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"
20 import CoreUnfold ( mkUnfolding, mkInlineRule )
25 import CoreUtils ( exprIsTrivial, applyTypeToArgs, mkPiTypes )
26 import CoreFVs ( exprFreeVars, exprsFreeVars, idFreeVars )
27 import UniqSupply ( UniqSupply, UniqSM, initUs_, MonadUnique(..) )
29 import MkId ( voidArgId, realWorldPrimId )
31 import Maybes ( catMaybes, isJust )
32 import BasicTypes ( isNeverActive, inlinePragmaActivation )
40 %************************************************************************
42 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
44 %************************************************************************
46 These notes describe how we implement specialisation to eliminate
49 The specialisation pass works on Core
50 syntax, complete with all the explicit dictionary application,
51 abstraction and construction as added by the type checker. The
52 existing type checker remains largely as it is.
54 One important thought: the {\em types} passed to an overloaded
55 function, and the {\em dictionaries} passed are mutually redundant.
56 If the same function is applied to the same type(s) then it is sure to
57 be applied to the same dictionary(s)---or rather to the same {\em
58 values}. (The arguments might look different but they will evaluate
61 Second important thought: we know that we can make progress by
62 treating dictionary arguments as static and worth specialising on. So
63 we can do without binding-time analysis, and instead specialise on
64 dictionary arguments and no others.
73 and suppose f is overloaded.
75 STEP 1: CALL-INSTANCE COLLECTION
77 We traverse <body>, accumulating all applications of f to types and
80 (Might there be partial applications, to just some of its types and
81 dictionaries? In principle yes, but in practice the type checker only
82 builds applications of f to all its types and dictionaries, so partial
83 applications could only arise as a result of transformation, and even
84 then I think it's unlikely. In any case, we simply don't accumulate such
85 partial applications.)
90 So now we have a collection of calls to f:
94 Notice that f may take several type arguments. To avoid ambiguity, we
95 say that f is called at type t1/t2 and t3/t4.
97 We take equivalence classes using equality of the *types* (ignoring
98 the dictionary args, which as mentioned previously are redundant).
100 STEP 3: SPECIALISATION
102 For each equivalence class, choose a representative (f t1 t2 d1 d2),
103 and create a local instance of f, defined thus:
105 f@t1/t2 = <f_rhs> t1 t2 d1 d2
107 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
108 of simplification will now result. However we don't actually *do* that
109 simplification. Rather, we leave it for the simplifier to do. If we
110 *did* do it, though, we'd get more call instances from the specialised
111 RHS. We can work out what they are by instantiating the call-instance
112 set from f's RHS with the types t1, t2.
114 Add this new id to f's IdInfo, to record that f has a specialised version.
116 Before doing any of this, check that f's IdInfo doesn't already
117 tell us about an existing instance of f at the required type/s.
118 (This might happen if specialisation was applied more than once, or
119 it might arise from user SPECIALIZE pragmas.)
123 Wait a minute! What if f is recursive? Then we can't just plug in
124 its right-hand side, can we?
126 But it's ok. The type checker *always* creates non-recursive definitions
127 for overloaded recursive functions. For example:
129 f x = f (x+x) -- Yes I know its silly
133 f a (d::Num a) = let p = +.sel a d
135 letrec fl (y::a) = fl (p y y)
139 We still have recusion for non-overloaded functions which we
140 speciailise, but the recursive call should get specialised to the
141 same recursive version.
147 All this is crystal clear when the function is applied to *constant
148 types*; that is, types which have no type variables inside. But what if
149 it is applied to non-constant types? Suppose we find a call of f at type
150 t1/t2. There are two possibilities:
152 (a) The free type variables of t1, t2 are in scope at the definition point
153 of f. In this case there's no problem, we proceed just as before. A common
154 example is as follows. Here's the Haskell:
159 After typechecking we have
161 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
162 in +.sel a d (f a d y) (f a d y)
164 Notice that the call to f is at type type "a"; a non-constant type.
165 Both calls to f are at the same type, so we can specialise to give:
167 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
168 in +.sel a d (f@a y) (f@a y)
171 (b) The other case is when the type variables in the instance types
172 are *not* in scope at the definition point of f. The example we are
173 working with above is a good case. There are two instances of (+.sel a d),
174 but "a" is not in scope at the definition of +.sel. Can we do anything?
175 Yes, we can "common them up", a sort of limited common sub-expression deal.
178 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
179 f@a (x::a) = +.sel@a x x
180 in +.sel@a (f@a y) (f@a y)
182 This can save work, and can't be spotted by the type checker, because
183 the two instances of +.sel weren't originally at the same type.
187 * There are quite a few variations here. For example, the defn of
188 +.sel could be floated ouside the \y, to attempt to gain laziness.
189 It certainly mustn't be floated outside the \d because the d has to
192 * We don't want to inline f_rhs in this case, because
193 that will duplicate code. Just commoning up the call is the point.
195 * Nothing gets added to +.sel's IdInfo.
197 * Don't bother unless the equivalence class has more than one item!
199 Not clear whether this is all worth it. It is of course OK to
200 simply discard call-instances when passing a big lambda.
202 Polymorphism 2 -- Overloading
204 Consider a function whose most general type is
206 f :: forall a b. Ord a => [a] -> b -> b
208 There is really no point in making a version of g at Int/Int and another
209 at Int/Bool, because it's only instancing the type variable "a" which
210 buys us any efficiency. Since g is completely polymorphic in b there
211 ain't much point in making separate versions of g for the different
214 That suggests that we should identify which of g's type variables
215 are constrained (like "a") and which are unconstrained (like "b").
216 Then when taking equivalence classes in STEP 2, we ignore the type args
217 corresponding to unconstrained type variable. In STEP 3 we make
218 polymorphic versions. Thus:
220 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
229 f a (d::Num a) = let g = ...
231 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
233 Here, g is only called at one type, but the dictionary isn't in scope at the
234 definition point for g. Usually the type checker would build a
235 definition for d1 which enclosed g, but the transformation system
236 might have moved d1's defn inward. Solution: float dictionary bindings
237 outwards along with call instances.
241 f x = let g p q = p==q
247 Before specialisation, leaving out type abstractions we have
249 f df x = let g :: Eq a => a -> a -> Bool
251 h :: Num a => a -> a -> (a, Bool)
252 h dh r s = let deq = eqFromNum dh
253 in (+ dh r s, g deq r s)
257 After specialising h we get a specialised version of h, like this:
259 h' r s = let deq = eqFromNum df
260 in (+ df r s, g deq r s)
262 But we can't naively make an instance for g from this, because deq is not in scope
263 at the defn of g. Instead, we have to float out the (new) defn of deq
264 to widen its scope. Notice that this floating can't be done in advance -- it only
265 shows up when specialisation is done.
267 User SPECIALIZE pragmas
268 ~~~~~~~~~~~~~~~~~~~~~~~
269 Specialisation pragmas can be digested by the type checker, and implemented
270 by adding extra definitions along with that of f, in the same way as before
272 f@t1/t2 = <f_rhs> t1 t2 d1 d2
274 Indeed the pragmas *have* to be dealt with by the type checker, because
275 only it knows how to build the dictionaries d1 and d2! For example
277 g :: Ord a => [a] -> [a]
278 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
280 Here, the specialised version of g is an application of g's rhs to the
281 Ord dictionary for (Tree Int), which only the type checker can conjure
282 up. There might not even *be* one, if (Tree Int) is not an instance of
283 Ord! (All the other specialision has suitable dictionaries to hand
286 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
287 it is buried in a complex (as-yet-un-desugared) binding group.
290 f@t1/t2 = f* t1 t2 d1 d2
292 where f* is the Id f with an IdInfo which says "inline me regardless!".
293 Indeed all the specialisation could be done in this way.
294 That in turn means that the simplifier has to be prepared to inline absolutely
295 any in-scope let-bound thing.
298 Again, the pragma should permit polymorphism in unconstrained variables:
300 h :: Ord a => [a] -> b -> b
301 {-# SPECIALIZE h :: [Int] -> b -> b #-}
303 We *insist* that all overloaded type variables are specialised to ground types,
304 (and hence there can be no context inside a SPECIALIZE pragma).
305 We *permit* unconstrained type variables to be specialised to
307 - or left as a polymorphic type variable
308 but nothing in between. So
310 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
312 is *illegal*. (It can be handled, but it adds complication, and gains the
316 SPECIALISING INSTANCE DECLARATIONS
317 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
320 instance Foo a => Foo [a] where
322 {-# SPECIALIZE instance Foo [Int] #-}
324 The original instance decl creates a dictionary-function
327 dfun.Foo.List :: forall a. Foo a -> Foo [a]
329 The SPECIALIZE pragma just makes a specialised copy, just as for
330 ordinary function definitions:
332 dfun.Foo.List@Int :: Foo [Int]
333 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
335 The information about what instance of the dfun exist gets added to
336 the dfun's IdInfo in the same way as a user-defined function too.
339 Automatic instance decl specialisation?
340 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
341 Can instance decls be specialised automatically? It's tricky.
342 We could collect call-instance information for each dfun, but
343 then when we specialised their bodies we'd get new call-instances
344 for ordinary functions; and when we specialised their bodies, we might get
345 new call-instances of the dfuns, and so on. This all arises because of
346 the unrestricted mutual recursion between instance decls and value decls.
348 Still, there's no actual problem; it just means that we may not do all
349 the specialisation we could theoretically do.
351 Furthermore, instance decls are usually exported and used non-locally,
352 so we'll want to compile enough to get those specialisations done.
354 Lastly, there's no such thing as a local instance decl, so we can
355 survive solely by spitting out *usage* information, and then reading that
356 back in as a pragma when next compiling the file. So for now,
357 we only specialise instance decls in response to pragmas.
360 SPITTING OUT USAGE INFORMATION
361 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
363 To spit out usage information we need to traverse the code collecting
364 call-instance information for all imported (non-prelude?) functions
365 and data types. Then we equivalence-class it and spit it out.
367 This is done at the top-level when all the call instances which escape
368 must be for imported functions and data types.
370 *** Not currently done ***
373 Partial specialisation by pragmas
374 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
375 What about partial specialisation:
377 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
378 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
382 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
384 Seems quite reasonable. Similar things could be done with instance decls:
386 instance (Foo a, Foo b) => Foo (a,b) where
388 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
389 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
391 Ho hum. Things are complex enough without this. I pass.
394 Requirements for the simplifer
395 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
396 The simplifier has to be able to take advantage of the specialisation.
398 * When the simplifier finds an application of a polymorphic f, it looks in
399 f's IdInfo in case there is a suitable instance to call instead. This converts
401 f t1 t2 d1 d2 ===> f_t1_t2
403 Note that the dictionaries get eaten up too!
405 * Dictionary selection operations on constant dictionaries must be
408 +.sel Int d ===> +Int
410 The obvious way to do this is in the same way as other specialised
411 calls: +.sel has inside it some IdInfo which tells that if it's applied
412 to the type Int then it should eat a dictionary and transform to +Int.
414 In short, dictionary selectors need IdInfo inside them for constant
417 * Exactly the same applies if a superclass dictionary is being
420 Eq.sel Int d ===> dEqInt
422 * Something similar applies to dictionary construction too. Suppose
423 dfun.Eq.List is the function taking a dictionary for (Eq a) to
424 one for (Eq [a]). Then we want
426 dfun.Eq.List Int d ===> dEq.List_Int
428 Where does the Eq [Int] dictionary come from? It is built in
429 response to a SPECIALIZE pragma on the Eq [a] instance decl.
431 In short, dfun Ids need IdInfo with a specialisation for each
432 constant instance of their instance declaration.
434 All this uses a single mechanism: the SpecEnv inside an Id
437 What does the specialisation IdInfo look like?
438 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
440 The SpecEnv of an Id maps a list of types (the template) to an expression
444 For example, if f has this SpecInfo:
446 [Int, a] -> \d:Ord Int. f' a
448 it means that we can replace the call
450 f Int t ===> (\d. f' t)
452 This chucks one dictionary away and proceeds with the
453 specialised version of f, namely f'.
456 What can't be done this way?
457 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
458 There is no way, post-typechecker, to get a dictionary for (say)
459 Eq a from a dictionary for Eq [a]. So if we find
463 we can't transform to
468 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
470 Of course, we currently have no way to automatically derive
471 eqList, nor to connect it to the Eq [a] instance decl, but you
472 can imagine that it might somehow be possible. Taking advantage
473 of this is permanently ruled out.
475 Still, this is no great hardship, because we intend to eliminate
476 overloading altogether anyway!
478 A note about non-tyvar dictionaries
479 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
480 Some Ids have types like
482 forall a,b,c. Eq a -> Ord [a] -> tau
484 This seems curious at first, because we usually only have dictionary
485 args whose types are of the form (C a) where a is a type variable.
486 But this doesn't hold for the functions arising from instance decls,
487 which sometimes get arguements with types of form (C (T a)) for some
490 Should we specialise wrt this compound-type dictionary? We used to say
492 "This is a heuristic judgement, as indeed is the fact that we
493 specialise wrt only dictionaries. We choose *not* to specialise
494 wrt compound dictionaries because at the moment the only place
495 they show up is in instance decls, where they are simply plugged
496 into a returned dictionary. So nothing is gained by specialising
499 But it is simpler and more uniform to specialise wrt these dicts too;
500 and in future GHC is likely to support full fledged type signatures
502 f :: Eq [(a,b)] => ...
505 %************************************************************************
507 \subsubsection{The new specialiser}
509 %************************************************************************
511 Our basic game plan is this. For let(rec) bound function
512 f :: (C a, D c) => (a,b,c,d) -> Bool
514 * Find any specialised calls of f, (f ts ds), where
515 ts are the type arguments t1 .. t4, and
516 ds are the dictionary arguments d1 .. d2.
518 * Add a new definition for f1 (say):
520 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
522 Note that we abstract over the unconstrained type arguments.
526 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
528 to the specialisations of f. This will be used by the
529 simplifier to replace calls
530 (f t1 t2 t3 t4) da db
532 (\d1 d1 -> f1 t2 t4) da db
534 All the stuff about how many dictionaries to discard, and what types
535 to apply the specialised function to, are handled by the fact that the
536 SpecEnv contains a template for the result of the specialisation.
538 We don't build *partial* specialisations for f. For example:
540 f :: Eq a => a -> a -> Bool
541 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
543 Here, little is gained by making a specialised copy of f.
544 There's a distinct danger that the specialised version would
545 first build a dictionary for (Eq b, Eq c), and then select the (==)
546 method from it! Even if it didn't, not a great deal is saved.
