[GHC] #12161: Panic when literal is coerced into function

7 Jun 2016

      #12161: Panic when literal is coerced into function
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  lowest         |         Milestone:  ⊥
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I don't think there is ever a legitimate use-case for any code which
 triggers this error, but it does seem to indicate some sort of problem
 with how ANF Core is defined. Consider:

 {{{
 {-# LANGUAGE MagicHash #-}
 module G where
 import GHC.Prim

 f :: a -> a
 f = unsafeCoerce# 5#

 g = case f True of
         True -> ()
         False -> ()
 }}}

 When I build this I get:

 {{{
 ezyang@sabre:~$ ghc-8.0 -c G.hs -fforce-recomp
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
         CoreToStg.myCollectArgs
   (5#
    `cast` (UnsafeCo representational Int# (Bool -> Bool)
            :: Int# ~R# (Bool -> Bool)))
     True

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

 According to the specification of ANF Core in `CorePrep`, this Core is
 perfectly legitimate:

 {{{
     Trivial expressions
        triv ::= lit |  var
               | triv ty  |  /\a. triv
               | truv co  |  /\c. triv  |  triv |> co

     Applications
        app ::= lit  |  var  |  app triv  |  app ty  | app co | app |> co

     Expressions
        body ::= app
               | let(rec) x = rhs in body     -- Boxed only
               | case body of pat -> body
               | /\a. body | /\c. body
               | body |> co

     Right hand sides (only place where value lambdas can occur)
        rhs ::= /\a.rhs  |  \x.rhs  |  body
 }}}

 as the terminal productions for an `app` include both literals and
 variables. However, `myCollectArgs` assumes that there are only variables
 in the function position. So it seems that in practice we need a stronger
 invariant on ANF.

 An obvious fix is to remove `lit` from `app`, but that is not quite enough
 because then there is no way to represent expressions of the form `let x =
 lit in body` (in particular, `MachStr` and `LitInteger` are not considered
 trivial and may very well be let-bound). So perhaps the right way to do
 this is to remove `lit` from `app`, and add it to `body`:

 {{{

     Trivial expressions
        triv ::= lit |  var
               | triv ty  |  /\a. triv
               | truv co  |  /\c. triv  |  triv |> co

     Applications (removed lit)
        app ::= var  |  app triv  |  app ty  | app co | app |> co

     Expressions (added lit)
        body ::= app
               | lit
               | let(rec) x = rhs in body     -- Boxed only
               | case body of pat -> body
               | /\a. body | /\c. body
               | body |> co

     Right hand sides (only place where value lambdas can occur)
        rhs ::= /\a.rhs  |  \x.rhs  |  body
 }}}

 the point being that we never have an application with a literal in
 function position.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12161
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler