Major patch to fix reporting of unused imports

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

diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs
index 30cb321..b38bdc8 100644 (file)
--- a/compiler/simplCore/Simplify.lhs
+++ b/compiler/simplCore/Simplify.lhs
@@ -13,9 +13,9 @@ import SimplMonad
 import Type hiding      ( substTy, extendTvSubst )
 import SimplEnv
 import SimplUtils
 import Type hiding      ( substTy, extendTvSubst )
 import SimplEnv
 import SimplUtils

-import MkId            ( rUNTIME_ERROR_ID )

 import FamInstEnv      ( FamInstEnv )
 import Id
 import FamInstEnv      ( FamInstEnv )
 import Id

+import MkId            ( mkImpossibleExpr, seqId )

 import Var
 import IdInfo
 import Coercion
 import Var
 import IdInfo
 import Coercion

@@ -26,8 +26,9 @@ 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

+import CoreArity       ( exprArity )

 import Rules            ( lookupRule, getRules )
 import Rules            ( lookupRule, getRules )

-import BasicTypes       ( isMarkedStrict )
+import BasicTypes       ( isMarkedStrict, Arity )

 import CostCentre       ( currentCCS )
 import TysPrim          ( realWorldStatePrimTy )
 import PrelInfo         ( realWorldPrimId )
 import CostCentre       ( currentCCS )
 import TysPrim          ( realWorldStatePrimTy )
 import PrelInfo         ( realWorldPrimId )

@@ -35,8 +36,6 @@ 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}

@@ -341,7 +340,7 @@ simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
         ; (env', rhs')
             <-  if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
                 then                            -- No floating, just wrap up!
         ; (env', rhs')
             <-  if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
                 then                            -- No floating, just wrap up!

-                     do { rhs' <- mkLam tvs' (wrapFloats body_env2 body2)
+                     do { rhs' <- mkLam env tvs' (wrapFloats body_env2 body2)

                         ; return (env, rhs') }
 
                 else if null tvs then           -- Simple floating
                         ; return (env, rhs') }
 
                 else if null tvs then           -- Simple floating

@@ -351,8 +350,8 @@ simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
                 else                            -- Do type-abstraction first
                      do { tick LetFloatFromLet
                         ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
                 else                            -- Do type-abstraction first
                      do { tick LetFloatFromLet
                         ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2

-                        ; rhs' <- mkLam tvs' body3
-                        ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
+                        ; rhs' <- mkLam env tvs' body3
+                        ; let env' = foldl (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' }

@@ -462,7 +461,7 @@ prepareRhs env0 rhs0
         where
           is_val = n_val_args > 0       -- There is at least one arg
                                         -- ...and the fun a constructor or PAP
         where
           is_val = n_val_args > 0       -- There is at least one arg
                                         -- ...and the fun a constructor or PAP

-                 && (isDataConWorkId fun || n_val_args < idArity fun)
+                 && (isConLikeId fun || n_val_args < idArity fun)

     go _ env other
         = return (False, env, other)
 \end{code}
     go _ env other
         = return (False, env, other)
 \end{code}

@@ -567,23 +566,29 @@ 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

-  = do { let old_info = idInfo old_bndr
-             old_unf  = unfoldingInfo old_info
-             occ_info = occInfo old_info
+  | 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

 
 

-       ; 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
+  | 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

 -- 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

@@ -596,92 +601,95 @@ addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM 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)

-  = 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) }
+  = 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

 
 

-addPolyBind _ env bind@(Rec _) = return (extendFloats env bind)
+addPolyBind _ env bind@(Rec _) = 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           -- 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
-       
-              -- 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 )
+                 -> 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
+  = ASSERT( isId new_bndr )

     WARN( new_arity < old_arity || new_arity < dmd_arity, 
     WARN( new_arity < old_arity || new_arity < dmd_arity, 