548 We do, however, generate polymorphic, but not overloaded, specialisations:
550 f :: Eq a => [a] -> b -> b -> b
551 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
553 Hence, the invariant is this:
555 *** no specialised version is overloaded ***
558 %************************************************************************
560 \subsubsection{The exported function}
562 %************************************************************************
565 specProgram :: UniqSupply -> [CoreBind] -> [CoreBind]
566 specProgram us binds = initSM us $
567 do { (binds', uds') <- go binds
568 ; return (wrapDictBinds (ud_binds uds') binds') }
570 -- We need to start with a Subst that knows all the things
571 -- that are in scope, so that the substitution engine doesn't
572 -- accidentally re-use a unique that's already in use
573 -- Easiest thing is to do it all at once, as if all the top-level
574 -- decls were mutually recursive
575 top_subst = mkEmptySubst (mkInScopeSet (mkVarSet (bindersOfBinds binds)))
577 go [] = return ([], emptyUDs)
578 go (bind:binds) = do (binds', uds) <- go binds
579 (bind', uds') <- specBind top_subst bind uds
580 return (bind' ++ binds', uds')
583 %************************************************************************
585 \subsubsection{@specExpr@: the main function}
587 %************************************************************************
590 specVar :: Subst -> Id -> CoreExpr
591 specVar subst v = lookupIdSubst (text "specVar") subst v
593 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
594 -- We carry a substitution down:
595 -- a) we must clone any binding that might float outwards,
596 -- to avoid name clashes
597 -- b) we carry a type substitution to use when analysing
598 -- the RHS of specialised bindings (no type-let!)
600 ---------------- First the easy cases --------------------
601 specExpr subst (Type ty) = return (Type (CoreSubst.substTy subst ty), emptyUDs)
602 specExpr subst (Var v) = return (specVar subst v, emptyUDs)
603 specExpr _ (Lit lit) = return (Lit lit, emptyUDs)
604 specExpr subst (Cast e co) = do
605 (e', uds) <- specExpr subst e
606 return ((Cast e' (CoreSubst.substTy subst co)), uds)
607 specExpr subst (Note note body) = do
608 (body', uds) <- specExpr subst body
609 return (Note (specNote subst note) body', uds)
612 ---------------- Applications might generate a call instance --------------------
613 specExpr subst expr@(App {})
616 go (App fun arg) args = do (arg', uds_arg) <- specExpr subst arg
617 (fun', uds_app) <- go fun (arg':args)
618 return (App fun' arg', uds_arg `plusUDs` uds_app)
620 go (Var f) args = case specVar subst f of
621 Var f' -> return (Var f', mkCallUDs f' args)
622 e' -> return (e', emptyUDs) -- I don't expect this!
623 go other _ = specExpr subst other
625 ---------------- Lambda/case require dumping of usage details --------------------
626 specExpr subst e@(Lam _ _) = do
627 (body', uds) <- specExpr subst' body
628 let (free_uds, dumped_dbs) = dumpUDs bndrs' uds
629 return (mkLams bndrs' (wrapDictBindsE dumped_dbs body'), free_uds)
631 (bndrs, body) = collectBinders e
632 (subst', bndrs') = substBndrs subst bndrs
633 -- More efficient to collect a group of binders together all at once
634 -- and we don't want to split a lambda group with dumped bindings
636 specExpr subst (Case scrut case_bndr ty alts) = do
637 (scrut', uds_scrut) <- specExpr subst scrut
638 (alts', uds_alts) <- mapAndCombineSM spec_alt alts
639 return (Case scrut' case_bndr' (CoreSubst.substTy subst ty) alts',
640 uds_scrut `plusUDs` uds_alts)
642 (subst_alt, case_bndr') = substBndr subst case_bndr
643 -- No need to clone case binder; it can't float like a let(rec)
645 spec_alt (con, args, rhs) = do
646 (rhs', uds) <- specExpr subst_rhs rhs
647 let (free_uds, dumped_dbs) = dumpUDs args' uds
648 return ((con, args', wrapDictBindsE dumped_dbs rhs'), free_uds)
650 (subst_rhs, args') = substBndrs subst_alt args
652 ---------------- Finally, let is the interesting case --------------------
653 specExpr subst (Let bind body) = do
655 (rhs_subst, body_subst, bind') <- cloneBindSM subst bind
657 -- Deal with the body
658 (body', body_uds) <- specExpr body_subst body
660 -- Deal with the bindings
661 (binds', uds) <- specBind rhs_subst bind' body_uds
664 return (foldr Let body' binds', uds)
666 -- Must apply the type substitution to coerceions
667 specNote :: Subst -> Note -> Note
668 specNote _ note = note
671 %************************************************************************
673 \subsubsection{Dealing with a binding}
675 %************************************************************************
678 specBind :: Subst -- Use this for RHSs
680 -> UsageDetails -- Info on how the scope of the binding
681 -> SpecM ([CoreBind], -- New bindings
682 UsageDetails) -- And info to pass upstream
684 -- Returned UsageDetails:
685 -- No calls for binders of this bind
686 specBind rhs_subst (NonRec fn rhs) body_uds
687 = do { (rhs', rhs_uds) <- specExpr rhs_subst rhs
688 ; (fn', spec_defns, body_uds1) <- specDefn rhs_subst body_uds fn rhs
690 ; let pairs = spec_defns ++ [(fn', rhs')]
691 -- fn' mentions the spec_defns in its rules,
692 -- so put the latter first
694 combined_uds = body_uds1 `plusUDs` rhs_uds
695 -- This way round a call in rhs_uds of a function f
696 -- at type T will override a call of f at T in body_uds1; and
697 -- that is good because it'll tend to keep "earlier" calls
698 -- See Note [Specialisation of dictionary functions]
700 (free_uds, dump_dbs, float_all) = dumpBindUDs [fn] combined_uds
701 -- See Note [From non-recursive to recursive]
703 final_binds | isEmptyBag dump_dbs = [NonRec b r | (b,r) <- pairs]
704 | otherwise = [Rec (flattenDictBinds dump_dbs pairs)]
707 -- Rather than discard the calls mentioning the bound variables
708 -- we float this binding along with the others
709 return ([], free_uds `snocDictBinds` final_binds)
711 -- No call in final_uds mentions bound variables,
712 -- so we can just leave the binding here
713 return (final_binds, free_uds) }
716 specBind rhs_subst (Rec pairs) body_uds
717 -- Note [Specialising a recursive group]
718 = do { let (bndrs,rhss) = unzip pairs
719 ; (rhss', rhs_uds) <- mapAndCombineSM (specExpr rhs_subst) rhss
720 ; let scope_uds = body_uds `plusUDs` rhs_uds
721 -- Includes binds and calls arising from rhss
723 ; (bndrs1, spec_defns1, uds1) <- specDefns rhs_subst scope_uds pairs
725 ; (bndrs3, spec_defns3, uds3)
726 <- if null spec_defns1 -- Common case: no specialisation
727 then return (bndrs1, [], uds1)
728 else do { -- Specialisation occurred; do it again
729 (bndrs2, spec_defns2, uds2)
730 <- specDefns rhs_subst uds1 (bndrs1 `zip` rhss)
731 ; return (bndrs2, spec_defns2 ++ spec_defns1, uds2) }
733 ; let (final_uds, dumped_dbs, float_all) = dumpBindUDs bndrs uds3
734 bind = Rec (flattenDictBinds dumped_dbs $
735 spec_defns3 ++ zip bndrs3 rhss')
738 return ([], final_uds `snocDictBind` bind)
740 return ([bind], final_uds) }
743 ---------------------------
745 -> UsageDetails -- Info on how it is used in its scope
746 -> [(Id,CoreExpr)] -- The things being bound and their un-processed RHS
747 -> SpecM ([Id], -- Original Ids with RULES added
748 [(Id,CoreExpr)], -- Extra, specialised bindings
749 UsageDetails) -- Stuff to fling upwards from the specialised versions
751 -- Specialise a list of bindings (the contents of a Rec), but flowing usages
752 -- upwards binding by binding. Example: { f = ...g ...; g = ...f .... }
753 -- Then if the input CallDetails has a specialised call for 'g', whose specialisation
754 -- in turn generates a specialised call for 'f', we catch that in this one sweep.
