% % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[SpecConstr]{Specialise over constructors} \begin{code}

    -- The above warning supression flag is a temporary kludge.
    -- While working on this module you are encouraged to remove it and fix
    -- any warnings in the module. See
    --     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
    -- for details

    module SpecConstr(
    	specConstrProgram
    #ifdef GHCI
            , SpecConstrAnnotation(..)
    #endif
        ) where

    #include "HsVersions.h"

    import CoreSyn
    import CoreSubst
    import CoreUtils
    import CoreUnfold	( couldBeSmallEnoughToInline )
    import CoreFVs 		( exprsFreeVars )
    import CoreMonad
    import HscTypes         ( ModGuts(..) )
    import WwLib		( mkWorkerArgs )
    import DataCon
    import Coercion
    import Rules
    import Type		hiding( substTy )
    import Id
    import MkCore		( mkImpossibleExpr )
    import Var
    import VarEnv
    import VarSet
    import Name
    import BasicTypes
    import DynFlags		( DynFlags(..) )
    import StaticFlags	( opt_PprStyle_Debug )
    import Maybes		( orElse, catMaybes, isJust, isNothing )
    import Demand
    import DmdAnal		( both )
    import Serialized       ( deserializeWithData )
    import Util
    import UniqSupply
    import Outputable
    import FastString
    import UniqFM
    import MonadUtils
    import Control.Monad	( zipWithM )
    import Data.List

    -- See Note [SpecConstrAnnotation]
    #ifndef GHCI
    type SpecConstrAnnotation = ()
    #else
    import Literal          ( literalType )
    import TyCon            ( TyCon )
    import GHC.Exts( SpecConstrAnnotation(..) )
    #endif

