2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[Specialise]{Stamping out overloading, and (optionally) polymorphism}
7 -- The above warning supression flag is a temporary kludge.
8 -- While working on this module you are encouraged to remove it and fix
9 -- any warnings in the module. See
10 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
13 module Specialise ( specProgram ) where
15 #include "HsVersions.h"
17 import Id ( Id, idName, idType, mkUserLocal, idCoreRules,
18 idInlinePragma, setInlinePragma, setIdUnfolding,
20 import TcType ( Type, mkTyVarTy, tcSplitSigmaTy,
21 tyVarsOfTypes, tyVarsOfTheta, isClassPred,
22 tcCmpType, isUnLiftedType
24 import CoreSubst ( Subst, mkEmptySubst, extendTvSubstList, lookupIdSubst,
25 substBndr, substBndrs, substTy, substInScope,
26 cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs,
29 import CoreUnfold ( mkUnfolding )
30 import SimplUtils ( interestingArg )
36 import CoreUtils ( exprIsTrivial, applyTypeToArgs, mkPiTypes )
37 import CoreFVs ( exprFreeVars, exprsFreeVars, idFreeVars )
38 import UniqSupply ( UniqSupply,
43 import MkId ( voidArgId, realWorldPrimId )
45 import Maybes ( catMaybes, isJust )
53 %************************************************************************
55 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
57 %************************************************************************
59 These notes describe how we implement specialisation to eliminate
62 The specialisation pass works on Core
63 syntax, complete with all the explicit dictionary application,
64 abstraction and construction as added by the type checker. The
65 existing type checker remains largely as it is.
67 One important thought: the {\em types} passed to an overloaded
68 function, and the {\em dictionaries} passed are mutually redundant.
69 If the same function is applied to the same type(s) then it is sure to
70 be applied to the same dictionary(s)---or rather to the same {\em
71 values}. (The arguments might look different but they will evaluate
74 Second important thought: we know that we can make progress by
75 treating dictionary arguments as static and worth specialising on. So
76 we can do without binding-time analysis, and instead specialise on
77 dictionary arguments and no others.
86 and suppose f is overloaded.
88 STEP 1: CALL-INSTANCE COLLECTION
90 We traverse <body>, accumulating all applications of f to types and
93 (Might there be partial applications, to just some of its types and
94 dictionaries? In principle yes, but in practice the type checker only
95 builds applications of f to all its types and dictionaries, so partial
96 applications could only arise as a result of transformation, and even
97 then I think it's unlikely. In any case, we simply don't accumulate such
98 partial applications.)
103 So now we have a collection of calls to f:
107 Notice that f may take several type arguments. To avoid ambiguity, we
108 say that f is called at type t1/t2 and t3/t4.
110 We take equivalence classes using equality of the *types* (ignoring
111 the dictionary args, which as mentioned previously are redundant).
113 STEP 3: SPECIALISATION
115 For each equivalence class, choose a representative (f t1 t2 d1 d2),
116 and create a local instance of f, defined thus:
118 f@t1/t2 = <f_rhs> t1 t2 d1 d2
120 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
121 of simplification will now result. However we don't actually *do* that
122 simplification. Rather, we leave it for the simplifier to do. If we
123 *did* do it, though, we'd get more call instances from the specialised
124 RHS. We can work out what they are by instantiating the call-instance
125 set from f's RHS with the types t1, t2.
127 Add this new id to f's IdInfo, to record that f has a specialised version.
129 Before doing any of this, check that f's IdInfo doesn't already
130 tell us about an existing instance of f at the required type/s.
131 (This might happen if specialisation was applied more than once, or
132 it might arise from user SPECIALIZE pragmas.)
136 Wait a minute! What if f is recursive? Then we can't just plug in
137 its right-hand side, can we?
139 But it's ok. The type checker *always* creates non-recursive definitions
140 for overloaded recursive functions. For example:
142 f x = f (x+x) -- Yes I know its silly
146 f a (d::Num a) = let p = +.sel a d
148 letrec fl (y::a) = fl (p y y)
152 We still have recusion for non-overloaded functions which we
153 speciailise, but the recursive call should get specialised to the
154 same recursive version.
160 All this is crystal clear when the function is applied to *constant
161 types*; that is, types which have no type variables inside. But what if
162 it is applied to non-constant types? Suppose we find a call of f at type
163 t1/t2. There are two possibilities:
165 (a) The free type variables of t1, t2 are in scope at the definition point
166 of f. In this case there's no problem, we proceed just as before. A common
167 example is as follows. Here's the Haskell:
172 After typechecking we have
174 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
175 in +.sel a d (f a d y) (f a d y)
177 Notice that the call to f is at type type "a"; a non-constant type.
178 Both calls to f are at the same type, so we can specialise to give:
180 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
181 in +.sel a d (f@a y) (f@a y)
184 (b) The other case is when the type variables in the instance types
185 are *not* in scope at the definition point of f. The example we are
186 working with above is a good case. There are two instances of (+.sel a d),
187 but "a" is not in scope at the definition of +.sel. Can we do anything?
188 Yes, we can "common them up", a sort of limited common sub-expression deal.
191 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
192 f@a (x::a) = +.sel@a x x
193 in +.sel@a (f@a y) (f@a y)
195 This can save work, and can't be spotted by the type checker, because
196 the two instances of +.sel weren't originally at the same type.
200 * There are quite a few variations here. For example, the defn of
201 +.sel could be floated ouside the \y, to attempt to gain laziness.
202 It certainly mustn't be floated outside the \d because the d has to
205 * We don't want to inline f_rhs in this case, because
206 that will duplicate code. Just commoning up the call is the point.
208 * Nothing gets added to +.sel's IdInfo.
210 * Don't bother unless the equivalence class has more than one item!
212 Not clear whether this is all worth it. It is of course OK to
213 simply discard call-instances when passing a big lambda.
215 Polymorphism 2 -- Overloading
217 Consider a function whose most general type is
219 f :: forall a b. Ord a => [a] -> b -> b
221 There is really no point in making a version of g at Int/Int and another
222 at Int/Bool, because it's only instancing the type variable "a" which
223 buys us any efficiency. Since g is completely polymorphic in b there
224 ain't much point in making separate versions of g for the different
227 That suggests that we should identify which of g's type variables
228 are constrained (like "a") and which are unconstrained (like "b").
