Completely new treatment of INLINE pragmas (big patch)

[ghc-hetmet.git] / compiler / simplCore / Simplify.lhs

diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs
index e1a8492..5bcda0c 100644 (file)
--- a/compiler/simplCore/Simplify.lhs
+++ b/compiler/simplCore/Simplify.lhs
@@ -14,6 +14,7 @@ import Type hiding      ( substTy, extendTvSubst )
 import SimplEnv
 import SimplUtils
 import MkId            ( rUNTIME_ERROR_ID )
 import SimplEnv
 import SimplUtils
 import MkId            ( rUNTIME_ERROR_ID )

+import FamInstEnv      ( FamInstEnv )

 import Id
 import Var
 import IdInfo
 import Id
 import Var
 import IdInfo

@@ -21,7 +22,7 @@ import Coercion
 import FamInstEnv       ( topNormaliseType )
 import DataCon          ( dataConRepStrictness, dataConUnivTyVars )
 import CoreSyn
 import FamInstEnv       ( topNormaliseType )
 import DataCon          ( dataConRepStrictness, dataConUnivTyVars )
 import CoreSyn

-import NewDemand        ( isStrictDmd )
+import NewDemand        ( isStrictDmd, splitStrictSig )

 import PprCore          ( pprParendExpr, pprCoreExpr )
 import CoreUnfold       ( mkUnfolding, callSiteInline, CallCtxt(..) )
 import CoreUtils
 import PprCore          ( pprParendExpr, pprCoreExpr )
 import CoreUnfold       ( mkUnfolding, callSiteInline, CallCtxt(..) )
 import CoreUtils

@@ -34,6 +35,8 @@ import BasicTypes       ( TopLevelFlag(..), isTopLevel,
                           RecFlag(..), isNonRuleLoopBreaker )
 import Maybes           ( orElse )
 import Data.List        ( mapAccumL )
                           RecFlag(..), isNonRuleLoopBreaker )
 import Maybes           ( orElse )
 import Data.List        ( mapAccumL )

+import MonadUtils      ( foldlM )
+import StaticFlags     ( opt_PassCaseBndrToJoinPoints )

 import Outputable
 import FastString
 \end{code}
 import Outputable
 import FastString
 \end{code}

@@ -349,7 +352,7 @@ simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
                      do { tick LetFloatFromLet
                         ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
                         ; rhs' <- mkLam tvs' body3
                      do { tick LetFloatFromLet
                         ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
                         ; rhs' <- mkLam tvs' body3

-                        ; let env' = foldl (addPolyBind top_lvl) env poly_binds
+                        ; env' <- foldlM (addPolyBind top_lvl) env poly_binds

                         ; return (env', rhs') }
 
         ; completeBind env' top_lvl bndr bndr1 rhs' }
                         ; return (env', rhs') }
 
         ; completeBind env' top_lvl bndr bndr1 rhs' }

@@ -365,6 +368,9 @@ simplNonRecX :: SimplEnv
              -> SimplM SimplEnv
 
 simplNonRecX env bndr new_rhs
              -> SimplM SimplEnv
 
 simplNonRecX env bndr new_rhs

+  | isDeadBinder bndr  -- Not uncommon; e.g. case (a,b) of b { (p,q) -> p }
+  = return env         --               Here b is dead, and we avoid creating
+  | otherwise          --               the binding b = (a,b)

   = do  { (env', bndr') <- simplBinder env bndr
         ; completeNonRecX env' (isStrictId bndr) bndr bndr' new_rhs }
 
   = do  { (env', bndr') <- simplBinder env bndr
         ; completeNonRecX env' (isStrictId bndr) bndr bndr' new_rhs }
 

@@ -510,7 +516,18 @@ makeTrivial env expr
   | otherwise           -- See Note [Take care] below
   = do  { var <- newId (fsLit "a") (exprType expr)
         ; env' <- completeNonRecX env False var var expr
   | otherwise           -- See Note [Take care] below
   = do  { var <- newId (fsLit "a") (exprType expr)
         ; env' <- completeNonRecX env False var var expr

-        ; return (env', substExpr env' (Var var)) }
+--       pprTrace "makeTrivial" (vcat [ppr var <+> ppr (exprArity (substExpr env' (Var var)))
+--                                    , ppr expr
+--                                    , ppr (substExpr env' (Var var))
+--                                    , ppr (idArity (fromJust (lookupInScope (seInScope env') var))) ]) $
+       ; return (env', substExpr env' (Var var)) }
+       -- The substitution is needed becase we're constructing a new binding
+       --     a = rhs
+       -- And if rhs is of form (rhs1 |> co), then we might get
+       --     a1 = rhs1
+       --     a = a1 |> co
+       -- and now a's RHS is trivial and can be substituted out, and that
+       -- is what completeNonRecX will do

 \end{code}
 
 
 \end{code}
 
 

@@ -550,29 +567,23 @@ completeBind :: SimplEnv
 --      * or by adding to the floats in the envt
 
 completeBind env top_lvl old_bndr new_bndr new_rhs
 --      * or by adding to the floats in the envt
 
 completeBind env top_lvl old_bndr new_bndr new_rhs

-  | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
-                -- Inline and discard the binding
-  = do  { tick (PostInlineUnconditionally old_bndr)
-        ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
-          return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
-        -- Use the substitution to make quite, quite sure that the
-        -- substitution will happen, since we are going to discard the binding
+  = do { let old_info = idInfo old_bndr
+             old_unf  = unfoldingInfo old_info
+             occ_info = occInfo old_info

 
 

-  | otherwise
-  = return (addNonRecWithUnf env new_bndr new_rhs unfolding wkr)
-  where
-    unfolding | omit_unfolding = NoUnfolding
-             | otherwise      = mkUnfolding  (isTopLevel top_lvl) new_rhs
-    old_info    = idInfo old_bndr
-    occ_info    = occInfo old_info
-    wkr                = substWorker env (workerInfo old_info)
-    omit_unfolding = isNonRuleLoopBreaker occ_info 
-                  --       or not (activeInline env old_bndr)
-                  -- Do *not* trim the unfolding in SimplGently, else
-                  -- the specialiser can't see it!
-
------------------
-addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplEnv
+       ; new_unfolding <- simplUnfolding env top_lvl old_bndr occ_info old_unf new_rhs
+
+       ; if postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs new_unfolding
+                       -- Inline and discard the binding
+         then do  { tick (PostInlineUnconditionally old_bndr)
+                   ; return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
+               -- Use the substitution to make quite, quite sure that the
+               -- substitution will happen, since we are going to discard the binding
+
+         else return (addNonRecWithUnf env new_bndr new_rhs new_unfolding) }
+
+------------------------------
+addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv

 -- Add a new binding to the environment, complete with its unfolding
 -- but *do not* do postInlineUnconditionally, because we have already
 -- processed some of the scope of the binding
 -- Add a new binding to the environment, complete with its unfolding
 -- but *do not* do postInlineUnconditionally, because we have already
 -- processed some of the scope of the binding

@@ -585,65 +596,95 @@ addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplEnv
 -- opportunity to inline 'y' too.
 
 addPolyBind top_lvl env (NonRec poly_id rhs)
 -- opportunity to inline 'y' too.
 
 addPolyBind top_lvl env (NonRec poly_id rhs)

-  = addNonRecWithUnf env poly_id rhs unfolding NoWorker
-  where
-    unfolding | not (activeInline env poly_id) = NoUnfolding
-             | otherwise                      = mkUnfolding (isTopLevel top_lvl) rhs
-               -- addNonRecWithInfo adds the new binding in the
-               -- proper way (ie complete with unfolding etc),
-               -- and extends the in-scope set
+  = do { unfolding <- simplUnfolding env top_lvl poly_id NoOccInfo noUnfolding rhs
+                       -- Assumes that poly_id did not have an INLINE prag
+                       -- which is perhaps wrong.  ToDo: think about this
+        ; return (addNonRecWithUnf env poly_id rhs unfolding) }

 
 

-addPolyBind _ env bind@(Rec _) = extendFloats env bind
+addPolyBind _ env bind@(Rec _) = return (extendFloats env bind)

                -- Hack: letrecs are more awkward, so we extend "by steam"
                -- without adding unfoldings etc.  At worst this leads to
                -- more simplifier iterations
 
                -- Hack: letrecs are more awkward, so we extend "by steam"
                -- without adding unfoldings etc.  At worst this leads to
                -- more simplifier iterations
 

------------------
+------------------------------

 addNonRecWithUnf :: SimplEnv
 addNonRecWithUnf :: SimplEnv

-                 -> OutId -> OutExpr        -- New binder and RHS
-                 -> Unfolding -> WorkerInfo -- and unfolding
-                 -> SimplEnv
--- Add suitable IdInfo to the Id, add the binding to the floats, and extend the in-scope set
-addNonRecWithUnf env new_bndr rhs unfolding wkr
-  = final_id `seq`      -- This seq forces the Id, and hence its IdInfo,
-                       -- and hence any inner substitutions
-    addNonRec env final_id rhs
-       -- The addNonRec adds it to the in-scope set too
-  where
-        --      Arity info
-        new_bndr_info = idInfo new_bndr `setArityInfo` exprArity rhs
-
-        --      Unfolding info
-        -- Add the unfolding *only* for non-loop-breakers
-        -- Making loop breakers not have an unfolding at all
-        -- means that we can avoid tests in exprIsConApp, for example.
-        -- This is important: if exprIsConApp says 'yes' for a recursive
-        -- thing, then we can get into an infinite loop
-
-        --      Demand info
-        -- If the unfolding is a value, the demand info may
-        -- go pear-shaped, so we nuke it.  Example:
-        --      let x = (a,b) in
-        --      case x of (p,q) -> h p q x
-        -- Here x is certainly demanded. But after we've nuked
-        -- the case, we'll get just
-        --      let x = (a,b) in h a b x
-        -- and now x is not demanded (I'm assuming h is lazy)
-        -- This really happens.  Similarly
-        --      let f = \x -> e in ...f..f...
-        -- After inlining f at some of its call sites the original binding may
-        -- (for example) be no longer strictly demanded.
-        -- The solution here is a bit ad hoc...
-        info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
-                                  `setWorkerInfo`    wkr
-
-        final_info | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
-                   | otherwise                  = info_w_unf
+                -> OutId -> OutExpr    -- New binder and RHS
+                -> Unfolding           -- New unfolding
+                -> SimplEnv
+addNonRecWithUnf env new_bndr new_rhs new_unfolding
+  = let new_arity = exprArity new_rhs
+       old_arity = idArity new_bndr
+        info1 = idInfo new_bndr `setArityInfo` new_arity

        
        

-        final_id = new_bndr `setIdInfo` final_info
+              -- Unfolding info: Note [Setting the new unfolding]
+       info2 = info1 `setUnfoldingInfo` new_unfolding
+
+        -- Demand info: Note [Setting the demand info]
+        info3 | isEvaldUnfolding new_unfolding = zapDemandInfo info2 `orElse` info2
+              | otherwise                      = info2
+
+        final_id = new_bndr `setIdInfo` info3
+       dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr
+    in
+    ASSERT( isId new_bndr )
+    WARN( new_arity < old_arity || new_arity < dmd_arity, 
+          (ppr final_id <+> ppr old_arity <+> ppr new_arity <+> ppr dmd_arity) $$ ppr new_rhs )
+
+    final_id `seq`   -- This seq forces the Id, and hence its IdInfo,
+                    -- and hence any inner substitutions
+           -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
+    addNonRec env final_id new_rhs
+               -- The addNonRec adds it to the in-scope set too
+
+
+------------------------------
+simplUnfolding :: SimplEnv-> TopLevelFlag
+              -> Id    -- Debug output only
+              -> OccInfo -> Unfolding -> OutExpr
+              -> SimplM Unfolding
+simplUnfolding env top_lvl bndr occ_info old_unf new_rhs       -- Note [Setting the new unfolding]
+  | omit_unfolding = WARN( is_inline_rule, ppr bndr ) return NoUnfolding       
+  | is_inline_rule = return (substUnfolding env is_top_lvl old_unf)
+  | otherwise     = return (mkUnfolding is_top_lvl new_rhs)
+  where
+    is_top_lvl     = isTopLevel top_lvl
+    is_inline_rule = isInlineRule old_unf
+    omit_unfolding = isNonRuleLoopBreaker occ_info