\end{code} ----------------------------------------------------- Game plan ----------------------------------------------------- Consider drop n [] = [] drop 0 xs = [] drop n (x:xs) = drop (n-1) xs After the first time round, we could pass n unboxed. This happens in numerical code too. Here's what it looks like in Core: drop n xs = case xs of [] -> [] (y:ys) -> case n of I# n# -> case n# of 0 -> [] _ -> drop (I# (n# -# 1#)) xs Notice that the recursive call has an explicit constructor as argument. Noticing this, we can make a specialised version of drop RULE: drop (I# n#) xs ==> drop' n# xs drop' n# xs = let n = I# n# in ...orig RHS... Now the simplifier will apply the specialisation in the rhs of drop', giving drop' n# xs = case xs of [] -> [] (y:ys) -> case n# of 0 -> [] _ -> drop (n# -# 1#) xs Much better! We'd also like to catch cases where a parameter is carried along unchanged, but evaluated each time round the loop: f i n = if i>0 || i>n then i else f (i*2) n Here f isn't strict in n, but we'd like to avoid evaluating it each iteration. In Core, by the time we've w/wd (f is strict in i) we get f i# n = case i# ># 0 of False -> I# i# True -> case n of n' { I# n# -> case i# ># n# of False -> I# i# True -> f (i# *# 2#) n' At the call to f, we see that the argument, n is know to be (I# n#), and n is evaluated elsewhere in the body of f, so we can play the same trick as above. Note [Reboxing] ~~~~~~~~~~~~~~~ We must be careful not to allocate the same constructor twice. Consider f p = (...(case p of (a,b) -> e)...p..., ...let t = (r,s) in ...t...(f t)...) At the recursive call to f, we can see that t is a pair. But we do NOT want to make a specialised copy: f' a b = let p = (a,b) in (..., ...) because now t is allocated by the caller, then r and s are passed to the recursive call, which allocates the (r,s) pair again. This happens if (a) the argument p is used in other than a case-scrutinsation way. (b) the argument to the call is not a 'fresh' tuple; you have to look into its unfolding to see that it's a tuple Hence the "OR" part of Note [Good arguments] below. ALTERNATIVE 2: pass both boxed and unboxed versions. This no longer saves allocation, but does perhaps save evals. In the RULE we'd have something like f (I# x#) = f' (I# x#) x# If at the call site the (I# x) was an unfolding, then we'd have to rely on CSE to eliminate the duplicate allocation.... This alternative doesn't look attractive enough to pursue. ALTERNATIVE 3: ignore the reboxing problem. The trouble is that the conservative reboxing story prevents many useful functions from being specialised. Example: foo :: Maybe Int -> Int -> Int foo (Just m) 0 = 0 foo x@(Just m) n = foo x (n-m) Here the use of 'x' will clearly not require boxing in the specialised function. The strictness analyser has the same problem, in fact. Example: f p@(a,b) = ... If we pass just 'a' and 'b' to the worker, it might need to rebox the pair to create (a,b). A more sophisticated analysis might figure out precisely the cases in which this could happen, but the strictness analyser does no such analysis; it just passes 'a' and 'b', and hopes for the best. So my current choice is to make SpecConstr similarly aggressive, and ignore the bad potential of reboxing. Note [Good arguments] ~~~~~~~~~~~~~~~~~~~~~ So we look for * A self-recursive function. Ignore mutual recursion for now, because it's less common, and the code is simpler for self-recursion. * EITHER a) At a recursive call, one or more parameters is an explicit constructor application AND That same parameter is scrutinised by a case somewhere in the RHS of the function OR b) At a recursive call, one or more parameters has an unfolding that is an explicit constructor application AND That same parameter is scrutinised by a case somewhere in the RHS of the function AND Those are the only uses of the parameter (see Note [Reboxing]) What to abstract over ~~~~~~~~~~~~~~~~~~~~~ There's a bit of a complication with type arguments. If the call site looks like f p = ...f ((:) [a] x xs)... then our specialised function look like f_spec x xs = let p = (:) [a] x xs in ....as before.... This only makes sense if either a) the type variable 'a' is in scope at the top of f, or b) the type variable 'a' is an argument to f (and hence fs) Actually, (a) may hold for value arguments too, in which case we may not want to pass them. Supose 'x' is in scope at f's defn, but xs is not. Then we'd like f_spec xs = let p = (:) [a] x xs in ....as before.... Similarly (b) may hold too. If x is already an argument at the call, no need to pass it again. Finally, if 'a' is not in scope at the call site, we could abstract it as we do the term variables: f_spec a x xs = let p = (:) [a] x xs in ...as before... So the grand plan is: * abstract the call site to a constructor-only pattern e.g. C x (D (f p) (g q)) ==> C s1 (D s2 s3) * Find the free variables of the abstracted pattern * Pass these variables, less any that are in scope at the fn defn. But see Note [Shadowing] below. NOTICE that we only abstract over variables that are not in scope, so we're in no danger of shadowing variables used in "higher up" in f_spec's RHS. Note [Shadowing] ~~~~~~~~~~~~~~~~ In this pass we gather up usage information that may mention variables that are bound between the usage site and the definition site; or (more seriously) may be bound to something different at the definition site. For example: f x = letrec g y v = let x = ... in ...(g (a,b) x)... Since 'x' is in scope at the call site, we may make a rewrite rule that looks like RULE forall a,b. g (a,b) x = ... But this rule will never match, because it's really a different 'x' at the call site -- and that difference will be manifest by the time the simplifier gets to it. [A worry: the simplifier doesn't *guarantee* no-shadowing, so perhaps it may not be distinct?] Anyway, the rule isn't actually wrong, it's just not useful. One possibility is to run deShadowBinds before running SpecConstr, but instead we run the simplifier. That gives the simplest possible program for SpecConstr to chew on; and it virtually guarantees no shadowing. Note [Specialising for constant parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This one is about specialising on a *constant* (but not necessarily constructor) argument foo :: Int -> (Int -> Int) -> Int foo 0 f = 0 foo m f = foo (f m) (+1) It produces lvl_rmV :: GHC.Base.Int -> GHC.Base.Int lvl_rmV = \ (ds_dlk :: GHC.Base.Int) -> case ds_dlk of wild_alH { GHC.Base.I# x_alG -> GHC.Base.I# (GHC.Prim.+# x_alG 1) T.$wfoo :: GHC.Prim.Int# -> (GHC.Base.Int -> GHC.Base.Int) -> GHC.Prim.Int# T.$wfoo = \ (ww_sme :: GHC.Prim.Int#) (w_smg :: GHC.Base.Int -> GHC.Base.Int) -> case ww_sme of ds_Xlw { __DEFAULT -> case w_smg (GHC.Base.I# ds_Xlw) of w1_Xmo { GHC.Base.I# ww1_Xmz -> T.$wfoo ww1_Xmz lvl_rmV }; 0 -> 0 } The recursive call has lvl_rmV as its argument, so we could create a specialised copy with that argument baked in; that is, not passed at all. Now it can perhaps be inlined. When is this worth it? Call the constant 'lvl' - If 'lvl' has an unfolding that is a constructor, see if the corresponding parameter is scrutinised anywhere in the body. - If 'lvl' has an unfolding that is a inlinable function, see if the corresponding parameter is applied (...to enough arguments...?) Also do this is if the function has RULES? Also Note [Specialising for lambda parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ foo :: Int -> (Int -> Int) -> Int foo 0 f = 0 foo m f = foo (f m) (\n -> n-m) This is subtly different from the previous one in that we get an explicit lambda as the argument: T.$wfoo :: GHC.Prim.Int# -> (GHC.Base.Int -> GHC.Base.Int) -> GHC.Prim.Int# T.$wfoo = \ (ww_sm8 :: GHC.Prim.Int#) (w_sma :: GHC.Base.Int -> GHC.Base.Int) -> case ww_sm8 of ds_Xlr { __DEFAULT -> case w_sma (GHC.Base.I# ds_Xlr) of w1_Xmf { GHC.Base.I# ww1_Xmq -> T.$wfoo ww1_Xmq (\ (n_ad3 :: GHC.Base.Int) -> case n_ad3 of wild_alB { GHC.Base.I# x_alA -> GHC.Base.I# (GHC.Prim.-# x_alA ds_Xlr) }) }; 0 -> 0 } I wonder if SpecConstr couldn't be extended to handle this? After all, lambda is a sort of constructor for functions and perhaps it already has most of the necessary machinery? Furthermore, there's an immediate win, because you don't need to allocate the lamda at the call site; and if perchance it's called in the recursive call, then you may avoid allocating it altogether. Just like for constructors. Looks cool, but probably rare...but it might be easy to implement. Note [SpecConstr for casts] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data family T a :: * data instance T Int = T Int foo n = ... where go (T 0) = 0 go (T n) = go (T (n-1)) The recursive call ends up looking like go (T (I# ...) `cast` g) So we want to spot the construtor application inside the cast. That's why we have the Cast case in argToPat Note [Local recursive groups] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For a *local* recursive group, we can see all the calls to the function, so we seed the specialisation loop from the calls in the body, not from the calls in the RHS. Consider: bar m n = foo n (n,n) (n,n) (n,n) (n,n) where foo n p q r s | n == 0 = m | n > 3000 = case p of { (p1,p2) -> foo (n-1) (p2,p1) q r s } | n > 2000 = case q of { (q1,q2) -> foo (n-1) p (q2,q1) r s } | n > 1000 = case r of { (r1,r2) -> foo (n-1) p q (r2,r1) s } | otherwise = case s of { (s1,s2) -> foo (n-1) p q r (s2,s1) } If we start with the RHSs of 'foo', we get lots and lots of specialisations, most of which are not needed. But if we start with the (single) call in the rhs of 'bar' we get exactly one fully-specialised copy, and all the recursive calls go to this fully-specialised copy. Indeed, the original function is later collected as dead code. This is very important in specialising the loops arising from stream fusion, for example in NDP where we were getting literally hundreds of (mostly unused) specialisations of a local function. Note [Do not specialise diverging functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Specialising a function that just diverges is a waste of code. Furthermore, it broke GHC (simpl014) thus: {-# STR Sb #-} f = \x. case x of (a,b) -> f x If we specialise f we get f = \x. case x of (a,b) -> fspec a b But fspec doesn't have decent strictnes info. As it happened, (f x) :: IO t, so the state hack applied and we eta expanded fspec, and hence f. But now f's strictness is less than its arity, which breaks an invariant. Note [SpecConstrAnnotation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ SpecConstrAnnotation is defined in GHC.Exts, and is only guaranteed to be available in stage 2 (well, until the bootstrap compiler can be guaranteed to have it) So we define it to be () in stage1 (ie when GHCI is undefined), and '#ifdef' out the code that uses it. See also Note [Forcing specialisation] Note [Forcing specialisation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ With stream fusion and in other similar cases, we want to fully specialise some (but not necessarily all!) loops regardless of their size and the number of specialisations. We allow a library to specify this by annotating a type with ForceSpecConstr and then adding a parameter of that type to the loop. Here is a (simplified) example from the vector library: data SPEC = SPEC | SPEC2 {-# ANN type SPEC ForceSpecConstr #-} foldl :: (a -> b -> a) -> a -> Stream b -> a {-# INLINE foldl #-} foldl f z (Stream step s _) = foldl_loop SPEC z s where foldl_loop SPEC z s = case step s of Yield x s' -> foldl_loop SPEC (f z x) s' Skip -> foldl_loop SPEC z s' Done -> z SpecConstr will spot the SPEC parameter and always fully specialise foldl_loop. Note that we can't just annotate foldl_loop since it isn't a top-level function but even if we could, inlining etc. could easily drop the annotation. We also have to prevent the SPEC argument from being removed by w/w which is why SPEC is a sum type. This is all quite ugly; we ought to come up with a better design. ForceSpecConstr arguments are spotted in scExpr' and scTopBinds which then set force_spec to True when calling specLoop. This flag makes specLoop and specialise ignore specConstrCount and specConstrThreshold when deciding whether to specialise a function. ----------------------------------------------------- Stuff not yet handled ----------------------------------------------------- Here are notes arising from Roman's work that I don't want to lose. Example 1 ~~~~~~~~~ data T a = T !a foo :: Int -> T Int -> Int foo 0 t = 0 foo x t | even x = case t of { T n -> foo (x-n) t } | otherwise = foo (x-1) t SpecConstr does no specialisation, because the second recursive call looks like a boxed use of the argument. A pity. $wfoo_sFw :: GHC.Prim.Int# -> T.T GHC.Base.Int -> GHC.Prim.Int# $wfoo_sFw = \ (ww_sFo [Just L] :: GHC.Prim.Int#) (w_sFq [Just L] :: T.T GHC.Base.Int) -> case ww_sFo of ds_Xw6 [Just L] { __DEFAULT -> case GHC.Prim.remInt# ds_Xw6 2 of wild1_aEF [Dead Just A] { __DEFAULT -> $wfoo_sFw (GHC.Prim.-# ds_Xw6 1) w_sFq; 0 -> case w_sFq of wild_Xy [Just L] { T.T n_ad5 [Just U(L)] -> case n_ad5 of wild1_aET [Just A] { GHC.Base.I# y_aES [Just L] -> $wfoo_sFw (GHC.Prim.-# ds_Xw6 y_aES) wild_Xy } } }; 0 -> 0 Example 2 ~~~~~~~~~ data a :*: b = !a :*: !b data T a = T !a foo :: (Int :*: T Int) -> Int foo (0 :*: t) = 0 foo (x :*: t) | even x = case t of { T n -> foo ((x-n) :*: t) } | otherwise = foo ((x-1) :*: t) Very similar to the previous one, except that the parameters are now in a strict tuple. Before SpecConstr, we have $wfoo_sG3 :: GHC.Prim.Int# -> T.T GHC.Base.Int -> GHC.Prim.Int# $wfoo_sG3 = \ (ww_sFU [Just L] :: GHC.Prim.Int#) (ww_sFW [Just L] :: T.T GHC.Base.Int) -> case ww_sFU of ds_Xws [Just L] { __DEFAULT -> case GHC.Prim.remInt# ds_Xws 2 of wild1_aEZ [Dead Just A] { __DEFAULT -> case ww_sFW of tpl_B2 [Just L] { T.T a_sFo [Just A] -> $wfoo_sG3 (GHC.Prim.-# ds_Xws 1) tpl_B2 -- $wfoo1 }; 0 -> case ww_sFW of wild_XB [Just A] { T.T n_ad7 [Just S(L)] -> case n_ad7 of wild1_aFd [Just L] { GHC.Base.I# y_aFc [Just L] -> $wfoo_sG3 (GHC.Prim.-# ds_Xws y_aFc) wild_XB -- $wfoo2 } } }; 0 -> 0 } We get two specialisations: "SC:$wfoo1" [0] __forall {a_sFB :: GHC.Base.Int sc_sGC :: GHC.Prim.Int#} Foo.$wfoo sc_sGC (Foo.T @ GHC.Base.Int a_sFB) = Foo.$s$wfoo1 a_sFB sc_sGC ; "SC:$wfoo2" [0] __forall {y_aFp :: GHC.Prim.Int# sc_sGC :: GHC.Prim.Int#} Foo.$wfoo sc_sGC (Foo.T @ GHC.Base.Int (GHC.Base.I# y_aFp)) = Foo.$s$wfoo y_aFp sc_sGC ; But perhaps the first one isn't good. After all, we know that tpl_B2 is a T (I# x) really, because T is strict and Int has one constructor. (We can't unbox the strict fields, becuase T is polymorphic!) %************************************************************************ %* * \subsection{Top level wrapper stuff} %* * %************************************************************************ \begin{code}

    specConstrProgram :: ModGuts -> CoreM ModGuts
    specConstrProgram guts
      = do
          dflags <- getDynFlags
          us     <- getUniqueSupplyM
          annos  <- getFirstAnnotations deserializeWithData guts
          let binds' = fst $ initUs us (go (initScEnv dflags annos) (mg_binds guts))
          return (guts { mg_binds = binds' })
      where
        go _   []	        = return []
        go env (bind:binds) = do (env', bind') <- scTopBind env bind
                                 binds' <- go env' binds
                                 return (bind' : binds')

