2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1996
4 \section[Specialise]{Stamping out overloading, and (optionally) polymorphism}
12 #include "HsVersions.h"
14 import MkId ( mkUserLocal )
15 import Id ( Id, DictVar, idType, mkTemplateLocals,
17 getIdSpecialisation, setIdSpecialisation, isSpecPragmaId,
19 IdSet, mkIdSet, addOneToIdSet, intersectIdSets, isEmptyIdSet,
20 emptyIdSet, unionIdSets, minusIdSet, unitIdSet, elementOfIdSet,
22 IdEnv, mkIdEnv, lookupIdEnv, addOneToIdEnv, delOneFromIdEnv
25 import Type ( Type, mkTyVarTy, splitSigmaTy, instantiateTy, isDictTy,
26 tyVarsOfType, tyVarsOfTypes, applyTys, mkForAllTys
28 import TyCon ( TyCon )
29 import TyVar ( TyVar, mkTyVar,
30 TyVarSet, mkTyVarSet, isEmptyTyVarSet, intersectTyVarSets,
31 elementOfTyVarSet, unionTyVarSets, emptyTyVarSet,
33 TyVarEnv, mkTyVarEnv, delFromTyVarEnv
35 import Kind ( mkBoxedTypeKind )
37 import FreeVars ( exprFreeVars )
38 import PprCore () -- Instances
39 import Name ( NamedThing(..), getSrcLoc, mkSysLocalName, isLocallyDefined )
40 import SrcLoc ( noSrcLoc )
41 import SpecEnv ( addToSpecEnv, lookupSpecEnv, specEnvValues )
43 import UniqSupply ( UniqSupply,
44 UniqSM, initUs, thenUs, returnUs, getUnique, mapUs
46 import Unique ( mkAlphaTyVarUnique )
48 import Maybes ( MaybeErr(..), maybeToBool )
50 import List ( partition )
51 import Util ( zipEqual )
58 %************************************************************************
60 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
62 %************************************************************************
64 These notes describe how we implement specialisation to eliminate
67 The specialisation pass works on Core
68 syntax, complete with all the explicit dictionary application,
69 abstraction and construction as added by the type checker. The
70 existing type checker remains largely as it is.
72 One important thought: the {\em types} passed to an overloaded
73 function, and the {\em dictionaries} passed are mutually redundant.
74 If the same function is applied to the same type(s) then it is sure to
75 be applied to the same dictionary(s)---or rather to the same {\em
76 values}. (The arguments might look different but they will evaluate
79 Second important thought: we know that we can make progress by
80 treating dictionary arguments as static and worth specialising on. So
81 we can do without binding-time analysis, and instead specialise on
82 dictionary arguments and no others.
91 and suppose f is overloaded.
93 STEP 1: CALL-INSTANCE COLLECTION
95 We traverse <body>, accumulating all applications of f to types and
98 (Might there be partial applications, to just some of its types and
99 dictionaries? In principle yes, but in practice the type checker only
100 builds applications of f to all its types and dictionaries, so partial
101 applications could only arise as a result of transformation, and even
102 then I think it's unlikely. In any case, we simply don't accumulate such
103 partial applications.)
105 There's a choice of whether to collect details of all *polymorphic* functions
106 or simply all *overloaded* ones. How to sort this out?
107 Pass in a predicate on the function to say if it is "interesting"?
108 This is dependent on the user flags: SpecialiseOverloaded
114 So now we have a collection of calls to f:
118 Notice that f may take several type arguments. To avoid ambiguity, we
119 say that f is called at type t1/t2 and t3/t4.
121 We take equivalence classes using equality of the *types* (ignoring
122 the dictionary args, which as mentioned previously are redundant).
124 STEP 3: SPECIALISATION
126 For each equivalence class, choose a representative (f t1 t2 d1 d2),
127 and create a local instance of f, defined thus:
129 f@t1/t2 = <f_rhs> t1 t2 d1 d2
131 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
132 of simplification will now result. However we don't actually *do* that
133 simplification. Rather, we leave it for the simplifier to do. If we
134 *did* do it, though, we'd get more call instances from the specialised
135 RHS. We can work out what they are by instantiating the call-instance
136 set from f's RHS with the types t1, t2.
138 Add this new id to f's IdInfo, to record that f has a specialised version.
140 Before doing any of this, check that f's IdInfo doesn't already
141 tell us about an existing instance of f at the required type/s.
142 (This might happen if specialisation was applied more than once, or
143 it might arise from user SPECIALIZE pragmas.)
147 Wait a minute! What if f is recursive? Then we can't just plug in
148 its right-hand side, can we?
150 But it's ok. The type checker *always* creates non-recursive definitions
151 for overloaded recursive functions. For example:
153 f x = f (x+x) -- Yes I know its silly
157 f a (d::Num a) = let p = +.sel a d
159 letrec fl (y::a) = fl (p y y)
163 We still have recusion for non-overloaded functions which we
164 speciailise, but the recursive call should get specialised to the
165 same recursive version.
171 All this is crystal clear when the function is applied to *constant
172 types*; that is, types which have no type variables inside. But what if
173 it is applied to non-constant types? Suppose we find a call of f at type
174 t1/t2. There are two possibilities:
176 (a) The free type variables of t1, t2 are in scope at the definition point
177 of f. In this case there's no problem, we proceed just as before. A common
178 example is as follows. Here's the Haskell:
183 After typechecking we have
185 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
186 in +.sel a d (f a d y) (f a d y)
188 Notice that the call to f is at type type "a"; a non-constant type.
189 Both calls to f are at the same type, so we can specialise to give:
191 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
192 in +.sel a d (f@a y) (f@a y)
195 (b) The other case is when the type variables in the instance types
196 are *not* in scope at the definition point of f. The example we are
197 working with above is a good case. There are two instances of (+.sel a d),
198 but "a" is not in scope at the definition of +.sel. Can we do anything?
199 Yes, we can "common them up", a sort of limited common sub-expression deal.
202 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
203 f@a (x::a) = +.sel@a x x
204 in +.sel@a (f@a y) (f@a y)
206 This can save work, and can't be spotted by the type checker, because
207 the two instances of +.sel weren't originally at the same type.
211 * There are quite a few variations here. For example, the defn of
212 +.sel could be floated ouside the \y, to attempt to gain laziness.
213 It certainly mustn't be floated outside the \d because the d has to
216 * We don't want to inline f_rhs in this case, because
217 that will duplicate code. Just commoning up the call is the point.
219 * Nothing gets added to +.sel's IdInfo.
221 * Don't bother unless the equivalence class has more than one item!
223 Not clear whether this is all worth it. It is of course OK to
224 simply discard call-instances when passing a big lambda.
226 Polymorphism 2 -- Overloading
228 Consider a function whose most general type is
230 f :: forall a b. Ord a => [a] -> b -> b
232 There is really no point in making a version of g at Int/Int and another
233 at Int/Bool, because it's only instancing the type variable "a" which
234 buys us any efficiency. Since g is completely polymorphic in b there
235 ain't much point in making separate versions of g for the different
238 That suggests that we should identify which of g's type variables
239 are constrained (like "a") and which are unconstrained (like "b").
240 Then when taking equivalence classes in STEP 2, we ignore the type args
241 corresponding to unconstrained type variable. In STEP 3 we make
242 polymorphic versions. Thus:
244 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
253 f a (d::Num a) = let g = ...
255 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
257 Here, g is only called at one type, but the dictionary isn't in scope at the
258 definition point for g. Usually the type checker would build a
259 definition for d1 which enclosed g, but the transformation system
260 might have moved d1's defn inward. Solution: float dictionary bindings
261 outwards along with call instances.
265 f x = let g p q = p==q
271 Before specialisation, leaving out type abstractions we have
273 f df x = let g :: Eq a => a -> a -> Bool
275 h :: Num a => a -> a -> (a, Bool)
276 h dh r s = let deq = eqFromNum dh
277 in (+ dh r s, g deq r s)
281 After specialising h we get a specialised version of h, like this:
283 h' r s = let deq = eqFromNum df
284 in (+ df r s, g deq r s)
286 But we can't naively make an instance for g from this, because deq is not in scope
287 at the defn of g. Instead, we have to float out the (new) defn of deq
288 to widen its scope. Notice that this floating can't be done in advance -- it only
289 shows up when specialisation is done.
291 User SPECIALIZE pragmas
292 ~~~~~~~~~~~~~~~~~~~~~~~
293 Specialisation pragmas can be digested by the type checker, and implemented
294 by adding extra definitions along with that of f, in the same way as before
296 f@t1/t2 = <f_rhs> t1 t2 d1 d2
298 Indeed the pragmas *have* to be dealt with by the type checker, because
299 only it knows how to build the dictionaries d1 and d2! For example
301 g :: Ord a => [a] -> [a]
302 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
304 Here, the specialised version of g is an application of g's rhs to the
305 Ord dictionary for (Tree Int), which only the type checker can conjure
306 up. There might not even *be* one, if (Tree Int) is not an instance of
307 Ord! (All the other specialision has suitable dictionaries to hand
310 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
311 it is buried in a complex (as-yet-un-desugared) binding group.
314 f@t1/t2 = f* t1 t2 d1 d2
316 where f* is the Id f with an IdInfo which says "inline me regardless!".
317 Indeed all the specialisation could be done in this way.
318 That in turn means that the simplifier has to be prepared to inline absolutely
319 any in-scope let-bound thing.
322 Again, the pragma should permit polymorphism in unconstrained variables:
324 h :: Ord a => [a] -> b -> b
325 {-# SPECIALIZE h :: [Int] -> b -> b #-}
327 We *insist* that all overloaded type variables are specialised to ground types,
328 (and hence there can be no context inside a SPECIALIZE pragma).
329 We *permit* unconstrained type variables to be specialised to
331 - or left as a polymorphic type variable
332 but nothing in between. So
334 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
336 is *illegal*. (It can be handled, but it adds complication, and gains the
340 SPECIALISING INSTANCE DECLARATIONS
341 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
344 instance Foo a => Foo [a] where
346 {-# SPECIALIZE instance Foo [Int] #-}
348 The original instance decl creates a dictionary-function
351 dfun.Foo.List :: forall a. Foo a -> Foo [a]
353 The SPECIALIZE pragma just makes a specialised copy, just as for
354 ordinary function definitions:
356 dfun.Foo.List@Int :: Foo [Int]
357 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
359 The information about what instance of the dfun exist gets added to
360 the dfun's IdInfo in the same way as a user-defined function too.
363 Automatic instance decl specialisation?
364 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
365 Can instance decls be specialised automatically? It's tricky.
366 We could collect call-instance information for each dfun, but
367 then when we specialised their bodies we'd get new call-instances
368 for ordinary functions; and when we specialised their bodies, we might get
369 new call-instances of the dfuns, and so on. This all arises because of
370 the unrestricted mutual recursion between instance decls and value decls.
372 Still, there's no actual problem; it just means that we may not do all
373 the specialisation we could theoretically do.
375 Furthermore, instance decls are usually exported and used non-locally,
376 so we'll want to compile enough to get those specialisations done.
378 Lastly, there's no such thing as a local instance decl, so we can
379 survive solely by spitting out *usage* information, and then reading that
380 back in as a pragma when next compiling the file. So for now,
381 we only specialise instance decls in response to pragmas.
384 SPITTING OUT USAGE INFORMATION
385 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
387 To spit out usage information we need to traverse the code collecting
388 call-instance information for all imported (non-prelude?) functions
389 and data types. Then we equivalence-class it and spit it out.
391 This is done at the top-level when all the call instances which escape
392 must be for imported functions and data types.
394 *** Not currently done ***
397 Partial specialisation by pragmas
398 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
399 What about partial specialisation:
401 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
402 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
406 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
408 Seems quite reasonable. Similar things could be done with instance decls:
410 instance (Foo a, Foo b) => Foo (a,b) where
412 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
413 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
415 Ho hum. Things are complex enough without this. I pass.
418 Requirements for the simplifer
419 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
420 The simplifier has to be able to take advantage of the specialisation.
422 * When the simplifier finds an application of a polymorphic f, it looks in
423 f's IdInfo in case there is a suitable instance to call instead. This converts
425 f t1 t2 d1 d2 ===> f_t1_t2
427 Note that the dictionaries get eaten up too!
429 * Dictionary selection operations on constant dictionaries must be
432 +.sel Int d ===> +Int
434 The obvious way to do this is in the same way as other specialised
435 calls: +.sel has inside it some IdInfo which tells that if it's applied
436 to the type Int then it should eat a dictionary and transform to +Int.
438 In short, dictionary selectors need IdInfo inside them for constant
441 * Exactly the same applies if a superclass dictionary is being
444 Eq.sel Int d ===> dEqInt
446 * Something similar applies to dictionary construction too. Suppose
447 dfun.Eq.List is the function taking a dictionary for (Eq a) to
448 one for (Eq [a]). Then we want
450 dfun.Eq.List Int d ===> dEq.List_Int
452 Where does the Eq [Int] dictionary come from? It is built in
453 response to a SPECIALIZE pragma on the Eq [a] instance decl.
455 In short, dfun Ids need IdInfo with a specialisation for each
456 constant instance of their instance declaration.
458 All this uses a single mechanism: the SpecEnv inside an Id
461 What does the specialisation IdInfo look like?
462 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
464 The SpecEnv of an Id maps a list of types (the template) to an expression
468 For example, if f has this SpecInfo:
470 [Int, a] -> \d:Ord Int. f' a
472 it means that we can replace the call
474 f Int t ===> (\d. f' t)
476 This chucks one dictionary away and proceeds with the
477 specialised version of f, namely f'.
480 What can't be done this way?
481 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
482 There is no way, post-typechecker, to get a dictionary for (say)
483 Eq a from a dictionary for Eq [a]. So if we find
487 we can't transform to
492 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
494 Of course, we currently have no way to automatically derive
495 eqList, nor to connect it to the Eq [a] instance decl, but you
496 can imagine that it might somehow be possible. Taking advantage
497 of this is permanently ruled out.
499 Still, this is no great hardship, because we intend to eliminate
500 overloading altogether anyway!
504 A note about non-tyvar dictionaries
505 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
506 Some Ids have types like
508 forall a,b,c. Eq a -> Ord [a] -> tau
510 This seems curious at first, because we usually only have dictionary
511 args whose types are of the form (C a) where a is a type variable.
512 But this doesn't hold for the functions arising from instance decls,
513 which sometimes get arguements with types of form (C (T a)) for some
516 Should we specialise wrt this compound-type dictionary? We used to say
518 "This is a heuristic judgement, as indeed is the fact that we
519 specialise wrt only dictionaries. We choose *not* to specialise
520 wrt compound dictionaries because at the moment the only place
521 they show up is in instance decls, where they are simply plugged
522 into a returned dictionary. So nothing is gained by specialising
525 But it is simpler and more uniform to specialise wrt these dicts too;
526 and in future GHC is likely to support full fledged type signatures
528 f ;: Eq [(a,b)] => ...
531 %************************************************************************
533 \subsubsection{The new specialiser}
535 %************************************************************************
537 Our basic game plan is this. For let(rec) bound function
538 f :: (C a, D c) => (a,b,c,d) -> Bool
540 * Find any specialised calls of f, (f ts ds), where
541 ts are the type arguments t1 .. t4, and
542 ds are the dictionary arguments d1 .. d2.
544 * Add a new definition for f1 (say):
546 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
548 Note that we abstract over the unconstrained type arguments.
552 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
554 to the specialisations of f. This will be used by the
555 simplifier to replace calls
556 (f t1 t2 t3 t4) da db
558 (\d1 d1 -> f1 t2 t4) da db
560 All the stuff about how many dictionaries to discard, and what types
561 to apply the specialised function to, are handled by the fact that the
562 SpecEnv contains a template for the result of the specialisation.
564 We don't build *partial* specialisations for f. For example:
566 f :: Eq a => a -> a -> Bool
567 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
569 Here, little is gained by making a specialised copy of f.
570 There's a distinct danger that the specialised version would
571 first build a dictionary for (Eq b, Eq c), and then select the (==)
572 method from it! Even if it didn't, not a great deal is saved.
574 We do, however, generate polymorphic, but not overloaded, specialisations:
576 f :: Eq a => [a] -> b -> b -> b
577 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
579 Hence, the invariant is this:
581 *** no specialised version is overloaded ***
584 %************************************************************************
586 \subsubsection{The exported function}
588 %************************************************************************
591 specProgram :: UniqSupply -> [CoreBinding] -> [CoreBinding]
593 = initSM us (go binds `thenSM` \ (binds', uds') ->
594 returnSM (dumpAllDictBinds uds' binds')
597 go [] = returnSM ([], emptyUDs)
598 go (bind:binds) = go binds `thenSM` \ (binds', uds) ->
599 specBind bind uds `thenSM` \ (bind', uds') ->
600 returnSM (bind' ++ binds', uds')
603 %************************************************************************
605 \subsubsection{@specExpr@: the main function}
607 %************************************************************************
610 specExpr :: CoreExpr -> SpecM (CoreExpr, UsageDetails)
612 ---------------- First the easy cases --------------------
613 specExpr e@(Var _) = returnSM (e, emptyUDs)
614 specExpr e@(Lit _) = returnSM (e, emptyUDs)
615 specExpr e@(Con _ _) = returnSM (e, emptyUDs)
616 specExpr e@(Prim _ _) = returnSM (e, emptyUDs)
618 specExpr (Note note body)
619 = specExpr body `thenSM` \ (body', uds) ->
620 returnSM (Note note body', uds)
623 ---------------- Applications might generate a call instance --------------------
624 specExpr e@(App fun arg)
627 go (App fun arg) args = go fun (arg:args)
628 go (Var f) args = returnSM (e, mkCallUDs f args)
629 go other args = specExpr other `thenSM` \ (e', uds) ->
630 returnSM (foldl App e' args, uds)
632 ---------------- Lambda/case require dumping of usage details --------------------
634 = specExpr body `thenSM` \ (body', uds) ->
636 (filtered_uds, body'') = dumpUDs bndrs uds body'
638 returnSM (foldr Lam body'' bndrs, filtered_uds)
640 (bndrs, body) = go [] e
642 -- More efficient to collect a group of binders together all at once
643 go bndrs (Lam bndr e) = go (bndr:bndrs) e
644 go bndrs e = (reverse bndrs, e)
647 specExpr (Case scrut alts)
648 = specExpr scrut `thenSM` \ (scrut', uds_scrut) ->
649 spec_alts alts `thenSM` \ (alts', uds_alts) ->
650 returnSM (Case scrut' alts', uds_scrut `plusUDs` uds_alts)
652 spec_alts (AlgAlts alts deflt)
653 = mapAndCombineSM spec_alg_alt alts `thenSM` \ (alts', uds1) ->
654 spec_deflt deflt `thenSM` \ (deflt', uds2) ->
655 returnSM (AlgAlts alts' deflt', uds1 `plusUDs` uds2)
657 spec_alts (PrimAlts alts deflt)
658 = mapAndCombineSM spec_prim_alt alts `thenSM` \ (alts', uds1) ->
659 spec_deflt deflt `thenSM` \ (deflt', uds2) ->
660 returnSM (PrimAlts alts' deflt', uds1 `plusUDs` uds2)
662 spec_alg_alt (con, args, rhs)
663 = specExpr rhs `thenSM` \ (rhs', uds) ->
665 (uds', rhs'') = dumpUDs (map ValBinder args) uds rhs'
667 returnSM ((con, args, rhs''), uds')
669 spec_prim_alt (lit, rhs)
670 = specExpr rhs `thenSM` \ (rhs', uds) ->
671 returnSM ((lit, rhs'), uds)
673 spec_deflt NoDefault = returnSM (NoDefault, emptyUDs)
674 spec_deflt (BindDefault arg rhs)
675 = specExpr rhs `thenSM` \ (rhs', uds) ->
677 (uds', rhs'') = dumpUDs [ValBinder arg] uds rhs'
679 returnSM (BindDefault arg rhs'', uds')
681 ---------------- Finally, let is the interesting case --------------------
682 specExpr (Let bind body)
683 = -- Deal with the body
684 specExpr body `thenSM` \ (body', body_uds) ->
686 -- Deal with the bindings
687 specBind bind body_uds `thenSM` \ (binds', uds) ->
690 returnSM (foldr Let body' binds', uds)
693 %************************************************************************
695 \subsubsection{Dealing with a binding}
697 %************************************************************************
700 specBind :: CoreBinding
701 -> UsageDetails -- Info on how the scope of the binding
702 -> SpecM ([CoreBinding], -- New bindings
703 UsageDetails) -- And info to pass upstream
705 specBind (NonRec bndr rhs) body_uds
706 | isDictTy (idType bndr)
707 = -- It's a dictionary binding
708 -- Pick it up and float it outwards.
709 specExpr rhs `thenSM` \ (rhs', rhs_uds) ->
711 all_uds = rhs_uds `plusUDs` addDictBind body_uds bndr rhs'
713 returnSM ([], all_uds)
715 | isSpecPragmaId bndr
716 = specExpr rhs `thenSM` \ (rhs', rhs_uds) ->
717 returnSM ([], rhs_uds `plusUDs` body_uds)
720 = -- Deal with the RHS, specialising it according
721 -- to the calls found in the body
722 specDefn (calls body_uds) (bndr,rhs) `thenSM` \ ((bndr',rhs'), spec_defns, spec_uds) ->
724 (all_uds, (dict_binds, dump_calls))
725 = splitUDs [ValBinder bndr]
726 (body_uds `plusUDs` spec_uds)
727 -- It's important that the `plusUDs` is this way round,
728 -- because body_uds may bind dictionaries that are
729 -- used in the calls passed to specDefn. So the
730 -- dictionary bindings in spec_uds may mention
731 -- dictionaries bound in body_uds.
733 -- If we make specialisations then we Rec the whole lot together
734 -- If not, leave it as a NonRec
735 new_bind | null spec_defns = NonRec bndr' rhs'
736 | otherwise = Rec ((bndr',rhs'):spec_defns)
738 returnSM ( new_bind : mkDictBinds dict_binds, all_uds )
740 specBind (Rec pairs) body_uds
741 = mapSM (specDefn (calls body_uds)) pairs `thenSM` \ stuff ->
743 (pairs', spec_defns_s, spec_uds_s) = unzip3 stuff
744 spec_defns = concat spec_defns_s
745 spec_uds = plusUDList spec_uds_s
747 (all_uds, (dict_binds, dump_calls))
748 = splitUDs (map (ValBinder . fst) pairs)
749 (body_uds `plusUDs` spec_uds)
750 -- See notes for non-rec case
752 new_bind = Rec (spec_defns ++ pairs')
754 returnSM ( new_bind : mkDictBinds dict_binds, all_uds )
756 specDefn :: CallDetails -- Info on how it is used in its scope
757 -> (Id, CoreExpr) -- The thing being bound and its un-processed RHS
758 -> SpecM ((Id, CoreExpr), -- The thing and its processed RHS
759 -- the Id may now have specialisations attached
760 [(Id,CoreExpr)], -- Extra, specialised bindings
761 UsageDetails -- Stuff to fling upwards from the RHS and its
762 ) -- specialised versions
764 specDefn calls (fn, rhs)
765 -- The first case is the interesting one
766 | n_tyvars == length rhs_tyvars -- Rhs of fn's defn has right number of big lambdas
767 && n_dicts <= length rhs_bndrs -- and enough dict args
768 && not (null calls_for_me) -- And there are some calls to specialise
769 = -- Specialise the body of the function
770 specExpr body `thenSM` \ (body', body_uds) ->
772 (float_uds, bound_uds@(dict_binds,_)) = splitUDs rhs_bndrs body_uds
775 -- Make a specialised version for each call in calls_for_me
776 mapSM (spec_call bound_uds) calls_for_me `thenSM` \ stuff ->
778 (spec_defns, spec_uds, spec_env_stuff) = unzip3 stuff
780 fn' = addIdSpecialisations fn spec_env_stuff
781 rhs' = foldr Lam (mkDictLets dict_binds body') rhs_bndrs
783 returnSM ((fn',rhs'),
785 float_uds `plusUDs` plusUDList spec_uds)
787 | otherwise -- No calls or RHS doesn't fit our preconceptions
788 = specExpr rhs `thenSM` \ (rhs', rhs_uds) ->
789 returnSM ((fn, rhs'), [], rhs_uds)
793 (tyvars, theta, tau) = splitSigmaTy fn_type
794 n_tyvars = length tyvars
795 n_dicts = length theta
796 mk_spec_tys call_ts = zipWith mk_spec_ty call_ts tyVarTemplates
798 mk_spec_ty (Just ty) _ = ty
799 mk_spec_ty Nothing tyvar = mkTyVarTy tyvar
801 (rhs_tyvars, rhs_ids, rhs_body) = collectBinders rhs
802 rhs_dicts = take n_dicts rhs_ids
803 rhs_bndrs = map TyBinder rhs_tyvars ++ map ValBinder rhs_dicts
804 body = mkValLam (drop n_dicts rhs_ids) rhs_body
805 -- Glue back on the non-dict lambdas
807 calls_for_me = case lookupFM calls fn of
809 Just cs -> fmToList cs
811 ----------------------------------------------------------
812 -- Specialise to one particular call pattern
813 spec_call :: ProtoUsageDetails -- From the original body, captured by
814 -- the dictionary lambdas
815 -> ([Maybe Type], [DictVar]) -- Call instance
816 -> SpecM ((Id,CoreExpr), -- Specialised definition
817 UsageDetails, -- Usage details from specialised body
818 ([TyVar], [Type], CoreExpr)) -- Info for the Id's SpecEnv
819 spec_call bound_uds (call_ts, call_ds)
820 = ASSERT( length call_ts == n_tyvars && length call_ds == n_dicts )
821 -- Calls are only recorded for properly-saturated applications
823 -- Supppose the call is for f [Just t1, Nothing, Just t3, Nothing] [d1, d2]
825 -- Construct the new binding
826 -- f1 = /\ b d -> (..rhs of f..) t1 b t3 d d1 d2
827 -- and the type of this binder
829 spec_tyvars = [tyvar | (tyvar, Nothing) <- tyVarTemplates `zip` call_ts]
830 spec_tys = mk_spec_tys call_ts
831 spec_rhs = mkTyLam spec_tyvars $
832 mkGenApp rhs (map TyArg spec_tys ++ map VarArg call_ds)
833 spec_id_ty = mkForAllTys spec_tyvars (instantiateTy ty_env tau)
834 ty_env = mkTyVarEnv (zipEqual "spec_call" tyvars spec_tys)
837 newIdSM fn spec_id_ty `thenSM` \ spec_f ->
840 -- Construct the stuff for f's spec env
841 -- [b,d] [t1,b,t3,d] |-> \d1 d2 -> f1 b d
842 -- The only awkward bit is that d1,d2 might well be global
843 -- dictionaries, so it's tidier to make new local variables
844 -- for the lambdas in the RHS, rather than lambda-bind the
845 -- dictionaries themselves.
847 -- In fact we use the standard template locals, so that the
848 -- they don't need to be "tidied" before putting in interface files
850 arg_ds = mkTemplateLocals (map idType call_ds)
851 spec_env_rhs = mkValLam arg_ds $
852 mkTyApp (Var spec_f) $
853 map mkTyVarTy spec_tyvars
854 spec_env_info = (spec_tyvars, spec_tys, spec_env_rhs)
857 -- Specialise the UDs from f's RHS
859 -- Only the overloaded tyvars should be free in the uds
860 ty_env = [ (rhs_tyvar,ty)
861 | (rhs_tyvar, Just ty) <- zipEqual "specUDs1" rhs_tyvars call_ts
863 dict_env = zipEqual "specUDs2" rhs_dicts call_ds
865 specUDs ty_env dict_env bound_uds `thenSM` \ spec_uds ->
867 returnSM ((spec_f, spec_rhs),
873 %************************************************************************
875 \subsubsection{UsageDetails and suchlike}
877 %************************************************************************
880 type FreeDicts = IdSet
884 dict_binds :: !(Bag DictBind),
885 -- Floated dictionary bindings
886 -- The order is important;
887 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
888 -- (Remember, Bags preserve order in GHC.)
889 -- The FreeDicts is the free vars of the RHS
891 calls :: !CallDetails
894 type DictBind = (DictVar, CoreExpr, TyVarSet, FreeDicts)
895 -- The FreeDicts are the free dictionaries (only)
896 -- of the RHS of the dictionary bindings
897 -- Similarly the TyVarSet
899 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM }
901 type ProtoUsageDetails = ([DictBind],
902 [(Id, [Maybe Type], [DictVar])]
905 ------------------------------------------------------------
906 type CallDetails = FiniteMap Id CallInfo
907 type CallInfo = FiniteMap [Maybe Type] -- Nothing => unconstrained type argument
908 [DictVar] -- Dict args
909 -- The finite maps eliminate duplicates
910 -- The list of types and dictionaries is guaranteed to
911 -- match the type of f
913 callDetailsToList calls = [ (id,tys,dicts)
914 | (id,fm) <- fmToList calls,
915 (tys,dicts) <- fmToList fm
918 listToCallDetails calls = foldr (unionCalls . singleCall) emptyFM calls
920 unionCalls :: CallDetails -> CallDetails -> CallDetails
921 unionCalls c1 c2 = plusFM_C plusFM c1 c2
923 singleCall (id, tys, dicts) = unitFM id (unitFM tys dicts)
927 || length spec_tys /= n_tyvars
928 || length dicts /= n_dicts
929 = emptyUDs -- Not overloaded
932 = MkUD {dict_binds = emptyBag,
933 calls = singleCall (f, spec_tys, dicts)
936 (tyvars, theta, tau) = splitSigmaTy (idType f)
937 constrained_tyvars = foldr (unionTyVarSets . tyVarsOfTypes . snd) emptyTyVarSet theta
938 n_tyvars = length tyvars
939 n_dicts = length theta
941 spec_tys = [mk_spec_ty tv ty | (tv, TyArg ty) <- tyvars `zip` args]
942 dicts = [d | (_, VarArg d) <- theta `zip` (drop n_tyvars args)]
944 mk_spec_ty tyvar ty | tyvar `elementOfTyVarSet` constrained_tyvars
949 ------------------------------------------------------------
950 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
951 plusUDs (MkUD {dict_binds = db1, calls = calls1})
952 (MkUD {dict_binds = db2, calls = calls2})
953 = MkUD {dict_binds, calls}
955 dict_binds = db1 `unionBags` db2
956 calls = calls1 `unionCalls` calls2
958 plusUDList = foldr plusUDs emptyUDs
960 mkDB dict rhs = (dict, rhs, db_ftvs, db_fvs)
962 db_ftvs = tyVarsOfType (idType dict) -- Superset of RHS fvs
963 db_fvs = dictRhsFVs rhs
965 addDictBind uds dict rhs = uds { dict_binds = mkDB dict rhs `consBag` dict_binds uds }
967 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
968 = foldrBag add binds dbs
970 add (dict,rhs,_,_) binds = NonRec dict rhs : binds
972 mkDictBinds :: [DictBind] -> [CoreBinding]
973 mkDictBinds = map (\(d,r,_,_) -> NonRec d r)
975 mkDictLets :: [DictBind] -> CoreExpr -> CoreExpr
976 mkDictLets dbs body = foldr mk body dbs
978 mk (d,r,_,_) e = Let (NonRec d r) e
980 dumpUDs :: [CoreBinder]
981 -> UsageDetails -> CoreExpr
982 -> (UsageDetails, CoreExpr)
983 dumpUDs bndrs uds body
984 = (free_uds, mkDictLets dict_binds body)
986 (free_uds, (dict_binds, _)) = splitUDs bndrs uds
988 splitUDs :: [CoreBinder]
990 -> (UsageDetails, -- These don't mention the binders
991 ProtoUsageDetails) -- These do
993 splitUDs bndrs uds@(MkUD {dict_binds = orig_dbs,
996 = if isEmptyBag dump_dbs && null dump_calls then
997 -- Common case: binder doesn't affect floats
1001 -- Binders bind some of the fvs of the floats
1002 (MkUD {dict_binds = free_dbs,
1003 calls = listToCallDetails free_calls},
1004 (bagToList dump_dbs, dump_calls)
1008 tyvar_set = mkTyVarSet [tv | TyBinder tv <- bndrs]
1009 id_set = mkIdSet [id | ValBinder id <- bndrs]
1011 (free_dbs, dump_dbs, dump_idset)
1012 = foldlBag dump_db (emptyBag, emptyBag, id_set) orig_dbs
1013 -- Important that it's foldl not foldr;
1014 -- we're accumulating the set of dumped ids in dump_set
1016 -- Filter out any calls that mention things that are being dumped
1017 -- Don't need to worry about the tyvars because the dicts will
1018 -- spot the captured ones; any fully polymorphic arguments will
1019 -- be Nothings in the call details
1020 orig_call_list = callDetailsToList orig_calls
1021 (dump_calls, free_calls) = partition captured orig_call_list
1022 captured (id,tys,dicts) = any (`elementOfIdSet` dump_idset) (id:dicts)
1024 dump_db (free_dbs, dump_dbs, dump_idset) db@(dict, rhs, ftvs, fvs)
1025 | isEmptyIdSet (dump_idset `intersectIdSets` fvs)
1026 && isEmptyTyVarSet (tyvar_set `intersectTyVarSets` ftvs)
1027 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1029 | otherwise -- Dump it
1030 = (free_dbs, dump_dbs `snocBag` db,
1031 dump_idset `addOneToIdSet` dict)
1034 Given a type and value substitution, specUDs creates a specialised copy of
1038 specUDs :: [(TyVar,Type)] -> [(DictVar,DictVar)] -> ProtoUsageDetails -> SpecM UsageDetails
1039 specUDs tv_env_list dict_env_list (dbs, calls)
1040 = specDBs dict_env_list dbs `thenSM` \ (dict_env_list', dbs') ->
1042 dict_env = mkIdEnv dict_env_list'
1044 returnSM (MkUD { dict_binds = dbs',
1045 calls = listToCallDetails (map (inst_call dict_env) calls)
1048 bound_tyvars = mkTyVarSet (map fst tv_env_list)
1049 tv_env = mkTyVarEnv tv_env_list -- Doesn't change
1051 inst_call dict_env (id, tys, dicts) = (id, map inst_maybe_ty tys,
1052 map (lookupId dict_env) dicts)
1054 inst_maybe_ty Nothing = Nothing
1055 inst_maybe_ty (Just ty) = Just (instantiateTy tv_env ty)
1058 = returnSM (dict_env, emptyBag)
1059 specDBs dict_env ((dict, rhs, ftvs, fvs) : dbs)
1060 = newIdSM dict (instantiateTy tv_env (idType dict)) `thenSM` \ dict' ->
1062 rhs' = foldl App (foldr Lam rhs (t_bndrs ++ d_bndrs)) (t_args ++ d_args)
1063 (t_bndrs, t_args) = unzip [(TyBinder tv, TyArg ty) | (tv,ty) <- tv_env_list,
1064 tv `elementOfTyVarSet` ftvs]
1065 (d_bndrs, d_args) = unzip [(ValBinder d, VarArg d') | (d,d') <- dict_env,
1066 d `elementOfIdSet` fvs]
1067 dict_env' = (dict,dict') : dict_env
1068 ftvs' = tyVarsOfTypes [ty | TyArg ty <- t_args] `unionTyVarSets`
1069 (ftvs `minusTyVarSet` bound_tyvars)
1070 fvs' = mkIdSet [d | VarArg d <- d_args] `unionIdSets`
1071 (fvs `minusIdSet` mkIdSet [d | ValBinder d <- d_bndrs])
1073 specDBs dict_env' dbs `thenSM` \ (dict_env'', dbs') ->
1074 returnSM ( dict_env'', (dict', rhs', ftvs', fvs') `consBag` dbs' )
1077 %************************************************************************
1079 \subsubsection{Boring helper functions}
1081 %************************************************************************
1084 tyVarTemplates :: [TyVar]
1085 tyVarTemplates = map mk [1..]
1087 mk i = mkTyVar (mkSysLocalName uniq occ noSrcLoc) mkBoxedTypeKind
1089 uniq = mkAlphaTyVarUnique i
1090 occ = _PK_ ("$t" ++ show i)
1094 lookupId:: IdEnv Id -> Id -> Id
1095 lookupId env id = case lookupIdEnv env id of
1099 dictRhsFVs :: CoreExpr -> IdSet
1100 dictRhsFVs e = exprFreeVars isLocallyDefined e
1102 addIdSpecialisations id spec_stuff
1103 = (if not (null errs) then
1104 pprTrace "Duplicate specialisations" (vcat (map ppr errs))
1107 setIdSpecialisation id new_spec_env
1109 (new_spec_env, errs) = foldr add (getIdSpecialisation id, []) spec_stuff
1111 add (tyvars, tys, template) (spec_env, errs)
1112 = case addToSpecEnv True spec_env tyvars tys template of
1113 Succeeded spec_env' -> (spec_env', errs)
1114 Failed err -> (spec_env, err:errs)
1116 -- Given an Id, isSpecVars returns all its specialisations.
1117 -- We extract these from its SpecEnv.
1118 -- This is used by the occurrence analyser and free-var finder;
1119 -- we regard an Id's specialisations as free in the Id's definition.
1121 idSpecVars :: Id -> [Id]
1123 = map get_spec (specEnvValues (getIdSpecialisation id))
1125 -- get_spec is another cheapo function like dictRhsFVs
1126 -- It knows what these specialisation temlates look like,
1127 -- and just goes for the jugular
1128 get_spec (App f _) = get_spec f
1129 get_spec (Lam _ b) = get_spec b
1130 get_spec (Var v) = v
1132 ----------------------------------------
1133 type SpecM a = UniqSM a
1137 getUniqSM = getUnique
1141 mapAndCombineSM f [] = returnSM ([], emptyUDs)
1142 mapAndCombineSM f (x:xs) = f x `thenSM` \ (y, uds1) ->
1143 mapAndCombineSM f xs `thenSM` \ (ys, uds2) ->
1144 returnSM (y:ys, uds1 `plusUDs` uds2)
1146 newIdSM old_id new_ty
1147 = getUnique `thenSM` \ uniq ->
1148 returnSM (mkUserLocal (getOccName old_id)
1156 Old (but interesting) stuff about unboxed bindings
1157 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1159 What should we do when a value is specialised to a *strict* unboxed value?
1161 map_*_* f (x:xs) = let h = f x
1165 Could convert let to case:
1167 map_*_Int# f (x:xs) = case f x of h# ->
1171 This may be undesirable since it forces evaluation here, but the value
1172 may not be used in all branches of the body. In the general case this
1173 transformation is impossible since the mutual recursion in a letrec
1174 cannot be expressed as a case.
1176 There is also a problem with top-level unboxed values, since our
1177 implementation cannot handle unboxed values at the top level.
1179 Solution: Lift the binding of the unboxed value and extract it when it
1182 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1187 Now give it to the simplifier and the _Lifting will be optimised away.
1189 The benfit is that we have given the specialised "unboxed" values a
1190 very simplep lifted semantics and then leave it up to the simplifier to
1191 optimise it --- knowing that the overheads will be removed in nearly
1194 In particular, the value will only be evaluted in the branches of the
1195 program which use it, rather than being forced at the point where the
1196 value is bound. For example:
1198 filtermap_*_* p f (x:xs)
1205 filtermap_*_Int# p f (x:xs)
1206 = let h = case (f x) of h# -> _Lift h#
1209 True -> case h of _Lift h#
1213 The binding for h can still be inlined in the one branch and the
1214 _Lifting eliminated.
1217 Question: When won't the _Lifting be eliminated?
1219 Answer: When they at the top-level (where it is necessary) or when
1220 inlining would duplicate work (or possibly code depending on
1221 options). However, the _Lifting will still be eliminated if the
1222 strictness analyser deems the lifted binding strict.