% % (c) The AQUA Project, Glasgow University, 1993-1998 % \section[Simplify]{The main module of the simplifier} \begin{code}

    module Simplify ( simplTopBinds, simplExpr ) where

    #include "HsVersions.h"

    import DynFlags
    import SimplMonad
    import Type hiding      ( substTy, extendTvSubst, substTyVar )
    import SimplEnv
    import SimplUtils
    import FamInstEnv	( FamInstEnv )
    import Id
    import MkId		( seqId, realWorldPrimId )
    import MkCore		( mkImpossibleExpr )
    import Var
    import IdInfo
    import Name		( mkSystemVarName, isExternalName )
    import Coercion
    import OptCoercion	( optCoercion )
    import FamInstEnv       ( topNormaliseType )
    import DataCon          ( DataCon, dataConWorkId, dataConRepStrictness )
    import CoreMonad        ( Tick(..), SimplifierMode(..) )
    import CoreSyn
    import Demand           ( isStrictDmd )
    import PprCore          ( pprParendExpr, pprCoreExpr )
    import CoreUnfold       ( mkUnfolding, mkCoreUnfolding
                            , mkInlineUnfolding, mkSimpleUnfolding
                            , exprIsConApp_maybe, callSiteInline, CallCtxt(..) )
    import CoreUtils
    import qualified CoreSubst
    import CoreArity
    import Rules            ( lookupRule, getRules )
    import BasicTypes       ( isMarkedStrict, Arity )
    import CostCentre       ( currentCCS, pushCCisNop )
    import TysPrim          ( realWorldStatePrimTy )
    import BasicTypes       ( TopLevelFlag(..), isTopLevel, RecFlag(..) )
    import MonadUtils	( foldlM, mapAccumLM )
    import Maybes           ( orElse, isNothing )
    import Data.List        ( mapAccumL )
    import Outputable
    import FastString

\end{code} The guts of the simplifier is in this module, but the driver loop for the simplifier is in SimplCore.lhs. ----------------------------------------- *** IMPORTANT NOTE *** ----------------------------------------- The simplifier used to guarantee that the output had no shadowing, but it does not do so any more. (Actually, it never did!) The reason is documented with simplifyArgs. ----------------------------------------- *** IMPORTANT NOTE *** ----------------------------------------- Many parts of the simplifier return a bunch of "floats" as well as an expression. This is wrapped as a datatype SimplUtils.FloatsWith. All "floats" are let-binds, not case-binds, but some non-rec lets may be unlifted (with RHS ok-for-speculation). ----------------------------------------- ORGANISATION OF FUNCTIONS ----------------------------------------- simplTopBinds - simplify all top-level binders - for NonRec, call simplRecOrTopPair - for Rec, call simplRecBind ------------------------------ simplExpr (applied lambda) ==> simplNonRecBind simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind ------------------------------ simplRecBind [binders already simplfied] - use simplRecOrTopPair on each pair in turn simplRecOrTopPair [binder already simplified] Used for: recursive bindings (top level and nested) top-level non-recursive bindings Returns: - check for PreInlineUnconditionally - simplLazyBind simplNonRecBind Used for: non-top-level non-recursive bindings beta reductions (which amount to the same thing) Because it can deal with strict arts, it takes a "thing-inside" and returns an expression - check for PreInlineUnconditionally - simplify binder, including its IdInfo - if strict binding simplStrictArg mkAtomicArgs completeNonRecX else simplLazyBind addFloats simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder] Used for: binding case-binder and constr args in a known-constructor case - check for PreInLineUnconditionally - simplify binder - completeNonRecX ------------------------------ simplLazyBind: [binder already simplified, RHS not] Used for: recursive bindings (top level and nested) top-level non-recursive bindings non-top-level, but *lazy* non-recursive bindings [must not be strict or unboxed] Returns floats + an augmented environment, not an expression - substituteIdInfo and add result to in-scope [so that rules are available in rec rhs] - simplify rhs - mkAtomicArgs - float if exposes constructor or PAP - completeBind completeNonRecX: [binder and rhs both simplified] - if the the thing needs case binding (unlifted and not ok-for-spec) build a Case else completeBind addFloats completeBind: [given a simplified RHS] [used for both rec and non-rec bindings, top level and not] - try PostInlineUnconditionally - add unfolding [this is the only place we add an unfolding] - add arity Right hand sides and arguments ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In many ways we want to treat (a) the right hand side of a let(rec), and (b) a function argument in the same way. But not always! In particular, we would like to leave these arguments exactly as they are, so they will match a RULE more easily. f (g x, h x) g (+ x) It's harder to make the rule match if we ANF-ise the constructor, or eta-expand the PAP: f (let { a = g x; b = h x } in (a,b)) g (\y. + x y) On the other hand if we see the let-defns p = (g x, h x) q = + x then we *do* want to ANF-ise and eta-expand, so that p and q can be safely inlined. Even floating lets out is a bit dubious. For let RHS's we float lets out if that exposes a value, so that the value can be inlined more vigorously. For example r = let x = e in (x,x) Here, if we float the let out we'll expose a nice constructor. We did experiments that showed this to be a generally good thing. But it was a bad thing to float lets out unconditionally, because that meant they got allocated more often. For function arguments, there's less reason to expose a constructor (it won't get inlined). Just possibly it might make a rule match, but I'm pretty skeptical. So for the moment we don't float lets out of function arguments either. Eta expansion ~~~~~~~~~~~~~~ For eta expansion, we want to catch things like case e of (a,b) -> \x -> case a of (p,q) -> \y -> r If the \x was on the RHS of a let, we'd eta expand to bring the two lambdas together. And in general that's a good thing to do. Perhaps we should eta expand wherever we find a (value) lambda? Then the eta expansion at a let RHS can concentrate solely on the PAP case. %************************************************************************ %* * \subsection{Bindings} %* * %************************************************************************ \begin{code}

    simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv

    simplTopBinds env0 binds0
      = do  {       -- Put all the top-level binders into scope at the start
                    -- so that if a transformation rule has unexpectedly brought
                    -- anything into scope, then we don't get a complaint about that.
                    -- It's rather as if the top-level binders were imported.
            ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
            ; dflags <- getDOptsSmpl
            ; let dump_flag = dopt Opt_D_verbose_core2core dflags
            ; env2 <- simpl_binds dump_flag env1 binds0
            ; freeTick SimplifierDone
            ; return env2 }
      where
            -- We need to track the zapped top-level binders, because
            -- they should have their fragile IdInfo zapped (notably occurrence info)
            -- That's why we run down binds and bndrs' simultaneously.
            --
            -- The dump-flag emits a trace for each top-level binding, which
            -- helps to locate the tracing for inlining and rule firing
        simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
        simpl_binds _    env []           = return env
        simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
                                                         simpl_bind env bind
                                               ; simpl_binds dump env' binds }

        trace_bind True  bind = pprTrace "SimplBind" (ppr (bindersOf bind))
        trace_bind False _    = \x -> x

        simpl_bind env (Rec pairs)  = simplRecBind      env  TopLevel pairs
        simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel NonRecursive b b' r
            where
              (env', b') = addBndrRules env b (lookupRecBndr env b)

\end{code} %************************************************************************ %* * \subsection{Lazy bindings} %* * %************************************************************************ simplRecBind is used for * recursive bindings only \begin{code}

    simplRecBind :: SimplEnv -> TopLevelFlag
                 -> [(InId, InExpr)]
                 -> SimplM SimplEnv
    simplRecBind env0 top_lvl pairs0
      = do  { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
            ; env1 <- go (zapFloats env_with_info) triples
            ; return (env0 `addRecFloats` env1) }
            -- addFloats adds the floats from env1,
            -- _and_ updates env0 with the in-scope set from env1
      where
        add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
            -- Add the (substituted) rules to the binder
        add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
            where
              (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)

        go env [] = return env

        go env ((old_bndr, new_bndr, rhs) : pairs)
            = do { env' <- simplRecOrTopPair env top_lvl Recursive old_bndr new_bndr rhs
                 ; go env' pairs }

