Re: [GHC] #10083: ghc: panic! (the 'impossible' happened)

9 Mar 2015

      #10083: ghc: panic! (the 'impossible' happened)
-------------------------------------+-------------------------------------
        Reporter:  hedayaty          |                   Owner:
            Type:  bug               |                  Status:  infoneeded
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 OK, good.  With your reduced test case I can reproduce the bug.  I've
 produced a much smaller version, in two variants
 {{{
 ---------- RSR.hs-boot ------------
 module RSR where
   data RSR
   instance Eq RSR

 ---------- SR.hs ------------
 module SR where
   import {-# SOURCE #-} RSR
   data SR = MkSR RSR deriving( Eq )

 ---------- SR.hs ------------
 module RSR where
   import SR
   data RSR = MkRSR SR deriving( Eq )
 }}}
 Now compile like this
 {{{
 ghc -c -O RSR.hs-boot
 ghc -c -O SR.hs
 ghc -c -O RSR.hs
 }}}
 Indeed, compiling `RSR` causes infinite inlining.

 Here's a version that doesn't use instances and so is a bit clearer
 {{{
 ---------- RSR.hs-boot ------------
 module RSR where
   data RSR
   eqRSR :: RSR -> RSR -> Bool

 ---------- SR.hs ------------
 module SR where
   import {-# SOURCE #-} RSR
   data SR = MkSR RSR
   eqSR (MkSR r1) (MkSR r2) = eqRSR r1 r2

 ---------- SR.hs ------------
 module RSR where
   import SR
   data RSR = MkRSR SR -- deriving( Eq )
   eqRSR (MkRSR s1) (MkRSR s2) = (eqSR s1 s2)
   foo x y = not (eqRSR x y)
 }}}
 This fails in the same way.  The problem is this.  When compiling `RSR` we
 get this code
 {{{
 RSR.eqRSR :: RSR.RSR -> RSR.RSR -> GHC.Types.Bool
 RSR.eqRSR =
   \ (ds_dkA [Occ=Once!] :: RSR.RSR)
     (ds_dkB [Occ=Once!] :: RSR.RSR) ->
     case ds_dkA of _ { RSR.MkRSR s1_aeO [Occ=Once] ->
     case ds_dkB of _ { RSR.MkRSR s2_aeP [Occ=Once] ->
     SR.eqSR s1_aeO s2_aeP
     }
     }

 RSR.foo :: RSR.RSR -> RSR.RSR -> GHC.Types.Bool
 RSR.foo =
   \ (x_aeQ [Occ=Once] :: RSR.RSR) (y_aeR [Occ=Once] :: RSR.RSR) ->
     GHC.Classes.not (RSR.eqRSR x_aeQ y_aeR)
 }}}
 Notice that neither are (apprently) recursive, and neither is a loop
 breaker.
 Now, when optimising `foo`:
  * Inline `eqRSR`
  * Inline `eqSR`
 but the result of inlining `eqSR` from `SR` is another call to `eqRSR`, so
 everything repeats.

 It's pretty simple, so I'm quite surprised that this hasn't bitten us
 before now!

 Next: figure out a solution.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10083#comment:12
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