% % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998 % \section[Specialise]{Stamping out overloading, and (optionally) polymorphism} \begin{code}

    module Specialise ( specProgram ) where

    #include "HsVersions.h"

    import Id
    import TcType
    import CoreMonad
    import CoreSubst
    import CoreUnfold
    import VarSet
    import VarEnv
    import CoreSyn
    import Rules
    import CoreUtils	( exprIsTrivial, applyTypeToArgs, mkPiTypes )
    import CoreFVs		( exprFreeVars, exprsFreeVars, idFreeVars )
    import UniqSupply	( UniqSM, initUs_, MonadUnique(..) )
    import Name
    import MkId		( voidArgId, realWorldPrimId )
    import Maybes		( catMaybes, isJust )
    import BasicTypes
    import HscTypes
    import Bag
    import Util
    import Outputable
    import FastString

    import Data.Map (Map)
    import qualified Data.Map as Map
    import qualified FiniteMap as Map

\end{code} %************************************************************************ %* * \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]} %* * %************************************************************************ These notes describe how we implement specialisation to eliminate overloading. The specialisation pass works on Core syntax, complete with all the explicit dictionary application, abstraction and construction as added by the type checker. The existing type checker remains largely as it is. One important thought: the {\em types} passed to an overloaded function, and the {\em dictionaries} passed are mutually redundant. If the same function is applied to the same type(s) then it is sure to be applied to the same dictionary(s)---or rather to the same {\em values}. (The arguments might look different but they will evaluate to the same value.) Second important thought: we know that we can make progress by treating dictionary arguments as static and worth specialising on. So we can do without binding-time analysis, and instead specialise on dictionary arguments and no others. The basic idea ~~~~~~~~~~~~~~ Suppose we have let f = <f_rhs> in
and suppose f is overloaded. STEP 1: CALL-INSTANCE COLLECTION We traverse
, accumulating all applications of f to types and dictionaries. (Might there be partial applications, to just some of its types and dictionaries? In principle yes, but in practice the type checker only builds applications of f to all its types and dictionaries, so partial applications could only arise as a result of transformation, and even then I think it's unlikely. In any case, we simply don't accumulate such partial applications.) STEP 2: EQUIVALENCES So now we have a collection of calls to f: f t1 t2 d1 d2 f t3 t4 d3 d4 ... Notice that f may take several type arguments. To avoid ambiguity, we say that f is called at type t1/t2 and t3/t4. We take equivalence classes using equality of the *types* (ignoring the dictionary args, which as mentioned previously are redundant). STEP 3: SPECIALISATION For each equivalence class, choose a representative (f t1 t2 d1 d2), and create a local instance of f, defined thus: f@t1/t2 = <f_rhs> t1 t2 d1 d2 f_rhs presumably has some big lambdas and dictionary lambdas, so lots of simplification will now result. However we don't actually *do* that simplification. Rather, we leave it for the simplifier to do. If we *did* do it, though, we'd get more call instances from the specialised RHS. We can work out what they are by instantiating the call-instance set from f's RHS with the types t1, t2. Add this new id to f's IdInfo, to record that f has a specialised version. Before doing any of this, check that f's IdInfo doesn't already tell us about an existing instance of f at the required type/s. (This might happen if specialisation was applied more than once, or it might arise from user SPECIALIZE pragmas.) Recursion ~~~~~~~~~ Wait a minute! What if f is recursive? Then we can't just plug in its right-hand side, can we? But it's ok. The type checker *always* creates non-recursive definitions for overloaded recursive functions. For example: f x = f (x+x) -- Yes I know its silly becomes f a (d::Num a) = let p = +.sel a d in letrec fl (y::a) = fl (p y y) in fl We still have recusion for non-overloaded functions which we speciailise, but the recursive call should get specialised to the same recursive version. Polymorphism 1 ~~~~~~~~~~~~~~ All this is crystal clear when the function is applied to *constant types*; that is, types which have no type variables inside. But what if it is applied to non-constant types? Suppose we find a call of f at type t1/t2. There are two possibilities: (a) The free type variables of t1, t2 are in scope at the definition point of f. In this case there's no problem, we proceed just as before. A common example is as follows. Here's the Haskell: g y = let f x = x+x in f y + f y After typechecking we have g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x in +.sel a d (f a d y) (f a d y) Notice that the call to f is at type type "a"; a non-constant type. Both calls to f are at the same type, so we can specialise to give: g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x in +.sel a d (f@a y) (f@a y) (b) The other case is when the type variables in the instance types are *not* in scope at the definition point of f. The example we are working with above is a good case. There are two instances of (+.sel a d), but "a" is not in scope at the definition of +.sel. Can we do anything? Yes, we can "common them up", a sort of limited common sub-expression deal. This would give: g a (d::Num a) (y::a) = let +.sel@a = +.sel a d f@a (x::a) = +.sel@a x x in +.sel@a (f@a y) (f@a y) This can save work, and can't be spotted by the type checker, because the two instances of +.sel weren't originally at the same type. Further notes on (b) * There are quite a few variations here. For example, the defn of +.sel could be floated ouside the \y, to attempt to gain laziness. It certainly mustn't be floated outside the \d because the d has to be in scope too. * We don't want to inline f_rhs in this case, because that will duplicate code. Just commoning up the call is the point. * Nothing gets added to +.sel's IdInfo. * Don't bother unless the equivalence class has more than one item! Not clear whether this is all worth it. It is of course OK to simply discard call-instances when passing a big lambda. Polymorphism 2 -- Overloading ~~~~~~~~~~~~~~ Consider a function whose most general type is f :: forall a b. Ord a => [a] -> b -> b There is really no point in making a version of g at Int/Int and another at Int/Bool, because it's only instancing the type variable "a" which buys us any efficiency. Since g is completely polymorphic in b there ain't much point in making separate versions of g for the different b types. That suggests that we should identify which of g's type variables are constrained (like "a") and which are unconstrained (like "b"). Then when taking equivalence classes in STEP 2, we ignore the type args corresponding to unconstrained type variable. In STEP 3 we make polymorphic versions. Thus: f@t1/ = /\b -> <f_rhs> t1 b d1 d2 We do this. Dictionary floating ~~~~~~~~~~~~~~~~~~~ Consider this f a (d::Num a) = let g = ... in ...(let d1::Ord a = Num.Ord.sel a d in g a d1)... Here, g is only called at one type, but the dictionary isn't in scope at the definition point for g. Usually the type checker would build a definition for d1 which enclosed g, but the transformation system might have moved d1's defn inward. Solution: float dictionary bindings outwards along with call instances. Consider f x = let g p q = p==q h r s = (r+s, g r s) in h x x Before specialisation, leaving out type abstractions we have f df x = let g :: Eq a => a -> a -> Bool g dg p q = == dg p q h :: Num a => a -> a -> (a, Bool) h dh r s = let deq = eqFromNum dh in (+ dh r s, g deq r s) in h df x x After specialising h we get a specialised version of h, like this: h' r s = let deq = eqFromNum df in (+ df r s, g deq r s) But we can't naively make an instance for g from this, because deq is not in scope at the defn of g. Instead, we have to float out the (new) defn of deq to widen its scope. Notice that this floating can't be done in advance -- it only shows up when specialisation is done. User SPECIALIZE pragmas ~~~~~~~~~~~~~~~~~~~~~~~ Specialisation pragmas can be digested by the type checker, and implemented by adding extra definitions along with that of f, in the same way as before f@t1/t2 = <f_rhs> t1 t2 d1 d2 Indeed the pragmas *have* to be dealt with by the type checker, because only it knows how to build the dictionaries d1 and d2! For example g :: Ord a => [a] -> [a] {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-} Here, the specialised version of g is an application of g's rhs to the Ord dictionary for (Tree Int), which only the type checker can conjure up. There might not even *be* one, if (Tree Int) is not an instance of Ord! (All the other specialision has suitable dictionaries to hand from actual calls.) Problem. The type checker doesn't have to hand a convenient <f_rhs>, because it is buried in a complex (as-yet-un-desugared) binding group. Maybe we should say f@t1/t2 = f* t1 t2 d1 d2 where f* is the Id f with an IdInfo which says "inline me regardless!". Indeed all the specialisation could be done in this way. That in turn means that the simplifier has to be prepared to inline absolutely any in-scope let-bound thing. Again, the pragma should permit polymorphism in unconstrained variables: h :: Ord a => [a] -> b -> b {-# SPECIALIZE h :: [Int] -> b -> b #-} We *insist* that all overloaded type variables are specialised to ground types, (and hence there can be no context inside a SPECIALIZE pragma). We *permit* unconstrained type variables to be specialised to - a ground type - or left as a polymorphic type variable but nothing in between. So {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-} is *illegal*. (It can be handled, but it adds complication, and gains the programmer nothing.) SPECIALISING INSTANCE DECLARATIONS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider instance Foo a => Foo [a] where ... {-# SPECIALIZE instance Foo [Int] #-} The original instance decl creates a dictionary-function definition: dfun.Foo.List :: forall a. Foo a -> Foo [a] The SPECIALIZE pragma just makes a specialised copy, just as for ordinary function definitions: dfun.Foo.List@Int :: Foo [Int] dfun.Foo.List@Int = dfun.Foo.List Int dFooInt The information about what instance of the dfun exist gets added to the dfun's IdInfo in the same way as a user-defined function too. Automatic instance decl specialisation? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Can instance decls be specialised automatically? It's tricky. We could collect call-instance information for each dfun, but then when we specialised their bodies we'd get new call-instances for ordinary functions; and when we specialised their bodies, we might get new call-instances of the dfuns, and so on. This all arises because of the unrestricted mutual recursion between instance decls and value decls. Still, there's no actual problem; it just means that we may not do all the specialisation we could theoretically do. Furthermore, instance decls are usually exported and used non-locally, so we'll want to compile enough to get those specialisations done. Lastly, there's no such thing as a local instance decl, so we can survive solely by spitting out *usage* information, and then reading that back in as a pragma when next compiling the file. So for now, we only specialise instance decls in response to pragmas. SPITTING OUT USAGE INFORMATION ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ To spit out usage information we need to traverse the code collecting call-instance information for all imported (non-prelude?) functions and data types. Then we equivalence-class it and spit it out. This is done at the top-level when all the call instances which escape must be for imported functions and data types. *** Not currently done *** Partial specialisation by pragmas ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ What about partial specialisation: k :: (Ord a, Eq b) => [a] -> b -> b -> [a] {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-} or even {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-} Seems quite reasonable. Similar things could be done with instance decls: instance (Foo a, Foo b) => Foo (a,b) where ... {-# SPECIALIZE instance Foo a => Foo (a,Int) #-} {-# SPECIALIZE instance Foo b => Foo (Int,b) #-} Ho hum. Things are complex enough without this. I pass. Requirements for the simplifer ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The simplifier has to be able to take advantage of the specialisation. * When the simplifier finds an application of a polymorphic f, it looks in f's IdInfo in case there is a suitable instance to call instead. This converts f t1 t2 d1 d2 ===> f_t1_t2 Note that the dictionaries get eaten up too! * Dictionary selection operations on constant dictionaries must be short-circuited: +.sel Int d ===> +Int The obvious way to do this is in the same way as other specialised calls: +.sel has inside it some IdInfo which tells that if it's applied to the type Int then it should eat a dictionary and transform to +Int. In short, dictionary selectors need IdInfo inside them for constant methods. * Exactly the same applies if a superclass dictionary is being extracted: Eq.sel Int d ===> dEqInt * Something similar applies to dictionary construction too. Suppose dfun.Eq.List is the function taking a dictionary for (Eq a) to one for (Eq [a]). Then we want dfun.Eq.List Int d ===> dEq.List_Int Where does the Eq [Int] dictionary come from? It is built in response to a SPECIALIZE pragma on the Eq [a] instance decl. In short, dfun Ids need IdInfo with a specialisation for each constant instance of their instance declaration. All this uses a single mechanism: the SpecEnv inside an Id What does the specialisation IdInfo look like? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The SpecEnv of an Id maps a list of types (the template) to an expression [Type] |-> Expr For example, if f has this SpecInfo: [Int, a] -> \d:Ord Int. f' a it means that we can replace the call f Int t ===> (\d. f' t) This chucks one dictionary away and proceeds with the specialised version of f, namely f'. What can't be done this way? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There is no way, post-typechecker, to get a dictionary for (say) Eq a from a dictionary for Eq [a]. So if we find ==.sel [t] d we can't transform to eqList (==.sel t d') where eqList :: (a->a->Bool) -> [a] -> [a] -> Bool Of course, we currently have no way to automatically derive eqList, nor to connect it to the Eq [a] instance decl, but you can imagine that it might somehow be possible. Taking advantage of this is permanently ruled out. Still, this is no great hardship, because we intend to eliminate overloading altogether anyway! A note about non-tyvar dictionaries ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Some Ids have types like forall a,b,c. Eq a -> Ord [a] -> tau This seems curious at first, because we usually only have dictionary args whose types are of the form (C a) where a is a type variable. But this doesn't hold for the functions arising from instance decls, which sometimes get arguements with types of form (C (T a)) for some type constructor T. Should we specialise wrt this compound-type dictionary? We used to say "no", saying: "This is a heuristic judgement, as indeed is the fact that we specialise wrt only dictionaries. We choose *not* to specialise wrt compound dictionaries because at the moment the only place they show up is in instance decls, where they are simply plugged into a returned dictionary. So nothing is gained by specialising wrt them." But it is simpler and more uniform to specialise wrt these dicts too; and in future GHC is likely to support full fledged type signatures like f :: Eq [(a,b)] => ... %************************************************************************ %* * \subsubsection{The new specialiser} %* * %************************************************************************ Our basic game plan is this. For let(rec) bound function f :: (C a, D c) => (a,b,c,d) -> Bool * Find any specialised calls of f, (f ts ds), where ts are the type arguments t1 .. t4, and ds are the dictionary arguments d1 .. d2. * Add a new definition for f1 (say): f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2 Note that we abstract over the unconstrained type arguments. * Add the mapping [t1,b,t3,d] |-> \d1 d2 -> f1 b d to the specialisations of f. This will be used by the simplifier to replace calls (f t1 t2 t3 t4) da db by (\d1 d1 -> f1 t2 t4) da db All the stuff about how many dictionaries to discard, and what types to apply the specialised function to, are handled by the fact that the SpecEnv contains a template for the result of the specialisation. We don't build *partial* specialisations for f. For example: f :: Eq a => a -> a -> Bool {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-} Here, little is gained by making a specialised copy of f. There's a distinct danger that the specialised version would first build a dictionary for (Eq b, Eq c), and then select the (==) method from it! Even if it didn't, not a great deal is saved. We do, however, generate polymorphic, but not overloaded, specialisations: f :: Eq a => [a] -> b -> b -> b {#- SPECIALISE f :: [Int] -> b -> b -> b #-} Hence, the invariant is this: *** no specialised version is overloaded *** %************************************************************************ %* * \subsubsection{The exported function} %* * %************************************************************************ \begin{code}

