Need help with debugging "Impossible case alternative"

[Jan Stolarek]

Thanks Simon. I think I tracked down the source of the problem.

When I modified primops to return Int# instead of Bool I assumed that the definition of what 
is "True" and "False" changes as well. So I changed (prelude/PrelRules.lhs):

trueVal, falseVal :: Expr CoreBndr
trueVal       = Var trueDataConId
falseVal      = Var falseDataConId 

to:

trueVal, falseVal :: DynFlags -> Expr CoreBndr
trueVal  dflags = Lit $ onei  dflags
falseVal dflags = Lit $ zeroi dflags 

I assumed that this is a single source of truth about what is "True" and "False". I just realized 
that this probably is an incorrect assumption and some parts of the compiler (mostly desugarer?) 
generate True and False instead of 1# and 0# during the transformations. I'll be trying to verify 
this. 

At the moment I am not sure how deep does this change goes. Should all "True" and "False" values 
generated by the compiler during transformations be replaced with 1# and 0#, so that only Bools 
appearing at the Core level are the ones declared explicitly by the user?

Janek

Dnia piątek, 10 maja 2013, Simon Peyton-Jones napisał:
> It's hard to help without being able to reproduce it.
>
> | If I understand correctly this means that the case tries to scrutinize
> | integer 1 against True and
> | False, which obviously fails (surprisingly, compiling with -dcore-lint
> | and -dcmm-lint doesn't
> | detect any problems).
>
> That sounds right.  So what is happening right now is that
>
> 	case 1 of { True -> e1; False -> e2 }
>
> is getting converted to
>
> 	error "Missing alternative"
>
> which isn't much help.  Better perhaps to leave it as
> 		case 1 of { True -> e1; False -> e2 }
>
> and then the next run of CoreLint will complain, and you can see where. You
> can do that by replacing the call to missingALt with a call to
> reallyRebuildCase env scrut case_bndr alts cont
> as in the last equation for rebuildCase
>
> Then you should get Core Lint errors in the offending library. (Make sure
> -dcore-lint is on all the time)
>
> S
>
> | -----Original Message-----
> | From: ghc-devs-bounces at haskell.org [mailto:ghc-devs-bounces at haskell.org]
> | On Behalf Of Jan Stolarek
> | Sent: 10 May 2013 11:52
> | To: ghc-devs at haskell.org
> | Subject: Need help with debugging "Impossible case alternative"
> |
> | I'm working on modifying comparison primops so that they return Int#
> | instead of Bool. I've been
> | stuck with one bug for about two weeks now and I just reached a point
> | where I don't have any more
> | ideas what to do. I would appreciate any hints or ideas.
> |
> | So far I changed the primops to return Int# and created a module
> | GHC.PrimWrappers that contains
> | wrappers around the new primops:
> |
> | (>#) :: Int# -> Int# -> Bool
> | (>#) a b = tagToEnum# (a >$# b)  -- (>$#) :: Int# -> Int# -> Int#
> |
> | I also made changes to base, integer-gmp, primitive and ghc-prim
> | libraries
> |
> | The problem is that many compiled programs produce "Impossible case
> | alternative" message when they
> | are run. This is generated by missingAlt function, which is called by
> | rebuildCase function on
> | line 1785 in simplCore/Simplify.lhs. I modified the message displayed by
> | missingAlt to get
> | information about the scrutinee, binder and case alternatives:
> |
> | missingAlt env scrut@(LitAlt _) case_bndr alts cont
> |   = WARN( True, ptext (sLit "missing LitAlt: ") <+> ppr case_bndr )
> |     return (env, mkImpossibleExpr_REMOVE_ME (contResultType cont)
> | (showSDoc unsafeGlobalDynFlags $
> | ptext (sLit "missing LitAlt : ") <+> ppr scrut <+> ppr case_bndr <+> ppr
> | alts))
> |
> | Here's an example error message:
> |
> | Impossible case alternative: missing LitAlt :  1 wild [(GHC.Types.False,
> | [], GHC.Types.False),
> |                           (GHC.Types.True, [], x)]
> |
> | If I understand correctly this means that the case tries to scrutinize
> | integer 1 against True and
> | False, which obviously fails (surprisingly, compiling with -dcore-lint
> | and -dcmm-lint doesn't
> | detect any problems). My theory is that this integer is supposed to be a
> | tag of a Bool data type
> | (in the example message this would be True) and the call to tagToEnum#
> | somehow got eliminated or
> | floated out of the case scrutinee. Is that a reasonable theory? Is there
> | a transform that could
> | perform such floating of tagToEnum# and if so how to prevent it?
> |
> | I am unable to create a minimal code example that would cause this
> | problem. I noticed that the
> | problem happens on every attempt to print a floating point number (this
> | causes tens of tests in
> | the testsuite to fail). I extracted all the code responsible for
> | converting Double to String from
> | the GHC.Float and GHC.Show modules so that I have a self-contained
> | program (except the call to
> | print). Surprisingly that extracted code does not produce the error
> | message. I'm clueless and
> | will really appreciate any ideas.
> |
> | Janek
> |
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