2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[Specialise]{Stamping out overloading, and (optionally) polymorphism}
7 module Specialise ( specProgram ) where
9 #include "HsVersions.h"
11 import CmdLineOpts ( DynFlags, DynFlag(..) )
12 import Id ( Id, idName, idType, mkUserLocal, idSpecialisation )
13 import TcType ( Type, mkTyVarTy, tcSplitSigmaTy,
14 tyVarsOfTypes, tyVarsOfTheta,
15 mkForAllTys, tcCmpType
17 import Subst ( Subst, mkSubst, substTy, mkSubst, extendSubstList, mkInScopeSet,
18 simplBndr, simplBndrs,
19 substAndCloneId, substAndCloneIds, substAndCloneRecIds,
20 lookupIdSubst, substInScope
22 import Var ( zapSpecPragmaId )
26 import CoreUtils ( applyTypeToArgs )
27 import CoreUnfold ( certainlyWillInline )
28 import CoreFVs ( exprFreeVars, exprsFreeVars )
29 import CoreLint ( showPass, endPass )
30 import PprCore ( pprCoreRules )
31 import Rules ( addIdSpecialisations, lookupRule )
33 import UniqSupply ( UniqSupply,
34 UniqSM, initUs_, thenUs, returnUs, getUniqueUs,
37 import Name ( nameOccName, mkSpecOcc, getSrcLoc )
39 import Maybes ( catMaybes, maybeToBool )
40 import ErrUtils ( dumpIfSet_dyn )
42 import List ( partition )
43 import Util ( zipEqual, zipWithEqual, cmpList )
50 %************************************************************************
52 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
54 %************************************************************************
56 These notes describe how we implement specialisation to eliminate
59 The specialisation pass works on Core
60 syntax, complete with all the explicit dictionary application,
61 abstraction and construction as added by the type checker. The
62 existing type checker remains largely as it is.
64 One important thought: the {\em types} passed to an overloaded
65 function, and the {\em dictionaries} passed are mutually redundant.
66 If the same function is applied to the same type(s) then it is sure to
67 be applied to the same dictionary(s)---or rather to the same {\em
68 values}. (The arguments might look different but they will evaluate
71 Second important thought: we know that we can make progress by
72 treating dictionary arguments as static and worth specialising on. So
73 we can do without binding-time analysis, and instead specialise on
74 dictionary arguments and no others.
83 and suppose f is overloaded.
85 STEP 1: CALL-INSTANCE COLLECTION
87 We traverse <body>, accumulating all applications of f to types and
90 (Might there be partial applications, to just some of its types and
91 dictionaries? In principle yes, but in practice the type checker only
92 builds applications of f to all its types and dictionaries, so partial
93 applications could only arise as a result of transformation, and even
94 then I think it's unlikely. In any case, we simply don't accumulate such
95 partial applications.)
100 So now we have a collection of calls to f:
104 Notice that f may take several type arguments. To avoid ambiguity, we
105 say that f is called at type t1/t2 and t3/t4.
107 We take equivalence classes using equality of the *types* (ignoring
108 the dictionary args, which as mentioned previously are redundant).
110 STEP 3: SPECIALISATION
112 For each equivalence class, choose a representative (f t1 t2 d1 d2),
113 and create a local instance of f, defined thus:
115 f@t1/t2 = <f_rhs> t1 t2 d1 d2
117 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
118 of simplification will now result. However we don't actually *do* that
119 simplification. Rather, we leave it for the simplifier to do. If we
120 *did* do it, though, we'd get more call instances from the specialised
121 RHS. We can work out what they are by instantiating the call-instance
122 set from f's RHS with the types t1, t2.
124 Add this new id to f's IdInfo, to record that f has a specialised version.
126 Before doing any of this, check that f's IdInfo doesn't already
127 tell us about an existing instance of f at the required type/s.
128 (This might happen if specialisation was applied more than once, or
129 it might arise from user SPECIALIZE pragmas.)
133 Wait a minute! What if f is recursive? Then we can't just plug in
134 its right-hand side, can we?
136 But it's ok. The type checker *always* creates non-recursive definitions
137 for overloaded recursive functions. For example:
139 f x = f (x+x) -- Yes I know its silly
143 f a (d::Num a) = let p = +.sel a d
145 letrec fl (y::a) = fl (p y y)
149 We still have recusion for non-overloaded functions which we
150 speciailise, but the recursive call should get specialised to the
151 same recursive version.
157 All this is crystal clear when the function is applied to *constant
158 types*; that is, types which have no type variables inside. But what if
159 it is applied to non-constant types? Suppose we find a call of f at type
160 t1/t2. There are two possibilities:
162 (a) The free type variables of t1, t2 are in scope at the definition point
163 of f. In this case there's no problem, we proceed just as before. A common
164 example is as follows. Here's the Haskell:
169 After typechecking we have
171 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
172 in +.sel a d (f a d y) (f a d y)
174 Notice that the call to f is at type type "a"; a non-constant type.
175 Both calls to f are at the same type, so we can specialise to give:
177 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
178 in +.sel a d (f@a y) (f@a y)
181 (b) The other case is when the type variables in the instance types
182 are *not* in scope at the definition point of f. The example we are
183 working with above is a good case. There are two instances of (+.sel a d),
184 but "a" is not in scope at the definition of +.sel. Can we do anything?
185 Yes, we can "common them up", a sort of limited common sub-expression deal.
188 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
189 f@a (x::a) = +.sel@a x x
190 in +.sel@a (f@a y) (f@a y)
192 This can save work, and can't be spotted by the type checker, because
193 the two instances of +.sel weren't originally at the same type.
197 * There are quite a few variations here. For example, the defn of
198 +.sel could be floated ouside the \y, to attempt to gain laziness.
199 It certainly mustn't be floated outside the \d because the d has to
202 * We don't want to inline f_rhs in this case, because
203 that will duplicate code. Just commoning up the call is the point.
205 * Nothing gets added to +.sel's IdInfo.
207 * Don't bother unless the equivalence class has more than one item!
209 Not clear whether this is all worth it. It is of course OK to
210 simply discard call-instances when passing a big lambda.
212 Polymorphism 2 -- Overloading
214 Consider a function whose most general type is
216 f :: forall a b. Ord a => [a] -> b -> b
218 There is really no point in making a version of g at Int/Int and another
219 at Int/Bool, because it's only instancing the type variable "a" which
220 buys us any efficiency. Since g is completely polymorphic in b there
221 ain't much point in making separate versions of g for the different
224 That suggests that we should identify which of g's type variables
225 are constrained (like "a") and which are unconstrained (like "b").
226 Then when taking equivalence classes in STEP 2, we ignore the type args
227 corresponding to unconstrained type variable. In STEP 3 we make
228 polymorphic versions. Thus:
230 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
239 f a (d::Num a) = let g = ...
241 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
243 Here, g is only called at one type, but the dictionary isn't in scope at the
244 definition point for g. Usually the type checker would build a
245 definition for d1 which enclosed g, but the transformation system
246 might have moved d1's defn inward. Solution: float dictionary bindings
247 outwards along with call instances.
251 f x = let g p q = p==q
257 Before specialisation, leaving out type abstractions we have
259 f df x = let g :: Eq a => a -> a -> Bool
261 h :: Num a => a -> a -> (a, Bool)
262 h dh r s = let deq = eqFromNum dh
263 in (+ dh r s, g deq r s)
267 After specialising h we get a specialised version of h, like this:
269 h' r s = let deq = eqFromNum df
270 in (+ df r s, g deq r s)
272 But we can't naively make an instance for g from this, because deq is not in scope
273 at the defn of g. Instead, we have to float out the (new) defn of deq
274 to widen its scope. Notice that this floating can't be done in advance -- it only
275 shows up when specialisation is done.
277 User SPECIALIZE pragmas
278 ~~~~~~~~~~~~~~~~~~~~~~~
279 Specialisation pragmas can be digested by the type checker, and implemented
280 by adding extra definitions along with that of f, in the same way as before
282 f@t1/t2 = <f_rhs> t1 t2 d1 d2
284 Indeed the pragmas *have* to be dealt with by the type checker, because
285 only it knows how to build the dictionaries d1 and d2! For example
287 g :: Ord a => [a] -> [a]
288 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
290 Here, the specialised version of g is an application of g's rhs to the
291 Ord dictionary for (Tree Int), which only the type checker can conjure
292 up. There might not even *be* one, if (Tree Int) is not an instance of
293 Ord! (All the other specialision has suitable dictionaries to hand
296 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
297 it is buried in a complex (as-yet-un-desugared) binding group.
300 f@t1/t2 = f* t1 t2 d1 d2
302 where f* is the Id f with an IdInfo which says "inline me regardless!".
303 Indeed all the specialisation could be done in this way.
304 That in turn means that the simplifier has to be prepared to inline absolutely
305 any in-scope let-bound thing.
308 Again, the pragma should permit polymorphism in unconstrained variables:
310 h :: Ord a => [a] -> b -> b
311 {-# SPECIALIZE h :: [Int] -> b -> b #-}
313 We *insist* that all overloaded type variables are specialised to ground types,
314 (and hence there can be no context inside a SPECIALIZE pragma).
315 We *permit* unconstrained type variables to be specialised to
317 - or left as a polymorphic type variable
318 but nothing in between. So
320 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
322 is *illegal*. (It can be handled, but it adds complication, and gains the
326 SPECIALISING INSTANCE DECLARATIONS
327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
330 instance Foo a => Foo [a] where
332 {-# SPECIALIZE instance Foo [Int] #-}
334 The original instance decl creates a dictionary-function
337 dfun.Foo.List :: forall a. Foo a -> Foo [a]
339 The SPECIALIZE pragma just makes a specialised copy, just as for
340 ordinary function definitions:
342 dfun.Foo.List@Int :: Foo [Int]
343 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
345 The information about what instance of the dfun exist gets added to
346 the dfun's IdInfo in the same way as a user-defined function too.
