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