 \end{code}
 
 
 \end{code}
 
 

+Note [Setting the new unfolding]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+* If there's an INLINE pragma, we use substUnfolding to retain the 
+  supplied inlining
+
+* If not, we make an unfolding from the new RHS.  But *only* for
+  non-loop-breakers. Making loop breakers not have an unfolding at all
+  means that we can avoid tests in exprIsConApp, for example.  This is
+  important: if exprIsConApp says 'yes' for a recursive thing, then we
+  can get into an infinite loop
+
+If there's an INLINE pragma on a loop breaker, we simply discard it 
+(with a DEBUG warning).  The desugarer complains about binding groups
+that look likely to trigger this behaviour.
+
+
+Note [Setting the demand info]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If the unfolding is a value, the demand info may
+go pear-shaped, so we nuke it.  Example:
+     let x = (a,b) in
+     case x of (p,q) -> h p q x
+Here x is certainly demanded. But after we've nuked
+the case, we'll get just
+     let x = (a,b) in h a b x
+and now x is not demanded (I'm assuming h is lazy)
+This really happens.  Similarly
+     let f = \x -> e in ...f..f...
+After inlining f at some of its call sites the original binding may
+(for example) be no longer strictly demanded.
+The solution here is a bit ad hoc...
+

 
 %************************************************************************
 %*                                                                      *
 
 %************************************************************************
 %*                                                                      *

@@ -822,10 +863,10 @@ simplCast env body co0 cont0
 
        add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
          | (_l1, t1) <- coercionKind co2
 
        add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
          | (_l1, t1) <- coercionKind co2

-                --      coerce T1 S1 (coerce S1 K1 e)
+               --      e |> (g1 :: S1~L) |> (g2 :: L~T1)

                 -- ==>
                 -- ==>

-                --      e,                      if T1=K1
-                --      coerce T1 K1 e,         otherwise
+                --      e,                       if T1=T2
+                --      e |> (g1 . g2 :: T1~T2)  otherwise

                 --
                 -- For example, in the initial form of a worker
                 -- we may find  (coerce T (coerce S (\x.e))) y
                 --
                 -- For example, in the initial form of a worker
                 -- we may find  (coerce T (coerce S (\x.e))) y

@@ -835,7 +876,7 @@ simplCast env body co0 cont0
          | otherwise           = CoerceIt (mkTransCoercion co1 co2) cont
 
        add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
          | otherwise           = CoerceIt (mkTransCoercion co1 co2) cont
 
        add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)

-                -- (f `cast` g) ty  --->   (f ty) `cast` (g @ ty)
+                -- (f |> g) ty  --->   (f ty) |> (g @ ty)

                 -- This implements the PushT rule from the paper
          | Just (tyvar,_) <- splitForAllTy_maybe s1s2
          , not (isCoVar tyvar)
                 -- This implements the PushT rule from the paper
          | Just (tyvar,_) <- splitForAllTy_maybe s1s2
          , not (isCoVar tyvar)

@@ -848,12 +889,12 @@ simplCast env body co0 cont0
        add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
          | not (isTypeArg arg)  -- This implements the Push rule from the paper
          , isFunTy s1s2   -- t1t2 must be a function type, becuase it's applied
        add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
          | not (isTypeArg arg)  -- This implements the Push rule from the paper
          , isFunTy s1s2   -- t1t2 must be a function type, becuase it's applied

-                -- co : s1s2 :=: t1t2
-                --      (coerce (T1->T2) (S1->S2) F) E
+                --      (e |> (g :: s1s2 ~ t1->t2)) f

                 -- ===>
                 -- ===>

-                --      coerce T2 S2 (F (coerce S1 T1 E))
+                --      (e (f |> (arg g :: t1~s1))
+               --      |> (res g :: s2->t2)

                 --
                 --

-                -- t1t2 must be a function type, T1->T2, because it's applied
+                -- t1t2 must be a function type, t1->t2, because it's applied

                 -- to something but s1s2 might conceivably not be
                 --
                 -- When we build the ApplyTo we can't mix the out-types
                 -- to something but s1s2 might conceivably not be
                 --
                 -- When we build the ApplyTo we can't mix the out-types

@@ -864,9 +905,9 @@ simplCast env body co0 cont0
                 -- Example of use: Trac #995
          = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
          where
                 -- Example of use: Trac #995
          = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
          where

-           -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
-           -- t2 :=: s2 with left and right on the curried form:
-           --    (->) t1 t2 :=: (->) s1 s2
+           -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
+           -- t2 ~ s2 with left and right on the curried form:
+           --    (->) t1 t2 ~ (->) s1 s2

            [co1, co2] = decomposeCo 2 co
            new_arg    = mkCoerce (mkSymCoercion co1) arg'
            arg'       = substExpr (arg_se `setInScope` env) arg
            [co1, co2] = decomposeCo 2 co
            new_arg    = mkCoerce (mkSymCoercion co1) arg'
            arg'       = substExpr (arg_se `setInScope` env) arg

@@ -901,7 +942,7 @@ simplLam env bndrs body cont
 
 ------------------
 simplNonRecE :: SimplEnv
 
 ------------------
 simplNonRecE :: SimplEnv

-             -> InId                    -- The binder
+             -> InBndr                  -- The binder

              -> (InExpr, SimplEnv)      -- Rhs of binding (or arg of lambda)
              -> ([InBndr], InExpr)      -- Body of the let/lambda
                                         --      \xs.e
              -> (InExpr, SimplEnv)      -- Rhs of binding (or arg of lambda)
              -> ([InBndr], InExpr)      -- Body of the let/lambda
                                         --      \xs.e

@@ -937,7 +978,8 @@ simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
                      (StrictBind bndr bndrs body env cont) }
 
   | otherwise
                      (StrictBind bndr bndrs body env cont) }
 
   | otherwise

-  = do  { (env1, bndr1) <- simplNonRecBndr env bndr
+  = ASSERT( not (isTyVar bndr) )
+    do  { (env1, bndr1) <- simplNonRecBndr env bndr

         ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
         ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
         ; simplLam env3 bndrs body cont }
         ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
         ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
         ; simplLam env3 bndrs body cont }

