Need help with debugging "Impossible case alternative"

[Jan Stolarek]

Oh, I forgot about this one:

http://hpaste.org/87885#line391

That's what I got from -dcore-lint after replacing missingAlt with reallyRebuildCase. This is 
reported while building GHC.Float module in base package. Build log is here:

http://hpaste.org/87886

Janek

Dnia poniedziałek, 13 maja 2013, Jan Stolarek napisał:
> 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
> > |
> > | _______________________________________________
> > | ghc-devs mailing list
> > | ghc-devs at haskell.org
> > | http://www.haskell.org/mailman/listinfo/ghc-devs
>
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