349 Automatic instance decl specialisation?
350 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
351 Can instance decls be specialised automatically? It's tricky.
352 We could collect call-instance information for each dfun, but
353 then when we specialised their bodies we'd get new call-instances
354 for ordinary functions; and when we specialised their bodies, we might get
355 new call-instances of the dfuns, and so on. This all arises because of
356 the unrestricted mutual recursion between instance decls and value decls.
358 Still, there's no actual problem; it just means that we may not do all
359 the specialisation we could theoretically do.
361 Furthermore, instance decls are usually exported and used non-locally,
362 so we'll want to compile enough to get those specialisations done.
364 Lastly, there's no such thing as a local instance decl, so we can
365 survive solely by spitting out *usage* information, and then reading that
366 back in as a pragma when next compiling the file. So for now,
367 we only specialise instance decls in response to pragmas.
370 SPITTING OUT USAGE INFORMATION
371 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
373 To spit out usage information we need to traverse the code collecting
374 call-instance information for all imported (non-prelude?) functions
375 and data types. Then we equivalence-class it and spit it out.
377 This is done at the top-level when all the call instances which escape
378 must be for imported functions and data types.
380 *** Not currently done ***
383 Partial specialisation by pragmas
384 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
385 What about partial specialisation:
387 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
388 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
392 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
394 Seems quite reasonable. Similar things could be done with instance decls:
396 instance (Foo a, Foo b) => Foo (a,b) where
398 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
399 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
401 Ho hum. Things are complex enough without this. I pass.
404 Requirements for the simplifer
405 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
406 The simplifier has to be able to take advantage of the specialisation.
408 * When the simplifier finds an application of a polymorphic f, it looks in
409 f's IdInfo in case there is a suitable instance to call instead. This converts
411 f t1 t2 d1 d2 ===> f_t1_t2
413 Note that the dictionaries get eaten up too!
415 * Dictionary selection operations on constant dictionaries must be
418 +.sel Int d ===> +Int
420 The obvious way to do this is in the same way as other specialised
421 calls: +.sel has inside it some IdInfo which tells that if it's applied
422 to the type Int then it should eat a dictionary and transform to +Int.
424 In short, dictionary selectors need IdInfo inside them for constant
427 * Exactly the same applies if a superclass dictionary is being
430 Eq.sel Int d ===> dEqInt
432 * Something similar applies to dictionary construction too. Suppose
433 dfun.Eq.List is the function taking a dictionary for (Eq a) to
434 one for (Eq [a]). Then we want
436 dfun.Eq.List Int d ===> dEq.List_Int
438 Where does the Eq [Int] dictionary come from? It is built in
439 response to a SPECIALIZE pragma on the Eq [a] instance decl.
441 In short, dfun Ids need IdInfo with a specialisation for each
442 constant instance of their instance declaration.
444 All this uses a single mechanism: the SpecEnv inside an Id
447 What does the specialisation IdInfo look like?
448 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
450 The SpecEnv of an Id maps a list of types (the template) to an expression
454 For example, if f has this SpecInfo:
456 [Int, a] -> \d:Ord Int. f' a
458 it means that we can replace the call
460 f Int t ===> (\d. f' t)
462 This chucks one dictionary away and proceeds with the
463 specialised version of f, namely f'.
466 What can't be done this way?
467 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
468 There is no way, post-typechecker, to get a dictionary for (say)
469 Eq a from a dictionary for Eq [a]. So if we find
473 we can't transform to
478 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
480 Of course, we currently have no way to automatically derive
481 eqList, nor to connect it to the Eq [a] instance decl, but you
482 can imagine that it might somehow be possible. Taking advantage
483 of this is permanently ruled out.
485 Still, this is no great hardship, because we intend to eliminate
486 overloading altogether anyway!
490 A note about non-tyvar dictionaries
491 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
492 Some Ids have types like
494 forall a,b,c. Eq a -> Ord [a] -> tau
496 This seems curious at first, because we usually only have dictionary
497 args whose types are of the form (C a) where a is a type variable.
498 But this doesn't hold for the functions arising from instance decls,
499 which sometimes get arguements with types of form (C (T a)) for some
502 Should we specialise wrt this compound-type dictionary? We used to say
504 "This is a heuristic judgement, as indeed is the fact that we
505 specialise wrt only dictionaries. We choose *not* to specialise
506 wrt compound dictionaries because at the moment the only place
507 they show up is in instance decls, where they are simply plugged
508 into a returned dictionary. So nothing is gained by specialising
511 But it is simpler and more uniform to specialise wrt these dicts too;
512 and in future GHC is likely to support full fledged type signatures
514 f ;: Eq [(a,b)] => ...