@@ -959,21 +1001,9 @@ simplNote env (SCC cc) e cont
   = do  { e' <- simplExpr (setEnclosingCC env currentCCS) e
         ; rebuild env (mkSCC cc e') cont }
 
   = do  { e' <- simplExpr (setEnclosingCC env currentCCS) e
         ; rebuild env (mkSCC cc e') cont }
 

--- See notes with SimplMonad.inlineMode
-simplNote env InlineMe e cont
-  | Just (inside, outside) <- splitInlineCont cont  -- Boring boring continuation; see notes above
-  = do  {                       -- Don't inline inside an INLINE expression
-          e' <- simplExprC (setMode inlineMode env) e inside
-        ; rebuild env (mkInlineMe e') outside }
-
-  | otherwise   -- Dissolve the InlineMe note if there's
-                -- an interesting context of any kind to combine with
-                -- (even a type application -- anything except Stop)
-  = simplExprF env e cont
-
-simplNote env (CoreNote s) e cont = do
-    e' <- simplExpr env e
-    rebuild env (Note (CoreNote s) e') cont
+simplNote env (CoreNote s) e cont
+  = do { e' <- simplExpr env e
+       ; rebuild env (Note (CoreNote s) e') cont }

 \end{code}
 
 
 \end{code}
 
 

@@ -1070,7 +1100,7 @@ completeCall env var cont
             Just unfolding      -- There is an inlining!
               ->  do { tick (UnfoldingDone var)
                      ; (if dopt Opt_D_dump_inlinings dflags then
             Just unfolding      -- There is an inlining!
               ->  do { tick (UnfoldingDone var)
                      ; (if dopt Opt_D_dump_inlinings dflags then

-                           pprTrace ("Inlining done" ++ showSDoc (ppr var)) (vcat [
+                           pprTrace ("Inlining done: " ++ showSDoc (ppr var)) (vcat [

                                 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
                                 text "Inlined fn: " <+> nest 2 (ppr unfolding),
                                 text "Cont:  " <+> ppr call_cont])
                                 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
                                 text "Inlined fn: " <+> nest 2 (ppr unfolding),
                                 text "Cont:  " <+> ppr call_cont])

@@ -1172,7 +1202,91 @@ all this at once is TOO HARD!
 %*                                                                      *
 %************************************************************************
 
 %*                                                                      *
 %************************************************************************
 

-Blob of helper functions for the "case-of-something-else" situation.
+Note [Case elimination]
+~~~~~~~~~~~~~~~~~~~~~~~
+The case-elimination transformation discards redundant case expressions.
+Start with a simple situation:
+
+        case x# of      ===>   e[x#/y#]
+          y# -> e
+
+(when x#, y# are of primitive type, of course).  We can't (in general)
+do this for algebraic cases, because we might turn bottom into
+non-bottom!
+
+The code in SimplUtils.prepareAlts has the effect of generalise this
+idea to look for a case where we're scrutinising a variable, and we
+know that only the default case can match.  For example:
+
+        case x of
+          0#      -> ...
+          DEFAULT -> ...(case x of
+                         0#      -> ...
+                         DEFAULT -> ...) ...
+
+Here the inner case is first trimmed to have only one alternative, the
+DEFAULT, after which it's an instance of the previous case.  This
+really only shows up in eliminating error-checking code.
+
+We also make sure that we deal with this very common case:
+
+        case e of
+          x -> ...x...
+
+Here we are using the case as a strict let; if x is used only once
+then we want to inline it.  We have to be careful that this doesn't
+make the program terminate when it would have diverged before, so we
+check that
+        - e is already evaluated (it may so if e is a variable)
+        - x is used strictly, or
+
+Lastly, the code in SimplUtils.mkCase combines identical RHSs.  So
+
+        case e of       ===> case e of DEFAULT -> r
+           True  -> r
+           False -> r
+
+Now again the case may be elminated by the CaseElim transformation.
+
+
+Further notes about case elimination
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider:       test :: Integer -> IO ()
+                test = print
+
+Turns out that this compiles to:
+    Print.test
+      = \ eta :: Integer
+          eta1 :: State# RealWorld ->
+          case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
+          case hPutStr stdout
+                 (PrelNum.jtos eta ($w[] @ Char))
+                 eta1
+          of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s  }}
+
+Notice the strange '<' which has no effect at all. This is a funny one.
+It started like this:
+
+f x y = if x < 0 then jtos x
+          else if y==0 then "" else jtos x
+
+At a particular call site we have (f v 1).  So we inline to get
+
+        if v < 0 then jtos x
+        else if 1==0 then "" else jtos x
+
+Now simplify the 1==0 conditional:
+
+        if v<0 then jtos v else jtos v
+
+Now common-up the two branches of the case:
+
+        case (v<0) of DEFAULT -> jtos v
+
+Why don't we drop the case?  Because it's strict in v.  It's technically
+wrong to drop even unnecessary evaluations, and in practice they
+may be a result of 'seq' so we *definitely* don't want to drop those.
+I don't really know how to improve this situation.

 
 \begin{code}
 ---------------------------------------------------------
 
 \begin{code}
 ---------------------------------------------------------

@@ -1206,7 +1320,7 @@ rebuildCase env scrut case_bndr alts cont
 
 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
   -- See if we can get rid of the case altogether
 
 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
   -- See if we can get rid of the case altogether

-  -- See the extensive notes on case-elimination above
+  -- See Note [Case eliminiation] 

   -- mkCase made sure that if all the alternatives are equal,
   -- then there is now only one (DEFAULT) rhs
  | all isDeadBinder bndrs       -- bndrs are [InId]
   -- mkCase made sure that if all the alternatives are equal,
   -- then there is now only one (DEFAULT) rhs
  | all isDeadBinder bndrs       -- bndrs are [InId]

@@ -1282,78 +1396,15 @@ try to eliminate uses of v in the RHSs in favour of case_bndr; that
 way, there's a chance that v will now only be used once, and hence
 inlined.
 
 way, there's a chance that v will now only be used once, and hence
 inlined.
 

