Fix case-merge bug that was breaking the HEAD

My re-org of the case-merging transformation introduced a bug, which led
to incorrect code.   This only showed up occasionally, but it generated
incorrect code for PprC.pprCastReg in the stage-2 compiler. As a result
the stage-2 compiler ran without crashing, but itself generated bogus C.

For a change, this is one that Core Lint couldn't find, so the trail was
a bit longer.  The fix is easy (and commented).

diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs
index 1ff6f8f..9985397 100644 (file)
--- a/compiler/simplCore/SimplUtils.lhs
+++ b/compiler/simplCore/SimplUtils.lhs
@@ -1192,20 +1192,19 @@ prepareAlts scrut case_bndr' alts
        ; let (alts_wo_default, maybe_deflt) = findDefault alts
              alt_cons = [con | (con,_,_) <- alts_wo_default]
              imposs_deflt_cons = nub (imposs_cons ++ alt_cons)
-               -- "imposs_deflt_cons" are handled either by the context, 
-               -- OR by a branch in this case expression.
-               -- Don't include DEFAULT!!
+               -- "imposs_deflt_cons" are handled 
+               --   EITHER by the context, 
+               --   OR by a non-DEFAULT branch in this case expression.
 
        ; default_alts <- prepareDefault dflags scrut case_bndr' mb_tc_app 
                                         imposs_deflt_cons maybe_deflt
 
        ; let trimmed_alts = filter possible_alt alts_wo_default
-             merged_alts = mergeAlts default_alts trimmed_alts
+             merged_alts = mergeAlts trimmed_alts default_alts
                -- We need the mergeAlts in case the new default_alt 
                -- has turned into a constructor alternative.
                -- The merge keeps the inner DEFAULT at the front, if there is one
-               -- and eliminates any inner_alts that are shadowed by the outer_alts
-
+               -- and interleaves the alternatives in the right order
 
        ; return (imposs_deflt_cons, merged_alts) }
   where
@@ -1262,7 +1261,17 @@ prepareDefault dflags scrut outer_bndr bndr_ty imposs_cons (Just deflt_rhs)
   = do { tick (CaseMerge outer_bndr)
 
        ; let munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
-       ; return [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts] }
+       ; return [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts,
+                                              not (con `elem` imposs_cons) ]
+               -- NB: filter out any imposs_cons.  Example:
+               --      case x of 
+               --        A -> e1
+               --        DEFAULT -> case x of 
+               --                      A -> e2
+               --                      B -> e3
+               -- When we merge, we must ensure that e1 takes 
+               -- precedence over e2 as the value for A!  
+       }
        -- Warning: don't call prepareAlts recursively!
        -- Firstly, there's no point, because inner alts have already had
        -- mkCase applied to them, so they won't have a case in their default