-          (ppr final_id <+> ppr old_arity <+> ppr new_arity <+> ppr dmd_arity) $$ ppr new_rhs )
-
-    final_id `seq`   -- This seq forces the Id, and hence its IdInfo,
-                    -- and hence any inner substitutions
-           -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
-    addNonRec env final_id new_rhs
-               -- The addNonRec adds it to the in-scope set too
-
-------------------------------
-simplUnfolding :: SimplEnv-> TopLevelFlag
-              -> Id    -- Debug output only
-              -> OccInfo -> Unfolding -> OutExpr
-              -> SimplM Unfolding
-simplUnfolding env top_lvl bndr occ_info old_unf new_rhs       -- Note [Setting the new unfolding]
-  | omit_unfolding = WARN( is_inline_rule, ppr bndr ) return NoUnfolding       
-  | is_inline_rule = return (substUnfolding env is_top_lvl old_unf)
-  | otherwise     = return (mkUnfolding is_top_lvl new_rhs)
+          (ptext (sLit "Arity decrease:") <+> ppr final_id <+> ppr old_arity
+               <+> ppr new_arity <+> ppr dmd_arity) $$ ppr rhs )
+       -- Note [Arity decrease]
+    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
   where

-    is_top_lvl     = isTopLevel top_lvl
-    is_inline_rule = isInlineRule old_unf
-    omit_unfolding = isNonRuleLoopBreaker occ_info
-\end{code}
-
-Note [Setting the new unfolding]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-* If there's an INLINE pragma, we use substUnfolding to retain the 
-  supplied inlining
+       dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr
+       old_arity = idArity new_bndr

 
 

-* 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
+        --      Arity info
+       new_arity = exprArity rhs
+        new_bndr_info = idInfo new_bndr `setArityInfo` new_arity
+
+        --      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
+       
+        final_id = new_bndr `setIdInfo` final_info
+\end{code}

 
 

-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 [Arity decrease]
+~~~~~~~~~~~~~~~~~~~~~
+Generally speaking the arity of a binding should not decrease.  But it *can* 
+legitimately happen becuase of RULES.  Eg
+       f = g Int
+where g has arity 2, will have arity 2.  But if there's a rewrite rule
+       g Int --> h
+where h has arity 1, then f's arity will decrease.  Here's a real-life example,
+which is in the output of Specialise:
+
+     Rec {
+       $dm {Arity 2} = \d.\x. op d
+       {-# RULES forall d. $dm Int d = $s$dm #-}
+       
+       dInt = MkD .... opInt ...
+       opInt {Arity 1} = $dm dInt

 
 

+       $s$dm {Arity 0} = \x. op dInt }

 
 

-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...
+Here opInt has arity 1; but when we apply the rule its arity drops to 0.
+That's why Specialise goes to a little trouble to pin the right arity
+on specialised functions too.

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

@@ -935,12 +943,12 @@ simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
 simplLam env bndrs body cont
   = do  { (env', bndrs') <- simplLamBndrs env bndrs
         ; body' <- simplExpr env' body
 simplLam env bndrs body cont
   = do  { (env', bndrs') <- simplLamBndrs env bndrs
         ; body' <- simplExpr env' body

-        ; new_lam <- mkLam bndrs' body'
+        ; new_lam <- mkLam env' bndrs' body'

         ; rebuild env' new_lam cont }
 
 ------------------
 simplNonRecE :: SimplEnv
         ; rebuild env' new_lam cont }
 
 ------------------
 simplNonRecE :: SimplEnv

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

@@ -999,9 +1007,21 @@ 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 }
 

-simplNote env (CoreNote s) e cont
-  = do { e' <- simplExpr env e
-       ; rebuild env (Note (CoreNote s) 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

 \end{code}
 
 
 \end{code}
 
 

@@ -1033,8 +1053,7 @@ simplVar env var cont
 
 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
 completeCall env var cont
 
 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
 completeCall env var cont

-  = do  { dflags <- getDOptsSmpl
-        ; let   (args,call_cont) = contArgs cont
+  = do  { let   (args,call_cont) = contArgs cont

                 -- The args are OutExprs, obtained by *lazily* substituting
                 -- in the args found in cont.  These args are only examined
                 -- to limited depth (unless a rule fires).  But we must do
                 -- The args are OutExprs, obtained by *lazily* substituting
                 -- in the args found in cont.  These args are only examined
                 -- to limited depth (unless a rule fires).  But we must do

@@ -1050,45 +1069,18 @@ completeCall env var cont
         -- We used to use the black-listing mechanism to ensure that inlining of
         -- the wrapper didn't occur for things that have specialisations till a
         -- later phase, so but now we just try RULES first
         -- We used to use the black-listing mechanism to ensure that inlining of
         -- the wrapper didn't occur for things that have specialisations till a
         -- later phase, so but now we just try RULES first

-        --
-        -- Note [Rules for recursive functions]
-        -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-        -- You might think that we shouldn't apply rules for a loop breaker:
-        -- doing so might give rise to an infinite loop, because a RULE is
-        -- rather like an extra equation for the function:
-        --      RULE:           f (g x) y = x+y
-        --      Eqn:            f a     y = a-y
-        --
-        -- But it's too drastic to disable rules for loop breakers.
-        -- Even the foldr/build rule would be disabled, because foldr
-        -- is recursive, and hence a loop breaker:
-        --      foldr k z (build g) = g k z
-        -- So it's up to the programmer: rules can cause divergence
-        ; rule_base <- getSimplRules
-        ; let   in_scope   = getInScope env
-               rules      = getRules rule_base var
-                maybe_rule = case activeRule dflags env of
-                                Nothing     -> Nothing  -- No rules apply
-                                Just act_fn -> lookupRule act_fn in_scope
-                                                          var args rules 
-        ; case maybe_rule of {
-            Just (rule, rule_rhs) -> do
-                tick (RuleFired (ru_name rule))
-                (if dopt Opt_D_dump_rule_firings dflags then
-                   pprTrace "Rule fired" (vcat [
-                        text "Rule:" <+> ftext (ru_name rule),
-                        text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
-                        text "After: " <+> pprCoreExpr rule_rhs,
-                        text "Cont:  " <+> ppr call_cont])
-                 else
-                        id)             $
-                 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
+       -- 
+       -- See also Note [Rules for recursive functions]
+       ; mb_rule <- tryRules env var args call_cont
+       ; case mb_rule of {
+            Just (n_args, rule_rhs) -> simplExprF env rule_rhs (dropArgs n_args cont) ;

                  -- The ruleArity says how many args the rule consumed
                  -- The ruleArity says how many args the rule consumed

+           ; Nothing -> do       -- No rules

 
 

-          ; Nothing -> do       -- No rules

 
         ------------- Next try inlining ----------------
 
         ------------- Next try inlining ----------------

-        { let   arg_infos = [interestingArg arg | arg <- args, isValArg arg]
+        { dflags <- getDOptsSmpl
+        ; let   arg_infos = [interestingArg arg | arg <- args, isValArg arg]

                 n_val_args = length arg_infos
                 interesting_cont = interestingCallContext call_cont
                 active_inline = activeInline env var
                 n_val_args = length arg_infos
                 interesting_cont = interestingCallContext call_cont
                 active_inline = activeInline env var

@@ -1194,6 +1186,58 @@ to get the effect that finding (error "foo") in a strict arg position will
 discard the entire application and replace it with (error "foo").  Getting
 all this at once is TOO HARD!
 
 discard the entire application and replace it with (error "foo").  Getting
 all this at once is TOO HARD!
 