-Note [no-case-of-case]
-~~~~~~~~~~~~~~~~~~~~~~
-We *used* to suppress the binder-swap in case expressoins when 
--fno-case-of-case is on.  Old remarks:
-    "This happens in the first simplifier pass,
-    and enhances full laziness.  Here's the bad case:
-            f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
-    If we eliminate the inner case, we trap it inside the I# v -> arm,
-    which might prevent some full laziness happening.  I've seen this
-    in action in spectral/cichelli/Prog.hs:
-             [(m,n) | m <- [1..max], n <- [1..max]]
-    Hence the check for NoCaseOfCase."
-However, now the full-laziness pass itself reverses the binder-swap, so this
-check is no longer necessary.
-
-Note [Suppressing the case binder-swap]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-There is another situation when it might make sense to suppress the
-case-expression binde-swap. If we have
-
-    case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
-                   ...other cases .... }
-
-We'll perform the binder-swap for the outer case, giving
-
-    case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
-                   ...other cases .... }
-
-But there is no point in doing it for the inner case, because w1 can't
-be inlined anyway.  Furthermore, doing the case-swapping involves
-zapping w2's occurrence info (see paragraphs that follow), and that
-forces us to bind w2 when doing case merging.  So we get
-
-    case x of w1 { A -> let w2 = w1 in e1
-                   B -> let w2 = w1 in e2
-                   ...other cases .... }
-
-This is plain silly in the common case where w2 is dead.
-
-Even so, I can't see a good way to implement this idea.  I tried
-not doing the binder-swap if the scrutinee was already evaluated
-but that failed big-time:
-
-        data T = MkT !Int
-
-        case v of w  { MkT x ->
-        case x of x1 { I# y1 ->
-        case x of x2 { I# y2 -> ...
-
-Notice that because MkT is strict, x is marked "evaluated".  But to
-eliminate the last case, we must either make sure that x (as well as
-x1) has unfolding MkT y1.  THe straightforward thing to do is to do
-the binder-swap.  So this whole note is a no-op.
+Historical note: we use to do the "case binder swap" in the Simplifier
+so there were additional complications if the scrutinee was a variable.
+Now the binder-swap stuff is done in the occurrence analyer; see
+OccurAnal Note [Binder swap].

 
 Note [zapOccInfo]
 ~~~~~~~~~~~~~~~~~
 
 Note [zapOccInfo]
 ~~~~~~~~~~~~~~~~~

-If we replace the scrutinee, v, by tbe case binder, then we have to nuke
-any occurrence info (eg IAmDead) in the case binder, because the
-case-binder now effectively occurs whenever v does.  AND we have to do
-the same for the pattern-bound variables!  Example:
-
-        (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
-
-Here, b and p are dead.  But when we move the argment inside the first
-case RHS, and eliminate the second case, we get
-
-        case x of { (a,b) -> a b }
-
-Urk! b is alive!  Reason: the scrutinee was a variable, and case elimination
-happened.
-
-Indeed, this can happen anytime the case binder isn't dead:
+If the case binder is not dead, then neither are the pattern bound
+variables:  

         case <any> of x { (a,b) ->
         case x of { (p,q) -> p } }
 Here (a,b) both look dead, but come alive after the inner case is eliminated.
         case <any> of x { (a,b) ->
         case x of { (p,q) -> p } }
 Here (a,b) both look dead, but come alive after the inner case is eliminated.

@@ -1362,6 +1413,10 @@ The point is that we bring into the envt a binding
 after the outer case, and that makes (a,b) alive.  At least we do unless
 the case binder is guaranteed dead.
 
 after the outer case, and that makes (a,b) alive.  At least we do unless
 the case binder is guaranteed dead.
 

+In practice, the scrutinee is almost always a variable, so we pretty
+much always zap the OccInfo of the binders.  It doesn't matter much though.
+
+

 Note [Case of cast]
 ~~~~~~~~~~~~~~~~~~~
 Consider        case (v `cast` co) of x { I# ->
 Note [Case of cast]
 ~~~~~~~~~~~~~~~~~~~
 Consider        case (v `cast` co) of x { I# ->

@@ -1401,121 +1456,78 @@ At one point I did transformation in LiberateCase, but it's more robust here.
 (Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
 LiberateCase gets to see it.)
 
 (Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
 LiberateCase gets to see it.)
 

-Note [Case elimination]
-~~~~~~~~~~~~~~~~~~~~~~~
-The case-elimination transformation discards redundant case expressions.
-Start with a simple situation:
-
-        case x# of      ===>   e[x#/y#]
-          y# -> e
-
-(when x#, y# are of primitive type, of course).  We can't (in general)
-do this for algebraic cases, because we might turn bottom into
-non-bottom!
-
-The code in SimplUtils.prepareAlts has the effect of generalise this
-idea to look for a case where we're scrutinising a variable, and we
-know that only the default case can match.  For example:
-
-        case x of
-          0#      -> ...
-          DEFAULT -> ...(case x of
-                         0#      -> ...
-                         DEFAULT -> ...) ...
-
-Here the inner case is first trimmed to have only one alternative, the
-DEFAULT, after which it's an instance of the previous case.  This
-really only shows up in eliminating error-checking code.
-
-We also make sure that we deal with this very common case:
-
-        case e of
-          x -> ...x...
-
-Here we are using the case as a strict let; if x is used only once
-then we want to inline it.  We have to be careful that this doesn't
-make the program terminate when it would have diverged before, so we
-check that
-        - e is already evaluated (it may so if e is a variable)
-        - x is used strictly, or
-
-Lastly, the code in SimplUtils.mkCase combines identical RHSs.  So
-
-        case e of       ===> case e of DEFAULT -> r
-           True  -> r
-           False -> r
-
-Now again the case may be elminated by the CaseElim transformation.

 
 

+Historical note [no-case-of-case]
+~~~~~~~~~~~~~~~~~~~~~~
+We *used* to suppress the binder-swap in case expressoins when 
+-fno-case-of-case is on.  Old remarks:
+    "This happens in the first simplifier pass,
+    and enhances full laziness.  Here's the bad case:
+            f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
+    If we eliminate the inner case, we trap it inside the I# v -> arm,
+    which might prevent some full laziness happening.  I've seen this
+    in action in spectral/cichelli/Prog.hs:
+             [(m,n) | m <- [1..max], n <- [1..max]]
+    Hence the check for NoCaseOfCase."
+However, now the full-laziness pass itself reverses the binder-swap, so this
+check is no longer necessary.