\end{code} simplOrTopPair is used for * recursive bindings (whether top level or not) * top-level non-recursive bindings It assumes the binder has already been simplified, but not its IdInfo. \begin{code}

    simplRecOrTopPair :: SimplEnv
                      -> TopLevelFlag -> RecFlag
                      -> InId -> OutBndr -> InExpr  -- Binder and rhs
                      -> SimplM SimplEnv    -- Returns an env that includes the binding

    simplRecOrTopPair env top_lvl is_rec old_bndr new_bndr rhs
      | preInlineUnconditionally env top_lvl old_bndr rhs   -- Check for unconditional inline
      = do  { tick (PreInlineUnconditionally old_bndr)
            ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }

      | otherwise
      = simplLazyBind env top_lvl is_rec old_bndr new_bndr rhs env

\end{code} simplLazyBind is used for * [simplRecOrTopPair] recursive bindings (whether top level or not) * [simplRecOrTopPair] top-level non-recursive bindings * [simplNonRecE] non-top-level *lazy* non-recursive bindings Nota bene: 1. It assumes that the binder is *already* simplified, and is in scope, and its IdInfo too, except unfolding 2. It assumes that the binder type is lifted. 3. It does not check for pre-inline-unconditionallly; that should have been done already. \begin{code}

    simplLazyBind :: SimplEnv
                  -> TopLevelFlag -> RecFlag
                  -> InId -> OutId          -- Binder, both pre-and post simpl
                                            -- The OutId has IdInfo, except arity, unfolding
                  -> InExpr -> SimplEnv     -- The RHS and its environment
                  -> SimplM SimplEnv

    simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
      = -- pprTrace "simplLazyBind" ((ppr bndr <+> ppr bndr1) $$ ppr rhs $$ ppr (seIdSubst rhs_se)) $
        do  { let   rhs_env     = rhs_se `setInScope` env
    		(tvs, body) = case collectTyBinders rhs of
    			        (tvs, body) | not_lam body -> (tvs,body)
    					    | otherwise	   -> ([], rhs)
    		not_lam (Lam _ _) = False
    		not_lam _	  = True
    			-- Do not do the "abstract tyyvar" thing if there's
    			-- a lambda inside, becuase it defeats eta-reduction
    			--    f = /\a. \x. g a x
    			-- should eta-reduce

            ; (body_env, tvs') <- simplBinders rhs_env tvs
                    -- See Note [Floating and type abstraction] in SimplUtils

            -- Simplify the RHS
            ; (body_env1, body1) <- simplExprF body_env body mkRhsStop
            -- ANF-ise a constructor or PAP rhs
            ; (body_env2, body2) <- prepareRhs top_lvl body_env1 bndr1 body1

            ; (env', rhs')
                <-  if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
                    then                            -- No floating, revert to body1
                         do { rhs' <- mkLam env tvs' (wrapFloats body_env1 body1)
                            ; return (env, rhs') }

                    else if null tvs then           -- Simple floating
                         do { tick LetFloatFromLet
                            ; return (addFloats env body_env2, body2) }

                    else                            -- Do type-abstraction first
                         do { tick LetFloatFromLet
                            ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
                            ; rhs' <- mkLam env tvs' body3
                            ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
                            ; return (env', rhs') }

            ; completeBind env' top_lvl bndr bndr1 rhs' }

\end{code} A specialised variant of simplNonRec used when the RHS is already simplified, notably in knownCon. It uses case-binding where necessary. \begin{code}

    simplNonRecX :: SimplEnv
                 -> InId            -- Old binder
                 -> OutExpr         -- Simplified 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 NotTopLevel env' (isStrictId bndr) bndr bndr' new_rhs }
    		-- simplNonRecX is only used for NotTopLevel things

    completeNonRecX :: TopLevelFlag -> SimplEnv
                    -> Bool
                    -> InId                 -- Old binder
                    -> OutId                -- New binder
                    -> OutExpr              -- Simplified RHS
                    -> SimplM SimplEnv

    completeNonRecX top_lvl env is_strict old_bndr new_bndr new_rhs
      = do  { (env1, rhs1) <- prepareRhs top_lvl (zapFloats env) new_bndr new_rhs
            ; (env2, rhs2) <-
                    if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
                    then do { tick LetFloatFromLet
                            ; return (addFloats env env1, rhs1) }   -- Add the floats to the main env
                    else return (env, wrapFloats env1 rhs1)         -- Wrap the floats around the RHS
            ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }

\end{code} {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX Doing so risks exponential behaviour, because new_rhs has been simplified once already In the cases described by the folowing commment, postInlineUnconditionally will catch many of the relevant cases. -- This happens; for example, the case_bndr during case of -- known constructor: case (a,b) of x { (p,q) -> ... } -- Here x isn't mentioned in the RHS, so we don't want to -- create the (dead) let-binding let x = (a,b) in ... -- -- Similarly, single occurrences can be inlined vigourously -- e.g. case (f x, g y) of (a,b) -> .... -- If a,b occur once we can avoid constructing the let binding for them. Furthermore in the case-binding case preInlineUnconditionally risks extra thunks -- Consider case I# (quotInt# x y) of -- I# v -> let w = J# v in ... -- If we gaily inline (quotInt# x y) for v, we end up building an -- extra thunk: -- let w = J# (quotInt# x y) in ... -- because quotInt# can fail. | preInlineUnconditionally env NotTopLevel bndr new_rhs = thing_inside (extendIdSubst env bndr (DoneEx new_rhs)) -} ---------------------------------- prepareRhs takes a putative RHS, checks whether it's a PAP or constructor application and, if so, converts it to ANF, so that the resulting thing can be inlined more easily. Thus x = (f a, g b) becomes t1 = f a t2 = g b x = (t1,t2) We also want to deal well cases like this v = (f e1 `cast` co) e2 Here we want to make e1,e2 trivial and get x1 = e1; x2 = e2; v = (f x1 `cast` co) v2 That's what the 'go' loop in prepareRhs does \begin{code}

    prepareRhs :: TopLevelFlag -> SimplEnv -> OutId -> OutExpr -> SimplM (SimplEnv, OutExpr)
    -- Adds new floats to the env iff that allows us to return a good RHS
    prepareRhs top_lvl env id (Cast rhs co)    -- Note [Float coercions]
      | (ty1, _ty2) <- coercionKind co       -- Do *not* do this if rhs has an unlifted type
      , not (isUnLiftedType ty1)            -- see Note [Float coercions (unlifted)]
      = do  { (env', rhs') <- makeTrivialWithInfo top_lvl env sanitised_info rhs
            ; return (env', Cast rhs' co) }
      where
        sanitised_info = vanillaIdInfo `setStrictnessInfo` strictnessInfo info
                                       `setDemandInfo`     demandInfo info
        info = idInfo id

    prepareRhs top_lvl env0 _ rhs0
      = do  { (_is_exp, env1, rhs1) <- go 0 env0 rhs0
            ; return (env1, rhs1) }
      where
        go n_val_args env (Cast rhs co)
            = do { (is_exp, env', rhs') <- go n_val_args env rhs
                 ; return (is_exp, env', Cast rhs' co) }
        go n_val_args env (App fun (Type ty))
            = do { (is_exp, env', rhs') <- go n_val_args env fun
                 ; return (is_exp, env', App rhs' (Type ty)) }
        go n_val_args env (App fun arg)
            = do { (is_exp, env', fun') <- go (n_val_args+1) env fun
                 ; case is_exp of
                    True -> do { (env'', arg') <- makeTrivial top_lvl env' arg
                               ; return (True, env'', App fun' arg') }
                    False -> return (False, env, App fun arg) }
        go n_val_args env (Var fun)
            = return (is_exp, env, Var fun)
            where
              is_exp = isExpandableApp fun n_val_args   -- The fun a constructor or PAP
    		        -- See Note [CONLIKE pragma] in BasicTypes
    			-- The definition of is_exp should match that in
    	                -- OccurAnal.occAnalApp

        go _ env other
            = return (False, env, other)

\end{code} Note [Float coercions] ~~~~~~~~~~~~~~~~~~~~~~ When we find the binding x = e `cast` co we'd like to transform it to x' = e x = x `cast` co -- A trivial binding There's a chance that e will be a constructor application or function, or something like that, so moving the coerion to the usage site may well cancel the coersions and lead to further optimisation. Example: data family T a :: * data instance T Int = T Int foo :: Int -> Int -> Int foo m n = ... where x = T m go 0 = 0 go n = case x of { T m -> go (n-m) } -- This case should optimise Note [Preserve strictness when floating coercions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the Note [Float coercions] transformation, keep the strictness info. Eg f = e `cast` co -- f has strictness SSL When we transform to f' = e -- f' also has strictness SSL f = f' `cast` co -- f still has strictness SSL Its not wrong to drop it on the floor, but better to keep it. Note [Float coercions (unlifted)] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ BUT don't do [Float coercions] if 'e' has an unlifted type. This *can* happen: foo :: Int = (error (# Int,Int #) "urk") `cast` CoUnsafe (# Int,Int #) Int If do the makeTrivial thing to the error call, we'll get foo = case error (# Int,Int #) "urk" of v -> v `cast` ... But 'v' isn't in scope! These strange casts can happen as a result of case-of-case bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of (# p,q #) -> p+q \begin{code}

    makeTrivial :: TopLevelFlag -> SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
    -- Binds the expression to a variable, if it's not trivial, returning the variable
    makeTrivial top_lvl env expr = makeTrivialWithInfo top_lvl env vanillaIdInfo expr

    makeTrivialWithInfo :: TopLevelFlag -> SimplEnv -> IdInfo
                        -> OutExpr -> SimplM (SimplEnv, OutExpr)
    -- Propagate strictness and demand info to the new binder
    -- Note [Preserve strictness when floating coercions]
    -- Returned SimplEnv has same substitution as incoming one
    makeTrivialWithInfo top_lvl env info expr
      | exprIsTrivial expr 				-- Already trivial
      || not (bindingOk top_lvl expr expr_ty)	-- Cannot trivialise
    						--   See Note [Cannot trivialise]
      = return (env, expr)
      | otherwise           -- See Note [Take care] below
      = do  { uniq <- getUniqueM
            ; let name = mkSystemVarName uniq (fsLit "a")
                  var = mkLocalIdWithInfo name expr_ty info
            ; env'  <- completeNonRecX top_lvl env False var var expr
    	; expr' <- simplVar env' var
            ; return (env', expr') }
    	-- The simplVar 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
    	-- To put it another way, it's as if we'd simplified
    	--    let var = e in var
      where
        expr_ty = exprType expr

    bindingOk :: TopLevelFlag -> CoreExpr -> Type -> Bool
    -- True iff we can have a binding of this expression at this level
    -- Precondition: the type is the type of the expression
    bindingOk top_lvl _ expr_ty
      | isTopLevel top_lvl = not (isUnLiftedType expr_ty)
      | otherwise          = True

