[GHC] #13429: Optimizer produces Core with an infinite <<loop>>

[GHC]

#13429: Optimizer produces Core with an infinite <<loop>>
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        Reporter:  lehins            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  7.10.4
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 I reduced the test case again. See `LoopReduced2.hs` and `mainReduced2.hs`
 (you'll have to rename the `LoopReduced2.hs` file to `Loop.hs` to
 compile). I attached `-dverbose-core2core` results before and after the
 early inlining patch as `dumpbefore.tgz` and `dumpafter.tgz`. I haven't
 figured out where things go wrong just yet, but if you look at
 `dumpbefore/main.dump-simpl`, you'll see

 {{{#!hs
 Rec {
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 Main.$s$fArrayXe_$dNum [Occ=LoopBreaker] :: Num Word8
 [GblId, Str=b]
 Main.$s$fArrayXe_$dNum = Main.$s$fArrayXe_$dNum
 end Rec }
 }}}

 which is very obviously very bad. For some reason we're "optimizing" a
 dictionary into a reference to itself! My vague guess is that GHC sees
 that it can get a `Num Word8` dictionary from an `Array` or `ColorSpace`
 dictionary, and that it can get `ColorSpace` and `Array` dictionaries from
 `Num` ones, and makes some very bad decisions. I have no idea why the
 early inline patch fixes this, but I'm rather nervous about it being a
 proper fix.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13429#comment:13>
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