 
 

-Further notes about case elimination
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider:       test :: Integer -> IO ()
-                test = print
+Historical note [Suppressing the case binder-swap]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+There is another situation when it might make sense to suppress the
+case-expression binde-swap. If we have

 
 

-Turns out that this compiles to:
-    Print.test
-      = \ eta :: Integer
-          eta1 :: State# RealWorld ->
-          case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
-          case hPutStr stdout
-                 (PrelNum.jtos eta ($w[] @ Char))
-                 eta1
-          of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s  }}
+    case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
+                   ...other cases .... }

 
 

-Notice the strange '<' which has no effect at all. This is a funny one.
-It started like this:
+We'll perform the binder-swap for the outer case, giving

 
 

-f x y = if x < 0 then jtos x
-          else if y==0 then "" else jtos x
+    case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
+                   ...other cases .... }

 
 

-At a particular call site we have (f v 1).  So we inline to get
+But there is no point in doing it for the inner case, because w1 can't
+be inlined anyway.  Furthermore, doing the case-swapping involves
+zapping w2's occurrence info (see paragraphs that follow), and that
+forces us to bind w2 when doing case merging.  So we get

 
 

-        if v < 0 then jtos x
-        else if 1==0 then "" else jtos x
+    case x of w1 { A -> let w2 = w1 in e1
+                   B -> let w2 = w1 in e2
+                   ...other cases .... }

 
 

-Now simplify the 1==0 conditional:
+This is plain silly in the common case where w2 is dead.

 
 

-        if v<0 then jtos v else jtos v
+Even so, I can't see a good way to implement this idea.  I tried
+not doing the binder-swap if the scrutinee was already evaluated
+but that failed big-time:

 
 

-Now common-up the two branches of the case:
+        data T = MkT !Int

 
 

-        case (v<0) of DEFAULT -> jtos v
+        case v of w  { MkT x ->
+        case x of x1 { I# y1 ->
+        case x of x2 { I# y2 -> ...

 
 

-Why don't we drop the case?  Because it's strict in v.  It's technically
-wrong to drop even unnecessary evaluations, and in practice they
-may be a result of 'seq' so we *definitely* don't want to drop those.
-I don't really know how to improve this situation.
+Notice that because MkT is strict, x is marked "evaluated".  But to
+eliminate the last case, we must either make sure that x (as well as
+x1) has unfolding MkT y1.  THe straightforward thing to do is to do
+the binder-swap.  So this whole note is a no-op.

 
 
 \begin{code}
 
 
 \begin{code}

-simplCaseBinder :: SimplEnv -> OutExpr -> OutId -> [InAlt]
-                -> SimplM (SimplEnv, OutExpr, OutId)
-simplCaseBinder env0 scrut0 case_bndr0 alts
-  = do  { (env1, case_bndr1) <- simplBinder env0 case_bndr0
-
-        ; fam_envs <- getFamEnvs
-        ; (env2, scrut2, case_bndr2) <- improve_seq fam_envs env1 scrut0
-                                                case_bndr0 case_bndr1 alts
-                        -- Note [Improving seq]
-
-        ; let (env3, case_bndr3) = improve_case_bndr env2 scrut2 case_bndr2
-                        -- Note [Case of cast]
-
-        ; return (env3, scrut2, case_bndr3) }
-  where
-
-    improve_seq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
-        | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
-        =  do { case_bndr2 <- newId (fsLit "nt") ty2
-              ; let rhs  = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
-                    env2 = extendIdSubst env case_bndr rhs
-              ; return (env2, scrut `Cast` co, case_bndr2) }
-
-    improve_seq _ env scrut _ case_bndr1 _
-        = return (env, scrut, case_bndr1)
-
-
+improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
+          -> OutExpr -> InId -> OutId -> [InAlt]
+          -> SimplM (SimplEnv, OutExpr, OutId)
+-- Note [Improving seq]
+improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
+  | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
+  =  do { case_bndr2 <- newId (fsLit "nt") ty2
+        ; let rhs  = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
+              env2 = extendIdSubst env case_bndr rhs
+        ; return (env2, scrut `Cast` co, case_bndr2) }
+
+improveSeq _ env scrut _ case_bndr1 _
+  = return (env, scrut, case_bndr1)
+
+{-

     improve_case_bndr env scrut case_bndr
         -- See Note [no-case-of-case]
        --  | switchIsOn (getSwitchChecker env) NoCaseOfCase
     improve_case_bndr env scrut case_bndr
         -- See Note [no-case-of-case]
        --  | switchIsOn (getSwitchChecker env) NoCaseOfCase

@@ -1536,12 +1548,9 @@ simplCaseBinder env0 scrut0 case_bndr0 alts
 
             _ -> (env, case_bndr)
         where
 
             _ -> (env, case_bndr)
         where

-          case_bndr' = zapOccInfo case_bndr
+          case_bndr' = zapIdOccInfo case_bndr

           env1       = modifyInScope env case_bndr case_bndr'
           env1       = modifyInScope env case_bndr case_bndr'

-
-
-zapOccInfo :: InId -> InId      -- See Note [zapOccInfo]
-zapOccInfo b = b `setIdOccInfo` NoOccInfo
+-}

 \end{code}
 
 
 \end{code}
 
 

@@ -1597,10 +1606,15 @@ simplAlts :: SimplEnv
 
 simplAlts env scrut case_bndr alts cont'
   = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
 
 simplAlts env scrut case_bndr alts cont'
   = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $

-    do  { let alt_env = zapFloats env
-        ; (alt_env', scrut', case_bndr') <- simplCaseBinder alt_env scrut case_bndr alts
+    do  { let env0 = zapFloats env

 
 

-        ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env' scrut case_bndr' alts
+        ; (env1, case_bndr1) <- simplBinder env0 case_bndr
+
+        ; fam_envs <- getFamEnvs
+       ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut 
+                                                      case_bndr case_bndr1 alts
+
+        ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env' scrut' case_bndr' alts

 
         ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
         ; return (scrut', case_bndr', alts') }
 
         ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
         ; return (scrut', case_bndr', alts') }

@@ -1666,6 +1680,7 @@ simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
               evald_v  = zapped_v `setIdUnfolding` evaldUnfolding
           go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
 
               evald_v  = zapped_v `setIdUnfolding` evaldUnfolding
           go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
 

+       -- See Note [zapOccInfo]

         -- zap_occ_info: if the case binder is alive, then we add the unfolding
         --      case_bndr = C vs
         -- to the envt; so vs are now very much alive
         -- zap_occ_info: if the case binder is alive, then we add the unfolding
         --      case_bndr = C vs
         -- to the envt; so vs are now very much alive

@@ -1673,16 +1688,23 @@ simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
         --        case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
         --   ==>  case e of t { (a,b) -> ...(a)... }
         -- Look, Ma, a is alive now.
         --        case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
         --   ==>  case e of t { (a,b) -> ...(a)... }
         -- Look, Ma, a is alive now.