\end{code} %************************************************************************ %* * \subsection{Environment: goes downwards} %* * %************************************************************************ \begin{code}

    data ScEnv = SCE { sc_size  :: Maybe Int,	-- Size threshold
    		   sc_count :: Maybe Int,	-- Max # of specialisations for any one fn
    						-- See Note [Avoiding exponential blowup]

    		   sc_subst :: Subst,   	-- Current substitution
    						-- Maps InIds to OutExprs

    		   sc_how_bound :: HowBoundEnv,
    			-- Binds interesting non-top-level variables
    			-- Domain is OutVars (*after* applying the substitution)

    		   sc_vals  :: ValueEnv,
    			-- Domain is OutIds (*after* applying the substitution)
    			-- Used even for top-level bindings (but not imported ones)

                       sc_annotations :: UniqFM SpecConstrAnnotation
    	     }

    ---------------------
    -- As we go, we apply a substitution (sc_subst) to the current term
    type InExpr = CoreExpr		-- _Before_ applying the subst
    type InVar  = Var

    type OutExpr = CoreExpr		-- _After_ applying the subst
    type OutId   = Id
    type OutVar  = Var

    ---------------------
    type HowBoundEnv = VarEnv HowBound	-- Domain is OutVars

    ---------------------
    type ValueEnv = IdEnv Value		-- Domain is OutIds
    data Value    = ConVal AltCon [CoreArg]	-- _Saturated_ constructors
         	      	       	      		--   The AltCon is never DEFAULT
    	      | LambdaVal		-- Inlinable lambdas or PAPs

    instance Outputable Value where
       ppr (ConVal con args) = ppr con <+> interpp'SP args
       ppr LambdaVal	 = ptext (sLit "<Lambda>")

    ---------------------
    initScEnv :: DynFlags -> UniqFM SpecConstrAnnotation -> ScEnv
    initScEnv dflags anns
      = SCE { sc_size = specConstrThreshold dflags,
    	  sc_count = specConstrCount dflags,
    	  sc_subst = emptySubst,
    	  sc_how_bound = emptyVarEnv,
    	  sc_vals = emptyVarEnv,
              sc_annotations = anns }

    data HowBound = RecFun	-- These are the recursive functions for which
    			-- we seek interesting call patterns

    	      | RecArg	-- These are those functions' arguments, or their sub-components;
    			-- we gather occurrence information for these

    instance Outputable HowBound where
      ppr RecFun = text "RecFun"
      ppr RecArg = text "RecArg"

    lookupHowBound :: ScEnv -> Id -> Maybe HowBound
    lookupHowBound env id = lookupVarEnv (sc_how_bound env) id

    scSubstId :: ScEnv -> Id -> CoreExpr
    scSubstId env v = lookupIdSubst (text "scSubstId") (sc_subst env) v

    scSubstTy :: ScEnv -> Type -> Type
    scSubstTy env ty = substTy (sc_subst env) ty

    zapScSubst :: ScEnv -> ScEnv
    zapScSubst env = env { sc_subst = zapSubstEnv (sc_subst env) }

    extendScInScope :: ScEnv -> [Var] -> ScEnv
    	-- Bring the quantified variables into scope
    extendScInScope env qvars = env { sc_subst = extendInScopeList (sc_subst env) qvars }

    	-- Extend the substitution
    extendScSubst :: ScEnv -> Var -> OutExpr -> ScEnv
    extendScSubst env var expr = env { sc_subst = extendSubst (sc_subst env) var expr }

    extendScSubstList :: ScEnv -> [(Var,OutExpr)] -> ScEnv
    extendScSubstList env prs = env { sc_subst = extendSubstList (sc_subst env) prs }

    extendHowBound :: ScEnv -> [Var] -> HowBound -> ScEnv
    extendHowBound env bndrs how_bound
      = env { sc_how_bound = extendVarEnvList (sc_how_bound env)
    			    [(bndr,how_bound) | bndr <- bndrs] }

    extendBndrsWith :: HowBound -> ScEnv -> [Var] -> (ScEnv, [Var])
    extendBndrsWith how_bound env bndrs
      = (env { sc_subst = subst', sc_how_bound = hb_env' }, bndrs')
      where
        (subst', bndrs') = substBndrs (sc_subst env) bndrs
        hb_env' = sc_how_bound env `extendVarEnvList`
    		    [(bndr,how_bound) | bndr <- bndrs']

    extendBndrWith :: HowBound -> ScEnv -> Var -> (ScEnv, Var)
    extendBndrWith how_bound env bndr
      = (env { sc_subst = subst', sc_how_bound = hb_env' }, bndr')
      where
        (subst', bndr') = substBndr (sc_subst env) bndr
        hb_env' = extendVarEnv (sc_how_bound env) bndr' how_bound

    extendRecBndrs :: ScEnv -> [Var] -> (ScEnv, [Var])
    extendRecBndrs env bndrs  = (env { sc_subst = subst' }, bndrs')
    		      where
    			(subst', bndrs') = substRecBndrs (sc_subst env) bndrs

    extendBndr :: ScEnv -> Var -> (ScEnv, Var)
    extendBndr  env bndr  = (env { sc_subst = subst' }, bndr')
    		      where
    			(subst', bndr') = substBndr (sc_subst env) bndr

    extendValEnv :: ScEnv -> Id -> Maybe Value -> ScEnv
    extendValEnv env _  Nothing   = env
    extendValEnv env id (Just cv) = env { sc_vals = extendVarEnv (sc_vals env) id cv }

    extendCaseBndrs :: ScEnv -> OutExpr -> OutId -> AltCon -> [Var] -> (ScEnv, [Var])
    -- When we encounter
    --	case scrut of b
    --	    C x y -> ...
    -- we want to bind b, to (C x y)
    -- NB1: Extends only the sc_vals part of the envt
    -- NB2: Kill the dead-ness info on the pattern binders x,y, since
    --      they are potentially made alive by the [b -> C x y] binding
    extendCaseBndrs env scrut case_bndr con alt_bndrs
       = (env2, alt_bndrs')
     where
       live_case_bndr = not (isDeadBinder case_bndr)
       env1 | Var v <- scrut = extendValEnv env v cval
            | otherwise      = env	-- See Note [Add scrutinee to ValueEnv too]
       env2 | live_case_bndr = extendValEnv env1 case_bndr cval
            | otherwise      = env1

       alt_bndrs' | case scrut of { Var {} -> True; _ -> live_case_bndr }
                  = map zap alt_bndrs
                  | otherwise
                  = alt_bndrs

       cval = case con of
    		DEFAULT    -> Nothing
    		LitAlt {}  -> Just (ConVal con [])
    		DataAlt {} -> Just (ConVal con vanilla_args)
    		      where
    		       	vanilla_args = map Type (tyConAppArgs (idType case_bndr)) ++
    				       varsToCoreExprs alt_bndrs

       zap v | isTyCoVar v = v		-- See NB2 above
             | otherwise = zapIdOccInfo v

    decreaseSpecCount :: ScEnv -> Int -> ScEnv
    -- See Note [Avoiding exponential blowup]
    decreaseSpecCount env n_specs
      = env { sc_count = case sc_count env of
                           Nothing -> Nothing
                           Just n  -> Just (n `div` (n_specs + 1)) }
    	-- The "+1" takes account of the original function;
    	-- See Note [Avoiding exponential blowup]

    ---------------------------------------------------
    -- See Note [SpecConstrAnnotation]
    ignoreType    :: ScEnv -> Type   -> Bool
    ignoreAltCon  :: ScEnv -> AltCon -> Bool
    forceSpecBndr :: ScEnv -> Var    -> Bool
    #ifndef GHCI
    ignoreType    _ _ = False
    ignoreAltCon  _ _ = False
    forceSpecBndr _ _ = False

