Zap dead-ness info appropriately in SpecConstr

SpecConstr can make pattern binders come alive, so we must remember
to zap their dead-variable annotation.  See extendCaseBndrs.

(This was triggering a Core Lint failure in DPH.)

diff --git a/compiler/specialise/SpecConstr.lhs b/compiler/specialise/SpecConstr.lhs
index bdd9a16..0280255 100644 (file)
--- a/compiler/specialise/SpecConstr.lhs
+++ b/compiler/specialise/SpecConstr.lhs
@@ -27,8 +27,7 @@ import DataCon                ( dataConRepArity, dataConUnivTyVars )
 import Coercion        
 import Rules
 import Type            hiding( substTy )
 import Coercion        
 import Rules
 import Type            hiding( substTy )

-import Id              ( Id, idName, idType, isDataConWorkId_maybe, idArity,
-                         mkUserLocal, mkSysLocal, idUnfolding, isLocalId )
+import Id

 import Var
 import VarEnv
 import VarSet
 import Var
 import VarEnv
 import VarSet

@@ -591,17 +590,28 @@ extendValEnv :: ScEnv -> Id -> Maybe Value -> ScEnv
 extendValEnv env _  Nothing   = env
 extendValEnv env id (Just cv) = env { sc_vals = extendVarEnv (sc_vals env) id cv }
 
 extendValEnv env _  Nothing   = env
 extendValEnv env id (Just cv) = env { sc_vals = extendVarEnv (sc_vals env) id cv }
 

-extendCaseBndrs :: ScEnv -> CoreExpr -> Id -> AltCon -> [Var] -> ScEnv
+extendCaseBndrs :: ScEnv -> Id -> AltCon -> [Var] -> (ScEnv, [Var])

 -- When we encounter
 --     case scrut of b
 --         C x y -> ...
 -- When we encounter
 --     case scrut of b
 --         C x y -> ...

--- we want to bind b, and perhaps scrut too, to (C x y)
--- NB: Extends only the sc_vals part of the envt
-extendCaseBndrs env scrut case_bndr con alt_bndrs
-  = case scrut of
-       Var v  -> extendValEnv env1 v cval
-       _other -> env1
+-- we want to bind b, to (C x y)
+-- NB1: Extends only the sc_vals part of the envt
+-- NB2: Kill the dead-ness info on the pattern binders x,y, since
+--      they are potentially made alive by the [b -> C x y] binding
+extendCaseBndrs env case_bndr con alt_bndrs
+  | isDeadBinder case_bndr
+  = (env, alt_bndrs)
+  | otherwise
+  = (env1, map zap alt_bndrs)
+       -- NB: We used to bind v too, if scrut = (Var v); but
+        --     the simplifer has already done this so it seems
+        --     redundant to do so here
+       -- case scrut of
+       --      Var v  -> extendValEnv env1 v cval
+       --      _other -> env1

  where
  where

+   zap v | isTyVar v = v               -- See NB2 above
+         | otherwise = zapIdOccInfo v

    env1 = extendValEnv env case_bndr cval
    cval = case con of
                DEFAULT    -> Nothing
    env1 = extendValEnv env case_bndr cval
    cval = case con of
                DEFAULT    -> Nothing

@@ -788,15 +798,15 @@ scExpr' env (Case scrut b ty alts)
          ; return (alt_usg `combineUsage` scrut_usg',
                    Case scrut' b' (scSubstTy env ty) alts') }
 
          ; return (alt_usg `combineUsage` scrut_usg',
                    Case scrut' b' (scSubstTy env ty) alts') }
 

-    sc_alt env scrut' b' (con,bs,rhs)
-      = do { let (env1, bs') = extendBndrsWith RecArg env bs
-                env2        = extendCaseBndrs env1 scrut' b' con bs'
+    sc_alt env _scrut' b' (con,bs,rhs)
+      = do { let (env1, bs1)  = extendBndrsWith RecArg env bs
+                (env2, bs2) = extendCaseBndrs env1 b' con bs1

           ; (usg,rhs') <- scExpr env2 rhs
           ; (usg,rhs') <- scExpr env2 rhs

-          ; let (usg', arg_occs) = lookupOccs usg bs'
+          ; let (usg', arg_occs) = lookupOccs usg bs2

                 scrut_occ = case con of
                                DataAlt dc -> ScrutOcc (unitUFM dc arg_occs)
                                _          -> ScrutOcc emptyUFM
                 scrut_occ = case con of
                                DataAlt dc -> ScrutOcc (unitUFM dc arg_occs)
                                _          -> ScrutOcc emptyUFM

-          ; return (usg', scrut_occ, (con,bs',rhs')) }
+          ; return (usg', scrut_occ, (con, bs2, rhs')) }

 
 scExpr' env (Let (NonRec bndr rhs) body)
   | isTyVar bndr       -- Type-lets may be created by doBeta
 
 scExpr' env (Let (NonRec bndr rhs) body)
   | isTyVar bndr       -- Type-lets may be created by doBeta