    specProgram :: ModGuts -> CoreM ModGuts
    specProgram guts
      = do { hpt_rules <- getRuleBase
           ; let local_rules = mg_rules guts
                 rule_base = extendRuleBaseList hpt_rules (mg_rules guts)

    	     -- Specialise the bindings of this module
           ; (binds', uds) <- runSpecM (go (mg_binds guts))

    	     -- Specialise imported functions
           ; (new_rules, spec_binds) <- specImports emptyVarSet rule_base uds

           ; let final_binds | null spec_binds = binds'
                             | otherwise       = Rec (flattenBinds spec_binds) : binds'
        	   	   -- Note [Glom the bindings if imported functions are specialised]

           ; return (guts { mg_binds = final_binds
                          , mg_rules = new_rules ++ local_rules }) }
      where
    	-- We need to start with a Subst that knows all the things
    	-- that are in scope, so that the substitution engine doesn't
    	-- accidentally re-use a unique that's already in use
    	-- Easiest thing is to do it all at once, as if all the top-level
    	-- decls were mutually recursive
        top_subst = mkEmptySubst $ mkInScopeSet $ mkVarSet $
                    bindersOfBinds $ mg_binds guts

        go []           = return ([], emptyUDs)
        go (bind:binds) = do (binds', uds) <- go binds
                             (bind', uds') <- specBind top_subst bind uds
                             return (bind' ++ binds', uds')

    specImports :: VarSet	 	-- Don't specialise these ones
    	       			-- See Note [Avoiding recursive specialisation]
                -> RuleBase	 	-- Rules from this module and the home package
    	       			-- (but not external packages, which can change)
                -> UsageDetails	-- Calls for imported things, and floating bindings
                -> CoreM ( [CoreRule]   -- New rules
                         , [CoreBind] ) -- Specialised bindings and floating bindings
    -- See Note [Specialise imported INLINABLE things]
    specImports done rb uds
      = do { let import_calls = varEnvElts (ud_calls uds)
           ; (rules, spec_binds) <- go rb import_calls
           ; return (rules, wrapDictBinds (ud_binds uds) spec_binds) }
      where
        go _ [] = return ([], [])
        go rb (CIS fn calls_for_fn : other_calls)
          = do { (rules1, spec_binds1) <- specImport done rb fn (Map.toList calls_for_fn)
               ; (rules2, spec_binds2) <- go (extendRuleBaseList rb rules1) other_calls
               ; return (rules1 ++ rules2, spec_binds1 ++ spec_binds2) }

    specImport :: VarSet	    	    -- Don't specialise these
    	       			    -- See Note [Avoiding recursive specialisation]
               -> RuleBase	    	    -- Rules from this module
               -> Id -> [CallInfo]	    -- Imported function and calls for it
               -> CoreM ( [CoreRule]    -- New rules
                        , [CoreBind] )  -- Specialised bindings
    specImport done rb fn calls_for_fn
      | fn `elemVarSet` done
      = return ([], [])     -- No warning.  This actually happens all the time
        	   		-- when specialising a recursive function, becuase
    			-- the RHS of the specialised function contains a recursive
    			-- call to the original function

      | isInlinablePragma (idInlinePragma fn)
      , Just rhs <- maybeUnfoldingTemplate (realIdUnfolding fn)
      = do {     -- Get rules from the external package state
           	     -- We keep doing this in case we "page-fault in"
    	     -- more rules as we go along
           ; hsc_env <- getHscEnv
           ; eps <- liftIO $ hscEPS hsc_env
           ; let full_rb = unionRuleBase rb (eps_rule_base eps)
                 rules_for_fn = getRules full_rb fn