    #else /* GHCI */

    ignoreAltCon env (DataAlt dc) = ignoreTyCon env (dataConTyCon dc)
    ignoreAltCon env (LitAlt lit) = ignoreType env (literalType lit)
    ignoreAltCon _   DEFAULT      = panic "ignoreAltCon"  -- DEFAULT cannot be in a ConVal

    ignoreType env ty
      = case splitTyConApp_maybe ty of
          Just (tycon, _) -> ignoreTyCon env tycon
          _               -> False

    ignoreTyCon :: ScEnv -> TyCon -> Bool
    ignoreTyCon env tycon
      = lookupUFM (sc_annotations env) tycon == Just NoSpecConstr

    forceSpecBndr env var = forceSpecFunTy env . snd . splitForAllTys . varType $ var

    forceSpecFunTy :: ScEnv -> Type -> Bool
    forceSpecFunTy env = any (forceSpecArgTy env) . fst . splitFunTys

    forceSpecArgTy :: ScEnv -> Type -> Bool
    forceSpecArgTy env ty
      | Just ty' <- coreView ty = forceSpecArgTy env ty'

    forceSpecArgTy env ty
      | Just (tycon, tys) <- splitTyConApp_maybe ty
      , tycon /= funTyCon
          = lookupUFM (sc_annotations env) tycon == Just ForceSpecConstr
            || any (forceSpecArgTy env) tys

    forceSpecArgTy _ _ = False
    #endif /* GHCI */

\end{code} Note [Add scrutinee to ValueEnv too] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this: case x of y (a,b) -> case b of c I# v -> ...(f y)... By the time we get to the call (f y), the ValueEnv will have a binding for y, and for c y -> (a,b) c -> I# v BUT that's not enough! Looking at the call (f y) we see that y is pair (a,b), but we also need to know what 'b' is. So in extendCaseBndrs we must *also* add the binding b -> I# v else we lose a useful specialisation for f. This is necessary even though the simplifier has systematically replaced uses of 'x' with 'y' and 'b' with 'c' in the code. The use of 'b' in the ValueEnv came from outside the case. See Trac #4908 for the live example. Note [Avoiding exponential blowup] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The sc_count field of the ScEnv says how many times we are prepared to duplicate a single function. But we must take care with recursive specialiations. Consider let $j1 = let $j2 = let $j3 = ... in ...$j3... in ...$j2... in ...$j1... If we specialise $j1 then in each specialisation (as well as the original) we can specialise $j2, and similarly $j3. Even if we make just *one* specialisation of each, becuase we also have the original we'll get 2^n copies of $j3, which is not good. So when recursively specialising we divide the sc_count by the number of copies we are making at this level, including the original. %************************************************************************ %* * \subsection{Usage information: flows upwards} %* * %************************************************************************ \begin{code}

    data ScUsage
       = SCU {
    	scu_calls :: CallEnv,		-- Calls
    					-- The functions are a subset of the
    					-- 	RecFuns in the ScEnv

    	scu_occs :: !(IdEnv ArgOcc)	-- Information on argument occurrences
         }					-- The domain is OutIds

    type CallEnv = IdEnv [Call]
    type Call = (ValueEnv, [CoreArg])
    	-- The arguments of the call, together with the
    	-- env giving the constructor bindings at the call site

    nullUsage :: ScUsage
    nullUsage = SCU { scu_calls = emptyVarEnv, scu_occs = emptyVarEnv }

    combineCalls :: CallEnv -> CallEnv -> CallEnv
    combineCalls = plusVarEnv_C (++)

    combineUsage :: ScUsage -> ScUsage -> ScUsage
    combineUsage u1 u2 = SCU { scu_calls = combineCalls (scu_calls u1) (scu_calls u2),
    			   scu_occs  = plusVarEnv_C combineOcc (scu_occs u1) (scu_occs u2) }

    combineUsages :: [ScUsage] -> ScUsage
    combineUsages [] = nullUsage
    combineUsages us = foldr1 combineUsage us

    lookupOcc :: ScUsage -> OutVar -> (ScUsage, ArgOcc)
    lookupOcc (SCU { scu_calls = sc_calls, scu_occs = sc_occs }) bndr
      = (SCU {scu_calls = sc_calls, scu_occs = delVarEnv sc_occs bndr},
         lookupVarEnv sc_occs bndr `orElse` NoOcc)

    lookupOccs :: ScUsage -> [OutVar] -> (ScUsage, [ArgOcc])
    lookupOccs (SCU { scu_calls = sc_calls, scu_occs = sc_occs }) bndrs
      = (SCU {scu_calls = sc_calls, scu_occs = delVarEnvList sc_occs bndrs},
         [lookupVarEnv sc_occs b `orElse` NoOcc | b <- bndrs])

    data ArgOcc = NoOcc	-- Doesn't occur at all; or a type argument
    	    | UnkOcc	-- Used in some unknown way

    	    | ScrutOcc (UniqFM [ArgOcc])	-- See Note [ScrutOcc]

    	    | BothOcc	-- Definitely taken apart, *and* perhaps used in some other way

    {-	Note  [ScrutOcc]

    An occurrence of ScrutOcc indicates that the thing, or a `cast` version of the thing,
    is *only* taken apart or applied.

      Functions, literal: ScrutOcc emptyUFM
      Data constructors:  ScrutOcc subs,

    where (subs :: UniqFM [ArgOcc]) gives usage of the *pattern-bound* components,
    The domain of the UniqFM is the Unique of the data constructor

    The [ArgOcc] is the occurrences of the *pattern-bound* components
    of the data structure.  E.g.
    	data T a = forall b. MkT a b (b->a)
    A pattern binds b, x::a, y::b, z::b->a, but not 'a'!

    -}

    instance Outputable ArgOcc where
      ppr (ScrutOcc xs) = ptext (sLit "scrut-occ") <> ppr xs
      ppr UnkOcc 	    = ptext (sLit "unk-occ")
      ppr BothOcc 	    = ptext (sLit "both-occ")
      ppr NoOcc    	    = ptext (sLit "no-occ")

    -- Experimentally, this vesion of combineOcc makes ScrutOcc "win", so
    -- that if the thing is scrutinised anywhere then we get to see that
    -- in the overall result, even if it's also used in a boxed way
    -- This might be too agressive; see Note [Reboxing] Alternative 3
    combineOcc :: ArgOcc -> ArgOcc -> ArgOcc
    combineOcc NoOcc	 occ 	       = occ
    combineOcc occ 		 NoOcc	       = occ
    combineOcc (ScrutOcc xs) (ScrutOcc ys) = ScrutOcc (plusUFM_C combineOccs xs ys)
    combineOcc _occ          (ScrutOcc ys) = ScrutOcc ys
    combineOcc (ScrutOcc xs) _occ	       = ScrutOcc xs
    combineOcc UnkOcc        UnkOcc        = UnkOcc
    combineOcc _	    _	     	       = BothOcc

    combineOccs :: [ArgOcc] -> [ArgOcc] -> [ArgOcc]
    combineOccs xs ys = zipWithEqual "combineOccs" combineOcc xs ys

    setScrutOcc :: ScEnv -> ScUsage -> OutExpr -> ArgOcc -> ScUsage
    -- _Overwrite_ the occurrence info for the scrutinee, if the scrutinee
    -- is a variable, and an interesting variable
    setScrutOcc env usg (Cast e _) occ = setScrutOcc env usg e occ
    setScrutOcc env usg (Note _ e) occ = setScrutOcc env usg e occ
    setScrutOcc env usg (Var v)    occ
      | Just RecArg <- lookupHowBound env v = usg { scu_occs = extendVarEnv (scu_occs usg) v occ }
      | otherwise				= usg
    setScrutOcc _env usg _other _occ	-- Catch-all
      = usg

    conArgOccs :: ArgOcc -> AltCon -> [ArgOcc]
    -- Find usage of components of data con; returns [UnkOcc...] if unknown
    -- See Note [ScrutOcc] for the extra UnkOccs in the vanilla datacon case

    conArgOccs (ScrutOcc fm) (DataAlt dc)
      | Just pat_arg_occs <- lookupUFM fm dc
      = [UnkOcc | _ <- dataConUnivTyVars dc] ++ pat_arg_occs

    conArgOccs _other _con = repeat UnkOcc

\end{code} %************************************************************************ %* * \subsection{The main recursive function} %* * %************************************************************************ The main recursive function gathers up usage information, and creates specialised versions of functions. \begin{code}