+
+%************************************************************************
+%*                                                                      *
+                Rewrite rules
+%*                                                                      *
+%************************************************************************
+
+\begin{code}
+tryRules :: SimplEnv -> Id -> [OutExpr] -> SimplCont 
+        -> SimplM (Maybe (Arity, CoreExpr))         -- The arity is the number of
+                                                    -- args consumed by the rule
+tryRules env fn args call_cont
+  = do {  dflags <- getDOptsSmpl
+        ; rule_base <- getSimplRules
+        ; let   in_scope   = getInScope env
+               rules      = getRules rule_base fn
+                maybe_rule = case activeRule dflags env of
+                                Nothing     -> Nothing  -- No rules apply
+                                Just act_fn -> lookupRule act_fn in_scope
+                                                          fn args rules 
+        ; case (rules, maybe_rule) of {
+           ([], _)                     -> return Nothing ;
+           (_,  Nothing)               -> return Nothing ;
+            (_,  Just (rule, rule_rhs)) -> do
+
+        { tick (RuleFired (ru_name rule))
+        ; (if dopt Opt_D_dump_rule_firings dflags then
+                   pprTrace "Rule fired" (vcat [
+                        text "Rule:" <+> ftext (ru_name rule),
+                        text "Before:" <+> ppr fn <+> sep (map pprParendExpr args),
+                        text "After: " <+> pprCoreExpr rule_rhs,
+                        text "Cont:  " <+> ppr call_cont])
+                 else
+                        id)             $
+           return (Just (ruleArity rule, rule_rhs)) }}}
+\end{code}
+
+Note [Rules for recursive functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+You might think that we shouldn't apply rules for a loop breaker:
+doing so might give rise to an infinite loop, because a RULE is
+rather like an extra equation for the function:
+     RULE:           f (g x) y = x+y
+     Eqn:            f a     y = a-y
+
+But it's too drastic to disable rules for loop breakers.
+Even the foldr/build rule would be disabled, because foldr
+is recursive, and hence a loop breaker:
+     foldr k z (build g) = g k z
+So it's up to the programmer: rules can cause divergence
+
+

 %************************************************************************
 %*                                                                      *
                 Rebuilding a cse expression
 %************************************************************************
 %*                                                                      *
                 Rebuilding a cse expression

@@ -1290,12 +1334,13 @@ I don't really know how to improve this situation.
 ---------------------------------------------------------
 --      Eliminate the case if possible
 
 ---------------------------------------------------------
 --      Eliminate the case if possible
 

-rebuildCase :: SimplEnv
-            -> OutExpr          -- Scrutinee
-            -> InId             -- Case binder
-            -> [InAlt]          -- Alternatives (inceasing order)
-            -> SimplCont
-            -> SimplM (SimplEnv, OutExpr)
+rebuildCase, reallyRebuildCase
+   :: SimplEnv
+   -> OutExpr          -- Scrutinee
+   -> InId             -- Case binder
+   -> [InAlt]          -- Alternatives (inceasing order)
+   -> SimplCont
+   -> SimplM (SimplEnv, OutExpr)

 
 --------------------------------------------------
 --      1. Eliminate the case if there's a known constructor
 
 --------------------------------------------------
 --      1. Eliminate the case if there's a known constructor

@@ -1356,12 +1401,28 @@ rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
                                     -- exprOkForSpeculation was intended for.
     var_demanded_later _       = False
 
                                     -- exprOkForSpeculation was intended for.
     var_demanded_later _       = False
 

+rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
+  | all isDeadBinder (case_bndr : bndrs)  -- So this is just 'seq'
+  =    -- For this case, see Note [Rules for seq] in MkId
+    do { let rhs' = substExpr env rhs
+             out_args = [Type (substTy env (idType case_bndr)), 
+                        Type (exprType rhs'), scrut, rhs']
+                     -- Lazily evaluated, so we don't do most of this
+       ; mb_rule <- tryRules env seqId out_args cont
+       ; case mb_rule of 
+           Just (n_args, res) -> simplExprF (zapSubstEnv env) 
+                                           (mkApps res (drop n_args out_args))
+                                            cont
+          Nothing -> reallyRebuildCase env scrut case_bndr alts cont }
+
+rebuildCase env scrut case_bndr alts cont
+  = reallyRebuildCase env scrut case_bndr alts cont

