[GHC] #11973: Simplifier ticks exhausted with complex list literal.

[GHC]

#11973: Simplifier ticks exhausted with complex list literal.
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           Reporter:  Lokathor       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Windows
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 So I had a working version of a module that was too slow:
 https://github.com/Lokathor/ludolib/blob/6e69f87762c88d7909ca259ffe69141ea4b707c0/src/Util/AutomataGen.hs#L167

 and then I tried to rearrange things so that the checking could short-
 circuit a bit and run faster. I came up with this:
 https://gist.github.com/Lokathor/d1e4a8cd6268835a2403423fb180a71d#file-
 automatagencrashesghc-hs-L172

 Which loads and runs just fine in GHCi, but then when I go to compile it
 into a binary for benchmarking I got the following:
 {{{
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-mingw32):
         Simplifier ticks exhausted
   When trying UnfoldingDone a_siVC
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   To see detailed counts use -ddump-simpl-stats
   Total ticks: 294802

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Setting the factor up to 1k caused it to crash out with the same problem.
 Setting the factor up to 10k caused GHC to just eat up nearly all of my
 RAM (11GB) and then stall out until I killed it a few minutes later.

 I tried not using -O2 during compilation, but it had the same problem
 anyway. With some IRC help, it was suggested to add the `-fsimple-list-
 literals` flag, which makes things compile and run just fine (and with the
 speed gain that I'd hoped for).

 So things work now, for some definition of "work", though I was advised to
 report this bad behavior anyway.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11973>
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