    scExpr, scExpr' :: ScEnv -> CoreExpr -> UniqSM (ScUsage, CoreExpr)
    	-- The unique supply is needed when we invent
    	-- a new name for the specialised function and its args

    scExpr env e = scExpr' env e

    scExpr' env (Var v)     = case scSubstId env v of
    		            Var v' -> return (varUsage env v' UnkOcc, Var v')
    		            e'     -> scExpr (zapScSubst env) e'

    scExpr' env (Type t)    = return (nullUsage, Type (scSubstTy env t))
    scExpr' _   e@(Lit {})  = return (nullUsage, e)
    scExpr' env (Note n e)  = do (usg,e') <- scExpr env e
                                 return (usg, Note n e')
    scExpr' env (Cast e co) = do (usg, e') <- scExpr env e
                                 return (usg, Cast e' (scSubstTy env co))
    scExpr' env e@(App _ _) = scApp env (collectArgs e)
    scExpr' env (Lam b e)   = do let (env', b') = extendBndr env b
                                 (usg, e') <- scExpr env' e
                                 return (usg, Lam b' e')

    scExpr' env (Case scrut b ty alts)
      = do	{ (scrut_usg, scrut') <- scExpr env scrut
    	; case isValue (sc_vals env) scrut' of
    		Just (ConVal con args) -> sc_con_app con args scrut'
    		_other		       -> sc_vanilla scrut_usg scrut'
    	}
      where
        sc_con_app con args scrut' 	-- Known constructor; simplify
    	= do { let (_, bs, rhs) = findAlt con alts
    	       	   	          `orElse` (DEFAULT, [], mkImpossibleExpr (coreAltsType alts))
    		   alt_env'  = extendScSubstList env ((b,scrut') : bs `zip` trimConArgs con args)
    	     ; scExpr alt_env' rhs }

        sc_vanilla scrut_usg scrut'	-- Normal case
         = do { let (alt_env,b') = extendBndrWith RecArg env b
    			-- Record RecArg for the components

    	  ; (alt_usgs, alt_occs, alts')
    		<- mapAndUnzip3M (sc_alt alt_env scrut' b') alts

    	  ; let (alt_usg, b_occ) = lookupOcc (combineUsages alt_usgs) b'
    		scrut_occ        = foldr combineOcc b_occ alt_occs
    		scrut_usg'       = setScrutOcc env scrut_usg scrut' scrut_occ
    	  	-- The combined usage of the scrutinee is given
    	  	-- by scrut_occ, which is passed to scScrut, which
    	  	-- in turn treats a bare-variable scrutinee specially

    	  ; return (alt_usg `combineUsage` scrut_usg',
    	  	    Case scrut' b' (scSubstTy env ty) alts') }

        sc_alt env scrut' b' (con,bs,rhs)
          = do { let (env1, bs1) = extendBndrsWith RecArg env bs
    		 (env2, bs2) = extendCaseBndrs env1 scrut' b' con bs1
    	   ; (usg,rhs') <- scExpr env2 rhs
    	   ; let (usg', arg_occs) = lookupOccs usg bs2
    		 scrut_occ = case con of
    				DataAlt dc -> ScrutOcc (unitUFM dc arg_occs)
    				_      	   -> ScrutOcc emptyUFM
    	   ; return (usg', scrut_occ, (con, bs2, rhs')) }

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

      | otherwise
      = do	{ let (body_env, bndr') = extendBndr env bndr
    	; (rhs_usg, rhs_info) <- scRecRhs env (bndr',rhs)

    	; let body_env2 = extendHowBound body_env [bndr'] RecFun
    	      			   -- Note [Local let bindings]
    	      RI _ rhs' _ _ _ = rhs_info
                  body_env3 = extendValEnv body_env2 bndr' (isValue (sc_vals env) rhs')

    	; (body_usg, body') <- scExpr body_env3 body

              -- NB: We don't use the ForceSpecConstr mechanism (see
              -- Note [Forcing specialisation]) for non-recursive bindings
              -- at the moment. I'm not sure if this is the right thing to do.
    	; let force_spec = False
    	; (spec_usg, specs) <- specialise env force_spec
                                              (scu_calls body_usg)
    					  rhs_info
                                              (SI [] 0 (Just rhs_usg))

    	; return (body_usg { scu_calls = scu_calls body_usg `delVarEnv` bndr' }
    	            `combineUsage` spec_usg,
    		  mkLets [NonRec b r | (b,r) <- specInfoBinds rhs_info specs] body')
    	}

    -- A *local* recursive group: see Note [Local recursive groups]
    scExpr' env (Let (Rec prs) body)
      = do	{ let (bndrs,rhss) = unzip prs
    	      (rhs_env1,bndrs') = extendRecBndrs env bndrs
    	      rhs_env2 = extendHowBound rhs_env1 bndrs' RecFun
                  force_spec = any (forceSpecBndr env) bndrs'
                    -- Note [Forcing specialisation]

    	; (rhs_usgs, rhs_infos) <- mapAndUnzipM (scRecRhs rhs_env2) (bndrs' `zip` rhss)
    	; (body_usg, body')     <- scExpr rhs_env2 body

    	-- NB: start specLoop from body_usg
    	; (spec_usg, specs) <- specLoop rhs_env2 force_spec
                                            (scu_calls body_usg) rhs_infos nullUsage
    					[SI [] 0 (Just usg) | usg <- rhs_usgs]
    		-- Do not unconditionally use rhs_usgs.
    		-- Instead use them only if we find an unspecialised call
    		-- See Note [Local recursive groups]

    	; let all_usg = spec_usg `combineUsage` body_usg
    	      bind'   = Rec (concat (zipWith specInfoBinds rhs_infos specs))

    	; return (all_usg { scu_calls = scu_calls all_usg `delVarEnvList` bndrs' },
    	          Let bind' body') }

\end{code} Note [Local let bindings] ~~~~~~~~~~~~~~~~~~~~~~~~~ It is not uncommon to find this let $j = \x. <blah> in ...$j True...$j True... Here $j is an arbitrary let-bound function, but it often comes up for join points. We might like to specialise $j for its call patterns. Notice the difference from a letrec, where we look for call patterns in the *RHS* of the function. Here we look for call patterns in the *body* of the let. At one point I predicated this on the RHS mentioning the outer recursive function, but that's not essential and might even be harmful. I'm not sure. \begin{code}

    scApp :: ScEnv -> (InExpr, [InExpr]) -> UniqSM (ScUsage, CoreExpr)

    scApp env (Var fn, args)	-- Function is a variable
      = ASSERT( not (null args) )
        do	{ args_w_usgs <- mapM (scExpr env) args
    	; let (arg_usgs, args') = unzip args_w_usgs
    	      arg_usg = combineUsages arg_usgs
    	; case scSubstId env fn of
    	    fn'@(Lam {}) -> scExpr (zapScSubst env) (doBeta fn' args')
    			-- Do beta-reduction and try again

    	    Var fn' -> return (arg_usg `combineUsage` fn_usg, mkApps (Var fn') args')
    		where
    		  fn_usg = case lookupHowBound env fn' of
    				Just RecFun -> SCU { scu_calls = unitVarEnv fn' [(sc_vals env, args')],
    					             scu_occs  = emptyVarEnv }
    				Just RecArg -> SCU { scu_calls = emptyVarEnv,
    					             scu_occs  = unitVarEnv fn' (ScrutOcc emptyUFM) }
    				Nothing     -> nullUsage

    	    other_fn' -> return (arg_usg, mkApps other_fn' args') }
    		-- NB: doing this ignores any usage info from the substituted
    		--     function, but I don't think that matters.  If it does
    		--     we can fix it.
      where
        doBeta :: OutExpr -> [OutExpr] -> OutExpr
        -- ToDo: adjust for System IF
        doBeta (Lam bndr body) (arg : args) = Let (NonRec bndr arg) (doBeta body args)
        doBeta fn	           args	        = mkApps fn args

    -- The function is almost always a variable, but not always.
    -- In particular, if this pass follows float-in,
    -- which it may, we can get
    --	(let f = ...f... in f) arg1 arg2
    scApp env (other_fn, args)
      = do 	{ (fn_usg,   fn')   <- scExpr env other_fn
    	; (arg_usgs, args') <- mapAndUnzipM (scExpr env) args
    	; return (combineUsages arg_usgs `combineUsage` fn_usg, mkApps fn' args') }