-    zap_occ_info | isDeadBinder case_bndr' = \ident -> ident
-                 | otherwise               = zapOccInfo
+    zap_occ_info = zapCasePatIdOcc case_bndr'

 
 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
 addBinderUnfolding env bndr rhs
 
 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
 addBinderUnfolding env bndr rhs

-  = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
+  = modifyInScope env (bndr `setIdUnfolding` mkUnfolding False rhs)

 
 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
 addBinderOtherCon env bndr cons
 
 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
 addBinderOtherCon env bndr cons

-  = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
+  = modifyInScope env (bndr `setIdUnfolding` mkOtherCon cons)
+
+zapCasePatIdOcc :: Id -> Id -> Id
+-- Consider  case e of b { (a,b) -> ... }
+-- Then if we bind b to (a,b) in "...", and b is not dead,
+-- then we must zap the deadness info on a,b
+zapCasePatIdOcc case_bndr
+  | isDeadBinder case_bndr = \ pat_id -> pat_id
+  | otherwise             = \ pat_id -> zapIdOccInfo pat_id

 \end{code}
 
 
 \end{code}
 
 

@@ -1732,9 +1754,8 @@ knownAlt env scrut _ bndr (LitAlt _, bs, rhs) cont
         ; simplExprF env' rhs cont }
 
 knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
         ; simplExprF env' rhs cont }
 
 knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont

-  = do  { let dead_bndr  = isDeadBinder bndr    -- bndr is an InId
-              n_drop_tys = length (dataConUnivTyVars dc)
-        ; env' <- bind_args env dead_bndr bs (drop n_drop_tys the_args)
+  = do  { let n_drop_tys = length (dataConUnivTyVars dc)
+        ; env' <- bind_args env bs (drop n_drop_tys the_args)

         ; let
                 -- It's useful to bind bndr to scrut, rather than to a fresh
                 -- binding      x = Con arg1 .. argn
         ; let
                 -- It's useful to bind bndr to scrut, rather than to a fresh
                 -- binding      x = Con arg1 .. argn

@@ -1751,28 +1772,29 @@ knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
                                 -- args are aready OutExprs, but bs are InIds
 
         ; env'' <- simplNonRecX env' bndr bndr_rhs
                                 -- args are aready OutExprs, but bs are InIds
 
         ; env'' <- simplNonRecX env' bndr bndr_rhs

-        ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env'')) $
-          simplExprF env'' rhs cont }
+        ; simplExprF env'' rhs cont }

   where
   where

-    -- Ugh!
-    bind_args env' _ [] _  = return env'
+    zap_occ = zapCasePatIdOcc bndr    -- bndr is an InId
+
+                  -- Ugh!
+    bind_args env' [] _  = return env'

 
 

-    bind_args env' dead_bndr (b:bs') (Type ty : args)
+    bind_args env' (b:bs') (Type ty : args)

       = ASSERT( isTyVar b )
       = ASSERT( isTyVar b )

-        bind_args (extendTvSubst env' b ty) dead_bndr bs' args
+        bind_args (extendTvSubst env' b ty) bs' args

 
 

-    bind_args env' dead_bndr (b:bs') (arg : args)
+    bind_args env' (b:bs') (arg : args)

       = ASSERT( isId b )
       = ASSERT( isId b )

-        do { let b' = if dead_bndr then b else zapOccInfo b
+        do { let b' = zap_occ b

              -- Note that the binder might be "dead", because it doesn't
              -- occur in the RHS; and simplNonRecX may therefore discard
              -- it via postInlineUnconditionally.
              -- Nevertheless we must keep it if the case-binder is alive,
              -- because it may be used in the con_app.  See Note [zapOccInfo]
            ; env'' <- simplNonRecX env' b' arg
              -- Note that the binder might be "dead", because it doesn't
              -- occur in the RHS; and simplNonRecX may therefore discard
              -- it via postInlineUnconditionally.
              -- Nevertheless we must keep it if the case-binder is alive,
              -- because it may be used in the con_app.  See Note [zapOccInfo]
            ; env'' <- simplNonRecX env' b' arg

-           ; bind_args env'' dead_bndr bs' args }
+           ; bind_args env'' bs' args }

 
 

-    bind_args _ _ _ _ =
+    bind_args _ _ _ =

       pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
                              text "scrut:" <+> ppr scrut
 \end{code}
       pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
                              text "scrut:" <+> ppr scrut
 \end{code}

@@ -1827,7 +1849,7 @@ mkDupableCont env (ApplyTo _ arg se cont)
     do  { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
         ; arg' <- simplExpr (se `setInScope` env') arg
         ; (env'', arg'') <- makeTrivial env' arg'
     do  { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
         ; arg' <- simplExpr (se `setInScope` env') arg
         ; (env'', arg'') <- makeTrivial env' arg'

-        ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env') dup_cont
+        ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont

         ; return (env'', app_cont, nodup_cont) }
 
 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
         ; return (env'', app_cont, nodup_cont) }
 
 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)

@@ -1886,40 +1908,97 @@ mkDupableAlts env case_bndr' the_alts
 
 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
               -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
 
 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
               -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))

