[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:  8.8.1
       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):

 Just to record that we decided that the experiment in comment:68 is
 misleading.

 There are two main changes in Richard's patch

 1.  Do `closeOverKinds` at the end, here:
 {{{
 -- NEW
 tyCoVarsOfType = closeOverKinds . tcvs_of_type
 }}}
     rather than independently at each leaf:
 {{{
 -- OLD
 tyCoFVsOfType (TyVarTy v)        a b c = (unitFV v `unionFV` tyCoFVsOfType
 (tyVarKind v)) a b c

 -- NEW
 tcvs_of_type (TyVarTy v) = unitVarSet v
 }}}

 2. Insetad of using `FV` and then taking its `VarSet`, use `VarSets`
 directly. This jolly well ought to be faster.

 But in comment:68, Tobias used the new (1) but the old (2) and thus closed
 over the kind vars ''both'' at the
 leaves ''and'' at the end.  That's not going to be fast.

 We agreed that a it'd be iluminating to start from the existing GHC
 baseline and try (2) alone: is
 `VarSet` faster than `FV`?

 Then add (1).

 NB: it is (1) that fixes the actual bug in this ticket.  Ultimately it is
 not optional.  But (1) should
 be a perf boost too!  If a variable occurs N times, we'll take the free
 vars of its kind once instead
 of N times.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:70>
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