755 -- But not vice versa (it's a fixpoint problem).
757 specDefns _subst uds []
758 = return ([], [], uds)
759 specDefns subst uds ((bndr,rhs):pairs)
760 = do { (bndrs1, spec_defns1, uds1) <- specDefns subst uds pairs
761 ; (bndr1, spec_defns2, uds2) <- specDefn subst uds1 bndr rhs
762 ; return (bndr1 : bndrs1, spec_defns1 ++ spec_defns2, uds2) }
764 ---------------------------
766 -> UsageDetails -- Info on how it is used in its scope
767 -> Id -> CoreExpr -- The thing being bound and its un-processed RHS
768 -> SpecM (Id, -- Original Id with added RULES
769 [(Id,CoreExpr)], -- Extra, specialised bindings
770 UsageDetails) -- Stuff to fling upwards from the specialised versions
772 specDefn subst body_uds fn rhs
773 -- The first case is the interesting one
774 | rhs_tyvars `lengthIs` n_tyvars -- Rhs of fn's defn has right number of big lambdas
775 && rhs_ids `lengthAtLeast` n_dicts -- and enough dict args
776 && notNull calls_for_me -- And there are some calls to specialise
777 && not (isNeverActive (idInlineActivation fn))
778 -- Don't specialise NOINLINE things
779 -- See Note [Auto-specialisation and RULES]
781 -- && not (certainlyWillInline (idUnfolding fn)) -- And it's not small
782 -- See Note [Inline specialisation] for why we do not
783 -- switch off specialisation for inline functions
785 = do { -- Make a specialised version for each call in calls_for_me
786 stuff <- mapM spec_call calls_for_me
787 ; let (spec_defns, spec_uds, spec_rules) = unzip3 (catMaybes stuff)
788 fn' = addIdSpecialisations fn spec_rules
789 final_uds = body_uds_without_me `plusUDs` plusUDList spec_uds
790 -- It's important that the `plusUDs` is this way
791 -- round, because body_uds_without_me may bind
792 -- dictionaries that are used in calls_for_me passed
793 -- to specDefn. So the dictionary bindings in
794 -- spec_uds may mention dictionaries bound in
795 -- body_uds_without_me
797 ; return (fn', spec_defns, final_uds) }
799 | otherwise -- No calls or RHS doesn't fit our preconceptions
800 = WARN( notNull calls_for_me, ptext (sLit "Missed specialisation opportunity for") <+> ppr fn )
801 -- Note [Specialisation shape]
802 return (fn, [], body_uds_without_me)
806 fn_arity = idArity fn
807 fn_unf = realIdUnfolding fn -- Ignore loop-breaker-ness here
808 (tyvars, theta, _) = tcSplitSigmaTy fn_type
809 n_tyvars = length tyvars
810 n_dicts = length theta
811 inl_act = inlinePragmaActivation (idInlinePragma fn)
813 -- Figure out whether the function has an INLINE pragma
814 -- See Note [Inline specialisations]
815 fn_has_inline_rule :: Maybe Bool -- Derive sat-flag from existing thing
816 fn_has_inline_rule = case isInlineRule_maybe fn_unf of
817 Just (_,sat) -> Just sat
820 spec_arity = unfoldingArity fn_unf - n_dicts -- Arity of the *specialised* inline rule
822 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs
824 (body_uds_without_me, calls_for_me) = callsForMe fn body_uds
826 rhs_dict_ids = take n_dicts rhs_ids
827 body = mkLams (drop n_dicts rhs_ids) rhs_body
828 -- Glue back on the non-dict lambdas
830 already_covered :: [CoreExpr] -> Bool
831 already_covered args -- Note [Specialisations already covered]
832 = isJust (lookupRule (const True) realIdUnfolding
834 fn args (idCoreRules fn))
836 mk_ty_args :: [Maybe Type] -> [CoreExpr]
837 mk_ty_args call_ts = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
839 mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
840 mk_ty_arg _ (Just ty) = Type ty
842 ----------------------------------------------------------
843 -- Specialise to one particular call pattern
844 spec_call :: CallInfo -- Call instance
845 -> SpecM (Maybe ((Id,CoreExpr), -- Specialised definition
846 UsageDetails, -- Usage details from specialised body
847 CoreRule)) -- Info for the Id's SpecEnv
848 spec_call (CallKey call_ts, (call_ds, _))
849 = ASSERT( call_ts `lengthIs` n_tyvars && call_ds `lengthIs` n_dicts )
851 -- Suppose f's defn is f = /\ a b c -> \ d1 d2 -> rhs
852 -- Supppose the call is for f [Just t1, Nothing, Just t3] [dx1, dx2]
854 -- Construct the new binding
855 -- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b -> rhs)
856 -- PLUS the usage-details
857 -- { d1' = dx1; d2' = dx2 }
858 -- where d1', d2' are cloned versions of d1,d2, with the type substitution
859 -- applied. These auxiliary bindings just avoid duplication of dx1, dx2
861 -- Note that the substitution is applied to the whole thing.
