[GHC] #10372: panic! compiling Y combinator with optimizations

1 May 2015


      #10372: panic! compiling Y combinator with optimizations
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              Reporter:  cdk         |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:  panic,      |  Operating System:  Unknown/Multiple
  simplifier                         |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 The following causes GHC 7.6.3, 7.8.3 and 7.10.1 (at least) to panic when
 compiling with optimizations:

 {{{#!hs
 module Y (y) where

 newtype Rec a = Rec { out :: Rec a -> a}

 k x = out x x

 y f = (f . k) (Rec (f . k))
 }}}

 {{{
 /tmp ««« ghc -O y.hs -fforce-recomp
 [1 of 1] Compiling Y                ( y.hs, y.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.1 for x86_64-unknown-linux):
         Simplifier ticks exhausted
   When trying UnfoldingDone a_sof
   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: 5282
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10372
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