           ; (rules1, spec_pairs, uds) <- runSpecM $
                  specCalls emptySubst rules_for_fn calls_for_fn fn rhs
           ; let spec_binds1 = [NonRec b r | (b,r) <- spec_pairs]
           	     -- After the rules kick in we may get recursion, but
    	     -- we rely on a global GlomBinds to sort that out later
                 -- See Note [Glom the bindings if imported functions are specialised]

    	      -- Now specialise any cascaded calls
           ; (rules2, spec_binds2) <- specImports (extendVarSet done fn)
                                                  (extendRuleBaseList rb rules1)
                                                  uds

           ; return (rules2 ++ rules1, spec_binds2 ++ spec_binds1) }

      | otherwise
      = WARN( True, ptext (sLit "specImport discard") <+> ppr fn <+> ppr calls_for_fn )
        return ([], [])

\end{code} Note [Specialise imported INLINABLE things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We specialise INLINABLE things but not INLINE things. The latter should be inlined bodily, so not much point in specialising them. Moreover, we risk lots of orphan modules from vigorous specialisation. Note [Glom the bindings if imported functions are specialised] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have an imported, *recursive*, INLINABLE function f :: Eq a => a -> a f = /\a \d x. ...(f a d)... In the module being compiled we have g x = f (x::Int) Now we'll make a specialised function f_spec :: Int -> Int f_spec = \x -> ...(f Int dInt)... {-# RULE f Int _ = f_spec #-} g = \x. f Int dInt x Note that f_spec doesn't look recursive After rewriting with the RULE, we get f_spec = \x -> ...(f_spec)... BUT since f_spec was non-recursive before it'll *stay* non-recursive. The occurrence analyser never turns a NonRec into a Rec. So we must make sure that f_spec is recursive. Easiest thing is to make all the specialisations for imported bindings recursive. Note [Avoiding recursive specialisation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we specialise 'f' we may find new overloaded calls to 'g', 'h' in 'f's RHS. So we want to specialise g,h. But we don't want to specialise f any more! It's possible that f's RHS might have a recursive yet-more-specialised call, so we'd diverge in that case. And if the call is to the same type, one specialisation is enough. Avoiding this recursive specialisation loop is the reason for the 'done' VarSet passed to specImports and specImport. %************************************************************************ %* * \subsubsection{@specExpr@: the main function} %* * %************************************************************************ \begin{code}

    specVar :: Subst -> Id -> CoreExpr
    specVar subst v = lookupIdSubst (text "specVar") subst v

    specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
    -- We carry a substitution down:
    --	a) we must clone any binding that might float outwards,
    --	   to avoid name clashes
    --	b) we carry a type substitution to use when analysing
    --	   the RHS of specialised bindings (no type-let!)

    ---------------- First the easy cases --------------------
    specExpr subst (Type ty) = return (Type (CoreSubst.substTy subst ty), emptyUDs)
    specExpr subst (Var v)   = return (specVar subst v,         emptyUDs)
    specExpr _     (Lit lit) = return (Lit lit,                 emptyUDs)
    specExpr subst (Cast e co) = do
        (e', uds) <- specExpr subst e
        return ((Cast e' (CoreSubst.substTy subst co)), uds)
    specExpr subst (Note note body) = do
        (body', uds) <- specExpr subst body
        return (Note (specNote subst note) body', uds)

    ---------------- Applications might generate a call instance --------------------
    specExpr subst expr@(App {})
      = go expr []
      where
        go (App fun arg) args = do (arg', uds_arg) <- specExpr subst arg
                                   (fun', uds_app) <- go fun (arg':args)
                                   return (App fun' arg', uds_arg `plusUDs` uds_app)

        go (Var f)       args = case specVar subst f of
                                    Var f' -> return (Var f', mkCallUDs f' args)
                                    e'     -> return (e', emptyUDs)	-- I don't expect this!
        go other	     _    = specExpr subst other

    ---------------- Lambda/case require dumping of usage details --------------------
    specExpr subst e@(Lam _ _) = do
        (body', uds) <- specExpr subst' body
        let (free_uds, dumped_dbs) = dumpUDs bndrs' uds
        return (mkLams bndrs' (wrapDictBindsE dumped_dbs body'), free_uds)
      where
        (bndrs, body) = collectBinders e
        (subst', bndrs') = substBndrs subst bndrs
    	-- More efficient to collect a group of binders together all at once
    	-- and we don't want to split a lambda group with dumped bindings

    specExpr subst (Case scrut case_bndr ty alts)
      = do { (scrut', scrut_uds) <- specExpr subst scrut
           ; (scrut'', case_bndr', alts', alts_uds)
                 <- specCase subst scrut' case_bndr alts
           ; return (Case scrut'' case_bndr' (CoreSubst.substTy subst ty) alts'
                    , scrut_uds `plusUDs` alts_uds) }

    ---------------- Finally, let is the interesting case --------------------
    specExpr subst (Let bind body) = do
       	-- Clone binders
        (rhs_subst, body_subst, bind') <- cloneBindSM subst bind

            -- Deal with the body
        (body', body_uds) <- specExpr body_subst body

            -- Deal with the bindings
        (binds', uds) <- specBind rhs_subst bind' body_uds

            -- All done
        return (foldr Let body' binds', uds)

    -- Must apply the type substitution to coerceions
    specNote :: Subst -> Note -> Note
    specNote _ note = note

    specCase :: Subst
             -> CoreExpr	 	-- Scrutinee, already done
             -> Id -> [CoreAlt]
             -> SpecM ( CoreExpr	-- New scrutinee
    	    	  , Id
    	    	  , [CoreAlt]
                      , UsageDetails)
    specCase subst scrut' case_bndr [(con, args, rhs)]
      | isDictId case_bndr  	 -- See Note [Floating dictionaries out of cases]
      , interestingDict scrut'
      , not (isDeadBinder case_bndr && null sc_args')
      = do { (case_bndr_flt : sc_args_flt) <- mapM clone_me (case_bndr' : sc_args')

           ; let sc_rhss = [ Case (Var case_bndr_flt) case_bndr' (idType sc_arg')
                                  [(con, args', Var sc_arg')]
                           | sc_arg' <- sc_args' ]

    	     -- Extend the substitution for RHS to map the *original* binders
    	     -- to their floated verions.  Attach an unfolding to these floated
    	     -- binders so they look interesting to interestingDict
    	     mb_sc_flts :: [Maybe DictId]
                 mb_sc_flts = map (lookupVarEnv clone_env) args'
                 clone_env  = zipVarEnv sc_args' (zipWith add_unf sc_args_flt sc_rhss)
                 subst_prs  = (case_bndr, Var (add_unf case_bndr_flt scrut'))
                            : [ (arg, Var sc_flt)
                              | (arg, Just sc_flt) <- args `zip` mb_sc_flts ]
                 subst_rhs' = extendIdSubstList subst_rhs subst_prs