    ----------------------
    scTopBind :: ScEnv -> CoreBind -> UniqSM (ScEnv, CoreBind)
    scTopBind env (Rec prs)
      | Just threshold <- sc_size env
      , not force_spec
      , not (all (couldBeSmallEnoughToInline threshold) rhss)
    		-- No specialisation
      = do	{ let (rhs_env,bndrs') = extendRecBndrs env bndrs
    	; (_, rhss') <- mapAndUnzipM (scExpr rhs_env) rhss
    	; return (rhs_env, Rec (bndrs' `zip` rhss')) }
      | otherwise	-- Do specialisation
      = do	{ let (rhs_env1,bndrs') = extendRecBndrs env bndrs
    	      rhs_env2 = extendHowBound rhs_env1 bndrs' RecFun

    	; (rhs_usgs, rhs_infos) <- mapAndUnzipM (scRecRhs rhs_env2) (bndrs' `zip` rhss)
    	; let rhs_usg = combineUsages rhs_usgs

    	; (_, specs) <- specLoop rhs_env2 force_spec
                                     (scu_calls rhs_usg) rhs_infos nullUsage
    				 [SI [] 0 Nothing | _ <- bndrs]

    	; return (rhs_env1,  -- For the body of the letrec, delete the RecFun business
    		  Rec (concat (zipWith specInfoBinds rhs_infos specs))) }
      where
        (bndrs,rhss) = unzip prs
        force_spec = any (forceSpecBndr env) bndrs
          -- Note [Forcing specialisation]

    scTopBind env (NonRec bndr rhs)
      = do	{ (_, rhs') <- scExpr env rhs
    	; let (env1, bndr') = extendBndr env bndr
    	      env2 = extendValEnv env1 bndr' (isValue (sc_vals env) rhs')
    	; return (env2, NonRec bndr' rhs') }

    ----------------------
    scRecRhs :: ScEnv -> (OutId, InExpr) -> UniqSM (ScUsage, RhsInfo)
    scRecRhs env (bndr,rhs)
      = do	{ let (arg_bndrs,body) = collectBinders rhs
    	      (body_env, arg_bndrs') = extendBndrsWith RecArg env arg_bndrs
    	; (body_usg, body') <- scExpr body_env body
    	; let (rhs_usg, arg_occs) = lookupOccs body_usg arg_bndrs'
    	; return (rhs_usg, RI bndr (mkLams arg_bndrs' body')
                                       arg_bndrs body arg_occs) }
    		-- The arg_occs says how the visible,
    		-- lambda-bound binders of the RHS are used
    		-- (including the TyVar binders)
    	 	-- Two pats are the same if they match both ways

    ----------------------
    specInfoBinds :: RhsInfo -> SpecInfo -> [(Id,CoreExpr)]
    specInfoBinds (RI fn new_rhs _ _ _) (SI specs _ _)
      = [(id,rhs) | OS _ _ id rhs <- specs] ++
        [(fn `addIdSpecialisations` rules, new_rhs)]
      where
        rules = [r | OS _ r _ _ <- specs]

    ----------------------
    varUsage :: ScEnv -> OutVar -> ArgOcc -> ScUsage
    varUsage env v use
      | Just RecArg <- lookupHowBound env v = SCU { scu_calls = emptyVarEnv
    					      , scu_occs = unitVarEnv v use }
      | otherwise		   	        = nullUsage

\end{code} %************************************************************************ %* * The specialiser itself %* * %************************************************************************ \begin{code}

    data RhsInfo = RI OutId 		-- The binder
                      OutExpr		-- The new RHS
    		  [InVar] InExpr	-- The *original* RHS (\xs.body)
    					--   Note [Specialise original body]
                      [ArgOcc]		-- Info on how the xs occur in body

    data SpecInfo = SI [OneSpec]		-- The specialisations we have generated

    		   Int			-- Length of specs; used for numbering them

    		   (Maybe ScUsage)	-- Nothing => we have generated specialisations
    					--	      from calls in the *original* RHS
    					-- Just cs => we haven't, and this is the usage
    					--	      of the original RHS
    					-- See Note [Local recursive groups]

    	-- One specialisation: Rule plus definition
    data OneSpec  = OS CallPat 		-- Call pattern that generated this specialisation
    		   CoreRule		-- Rule connecting original id with the specialisation
    		   OutId OutExpr	-- Spec id + its rhs

    specLoop :: ScEnv
             -> Bool                                -- force specialisation?
                                                    -- Note [Forcing specialisation]
    	 -> CallEnv
    	 -> [RhsInfo]
    	 -> ScUsage -> [SpecInfo]		-- One per binder; acccumulating parameter
    	 -> UniqSM (ScUsage, [SpecInfo])	-- ...ditto...
    specLoop env force_spec all_calls rhs_infos usg_so_far specs_so_far
      = do	{ specs_w_usg <- zipWithM (specialise env force_spec all_calls) rhs_infos specs_so_far
    	; let (new_usg_s, all_specs) = unzip specs_w_usg
    	      new_usg   = combineUsages new_usg_s
    	      new_calls = scu_calls new_usg
    	      all_usg   = usg_so_far `combineUsage` new_usg
    	; if isEmptyVarEnv new_calls then
    		return (all_usg, all_specs)
     	  else
    		specLoop env force_spec new_calls rhs_infos all_usg all_specs }

    specialise
       :: ScEnv
       -> Bool                              -- force specialisation?
                                            --   Note [Forcing specialisation]
       -> CallEnv				-- Info on calls
       -> RhsInfo
       -> SpecInfo				-- Original RHS plus patterns dealt with
       -> UniqSM (ScUsage, SpecInfo)	-- New specialised versions and their usage

    -- Note: the rhs here is the optimised version of the original rhs
    -- So when we make a specialised copy of the RHS, we're starting
    -- from an RHS whose nested functions have been optimised already.

    specialise env force_spec bind_calls (RI fn _ arg_bndrs body arg_occs)
    			  spec_info@(SI specs spec_count mb_unspec)
      | not (isBottomingId fn)      -- Note [Do not specialise diverging functions]
      , not (isNeverActive (idInlineActivation fn))	-- See Note [Transfer activation]
      , notNull arg_bndrs		-- Only specialise functions
      , Just all_calls <- lookupVarEnv bind_calls fn
      = do	{ (boring_call, pats) <- callsToPats env specs arg_occs all_calls
    --	; pprTrace "specialise" (vcat [ ppr fn <+> text "with" <+> int (length pats) <+> text "good patterns"
    --                                      , text "arg_occs" <+> ppr arg_occs
    --	  			      ,	text "calls" <+> ppr all_calls
    --	  			      , text "good pats" <+> ppr pats])  $
    --	  return ()

    		-- Bale out if too many specialisations
    	; let n_pats      = length pats
                  spec_count' = n_pats + spec_count
    	; case sc_count env of
    	    Just max | not force_spec && spec_count' > max
    		-> pprTrace "SpecConstr" msg $
                       return (nullUsage, spec_info)
    		where
    		   msg = vcat [ sep [ ptext (sLit "Function") <+> quotes (ppr fn)
    		       	            , nest 2 (ptext (sLit "has") <+>
                                                  speakNOf spec_count' (ptext (sLit "call pattern")) <> comma <+>
                                                  ptext (sLit "but the limit is") <+> int max) ]
    			      , ptext (sLit "Use -fspec-constr-count=n to set the bound")
    			      , extra ]
    	           extra | not opt_PprStyle_Debug = ptext (sLit "Use -dppr-debug to see specialisations")
    		   	 | otherwise = ptext (sLit "Specialisations:") <+> ppr (pats ++ [p | OS p _ _ _ <- specs])

    	    _normal_case -> do {

              let spec_env = decreaseSpecCount env n_pats
    	; (spec_usgs, new_specs) <- mapAndUnzipM (spec_one spec_env fn arg_bndrs body)
    					         (pats `zip` [spec_count..])
    		-- See Note [Specialise original body]

    	; let spec_usg = combineUsages spec_usgs
    	      (new_usg, mb_unspec')
    		  = case mb_unspec of
    		      Just rhs_usg | boring_call -> (spec_usg `combineUsage` rhs_usg, Nothing)
    		      _			         -> (spec_usg,                      mb_unspec)

    	; return (new_usg, SI (new_specs ++ specs) spec_count' mb_unspec') } }
      | otherwise
      = return (nullUsage, spec_info)		-- The boring case

    ---------------------
    spec_one :: ScEnv
    	 -> OutId	-- Function
    	 -> [InVar]	-- Lambda-binders of RHS; should match patterns
    	 -> InExpr	-- Body of the original function
    	 -> (CallPat, Int)
    	 -> UniqSM (ScUsage, OneSpec)	-- Rule and binding

    -- spec_one creates a specialised copy of the function, together
    -- with a rule for using it.  I'm very proud of how short this
    -- function is, considering what it does :-).