\end{code} Note [Cannot trivialise] ~~~~~~~~~~~~~~~~~~~~~~~~ Consider tih f :: Int -> Addr# foo :: Bar foo = Bar (f 3) Then we can't ANF-ise foo, even though we'd like to, because we can't make a top-level binding for the Addr# (f 3). And if so we don't want to turn it into foo = let x = f 3 in Bar x because we'll just end up inlining x back, and that makes the simplifier loop. Better not to ANF-ise it at all. A case in point is literal strings (a MachStr is not regarded as trivial): foo = Ptr "blob"# We don't want to ANF-ise this. %************************************************************************ %* * \subsection{Completing a lazy binding} %* * %************************************************************************ completeBind * deals only with Ids, not TyVars * takes an already-simplified binder and RHS * is used for both recursive and non-recursive bindings * is used for both top-level and non-top-level bindings It does the following: - tries discarding a dead binding - tries PostInlineUnconditionally - add unfolding [this is the only place we add an unfolding] - add arity It does *not* attempt to do let-to-case. Why? Because it is used for - top-level bindings (when let-to-case is impossible) - many situations where the "rhs" is known to be a WHNF (so let-to-case is inappropriate). Nor does it do the atomic-argument thing \begin{code}

    completeBind :: SimplEnv
                 -> TopLevelFlag            -- Flag stuck into unfolding
                 -> InId                    -- Old binder
                 -> OutId -> OutExpr        -- New binder and RHS
                 -> SimplM SimplEnv
    -- completeBind may choose to do its work
    --      * by extending the substitution (e.g. let x = y in ...)
    --      * or by adding to the floats in the envt

    completeBind env top_lvl old_bndr new_bndr new_rhs
     = ASSERT( isId new_bndr )
       do { let old_info = idInfo old_bndr
    	    old_unf  = unfoldingInfo old_info
    	    occ_info = occInfo old_info

    	-- Do eta-expansion on the RHS of the binding
            -- See Note [Eta-expanding at let bindings] in SimplUtils
          ; (new_arity, final_rhs) <- tryEtaExpand env new_bndr new_rhs

          	-- Simplify the unfolding
          ; new_unfolding <- simplUnfolding env top_lvl old_bndr occ_info final_rhs old_unf

          ; if postInlineUnconditionally env top_lvl new_bndr occ_info final_rhs new_unfolding
    	                -- Inline and discard the binding
    	then do  { tick (PostInlineUnconditionally old_bndr)
    	         ; -- pprTrace "postInlineUnconditionally"
                       --         (ppr old_bndr <+> equals <+> ppr final_rhs $$ ppr occ_info) $
                       return (extendIdSubst env old_bndr (DoneEx final_rhs)) }
    	        -- Use the substitution to make quite, quite sure that the
    	        -- substitution will happen, since we are going to discard the binding
    	else
       do { let 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

          ; -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
            return (addNonRec env final_id final_rhs) } }
    		-- The addNonRec adds it to the in-scope set too

    ------------------------------
    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
    -- We still want the unfolding though.  Consider
    --	let
    --	      x = /\a. let y = ... in Just y
    --	in body
    -- Then we float the y-binding out (via abstractFloats and addPolyBind)
    -- but 'x' may well then be inlined in 'body' in which case we'd like the
    -- opportunity to inline 'y' too.

    addPolyBind top_lvl env (NonRec poly_id rhs)
      = do	{ unfolding <- simplUnfolding env top_lvl poly_id NoOccInfo rhs noUnfolding
        	  		-- Assumes that poly_id did not have an INLINE prag
    			-- which is perhaps wrong.  ToDo: think about this
            ; let final_id = setIdInfo poly_id $
                             idInfo poly_id `setUnfoldingInfo` unfolding
                                            `setArityInfo`     exprArity rhs

            ; return (addNonRec env final_id rhs) }

    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

    ------------------------------
    simplUnfolding :: SimplEnv-> TopLevelFlag
    	       -> Id
    	       -> OccInfo -> OutExpr
    	       -> Unfolding -> SimplM Unfolding
    -- Note [Setting the new unfolding]
    simplUnfolding env _ _ _ _ (DFunUnfolding ar con ops)
      = return (DFunUnfolding ar con ops')
      where
        ops' = map (fmap (substExpr (text "simplUnfolding") env)) ops

    simplUnfolding env top_lvl id _ _
        (CoreUnfolding { uf_tmpl = expr, uf_arity = arity
                       , uf_src = src, uf_guidance = guide })
      | isStableSource src
      = do { expr' <- simplExpr rule_env expr
           ; let src' = CoreSubst.substUnfoldingSource (mkCoreSubst (text "inline-unf") env) src
                 is_top_lvl = isTopLevel top_lvl
           ; case guide of
               UnfIfGoodArgs{} ->
                  -- We need to force bottoming, or the new unfolding holds
                  -- on to the old unfolding (which is part of the id).
                  let bottoming = isBottomingId id
                  in bottoming `seq` return (mkUnfolding src' is_top_lvl bottoming expr')
                    -- If the guidance is UnfIfGoodArgs, this is an INLINABLE
                    -- unfolding, and we need to make sure the guidance is kept up
                    -- to date with respect to any changes in the unfolding.
               _other ->
                  return (mkCoreUnfolding src' is_top_lvl expr' arity guide)
    		-- See Note [Top-level flag on inline rules] in CoreUnfold
           }
      where
        act      = idInlineActivation id
        rule_env = updMode (updModeForInlineRules act) env
           	       -- See Note [Simplifying inside InlineRules] in SimplUtils

    simplUnfolding _ top_lvl id _occ_info new_rhs _
      = -- We need to force bottoming, or the new unfolding holds
        -- on to the old unfolding (which is part of the id).
        let bottoming = isBottomingId id
        in bottoming `seq` return (mkUnfolding InlineRhs (isTopLevel top_lvl) bottoming new_rhs)
      -- We make an  unfolding *even for loop-breakers*.
      -- Reason: (a) It might be useful to know that they are WHNF
      -- 	     (b) In TidyPgm we currently assume that, if we want to
      --	         expose the unfolding then indeed we *have* an unfolding
      --		 to expose.  (We could instead use the RHS, but currently
      --		 we don't.)  The simple thing is always to have one.

\end{code} 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 } 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. Note [Setting the new unfolding] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * If there's an INLINE pragma, we simplify the RHS gently. Maybe we should do nothing at all, but simplifying gently might get rid of more crap. * 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 InlineRule on a loop breaker, we hang on to the inlining. It's pretty dodgy, but the user did say 'INLINE'. May need to revisit this choice. 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... %************************************************************************ %* * \subsection[Simplify-simplExpr]{The main function: simplExpr} %* * %************************************************************************ The reason for this OutExprStuff stuff is that we want to float *after* simplifying a RHS, not before. If we do so naively we get quadratic behaviour as things float out. To see why it's important to do it after, consider this (real) example: let t = f x in fst t ==> let t = let a = e1 b = e2 in (a,b) in fst t ==> let a = e1 b = e2 t = (a,b) in a -- Can't inline a this round, cos it appears twice ==> e1 Each of the ==> steps is a round of simplification. We'd save a whole round if we float first. This can cascade. Consider let f = g d in \x -> ...f... ==> let f = let d1 = ..d.. in \y -> e in \x -> ...f... ==> let d1 = ..d.. in \x -> ...(\y ->e)... Only in this second round can the \y be applied, and it might do the same again. \begin{code}

    simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
    simplExpr env expr = simplExprC env expr mkBoringStop

    simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
            -- Simplify an expression, given a continuation
    simplExprC env expr cont
      = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
        do  { (env', expr') <- simplExprF (zapFloats env) expr cont
            ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
              -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
              -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
              return (wrapFloats env' expr') }

