by simonpj):

 This is all very puzzling. But when we get to the bottom of it we may
 speed up a key piece of infrastructure!  So I think it's worth
 persevering.

 Some other ideas

 1.  Is `poly_merge` still high on the list in `ticky` in the accumulator
 version?  If so, why?

 2.  Can you post the accumulator version for types anyway?

 3.  Try switching back to FV.   That is, take the currently-deleted code
 for `tyCoFVsOfType`, and re-instate it, but modify it not to take the
 kinds at the variable occurrences.  Now use that instead of the new
 `fvs_of_type`.  It "obviously" should be a lot slower -- is it?

 4. In `unitFV` I see
 {{{
 unitFV :: Id -> FV
 unitFV var fv_cand in_scope acc@(have, haveSet)
   | var `elemVarSet` in_scope = acc
   | var `elemVarSet` haveSet = acc
   | fv_cand var = (var:have, extendVarSet haveSet var)
   | otherwise = acc
 }}}
 Notice that ''before inserting we test for membership''.  Could that
 possibly be a win?  If so, we can use it in the `VarSet` version.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:59>
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