862 -- This is convenient, but just slightly fragile. Notably:
863 -- * There had better be no name clashes in a/b/c
865 -- poly_tyvars = [b] in the example above
866 -- spec_tyvars = [a,c]
867 -- ty_args = [t1,b,t3]
868 poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
869 spec_tv_binds = [(tv,ty) | (tv, Just ty) <- rhs_tyvars `zip` call_ts]
870 spec_ty_args = map snd spec_tv_binds
871 ty_args = mk_ty_args call_ts
872 rhs_subst = CoreSubst.extendTvSubstList subst spec_tv_binds
874 ; (rhs_subst1, inst_dict_ids) <- newDictBndrs rhs_subst rhs_dict_ids
875 -- Clone rhs_dicts, including instantiating their types
877 ; let (rhs_subst2, dx_binds) = bindAuxiliaryDicts rhs_subst1 $
878 (my_zipEqual rhs_dict_ids inst_dict_ids call_ds)
879 inst_args = ty_args ++ map Var inst_dict_ids
881 ; if already_covered inst_args then
884 { -- Figure out the type of the specialised function
885 let body_ty = applyTypeToArgs rhs fn_type inst_args
886 (lam_args, app_args) -- Add a dummy argument if body_ty is unlifted
887 | isUnLiftedType body_ty -- C.f. WwLib.mkWorkerArgs
888 = (poly_tyvars ++ [voidArgId], poly_tyvars ++ [realWorldPrimId])
889 | otherwise = (poly_tyvars, poly_tyvars)
890 spec_id_ty = mkPiTypes lam_args body_ty
892 ; spec_f <- newSpecIdSM fn spec_id_ty
893 ; (spec_rhs, rhs_uds) <- specExpr rhs_subst2 (mkLams lam_args body)
895 -- The rule to put in the function's specialisation is:
896 -- forall b, d1',d2'. f t1 b t3 d1' d2' = f1 b
897 rule_name = mkFastString ("SPEC " ++ showSDoc (ppr fn <+> ppr spec_ty_args))
898 spec_env_rule = mkLocalRule
900 inl_act -- Note [Auto-specialisation and RULES]
902 (poly_tyvars ++ inst_dict_ids)
904 (mkVarApps (Var spec_f) app_args)
906 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
907 final_uds = foldr consDictBind rhs_uds dx_binds
909 -- Adding arity information just propagates it a bit faster
910 -- See Note [Arity decrease] in Simplify
911 -- Copy InlinePragma information from the parent Id.
912 -- So if f has INLINE[1] so does spec_f
913 spec_f_w_arity = spec_f `setIdArity` max 0 (fn_arity - n_dicts)
914 `setInlineActivation` inl_act
916 -- Add an InlineRule if the parent has one
917 -- See Note [Inline specialisations]
919 | Just sat <- fn_has_inline_rule
921 mb_spec_arity = if sat then Just spec_arity else Nothing
923 spec_f_w_arity `setIdUnfolding` mkInlineRule spec_rhs mb_spec_arity
927 ; return (Just ((final_spec_f, spec_rhs), final_uds, spec_env_rule)) } }
930 | debugIsOn && not (equalLength xs ys && equalLength ys zs)
931 = pprPanic "my_zipEqual" (vcat [ ppr xs, ppr ys
932 , ppr fn <+> ppr call_ts
933 , ppr (idType fn), ppr theta
934 , ppr n_dicts, ppr rhs_dict_ids
936 | otherwise = zip3 xs ys zs
940 -> [(DictId,DictId,CoreExpr)] -- (orig_dict, inst_dict, dx)
941 -> (Subst, -- Substitute for all orig_dicts
942 [CoreBind]) -- Auxiliary bindings
943 -- Bind any dictionary arguments to fresh names, to preserve sharing
944 -- Substitution already substitutes orig_dict -> inst_dict
945 bindAuxiliaryDicts subst triples = go subst [] triples
947 go subst binds [] = (subst, binds)
948 go subst binds ((d, dx_id, dx) : pairs)
949 | exprIsTrivial dx = go (extendIdSubst subst d dx) binds pairs
950 -- No auxiliary binding necessary
951 -- Note that we bind the *original* dict in the substitution,
952 -- overriding any d->dx_id binding put there by substBndrs
954 | otherwise = go subst_w_unf (NonRec dx_id dx : binds) pairs
956 dx_id1 = dx_id `setIdUnfolding` mkUnfolding False False dx
957 subst_w_unf = extendIdSubst subst d (Var dx_id1)
958 -- Important! We're going to substitute dx_id1 for d
959 -- and we want it to look "interesting", else we won't gather *any*
960 -- consequential calls. E.g.
962 -- If we specialise f for a call (f (dfun dNumInt)), we'll get
963 -- a consequent call (g d') with an auxiliary definition
965 -- We want that consequent call to look interesting
967 -- Again, note that we bind the *original* dict in the substitution,
968 -- overriding any d->dx_id binding put there by substBndrs
971 Note [From non-recursive to recursive]
972 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
973 Even in the non-recursive case, if any dict-binds depend on 'fn' we might
974 have built a recursive knot
977 MkUD { ud_binds = d7 = MkD ..f..
978 , ud_calls = ...(f T d7)... }
982 Rec { fs x = <blah>[T/a, d7/d]
987 Here the recursion is only through the RULE.
990 Note [Specialisation of dictionary functions]
991 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
992 Here is a nasty example that bit us badly: see Trac #3591
994 dfun a d = MkD a d (meth d)
1000 None of these definitions is recursive. What happened was that we
1001 generated a specialisation:
1003 RULE forall d. dfun T d = dT
1004 dT = (MkD a d (meth d)) [T/a, d1/d]
1005 = MkD T d1 (meth d1)
1007 But now we use the RULE on the RHS of d2, to get
1009 d2 = dT = MkD d1 (meth d1)
1012 and now d1 is bottom! The problem is that when specialising 'dfun' we
1013 should first dump "below" the binding all floated dictionary bindings
1014 that mention 'dfun' itself. So d2 and d3 (and hence d1) must be
1015 placed below 'dfun', and thus unavailable to it when specialising
1016 'dfun'. That in turn means that the call (dfun T d1) must be
1017 discarded. On the other hand, the call (dfun T d4) is fine, assuming
1018 d4 doesn't mention dfun.
1022 class C a where { foo,bar :: [a] -> [a] }
1024 instance C Int where
1028 r_bar :: C a => [a] -> [a]
1029 r_bar xs = bar (xs ++ xs)
1033 r_bar a (c::C a) (xs::[a]) = bar a d (xs ++ xs)
1035 Rec { $fCInt :: C Int = MkC foo_help reverse
1036 foo_help (xs::[Int]) = r_bar Int $fCInt xs }
1038 The call (r_bar $fCInt) mentions $fCInt,
1039 which mentions foo_help,
1040 which mentions r_bar
1041 But we DO want to specialise r_bar at Int:
1043 Rec { $fCInt :: C Int = MkC foo_help reverse
1044 foo_help (xs::[Int]) = r_bar Int $fCInt xs
1046 r_bar a (c::C a) (xs::[a]) = bar a d (xs ++ xs)
1047 RULE r_bar Int _ = r_bar_Int
1049 r_bar_Int xs = bar Int $fCInt (xs ++ xs)
1052 Note that, because of its RULE, r_bar joins the recursive
1053 group. (In this case it'll unravel a short moment later.)
1056 Conclusion: we catch the nasty case using filter_dfuns in
1057 callsForMe To be honest I'm not 100% certain that this is 100%
1058 right, but it works. Sigh.
1061 Note [Specialising a recursive group]
1062 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1064 let rec { f x = ...g x'...
1065 ; g y = ...f y'.... }
1067 Here we specialise 'f' at Char; but that is very likely to lead to
1068 a specialisation of 'g' at Char. We must do the latter, else the
1069 whole point of specialisation is lost.
1071 But we do not want to keep iterating to a fixpoint, because in the
1072 presence of polymorphic recursion we might generate an infinite number
1075 So we use the following heuristic:
1076 * Arrange the rec block in dependency order, so far as possible
1077 (the occurrence analyser already does this)
1079 * Specialise it much like a sequence of lets
1081 * Then go through the block a second time, feeding call-info from
1082 the RHSs back in the bottom, as it were
1084 In effect, the ordering maxmimises the effectiveness of each sweep,
1085 and we do just two sweeps. This should catch almost every case of
1086 monomorphic recursion -- the exception could be a very knotted-up
1087 recursion with multiple cycles tied up together.
