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,
13 idSpecialisation, modifyIdInfo
15 import TcType ( Type, mkTyVarTy, tcSplitSigmaTy,
16 tyVarsOfTypes, tyVarsOfTheta,
17 mkForAllTys, tcCmpType
19 import Subst ( Subst, mkSubst, substTy, mkSubst, extendSubstList, mkInScopeSet,
20 simplBndr, simplBndrs,
21 substAndCloneId, substAndCloneIds, substAndCloneRecIds,
22 lookupIdSubst, substInScope
24 import Var ( zapSpecPragmaId )
28 import CoreUtils ( applyTypeToArgs )
29 import CoreUnfold ( certainlyWillInline )
30 import CoreFVs ( exprFreeVars, exprsFreeVars )
31 import CoreLint ( showPass, endPass )
32 import PprCore ( pprCoreRules )
33 import Rules ( addIdSpecialisations, lookupRule )
35 import UniqSupply ( UniqSupply,
36 UniqSM, initUs_, thenUs, returnUs, getUniqueUs,
39 import Name ( nameOccName, mkSpecOcc, getSrcLoc )
41 import Maybes ( catMaybes, maybeToBool )
42 import ErrUtils ( dumpIfSet_dyn )
44 import List ( partition )
45 import Util ( zipEqual, zipWithEqual, cmpList )
52 %************************************************************************
54 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
56 %************************************************************************
58 These notes describe how we implement specialisation to eliminate
61 The specialisation pass works on Core
62 syntax, complete with all the explicit dictionary application,
63 abstraction and construction as added by the type checker. The
64 existing type checker remains largely as it is.
66 One important thought: the {\em types} passed to an overloaded
67 function, and the {\em dictionaries} passed are mutually redundant.
68 If the same function is applied to the same type(s) then it is sure to
69 be applied to the same dictionary(s)---or rather to the same {\em
70 values}. (The arguments might look different but they will evaluate
73 Second important thought: we know that we can make progress by
74 treating dictionary arguments as static and worth specialising on. So
75 we can do without binding-time analysis, and instead specialise on
76 dictionary arguments and no others.
85 and suppose f is overloaded.
87 STEP 1: CALL-INSTANCE COLLECTION
89 We traverse <body>, accumulating all applications of f to types and
92 (Might there be partial applications, to just some of its types and
93 dictionaries? In principle yes, but in practice the type checker only
94 builds applications of f to all its types and dictionaries, so partial
95 applications could only arise as a result of transformation, and even
96 then I think it's unlikely. In any case, we simply don't accumulate such
97 partial applications.)
102 So now we have a collection of calls to f:
106 Notice that f may take several type arguments. To avoid ambiguity, we
107 say that f is called at type t1/t2 and t3/t4.
109 We take equivalence classes using equality of the *types* (ignoring
110 the dictionary args, which as mentioned previously are redundant).
112 STEP 3: SPECIALISATION
114 For each equivalence class, choose a representative (f t1 t2 d1 d2),
115 and create a local instance of f, defined thus:
117 f@t1/t2 = <f_rhs> t1 t2 d1 d2
119 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
120 of simplification will now result. However we don't actually *do* that
121 simplification. Rather, we leave it for the simplifier to do. If we
122 *did* do it, though, we'd get more call instances from the specialised
123 RHS. We can work out what they are by instantiating the call-instance
124 set from f's RHS with the types t1, t2.
126 Add this new id to f's IdInfo, to record that f has a specialised version.
128 Before doing any of this, check that f's IdInfo doesn't already
129 tell us about an existing instance of f at the required type/s.
130 (This might happen if specialisation was applied more than once, or
131 it might arise from user SPECIALIZE pragmas.)
135 Wait a minute! What if f is recursive? Then we can't just plug in
136 its right-hand side, can we?
138 But it's ok. The type checker *always* creates non-recursive definitions
139 for overloaded recursive functions. For example:
141 f x = f (x+x) -- Yes I know its silly
145 f a (d::Num a) = let p = +.sel a d
147 letrec fl (y::a) = fl (p y y)
151 We still have recusion for non-overloaded functions which we
152 speciailise, but the recursive call should get specialised to the
153 same recursive version.