           ; (rhs',   rhs_uds)   <- specExpr subst_rhs' rhs
           ; let scrut_bind    = mkDB (NonRec case_bndr_flt scrut')
                 case_bndr_set = unitVarSet case_bndr_flt
                 sc_binds      = [(NonRec sc_arg_flt sc_rhs, case_bndr_set)
                                 | (sc_arg_flt, sc_rhs) <- sc_args_flt `zip` sc_rhss ]
                 flt_binds     = scrut_bind : sc_binds
                 (free_uds, dumped_dbs) = dumpUDs (case_bndr':args') rhs_uds
                 all_uds = flt_binds `addDictBinds` free_uds
                 alt'    = (con, args', wrapDictBindsE dumped_dbs rhs')
           ; return (Var case_bndr_flt, case_bndr', [alt'], all_uds) }
      where
        (subst_rhs, (case_bndr':args')) = substBndrs subst (case_bndr:args)
        sc_args' = filter is_flt_sc_arg args'

        clone_me bndr = do { uniq <- getUniqueM
                           ; return (mkUserLocal occ uniq ty loc) }
           where
             name = idName bndr
             ty   = idType bndr
             occ  = nameOccName name
             loc  = getSrcSpan name

        add_unf sc_flt sc_rhs  -- Sole purpose: make sc_flt respond True to interestingDictId
          = setIdUnfolding sc_flt (mkSimpleUnfolding sc_rhs)

        arg_set = mkVarSet args'
        is_flt_sc_arg var =  isId var
                          && not (isDeadBinder var)
    	              && isDictTy var_ty
                          && not (tyVarsOfType var_ty `intersectsVarSet` arg_set)
           where
             var_ty = idType var

    specCase subst scrut case_bndr alts
      = do { (alts', uds_alts) <- mapAndCombineSM spec_alt alts
           ; return (scrut, case_bndr', alts', uds_alts) }
      where
        (subst_alt, case_bndr') = substBndr subst case_bndr
        spec_alt (con, args, rhs) = do
              (rhs', uds) <- specExpr subst_rhs rhs
              let (free_uds, dumped_dbs) = dumpUDs (case_bndr' : args') uds
              return ((con, args', wrapDictBindsE dumped_dbs rhs'), free_uds)
            where
              (subst_rhs, args') = substBndrs subst_alt args

\end{code} Note [Floating dictionaries out of cases] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider g = \d. case d of { MkD sc ... -> ...(f sc)... } Naively we can't float d2's binding out of the case expression, because 'sc' is bound by the case, and that in turn means we can't specialise f, which seems a pity. So we invert the case, by floating out a binding for 'sc_flt' thus: sc_flt = case d of { MkD sc ... -> sc } Now we can float the call instance for 'f'. Indeed this is just what'll happen if 'sc' was originally bound with a let binding, but case is more efficient, and necessary with equalities. So it's good to work with both. You might think that this won't make any difference, because the call instance will only get nuked by the \d. BUT if 'g' itself is specialised, then transitively we should be able to specialise f. In general, given case e of cb { MkD sc ... -> ...(f sc)... } we transform to let cb_flt = e sc_flt = case cb_flt of { MkD sc ... -> sc } in case cb_flt of bg { MkD sc ... -> ....(f sc_flt)... } The "_flt" things are the floated binds; we use the current substitution to substitute sc -> sc_flt in the RHS %************************************************************************ %* * Dealing with a binding %* * %************************************************************************ \begin{code}

    specBind :: Subst			-- Use this for RHSs
    	 -> CoreBind
    	 -> UsageDetails		-- Info on how the scope of the binding
    	 -> SpecM ([CoreBind],		-- New bindings
    		   UsageDetails)	-- And info to pass upstream

    -- Returned UsageDetails:
    --    No calls for binders of this bind
    specBind rhs_subst (NonRec fn rhs) body_uds
      = do { (rhs', rhs_uds) <- specExpr rhs_subst rhs
           ; (fn', spec_defns, body_uds1) <- specDefn rhs_subst body_uds fn rhs

           ; let pairs = spec_defns ++ [(fn', rhs')]
    	 		-- fn' mentions the spec_defns in its rules,
    			-- so put the latter first

                 combined_uds = body_uds1 `plusUDs` rhs_uds
    		-- This way round a call in rhs_uds of a function f
    		-- at type T will override a call of f at T in body_uds1; and
    		-- that is good because it'll tend to keep "earlier" calls
    		-- See Note [Specialisation of dictionary functions]

    	     (free_uds, dump_dbs, float_all) = dumpBindUDs [fn] combined_uds
    	        -- See Note [From non-recursive to recursive]

                 final_binds | isEmptyBag dump_dbs = [NonRec b r | (b,r) <- pairs]
                             | otherwise = [Rec (flattenDictBinds dump_dbs pairs)]

    	 ; if float_all then
    	     -- Rather than discard the calls mentioning the bound variables
    	     -- we float this binding along with the others
    	      return ([], free_uds `snocDictBinds` final_binds)
               else
    	     -- No call in final_uds mentions bound variables,
    	     -- so we can just leave the binding here
    	      return (final_binds, free_uds) }

    specBind rhs_subst (Rec pairs) body_uds
           -- Note [Specialising a recursive group]
      = do { let (bndrs,rhss) = unzip pairs
           ; (rhss', rhs_uds) <- mapAndCombineSM (specExpr rhs_subst) rhss
           ; let scope_uds = body_uds `plusUDs` rhs_uds
           	     	       -- Includes binds and calls arising from rhss

           ; (bndrs1, spec_defns1, uds1) <- specDefns rhs_subst scope_uds pairs

           ; (bndrs3, spec_defns3, uds3)
                 <- if null spec_defns1  -- Common case: no specialisation
    	        then return (bndrs1, [], uds1)
    		else do {      	     -- Specialisation occurred; do it again
                              (bndrs2, spec_defns2, uds2)
                                  <- specDefns rhs_subst uds1 (bndrs1 `zip` rhss)
                            ; return (bndrs2, spec_defns2 ++ spec_defns1, uds2) }

           ; let (final_uds, dumped_dbs, float_all) = dumpBindUDs bndrs uds3
                 bind = Rec (flattenDictBinds dumped_dbs $
                             spec_defns3 ++ zip bndrs3 rhss')

           ; if float_all then
    	      return ([], final_uds `snocDictBind` bind)
               else
    	      return ([bind], final_uds) }

    ---------------------------
    specDefns :: Subst
    	  -> UsageDetails		-- Info on how it is used in its scope
    	  -> [(Id,CoreExpr)]		-- The things being bound and their un-processed RHS
    	  -> SpecM ([Id],		-- Original Ids with RULES added
    	     	    [(Id,CoreExpr)],	-- Extra, specialised bindings
    		    UsageDetails)	-- Stuff to fling upwards from the specialised versions

    -- Specialise a list of bindings (the contents of a Rec), but flowing usages
    -- upwards binding by binding.  Example: { f = ...g ...; g = ...f .... }
    -- Then if the input CallDetails has a specialised call for 'g', whose specialisation
    -- in turn generates a specialised call for 'f', we catch that in this one sweep.
    -- But not vice versa (it's a fixpoint problem).

    specDefns _subst uds []
      = return ([], [], uds)
    specDefns subst uds ((bndr,rhs):pairs)
      = do { (bndrs1, spec_defns1, uds1) <- specDefns subst uds pairs
           ; (bndr1, spec_defns2, uds2)  <- specDefn subst uds1 bndr rhs
           ; return (bndr1 : bndrs1, spec_defns1 ++ spec_defns2, uds2) }

    ---------------------------
    specDefn :: Subst
    	 -> UsageDetails		-- Info on how it is used in its scope
    	 -> Id -> CoreExpr		-- The thing being bound and its un-processed RHS
    	 -> SpecM (Id,			-- Original Id with added RULES
    		   [(Id,CoreExpr)],	-- Extra, specialised bindings
    	    	   UsageDetails)	-- Stuff to fling upwards from the specialised versions

    specDefn subst body_uds fn rhs
      = do { let (body_uds_without_me, calls_for_me) = callsForMe fn body_uds
                 rules_for_me = idCoreRules fn
           ; (rules, spec_defns, spec_uds) <- specCalls subst rules_for_me
                                                        calls_for_me fn rhs
           ; return ( fn `addIdSpecialisations` rules
                    , spec_defns
                    , body_uds_without_me `plusUDs` spec_uds) }
    		-- It's important that the `plusUDs` is this way
    		-- round, because body_uds_without_me may bind
    		-- dictionaries that are used in calls_for_me passed
    		-- to specDefn.  So the dictionary bindings in
    		-- spec_uds may mention dictionaries bound in
    		-- body_uds_without_me

    ---------------------------
    specCalls :: Subst
              -> [CoreRule]			-- Existing RULES for the fn
    	  -> [CallInfo]
    	  -> Id -> CoreExpr
    	  -> SpecM ([CoreRule],		-- New RULES for the fn
    		    [(Id,CoreExpr)],	-- Extra, specialised bindings
    	    	    UsageDetails)	-- New usage details from the specialised RHSs

    -- This function checks existing rules, and does not create
    -- duplicate ones. So the caller does not need to do this filtering.
    -- See 'already_covered'

    specCalls subst rules_for_me calls_for_me fn rhs
    	-- The first case is the interesting one
      |  rhs_tyvars `lengthIs`     n_tyvars -- Rhs of fn's defn has right number of big lambdas
      && rhs_ids    `lengthAtLeast` n_dicts	-- and enough dict args
      && notNull calls_for_me		-- And there are some calls to specialise
      && not (isNeverActive (idInlineActivation fn))
    	-- Don't specialise NOINLINE things
    	-- See Note [Auto-specialisation and RULES]