    --------------------------------------------------
    simplExprF :: SimplEnv -> InExpr -> SimplCont
               -> SimplM (SimplEnv, OutExpr)

    simplExprF env e cont
      = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
        simplExprF' env e cont

    simplExprF' :: SimplEnv -> InExpr -> SimplCont
                -> SimplM (SimplEnv, OutExpr)
    simplExprF' env (Var v)        cont = simplVarF env v cont
    simplExprF' env (Lit lit)      cont = rebuild env (Lit lit) cont
    simplExprF' env (Note n expr)  cont = simplNote env n expr cont
    simplExprF' env (Cast body co) cont = simplCast env body co cont
    simplExprF' env (App fun arg)  cont = simplExprF env fun $
                                          ApplyTo NoDup arg env cont

    simplExprF' env expr@(Lam _ _) cont
      = simplLam env zapped_bndrs body cont
            -- The main issue here is under-saturated lambdas
            --   (\x1. \x2. e) arg1
            -- Here x1 might have "occurs-once" occ-info, because occ-info
            -- is computed assuming that a group of lambdas is applied
            -- all at once.  If there are too few args, we must zap the
            -- occ-info, UNLESS the remaining binders are one-shot
      where
        (bndrs, body) = collectBinders expr
        zapped_bndrs | need_to_zap = map zap bndrs
                     | otherwise   = bndrs

        need_to_zap = any zappable_bndr (drop n_args bndrs)
        n_args = countArgs cont
            -- NB: countArgs counts all the args (incl type args)
            -- and likewise drop counts all binders (incl type lambdas)

        zappable_bndr b = isId b && not (isOneShotBndr b)
        zap b | isTyCoVar b = b
              | otherwise   = zapLamIdInfo b

    simplExprF' env (Type ty) cont
      = ASSERT( contIsRhsOrArg cont )
        do  { ty' <- simplCoercion env ty
            ; rebuild env (Type ty') cont }

    simplExprF' env (Case scrut bndr _ alts) cont
      | sm_case_case (getMode env)
      =     -- Simplify the scrutinee with a Select continuation
        simplExprF env scrut (Select NoDup bndr alts env cont)

      | otherwise
      =     -- If case-of-case is off, simply simplify the case expression
            -- in a vanilla Stop context, and rebuild the result around it
        do  { case_expr' <- simplExprC env scrut
                                 (Select NoDup bndr alts env mkBoringStop)
            ; rebuild env case_expr' cont }

    simplExprF' env (Let (Rec pairs) body) cont
      = do  { env' <- simplRecBndrs env (map fst pairs)
                    -- NB: bndrs' don't have unfoldings or rules
                    -- We add them as we go down

            ; env'' <- simplRecBind env' NotTopLevel pairs
            ; simplExprF env'' body cont }

    simplExprF' env (Let (NonRec bndr rhs) body) cont
      = simplNonRecE env bndr (rhs, env) ([], body) cont

    ---------------------------------
    simplType :: SimplEnv -> InType -> SimplM OutType
            -- Kept monadic just so we can do the seqType
    simplType env ty
      = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
        seqType new_ty `seq` return new_ty
      where
        new_ty = substTy env ty

    ---------------------------------
    simplCoercion :: SimplEnv -> InType -> SimplM OutType
    -- The InType isn't *necessarily* a coercion, but it might be
    -- (in a type application, say) and optCoercion is a no-op on types
    simplCoercion env co
      = seqType new_co `seq` return new_co
      where
        new_co = optCoercion (getTvSubst env) co

\end{code} %************************************************************************ %* * \subsection{The main rebuilder} %* * %************************************************************************ \begin{code}

    rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
    -- At this point the substitution in the SimplEnv should be irrelevant
    -- only the in-scope set and floats should matter
    rebuild env expr cont
      = case cont of
          Stop {}                      -> return (env, expr)
          CoerceIt co cont             -> rebuild env (mkCoerce co expr) cont
          Select _ bndr alts se cont   -> rebuildCase (se `setFloats` env) expr bndr alts cont
          StrictArg info _ cont        -> rebuildCall env (info `addArgTo` expr) cont
          StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
                                             ; simplLam env' bs body cont }
          ApplyTo dup_flag arg se cont -- See Note [Avoid redundant simplification]
            | isSimplified dup_flag    -> rebuild env (App expr arg) cont
            | otherwise                -> do { arg' <- simplExpr (se `setInScope` env) arg
                                             ; rebuild env (App expr arg') cont }

\end{code} %************************************************************************ %* * \subsection{Lambdas} %* * %************************************************************************ \begin{code}

    simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
              -> SimplM (SimplEnv, OutExpr)
    simplCast env body co0 cont0
      = do  { co1 <- simplCoercion env co0
            ; simplExprF env body (addCoerce co1 cont0) }
      where
           addCoerce co cont = add_coerce co (coercionKind co) cont

           add_coerce _co (s1, k1) cont     -- co :: ty~ty
             | s1 `coreEqType` k1 = cont    -- is a no-op

           add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
             | (_l1, t1) <- coercionKind co2
    		-- 	e |> (g1 :: S1~L) |> (g2 :: L~T1)
                    -- ==>
                    --      e,                       if S1=T1
                    --      e |> (g1 . g2 :: S1~T1)  otherwise
                    --
                    -- For example, in the initial form of a worker
                    -- we may find  (coerce T (coerce S (\x.e))) y
                    -- and we'd like it to simplify to e[y/x] in one round
                    -- of simplification
             , s1 `coreEqType` t1  = cont            -- The coerces cancel out
             | otherwise           = CoerceIt (mkTransCoercion co1 co2) cont

           add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
                    -- (f |> g) ty  --->   (f ty) |> (g @ ty)
                    -- This implements the PushT and PushC rules from the paper
             | Just (tyvar,_) <- splitForAllTy_maybe s1s2
             = let
                 (new_arg_ty, new_cast)
                   | isCoVar tyvar = (new_arg_co, mkCselRCoercion co)       -- PushC rule
                   | otherwise     = (ty',        mkInstCoercion co ty')    -- PushT rule
               in
               ApplyTo dup (Type new_arg_ty) (zapSubstEnv arg_se) (addCoerce new_cast cont)
             where
               ty' = substTy (arg_se `setInScope` env) arg_ty
    	   new_arg_co = mkCsel1Coercion co  `mkTransCoercion`
                                  ty'           `mkTransCoercion`
                            mkSymCoercion (mkCsel2Coercion co)

           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
                    --      (e |> (g :: s1s2 ~ t1->t2)) f
                    -- ===>
                    --      (e (f |> (arg g :: t1~s1))
    		--	|> (res g :: s2->t2)
                    --
                    -- 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
                    -- with the InExpr in the argument, so we simply substitute
                    -- to make it all consistent.  It's a bit messy.
                    -- But it isn't a common case.
                    --
                    -- Example of use: Trac #995
             = ApplyTo dup new_arg (zapSubstEnv arg_se) (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
               [co1, co2] = decomposeCo 2 co
               new_arg    = mkCoerce (mkSymCoercion co1) arg'
               arg'       = substExpr (text "move-cast") (arg_se `setInScope` env) arg

           add_coerce co _ cont = CoerceIt co cont

\end{code} %************************************************************************ %* * \subsection{Lambdas} %* * %************************************************************************ \begin{code}

    simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
             -> SimplM (SimplEnv, OutExpr)

    simplLam env [] body cont = simplExprF env body cont

            -- Beta reduction
    simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
      = do  { tick (BetaReduction bndr)
            ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }

            -- Not enough args, so there are real lambdas left to put in the result
    simplLam env bndrs body cont
      = do  { (env', bndrs') <- simplLamBndrs env bndrs
            ; body' <- simplExpr env' body
            ; new_lam <- mkLam env' bndrs' body'
            ; rebuild env' new_lam cont }

    ------------------
    simplNonRecE :: SimplEnv
                 -> InBndr                  -- The binder
                 -> (InExpr, SimplEnv)      -- Rhs of binding (or arg of lambda)
                 -> ([InBndr], InExpr)      -- Body of the let/lambda
                                            --      \xs.e
                 -> SimplCont
                 -> SimplM (SimplEnv, OutExpr)

    -- simplNonRecE is used for
    --  * non-top-level non-recursive lets in expressions
    --  * beta reduction
    --
    -- It deals with strict bindings, via the StrictBind continuation,
    -- which may abort the whole process
    --
    -- The "body" of the binding comes as a pair of ([InId],InExpr)
    -- representing a lambda; so we recurse back to simplLam
    -- Why?  Because of the binder-occ-info-zapping done before
    --       the call to simplLam in simplExprF (Lam ...)