517 %************************************************************************
519 \subsubsection{The new specialiser}
521 %************************************************************************
523 Our basic game plan is this. For let(rec) bound function
524 f :: (C a, D c) => (a,b,c,d) -> Bool
526 * Find any specialised calls of f, (f ts ds), where
527 ts are the type arguments t1 .. t4, and
528 ds are the dictionary arguments d1 .. d2.
530 * Add a new definition for f1 (say):
532 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
534 Note that we abstract over the unconstrained type arguments.
538 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
540 to the specialisations of f. This will be used by the
541 simplifier to replace calls
542 (f t1 t2 t3 t4) da db
544 (\d1 d1 -> f1 t2 t4) da db
546 All the stuff about how many dictionaries to discard, and what types
547 to apply the specialised function to, are handled by the fact that the
548 SpecEnv contains a template for the result of the specialisation.
550 We don't build *partial* specialisations for f. For example:
552 f :: Eq a => a -> a -> Bool
553 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
555 Here, little is gained by making a specialised copy of f.
556 There's a distinct danger that the specialised version would
557 first build a dictionary for (Eq b, Eq c), and then select the (==)
558 method from it! Even if it didn't, not a great deal is saved.
560 We do, however, generate polymorphic, but not overloaded, specialisations:
562 f :: Eq a => [a] -> b -> b -> b
563 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
565 Hence, the invariant is this:
567 *** no specialised version is overloaded ***
570 %************************************************************************
572 \subsubsection{The exported function}
574 %************************************************************************
577 specProgram :: DynFlags -> UniqSupply -> [CoreBind] -> IO [CoreBind]
578 specProgram dflags us binds
580 showPass dflags "Specialise"
582 let binds' = initSM us (go binds `thenSM` \ (binds', uds') ->
583 returnSM (dumpAllDictBinds uds' binds'))
585 endPass dflags "Specialise" Opt_D_dump_spec binds'
587 dumpIfSet_dyn dflags Opt_D_dump_rules "Top-level specialisations"
588 (vcat (map dump_specs (concat (map bindersOf binds'))))
592 -- We need to start with a Subst that knows all the things
593 -- that are in scope, so that the substitution engine doesn't
594 -- accidentally re-use a unique that's already in use
595 -- Easiest thing is to do it all at once, as if all the top-level
596 -- decls were mutually recursive
597 top_subst = mkSubst (mkInScopeSet (mkVarSet (bindersOfBinds binds))) emptySubstEnv
599 go [] = returnSM ([], emptyUDs)
600 go (bind:binds) = go binds `thenSM` \ (binds', uds) ->
601 specBind top_subst bind uds `thenSM` \ (bind', uds') ->
602 returnSM (bind' ++ binds', uds')
604 dump_specs var = pprCoreRules var (idSpecialisation var)
607 %************************************************************************
609 \subsubsection{@specExpr@: the main function}
611 %************************************************************************
614 specVar :: Subst -> Id -> CoreExpr
615 specVar subst v = case lookupIdSubst subst v of
619 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
620 -- We carry a substitution down:
621 -- a) we must clone any binding that might flaot outwards,
622 -- to avoid name clashes
623 -- b) we carry a type substitution to use when analysing
624 -- the RHS of specialised bindings (no type-let!)
626 ---------------- First the easy cases --------------------
627 specExpr subst (Type ty) = returnSM (Type (substTy subst ty), emptyUDs)
628 specExpr subst (Var v) = returnSM (specVar subst v, emptyUDs)
629 specExpr subst (Lit lit) = returnSM (Lit lit, emptyUDs)
631 specExpr subst (Note note body)
632 = specExpr subst body `thenSM` \ (body', uds) ->
633 returnSM (Note (specNote subst note) body', uds)
636 ---------------- Applications might generate a call instance --------------------
637 specExpr subst expr@(App fun arg)
640 go (App fun arg) args = specExpr subst arg `thenSM` \ (arg', uds_arg) ->
641 go fun (arg':args) `thenSM` \ (fun', uds_app) ->
642 returnSM (App fun' arg', uds_arg `plusUDs` uds_app)
644 go (Var f) args = case specVar subst f of
645 Var f' -> returnSM (Var f', mkCallUDs subst f' args)
646 e' -> returnSM (e', emptyUDs) -- I don't expect this!