    --   && not (certainlyWillInline (idUnfolding fn))	-- And it's not small
    --	See Note [Inline specialisation] for why we do not
    --	switch off specialisation for inline functions

      = -- pprTrace "specDefn: some" (ppr fn $$ ppr calls_for_me $$ ppr rules_for_me) $
        do { stuff <- mapM spec_call calls_for_me
           ; let (spec_defns, spec_uds, spec_rules) = unzip3 (catMaybes stuff)
           ; return (spec_rules, spec_defns, plusUDList spec_uds) }

      | otherwise	-- No calls or RHS doesn't fit our preconceptions
      = WARN( notNull calls_for_me, ptext (sLit "Missed specialisation opportunity for")
                                     <+> ppr fn $$ _trace_doc )
    	  -- Note [Specialisation shape]
        -- pprTrace "specDefn: none" (ppr fn $$ ppr calls_for_me) $
        return ([], [], emptyUDs)
      where
        _trace_doc = vcat [ ppr rhs_tyvars, ppr n_tyvars
                          , ppr rhs_ids, ppr n_dicts
                          , ppr (idInlineActivation fn) ]

        fn_type	       = idType fn
        fn_arity	       = idArity fn
        fn_unf             = realIdUnfolding fn	-- Ignore loop-breaker-ness here
        (tyvars, theta, _) = tcSplitSigmaTy fn_type
        n_tyvars	       = length tyvars
        n_dicts	       = length theta
        inl_prag           = idInlinePragma fn
        inl_act            = inlinePragmaActivation inl_prag
        is_local           = isLocalId fn

    	-- Figure out whether the function has an INLINE pragma
    	-- See Note [Inline specialisations]

        spec_arity = unfoldingArity fn_unf - n_dicts  -- Arity of the *specialised* inline rule

        (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs

        rhs_dict_ids = take n_dicts rhs_ids
        body         = mkLams (drop n_dicts rhs_ids) rhs_body
    		-- Glue back on the non-dict lambdas

        already_covered :: [CoreExpr] -> Bool
        already_covered args	  -- Note [Specialisations already covered]
           = isJust (lookupRule (const True) realIdUnfolding
                                (substInScope subst)
           	 		    fn args rules_for_me)

        mk_ty_args :: [Maybe Type] -> [CoreExpr]
        mk_ty_args call_ts = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
        	       where
    	          mk_ty_arg rhs_tyvar Nothing   = Type (mkTyVarTy rhs_tyvar)
    		  mk_ty_arg _	      (Just ty) = Type ty

        ----------------------------------------------------------
    	-- Specialise to one particular call pattern
        spec_call :: CallInfo                         -- Call instance
                  -> SpecM (Maybe ((Id,CoreExpr),	  -- Specialised definition
    	                       UsageDetails, 	  -- Usage details from specialised body
    			       CoreRule))	  -- Info for the Id's SpecEnv
        spec_call (CallKey call_ts, (call_ds, _))
          = ASSERT( call_ts `lengthIs` n_tyvars  && call_ds `lengthIs` n_dicts )

    	-- Suppose f's defn is 	f = /\ a b c -> \ d1 d2 -> rhs
            -- Supppose the call is for f [Just t1, Nothing, Just t3] [dx1, dx2]

    	-- Construct the new binding
    	-- 	f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b -> rhs)
    	-- PLUS the usage-details
    	--	{ d1' = dx1; d2' = dx2 }
    	-- where d1', d2' are cloned versions of d1,d2, with the type substitution
    	-- applied.  These auxiliary bindings just avoid duplication of dx1, dx2
    	--
    	-- Note that the substitution is applied to the whole thing.
    	-- This is convenient, but just slightly fragile.  Notably:
    	--	* There had better be no name clashes in a/b/c
            do { let
    		-- poly_tyvars = [b] in the example above
    		-- spec_tyvars = [a,c]
    		-- ty_args     = [t1,b,t3]
    		poly_tyvars   = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
               	spec_tv_binds = [(tv,ty) | (tv, Just ty) <- rhs_tyvars `zip` call_ts]
    		spec_ty_args  = map snd spec_tv_binds
    	   	ty_args       = mk_ty_args call_ts
    	   	rhs_subst     = CoreSubst.extendTvSubstList subst spec_tv_binds

    	   ; (rhs_subst1, inst_dict_ids) <- newDictBndrs rhs_subst rhs_dict_ids
    	     		  -- Clone rhs_dicts, including instantiating their types

    	   ; let (rhs_subst2, dx_binds) = bindAuxiliaryDicts rhs_subst1 $
    	     	 	      		  (my_zipEqual rhs_dict_ids inst_dict_ids call_ds)
    	         inst_args = ty_args ++ map Var inst_dict_ids

    	   ; if already_covered inst_args then
    	        return Nothing
    	     else do
    	   {	-- Figure out the type of the specialised function
    	     let body_ty = applyTypeToArgs rhs fn_type inst_args
    	         (lam_args, app_args) 		-- Add a dummy argument if body_ty is unlifted
    		   | isUnLiftedType body_ty	-- C.f. WwLib.mkWorkerArgs
    		   = (poly_tyvars ++ [voidArgId], poly_tyvars ++ [realWorldPrimId])
    		   | otherwise = (poly_tyvars, poly_tyvars)
    	         spec_id_ty = mkPiTypes lam_args body_ty

               ; spec_f <- newSpecIdSM fn spec_id_ty
               ; (spec_rhs, rhs_uds) <- specExpr rhs_subst2 (mkLams lam_args body)
    	   ; let
    		-- The rule to put in the function's specialisation is:
    		--	forall b, d1',d2'.  f t1 b t3 d1' d2' = f1 b
    	        rule_name = mkFastString ("SPEC " ++ showSDoc (ppr fn <+> ppr spec_ty_args))
      		spec_env_rule = mkRule True {- Auto generated -} is_local
                                      rule_name
    				  inl_act 	-- Note [Auto-specialisation and RULES]
    			 	  (idName fn)
    			          (poly_tyvars ++ inst_dict_ids)
    				  inst_args
    				  (mkVarApps (Var spec_f) app_args)

    		-- Add the { d1' = dx1; d2' = dx2 } usage stuff
    	   	final_uds = foldr consDictBind rhs_uds dx_binds

    		--------------------------------------
    		-- Add a suitable unfolding if the spec_inl_prag says so
    		-- See Note [Inline specialisations]
    		spec_inl_prag
    		  = case inl_prag of
                           InlinePragma { inl_inline = Inlinable }
                              -> inl_prag { inl_inline = EmptyInlineSpec }
    		       _  -> inl_prag

    		spec_unf
                      = case inlinePragmaSpec spec_inl_prag of
                          Inline    -> mkInlineUnfolding (Just spec_arity) spec_rhs
                          Inlinable -> mkInlinableUnfolding spec_rhs
                          _         -> NoUnfolding

    		--------------------------------------
    		-- Adding arity information just propagates it a bit faster
    		-- 	See Note [Arity decrease] in Simplify
    		-- Copy InlinePragma information from the parent Id.
    		-- So if f has INLINE[1] so does spec_f
      	        spec_f_w_arity = spec_f `setIdArity`      max 0 (fn_arity - n_dicts)
                                            `setInlinePragma` spec_inl_prag
                                            `setIdUnfolding`  spec_unf

    	   ; return (Just ((spec_f_w_arity, spec_rhs), final_uds, spec_env_rule)) } }
          where
    	my_zipEqual xs ys zs
    	 | debugIsOn && not (equalLength xs ys && equalLength ys zs)
                 = pprPanic "my_zipEqual" (vcat [ ppr xs, ppr ys
    					    , ppr fn <+> ppr call_ts
    					    , ppr (idType fn), ppr theta
    					    , ppr n_dicts, ppr rhs_dict_ids
    					    , ppr rhs])
        	 | otherwise = zip3 xs ys zs

    bindAuxiliaryDicts
    	:: Subst
    	-> [(DictId,DictId,CoreExpr)]	-- (orig_dict, inst_dict, dx)
    	-> (Subst,			-- Substitute for all orig_dicts
    	    [CoreBind])			-- Auxiliary bindings
    -- Bind any dictionary arguments to fresh names, to preserve sharing
    -- Substitution already substitutes orig_dict -> inst_dict
    bindAuxiliaryDicts subst triples = go subst [] triples
      where
        go subst binds []    = (subst, binds)
        go subst binds ((d, dx_id, dx) : pairs)
          | exprIsTrivial dx = go (extendIdSubst subst d dx) binds pairs
                 -- No auxiliary binding necessary
    	     -- Note that we bind the *original* dict in the substitution,
    	     -- overriding any d->dx_id binding put there by substBndrs

          | otherwise        = go subst_w_unf (NonRec dx_id dx : binds) pairs
          where
            dx_id1 = dx_id `setIdUnfolding` mkSimpleUnfolding dx
    	subst_w_unf = extendIdSubst subst d (Var dx_id1)
           	     -- Important!  We're going to substitute dx_id1 for d
    	     -- and we want it to look "interesting", else we won't gather *any*
    	     -- consequential calls. E.g.
    	     --	    f d = ...g d....
    	     -- If we specialise f for a call (f (dfun dNumInt)), we'll get
    	     -- a consequent call (g d') with an auxiliary definition
    	     --	    d' = df dNumInt
      	     -- We want that consequent call to look interesting
    	     --
    	     -- Again, note that we bind the *original* dict in the substitution,
    	     -- overriding any d->dx_id binding put there by substBndrs