 
 --------------------------------------------------
 --      3. Catch-all case
 --------------------------------------------------
 
 
 --------------------------------------------------
 --      3. Catch-all case
 --------------------------------------------------
 

-rebuildCase env scrut case_bndr alts cont
+reallyRebuildCase env scrut case_bndr alts cont

   = do  {       -- Prepare the continuation;
                 -- The new subst_env is in place
           (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
   = do  {       -- Prepare the continuation;
                 -- The new subst_env is in place
           (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont

@@ -1370,17 +1431,7 @@ rebuildCase env scrut case_bndr alts cont
         ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
 
        -- Check for empty alternatives
         ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
 
        -- Check for empty alternatives

-       ; if null alts' then
-               -- This isn't strictly an error, although it is unusual. 
-               -- It's possible that the simplifer might "see" that 
-               -- an inner case has no accessible alternatives before 
-               -- it "sees" that the entire branch of an outer case is 
-               -- inaccessible.  So we simply put an error case here instead.
-           pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $
-           let res_ty' = contResultType env' (substTy env' (coreAltsType alts)) dup_cont
-               lit = mkStringLit "Impossible alternative"
-           in return (env', mkApps (Var rUNTIME_ERROR_ID) [Type res_ty', lit])
-
+       ; if null alts' then missingAlt env case_bndr alts cont

          else do
        { case_expr <- mkCase scrut' case_bndr' alts'
 
          else do
        { case_expr <- mkCase scrut' case_bndr' alts'
 

@@ -1411,19 +1462,6 @@ 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# ->
-                ... (case (v `cast` co) of {...}) ...
-We'd like to eliminate the inner case.  We can get this neatly by
-arranging that inside the outer case we add the unfolding
-        v |-> x `cast` (sym co)
-to v.  Then we should inline v at the inner case, cancel the casts, and away we go
-

 Note [Improving seq]
 ~~~~~~~~~~~~~~~~~~~
 Consider
 Note [Improving seq]
 ~~~~~~~~~~~~~~~~~~~
 Consider

@@ -1455,59 +1493,6 @@ At one point I did transformation in LiberateCase, but it's more robust here.
 LiberateCase gets to see it.)
 
 
 LiberateCase gets to see it.)
 
 

-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.
-
-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
-
-    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.

 
 
 \begin{code}
 
 
 \begin{code}

@@ -1733,23 +1718,15 @@ knownCon :: SimplEnv -> OutExpr -> AltCon
 
 knownCon env scrut con args bndr alts cont
   = do  { tick (KnownBranch bndr)
 
 knownCon env scrut con args bndr alts cont
   = do  { tick (KnownBranch bndr)

-        ; knownAlt env scrut args bndr (findAlt con alts) cont }
+        ; case findAlt con alts of
+           Nothing  -> missingAlt env bndr alts cont
+           Just alt -> knownAlt env scrut args bndr alt cont
+       }

 
 

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

 knownAlt :: SimplEnv -> OutExpr -> [OutExpr]
 knownAlt :: SimplEnv -> OutExpr -> [OutExpr]

-         -> InId -> (AltCon, [CoreBndr], InExpr) -> SimplCont
+         -> InId -> InAlt -> SimplCont

          -> SimplM (SimplEnv, OutExpr)
          -> SimplM (SimplEnv, OutExpr)

-knownAlt env scrut _ bndr (DEFAULT, bs, rhs) cont
-  = ASSERT( null bs )
-    do  { env' <- simplNonRecX env bndr scrut
-                -- This might give rise to a binding with non-atomic args
-                -- like x = Node (f x) (g x)
-                -- but simplNonRecX will atomic-ify it
-        ; simplExprF env' rhs cont }
-
-knownAlt env scrut _ bndr (LitAlt _, bs, rhs) cont
-  = ASSERT( null bs )
-    do  { env' <- simplNonRecX env bndr scrut
-        ; simplExprF env' rhs cont }

 
 knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
   = do  { let n_drop_tys = length (dataConUnivTyVars dc)
 
 knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
   = do  { let n_drop_tys = length (dataConUnivTyVars dc)