    {-
      Example

         In-scope: a, x::a
         f = /\b \y::[(a,b)] -> ....f (b,c) ((:) (a,(b,c)) (x,v) (h w))...
    	  [c::*, v::(b,c) are presumably bound by the (...) part]
      ==>
         f_spec = /\ b c \ v::(b,c) hw::[(a,(b,c))] ->
    		  (...entire body of f...) [b -> (b,c),
      					    y -> ((:) (a,(b,c)) (x,v) hw)]

         RULE:  forall b::* c::*,		-- Note, *not* forall a, x
    		   v::(b,c),
    		   hw::[(a,(b,c))] .

    	    f (b,c) ((:) (a,(b,c)) (x,v) hw) = f_spec b c v hw
    -}

    spec_one env fn arg_bndrs body (call_pat@(qvars, pats), rule_number)
      = do	{ spec_uniq <- getUniqueUs
            ; let spec_env = extendScSubstList (extendScInScope env qvars)
    				           (arg_bndrs `zip` pats)
    	      fn_name    = idName fn
    	      fn_loc     = nameSrcSpan fn_name
    	      spec_occ   = mkSpecOcc (nameOccName fn_name)
    	      rule_name  = mkFastString ("SC:" ++ showSDoc (ppr fn <> int rule_number))
    	      spec_name  = mkInternalName spec_uniq spec_occ fn_loc
    --	; pprTrace "{spec_one" (ppr (sc_count env) <+> ppr fn <+> ppr pats <+> text "-->" <+> ppr spec_name) $
    --	  return ()

    	-- Specialise the body
    	; (spec_usg, spec_body) <- scExpr spec_env body

    --	; pprTrace "done spec_one}" (ppr fn) $
    --	  return ()

    		-- And build the results
    	; let spec_id = mkLocalId spec_name (mkPiTypes spec_lam_args body_ty)
    	      		     `setIdStrictness` spec_str    	-- See Note [Transfer strictness]
    			     `setIdArity` count isId spec_lam_args
    	      spec_str   = calcSpecStrictness fn spec_lam_args pats
    	      (spec_lam_args, spec_call_args) = mkWorkerArgs qvars body_ty
    	      	-- Usual w/w hack to avoid generating
    	      	-- a spec_rhs of unlifted type and no args

                  spec_rhs   = mkLams spec_lam_args spec_body
    	      body_ty    = exprType spec_body
    	      rule_rhs   = mkVarApps (Var spec_id) spec_call_args
                  inline_act = idInlineActivation fn
    	      rule       = mkRule True {- Auto -} True {- Local -}
                                      rule_name inline_act fn_name qvars pats rule_rhs
    	      		   -- See Note [Transfer activation]
    	; return (spec_usg, OS call_pat rule spec_id spec_rhs) }

    calcSpecStrictness :: Id 		     -- The original function
                       -> [Var] -> [CoreExpr]    -- Call pattern
    		   -> StrictSig              -- Strictness of specialised thing
    -- See Note [Transfer strictness]
    calcSpecStrictness fn qvars pats
      = StrictSig (mkTopDmdType spec_dmds TopRes)
      where
        spec_dmds = [ lookupVarEnv dmd_env qv `orElse` lazyDmd | qv <- qvars, isId qv ]
        StrictSig (DmdType _ dmds _) = idStrictness fn

        dmd_env = go emptyVarEnv dmds pats

        go env ds (Type {} : pats) = go env ds pats
        go env (d:ds) (pat : pats) = go (go_one env d pat) ds pats
        go env _      _            = env

        go_one env d   (Var v) = extendVarEnv_C both env v d
        go_one env (Box d)   e = go_one env d e
        go_one env (Eval (Prod ds)) e
        	   | (Var _, args) <- collectArgs e = go env ds args
        go_one env _         _ = env

\end{code} Note [Specialise original body] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The RhsInfo for a binding keeps the *original* body of the binding. We must specialise that, *not* the result of applying specExpr to the RHS (which is also kept in RhsInfo). Otherwise we end up specialising a specialised RHS, and that can lead directly to exponential behaviour. Note [Transfer activation] ~~~~~~~~~~~~~~~~~~~~~~~~~~ This note is for SpecConstr, but exactly the same thing happens in the overloading specialiser; see Note [Auto-specialisation and RULES] in Specialise. In which phase should the specialise-constructor rules be active? Originally I made them always-active, but Manuel found that this defeated some clever user-written rules. Then I made them active only in Phase 0; after all, currently, the specConstr transformation is only run after the simplifier has reached Phase 0, but that meant that specialisations didn't fire inside wrappers; see test simplCore/should_compile/spec-inline. So now I just use the inline-activation of the parent Id, as the activation for the specialiation RULE, just like the main specialiser; This in turn means there is no point in specialising NOINLINE things, so we test for that. Note [Transfer strictness] ~~~~~~~~~~~~~~~~~~~~~~~~~~ We must transfer strictness information from the original function to the specialised one. Suppose, for example f has strictness SS and a RULE f (a:as) b = f_spec a as b Now we want f_spec to have strictess LLS, otherwise we'll use call-by-need when calling f_spec instead of call-by-value. And that can result in unbounded worsening in space (cf the classic foldl vs foldl') See Trac #3437 for a good example. The function calcSpecStrictness performs the calculation. %************************************************************************ %* * \subsection{Argument analysis} %* * %************************************************************************ This code deals with analysing call-site arguments to see whether they are constructor applications. \begin{code}

    type CallPat = ([Var], [CoreExpr])	-- Quantified variables and arguments

    callsToPats :: ScEnv -> [OneSpec] -> [ArgOcc] -> [Call] -> UniqSM (Bool, [CallPat])
    	-- Result has no duplicate patterns,
    	-- nor ones mentioned in done_pats
    	-- Bool indicates that there was at least one boring pattern
    callsToPats env done_specs bndr_occs calls
      = do	{ mb_pats <- mapM (callToPats env bndr_occs) calls

    	; let good_pats :: [([Var], [CoreArg])]
    	      good_pats = catMaybes mb_pats
    	      done_pats = [p | OS p _ _ _ <- done_specs]
    	      is_done p = any (samePat p) done_pats

    	; return (any isNothing mb_pats,
    		  filterOut is_done (nubBy samePat good_pats)) }

    callToPats :: ScEnv -> [ArgOcc] -> Call -> UniqSM (Maybe CallPat)
    	-- The [Var] is the variables to quantify over in the rule
    	--	Type variables come first, since they may scope
    	--	over the following term variables
    	-- The [CoreExpr] are the argument patterns for the rule
    callToPats env bndr_occs (con_env, args)
      | length args < length bndr_occs	-- Check saturated
      = return Nothing
      | otherwise
      = do	{ let in_scope = substInScope (sc_subst env)
    	; prs <- argsToPats env in_scope con_env (args `zip` bndr_occs)
    	; let (interesting_s, pats) = unzip prs
    	      pat_fvs = varSetElems (exprsFreeVars pats)
    	      qvars   = filterOut (`elemInScopeSet` in_scope) pat_fvs
    		-- Quantify over variables that are not in sccpe
    		-- at the call site
    		-- See Note [Shadowing] at the top

    	      (tvs, ids) = partition isTyCoVar qvars
    	      qvars'     = tvs ++ ids
    		-- Put the type variables first; the type of a term
    		-- variable may mention a type variable

    	; -- pprTrace "callToPats"  (ppr args $$ ppr prs $$ ppr bndr_occs) $
    	  if or interesting_s
    	  then return (Just (qvars', pats))
    	  else return Nothing }

        -- argToPat takes an actual argument, and returns an abstracted
        -- version, consisting of just the "constructor skeleton" of the
        -- argument, with non-constructor sub-expression replaced by new
        -- placeholder variables.  For example:
        --    C a (D (f x) (g y))  ==>  C p1 (D p2 p3)

    argToPat :: ScEnv
             -> InScopeSet			-- What's in scope at the fn defn site
    	 -> ValueEnv			-- ValueEnv at the call site
    	 -> CoreArg			-- A call arg (or component thereof)
    	 -> ArgOcc
    	 -> UniqSM (Bool, CoreArg)
    -- Returns (interesting, pat),
    -- where pat is the pattern derived from the argument
    --	      intersting=True if the pattern is non-trivial (not a variable or type)
    -- E.g.		x:xs	     --> (True, x:xs)
    --		f xs         --> (False, w)	   where w is a fresh wildcard
    --		(f xs, 'c')  --> (True, (w, 'c'))  where w is a fresh wildcard
    --		\x. x+y      --> (True, \x. x+y)
    --		lvl7	     --> (True, lvl7)	   if lvl7 is bound
    --						   somewhere further out

    argToPat _env _in_scope _val_env arg@(Type {}) _arg_occ
      = return (False, arg)

    argToPat env in_scope val_env (Note _ arg) arg_occ
      = argToPat env in_scope val_env arg arg_occ
    	-- Note [Notes in call patterns]
    	-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    	-- Ignore Notes.  In particular, we want to ignore any InlineMe notes
    	-- Perhaps we should not ignore profiling notes, but I'm going to
    	-- ride roughshod over them all for now.
    	--- See Note [Notes in RULE matching] in Rules

    argToPat env in_scope val_env (Let _ arg) arg_occ
      = argToPat env in_scope val_env arg arg_occ
    	-- See Note [Matching lets] in Rule.lhs
    	-- Look through let expressions
    	-- e.g.		f (let v = rhs in (v,w))
    	-- Here we can specialise for f (v,w)
    	-- because the rule-matcher will look through the let.