    	-- First deal with type applications and type lets
    	--   (/\a. e) (Type ty)   and   (let a = Type ty in e)
    simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
      = ASSERT( isTyCoVar bndr )
        do	{ ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
    	; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }

    simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
      | preInlineUnconditionally env NotTopLevel bndr rhs
      = do  { tick (PreInlineUnconditionally bndr)
            ; -- pprTrace "preInlineUncond" (ppr bndr <+> ppr rhs) $
              simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }

      | isStrictId bndr
      = do  { simplExprF (rhs_se `setFloats` env) rhs
                         (StrictBind bndr bndrs body env cont) }

      | otherwise
      = ASSERT( not (isTyCoVar 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 }

\end{code} %************************************************************************ %* * \subsection{Notes} %* * %************************************************************************ \begin{code}

    -- Hack alert: we only distinguish subsumed cost centre stacks for the
    -- purposes of inlining.  All other CCCSs are mapped to currentCCS.
    simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
              -> SimplM (SimplEnv, OutExpr)
    simplNote env (SCC cc) e cont
      | pushCCisNop cc (getEnclosingCC env)  -- scc "f" (...(scc "f" e)...)
      = simplExprF env e cont	         -- ==>  scc "f" (...e...)
      | otherwise
      = 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 }

\end{code} %************************************************************************ %* * Variables %* * %************************************************************************ \begin{code}

    simplVar :: SimplEnv -> InVar -> SimplM OutExpr
    -- Look up an InVar in the environment
    simplVar env var
      | isTyCoVar var
      = return (Type (substTyVar env var))
      | otherwise
      = case substId env var of
            DoneId var1      -> return (Var var1)
            DoneEx e         -> return e
            ContEx tvs ids e -> simplExpr (setSubstEnv env tvs ids) e

    simplVarF :: SimplEnv -> InId -> SimplCont -> SimplM (SimplEnv, OutExpr)
    simplVarF env var cont
      = case substId env var of
            DoneEx e         -> simplExprF (zapSubstEnv env) e cont
            ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
            DoneId var1      -> completeCall env var1 cont
                    -- Note [zapSubstEnv]
                    -- The template is already simplified, so don't re-substitute.
                    -- This is VITAL.  Consider
                    --      let x = e in
                    --      let y = \z -> ...x... in
                    --      \ x -> ...y...
                    -- We'll clone the inner \x, adding x->x' in the id_subst
                    -- Then when we inline y, we must *not* replace x by x' in
                    -- the inlined copy!!

    ---------------------------------------------------------
    --      Dealing with a call site

    completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
    completeCall env var cont
      = do  {   ------------- Try inlining ----------------
              dflags <- getDOptsSmpl
            ; let  (lone_variable, arg_infos, 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 substitution; rule matching on un-simplified args would
                    -- be bogus

                   n_val_args = length arg_infos
                   interesting_cont = interestingCallContext call_cont
                   unfolding    = activeUnfolding env var
                   maybe_inline = callSiteInline dflags var unfolding
                                                 lone_variable arg_infos interesting_cont
            ; case maybe_inline of {
                Just expr      -- There is an inlining!
                  ->  do { tick (UnfoldingDone var)
                         ; trace_inline dflags expr cont $
                           simplExprF (zapSubstEnv env) expr cont }

                ; Nothing -> do               -- No inlining!

            { rule_base <- getSimplRules
            ; let info = mkArgInfo var (getRules rule_base var) n_val_args call_cont
            ; rebuildCall env info cont
        }}}
      where
        trace_inline dflags unfolding cont stuff
          | not (dopt Opt_D_dump_inlinings dflags) = stuff
          | not (dopt Opt_D_verbose_core2core dflags)
          = if isExternalName (idName var) then
          	  pprTrace "Inlining done:" (ppr var) stuff
            else stuff
          | otherwise
          = pprTrace ("Inlining done: " ++ showSDoc (ppr var))
               (vcat [text "Inlined fn: " <+> nest 2 (ppr unfolding),
                      text "Cont:  " <+> ppr cont])
               stuff

    rebuildCall :: SimplEnv
                -> ArgInfo
                -> SimplCont
                -> SimplM (SimplEnv, OutExpr)
    rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_strs = [] }) cont
      -- When we run out of strictness args, it means
      -- that the call is definitely bottom; see SimplUtils.mkArgInfo
      -- Then we want to discard the entire strict continuation.  E.g.
      --    * case (error "hello") of { ... }
      --    * (error "Hello") arg
      --    * f (error "Hello") where f is strict
      --    etc
      -- Then, especially in the first of these cases, we'd like to discard
      -- the continuation, leaving just the bottoming expression.  But the
      -- type might not be right, so we may have to add a coerce.
      | not (contIsTrivial cont)     -- Only do this if there is a non-trivial
      = return (env, mk_coerce res)  -- contination to discard, else we do it
      where                          -- again and again!
        res     = mkApps (Var fun) (reverse rev_args)
        res_ty  = exprType res
        cont_ty = contResultType env res_ty cont
        co      = mkUnsafeCoercion res_ty cont_ty
        mk_coerce expr | cont_ty `coreEqType` res_ty = expr
                       | otherwise = mkCoerce co expr

    rebuildCall env info (ApplyTo _ (Type arg_ty) se cont)
      = do  { ty' <- simplCoercion (se `setInScope` env) arg_ty
            ; rebuildCall env (info `addArgTo` Type ty') cont }

    rebuildCall env info@(ArgInfo { ai_encl = encl_rules
                                  , ai_strs = str:strs, ai_discs = disc:discs })
                (ApplyTo dup_flag arg arg_se cont)
      | isSimplified dup_flag     -- See Note [Avoid redundant simplification]
      = rebuildCall env (addArgTo info' arg) cont

      | str 	        -- Strict argument
      = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
        simplExprF (arg_se `setFloats` env) arg
                   (StrictArg info' cci cont)
                    -- Note [Shadowing]

      | otherwise                           -- Lazy argument
            -- DO NOT float anything outside, hence simplExprC
            -- There is no benefit (unlike in a let-binding), and we'd
            -- have to be very careful about bogus strictness through
            -- floating a demanded let.
      = do  { arg' <- simplExprC (arg_se `setInScope` env) arg
                                 (mkLazyArgStop cci)
            ; rebuildCall env (addArgTo info' arg') cont }
      where
        info' = info { ai_strs = strs, ai_discs = discs }
        cci | encl_rules || disc > 0 = ArgCtxt encl_rules  -- Be keener here
            | otherwise              = BoringCtxt          -- Nothing interesting

    rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_rules = rules }) cont
      = do {  -- We've accumulated a simplified call in <fun,rev_args>
              -- so try rewrite rules; see Note [RULEs apply to simplified arguments]
    	  -- See also Note [Rules for recursive functions]
    	; let args = reverse rev_args
                  env' = zapSubstEnv env
    	; mb_rule <- tryRules env rules fun args cont
    	; case mb_rule of {
    	     Just (n_args, rule_rhs) -> simplExprF env' rule_rhs $
                                            pushSimplifiedArgs env' (drop n_args args) cont ;
                     -- n_args says how many args the rule consumed
               ; Nothing -> rebuild env (mkApps (Var fun) args) cont      -- No rules
        } }

\end{code} Note [RULES apply to simplified arguments] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's very desirable to try RULES once the arguments have been simplified, because doing so ensures that rule cascades work in one pass. Consider {-# RULES g (h x) = k x f (k x) = x #-} ...f (g (h x))... Then we want to rewrite (g (h x)) to (k x) and only then try f's rules. If we match f's rules against the un-simplified RHS, it won't match. This makes a particularly big difference when superclass selectors are involved: op ($p1 ($p2 (df d))) We want all this to unravel in one sweeep. Note [Avoid redundant simplification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Because RULES apply to simplified arguments, there's a danger of repeatedly simplifying already-simplified arguments. An important example is that of (>>=) d e1 e2 Here e1, e2 are simplified before the rule is applied, but don't really participate in the rule firing. So we mark them as Simplified to avoid re-simplifying them. Note [Shadowing] ~~~~~~~~~~~~~~~~ This part of the simplifier may break the no-shadowing invariant Consider f (...(\a -> e)...) (case y of (a,b) -> e') where f is strict in its second arg If we simplify the innermost one first we get (...(\a -> e)...) Simplifying the second arg makes us float the case out, so we end up with case y of (a,b) -> f (...(\a -> e)...) e' So the output does not have the no-shadowing invariant. However, there is no danger of getting name-capture, because when the first arg was simplified we used an in-scope set that at least mentioned all the variables free in its static environment, and that is enough. We can't just do innermost first, or we'd end up with a dual problem: case x of (a,b) -> f e (...(\a -> e')...) I spent hours trying to recover the no-shadowing invariant, but I just could not think of an elegant way to do it. The simplifier is already knee-deep in continuations. We have to keep the right in-scope set around; AND we have 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! %************************************************************************ %* * Rewrite rules %* * %************************************************************************ \begin{code}

    tryRules :: SimplEnv -> [CoreRule]
             -> Id -> [OutExpr] -> SimplCont
    	 -> SimplM (Maybe (Arity, CoreExpr))	     -- The arity is the number of
    	    	   	  	  		     -- args consumed by the rule
    tryRules env rules fn args call_cont
      | null rules
      = return Nothing
      | otherwise
      = do { dflags <- getDOptsSmpl
           ; case activeRule dflags env of {
               Nothing     -> return Nothing  ; -- No rules apply
               Just act_fn ->
             case lookupRule act_fn (getUnfoldingInRuleMatch env) (getInScope env) fn args rules of {
               Nothing               -> return Nothing ;   -- No rule matches
               Just (rule, rule_rhs) ->

                 do { tick (RuleFired (ru_name rule))
                    ; trace_dump dflags rule rule_rhs $
                      return (Just (ruleArity rule, rule_rhs)) }}}}
      where
        trace_dump dflags rule rule_rhs stuff
          | not (dopt Opt_D_dump_rule_firings dflags) = stuff
          | not (dopt Opt_D_verbose_core2core dflags)