1089 This plan is implemented in the Rec case of specBindItself.
1091 Note [Specialisations already covered]
1092 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1093 We obviously don't want to generate two specialisations for the same
1094 argument pattern. There are two wrinkles
1096 1. We do the already-covered test in specDefn, not when we generate
1097 the CallInfo in mkCallUDs. We used to test in the latter place, but
1098 we now iterate the specialiser somewhat, and the Id at the call site
1099 might therefore not have all the RULES that we can see in specDefn
1101 2. What about two specialisations where the second is an *instance*
1102 of the first? If the more specific one shows up first, we'll generate
1103 specialisations for both. If the *less* specific one shows up first,
1104 we *don't* currently generate a specialisation for the more specific
1105 one. (See the call to lookupRule in already_covered.) Reasons:
1106 (a) lookupRule doesn't say which matches are exact (bad reason)
1107 (b) if the earlier specialisation is user-provided, it's
1108 far from clear that we should auto-specialise further
1110 Note [Auto-specialisation and RULES]
1111 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1113 g :: Num a => a -> a
1116 f :: (Int -> Int) -> Int
1118 {-# RULE f g = 0 #-}
1120 Suppose that auto-specialisation makes a specialised version of
1121 g::Int->Int That version won't appear in the LHS of the RULE for f.
1122 So if the specialisation rule fires too early, the rule for f may
1125 It might be possible to add new rules, to "complete" the rewrite system.
1127 RULE forall d. g Int d = g_spec
1131 But that's a bit complicated. For now we ask the programmer's help,
1132 by *copying the INLINE activation pragma* to the auto-specialised
1133 rule. So if g says {-# NOINLINE[2] g #-}, then the auto-spec rule
1134 will also not be active until phase 2. And that's what programmers
1135 should jolly well do anyway, even aside from specialisation, to ensure
1136 that g doesn't inline too early.
1138 This in turn means that the RULE would never fire for a NOINLINE
1139 thing so not much point in generating a specialisation at all.
1141 Note [Specialisation shape]
1142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1143 We only specialise a function if it has visible top-level lambdas
1144 corresponding to its overloading. E.g. if
1145 f :: forall a. Eq a => ....
1146 then its body must look like
1149 Reason: when specialising the body for a call (f ty dexp), we want to
1150 substitute dexp for d, and pick up specialised calls in the body of f.
1152 This doesn't always work. One example I came across was this:
1153 newtype Gen a = MkGen{ unGen :: Int -> a }
1155 choose :: Eq a => a -> Gen a
1156 choose n = MkGen (\r -> n)
1158 oneof = choose (1::Int)
1160 It's a silly exapmle, but we get
1161 choose = /\a. g `cast` co
1162 where choose doesn't have any dict arguments. Thus far I have not
1163 tried to fix this (wait till there's a real example).
1165 Note [Inline specialisations]
1166 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1167 We transfer to the specialised function any INLINE stuff from the
1168 original. This means
1169 (a) the Activation for its inlining (from its InlinePragma)
1172 This is a change (Jun06). Previously the idea is that the point of
1173 inlining was precisely to specialise the function at its call site,
1174 and that's not so important for the specialised copies. But
1175 *pragma-directed* specialisation now takes place in the
1176 typechecker/desugarer, with manually specified INLINEs. The
1177 specialiation here is automatic. It'd be very odd if a function
1178 marked INLINE was specialised (because of some local use), and then
1179 forever after (including importing modules) the specialised version
1180 wasn't INLINEd. After all, the programmer said INLINE!
1182 You might wonder why we don't just not specialise INLINE functions.
1183 It's because even INLINE functions are sometimes not inlined, when
1184 they aren't applied to interesting arguments. But perhaps the type
1185 arguments alone are enough to specialise (even though the args are too
1186 boring to trigger inlining), and it's certainly better to call the
1187 specialised version.
1190 %************************************************************************
1192 \subsubsection{UsageDetails and suchlike}
1194 %************************************************************************
1199 ud_binds :: !(Bag DictBind),
1200 -- Floated dictionary bindings
1201 -- The order is important;
1202 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
1203 -- (Remember, Bags preserve order in GHC.)
1205 ud_calls :: !CallDetails
1207 -- INVARIANT: suppose bs = bindersOf ud_binds
1208 -- Then 'calls' may *mention* 'bs',
1209 -- but there should be no calls *for* bs
1212 instance Outputable UsageDetails where
1213 ppr (MkUD { ud_binds = dbs, ud_calls = calls })
1214 = ptext (sLit "MkUD") <+> braces (sep (punctuate comma
1215 [ptext (sLit "binds") <+> equals <+> ppr dbs,
1216 ptext (sLit "calls") <+> equals <+> ppr calls]))
1218 type DictBind = (CoreBind, VarSet)
1219 -- The set is the free vars of the binding
1220 -- both tyvars and dicts
1222 type DictExpr = CoreExpr
1224 emptyUDs :: UsageDetails
1225 emptyUDs = MkUD { ud_binds = emptyBag, ud_calls = emptyVarEnv }
1227 ------------------------------------------------------------
1228 type CallDetails = IdEnv CallInfoSet
1229 newtype CallKey = CallKey [Maybe Type] -- Nothing => unconstrained type argument
1231 -- CallInfo uses a FiniteMap, thereby ensuring that
1232 -- we record only one call instance for any key
1234 -- The list of types and dictionaries is guaranteed to
1235 -- match the type of f
1236 type CallInfoSet = FiniteMap CallKey ([DictExpr], VarSet)
1237 -- Range is dict args and the vars of the whole
1238 -- call (including tyvars)
1239 -- [*not* include the main id itself, of course]
1241 type CallInfo = (CallKey, ([DictExpr], VarSet))
1243 instance Outputable CallKey where
1244 ppr (CallKey ts) = ppr ts
1246 -- Type isn't an instance of Ord, so that we can control which
1247 -- instance we use. That's tiresome here. Oh well
1248 instance Eq CallKey where
1249 k1 == k2 = case k1 `compare` k2 of { EQ -> True; _ -> False }
1251 instance Ord CallKey where
1252 compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
1254 cmp Nothing Nothing = EQ
1255 cmp Nothing (Just _) = LT
1256 cmp (Just _) Nothing = GT
1257 cmp (Just t1) (Just t2) = tcCmpType t1 t2
1259 unionCalls :: CallDetails -> CallDetails -> CallDetails
1260 unionCalls c1 c2 = plusVarEnv_C plusFM c1 c2
1262 -- plusCalls :: UsageDetails -> CallDetails -> UsageDetails
1263 -- plusCalls uds call_ds = uds { ud_calls = ud_calls uds `unionCalls` call_ds }
1265 callDetailsFVs :: CallDetails -> VarSet
1266 callDetailsFVs calls = foldVarEnv (unionVarSet . callInfoFVs) emptyVarSet calls
1268 callInfoFVs :: CallInfoSet -> VarSet
1269 callInfoFVs call_info = foldFM (\_ (_,fv) vs -> unionVarSet fv vs) emptyVarSet call_info
1271 ------------------------------------------------------------
1272 singleCall :: Id -> [Maybe Type] -> [DictExpr] -> UsageDetails
1273 singleCall id tys dicts
1274 = MkUD {ud_binds = emptyBag,
1275 ud_calls = unitVarEnv id (unitFM (CallKey tys) (dicts, call_fvs)) }
1277 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
1278 tys_fvs = tyVarsOfTypes (catMaybes tys)
1279 -- The type args (tys) are guaranteed to be part of the dictionary
1280 -- types, because they are just the constrained types,
1281 -- and the dictionary is therefore sure to be bound
1282 -- inside the binding for any type variables free in the type;
1283 -- hence it's safe to neglect tyvars free in tys when making
1284 -- the free-var set for this call
1285 -- BUT I don't trust this reasoning; play safe and include tys_fvs
1287 -- We don't include the 'id' itself.
