[project @ 2002-04-24 11:06:10 by simonpj]

Fix an obscure corner situation in case-simplification (cg051)

diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs
index 90151b9..1d9b987 100644 (file)
--- a/ghc/compiler/simplCore/SimplUtils.lhs
+++ b/ghc/compiler/simplCore/SimplUtils.lhs
@@ -861,6 +861,16 @@ prepareDefault case_bndr handled_cons (Just rhs)
                                --      case x of { DEFAULT -> e }
                                -- and we don't want to fill in a default for them!
     Just all_cons <- tyConDataCons_maybe tycon,
+    not (null all_cons),       -- This is a tricky corner case.  If the data type has no constructors,
+                               -- which GHC allows, then the case expression will have at most a default
+                               -- alternative.  We don't want to eliminate that alternative, because the
+                               -- invariant is that there's always one alternative.  It's more convenient
+                               -- to leave     
+                               --      case x of { DEFAULT -> e }     
+                               -- as it is, rather than transform it to
+                               --      error "case cant match"
+                               -- which would be quite legitmate.  But it's a really obscure corner, and
+                               -- not worth wasting code on.
     let handled_data_cons = [data_con | DataAlt data_con <- handled_cons],
     let missing_cons      = [con | con <- all_cons, 
                                   not (con `elem` handled_data_cons)]
@@ -1170,6 +1180,16 @@ I don't really know how to improve this situation.
 
 \begin{code}
 --------------------------------------------------
+--     0. Check for empty alternatives
+--------------------------------------------------
+
+#ifdef DEBUG
+mkCase1 scrut case_bndr []
+  = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
+    returnSmpl scrut
+#endif
+
+--------------------------------------------------
 --     1. Eliminate the case altogether if poss
 --------------------------------------------------
 
@@ -1215,12 +1235,6 @@ mkCase1 scrut case_bndr [(con,bndrs,rhs)]
 --     2. Identity case
 --------------------------------------------------
 
-#ifdef DEBUG
-mkCase1 scrut case_bndr []
-  = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
-    returnSmpl scrut
-#endif
-
 mkCase1 scrut case_bndr alts   -- Identity case
   | all identity_alt alts
   = tick (CaseIdentity case_bndr)              `thenSmpl_`

diff --git a/ghc/compiler/simplCore/Simplify.lhs b/ghc/compiler/simplCore/Simplify.lhs
index 5cae204..0a61418 100644 (file)
--- a/ghc/compiler/simplCore/Simplify.lhs
+++ b/ghc/compiler/simplCore/Simplify.lhs
@@ -1240,7 +1240,7 @@ rebuildCase env scrut case_bndr alts cont
 
   | otherwise
   = prepareAlts scrut case_bndr alts           `thenSmpl` \ (better_alts, handled_cons) -> 
-
+       
        -- Deal with the case binder, and prepare the continuation;
        -- The new subst_env is in place
     prepareCaseCont env better_alts cont       `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->