647 go other args = specExpr subst other
649 ---------------- Lambda/case require dumping of usage details --------------------
650 specExpr subst e@(Lam _ _)
651 = specExpr subst' body `thenSM` \ (body', uds) ->
653 (filtered_uds, body'') = dumpUDs bndrs' uds body'
655 returnSM (mkLams bndrs' body'', filtered_uds)
657 (bndrs, body) = collectBinders e
658 (subst', bndrs') = simplBndrs subst bndrs
659 -- More efficient to collect a group of binders together all at once
660 -- and we don't want to split a lambda group with dumped bindings
662 specExpr subst (Case scrut case_bndr alts)
663 = specExpr subst scrut `thenSM` \ (scrut', uds_scrut) ->
664 mapAndCombineSM spec_alt alts `thenSM` \ (alts', uds_alts) ->
665 returnSM (Case scrut' case_bndr' alts', uds_scrut `plusUDs` uds_alts)
667 (subst_alt, case_bndr') = simplBndr subst case_bndr
668 -- No need to clone case binder; it can't float like a let(rec)
670 spec_alt (con, args, rhs)
671 = specExpr subst_rhs rhs `thenSM` \ (rhs', uds) ->
673 (uds', rhs'') = dumpUDs args uds rhs'
675 returnSM ((con, args', rhs''), uds')
677 (subst_rhs, args') = simplBndrs subst_alt args
679 ---------------- Finally, let is the interesting case --------------------
680 specExpr subst (Let bind body)
682 cloneBindSM subst bind `thenSM` \ (rhs_subst, body_subst, bind') ->
684 -- Deal with the body
685 specExpr body_subst body `thenSM` \ (body', body_uds) ->
687 -- Deal with the bindings
688 specBind rhs_subst bind' body_uds `thenSM` \ (binds', uds) ->
691 returnSM (foldr Let body' binds', uds)
693 -- Must apply the type substitution to coerceions
694 specNote subst (Coerce t1 t2) = Coerce (substTy subst t1) (substTy subst t2)
695 specNote subst note = note
698 %************************************************************************
700 \subsubsection{Dealing with a binding}
702 %************************************************************************
705 specBind :: Subst -- Use this for RHSs
707 -> UsageDetails -- Info on how the scope of the binding
708 -> SpecM ([CoreBind], -- New bindings
709 UsageDetails) -- And info to pass upstream
711 specBind rhs_subst bind body_uds
712 = specBindItself rhs_subst bind (calls body_uds) `thenSM` \ (bind', bind_uds) ->
714 bndrs = bindersOf bind
715 all_uds = zapCalls bndrs (body_uds `plusUDs` bind_uds)
716 -- It's important that the `plusUDs` is this way round,
717 -- because body_uds may bind dictionaries that are
718 -- used in the calls passed to specDefn. So the
719 -- dictionary bindings in bind_uds may mention
720 -- dictionaries bound in body_uds.
722 case splitUDs bndrs all_uds of
724 (_, ([],[])) -- This binding doesn't bind anything needed
725 -- in the UDs, so put the binding here
726 -- This is the case for most non-dict bindings, except
727 -- for the few that are mentioned in a dict binding
728 -- that is floating upwards in body_uds
729 -> returnSM ([bind'], all_uds)
731 (float_uds, (dict_binds, calls)) -- This binding is needed in the UDs, so float it out
732 -> returnSM ([], float_uds `plusUDs` mkBigUD bind' dict_binds calls)
735 -- A truly gruesome function
736 mkBigUD bind@(NonRec _ _) dbs calls
737 = -- Common case: non-recursive and no specialisations
738 -- (if there were any specialistions it would have been made recursive)
739 MkUD { dict_binds = listToBag (mkDB bind : dbs),
740 calls = listToCallDetails calls }
742 mkBigUD bind dbs calls
744 MkUD { dict_binds = unitBag (mkDB (Rec (bind_prs bind ++ dbsToPairs dbs))),
746 calls = listToCallDetails calls }
748 bind_prs (NonRec b r) = [(b,r)]
749 bind_prs (Rec prs) = prs
752 dbsToPairs ((bind,_):dbs) = bind_prs bind ++ dbsToPairs dbs
754 -- specBindItself deals with the RHS, specialising it according
755 -- to the calls found in the body (if any)
756 specBindItself rhs_subst (NonRec bndr rhs) call_info
757 = specDefn rhs_subst call_info (bndr,rhs) `thenSM` \ ((bndr',rhs'), spec_defns, spec_uds) ->
759 new_bind | null spec_defns = NonRec bndr' rhs'
760 | otherwise = Rec ((bndr',rhs'):spec_defns)
761 -- bndr' mentions the spec_defns in its SpecEnv
762 -- Not sure why we couln't just put the spec_defns first
764 returnSM (new_bind, spec_uds)
766 specBindItself rhs_subst (Rec pairs) call_info
767 = mapSM (specDefn rhs_subst call_info) pairs `thenSM` \ stuff ->
769 (pairs', spec_defns_s, spec_uds_s) = unzip3 stuff
770 spec_defns = concat spec_defns_s
771 spec_uds = plusUDList spec_uds_s
772 new_bind = Rec (spec_defns ++ pairs')
774 returnSM (new_bind, spec_uds)
777 specDefn :: Subst -- Subst to use for RHS
778 -> CallDetails -- Info on how it is used in its scope
779 -> (Id, CoreExpr) -- The thing being bound and its un-processed RHS
780 -> SpecM ((Id, CoreExpr), -- The thing and its processed RHS
781 -- the Id may now have specialisations attached
782 [(Id,CoreExpr)], -- Extra, specialised bindings
783 UsageDetails -- Stuff to fling upwards from the RHS and its
784 ) -- specialised versions
786 specDefn subst calls (fn, rhs)
787 -- The first case is the interesting one
788 | n_tyvars == length rhs_tyvars -- Rhs of fn's defn has right number of big lambdas
789 && n_dicts <= length rhs_bndrs -- and enough dict args
790 && not (null calls_for_me) -- And there are some calls to specialise
791 && not (certainlyWillInline fn) -- And it's not small
792 -- If it's small, it's better just to inline
793 -- it than to construct lots of specialisations
794 = -- Specialise the body of the function
795 specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
797 -- Make a specialised version for each call in calls_for_me
798 mapSM spec_call calls_for_me `thenSM` \ stuff ->
800 (spec_defns, spec_uds, spec_rules) = unzip3 stuff
802 fn' = addIdSpecialisations zapped_fn spec_rules
804 returnSM ((fn',rhs'),
806 rhs_uds `plusUDs` plusUDList spec_uds)
808 | otherwise -- No calls or RHS doesn't fit our preconceptions
809 = specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
810 returnSM ((zapped_fn, rhs'), [], rhs_uds)
813 zapped_fn = zapSpecPragmaId fn
814 -- If the fn is a SpecPragmaId, make it discardable
815 -- It's role as a holder for a call instance is o'er
816 -- But it might be alive for some other reason by now.
819 (tyvars, theta, _) = tcSplitSigmaTy fn_type
820 n_tyvars = length tyvars
821 n_dicts = length theta
823 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs
824 rhs_dicts = take n_dicts rhs_ids
825 rhs_bndrs = rhs_tyvars ++ rhs_dicts
826 body = mkLams (drop n_dicts rhs_ids) rhs_body
827 -- Glue back on the non-dict lambdas
829 calls_for_me = case lookupFM calls fn of
831 Just cs -> fmToList cs
833 ----------------------------------------------------------
834 -- Specialise to one particular call pattern
835 spec_call :: (CallKey, ([DictExpr], VarSet)) -- Call instance
836 -> SpecM ((Id,CoreExpr), -- Specialised definition
837 UsageDetails, -- Usage details from specialised body
838 CoreRule) -- Info for the Id's SpecEnv
839 spec_call (CallKey call_ts, (call_ds, call_fvs))
840 = ASSERT( length call_ts == n_tyvars && length call_ds == n_dicts )
841 -- Calls are only recorded for properly-saturated applications
843 -- Suppose f's defn is f = /\ a b c d -> \ d1 d2 -> rhs
844 -- Supppose the call is for f [Just t1, Nothing, Just t3, Nothing] [dx1, dx2]
846 -- Construct the new binding
847 -- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b d -> rhs)
848 -- PLUS the usage-details
849 -- { d1' = dx1; d2' = dx2 }
850 -- where d1', d2' are cloned versions of d1,d2, with the type substitution applied.
852 -- Note that the substitution is applied to the whole thing.
853 -- This is convenient, but just slightly fragile. Notably:
854 -- * There had better be no name clashes in a/b/c/d
857 -- poly_tyvars = [b,d] in the example above
858 -- spec_tyvars = [a,c]
859 -- ty_args = [t1,b,t3,d]
860 poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
861 spec_tyvars = [tv | (tv, Just _) <- rhs_tyvars `zip` call_ts]
862 ty_args = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
864 mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
865 mk_ty_arg rhs_tyvar (Just ty) = Type ty
866 rhs_subst = extendSubstList subst spec_tyvars [DoneTy ty | Just ty <- call_ts]
868 cloneBinders rhs_subst rhs_dicts `thenSM` \ (rhs_subst', rhs_dicts') ->
870 inst_args = ty_args ++ map Var rhs_dicts'
872 -- Figure out the type of the specialised function
873 spec_id_ty = mkForAllTys poly_tyvars (applyTypeToArgs rhs fn_type inst_args)
875 newIdSM fn spec_id_ty `thenSM` \ spec_f ->
876 specExpr rhs_subst' (mkLams poly_tyvars body) `thenSM` \ (spec_rhs, rhs_uds) ->
878 -- The rule to put in the function's specialisation is:
879 -- forall b,d, d1',d2'. f t1 b t3 d d1' d2' = f1 b d
880 spec_env_rule = Rule (_PK_ ("SPEC " ++ showSDoc (ppr fn)))
881 (poly_tyvars ++ rhs_dicts')
883 (mkTyApps (Var spec_f) (map mkTyVarTy poly_tyvars))
885 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
886 final_uds = foldr addDictBind rhs_uds (my_zipEqual "spec_call" rhs_dicts' call_ds)
888 returnSM ((spec_f, spec_rhs),
893 my_zipEqual doc xs ys
894 | length xs /= length ys = pprPanic "my_zipEqual" (ppr xs $$ ppr ys $$ (ppr fn <+> ppr call_ts) $$ ppr rhs)
895 | otherwise = zipEqual doc xs ys
898 %************************************************************************
900 \subsubsection{UsageDetails and suchlike}
902 %************************************************************************
907 dict_binds :: !(Bag DictBind),
908 -- Floated dictionary bindings
909 -- The order is important;
910 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
911 -- (Remember, Bags preserve order in GHC.)
913 calls :: !CallDetails
916 type DictBind = (CoreBind, VarSet)
917 -- The set is the free vars of the binding
918 -- both tyvars and dicts
920 type DictExpr = CoreExpr
922 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM }
924 type ProtoUsageDetails = ([DictBind],
925 [(Id, CallKey, ([DictExpr], VarSet))]
928 ------------------------------------------------------------
929 type CallDetails = FiniteMap Id CallInfo
930 newtype CallKey = CallKey [Maybe Type] -- Nothing => unconstrained type argument
931 type CallInfo = FiniteMap CallKey
932 ([DictExpr], VarSet) -- Dict args and the vars of the whole
933 -- call (including tyvars)
934 -- [*not* include the main id itself, of course]
935 -- The finite maps eliminate duplicates
936 -- The list of types and dictionaries is guaranteed to
937 -- match the type of f
939 -- Type isn't an instance of Ord, so that we can control which
940 -- instance we use. That's tiresome here. Oh well
941 instance Eq CallKey where
942 k1 == k2 = case k1 `compare` k2 of { EQ -> True; other -> False }
944 instance Ord CallKey where
945 compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
947 cmp Nothing Nothing = EQ
948 cmp Nothing (Just t2) = LT
949 cmp (Just t1) Nothing = GT
950 cmp (Just t1) (Just t2) = tcCmpType t1 t2
952 unionCalls :: CallDetails -> CallDetails -> CallDetails
953 unionCalls c1 c2 = plusFM_C plusFM c1 c2
955 singleCall :: Id -> [Maybe Type] -> [DictExpr] -> CallDetails
956 singleCall id tys dicts
957 = unitFM id (unitFM (CallKey tys) (dicts, call_fvs))
959 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
960 tys_fvs = tyVarsOfTypes (catMaybes tys)
961 -- The type args (tys) are guaranteed to be part of the dictionary
962 -- types, because they are just the constrained types,
963 -- and the dictionary is therefore sure to be bound
964 -- inside the binding for any type variables free in the type;
965 -- hence it's safe to neglect tyvars free in tys when making
966 -- the free-var set for this call
967 -- BUT I don't trust this reasoning; play safe and include tys_fvs
969 -- We don't include the 'id' itself.
971 listToCallDetails calls
972 = foldr (unionCalls . mk_call) emptyFM calls
974 mk_call (id, tys, dicts_w_fvs) = unitFM id (unitFM tys dicts_w_fvs)
975 -- NB: the free vars of the call are provided
977 callDetailsToList calls = [ (id,tys,dicts)
978 | (id,fm) <- fmToList calls,
979 (tys, dicts) <- fmToList fm
982 mkCallUDs subst f args
984 || length spec_tys /= n_tyvars
985 || length dicts /= n_dicts
986 || maybeToBool (lookupRule (substInScope subst) f args)
987 -- There's already a rule covering this call. A typical case
988 -- is where there's an explicit user-provided rule. Then
989 -- we don't want to create a specialised version
990 -- of the function that overlaps.
991 = emptyUDs -- Not overloaded, or no specialisation wanted
994 = MkUD {dict_binds = emptyBag,
995 calls = singleCall f spec_tys dicts
998 (tyvars, theta, _) = tcSplitSigmaTy (idType f)
999 constrained_tyvars = tyVarsOfTheta theta
1000 n_tyvars = length tyvars
1001 n_dicts = length theta
1003 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
1004 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
1006 mk_spec_ty tyvar ty | tyvar `elemVarSet` constrained_tyvars
1011 ------------------------------------------------------------
1012 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
1013 plusUDs (MkUD {dict_binds = db1, calls = calls1})
1014 (MkUD {dict_binds = db2, calls = calls2})
1015 = MkUD {dict_binds = d, calls = c}
1017 d = db1 `unionBags` db2
1018 c = calls1 `unionCalls` calls2
1020 plusUDList = foldr plusUDs emptyUDs
1022 -- zapCalls deletes calls to ids from uds
1023 zapCalls ids uds = uds {calls = delListFromFM (calls uds) ids}
1025 mkDB bind = (bind, bind_fvs bind)
1027 bind_fvs (NonRec bndr rhs) = exprFreeVars rhs
1028 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1031 rhs_fvs = unionVarSets [exprFreeVars rhs | (bndr,rhs) <- prs]
1033 addDictBind (dict,rhs) uds = uds { dict_binds = mkDB (NonRec dict rhs) `consBag` dict_binds uds }
1035 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
1036 = foldrBag add binds dbs
1038 add (bind,_) binds = bind : binds
1040 dumpUDs :: [CoreBndr]
1041 -> UsageDetails -> CoreExpr
1042 -> (UsageDetails, CoreExpr)
1043 dumpUDs bndrs uds body
1044 = (free_uds, foldr add_let body dict_binds)
1046 (free_uds, (dict_binds, _)) = splitUDs bndrs uds
1047 add_let (bind,_) body = Let bind body
1049 splitUDs :: [CoreBndr]
1051 -> (UsageDetails, -- These don't mention the binders