          = pprTrace "Rule fired:" (ftext (ru_name rule)) stuff
          | otherwise
          = 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])
               stuff

\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 %* * %************************************************************************ 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. Note [CaseElimination: lifted case] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We do not use exprOkForSpeculation in the lifted case. Consider case (case a ># b of { True -> (p,q); False -> (q,p) }) of r -> blah The scrutinee is ok-for-speculation (it looks inside cases), but we do not want to transform to let r = case a ># b of { True -> (p,q); False -> (q,p) } in blah because that builds an unnecessary thunk. 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}

    ---------------------------------------------------------
    --      Eliminate the case if possible

    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
    --------------------------------------------------

    rebuildCase env scrut case_bndr alts cont
      | Lit lit <- scrut    -- No need for same treatment as constructors
                            -- because literals are inlined more vigorously
      = do  { tick (KnownBranch case_bndr)
            ; case findAlt (LitAlt lit) alts of
    	    Nothing           -> missingAlt env case_bndr alts cont
    	    Just (_, bs, rhs) -> simple_rhs bs rhs }

      | Just (con, ty_args, other_args) <- exprIsConApp_maybe (getUnfoldingInRuleMatch env) scrut
            -- Works when the scrutinee is a variable with a known unfolding
            -- as well as when it's an explicit constructor application
      = do  { tick (KnownBranch case_bndr)
            ; case findAlt (DataAlt con) alts of
    	    Nothing  -> missingAlt env case_bndr alts cont
                Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs
    	    Just (_, bs, rhs)       -> knownCon env scrut con ty_args other_args
                                                    case_bndr bs rhs cont
    	}
      where
        simple_rhs bs rhs = ASSERT( null bs )
                            do { env' <- simplNonRecX env case_bndr scrut
        	                   ; simplExprF env' rhs cont }

    --------------------------------------------------
    --      2. Eliminate the case if scrutinee is evaluated
    --------------------------------------------------

    rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
      -- See if we can get rid of the case altogether
      -- See Note [Case elimination]
      -- mkCase made sure that if all the alternatives are equal,
      -- then there is now only one (DEFAULT) rhs
     | all isDeadBinder bndrs       -- bndrs are [InId]

            -- Check that the scrutinee can be let-bound instead of case-bound
     , if isUnLiftedType (idType case_bndr)
       then exprOkForSpeculation scrut
            -- Satisfy the let-binding invariant
            -- This includes things like (==# a# b#)::Bool
            -- so that we simplify
            --      case ==# a# b# of { True -> x; False -> x }
            -- to just
            --      x
            -- This particular example shows up in default methods for
            -- comparision operations (e.g. in (>=) for Int.Int32)

       else exprIsHNF scrut || var_demanded_later scrut
            -- It's already evaluated, or will be demanded later
            -- See Note [Case elimination: lifted case]
      = do  { tick (CaseElim case_bndr)
            ; env' <- simplNonRecX env case_bndr scrut
              -- If case_bndr is deads, simplNonRecX will discard
            ; simplExprF env' rhs cont }
      where
            -- The case binder is going to be evaluated later,
            -- and the scrutinee is a simple variable
        var_demanded_later (Var v) = isStrictDmd (idDemandInfo case_bndr)
                                     && not (isTickBoxOp v)
                                        -- ugly hack; covering this case is what
                                        -- exprOkForSpeculation was intended for.
        var_demanded_later _       = False

    --------------------------------------------------
    --      3. Try seq rules; see Note [User-defined RULES for seq] in MkId
    --------------------------------------------------

    rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
      | all isDeadBinder (case_bndr : bndrs)  -- So this is just 'seq'
      = do { let rhs' = substExpr (text "rebuild-case") 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

           ; rule_base <- getSimplRules
           ; mb_rule <- tryRules env (getRules rule_base seqId) 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
    --------------------------------------------------

    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

            -- Simplify the alternatives
            ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont

    	-- Check for empty alternatives
    	; if null alts' then missingAlt env case_bndr alts cont
    	  else do
            { dflags <- getDOptsSmpl
            ; case_expr <- mkCase dflags scrut' case_bndr' alts'

    	-- Notice that rebuild gets the in-scope set from env', not alt_env
    	-- (which in any case is only build in simplAlts)
    	-- The case binder *not* scope over the whole returned case-expression
    	; rebuild env' case_expr nodup_cont } }

\end{code} simplCaseBinder checks whether the scrutinee is a variable, v. If so, 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. 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] ~~~~~~~~~~~~~~~~~ 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. The point is that we bring into the envt a binding let x = (a,b) 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 [Improving seq] ~~~~~~~~~~~~~~~~~~~ Consider type family F :: * -> * type instance F Int = Int ... case e of x { DEFAULT -> rhs } ... where x::F Int. Then we'd like to rewrite (F Int) to Int, getting case e `cast` co of x'::Int I# x# -> let x = x' `cast` sym co in rhs so that 'rhs' can take advantage of the form of x'. Notice that Note [Case of cast] (in OccurAnal) may then apply to the result. Nota Bene: We only do the [Improving seq] transformation if the case binder 'x' is actually used in the rhs; that is, if the case is *not* a *pure* seq. a) There is no point in adding the cast to a pure seq. b) There is a good reason not to: doing so would interfere with seq rules (Note [Built-in RULES for seq] in MkId). In particular, this [Improving seq] thing *adds* a cast while [Built-in RULES for seq] *removes* one, so they just flip-flop. You might worry about case v of x { __DEFAULT -> ... case (v `cast` co) of y { I# -> ... }} This is a pure seq (since x is unused), so [Improving seq] won't happen. But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get case v of x { __DEFAULT -> ... case (x `cast` co) of y { I# -> ... }} Now the outer case is not a pure seq, so [Improving seq] will happen, and then the inner case will disappear. The need for [Improving seq] showed up in Roman's experiments. Example: foo :: F Int -> Int -> Int foo t n = t `seq` bar n where bar 0 = 0 bar n = bar (n - case t of TI i -> i) Here we'd like to avoid repeated evaluating t inside the loop, by taking advantage of the `seq`. 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.) \begin{code}

    simplAlts :: SimplEnv
              -> OutExpr
              -> InId                       -- Case binder
              -> [InAlt]			-- Non-empty
    	  -> SimplCont
              -> SimplM (OutExpr, OutId, [OutAlt])  -- Includes the continuation
    -- Like simplExpr, this just returns the simplified alternatives;
    -- it does not return an environment

    simplAlts env scrut case_bndr alts cont'
      = -- pprTrace "simplAlts" (ppr alts $$ ppr (seTvSubst env)) $
        do  { let env0 = zapFloats env

            ; (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 scrut' case_bndr' alts

    	; let mb_var_scrut = case scrut' of { Var v -> Just v; _ -> Nothing }
            ; alts' <- mapM (simplAlt alt_env' mb_var_scrut
                                 imposs_deflt_cons case_bndr' cont') in_alts
            ; return (scrut', case_bndr', alts') }

    ------------------------------------
    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,_,_)]
      | not (isDeadBinder case_bndr)	-- Not a pure seq!  See the Note!
      , 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)

    ------------------------------------
    simplAlt :: SimplEnv
    	 -> Maybe OutId    -- Scrutinee
             -> [AltCon]       -- These constructors can't be present when
                               -- matching the DEFAULT alternative
             -> OutId          -- The case binder
             -> SimplCont
             -> InAlt
             -> SimplM OutAlt

    simplAlt env scrut imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
      = ASSERT( null bndrs )
        do  { let env' = addBinderUnfolding env scrut case_bndr'
                                            (mkOtherCon imposs_deflt_cons)
                    -- Record the constructors that the case-binder *can't* be.
            ; rhs' <- simplExprC env' rhs cont'
            ; return (DEFAULT, [], rhs') }

    simplAlt env scrut _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
      = ASSERT( null bndrs )
        do  { let env' = addBinderUnfolding env scrut case_bndr'
                                            (mkSimpleUnfolding (Lit lit))
            ; rhs' <- simplExprC env' rhs cont'
            ; return (LitAlt lit, [], rhs') }

    simplAlt env scrut _ case_bndr' cont' (DataAlt con, vs, rhs)
      = do  {       -- Deal with the pattern-bound variables
                    -- Mark the ones that are in ! positions in the
                    -- data constructor as certainly-evaluated.
                    -- NB: simplLamBinders preserves this eval info
              let vs_with_evals = add_evals (dataConRepStrictness con)
            ; (env', vs') <- simplLamBndrs env vs_with_evals

                    -- Bind the case-binder to (con args)
            ; let inst_tys' = tyConAppArgs (idType case_bndr')
                  con_args  = map Type inst_tys' ++ varsToCoreExprs vs'
                  unf       = mkSimpleUnfolding (mkConApp con con_args)
                  env''     = addBinderUnfolding env' scrut case_bndr' unf

            ; rhs' <- simplExprC env'' rhs cont'
            ; return (DataAlt con, vs', rhs') }
      where
            -- add_evals records the evaluated-ness of the bound variables of
            -- a case pattern.  This is *important*.  Consider
            --      data T = T !Int !Int
            --
            --      case x of { T a b -> T (a+1) b }
            --
            -- We really must record that b is already evaluated so that we don't
            -- go and re-evaluate it when constructing the result.
            -- See Note [Data-con worker strictness] in MkId.lhs
        add_evals the_strs
            = go vs the_strs
            where
              go [] [] = []
              go (v:vs') strs | isTyCoVar v = v : go vs' strs
              go (v:vs') (str:strs)
                | isMarkedStrict str = evald_v  : go vs' strs
                | otherwise          = zapped_v : go vs' strs
                where
                  zapped_v = zapBndrOccInfo keep_occ_info v
                  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
            -- Note [Aug06] I can't see why this actually matters, but it's neater
            --        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.
        keep_occ_info = isDeadBinder case_bndr' && isNothing scrut

    addBinderUnfolding :: SimplEnv -> Maybe OutId -> Id -> Unfolding -> SimplEnv
    addBinderUnfolding env scrut bndr unf
      = case scrut of
           Just v -> modifyInScope env1 (v `setIdUnfolding` unf)
           _      -> env1
      where
        env1 = modifyInScope env bndr_w_unf
        bndr_w_unf = bndr `setIdUnfolding` unf

    zapBndrOccInfo :: Bool -> 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
    zapBndrOccInfo keep_occ_info pat_id
      | keep_occ_info = pat_id
      | otherwise     = zapIdOccInfo pat_id

\end{code} Note [Add unfolding for scrutinee] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general it's unlikely that a variable scrutinee will appear in the case alternatives case x of { ...x unlikely to appear... } because the binder-swap in OccAnal has got rid of all such occcurrences See Note [Binder swap] in OccAnal. BUT it is still VERY IMPORTANT to add a suitable unfolding for a variable scrutinee, in simplAlt. Here's why case x of y (a,b) -> case b of c I# v -> ...(f y)... There is no occurrence of 'b' in the (...(f y)...). But y gets the unfolding (a,b), and *that* mentions b. If f has a RULE RULE f (p, I# q) = ... we want that rule to match, so we must extend the in-scope env with a suitable unfolding for 'y'. It's *essential* for rule matching; but it's also good for case-elimintation -- suppose that 'f' was inlined and did multi-level case analysis, then we'd solve it in one simplifier sweep instead of two. Exactly the same issue arises in SpecConstr; see Note [Add scrutinee to ValueEnv too] in SpecConstr %************************************************************************ %* * \subsection{Known constructor} %* * %************************************************************************ We are a bit careful with occurrence info. Here's an example (\x* -> case x of (a*, b) -> f a) (h v, e) where the * means "occurs once". This effectively becomes case (h v, e) of (a*, b) -> f a) and then let a* = h v; b = e in f a and then f (h v) All this should happen in one sweep. \begin{code}

    knownCon :: SimplEnv
             -> OutExpr				-- The scrutinee
             -> DataCon -> [OutType] -> [OutExpr]	-- The scrutinee (in pieces)
             -> InId -> [InBndr] -> InExpr		-- The alternative
             -> SimplCont
             -> SimplM (SimplEnv, OutExpr)

    knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
      = do  { env'  <- bind_args env bs dc_args
            ; env'' <- bind_case_bndr env'
            ; simplExprF env'' rhs cont }
      where
        zap_occ = zapBndrOccInfo (isDeadBinder bndr)    -- bndr is an InId

                      -- Ugh!
        bind_args env' [] _  = return env'

        bind_args env' (b:bs') (Type ty : args)
          = ASSERT( isTyCoVar b )
            bind_args (extendTvSubst env' b ty) bs' args

        bind_args env' (b:bs') (arg : args)
          = ASSERT( isId 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
               ; bind_args env'' bs' args }

        bind_args _ _ _ =
          pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$
                                 text "scrut:" <+> ppr scrut

           -- It's useful to bind bndr to scrut, rather than to a fresh
           -- binding      x = Con arg1 .. argn
           -- because very often the scrut is a variable, so we avoid
           -- creating, and then subsequently eliminating, a let-binding
           -- BUT, if scrut is a not a variable, we must be careful
           -- about duplicating the arg redexes; in that case, make
           -- a new con-app from the args
        bind_case_bndr env
          | isDeadBinder bndr   = return env
          | exprIsTrivial scrut = return (extendIdSubst env bndr (DoneEx scrut))
          | otherwise           = do { dc_args <- mapM (simplVar env) bs
          			      	         -- dc_ty_args are aready OutTypes,
    					 -- but bs are InBndrs
    		    	         ; let con_app = Var (dataConWorkId dc)
                              			 `mkTyApps` dc_ty_args
                              			 `mkApps`   dc_args
          			         ; simplNonRecX env bndr con_app }

    -------------------
    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} %************************************************************************ %* * \subsection{Duplicating continuations} %* * %************************************************************************ \begin{code}

    prepareCaseCont :: SimplEnv
                    -> [InAlt] -> SimplCont
                    -> SimplM (SimplEnv, SimplCont,SimplCont)
                            -- Return a duplicatable continuation, a non-duplicable part
                            -- plus some extra bindings (that scope over the entire
                            -- continunation)

            -- No need to make it duplicatable if there's only one alternative
    prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
    prepareCaseCont env _   cont = mkDupableCont env cont

\end{code} \begin{code}

    mkDupableCont :: SimplEnv -> SimplCont
                  -> SimplM (SimplEnv, SimplCont, SimplCont)

    mkDupableCont env cont
      | contIsDupable cont
      = return (env, cont, mkBoringStop)

    mkDupableCont _   (Stop {}) = panic "mkDupableCont"     -- Handled by previous eqn

    mkDupableCont env (CoerceIt ty cont)
      = do  { (env', dup, nodup) <- mkDupableCont env cont
            ; return (env', CoerceIt ty dup, nodup) }

    mkDupableCont env cont@(StrictBind {})
      =  return (env, mkBoringStop, cont)
            -- See Note [Duplicating StrictBind]

    mkDupableCont env (StrictArg info cci cont)
            -- See Note [Duplicating StrictArg]
      = do { (env', dup, nodup) <- mkDupableCont env cont
           ; (env'', args')     <- mapAccumLM (makeTrivial NotTopLevel) env' (ai_args info)
           ; return (env'', StrictArg (info { ai_args = args' }) cci dup, nodup) }

    mkDupableCont env (ApplyTo _ arg se cont)
      =     -- e.g.         [...hole...] (...arg...)
            --      ==>
            --              let a = ...arg...
            --              in [...hole...] a
        do  { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
            ; arg' <- simplExpr (se `setInScope` env') arg
            ; (env'', arg'') <- makeTrivial NotTopLevel env' arg'
            ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
            ; return (env'', app_cont, nodup_cont) }

    mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
    --  See Note [Single-alternative case]
    --  | not (exprIsDupable rhs && contIsDupable case_cont)
    --  | not (isDeadBinder case_bndr)
      | all isDeadBinder bs  -- InIds
        && not (isUnLiftedType (idType case_bndr))
        -- Note [Single-alternative-unlifted]
      = return (env, mkBoringStop, cont)

    mkDupableCont env (Select _ case_bndr alts se cont)
      =     -- e.g.         (case [...hole...] of { pi -> ei })
            --      ===>
            --              let ji = \xij -> ei
            --              in case [...hole...] of { pi -> ji xij }
        do  { tick (CaseOfCase case_bndr)
            ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
                    -- NB: call mkDupableCont here, *not* prepareCaseCont
                    -- We must make a duplicable continuation, whereas prepareCaseCont
                    -- doesn't when there is a single case branch

            ; let alt_env = se `setInScope` env'
            ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
            ; alts' <- mapM (simplAlt alt_env' Nothing [] case_bndr' dup_cont) alts
            -- Safe to say that there are no handled-cons for the DEFAULT case
                    -- NB: simplBinder does not zap deadness occ-info, so
                    -- a dead case_bndr' will still advertise its deadness
                    -- This is really important because in
                    --      case e of b { (# p,q #) -> ... }
                    -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
                    -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
                    -- In the new alts we build, we have the new case binder, so it must retain
                    -- its deadness.
            -- NB: we don't use alt_env further; it has the substEnv for
            --     the alternatives, and we don't want that

            ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
            ; return (env'',  -- Note [Duplicated env]
                      Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
                      nodup_cont) }

    mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
                  -> SimplM (SimplEnv, [InAlt])
    -- Absorbs the continuation into the new alternatives

    mkDupableAlts env case_bndr' the_alts
      = go env the_alts
      where
        go env0 [] = return (env0, [])
        go env0 (alt:alts)
            = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
                 ; (env2, alts') <- go env1 alts
                 ; return (env2, alt' : alts' ) }

    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'))
      | otherwise
      = do  { let rhs_ty'  = exprType rhs'
        	      scrut_ty = idType case_bndr
        	      case_bndr_w_unf
                    = case con of
    		      DEFAULT    -> case_bndr
    	      	      DataAlt dc -> setIdUnfolding case_bndr unf
    		      	  where
    			     	 -- See Note [Case binders and join points]
    		      	     unf = mkInlineUnfolding Nothing rhs
    		      	     rhs = mkConApp dc (map Type (tyConAppArgs scrut_ty)
    			     	   	        ++ varsToCoreExprs bndrs')

    		      LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt")
    		      	     	   	        <+> ppr case_bndr <+> ppr con )
    			           case_bndr
    		      	   -- The case binder is alive but trivial, so why has
    		      	   -- it not been substituted away?

                  used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs'
    			  | otherwise		   = bndrs' ++ [case_bndr_w_unf]

                  abstract_over bndr
                      | isTyCoVar 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')
                        else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
                                ; return ([rw_id], [Var realWorldPrimId]) }

            ; 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
                    really_final_bndrs     = map one_shot final_bndrs'
                    one_shot v | isId v    = setOneShotLambda v
                               | otherwise = v
                    join_rhs  = mkLams really_final_bndrs rhs'
                    join_call = mkApps (Var join_bndr) final_args

    	; env' <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
            ; return (env', (con, bndrs', join_call)) }
                    -- See Note [Duplicated env]

\end{code} Note [Case binders and join points] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this case (case .. ) of c { I# c# -> ....c.... If we make a join point with c but not c# we get $j = \c -> ....c.... But if later inlining scrutines the c, thus $j = \c -> ... case c of { I# y -> ... } ... we won't see that 'c' has already been scrutinised. This actually happens in the 'tabulate' function in wave4main, and makes a significant difference to allocation. An alternative plan is this: $j = \c# -> let c = I# c# in ...c.... but that is bad if 'c' is *not* later scrutinised. So instead we do both: we pass 'c' and 'c#' , and record in c's inlining (an InlineRule) that it's really I# c#, thus $j = \c# -> \c[=I# c#] -> ...c.... Absence analysis may later discard 'c'. NB: take great care when doing strictness analysis; see Note [Lamba-bound unfoldings] in DmdAnal. Also note that we can still end up passing stuff that isn't used. Before strictness analysis we have let $j x y c{=(x,y)} = (h c, ...) in ... After strictness analysis we see that h is strict, we end up with let $j x y c{=(x,y)} = ($wh x y, ...) and c is unused. Note [Duplicated env] ~~~~~~~~~~~~~~~~~~~~~ Some of the alternatives are simplified, but have not been turned into a join point So they *must* have an zapped subst-env. So we can't use completeNonRecX to bind the join point, because it might to do PostInlineUnconditionally, and 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. 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 inline back in place. Quite often the rhs is just a variable or constructor. The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra iterations because the version with the let bindings looked big, and so wasn't inlined, but after the join points had been inlined it looked smaller, and so was inlined. NB: we have to check the size of rhs', not rhs. Duplicating a small InAlt might invalidate occurrence information However, if it *is* dupable, we return the *un* simplified alternative, because otherwise we'd need to pair it up with an empty subst-env.... but we only have one env shared between all the alts. (Remember we must zap the subst-env before re-simplifying something). Rather than do this we simply agree to re-simplify the original (small) thing later. Note [Funky mkPiTypes] ~~~~~~~~~~~~~~~~~~~~~~ Notice the funky mkPiTypes. If the contructor has existentials it's possible that the join point will be abstracted over type varaibles as well as term variables. Example: Suppose we have data T = forall t. C [t] Then faced with case (case e of ...) of C t xs::[t] -> rhs We get the join point let j :: forall t. [t] -> ... j = /\t \xs::[t] -> rhs in case (case e of ...) of C t xs::[t] -> j t xs Note [Join point abstaction] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we try to lift a primitive-typed something out for let-binding-purposes, we will *caseify* it (!), with potentially-disastrous strictness results. So instead we turn it into a function: \v -> e where v::State# RealWorld#. The value passed to this function is realworld#, which generates (almost) no code. There's a slight infelicity here: we pass the overall case_bndr to all the join points if it's used in *any* RHS, because we don't know its usage in each RHS separately We used to say "&& isUnLiftedType rhs_ty'" here, but now we make the join point into a function whenever used_bndrs' is empty. This makes the join-point more CPR friendly. Consider: let j = if .. then I# 3 else I# 4 in case .. of { A -> j; B -> j; C -> ... } Now CPR doesn't w/w j because it's a thunk, so that means that the enclosing function can't w/w either, which is a lose. Here's the example that happened in practice: kgmod :: Int -> Int -> Int kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0 then 78 else 5 I have seen a case alternative like this: True -> \v -> ... It's a bit silly to add the realWorld dummy arg in this case, making $j = \s v -> ... True -> $j s (the \v alone is enough to make CPR happy) but I think it's rare 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 let $j = \v -> && E v in simplExpr (case x of { T -> F; F -> T }) (ArgOf (\r -> $j r) And after simplifying more we get let $j = \v -> && E v 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. Note [Single-alternative cases] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This case is just like the ArgOf case. Here's an example: data T a = MkT !a ...(MkT (abs x))... Then we get case (case x of I# x' -> case x' <# 0# of True -> I# (negate# x') False -> I# x') of y { DEFAULT -> MkT y Because the (case x) has only one alternative, we'll transform to case x of I# x' -> case (case x' <# 0# of True -> I# (negate# x') False -> I# x') of y { DEFAULT -> MkT y But now we do *NOT* want to make a join point etc, giving case x of I# x' -> let $j = \y -> MkT y in case x' <# 0# of True -> $j (I# (negate# x')) False -> $j (I# x') In this case the $j will inline again, but suppose there was a big strict computation enclosing the orginal call to MkT. Then, it won't "see" the MkT any more, because it's big and won't get duplicated. And, what is worse, nothing was gained by the case-of-case transform. So, in circumstances like these, we don't want to build join points and push the outer case into the branches of the inner one. Instead, don't duplicate the continuation. When should we use this strategy? We should not use it on *every* single-alternative case: e.g. case (case ....) of (a,b) -> (# a,b #) Here we must push the outer case into the inner one! Other choices: * Match [(DEFAULT,_,_)], but in the common case of Int, the alternative-filling-in code turned the outer case into case (...) of y { I# _ -> MkT y } * Match on single alternative plus (not (isDeadBinder case_bndr)) Rationale: pushing the case inwards won't eliminate the construction. But there's a risk of case (...) of y { (a,b) -> let z=(a,b) in ... } Now y looks dead, but it'll come alive again. Still, this seems like the best option at the moment. * Match on single alternative plus (all (isDeadBinder bndrs)) Rationale: this is essentially seq. * Match when the rhs is *not* duplicable, and hence would lead to a join point. This catches the disaster-case above. We can test the *un-simplified* rhs, which is fine. It might get bigger or smaller after simplification; if it gets smaller, this case might fire next time round. NB also that we must test contIsDupable case_cont *too, because case_cont might be big! HOWEVER: I found that this version doesn't work well, because we can get let x = case (...) of { small } in ...case x... When x is inlined into its full context, we find that it was a bad idea to have pushed the outer case inside the (...) case. Note [Single-alternative-unlifted] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here's another single-alternative where we really want to do case-of-case: data Mk1 = Mk1 Int# data Mk1 = Mk2 Int# M1.f = \r [x_s74 y_s6X] case case y_s6X of tpl_s7m { M1.Mk1 ipv_s70 -> ipv_s70; M1.Mk2 ipv_s72 -> ipv_s72; } of wild_s7c { __DEFAULT -> case case x_s74 of tpl_s7n { M1.Mk1 ipv_s77 -> ipv_s77; M1.Mk2 ipv_s79 -> ipv_s79; } of wild1_s7b { __DEFAULT -> ==# [wild1_s7b wild_s7c]; }; }; So the outer case is doing *nothing at all*, other than serving as a join-point. In this case we really want to do case-of-case and decide whether to use a real join point or just duplicate the continuation. Hence: check whether the case binder's type is unlifted, because then the outer case is *not* a seq.