[GHC] #14565: Memory leak switching from -O1 to -O2

[GHC]

#14565: Memory leak switching from -O1 to -O2
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           Reporter:  dbeacham       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:  x86_64         |   Type of failure:  Runtime
  (amd64)                            |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #14379
Differential Rev(s):                 |         Wiki Page:
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 GHC seems to get stuck at SpecConstr stage when compiling. Cpu and memory
 usage then sky rocket.

 I've managed to get the example down to what appears to be minimal but
 making a number of minor changes makes the compilation go through quickly:

 * changing `toIdx` to `const 0`
 * removing one of the `V.forM_` layers.
 * using "-fno-spec-constr" (unsuprisingly).

 I can't reproduce it on 8.0.2.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14565>
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