@@ -1795,6 +1772,25 @@ knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
     bind_args _ _ _ =
       pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
                              text "scrut:" <+> ppr scrut
     bind_args _ _ _ =
       pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
                              text "scrut:" <+> ppr scrut

+
+knownAlt env scrut _ bndr (_, bs, rhs) cont
+  = ASSERT( null bs )    -- Works for LitAlt and DEFAULT
+    do  { env' <- simplNonRecX env bndr scrut
+        ; simplExprF env' rhs cont }
+
+
+-------------------
+missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr)
+               -- This isn't strictly an error, although it is unusual. 
+               -- It's possible that the simplifer might "see" that 
+               -- an inner case has no accessible alternatives before 
+               -- it "sees" that the entire branch of an outer case is 
+               -- inaccessible.  So we simply put an error case here instead.
+missingAlt env case_bndr alts cont
+  = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr )
+    return (env, mkImpossibleExpr res_ty)
+  where
+    res_ty = contResultType env (substTy env (coreAltsType alts)) cont

 \end{code}
 
 
 \end{code}
 
 

@@ -1833,11 +1829,20 @@ mkDupableCont env (CoerceIt ty cont)
 
 mkDupableCont env cont@(StrictBind {})
   =  return (env, mkBoringStop, cont)
 
 mkDupableCont env cont@(StrictBind {})
   =  return (env, mkBoringStop, cont)

-        -- See Note [Duplicating strict continuations]
+        -- See Note [Duplicating StrictBind]

 
 

-mkDupableCont env cont@(StrictArg {})
-  =  return (env, mkBoringStop, cont)
-        -- See Note [Duplicating strict continuations]
+mkDupableCont env (StrictArg fun cci ai cont)
+        -- See Note [Duplicating StrictArg]
+  = do { (env', dup, nodup) <- mkDupableCont env cont
+       ; (env'', fun') <- mk_dupable_call env' fun
+       ; return (env'', StrictArg fun' cci ai dup, nodup) }
+  where
+    mk_dupable_call env (Var v)       = return (env, Var v)
+    mk_dupable_call env (App fun arg) = do { (env', fun') <- mk_dupable_call env fun
+                                           ; (env'', arg') <- makeTrivial env' arg
+                                           ; return (env'', fun' `App` arg') }
+    mk_dupable_call _ other = pprPanic "mk_dupable_call" (ppr other)
+       -- The invariant of StrictArg is that the first arg is always an App chain

 
 mkDupableCont env (ApplyTo _ arg se cont)
   =     -- e.g.         [...hole...] (...arg...)
 
 mkDupableCont env (ApplyTo _ arg se cont)
   =     -- e.g.         [...hole...] (...arg...)

@@ -1906,97 +1911,40 @@ 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_bndr1 (con, bndrs1, rhs1)
-  | exprIsDupable rhs1  -- Note [Small alternative rhs]
-  = return (env, (con, bndrs1, rhs1))
+mkDupableAlt env case_bndr' (con, bndrs', rhs')
+  | exprIsDupable rhs'  -- Note [Small alternative rhs]
+  = return (env, (con, bndrs', rhs'))

   | otherwise
   | otherwise

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

-              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)
+        ; (final_bndrs', final_args)    -- Note [Join point abstraction]
+                <- if (any isId used_bndrs')
+                   then return (used_bndrs', varsToCoreExprs used_bndrs')

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

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

 
 

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

                 -- 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_bndrs1
+                really_final_bndrs     = map one_shot final_bndrs'

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

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

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

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

@@ -2006,7 +1954,7 @@ we'd lose that when zapping the subst-env.  We could have a per-alt subst-env,
 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
 at worst delays the join-point inlining.
 
 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
 at worst delays the join-point inlining.
 

-Note [Small alterantive rhs]
+Note [Small alternative rhs]

 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 It is worth checking for a small RHS because otherwise we
 get extra let bindings that may cause an extra iteration of the simplifier to
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 It is worth checking for a small RHS because otherwise we
 get extra let bindings that may cause an extra iteration of the simplifier to

@@ -2075,32 +2023,71 @@ It's a bit silly to add the realWorld dummy arg in this case, making
            True -> $j s
 (the \v alone is enough to make CPR happy) but I think it's rare
 
            True -> $j s
 (the \v alone is enough to make CPR happy) but I think it's rare
 

-Note [Duplicating strict continuations]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Do *not* duplicate StrictBind and StritArg continuations.  We gain
-nothing by propagating them into the expressions, and we do lose a
-lot.  Here's an example:
-        && (case x of { T -> F; F -> T }) E
+Note [Duplicating StrictArg]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The original plan had (where E is a big argument)
+e.g.    f E [..hole..]
+        ==>     let $j = \a -> f E a
+                in $j [..hole..]
+
+But this is terrible! Here's an example:
+        && E (case x of { T -> F; F -> T })

 Now, && is strict so we end up simplifying the case with
 an ArgOf continuation.  If we let-bind it, we get
 Now, && is strict so we end up simplifying the case with
 an ArgOf continuation.  If we let-bind it, we get

-
-        let $j = \v -> && v E
+        let $j = \v -> && E v

         in simplExpr (case x of { T -> F; F -> T })
                      (ArgOf (\r -> $j r)
 And after simplifying more we get
         in simplExpr (case x of { T -> F; F -> T })
                      (ArgOf (\r -> $j r)
 And after simplifying more we get

-
-        let $j = \v -> && v E
+        let $j = \v -> && E v

         in case x of { T -> $j F; F -> $j T }
 Which is a Very Bad Thing
 
         in case x of { T -> $j F; F -> $j T }
 Which is a Very Bad Thing
 

+What we do now is this
+       f E [..hole..]
+       ==>     let a = E
+               in f a [..hole..]
+Now if the thing in the hole is a case expression (which is when
+we'll call mkDupableCont), we'll push the function call into the
+branches, which is what we want.  Now RULES for f may fire, and
+call-pattern specialisation.  Here's an example from Trac #3116
+     go (n+1) (case l of
+                1  -> bs'
+                _  -> Chunk p fpc (o+1) (l-1) bs')
+If we can push the call for 'go' inside the case, we get
+call-pattern specialisation for 'go', which is *crucial* for 
+this program.
+
+Here is the (&&) example: 
+        && E (case x of { T -> F; F -> T })
+  ==>   let a = E in 
+        case x of { T -> && a F; F -> && a T }
+Much better!
+
+Notice that 
+  * Arguments to f *after* the strict one are handled by 
+    the ApplyTo case of mkDupableCont.  Eg
+       f [..hole..] E
+
+  * We can only do the let-binding of E because the function
+    part of a StrictArg continuation is an explicit syntax
+    tree.  In earlier versions we represented it as a function
+    (CoreExpr -> CoreEpxr) which we couldn't take apart.
+
+Do *not* duplicate StrictBind and StritArg continuations.  We gain
+nothing by propagating them into the expressions, and we do lose a
+lot.  
+
+The desire not to duplicate is the entire reason that
+mkDupableCont returns a pair of continuations.
+
+Note [Duplicating StrictBind]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Unlike StrictArg, there doesn't seem anything to gain from
+duplicating a StrictBind continuation, so we don't.
+

 The desire not to duplicate is the entire reason that
 mkDupableCont returns a pair of continuations.
 
 The desire not to duplicate is the entire reason that
 mkDupableCont returns a pair of continuations.
 

-The original plan had:
-e.g.    (...strict-fn...) [...hole...]
-        ==>
-                let $j = \a -> ...strict-fn...
-                in $j [...hole...]

 
 Note [Single-alternative cases]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Note [Single-alternative cases]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~