-mkDupableAlt env case_bndr' (con, bndrs', rhs')
-  | exprIsDupable rhs'  -- Note [Small alternative rhs]
-  = return (env, (con, bndrs', rhs'))
+mkDupableAlt env case_bndr1 (con, bndrs1, rhs1)
+  | exprIsDupable rhs1  -- Note [Small alternative rhs]
+  = return (env, (con, bndrs1, rhs1))

   | otherwise
   | otherwise

-  = do  { let rhs_ty'     = exprType rhs'
-              used_bndrs' = filter abstract_over (case_bndr' : bndrs')
-              abstract_over bndr
+  = do  { let abstract_over bndr

                   | isTyVar bndr = True -- Abstract over all type variables just in case
                   | otherwise    = not (isDeadBinder bndr)
                         -- The deadness info on the new Ids is preserved by simplBinders
 
                   | isTyVar bndr = True -- Abstract over all type variables just in case
                   | otherwise    = not (isDeadBinder bndr)
                         -- The deadness info on the new Ids is preserved by simplBinders
 

-        ; (final_bndrs', final_args)    -- Note [Join point abstraction]
-                <- if (any isId used_bndrs')
-                   then return (used_bndrs', varsToCoreExprs used_bndrs')
+              inst_tys1 = tyConAppArgs (idType case_bndr1)
+              con_app dc = mkConApp dc (map Type inst_tys1 ++ varsToCoreExprs bndrs1)
+
+             (rhs2, final_bndrs)   -- See Note [Passing the case binder to join points]
+                | isDeadBinder case_bndr1
+                = (rhs1, filter abstract_over bndrs1)
+                | opt_PassCaseBndrToJoinPoints, not (null bndrs1)
+                = (rhs1, (case_bndr1 : filter abstract_over bndrs1))
+                | otherwise 
+                 = case con of
+                    DataAlt dc -> (Let (NonRec case_bndr1 (con_app dc)) rhs1, bndrs1)
+                    LitAlt lit -> ASSERT( null bndrs1 ) (Let (NonRec case_bndr1 (Lit lit)) rhs1, [])
+                    DEFAULT    -> ASSERT( null bndrs1 ) (rhs1, [case_bndr1])
+
+        ; (final_bndrs1, final_args)    -- Note [Join point abstraction]
+                <- if (any isId final_bndrs)
+                   then return (final_bndrs, varsToCoreExprs final_bndrs)

                     else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
                     else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy

-                            ; return ([rw_id], [Var realWorldPrimId]) }
+                            ; return (rw_id : final_bndrs,  
+                                     Var realWorldPrimId : varsToCoreExprs final_bndrs) }

 
 

-        ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
+        ; let rhs_ty1 = exprType rhs1
+        ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs1 rhs_ty1)

                 -- Note [Funky mkPiTypes]
 
         ; let   -- We make the lambdas into one-shot-lambdas.  The
                 -- join point is sure to be applied at most once, and doing so
                 -- prevents the body of the join point being floated out by
                 -- the full laziness pass
                 -- Note [Funky mkPiTypes]
 
         ; let   -- We make the lambdas into one-shot-lambdas.  The
                 -- join point is sure to be applied at most once, and doing so
                 -- prevents the body of the join point being floated out by
                 -- the full laziness pass

-                really_final_bndrs     = map one_shot final_bndrs'
+                really_final_bndrs     = map one_shot final_bndrs1

                 one_shot v | isId v    = setOneShotLambda v
                            | otherwise = v
                 one_shot v | isId v    = setOneShotLambda v
                            | otherwise = v

-                join_rhs  = mkLams really_final_bndrs rhs'
+                join_rhs  = mkLams really_final_bndrs rhs2

                 join_call = mkApps (Var join_bndr) final_args
 
                 join_call = mkApps (Var join_bndr) final_args
 

-        ; return (addPolyBind NotTopLevel env (NonRec join_bndr join_rhs), (con, bndrs', join_call)) }
+       ; env1 <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
+        ; return (env1, (con, bndrs1, join_call)) }

                 -- See Note [Duplicated env]
 \end{code}
 
                 -- See Note [Duplicated env]
 \end{code}
 

+Note [Passing the case binder to join points]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we have
+   case e of cb { C1 -> r1[cb]; C2 x y z -> r2[cb,x] }
+and we want to make join points for the two alternatives, 
+which mention the case binder 'cb'.  Should we pass 'cb' to
+the join point, or reconstruct it? Here are the two alternatives 
+for the C2 alternative:
+
+  Plan A(pass cb):         j2 cb x = r2[cb,x]
+
+  Plan B(reconstruct cb):  j2 x y z = let cb = C2 x y z in r2[cb,x]
+
+The advantge of Plan B is that we can "see" the definition of cb
+in r2, and that may be important when we inline stuff in r2.  The
+disadvantage is that if this optimisation doesn't happen, we end up
+re-allocating C2, when it already exists.  This does happen occasionally;
+an example is the function nofib/spectral/cichelli/Auxil.$whinsert.
+
+Plan B is always better if the constructor is nullary.
+
+In both cases we don't have liveness info for cb on a branch-by-branch
+basis, and it's possible that 'cb' is used in some branches but not
+others.  Well, the absence analyser will find that out later, so it's
+not too bad.
+
+Sadly, at the time of writing, neither choice seems an unequivocal
+win. Here are nofib results, for adding -fpass-case-bndr-to-join-points
+(all others are zero effect):
+
+        Program           Size    Allocs   Runtime   Elapsed
+--------------------------------------------------------------------------------
+       cichelli          +0.0%     -4.4%      0.13      0.13
+            pic          +0.0%     -0.7%      0.01      0.04
+      transform          -0.0%     +2.8%     -0.4%     -9.1%
+      wave4main          +0.0%    +10.5%     +3.1%     +3.4%
+--------------------------------------------------------------------------------
+            Min          -0.0%     -4.4%     -7.0%    -31.9%
+            Max          +0.1%    +10.5%     +3.1%    +15.0%
+ Geometric Mean          +0.0%     +0.1%     -1.7%     -6.1%
+
+

 Note [Duplicated env]
 ~~~~~~~~~~~~~~~~~~~~~
 Some of the alternatives are simplified, but have not been turned into a join point
 Note [Duplicated env]
 ~~~~~~~~~~~~~~~~~~~~~
 Some of the alternatives are simplified, but have not been turned into a join point