159 All this is crystal clear when the function is applied to *constant
160 types*; that is, types which have no type variables inside. But what if
161 it is applied to non-constant types? Suppose we find a call of f at type
162 t1/t2. There are two possibilities:
164 (a) The free type variables of t1, t2 are in scope at the definition point
165 of f. In this case there's no problem, we proceed just as before. A common
166 example is as follows. Here's the Haskell:
171 After typechecking we have
173 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
174 in +.sel a d (f a d y) (f a d y)
176 Notice that the call to f is at type type "a"; a non-constant type.
177 Both calls to f are at the same type, so we can specialise to give:
179 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
180 in +.sel a d (f@a y) (f@a y)
183 (b) The other case is when the type variables in the instance types
184 are *not* in scope at the definition point of f. The example we are
185 working with above is a good case. There are two instances of (+.sel a d),
186 but "a" is not in scope at the definition of +.sel. Can we do anything?
187 Yes, we can "common them up", a sort of limited common sub-expression deal.
190 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
191 f@a (x::a) = +.sel@a x x
192 in +.sel@a (f@a y) (f@a y)
194 This can save work, and can't be spotted by the type checker, because
195 the two instances of +.sel weren't originally at the same type.
199 * There are quite a few variations here. For example, the defn of
200 +.sel could be floated ouside the \y, to attempt to gain laziness.
201 It certainly mustn't be floated outside the \d because the d has to
204 * We don't want to inline f_rhs in this case, because
205 that will duplicate code. Just commoning up the call is the point.
207 * Nothing gets added to +.sel's IdInfo.
209 * Don't bother unless the equivalence class has more than one item!
211 Not clear whether this is all worth it. It is of course OK to
212 simply discard call-instances when passing a big lambda.
214 Polymorphism 2 -- Overloading
216 Consider a function whose most general type is
218 f :: forall a b. Ord a => [a] -> b -> b
220 There is really no point in making a version of g at Int/Int and another
221 at Int/Bool, because it's only instancing the type variable "a" which
222 buys us any efficiency. Since g is completely polymorphic in b there
223 ain't much point in making separate versions of g for the different
226 That suggests that we should identify which of g's type variables
227 are constrained (like "a") and which are unconstrained (like "b").
228 Then when taking equivalence classes in STEP 2, we ignore the type args
229 corresponding to unconstrained type variable. In STEP 3 we make
230 polymorphic versions. Thus:
232 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
241 f a (d::Num a) = let g = ...
243 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
245 Here, g is only called at one type, but the dictionary isn't in scope at the
246 definition point for g. Usually the type checker would build a
247 definition for d1 which enclosed g, but the transformation system
248 might have moved d1's defn inward. Solution: float dictionary bindings
249 outwards along with call instances.
253 f x = let g p q = p==q
259 Before specialisation, leaving out type abstractions we have
261 f df x = let g :: Eq a => a -> a -> Bool
263 h :: Num a => a -> a -> (a, Bool)
264 h dh r s = let deq = eqFromNum dh
265 in (+ dh r s, g deq r s)
269 After specialising h we get a specialised version of h, like this:
271 h' r s = let deq = eqFromNum df
272 in (+ df r s, g deq r s)
274 But we can't naively make an instance for g from this, because deq is not in scope
275 at the defn of g. Instead, we have to float out the (new) defn of deq
276 to widen its scope. Notice that this floating can't be done in advance -- it only
277 shows up when specialisation is done.
279 User SPECIALIZE pragmas
280 ~~~~~~~~~~~~~~~~~~~~~~~
281 Specialisation pragmas can be digested by the type checker, and implemented
282 by adding extra definitions along with that of f, in the same way as before
284 f@t1/t2 = <f_rhs> t1 t2 d1 d2
286 Indeed the pragmas *have* to be dealt with by the type checker, because
287 only it knows how to build the dictionaries d1 and d2! For example
289 g :: Ord a => [a] -> [a]
290 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
292 Here, the specialised version of g is an application of g's rhs to the
293 Ord dictionary for (Tree Int), which only the type checker can conjure
294 up. There might not even *be* one, if (Tree Int) is not an instance of
295 Ord! (All the other specialision has suitable dictionaries to hand
298 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
299 it is buried in a complex (as-yet-un-desugared) binding group.
302 f@t1/t2 = f* t1 t2 d1 d2
304 where f* is the Id f with an IdInfo which says "inline me regardless!".
