id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
6078	GHC panic encountered for unlifted let bindings	SimonMeier		"The following code block makes GHC panic:

{{{

{-# LANGUAGE BangPatterns #-}
module GhcPanic where

import GHC.Ptr
import Foreign 

-- Occurred in a really low-level bytestring decoder implementation.
{-# INLINE byteStringSlice #-}
byteStringSlice len = \fpbuf ip0 ipe s0 ->
    let ip1p@(Ptr ip1) = Ptr ip0 `plusPtr` len
    -- Note that the panic goes away if we use a bang-pattern as follows
    -- let !ip1p@(Ptr ip1) = Ptr ip0 `plusPtr` len
    in  ip1p
}}}

The panic output is

{{{
ghc-7.4.1 GhcPanic.hs                                                           
[1 of 1] Compiling GhcPanic         ( GhcPanic.hs, GhcPanic.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.1 for x86_64-unknown-linux):
        dsLet: unlifted
    AbsBinds
    [( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )]
    []
    [( ip1p{v acI} [lid] :: forall ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ).
                            base:GHC.Ptr.Ptr{tc 33A}
                              ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ) ) <= ( ip1p{v acK} [lid] :: base:GHC.Ptr.Ptr{tc 33A}
                                                                                                                  ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ) )
       /\(@ ( t{tv acW} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )).
       (<>) @ ( t{tv acW} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ),
     ( ip1{v acJ} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33} ) <= ( ip1{v acP} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33} )
       (<>)
       @ (ghc-prim:GHC.Prim.Any{(w) tc 31N}
            ghc-prim:GHC.Prim.*{(w) tc 34d})]
      ( ip1p{v acI} [lid] :: forall ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ).
                             base:GHC.Ptr.Ptr{tc 33A}
                               ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ) )
        :: forall ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ).
           base:GHC.Ptr.Ptr{tc 33A}
             ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )
      [LclId]
      ( ip1{v acJ} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33} )
        :: ghc-prim:GHC.Prim.Addr#{(w) tc 33}
      [LclId]
      {GhcPanic.hs:10:9-46}
      {GhcPanic.hs:10:9-22}
      {GhcPanic.hs:10:9-12}
      ( ip1p{v acK} [lid] :: base:GHC.Ptr.Ptr{tc 33A}
                               ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ) )@({GhcPanic.hs:10:15-21}
                                                                                        {GhcPanic.hs:10:15-17}
                                                                                        base:GHC.Ptr.Ptr{d r3C}{EvBinds{}} ((( ip1{v acP} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33} )
                                                                                                                               :: ghc-prim:GHC.Prim.Addr#{(w) tc 33})))
        = {GhcPanic.hs:10:26-46}
          {GhcPanic.hs:10:34-42}
          (base:GHC.Ptr.plusPtr{v r3E})
          @ (ghc-prim:GHC.Prim.Any{(w) tc 31N}
               ghc-prim:GHC.Prim.*{(w) tc 34d})
          @ ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )
            ((base:GHC.Ptr.Ptr{v r39})
             @ (ghc-prim:GHC.Prim.Any{(w) tc 31N}
                  ghc-prim:GHC.Prim.*{(w) tc 34d})
               ip0{v acF})
            len{v acD}
    EvBinds{}
    ( ip1p{v acI} [lid] :: forall ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ).
                           base:GHC.Ptr.Ptr{tc 33A}
                             ( t{tv acU} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} ) )
      @ ( t{tv ad5} [sk] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )

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

}}}

It might be that its an instance of #4498."	bug	closed	normal		Compiler	7.4.1	fixed	unlifted, bang patterns		Unknown/Multiple	Unknown/Multiple	Compile-time crash	Unknown	typecheck/should_fail/T6078			4498