\end{code} Note [From non-recursive to recursive] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Even in the non-recursive case, if any dict-binds depend on 'fn' we might have built a recursive knot f a d x = <blah> MkUD { ud_binds = d7 = MkD ..f.. , ud_calls = ...(f T d7)... } The we generate Rec { fs x = <blah>[T/a, d7/d] f a d x = <blah> RULE f T _ = fs d7 = ...f... } Here the recursion is only through the RULE. Note [Specialisation of dictionary functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is a nasty example that bit us badly: see Trac #3591 dfun a d = MkD a d (meth d) d4 = <blah> d2 = dfun T d4 d1 = $p1 d2 d3 = dfun T d1 None of these definitions is recursive. What happened was that we generated a specialisation: RULE forall d. dfun T d = dT dT = (MkD a d (meth d)) [T/a, d1/d] = MkD T d1 (meth d1) But now we use the RULE on the RHS of d2, to get d2 = dT = MkD d1 (meth d1) d1 = $p1 d2 and now d1 is bottom! The problem is that when specialising 'dfun' we should first dump "below" the binding all floated dictionary bindings that mention 'dfun' itself. So d2 and d3 (and hence d1) must be placed below 'dfun', and thus unavailable to it when specialising 'dfun'. That in turn means that the call (dfun T d1) must be discarded. On the other hand, the call (dfun T d4) is fine, assuming d4 doesn't mention dfun. But look at this: class C a where { foo,bar :: [a] -> [a] } instance C Int where foo x = r_bar x bar xs = reverse xs r_bar :: C a => [a] -> [a] r_bar xs = bar (xs ++ xs) That translates to: r_bar a (c::C a) (xs::[a]) = bar a d (xs ++ xs) Rec { $fCInt :: C Int = MkC foo_help reverse foo_help (xs::[Int]) = r_bar Int $fCInt xs } The call (r_bar $fCInt) mentions $fCInt, which mentions foo_help, which mentions r_bar But we DO want to specialise r_bar at Int: Rec { $fCInt :: C Int = MkC foo_help reverse foo_help (xs::[Int]) = r_bar Int $fCInt xs r_bar a (c::C a) (xs::[a]) = bar a d (xs ++ xs) RULE r_bar Int _ = r_bar_Int r_bar_Int xs = bar Int $fCInt (xs ++ xs) } Note that, because of its RULE, r_bar joins the recursive group. (In this case it'll unravel a short moment later.) Conclusion: we catch the nasty case using filter_dfuns in callsForMe. To be honest I'm not 100% certain that this is 100% right, but it works. Sigh. Note [Specialising a recursive group] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider let rec { f x = ...g x'... ; g y = ...f y'.... } in f 'a' Here we specialise 'f' at Char; but that is very likely to lead to a specialisation of 'g' at Char. We must do the latter, else the whole point of specialisation is lost. But we do not want to keep iterating to a fixpoint, because in the presence of polymorphic recursion we might generate an infinite number of specialisations. So we use the following heuristic: * Arrange the rec block in dependency order, so far as possible (the occurrence analyser already does this) * Specialise it much like a sequence of lets * Then go through the block a second time, feeding call-info from the RHSs back in the bottom, as it were In effect, the ordering maxmimises the effectiveness of each sweep, and we do just two sweeps. This should catch almost every case of monomorphic recursion -- the exception could be a very knotted-up recursion with multiple cycles tied up together. This plan is implemented in the Rec case of specBindItself. Note [Specialisations already covered] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We obviously don't want to generate two specialisations for the same argument pattern. There are two wrinkles 1. We do the already-covered test in specDefn, not when we generate the CallInfo in mkCallUDs. We used to test in the latter place, but we now iterate the specialiser somewhat, and the Id at the call site might therefore not have all the RULES that we can see in specDefn 2. What about two specialisations where the second is an *instance* of the first? If the more specific one shows up first, we'll generate specialisations for both. If the *less* specific one shows up first, we *don't* currently generate a specialisation for the more specific one. (See the call to lookupRule in already_covered.) Reasons: (a) lookupRule doesn't say which matches are exact (bad reason) (b) if the earlier specialisation is user-provided, it's far from clear that we should auto-specialise further Note [Auto-specialisation and RULES] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider: g :: Num a => a -> a g = ... f :: (Int -> Int) -> Int f w = ... {-# RULE f g = 0 #-} Suppose that auto-specialisation makes a specialised version of g::Int->Int That version won't appear in the LHS of the RULE for f. So if the specialisation rule fires too early, the rule for f may never fire. It might be possible to add new rules, to "complete" the rewrite system. Thus when adding RULE forall d. g Int d = g_spec also add RULE f g_spec = 0 But that's a bit complicated. For now we ask the programmer's help, by *copying the INLINE activation pragma* to the auto-specialised rule. So if g says {-# NOINLINE[2] g #-}, then the auto-spec rule will also not be active until phase 2. And that's what programmers should jolly well do anyway, even aside from specialisation, to ensure that g doesn't inline too early. This in turn means that the RULE would never fire for a NOINLINE thing so not much point in generating a specialisation at all. Note [Specialisation shape] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ We only specialise a function if it has visible top-level lambdas corresponding to its overloading. E.g. if f :: forall a. Eq a => .... then its body must look like f = /\a. \d. ... Reason: when specialising the body for a call (f ty dexp), we want to substitute dexp for d, and pick up specialised calls in the body of f. This doesn't always work. One example I came across was this: newtype Gen a = MkGen{ unGen :: Int -> a } choose :: Eq a => a -> Gen a choose n = MkGen (\r -> n) oneof = choose (1::Int) It's a silly exapmle, but we get choose = /\a. g `cast` co where choose doesn't have any dict arguments. Thus far I have not tried to fix this (wait till there's a real example). Note [Inline specialisations] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is what we do with the InlinePragma of the original function * Activation/RuleMatchInfo: both transferred to the specialised function * InlineSpec: (a) An INLINE pragma is transferred (b) An INLINABLE pragma is *not* transferred Why (a)? Previously the idea is that the point of INLINE was precisely to specialise the function at its call site, and that's not so important for the specialised copies. But *pragma-directed* specialisation now takes place in the typechecker/desugarer, with manually specified INLINEs. The specialiation here is automatic. It'd be very odd if a function marked INLINE was specialised (because of some local use), and then forever after (including importing modules) the specialised version wasn't INLINEd. After all, the programmer said INLINE! You might wonder why we don't just not-specialise INLINE functions. It's because even INLINE functions are sometimes not inlined, when they aren't applied to interesting arguments. But perhaps the type arguments alone are enough to specialise (even though the args are too boring to trigger inlining), and it's certainly better to call the specialised version. Why (b)? See Trac #4874 for persuasive examples. Suppose we have {-# INLINABLE f #-} f :: Ord a => [a] -> Int f xs = letrec f' = ...f'... in f' Then, when f is specialised and optimised we might get wgo :: [Int] -> Int# wgo = ...wgo... f_spec :: [Int] -> Int f_spec xs = case wgo xs of { r -> I# r } and we clearly want to inline f_spec at call sites. But if we still have the big, un-optimised of f (albeit specialised) captured in an INLINABLE pragma for f_spec, we won't get that optimisation. So we simply drop INLINABLE pragmas when specialising. It's not really a complete solution; ignoring specalisation for now, INLINABLE functions don't get properly strictness analysed, for example. But it works well for examples involving specialisation, which is the dominant use of INLINABLE. See Trac #4874. %************************************************************************ %* * \subsubsection{UsageDetails and suchlike} %* * %************************************************************************ \begin{code}

    data UsageDetails
      = MkUD {
    	ud_binds :: !(Bag DictBind),
    			-- Floated dictionary bindings
    			-- The order is important;
    			-- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
    			-- (Remember, Bags preserve order in GHC.)

    	ud_calls :: !CallDetails

    	-- INVARIANT: suppose bs = bindersOf ud_binds
            -- Then 'calls' may *mention* 'bs',
            -- but there should be no calls *for* bs
        }

    instance Outputable UsageDetails where
      ppr (MkUD { ud_binds = dbs, ud_calls = calls })
    	= ptext (sLit "MkUD") <+> braces (sep (punctuate comma
    		[ptext (sLit "binds") <+> equals <+> ppr dbs,
    	 	 ptext (sLit "calls") <+> equals <+> ppr calls]))

    type DictBind = (CoreBind, VarSet)
    	-- The set is the free vars of the binding
    	-- both tyvars and dicts

    type DictExpr = CoreExpr

    emptyUDs :: UsageDetails
    emptyUDs = MkUD { ud_binds = emptyBag, ud_calls = emptyVarEnv }

    ------------------------------------------------------------
    type CallDetails  = IdEnv CallInfoSet
    newtype CallKey   = CallKey [Maybe Type]			-- Nothing => unconstrained type argument

    -- CallInfo uses a Map, thereby ensuring that
    -- we record only one call instance for any key
    --
    -- The list of types and dictionaries is guaranteed to
    -- match the type of f
    data CallInfoSet = CIS Id (Map CallKey ([DictExpr], VarSet))
         	      		-- Range is dict args and the vars of the whole
    			-- call (including tyvars)
    			-- [*not* include the main id itself, of course]

    type CallInfo = (CallKey, ([DictExpr], VarSet))

    instance Outputable CallInfoSet where
      ppr (CIS fn map) = hang (ptext (sLit "CIS") <+> ppr fn)
                            2 (ppr map)

    instance Outputable CallKey where
      ppr (CallKey ts) = ppr ts

    -- Type isn't an instance of Ord, so that we can control which
    -- instance we use.  That's tiresome here.  Oh well
    instance Eq CallKey where
      k1 == k2 = case k1 `compare` k2 of { EQ -> True; _ -> False }

    instance Ord CallKey where
      compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
    		where
    		  cmp Nothing   Nothing   = EQ
    		  cmp Nothing   (Just _)  = LT
    		  cmp (Just _)  Nothing   = GT
    		  cmp (Just t1) (Just t2) = tcCmpType t1 t2

    unionCalls :: CallDetails -> CallDetails -> CallDetails
    unionCalls c1 c2 = plusVarEnv_C unionCallInfoSet c1 c2

    unionCallInfoSet :: CallInfoSet -> CallInfoSet -> CallInfoSet
    unionCallInfoSet (CIS f calls1) (CIS _ calls2) = CIS f (calls1 `Map.union` calls2)

    callDetailsFVs :: CallDetails -> VarSet
    callDetailsFVs calls = foldVarEnv (unionVarSet . callInfoFVs) emptyVarSet calls

    callInfoFVs :: CallInfoSet -> VarSet
    callInfoFVs (CIS _ call_info) = Map.foldRight (\(_,fv) vs -> unionVarSet fv vs) emptyVarSet call_info

    ------------------------------------------------------------
    singleCall :: Id -> [Maybe Type] -> [DictExpr] -> UsageDetails
    singleCall id tys dicts
      = MkUD {ud_binds = emptyBag,
    	  ud_calls = unitVarEnv id $ CIS id $
                         Map.singleton (CallKey tys) (dicts, call_fvs) }
      where
        call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
        tys_fvs  = tyVarsOfTypes (catMaybes tys)
    	-- The type args (tys) are guaranteed to be part of the dictionary
    	-- types, because they are just the constrained types,
    	-- and the dictionary is therefore sure to be bound
    	-- inside the binding for any type variables free in the type;
    	-- hence it's safe to neglect tyvars free in tys when making
    	-- the free-var set for this call
    	-- BUT I don't trust this reasoning; play safe and include tys_fvs
    	--
    	-- We don't include the 'id' itself.

    mkCallUDs :: Id -> [CoreExpr] -> UsageDetails
    mkCallUDs f args
      | not (want_calls_for f)  -- Imported from elsewhere
      || null theta	   	    -- Not overloaded
      || not (all isClassPred theta)
    	-- Only specialise if all overloading is on class params.
    	-- In ptic, with implicit params, the type args
    	--  *don't* say what the value of the implicit param is!
      || not (spec_tys `lengthIs` n_tyvars)
      || not ( dicts   `lengthIs` n_dicts)
      || not (any interestingDict dicts)	-- Note [Interesting dictionary arguments]
      -- See also Note [Specialisations already covered]
      = -- pprTrace "mkCallUDs: discarding" _trace_doc
        emptyUDs	-- Not overloaded, or no specialisation wanted

      | otherwise
      = -- pprTrace "mkCallUDs: keeping" _trace_doc
        singleCall f spec_tys dicts
      where
        _trace_doc = vcat [ppr f, ppr args, ppr n_tyvars, ppr n_dicts
                          , ppr (map interestingDict dicts)]
        (tyvars, theta, _) = tcSplitSigmaTy (idType f)
        constrained_tyvars = tyVarsOfTheta theta
        n_tyvars	       = length tyvars
        n_dicts	       = length theta

        spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
        dicts    = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]

        mk_spec_ty tyvar ty
    	| tyvar `elemVarSet` constrained_tyvars = Just ty
    	| otherwise				= Nothing

        want_calls_for f = isLocalId f || isInlinablePragma (idInlinePragma f)

\end{code} Note [Interesting dictionary arguments] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this \a.\d:Eq a. let f = ... in ...(f d)... There really is not much point in specialising f wrt the dictionary d, because the code for the specialised f is not improved at all, because d is lambda-bound. We simply get junk specialisations. What is "interesting"? Just that it has *some* structure. \begin{code}

    interestingDict :: CoreExpr -> Bool
    -- A dictionary argument is interesting if it has *some* structure
    interestingDict (Var v) =  hasSomeUnfolding (idUnfolding v)
    			|| isDataConWorkId v
    interestingDict (Type _)	  = False
    interestingDict (App fn (Type _)) = interestingDict fn
    interestingDict (Note _ a)	  = interestingDict a
    interestingDict (Cast e _)	  = interestingDict e
    interestingDict _                 = True

\end{code} \begin{code}

    plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
    plusUDs (MkUD {ud_binds = db1, ud_calls = calls1})
    	(MkUD {ud_binds = db2, ud_calls = calls2})
      = MkUD { ud_binds = db1    `unionBags`   db2
             , ud_calls = calls1 `unionCalls`  calls2 }

    plusUDList :: [UsageDetails] -> UsageDetails
    plusUDList = foldr plusUDs emptyUDs

    -----------------------------
    _dictBindBndrs :: Bag DictBind -> [Id]
    _dictBindBndrs dbs = foldrBag ((++) . bindersOf . fst) [] dbs

    mkDB :: CoreBind -> DictBind
    mkDB bind = (bind, bind_fvs bind)

    bind_fvs :: CoreBind -> VarSet
    bind_fvs (NonRec bndr rhs) = pair_fvs (bndr,rhs)
    bind_fvs (Rec prs)	   = foldl delVarSet rhs_fvs bndrs
    			   where
    			     bndrs = map fst prs
    			     rhs_fvs = unionVarSets (map pair_fvs prs)

    pair_fvs :: (Id, CoreExpr) -> VarSet
    pair_fvs (bndr, rhs) = exprFreeVars rhs `unionVarSet` idFreeVars bndr
    	-- Don't forget variables mentioned in the
    	-- rules of the bndr.  C.f. OccAnal.addRuleUsage
    	-- Also tyvars mentioned in its type; they may not appear in the RHS
    	--	type T a = Int
    	--	x :: T a = 3

    flattenDictBinds :: Bag DictBind -> [(Id,CoreExpr)] -> [(Id,CoreExpr)]
    flattenDictBinds dbs pairs
      = foldrBag add pairs dbs
      where
        add (NonRec b r,_) pairs = (b,r) : pairs
        add (Rec prs1, _)  pairs = prs1 ++ pairs

    snocDictBinds :: UsageDetails -> [CoreBind] -> UsageDetails
    -- Add ud_binds to the tail end of the bindings in uds
    snocDictBinds uds dbs
      = uds { ud_binds = ud_binds uds `unionBags`
                         foldr (consBag . mkDB) emptyBag dbs }

    consDictBind :: CoreBind -> UsageDetails -> UsageDetails
    consDictBind bind uds = uds { ud_binds = mkDB bind `consBag` ud_binds uds }

    addDictBinds :: [DictBind] -> UsageDetails -> UsageDetails
    addDictBinds binds uds = uds { ud_binds = listToBag binds `unionBags` ud_binds uds }

    snocDictBind :: UsageDetails -> CoreBind -> UsageDetails
    snocDictBind uds bind = uds { ud_binds = ud_binds uds `snocBag` mkDB bind }

    wrapDictBinds :: Bag DictBind -> [CoreBind] -> [CoreBind]
    wrapDictBinds dbs binds
      = foldrBag add binds dbs
      where
        add (bind,_) binds = bind : binds

    wrapDictBindsE :: Bag DictBind -> CoreExpr -> CoreExpr
    wrapDictBindsE dbs expr
      = foldrBag add expr dbs
      where
        add (bind,_) expr = Let bind expr