1052 ProtoUsageDetails) -- These do
1054 splitUDs bndrs uds@(MkUD {dict_binds = orig_dbs,
1055 calls = orig_calls})
1057 = if isEmptyBag dump_dbs && null dump_calls then
1058 -- Common case: binder doesn't affect floats
1062 -- Binders bind some of the fvs of the floats
1063 (MkUD {dict_binds = free_dbs,
1064 calls = listToCallDetails free_calls},
1065 (bagToList dump_dbs, dump_calls)
1069 bndr_set = mkVarSet bndrs
1071 (free_dbs, dump_dbs, dump_idset)
1072 = foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
1073 -- Important that it's foldl not foldr;
1074 -- we're accumulating the set of dumped ids in dump_set
1076 -- Filter out any calls that mention things that are being dumped
1077 orig_call_list = callDetailsToList orig_calls
1078 (dump_calls, free_calls) = partition captured orig_call_list
1079 captured (id,tys,(dicts, fvs)) = fvs `intersectsVarSet` dump_idset
1080 || id `elemVarSet` dump_idset
1082 dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1083 | dump_idset `intersectsVarSet` fvs -- Dump it
1084 = (free_dbs, dump_dbs `snocBag` db,
1085 dump_idset `unionVarSet` mkVarSet (bindersOf bind))
1087 | otherwise -- Don't dump it
1088 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1092 %************************************************************************
1094 \subsubsection{Boring helper functions}
1096 %************************************************************************
1099 type SpecM a = UniqSM a
1103 getUniqSM = getUniqueUs
1107 mapAndCombineSM f [] = returnSM ([], emptyUDs)
1108 mapAndCombineSM f (x:xs) = f x `thenSM` \ (y, uds1) ->
1109 mapAndCombineSM f xs `thenSM` \ (ys, uds2) ->
1110 returnSM (y:ys, uds1 `plusUDs` uds2)
1112 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1113 -- Clone the binders of the bind; return new bind with the cloned binders
1114 -- Return the substitution to use for RHSs, and the one to use for the body
1115 cloneBindSM subst (NonRec bndr rhs)
1116 = getUs `thenUs` \ us ->
1118 (subst', bndr') = substAndCloneId subst us bndr
1120 returnUs (subst, subst', NonRec bndr' rhs)
1122 cloneBindSM subst (Rec pairs)
1123 = getUs `thenUs` \ us ->
1125 (subst', bndrs') = substAndCloneRecIds subst us (map fst pairs)
1127 returnUs (subst', subst', Rec (bndrs' `zip` map snd pairs))
1129 cloneBinders subst bndrs
1130 = getUs `thenUs` \ us ->
1131 returnUs (substAndCloneIds subst us bndrs)
1133 newIdSM old_id new_ty
1134 = getUniqSM `thenSM` \ uniq ->
1136 -- Give the new Id a similar occurrence name to the old one
1137 name = idName old_id
1138 new_id = mkUserLocal (mkSpecOcc (nameOccName name)) uniq new_ty (getSrcLoc name)
1144 Old (but interesting) stuff about unboxed bindings
1145 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1147 What should we do when a value is specialised to a *strict* unboxed value?
1149 map_*_* f (x:xs) = let h = f x
1153 Could convert let to case:
1155 map_*_Int# f (x:xs) = case f x of h# ->
1159 This may be undesirable since it forces evaluation here, but the value
1160 may not be used in all branches of the body. In the general case this
1161 transformation is impossible since the mutual recursion in a letrec
1162 cannot be expressed as a case.
1164 There is also a problem with top-level unboxed values, since our
1165 implementation cannot handle unboxed values at the top level.
1167 Solution: Lift the binding of the unboxed value and extract it when it
1170 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1175 Now give it to the simplifier and the _Lifting will be optimised away.
1177 The benfit is that we have given the specialised "unboxed" values a
1178 very simplep lifted semantics and then leave it up to the simplifier to
1179 optimise it --- knowing that the overheads will be removed in nearly
1182 In particular, the value will only be evaluted in the branches of the
1183 program which use it, rather than being forced at the point where the
1184 value is bound. For example:
1186 filtermap_*_* p f (x:xs)
1193 filtermap_*_Int# p f (x:xs)
1194 = let h = case (f x) of h# -> _Lift h#
1197 True -> case h of _Lift h#
1201 The binding for h can still be inlined in the one branch and the
1202 _Lifting eliminated.
1205 Question: When won't the _Lifting be eliminated?
1207 Answer: When they at the top-level (where it is necessary) or when
1208 inlining would duplicate work (or possibly code depending on
1209 options). However, the _Lifting will still be eliminated if the
1210 strictness analyser deems the lifted binding strict.