    {- Disabled; see Note [Matching cases] in Rule.lhs
    argToPat env in_scope val_env (Case scrut _ _ [(_, _, rhs)]) arg_occ
      | exprOkForSpeculation scrut	-- See Note [Matching cases] in Rule.hhs
      = argToPat env in_scope val_env rhs arg_occ
    -}

    argToPat env in_scope val_env (Cast arg co) arg_occ
      | not (ignoreType env ty2)
      = do	{ (interesting, arg') <- argToPat env in_scope val_env arg arg_occ
    	; if not interesting then
    		wildCardPat ty2
    	  else do
    	{ -- Make a wild-card pattern for the coercion
    	  uniq <- getUniqueUs
    	; let co_name = mkSysTvName uniq (fsLit "sg")
    	      co_var = mkCoVar co_name (mkCoKind ty1 ty2)
    	; return (interesting, Cast arg' (mkTyVarTy co_var)) } }
      where
        (ty1, ty2) = coercionKind co

    {-	Disabling lambda specialisation for now
    	It's fragile, and the spec_loop can be infinite
    argToPat in_scope val_env arg arg_occ
      | is_value_lam arg
      = return (True, arg)
      where
        is_value_lam (Lam v e) 	-- Spot a value lambda, even if
    	| isId v = True		-- it is inside a type lambda
    	| otherwise = is_value_lam e
        is_value_lam other = False
    -}

      -- Check for a constructor application
      -- NB: this *precedes* the Var case, so that we catch nullary constrs
    argToPat env in_scope val_env arg arg_occ
      | Just (ConVal dc args) <- isValue val_env arg
      , not (ignoreAltCon env dc)
      , case arg_occ of
    	ScrutOcc _ -> True		-- Used only by case scrutinee
    	BothOcc    -> case arg of	-- Used elsewhere
    			App {} -> True	--     see Note [Reboxing]
    			_other -> False
    	_other	   -> False	-- No point; the arg is not decomposed
      = do	{ args' <- argsToPats env in_scope val_env (args `zip` conArgOccs arg_occ dc)
    	; return (True, mk_con_app dc (map snd args')) }

      -- Check if the argument is a variable that
      -- is in scope at the function definition site
      -- It's worth specialising on this if
      --	(a) it's used in an interesting way in the body
      --	(b) we know what its value is
    argToPat env in_scope val_env (Var v) arg_occ
      | case arg_occ of { UnkOcc -> False; _other -> True },	-- (a)
        is_value,							-- (b)
        not (ignoreType env (varType v))
      = return (True, Var v)
      where
        is_value
    	| isLocalId v = v `elemInScopeSet` in_scope
    			&& isJust (lookupVarEnv val_env v)
    		-- Local variables have values in val_env
    	| otherwise   = isValueUnfolding (idUnfolding v)
    		-- Imports have unfoldings

    --	I'm really not sure what this comment means
    --	And by not wild-carding we tend to get forall'd
    --	variables that are in soope, which in turn can
    --	expose the weakness in let-matching
    --	See Note [Matching lets] in Rules

      -- Check for a variable bound inside the function.
      -- Don't make a wild-card, because we may usefully share
      --	e.g.  f a = let x = ... in f (x,x)
      -- NB: this case follows the lambda and con-app cases!!
    -- argToPat _in_scope _val_env (Var v) _arg_occ
    --   = return (False, Var v)
    	-- SLPJ : disabling this to avoid proliferation of versions
    	-- also works badly when thinking about seeding the loop
    	-- from the body of the let
    	--	 f x y = letrec g z = ... in g (x,y)
    	-- We don't want to specialise for that *particular* x,y

      -- The default case: make a wild-card
    argToPat _env _in_scope _val_env arg _arg_occ
      = wildCardPat (exprType arg)

    wildCardPat :: Type -> UniqSM (Bool, CoreArg)
    wildCardPat ty = do { uniq <- getUniqueUs
    		    ; let id = mkSysLocal (fsLit "sc") uniq ty
    		    ; return (False, Var id) }

    argsToPats :: ScEnv -> InScopeSet -> ValueEnv
    	   -> [(CoreArg, ArgOcc)]
    	   -> UniqSM [(Bool, CoreArg)]
    argsToPats env in_scope val_env args
      = mapM do_one args
      where
        do_one (arg,occ) = argToPat env in_scope val_env arg occ

\end{code} \begin{code}

    isValue :: ValueEnv -> CoreExpr -> Maybe Value
    isValue _env (Lit lit)
      = Just (ConVal (LitAlt lit) [])

    isValue env (Var v)
      | Just stuff <- lookupVarEnv env v
      = Just stuff	-- You might think we could look in the idUnfolding here
    		-- but that doesn't take account of which branch of a
    		-- case we are in, which is the whole point

      | not (isLocalId v) && isCheapUnfolding unf
      = isValue env (unfoldingTemplate unf)
      where
        unf = idUnfolding v
    	-- However we do want to consult the unfolding
    	-- as well, for let-bound constructors!

    isValue env (Lam b e)
      | isTyCoVar b = case isValue env e of
    		  Just _  -> Just LambdaVal
    		  Nothing -> Nothing
      | otherwise = Just LambdaVal

    isValue _env expr	-- Maybe it's a constructor application
      | (Var fun, args) <- collectArgs expr
      = case isDataConWorkId_maybe fun of

    	Just con | args `lengthAtLeast` dataConRepArity con
    		-- Check saturated; might be > because the
    		--		    arity excludes type args
    		-> Just (ConVal (DataAlt con) args)

    	_other | valArgCount args < idArity fun
    		-- Under-applied function
    	       -> Just LambdaVal	-- Partial application

    	_other -> Nothing

    isValue _env _expr = Nothing

    mk_con_app :: AltCon -> [CoreArg] -> CoreExpr
    mk_con_app (LitAlt lit)  []   = Lit lit
    mk_con_app (DataAlt con) args = mkConApp con args
    mk_con_app _other _args = panic "SpecConstr.mk_con_app"

    samePat :: CallPat -> CallPat -> Bool
    samePat (vs1, as1) (vs2, as2)
      = all2 same as1 as2
      where
        same (Var v1) (Var v2)
    	| v1 `elem` vs1 = v2 `elem` vs2
    	| v2 `elem` vs2 = False
    	| otherwise     = v1 == v2

        same (Lit l1)    (Lit l2)    = l1==l2
        same (App f1 a1) (App f2 a2) = same f1 f2 && same a1 a2

        same (Type {}) (Type {}) = True	-- Note [Ignore type differences]
        same (Note _ e1) e2	= same e1 e2	-- Ignore casts and notes
        same (Cast e1 _) e2	= same e1 e2
        same e1 (Note _ e2) = same e1 e2
        same e1 (Cast e2 _) = same e1 e2

        same e1 e2 = WARN( bad e1 || bad e2, ppr e1 $$ ppr e2)
    		 False 	-- Let, lambda, case should not occur
        bad (Case {}) = True
        bad (Let {})  = True
        bad (Lam {})  = True
        bad _other	  = False

\end{code} Note [Ignore type differences] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We do not want to generate specialisations where the call patterns differ only in their type arguments! Not only is it utterly useless, but it also means that (with polymorphic recursion) we can generate an infinite number of specialisations. Example is Data.Sequence.adjustTree, I think.