    ----------------------
    dumpUDs :: [CoreBndr] -> UsageDetails -> (UsageDetails, Bag DictBind)
    -- Used at a lambda or case binder; just dump anything mentioning the binder
    dumpUDs bndrs uds@(MkUD { ud_binds = orig_dbs, ud_calls = orig_calls })
      | null bndrs = (uds, emptyBag)  -- Common in case alternatives
      | otherwise  = -- pprTrace "dumpUDs" (ppr bndrs $$ ppr free_uds $$ ppr dump_dbs) $
                     (free_uds, dump_dbs)
      where
        free_uds = MkUD { ud_binds = free_dbs, ud_calls = free_calls }
        bndr_set = mkVarSet bndrs
        (free_dbs, dump_dbs, dump_set) = splitDictBinds orig_dbs bndr_set
        free_calls = deleteCallsMentioning dump_set $   -- Drop calls mentioning bndr_set on the floor
                     deleteCallsFor bndrs orig_calls    -- Discard calls for bndr_set; there should be
    		 			   	    -- no calls for any of the dicts in dump_dbs

    dumpBindUDs :: [CoreBndr] -> UsageDetails -> (UsageDetails, Bag DictBind, Bool)
    -- Used at a lambda or case binder; just dump anything mentioning the binder
    dumpBindUDs bndrs (MkUD { ud_binds = orig_dbs, ud_calls = orig_calls })
      = -- pprTrace "dumpBindUDs" (ppr bndrs $$ ppr free_uds $$ ppr dump_dbs) $
        (free_uds, dump_dbs, float_all)
      where
        free_uds = MkUD { ud_binds = free_dbs, ud_calls = free_calls }
        bndr_set = mkVarSet bndrs
        (free_dbs, dump_dbs, dump_set) = splitDictBinds orig_dbs bndr_set
        free_calls = deleteCallsFor bndrs orig_calls
        float_all = dump_set `intersectsVarSet` callDetailsFVs free_calls

    callsForMe :: Id -> UsageDetails -> (UsageDetails, [CallInfo])
    callsForMe fn (MkUD { ud_binds = orig_dbs, ud_calls = orig_calls })
      = -- pprTrace ("callsForMe")
        --         (vcat [ppr fn,
        --                text "Orig dbs ="     <+> ppr (_dictBindBndrs orig_dbs),
        --                text "Orig calls ="   <+> ppr orig_calls,
        --                text "Dep set ="      <+> ppr dep_set,
        --                text "Calls for me =" <+> ppr calls_for_me]) $
        (uds_without_me, calls_for_me)
      where
        uds_without_me = MkUD { ud_binds = orig_dbs, ud_calls = delVarEnv orig_calls fn }
        calls_for_me = case lookupVarEnv orig_calls fn of
    			Nothing -> []
    			Just (CIS _ calls) -> filter_dfuns (Map.toList calls)

        dep_set = foldlBag go (unitVarSet fn) orig_dbs
        go dep_set (db,fvs) | fvs `intersectsVarSet` dep_set
                            = extendVarSetList dep_set (bindersOf db)
                            | otherwise = dep_set

    	-- Note [Specialisation of dictionary functions]
        filter_dfuns | isDFunId fn = filter ok_call
                     | otherwise   = \cs -> cs

        ok_call (_, (_,fvs)) = not (fvs `intersectsVarSet` dep_set)

    ----------------------
    splitDictBinds :: Bag DictBind -> IdSet -> (Bag DictBind, Bag DictBind, IdSet)
    -- Returns (free_dbs, dump_dbs, dump_set)
    splitDictBinds dbs bndr_set
       = foldlBag split_db (emptyBag, emptyBag, bndr_set) dbs
    		-- Important that it's foldl not foldr;
    		-- we're accumulating the set of dumped ids in dump_set
       where
        split_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
    	| dump_idset `intersectsVarSet` fvs	-- Dump it
    	= (free_dbs, dump_dbs `snocBag` db,
    	   extendVarSetList dump_idset (bindersOf bind))

    	| otherwise	-- Don't dump it
    	= (free_dbs `snocBag` db, dump_dbs, dump_idset)

    ----------------------
    deleteCallsMentioning :: VarSet -> CallDetails -> CallDetails
    -- Remove calls *mentioning* bs
    deleteCallsMentioning bs calls
      = mapVarEnv filter_calls calls
      where
        filter_calls :: CallInfoSet -> CallInfoSet
        filter_calls (CIS f calls) = CIS f (Map.filter keep_call calls)
        keep_call (_, fvs) = not (fvs `intersectsVarSet` bs)

    deleteCallsFor :: [Id] -> CallDetails -> CallDetails
    -- Remove calls *for* bs
    deleteCallsFor bs calls = delVarEnvList calls bs

\end{code} %************************************************************************ %* * \subsubsection{Boring helper functions} %* * %************************************************************************ \begin{code}

    type SpecM a = UniqSM a

    runSpecM:: SpecM a -> CoreM a
    runSpecM spec = do { us <- getUniqueSupplyM
                       ; return (initUs_ us spec) }

    mapAndCombineSM :: (a -> SpecM (b, UsageDetails)) -> [a] -> SpecM ([b], UsageDetails)
    mapAndCombineSM _ []     = return ([], emptyUDs)
    mapAndCombineSM f (x:xs) = do (y, uds1) <- f x
                                  (ys, uds2) <- mapAndCombineSM f xs
                                  return (y:ys, uds1 `plusUDs` uds2)

    cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
    -- Clone the binders of the bind; return new bind with the cloned binders
    -- Return the substitution to use for RHSs, and the one to use for the body
    cloneBindSM subst (NonRec bndr rhs) = do
        us <- getUniqueSupplyM
        let (subst', bndr') = cloneIdBndr subst us bndr
        return (subst, subst', NonRec bndr' rhs)

    cloneBindSM subst (Rec pairs) = do
        us <- getUniqueSupplyM
        let (subst', bndrs') = cloneRecIdBndrs subst us (map fst pairs)
        return (subst', subst', Rec (bndrs' `zip` map snd pairs))

    newDictBndrs :: Subst -> [CoreBndr] -> SpecM (Subst, [CoreBndr])
    -- Make up completely fresh binders for the dictionaries
    -- Their bindings are going to float outwards
    newDictBndrs subst bndrs
      = do { bndrs' <- mapM new bndrs
           ; let subst' = extendIdSubstList subst
                            [(d, Var d') | (d,d') <- bndrs `zip` bndrs']
           ; return (subst', bndrs' ) }
      where
        new b = do { uniq <- getUniqueM
        	       ; let n   = idName b
                         ty' = CoreSubst.substTy subst (idType b)
                   ; return (mkUserLocal (nameOccName n) uniq ty' (getSrcSpan n)) }

    newSpecIdSM :: Id -> Type -> SpecM Id
        -- Give the new Id a similar occurrence name to the old one
    newSpecIdSM old_id new_ty
      = do	{ uniq <- getUniqueM
        	; let name    = idName old_id
    	      new_occ = mkSpecOcc (nameOccName name)
    	      new_id  = mkUserLocal new_occ uniq new_ty (getSrcSpan name)
            ; return new_id }

\end{code} Old (but interesting) stuff about unboxed bindings ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ What should we do when a value is specialised to a *strict* unboxed value? map_*_* f (x:xs) = let h = f x t = map f xs in h:t Could convert let to case: map_*_Int# f (x:xs) = case f x of h# -> let t = map f xs in h#:t This may be undesirable since it forces evaluation here, but the value may not be used in all branches of the body. In the general case this transformation is impossible since the mutual recursion in a letrec cannot be expressed as a case. There is also a problem with top-level unboxed values, since our implementation cannot handle unboxed values at the top level. Solution: Lift the binding of the unboxed value and extract it when it is used: map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h# t = map f xs in case h of _Lift h# -> h#:t Now give it to the simplifier and the _Lifting will be optimised away. The benfit is that we have given the specialised "unboxed" values a very simplep lifted semantics and then leave it up to the simplifier to optimise it --- knowing that the overheads will be removed in nearly all cases. In particular, the value will only be evaluted in the branches of the program which use it, rather than being forced at the point where the value is bound. For example: filtermap_*_* p f (x:xs) = let h = f x t = ... in case p x of True -> h:t False -> t ==> filtermap_*_Int# p f (x:xs) = let h = case (f x) of h# -> _Lift h# t = ... in case p x of True -> case h of _Lift h# -> h#:t False -> t The binding for h can still be inlined in the one branch and the _Lifting eliminated. Question: When won't the _Lifting be eliminated? Answer: When they at the top-level (where it is necessary) or when inlining would duplicate work (or possibly code depending on options). However, the _Lifting will still be eliminated if the strictness analyser deems the lifted binding strict.