1289 mkCallUDs :: Id -> [CoreExpr] -> UsageDetails
1291 | not (isLocalId f) -- Imported from elsewhere
1292 || null theta -- Not overloaded
1293 || not (all isClassPred theta)
1294 -- Only specialise if all overloading is on class params.
1295 -- In ptic, with implicit params, the type args
1296 -- *don't* say what the value of the implicit param is!
1297 || not (spec_tys `lengthIs` n_tyvars)
1298 || not ( dicts `lengthIs` n_dicts)
1299 || not (any interestingDict dicts) -- Note [Interesting dictionary arguments]
1300 -- See also Note [Specialisations already covered]
1301 = -- pprTrace "mkCallUDs: discarding" (vcat [ppr f, ppr args, ppr n_tyvars, ppr n_dicts, ppr (map interestingDict dicts)])
1302 emptyUDs -- Not overloaded, or no specialisation wanted
1305 = -- pprTrace "mkCallUDs: keeping" (vcat [ppr f, ppr args, ppr n_tyvars, ppr n_dicts, ppr (map interestingDict dicts)])
1306 singleCall f spec_tys dicts
1308 (tyvars, theta, _) = tcSplitSigmaTy (idType f)
1309 constrained_tyvars = tyVarsOfTheta theta
1310 n_tyvars = length tyvars
1311 n_dicts = length theta
1313 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
1314 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
1317 | tyvar `elemVarSet` constrained_tyvars = Just ty
1318 | otherwise = Nothing
1321 Note [Interesting dictionary arguments]
1322 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1324 \a.\d:Eq a. let f = ... in ...(f d)...
1325 There really is not much point in specialising f wrt the dictionary d,
1326 because the code for the specialised f is not improved at all, because
1327 d is lambda-bound. We simply get junk specialisations.
1329 What is "interesting"? Just that it has *some* structure.
1332 interestingDict :: CoreExpr -> Bool
1333 -- A dictionary argument is interesting if it has *some* structure
1334 interestingDict (Var v) = hasSomeUnfolding (idUnfolding v)
1335 || isDataConWorkId v
1336 interestingDict (Type _) = False
1337 interestingDict (App fn (Type _)) = interestingDict fn
1338 interestingDict (Note _ a) = interestingDict a
1339 interestingDict (Cast e _) = interestingDict e
1340 interestingDict _ = True
1344 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
1345 plusUDs (MkUD {ud_binds = db1, ud_calls = calls1})
1346 (MkUD {ud_binds = db2, ud_calls = calls2})
1347 = MkUD { ud_binds = db1 `unionBags` db2
1348 , ud_calls = calls1 `unionCalls` calls2 }
1350 plusUDList :: [UsageDetails] -> UsageDetails
1351 plusUDList = foldr plusUDs emptyUDs
1353 -----------------------------
1354 _dictBindBndrs :: Bag DictBind -> [Id]
1355 _dictBindBndrs dbs = foldrBag ((++) . bindersOf . fst) [] dbs
1357 mkDB :: CoreBind -> DictBind
1358 mkDB bind = (bind, bind_fvs bind)
1360 bind_fvs :: CoreBind -> VarSet
1361 bind_fvs (NonRec bndr rhs) = pair_fvs (bndr,rhs)
1362 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1365 rhs_fvs = unionVarSets (map pair_fvs prs)
1367 pair_fvs :: (Id, CoreExpr) -> VarSet
1368 pair_fvs (bndr, rhs) = exprFreeVars rhs `unionVarSet` idFreeVars bndr
1369 -- Don't forget variables mentioned in the
1370 -- rules of the bndr. C.f. OccAnal.addRuleUsage
1371 -- Also tyvars mentioned in its type; they may not appear in the RHS
1375 flattenDictBinds :: Bag DictBind -> [(Id,CoreExpr)] -> [(Id,CoreExpr)]
1376 flattenDictBinds dbs pairs
1377 = foldrBag add pairs dbs
1379 add (NonRec b r,_) pairs = (b,r) : pairs
1380 add (Rec prs1, _) pairs = prs1 ++ pairs
1382 snocDictBinds :: UsageDetails -> [CoreBind] -> UsageDetails
1383 -- Add ud_binds to the tail end of the bindings in uds
1384 snocDictBinds uds dbs
1385 = uds { ud_binds = ud_binds uds `unionBags`
1386 foldr (consBag . mkDB) emptyBag dbs }
1388 consDictBind :: CoreBind -> UsageDetails -> UsageDetails
1389 consDictBind bind uds = uds { ud_binds = mkDB bind `consBag` ud_binds uds }
1391 snocDictBind :: UsageDetails -> CoreBind -> UsageDetails
1392 snocDictBind uds bind = uds { ud_binds = ud_binds uds `snocBag` mkDB bind }
1394 wrapDictBinds :: Bag DictBind -> [CoreBind] -> [CoreBind]
1395 wrapDictBinds dbs binds
1396 = foldrBag add binds dbs
1398 add (bind,_) binds = bind : binds
1400 wrapDictBindsE :: Bag DictBind -> CoreExpr -> CoreExpr
1401 wrapDictBindsE dbs expr
1402 = foldrBag add expr dbs
1404 add (bind,_) expr = Let bind expr
1406 ----------------------
1407 dumpUDs :: [CoreBndr] -> UsageDetails -> (UsageDetails, Bag DictBind)
1408 -- Used at a lambda or case binder; just dump anything mentioning the binder
1409 dumpUDs bndrs uds@(MkUD { ud_binds = orig_dbs, ud_calls = orig_calls })
1410 | null bndrs = (uds, emptyBag) -- Common in case alternatives
1411 | otherwise = (free_uds, dump_dbs)
1413 free_uds = MkUD { ud_binds = free_dbs, ud_calls = free_calls }
1414 bndr_set = mkVarSet bndrs
1415 (free_dbs, dump_dbs, dump_set) = splitDictBinds orig_dbs bndr_set
1416 free_calls = deleteCallsMentioning dump_set $ -- Drop calls mentioning bndr_set on the floor
1417 deleteCallsFor bndrs orig_calls -- Discard calls for bndr_set; there should be
1418 -- no calls for any of the dicts in dump_dbs
1420 dumpBindUDs :: [CoreBndr] -> UsageDetails -> (UsageDetails, Bag DictBind, Bool)
1421 -- Used at a lambda or case binder; just dump anything mentioning the binder
1422 dumpBindUDs bndrs (MkUD { ud_binds = orig_dbs, ud_calls = orig_calls })
1423 = (free_uds, dump_dbs, float_all)
1425 free_uds = MkUD { ud_binds = free_dbs, ud_calls = free_calls }
1426 bndr_set = mkVarSet bndrs
1427 (free_dbs, dump_dbs, dump_set) = splitDictBinds orig_dbs bndr_set
1428 free_calls = deleteCallsFor bndrs orig_calls
1429 float_all = dump_set `intersectsVarSet` callDetailsFVs free_calls
1431 callsForMe :: Id -> UsageDetails -> (UsageDetails, [CallInfo])
1432 callsForMe fn (MkUD { ud_binds = orig_dbs, ud_calls = orig_calls })
1433 = -- pprTrace ("callsForMe")
