[GHC] #14880: GHC panic: updateRole

[GHC]

#14880: GHC panic: updateRole
-------------------------------------+-------------------------------------
        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 tdammers):

 Replying to [comment:78 goldfire]:
 > Before we continue drilling in that direction, are these results
 reproducible? Timing is notoriously fickle, and I think it would be wise
 to make sure we know what we're getting into here.

 Oh yes, absolutely right, and I just did exactly that.

 Turns out the switch from FV to VarSet increases allocations quite a bit,
 but not enough to explain the failure completely - only about halfway.
 Observe:

 baseline:
 {{{
 ("bytes allocated", "1145886664")
 }}}

 with VarSet instead of fv:
 {{{
 ("bytes allocated", "1176456040")
 }}}

 patch applied fully:
 {{{
 ("bytes allocated", "1207902048")
 }}}

 This uses the settings from the test suite directly, and a validate build.
 So it should be perfectly reproducible, something like this:

 {{{
 for NAME in T14880-baseline T14880-just-tvs T14880
 do
     echo "$NAME"
     git checkout "wip/$NAME" && (make -j2 && (/usr/bin/time --verbose
 ~/well-typed/devel/ghc-phab/inplace/bin/ghc-stage2 -c
 testsuite/tests/perf/compiler/T5631.hs -fforce-recomp +RTS -V0
 -tlogs/$NAME.stat --machine-readable -RTS) |& tee logs/$NAME.timing &&
 echo "$NAME")
 done
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:79>
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