229 Then when taking equivalence classes in STEP 2, we ignore the type args
230 corresponding to unconstrained type variable. In STEP 3 we make
231 polymorphic versions. Thus:
233 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
242 f a (d::Num a) = let g = ...
244 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
246 Here, g is only called at one type, but the dictionary isn't in scope at the
247 definition point for g. Usually the type checker would build a
248 definition for d1 which enclosed g, but the transformation system
249 might have moved d1's defn inward. Solution: float dictionary bindings
250 outwards along with call instances.
254 f x = let g p q = p==q
260 Before specialisation, leaving out type abstractions we have
262 f df x = let g :: Eq a => a -> a -> Bool
264 h :: Num a => a -> a -> (a, Bool)
265 h dh r s = let deq = eqFromNum dh
266 in (+ dh r s, g deq r s)
270 After specialising h we get a specialised version of h, like this:
272 h' r s = let deq = eqFromNum df
273 in (+ df r s, g deq r s)
275 But we can't naively make an instance for g from this, because deq is not in scope
276 at the defn of g. Instead, we have to float out the (new) defn of deq
277 to widen its scope. Notice that this floating can't be done in advance -- it only
278 shows up when specialisation is done.
280 User SPECIALIZE pragmas
281 ~~~~~~~~~~~~~~~~~~~~~~~
282 Specialisation pragmas can be digested by the type checker, and implemented
283 by adding extra definitions along with that of f, in the same way as before
285 f@t1/t2 = <f_rhs> t1 t2 d1 d2
287 Indeed the pragmas *have* to be dealt with by the type checker, because
288 only it knows how to build the dictionaries d1 and d2! For example
290 g :: Ord a => [a] -> [a]
291 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
293 Here, the specialised version of g is an application of g's rhs to the
294 Ord dictionary for (Tree Int), which only the type checker can conjure
295 up. There might not even *be* one, if (Tree Int) is not an instance of
296 Ord! (All the other specialision has suitable dictionaries to hand
299 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
300 it is buried in a complex (as-yet-un-desugared) binding group.
303 f@t1/t2 = f* t1 t2 d1 d2
305 where f* is the Id f with an IdInfo which says "inline me regardless!".
306 Indeed all the specialisation could be done in this way.
307 That in turn means that the simplifier has to be prepared to inline absolutely
308 any in-scope let-bound thing.
311 Again, the pragma should permit polymorphism in unconstrained variables:
313 h :: Ord a => [a] -> b -> b
314 {-# SPECIALIZE h :: [Int] -> b -> b #-}
316 We *insist* that all overloaded type variables are specialised to ground types,
317 (and hence there can be no context inside a SPECIALIZE pragma).
318 We *permit* unconstrained type variables to be specialised to
320 - or left as a polymorphic type variable
321 but nothing in between. So
323 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
325 is *illegal*. (It can be handled, but it adds complication, and gains the
329 SPECIALISING INSTANCE DECLARATIONS
330 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
333 instance Foo a => Foo [a] where
335 {-# SPECIALIZE instance Foo [Int] #-}
337 The original instance decl creates a dictionary-function
340 dfun.Foo.List :: forall a. Foo a -> Foo [a]
342 The SPECIALIZE pragma just makes a specialised copy, just as for
343 ordinary function definitions:
345 dfun.Foo.List@Int :: Foo [Int]
346 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
348 The information about what instance of the dfun exist gets added to
349 the dfun's IdInfo in the same way as a user-defined function too.
352 Automatic instance decl specialisation?
353 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
354 Can instance decls be specialised automatically? It's tricky.
355 We could collect call-instance information for each dfun, but
356 then when we specialised their bodies we'd get new call-instances
357 for ordinary functions; and when we specialised their bodies, we might get
358 new call-instances of the dfuns, and so on. This all arises because of
359 the unrestricted mutual recursion between instance decls and value decls.
361 Still, there's no actual problem; it just means that we may not do all
362 the specialisation we could theoretically do.
364 Furthermore, instance decls are usually exported and used non-locally,
365 so we'll want to compile enough to get those specialisations done.
367 Lastly, there's no such thing as a local instance decl, so we can
368 survive solely by spitting out *usage* information, and then reading that
369 back in as a pragma when next compiling the file. So for now,
370 we only specialise instance decls in response to pragmas.
373 SPITTING OUT USAGE INFORMATION
374 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
376 To spit out usage information we need to traverse the code collecting
377 call-instance information for all imported (non-prelude?) functions
378 and data types. Then we equivalence-class it and spit it out.
380 This is done at the top-level when all the call instances which escape
381 must be for imported functions and data types.
383 *** Not currently done ***
386 Partial specialisation by pragmas
387 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
388 What about partial specialisation:
390 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
391 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
395 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
397 Seems quite reasonable. Similar things could be done with instance decls:
399 instance (Foo a, Foo b) => Foo (a,b) where
401 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
402 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
404 Ho hum. Things are complex enough without this. I pass.
407 Requirements for the simplifer
408 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
409 The simplifier has to be able to take advantage of the specialisation.
411 * When the simplifier finds an application of a polymorphic f, it looks in
412 f's IdInfo in case there is a suitable instance to call instead. This converts
414 f t1 t2 d1 d2 ===> f_t1_t2
416 Note that the dictionaries get eaten up too!
418 * Dictionary selection operations on constant dictionaries must be
421 +.sel Int d ===> +Int
423 The obvious way to do this is in the same way as other specialised
424 calls: +.sel has inside it some IdInfo which tells that if it's applied
425 to the type Int then it should eat a dictionary and transform to +Int.
427 In short, dictionary selectors need IdInfo inside them for constant
430 * Exactly the same applies if a superclass dictionary is being
433 Eq.sel Int d ===> dEqInt
435 * Something similar applies to dictionary construction too. Suppose
436 dfun.Eq.List is the function taking a dictionary for (Eq a) to
437 one for (Eq [a]). Then we want
439 dfun.Eq.List Int d ===> dEq.List_Int
441 Where does the Eq [Int] dictionary come from? It is built in
442 response to a SPECIALIZE pragma on the Eq [a] instance decl.
444 In short, dfun Ids need IdInfo with a specialisation for each
445 constant instance of their instance declaration.
447 All this uses a single mechanism: the SpecEnv inside an Id
450 What does the specialisation IdInfo look like?
451 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
453 The SpecEnv of an Id maps a list of types (the template) to an expression
457 For example, if f has this SpecInfo:
459 [Int, a] -> \d:Ord Int. f' a
461 it means that we can replace the call
463 f Int t ===> (\d. f' t)
465 This chucks one dictionary away and proceeds with the
466 specialised version of f, namely f'.
469 What can't be done this way?
470 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
471 There is no way, post-typechecker, to get a dictionary for (say)
472 Eq a from a dictionary for Eq [a]. So if we find
476 we can't transform to
481 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
483 Of course, we currently have no way to automatically derive
484 eqList, nor to connect it to the Eq [a] instance decl, but you
485 can imagine that it might somehow be possible. Taking advantage
486 of this is permanently ruled out.
488 Still, this is no great hardship, because we intend to eliminate
489 overloading altogether anyway!
491 A note about non-tyvar dictionaries
492 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
493 Some Ids have types like
495 forall a,b,c. Eq a -> Ord [a] -> tau
497 This seems curious at first, because we usually only have dictionary
498 args whose types are of the form (C a) where a is a type variable.
499 But this doesn't hold for the functions arising from instance decls,
500 which sometimes get arguements with types of form (C (T a)) for some
503 Should we specialise wrt this compound-type dictionary? We used to say
505 "This is a heuristic judgement, as indeed is the fact that we
506 specialise wrt only dictionaries. We choose *not* to specialise
507 wrt compound dictionaries because at the moment the only place
508 they show up is in instance decls, where they are simply plugged
509 into a returned dictionary. So nothing is gained by specialising
512 But it is simpler and more uniform to specialise wrt these dicts too;
513 and in future GHC is likely to support full fledged type signatures
515 f :: Eq [(a,b)] => ...
518 %************************************************************************
520 \subsubsection{The new specialiser}
522 %************************************************************************
524 Our basic game plan is this. For let(rec) bound function
525 f :: (C a, D c) => (a,b,c,d) -> Bool
527 * Find any specialised calls of f, (f ts ds), where
528 ts are the type arguments t1 .. t4, and
529 ds are the dictionary arguments d1 .. d2.
531 * Add a new definition for f1 (say):
533 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
535 Note that we abstract over the unconstrained type arguments.
539 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
541 to the specialisations of f. This will be used by the
542 simplifier to replace calls
543 (f t1 t2 t3 t4) da db
545 (\d1 d1 -> f1 t2 t4) da db
547 All the stuff about how many dictionaries to discard, and what types
548 to apply the specialised function to, are handled by the fact that the
549 SpecEnv contains a template for the result of the specialisation.
551 We don't build *partial* specialisations for f. For example:
553 f :: Eq a => a -> a -> Bool
554 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
556 Here, little is gained by making a specialised copy of f.
557 There's a distinct danger that the specialised version would
558 first build a dictionary for (Eq b, Eq c), and then select the (==)
559 method from it! Even if it didn't, not a great deal is saved.
561 We do, however, generate polymorphic, but not overloaded, specialisations:
563 f :: Eq a => [a] -> b -> b -> b
564 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
566 Hence, the invariant is this:
568 *** no specialised version is overloaded ***
571 %************************************************************************
573 \subsubsection{The exported function}
575 %************************************************************************
578 specProgram :: UniqSupply -> [CoreBind] -> [CoreBind]
579 specProgram us binds = initSM us (do (binds', uds') <- go binds
580 return (dumpAllDictBinds uds' binds'))
582 -- We need to start with a Subst that knows all the things
583 -- that are in scope, so that the substitution engine doesn't
584 -- accidentally re-use a unique that's already in use
585 -- Easiest thing is to do it all at once, as if all the top-level
586 -- decls were mutually recursive
587 top_subst = mkEmptySubst (mkInScopeSet (mkVarSet (bindersOfBinds binds)))
589 go [] = return ([], emptyUDs)
590 go (bind:binds) = do (binds', uds) <- go binds
591 (bind', uds') <- specBind top_subst bind uds
592 return (bind' ++ binds', uds')
595 %************************************************************************
597 \subsubsection{@specExpr@: the main function}
599 %************************************************************************
602 specVar :: Subst -> Id -> CoreExpr
603 specVar subst v = lookupIdSubst subst v
605 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
606 -- We carry a substitution down:
607 -- a) we must clone any binding that might float outwards,
608 -- to avoid name clashes
609 -- b) we carry a type substitution to use when analysing
610 -- the RHS of specialised bindings (no type-let!)
612 ---------------- First the easy cases --------------------
613 specExpr subst (Type ty) = return (Type (substTy subst ty), emptyUDs)
614 specExpr subst (Var v) = return (specVar subst v, emptyUDs)
615 specExpr _ (Lit lit) = return (Lit lit, emptyUDs)
616 specExpr subst (Cast e co) = do
617 (e', uds) <- specExpr subst e
618 return ((Cast e' (substTy subst co)), uds)
619 specExpr subst (Note note body) = do
620 (body', uds) <- specExpr subst body
621 return (Note (specNote subst note) body', uds)
624 ---------------- Applications might generate a call instance --------------------
625 specExpr subst expr@(App {})
628 go (App fun arg) args = do (arg', uds_arg) <- specExpr subst arg
629 (fun', uds_app) <- go fun (arg':args)
630 return (App fun' arg', uds_arg `plusUDs` uds_app)
632 go (Var f) args = case specVar subst f of
633 Var f' -> return (Var f', mkCallUDs f' args)
634 e' -> return (e', emptyUDs) -- I don't expect this!
635 go other _ = specExpr subst other
637 ---------------- Lambda/case require dumping of usage details --------------------
638 specExpr subst e@(Lam _ _) = do
639 (body', uds) <- specExpr subst' body
640 let (filtered_uds, body'') = dumpUDs bndrs' uds body'
641 return (mkLams bndrs' body'', filtered_uds)
643 (bndrs, body) = collectBinders e
644 (subst', bndrs') = substBndrs subst bndrs
645 -- More efficient to collect a group of binders together all at once
646 -- and we don't want to split a lambda group with dumped bindings
648 specExpr subst (Case scrut case_bndr ty alts) = do
649 (scrut', uds_scrut) <- specExpr subst scrut
650 (alts', uds_alts) <- mapAndCombineSM spec_alt alts
651 return (Case scrut' case_bndr' (substTy subst ty) alts', uds_scrut `plusUDs` uds_alts)
653 (subst_alt, case_bndr') = substBndr subst case_bndr
654 -- No need to clone case binder; it can't float like a let(rec)
656 spec_alt (con, args, rhs) = do
657 (rhs', uds) <- specExpr subst_rhs rhs
658 let (uds', rhs'') = dumpUDs args uds rhs'
659 return ((con, args', rhs''), uds')
661 (subst_rhs, args') = substBndrs subst_alt args
663 ---------------- Finally, let is the interesting case --------------------
664 specExpr subst (Let bind body) = do
666 (rhs_subst, body_subst, bind') <- cloneBindSM subst bind
668 -- Deal with the body
669 (body', body_uds) <- specExpr body_subst body
671 -- Deal with the bindings
672 (binds', uds) <- specBind rhs_subst bind' body_uds
675 return (foldr Let body' binds', uds)
677 -- Must apply the type substitution to coerceions
678 specNote :: Subst -> Note -> Note
679 specNote _ note = note
682 %************************************************************************
684 \subsubsection{Dealing with a binding}
686 %************************************************************************
689 specBind :: Subst -- Use this for RHSs
691 -> UsageDetails -- Info on how the scope of the binding
692 -> SpecM ([CoreBind], -- New bindings
693 UsageDetails) -- And info to pass upstream
695 specBind rhs_subst bind body_uds
696 = do { (bind', bind_uds) <- specBindItself rhs_subst bind (calls body_uds)
697 ; return (finishSpecBind bind' bind_uds body_uds) }
699 finishSpecBind :: CoreBind -> UsageDetails -> UsageDetails -> ([CoreBind], UsageDetails)
701 (MkUD { dict_binds = rhs_dbs, calls = rhs_calls, ud_fvs = rhs_fvs })
702 (MkUD { dict_binds = body_dbs, calls = body_calls, ud_fvs = body_fvs })
703 | not (mkVarSet bndrs `intersectsVarSet` all_fvs)
704 -- Common case 1: the bound variables are not
705 -- mentioned in the dictionary bindings
706 = ([bind], MkUD { dict_binds = body_dbs `unionBags` rhs_dbs
707 -- It's important that the `unionBags` is this way round,
708 -- because body_uds may bind dictionaries that are
709 -- used in the calls passed to specDefn. So the
710 -- dictionary bindings in rhs_uds may mention
711 -- dictionaries bound in body_uds.
713 , ud_fvs = all_fvs })
715 | case bind of { NonRec {} -> True; Rec {} -> False }
716 -- Common case 2: no specialisation happened, and binding
717 -- is non-recursive. But the binding may be
718 -- mentioned in body_dbs, so we should put it first
719 = ([], MkUD { dict_binds = rhs_dbs `unionBags` ((bind, b_fvs) `consBag` body_dbs)
721 , ud_fvs = all_fvs `unionVarSet` b_fvs })
723 | otherwise -- General case: make a huge Rec (sigh)
724 = ([], MkUD { dict_binds = unitBag (Rec all_db_prs, all_db_fvs)
726 , ud_fvs = all_fvs `unionVarSet` b_fvs })
728 all_fvs = rhs_fvs `unionVarSet` body_fvs
729 all_calls = zapCalls bndrs (rhs_calls `unionCalls` body_calls)
731 bndrs = bindersOf bind
732 b_fvs = bind_fvs bind
734 (all_db_prs, all_db_fvs) = add (bind, b_fvs) $
735 foldrBag add ([], emptyVarSet) $
736 rhs_dbs `unionBags` body_dbs
737 add (NonRec b r, b_fvs) (prs, fvs) = ((b,r) : prs, b_fvs `unionVarSet` fvs)
738 add (Rec b_prs, b_fvs) (prs, fvs) = (b_prs ++ prs, b_fvs `unionVarSet` fvs)
740 ---------------------------
741 specBindItself :: Subst -> CoreBind -> CallDetails -> SpecM (CoreBind, UsageDetails)
743 -- specBindItself deals with the RHS, specialising it according
744 -- to the calls found in the body (if any)
745 specBindItself rhs_subst (NonRec fn rhs) call_info
746 = do { (rhs', rhs_uds) <- specExpr rhs_subst rhs -- Do RHS of original fn
747 ; (fn', spec_defns, spec_uds) <- specDefn rhs_subst call_info fn rhs
748 ; if null spec_defns then
749 return (NonRec fn rhs', rhs_uds)
751 return (Rec ((fn',rhs') : spec_defns), rhs_uds `plusUDs` spec_uds) }
752 -- bndr' mentions the spec_defns in its SpecEnv
753 -- Not sure why we couln't just put the spec_defns first
755 specBindItself rhs_subst (Rec pairs) call_info
756 -- Note [Specialising a recursive group]
757 = do { let (bndrs,rhss) = unzip pairs
758 ; (rhss', rhs_uds) <- mapAndCombineSM (specExpr rhs_subst) rhss
759 ; let all_calls = call_info `unionCalls` calls rhs_uds
760 ; (bndrs1, spec_defns1, spec_uds1) <- specDefns rhs_subst all_calls pairs
762 ; if null spec_defns1 then -- Common case: no specialisation
763 return (Rec (bndrs `zip` rhss'), rhs_uds)
764 else do -- Specialisation occurred; do it again
765 { (bndrs2, spec_defns2, spec_uds2) <-
766 -- pprTrace "specB" (ppr bndrs $$ ppr rhs_uds) $
767 specDefns rhs_subst (calls spec_uds1) (bndrs1 `zip` rhss)
769 ; let all_defns = spec_defns1 ++ spec_defns2 ++ zip bndrs2 rhss'
771 ; return (Rec all_defns, rhs_uds `plusUDs` spec_uds1 `plusUDs` spec_uds2) } }
774 ---------------------------
776 -> CallDetails -- Info on how it is used in its scope
777 -> [(Id,CoreExpr)] -- The things being bound and their un-processed RHS
778 -> SpecM ([Id], -- Original Ids with RULES added
779 [(Id,CoreExpr)], -- Extra, specialised bindings
780 UsageDetails) -- Stuff to fling upwards from the specialised versions
782 -- Specialise a list of bindings (the contents of a Rec), but flowing usages
783 -- upwards binding by binding. Example: { f = ...g ...; g = ...f .... }
784 -- Then if the input CallDetails has a specialised call for 'g', whose specialisation
785 -- in turn generates a specialised call for 'f', we catch that in this one sweep.
786 -- But not vice versa (it's a fixpoint problem).
788 specDefns _subst _call_info []
789 = return ([], [], emptyUDs)
790 specDefns subst call_info ((bndr,rhs):pairs)
791 = do { (bndrs', spec_defns, spec_uds) <- specDefns subst call_info pairs
792 ; let all_calls = call_info `unionCalls` calls spec_uds
793 ; (bndr', spec_defns1, spec_uds1) <- specDefn subst all_calls bndr rhs
794 ; return (bndr' : bndrs',
795 spec_defns1 ++ spec_defns,
796 spec_uds1 `plusUDs` spec_uds) }
798 ---------------------------
800 -> CallDetails -- Info on how it is used in its scope
801 -> Id -> CoreExpr -- The thing being bound and its un-processed RHS
802 -> SpecM (Id, -- Original Id with added RULES
803 [(Id,CoreExpr)], -- Extra, specialised bindings
804 UsageDetails) -- Stuff to fling upwards from the specialised versions
806 specDefn subst calls fn rhs
807 -- The first case is the interesting one
808 | rhs_tyvars `lengthIs` n_tyvars -- Rhs of fn's defn has right number of big lambdas
809 && rhs_ids `lengthAtLeast` n_dicts -- and enough dict args
810 && notNull calls_for_me -- And there are some calls to specialise
812 -- && not (certainlyWillInline (idUnfolding fn)) -- And it's not small
813 -- See Note [Inline specialisation] for why we do not
814 -- switch off specialisation for inline functions
816 = do { -- Make a specialised version for each call in calls_for_me
817 stuff <- mapM spec_call calls_for_me
818 ; let (spec_defns, spec_uds, spec_rules) = unzip3 (catMaybes stuff)
819 fn' = addIdSpecialisations fn spec_rules
820 ; return (fn', spec_defns, plusUDList spec_uds) }
822 | otherwise -- No calls or RHS doesn't fit our preconceptions
823 = WARN( notNull calls_for_me, ptext (sLit "Missed specialisation opportunity for") <+> ppr fn )
824 -- Note [Specialisation shape]
825 return (fn, [], emptyUDs)
829 (tyvars, theta, _) = tcSplitSigmaTy fn_type
830 n_tyvars = length tyvars
831 n_dicts = length theta
832 inline_prag = idInlinePragma fn
834 -- It's important that we "see past" any INLINE pragma
835 -- else we'll fail to specialise an INLINE thing
836 (inline_rhs, rhs_inside) = dropInline rhs
837 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs_inside
839 rhs_dict_ids = take n_dicts rhs_ids
840 body = mkLams (drop n_dicts rhs_ids) rhs_body
841 -- Glue back on the non-dict lambdas
843 calls_for_me = case lookupFM calls fn of
845 Just cs -> fmToList cs
847 already_covered :: [CoreExpr] -> Bool
848 already_covered args -- Note [Specialisations already covered]
849 = isJust (lookupRule (const True) (substInScope subst)
850 fn args (idCoreRules fn))
852 mk_ty_args :: [Maybe Type] -> [CoreExpr]
853 mk_ty_args call_ts = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
855 mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
856 mk_ty_arg _ (Just ty) = Type ty
858 ----------------------------------------------------------
859 -- Specialise to one particular call pattern
860 spec_call :: (CallKey, ([DictExpr], VarSet)) -- Call instance
861 -> SpecM (Maybe ((Id,CoreExpr), -- Specialised definition
862 UsageDetails, -- Usage details from specialised body
863 CoreRule)) -- Info for the Id's SpecEnv
864 spec_call (CallKey call_ts, (call_ds, _))
865 = ASSERT( call_ts `lengthIs` n_tyvars && call_ds `lengthIs` n_dicts )
867 -- Suppose f's defn is f = /\ a b c -> \ d1 d2 -> rhs
868 -- Supppose the call is for f [Just t1, Nothing, Just t3] [dx1, dx2]
870 -- Construct the new binding
871 -- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b -> rhs)
872 -- PLUS the usage-details
873 -- { d1' = dx1; d2' = dx2 }
874 -- where d1', d2' are cloned versions of d1,d2, with the type substitution
875 -- applied. These auxiliary bindings just avoid duplication of dx1, dx2
877 -- Note that the substitution is applied to the whole thing.
878 -- This is convenient, but just slightly fragile. Notably:
879 -- * There had better be no name clashes in a/b/c
881 -- poly_tyvars = [b] in the example above
882 -- spec_tyvars = [a,c]
883 -- ty_args = [t1,b,t3]
884 poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
885 spec_tv_binds = [(tv,ty) | (tv, Just ty) <- rhs_tyvars `zip` call_ts]
886 spec_ty_args = map snd spec_tv_binds
887 ty_args = mk_ty_args call_ts
888 rhs_subst = extendTvSubstList subst spec_tv_binds
890 ; (rhs_subst1, inst_dict_ids) <- cloneDictBndrs rhs_subst rhs_dict_ids
891 -- Clone rhs_dicts, including instantiating their types
893 ; let (rhs_subst2, dx_binds) = bindAuxiliaryDicts rhs_subst1 $
894 (my_zipEqual rhs_dict_ids inst_dict_ids call_ds)
895 inst_args = ty_args ++ map Var inst_dict_ids
897 ; if already_covered inst_args then
900 { -- Figure out the type of the specialised function
901 let body_ty = applyTypeToArgs rhs fn_type inst_args
902 (lam_args, app_args) -- Add a dummy argument if body_ty is unlifted
903 | isUnLiftedType body_ty -- C.f. WwLib.mkWorkerArgs
904 = (poly_tyvars ++ [voidArgId], poly_tyvars ++ [realWorldPrimId])
905 | otherwise = (poly_tyvars, poly_tyvars)
906 spec_id_ty = mkPiTypes lam_args body_ty
908 ; spec_f <- newSpecIdSM fn spec_id_ty
909 ; (spec_rhs, rhs_uds) <- specExpr rhs_subst2 (mkLams lam_args body)
911 -- The rule to put in the function's specialisation is:
912 -- forall b, d1',d2'. f t1 b t3 d1' d2' = f1 b
913 rule_name = mkFastString ("SPEC " ++ showSDoc (ppr fn <+> ppr spec_ty_args))
914 spec_env_rule = mkLocalRule
916 inline_prag -- Note [Auto-specialisation and RULES]
918 (poly_tyvars ++ inst_dict_ids)
920 (mkVarApps (Var spec_f) app_args)
922 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
923 final_uds = foldr addDictBind rhs_uds dx_binds
925 spec_pr | inline_rhs = (spec_f `setInlinePragma` inline_prag, Note InlineMe spec_rhs)
926 | otherwise = (spec_f, spec_rhs)
928 ; return (Just (spec_pr, final_uds, spec_env_rule)) } }
931 | debugIsOn && not (equalLength xs ys && equalLength ys zs)
932 = pprPanic "my_zipEqual" (vcat [ ppr xs, ppr ys
933 , ppr fn <+> ppr call_ts
934 , ppr (idType fn), ppr theta
935 , ppr n_dicts, ppr rhs_dict_ids
937 | otherwise = zip3 xs ys zs
941 -> [(DictId,DictId,CoreExpr)] -- (orig_dict, inst_dict, dx)
942 -> (Subst, -- Substitute for all orig_dicts
943 [(DictId, CoreExpr)]) -- Auxiliary bindings
944 -- Bind any dictionary arguments to fresh names, to preserve sharing
945 -- Substitution already substitutes orig_dict -> inst_dict
946 bindAuxiliaryDicts subst triples = go subst [] triples
948 go subst binds [] = (subst, binds)
949 go subst binds ((d, dx_id, dx) : pairs)
950 | exprIsTrivial dx = go (extendIdSubst subst d dx) binds pairs
951 -- No auxiliary binding necessary
952 | otherwise = go subst_w_unf ((dx_id,dx) : binds) pairs
954 dx_id1 = dx_id `setIdUnfolding` mkUnfolding False dx
955 subst_w_unf = extendIdSubst subst d (Var dx_id1)
956 -- Important! We're going to substitute dx_id1 for d
957 -- and we want it to look "interesting", else we won't gather *any*
958 -- consequential calls. E.g.
960 -- If we specialise f for a call (f (dfun dNumInt)), we'll get
961 -- a consequent call (g d') with an auxiliary definition
963 -- We want that consequent call to look interesting
966 Note [Specialising a recursive group]
967 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
969 let rec { f x = ...g x'...
970 ; g y = ...f y'.... }
972 Here we specialise 'f' at Char; but that is very likely to lead to
973 a specialisation of 'g' at Char. We must do the latter, else the
974 whole point of specialisation is lost.
976 But we do not want to keep iterating to a fixpoint, because in the
977 presence of polymorphic recursion we might generate an infinite number
980 So we use the following heuristic:
981 * Arrange the rec block in dependency order, so far as possible
982 (the occurrence analyser already does this)
984 * Specialise it much like a sequence of lets
986 * Then go through the block a second time, feeding call-info from
987 the RHSs back in the bottom, as it were
989 In effect, the ordering maxmimises the effectiveness of each sweep,
990 and we do just two sweeps. This should catch almost every case of
991 monomorphic recursion -- the exception could be a very knotted-up
992 recursion with multiple cycles tied up together.
994 This plan is implemented in the Rec case of specBindItself.
996 Note [Specialisations already covered]
997 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
998 We obviously don't want to generate two specialisations for the same
999 argument pattern. There are two wrinkles
1001 1. We do the already-covered test in specDefn, not when we generate
1002 the CallInfo in mkCallUDs. We used to test in the latter place, but
1003 we now iterate the specialiser somewhat, and the Id at the call site
1004 might therefore not have all the RULES that we can see in specDefn
1006 2. What about two specialisations where the second is an *instance*
1007 of the first? If the more specific one shows up first, we'll generate
1008 specialisations for both. If the *less* specific one shows up first,
1009 we *don't* currently generate a specialisation for the more specific
1010 one. (See the call to lookupRule in already_covered.) Reasons:
1011 (a) lookupRule doesn't say which matches are exact (bad reason)
1012 (b) if the earlier specialisation is user-provided, it's
1013 far from clear that we should auto-specialise further
1015 Note [Auto-specialisation and RULES]
1016 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1018 g :: Num a => a -> a
1021 f :: (Int -> Int) -> Int
1023 {-# RULE f g = 0 #-}
1025 Suppose that auto-specialisation makes a specialised version of
1026 g::Int->Int That version won't appear in the LHS of the RULE for f.
1027 So if the specialisation rule fires too early, the rule for f may
1030 It might be possible to add new rules, to "complete" the rewrite system.
1032 RULE forall d. g Int d = g_spec
1036 But that's a bit complicated. For now we ask the programmer's help,
1037 by *copying the INLINE activation pragma* to the auto-specialised rule.
1038 So if g says {-# NOINLINE[2] g #-}, then the auto-spec rule will also
1039 not be active until phase 2.
1042 Note [Specialisation shape]
1043 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1044 We only specialise a function if it has visible top-level lambdas
1045 corresponding to its overloading. E.g. if
1046 f :: forall a. Eq a => ....
1047 then its body must look like
1050 Reason: when specialising the body for a call (f ty dexp), we want to
1051 substitute dexp for d, and pick up specialised calls in the body of f.
1053 This doesn't always work. One example I came across was this:
1054 newtype Gen a = MkGen{ unGen :: Int -> a }
1056 choose :: Eq a => a -> Gen a
1057 choose n = MkGen (\r -> n)
1059 oneof = choose (1::Int)
1061 It's a silly exapmle, but we get
1062 choose = /\a. g `cast` co
1063 where choose doesn't have any dict arguments. Thus far I have not
1064 tried to fix this (wait till there's a real example).
1067 Note [Inline specialisations]
1068 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1069 We transfer to the specialised function any INLINE stuff from the
1070 original. This means (a) the Activation in the IdInfo, and (b) any
1071 InlineMe on the RHS.
1073 This is a change (Jun06). Previously the idea is that the point of
1074 inlining was precisely to specialise the function at its call site,
1075 and that's not so important for the specialised copies. But
1076 *pragma-directed* specialisation now takes place in the
1077 typechecker/desugarer, with manually specified INLINEs. The
1078 specialiation here is automatic. It'd be very odd if a function
1079 marked INLINE was specialised (because of some local use), and then
1080 forever after (including importing modules) the specialised version
1081 wasn't INLINEd. After all, the programmer said INLINE!
1083 You might wonder why we don't just not specialise INLINE functions.
1084 It's because even INLINE functions are sometimes not inlined, when
1085 they aren't applied to interesting arguments. But perhaps the type
1086 arguments alone are enough to specialise (even though the args are too
1087 boring to trigger inlining), and it's certainly better to call the
1088 specialised version.
1090 A case in point is dictionary functions, which are current marked
1091 INLINE, but which are worth specialising.
1094 dropInline :: CoreExpr -> (Bool, CoreExpr)
1095 dropInline (Note InlineMe rhs) = (True, rhs)
1096 dropInline rhs = (False, rhs)
1099 %************************************************************************
1101 \subsubsection{UsageDetails and suchlike}
1103 %************************************************************************
1108 dict_binds :: !(Bag DictBind),
1109 -- Floated dictionary bindings
1110 -- The order is important;
1111 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
1112 -- (Remember, Bags preserve order in GHC.)
1114 calls :: !CallDetails,
1116 ud_fvs :: !VarSet -- A superset of the variables mentioned in
1117 -- either dict_binds or calls
1120 instance Outputable UsageDetails where
1121 ppr (MkUD { dict_binds = dbs, calls = calls, ud_fvs = fvs })
1122 = ptext (sLit "MkUD") <+> braces (sep (punctuate comma
1123 [ptext (sLit "binds") <+> equals <+> ppr dbs,
1124 ptext (sLit "calls") <+> equals <+> ppr calls,
1125 ptext (sLit "fvs") <+> equals <+> ppr fvs]))
1127 type DictBind = (CoreBind, VarSet)
1128 -- The set is the free vars of the binding
1129 -- both tyvars and dicts
1131 type DictExpr = CoreExpr
1133 emptyUDs :: UsageDetails
1134 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM, ud_fvs = emptyVarSet }
1136 ------------------------------------------------------------
1137 type CallDetails = FiniteMap Id CallInfo
1138 newtype CallKey = CallKey [Maybe Type] -- Nothing => unconstrained type argument
1140 -- CallInfo uses a FiniteMap, thereby ensuring that
1141 -- we record only one call instance for any key
1143 -- The list of types and dictionaries is guaranteed to
1144 -- match the type of f
1145 type CallInfo = FiniteMap CallKey ([DictExpr], VarSet)
1146 -- Range is dict args and the vars of the whole
1147 -- call (including tyvars)
1148 -- [*not* include the main id itself, of course]
1150 instance Outputable CallKey where
1151 ppr (CallKey ts) = ppr ts
1153 -- Type isn't an instance of Ord, so that we can control which
1154 -- instance we use. That's tiresome here. Oh well
1155 instance Eq CallKey where
1156 k1 == k2 = case k1 `compare` k2 of { EQ -> True; _ -> False }
1158 instance Ord CallKey where
1159 compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
1161 cmp Nothing Nothing = EQ
1162 cmp Nothing (Just _) = LT
1163 cmp (Just _) Nothing = GT
1164 cmp (Just t1) (Just t2) = tcCmpType t1 t2
1166 unionCalls :: CallDetails -> CallDetails -> CallDetails
1167 unionCalls c1 c2 = plusFM_C plusFM c1 c2
1169 singleCall :: Id -> [Maybe Type] -> [DictExpr] -> UsageDetails
1170 singleCall id tys dicts
1171 = MkUD {dict_binds = emptyBag,
1172 calls = unitFM id (unitFM (CallKey tys) (dicts, call_fvs)),
1175 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
1176 tys_fvs = tyVarsOfTypes (catMaybes tys)
1177 -- The type args (tys) are guaranteed to be part of the dictionary
1178 -- types, because they are just the constrained types,
1179 -- and the dictionary is therefore sure to be bound
1180 -- inside the binding for any type variables free in the type;
1181 -- hence it's safe to neglect tyvars free in tys when making
1182 -- the free-var set for this call
1183 -- BUT I don't trust this reasoning; play safe and include tys_fvs
1185 -- We don't include the 'id' itself.
1187 mkCallUDs :: Id -> [CoreExpr] -> UsageDetails
1189 | not (isLocalId f) -- Imported from elsewhere
1190 || null theta -- Not overloaded
1191 || not (all isClassPred theta)
1192 -- Only specialise if all overloading is on class params.
1193 -- In ptic, with implicit params, the type args
1194 -- *don't* say what the value of the implicit param is!
1195 || not (spec_tys `lengthIs` n_tyvars)
1196 || not ( dicts `lengthIs` n_dicts)
1197 || not (any interestingArg dicts) -- Note [Interesting dictionary arguments]
1198 -- See also Note [Specialisations already covered]
1199 = -- pprTrace "mkCallUDs: discarding" (vcat [ppr f, ppr args, ppr n_tyvars, ppr n_dicts, ppr (map interestingArg dicts)])
1200 emptyUDs -- Not overloaded, or no specialisation wanted
1203 = -- pprTrace "mkCallUDs: keeping" (vcat [ppr f, ppr args, ppr n_tyvars, ppr n_dicts, ppr (map interestingArg dicts)])
1204 singleCall f spec_tys dicts
1206 (tyvars, theta, _) = tcSplitSigmaTy (idType f)
1207 constrained_tyvars = tyVarsOfTheta theta
1208 n_tyvars = length tyvars
1209 n_dicts = length theta
1211 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
1212 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
1215 | tyvar `elemVarSet` constrained_tyvars = Just ty
1216 | otherwise = Nothing
1219 Note [Interesting dictionary arguments]
1220 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1222 \a.\d:Eq a. let f = ... in ...(f d)...
1223 There really is not much point in specialising f wrt the dictionary d,
1224 because the code for the specialised f is not improved at all, because
1225 d is lambda-bound. We simply get junk specialisations.
1227 We re-use the function SimplUtils.interestingArg function to determine
1228 what sort of dictionary arguments have *some* information in them.
1232 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
1233 plusUDs (MkUD {dict_binds = db1, calls = calls1, ud_fvs = fvs1})
1234 (MkUD {dict_binds = db2, calls = calls2, ud_fvs = fvs2})
1235 = MkUD {dict_binds = d, calls = c, ud_fvs = fvs1 `unionVarSet` fvs2}
1237 d = db1 `unionBags` db2
1238 c = calls1 `unionCalls` calls2
1240 plusUDList :: [UsageDetails] -> UsageDetails
1241 plusUDList = foldr plusUDs emptyUDs
1243 -- zapCalls deletes calls to ids from uds
1244 zapCalls :: [Id] -> CallDetails -> CallDetails
1245 zapCalls ids calls = delListFromFM calls ids
1247 mkDB :: CoreBind -> DictBind
1248 mkDB bind = (bind, bind_fvs bind)
1250 bind_fvs :: CoreBind -> VarSet
1251 bind_fvs (NonRec bndr rhs) = pair_fvs (bndr,rhs)
1252 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1255 rhs_fvs = unionVarSets (map pair_fvs prs)
1257 pair_fvs :: (Id, CoreExpr) -> VarSet
1258 pair_fvs (bndr, rhs) = exprFreeVars rhs `unionVarSet` idFreeVars bndr
1259 -- Don't forget variables mentioned in the
1260 -- rules of the bndr. C.f. OccAnal.addRuleUsage
1261 -- Also tyvars mentioned in its type; they may not appear in the RHS
1265 addDictBind :: (Id,CoreExpr) -> UsageDetails -> UsageDetails
1266 addDictBind (dict,rhs) uds
1267 = uds { dict_binds = db `consBag` dict_binds uds
1268 , ud_fvs = ud_fvs uds `unionVarSet` fvs }
1270 db@(_, fvs) = mkDB (NonRec dict rhs)
1272 dumpAllDictBinds :: UsageDetails -> [CoreBind] -> [CoreBind]
1273 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
1274 = foldrBag add binds dbs
1276 add (bind,_) binds = bind : binds
1278 dumpUDs :: [CoreBndr]
1279 -> UsageDetails -> CoreExpr
1280 -> (UsageDetails, CoreExpr)
1281 dumpUDs bndrs (MkUD { dict_binds = orig_dbs
1282 , calls = orig_calls
1283 , ud_fvs = fvs}) body
1284 = (new_uds, foldrBag add_let body dump_dbs)
1285 -- This may delete fewer variables
1286 -- than in priciple possible
1289 MkUD { dict_binds = free_dbs
1290 , calls = free_calls
1291 , ud_fvs = fvs `minusVarSet` bndr_set}
1293 bndr_set = mkVarSet bndrs
1294 add_let (bind,_) body = Let bind body
1296 (free_dbs, dump_dbs, dump_set)
1297 = foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
1298 -- Important that it's foldl not foldr;
1299 -- we're accumulating the set of dumped ids in dump_set
1301 free_calls = filterCalls dump_set orig_calls
1303 dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1304 | dump_idset `intersectsVarSet` fvs -- Dump it
1305 = (free_dbs, dump_dbs `snocBag` db,
1306 extendVarSetList dump_idset (bindersOf bind))
1308 | otherwise -- Don't dump it
1309 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1311 filterCalls :: VarSet -> CallDetails -> CallDetails
1312 -- Remove any calls that mention the variables
1313 filterCalls bs calls
1314 = mapFM (\_ cs -> filter_calls cs) $
1315 filterFM (\k _ -> not (k `elemVarSet` bs)) calls
1317 filter_calls :: CallInfo -> CallInfo
1318 filter_calls = filterFM (\_ (_, fvs) -> not (fvs `intersectsVarSet` bs))
1322 %************************************************************************
1324 \subsubsection{Boring helper functions}
1326 %************************************************************************
1329 type SpecM a = UniqSM a
1331 initSM :: UniqSupply -> SpecM a -> a
1334 mapAndCombineSM :: (a -> SpecM (b, UsageDetails)) -> [a] -> SpecM ([b], UsageDetails)
1335 mapAndCombineSM _ [] = return ([], emptyUDs)
1336 mapAndCombineSM f (x:xs) = do (y, uds1) <- f x
1337 (ys, uds2) <- mapAndCombineSM f xs
1338 return (y:ys, uds1 `plusUDs` uds2)
1340 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1341 -- Clone the binders of the bind; return new bind with the cloned binders
1342 -- Return the substitution to use for RHSs, and the one to use for the body
1343 cloneBindSM subst (NonRec bndr rhs) = do
1344 us <- getUniqueSupplyM
1345 let (subst', bndr') = cloneIdBndr subst us bndr
1346 return (subst, subst', NonRec bndr' rhs)
1348 cloneBindSM subst (Rec pairs) = do
1349 us <- getUniqueSupplyM
1350 let (subst', bndrs') = cloneRecIdBndrs subst us (map fst pairs)
1351 return (subst', subst', Rec (bndrs' `zip` map snd pairs))
1353 cloneDictBndrs :: Subst -> [CoreBndr] -> SpecM (Subst, [CoreBndr])
1354 cloneDictBndrs subst bndrs
1355 = do { us <- getUniqueSupplyM
1356 ; return (cloneIdBndrs subst us bndrs) }
1358 newSpecIdSM :: Id -> Type -> SpecM Id
1359 -- Give the new Id a similar occurrence name to the old one
1360 newSpecIdSM old_id new_ty
1361 = do { uniq <- getUniqueM
1363 name = idName old_id
1364 new_occ = mkSpecOcc (nameOccName name)
1365 new_id = mkUserLocal new_occ uniq new_ty (getSrcSpan name)
1370 Old (but interesting) stuff about unboxed bindings
1371 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1373 What should we do when a value is specialised to a *strict* unboxed value?
1375 map_*_* f (x:xs) = let h = f x
1379 Could convert let to case:
1381 map_*_Int# f (x:xs) = case f x of h# ->
1385 This may be undesirable since it forces evaluation here, but the value
1386 may not be used in all branches of the body. In the general case this
1387 transformation is impossible since the mutual recursion in a letrec
1388 cannot be expressed as a case.
1390 There is also a problem with top-level unboxed values, since our
1391 implementation cannot handle unboxed values at the top level.
1393 Solution: Lift the binding of the unboxed value and extract it when it
1396 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1401 Now give it to the simplifier and the _Lifting will be optimised away.
1403 The benfit is that we have given the specialised "unboxed" values a
1404 very simplep lifted semantics and then leave it up to the simplifier to
1405 optimise it --- knowing that the overheads will be removed in nearly
1408 In particular, the value will only be evaluted in the branches of the
1409 program which use it, rather than being forced at the point where the
1410 value is bound. For example:
1412 filtermap_*_* p f (x:xs)
1419 filtermap_*_Int# p f (x:xs)
1420 = let h = case (f x) of h# -> _Lift h#
1423 True -> case h of _Lift h#
1427 The binding for h can still be inlined in the one branch and the
1428 _Lifting eliminated.
1431 Question: When won't the _Lifting be eliminated?
1433 Answer: When they at the top-level (where it is necessary) or when
1434 inlining would duplicate work (or possibly code depending on
1435 options). However, the _Lifting will still be eliminated if the
1436 strictness analyser deems the lifted binding strict.
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