1435 -- text "Orig dbs =" <+> ppr (_dictBindBndrs orig_dbs),
1436 -- text "Orig calls =" <+> ppr orig_calls,
1437 -- text "Dep set =" <+> ppr dep_set,
1438 -- text "Calls for me =" <+> ppr calls_for_me]) $
1439 (uds_without_me, calls_for_me)
1441 uds_without_me = MkUD { ud_binds = orig_dbs, ud_calls = delVarEnv orig_calls fn }
1442 calls_for_me = case lookupVarEnv orig_calls fn of
1444 Just cs -> filter_dfuns (fmToList cs)
1446 dep_set = foldlBag go (unitVarSet fn) orig_dbs
1447 go dep_set (db,fvs) | fvs `intersectsVarSet` dep_set
1448 = extendVarSetList dep_set (bindersOf db)
1451 -- Note [Specialisation of dictionary functions]
1452 filter_dfuns | isDFunId fn = filter ok_call
1453 | otherwise = \cs -> cs
1455 ok_call (_, (_,fvs)) = not (fvs `intersectsVarSet` dep_set)
1457 ----------------------
1458 splitDictBinds :: Bag DictBind -> IdSet -> (Bag DictBind, Bag DictBind, IdSet)
1459 -- Returns (free_dbs, dump_dbs, dump_set)
1460 splitDictBinds dbs bndr_set
1461 = foldlBag split_db (emptyBag, emptyBag, bndr_set) dbs
1462 -- Important that it's foldl not foldr;
1463 -- we're accumulating the set of dumped ids in dump_set
1465 split_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1466 | dump_idset `intersectsVarSet` fvs -- Dump it
1467 = (free_dbs, dump_dbs `snocBag` db,
1468 extendVarSetList dump_idset (bindersOf bind))
1470 | otherwise -- Don't dump it
1471 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1474 ----------------------
1475 deleteCallsMentioning :: VarSet -> CallDetails -> CallDetails
1476 -- Remove calls *mentioning* bs
1477 deleteCallsMentioning bs calls
1478 = mapVarEnv filter_calls calls
1480 filter_calls :: CallInfoSet -> CallInfoSet
1481 filter_calls = filterFM (\_ (_, fvs) -> not (fvs `intersectsVarSet` bs))
1483 deleteCallsFor :: [Id] -> CallDetails -> CallDetails
1484 -- Remove calls *for* bs
1485 deleteCallsFor bs calls = delVarEnvList calls bs
1489 %************************************************************************
1491 \subsubsection{Boring helper functions}
1493 %************************************************************************
1496 type SpecM a = UniqSM a
1498 initSM :: UniqSupply -> SpecM a -> a
1501 mapAndCombineSM :: (a -> SpecM (b, UsageDetails)) -> [a] -> SpecM ([b], UsageDetails)
1502 mapAndCombineSM _ [] = return ([], emptyUDs)
1503 mapAndCombineSM f (x:xs) = do (y, uds1) <- f x
1504 (ys, uds2) <- mapAndCombineSM f xs
1505 return (y:ys, uds1 `plusUDs` uds2)
1507 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1508 -- Clone the binders of the bind; return new bind with the cloned binders
1509 -- Return the substitution to use for RHSs, and the one to use for the body
1510 cloneBindSM subst (NonRec bndr rhs) = do
1511 us <- getUniqueSupplyM
1512 let (subst', bndr') = cloneIdBndr subst us bndr
1513 return (subst, subst', NonRec bndr' rhs)
1515 cloneBindSM subst (Rec pairs) = do
1516 us <- getUniqueSupplyM
1517 let (subst', bndrs') = cloneRecIdBndrs subst us (map fst pairs)
1518 return (subst', subst', Rec (bndrs' `zip` map snd pairs))
1520 newDictBndrs :: Subst -> [CoreBndr] -> SpecM (Subst, [CoreBndr])
1521 -- Make up completely fresh binders for the dictionaries
1522 -- Their bindings are going to float outwards
1523 newDictBndrs subst bndrs
1524 = do { bndrs' <- mapM new bndrs
1525 ; let subst' = extendIdSubstList subst
1526 [(d, Var d') | (d,d') <- bndrs `zip` bndrs']
1527 ; return (subst', bndrs' ) }
1529 new b = do { uniq <- getUniqueM
1531 ty' = CoreSubst.substTy subst (idType b)
1532 ; return (mkUserLocal (nameOccName n) uniq ty' (getSrcSpan n)) }
1534 newSpecIdSM :: Id -> Type -> SpecM Id
1535 -- Give the new Id a similar occurrence name to the old one
1536 newSpecIdSM old_id new_ty
1537 = do { uniq <- getUniqueM
1538 ; let name = idName old_id
1539 new_occ = mkSpecOcc (nameOccName name)
1540 new_id = mkUserLocal new_occ uniq new_ty (getSrcSpan name)
1545 Old (but interesting) stuff about unboxed bindings
1546 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1548 What should we do when a value is specialised to a *strict* unboxed value?
1550 map_*_* f (x:xs) = let h = f x
1554 Could convert let to case:
1556 map_*_Int# f (x:xs) = case f x of h# ->
1560 This may be undesirable since it forces evaluation here, but the value
1561 may not be used in all branches of the body. In the general case this
1562 transformation is impossible since the mutual recursion in a letrec
1563 cannot be expressed as a case.
1565 There is also a problem with top-level unboxed values, since our
1566 implementation cannot handle unboxed values at the top level.
1568 Solution: Lift the binding of the unboxed value and extract it when it
1571 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1576 Now give it to the simplifier and the _Lifting will be optimised away.
1578 The benfit is that we have given the specialised "unboxed" values a
1579 very simplep lifted semantics and then leave it up to the simplifier to
1580 optimise it --- knowing that the overheads will be removed in nearly
1583 In particular, the value will only be evaluted in the branches of the
1584 program which use it, rather than being forced at the point where the
1585 value is bound. For example:
1587 filtermap_*_* p f (x:xs)
1594 filtermap_*_Int# p f (x:xs)
1595 = let h = case (f x) of h# -> _Lift h#
1598 True -> case h of _Lift h#
1602 The binding for h can still be inlined in the one branch and the
1603 _Lifting eliminated.
1606 Question: When won't the _Lifting be eliminated?
1608 Answer: When they at the top-level (where it is necessary) or when
1609 inlining would duplicate work (or possibly code depending on
1610 options). However, the _Lifting will still be eliminated if the
1611 strictness analyser deems the lifted binding strict.