305 Indeed all the specialisation could be done in this way.
306 That in turn means that the simplifier has to be prepared to inline absolutely
307 any in-scope let-bound thing.
310 Again, the pragma should permit polymorphism in unconstrained variables:
312 h :: Ord a => [a] -> b -> b
313 {-# SPECIALIZE h :: [Int] -> b -> b #-}
315 We *insist* that all overloaded type variables are specialised to ground types,
316 (and hence there can be no context inside a SPECIALIZE pragma).
317 We *permit* unconstrained type variables to be specialised to
319 - or left as a polymorphic type variable
320 but nothing in between. So
322 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
324 is *illegal*. (It can be handled, but it adds complication, and gains the
328 SPECIALISING INSTANCE DECLARATIONS
329 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
332 instance Foo a => Foo [a] where
334 {-# SPECIALIZE instance Foo [Int] #-}
336 The original instance decl creates a dictionary-function
339 dfun.Foo.List :: forall a. Foo a -> Foo [a]
341 The SPECIALIZE pragma just makes a specialised copy, just as for
342 ordinary function definitions:
344 dfun.Foo.List@Int :: Foo [Int]
345 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
347 The information about what instance of the dfun exist gets added to
348 the dfun's IdInfo in the same way as a user-defined function too.
351 Automatic instance decl specialisation?
352 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
353 Can instance decls be specialised automatically? It's tricky.
354 We could collect call-instance information for each dfun, but
355 then when we specialised their bodies we'd get new call-instances
356 for ordinary functions; and when we specialised their bodies, we might get
357 new call-instances of the dfuns, and so on. This all arises because of
358 the unrestricted mutual recursion between instance decls and value decls.
360 Still, there's no actual problem; it just means that we may not do all
361 the specialisation we could theoretically do.
363 Furthermore, instance decls are usually exported and used non-locally,
364 so we'll want to compile enough to get those specialisations done.
366 Lastly, there's no such thing as a local instance decl, so we can
367 survive solely by spitting out *usage* information, and then reading that
368 back in as a pragma when next compiling the file. So for now,
369 we only specialise instance decls in response to pragmas.
372 SPITTING OUT USAGE INFORMATION
373 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
375 To spit out usage information we need to traverse the code collecting
376 call-instance information for all imported (non-prelude?) functions
377 and data types. Then we equivalence-class it and spit it out.
379 This is done at the top-level when all the call instances which escape
380 must be for imported functions and data types.
382 *** Not currently done ***
385 Partial specialisation by pragmas
386 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
387 What about partial specialisation:
389 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
390 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
394 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
396 Seems quite reasonable. Similar things could be done with instance decls:
398 instance (Foo a, Foo b) => Foo (a,b) where
400 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
401 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
403 Ho hum. Things are complex enough without this. I pass.
406 Requirements for the simplifer
407 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
408 The simplifier has to be able to take advantage of the specialisation.
410 * When the simplifier finds an application of a polymorphic f, it looks in
411 f's IdInfo in case there is a suitable instance to call instead. This converts
413 f t1 t2 d1 d2 ===> f_t1_t2
415 Note that the dictionaries get eaten up too!
417 * Dictionary selection operations on constant dictionaries must be
420 +.sel Int d ===> +Int
422 The obvious way to do this is in the same way as other specialised
423 calls: +.sel has inside it some IdInfo which tells that if it's applied
424 to the type Int then it should eat a dictionary and transform to +Int.
426 In short, dictionary selectors need IdInfo inside them for constant
429 * Exactly the same applies if a superclass dictionary is being
432 Eq.sel Int d ===> dEqInt
434 * Something similar applies to dictionary construction too. Suppose
435 dfun.Eq.List is the function taking a dictionary for (Eq a) to
436 one for (Eq [a]). Then we want
438 dfun.Eq.List Int d ===> dEq.List_Int
440 Where does the Eq [Int] dictionary come from? It is built in
441 response to a SPECIALIZE pragma on the Eq [a] instance decl.
443 In short, dfun Ids need IdInfo with a specialisation for each
444 constant instance of their instance declaration.
446 All this uses a single mechanism: the SpecEnv inside an Id
449 What does the specialisation IdInfo look like?
450 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
452 The SpecEnv of an Id maps a list of types (the template) to an expression
456 For example, if f has this SpecInfo:
458 [Int, a] -> \d:Ord Int. f' a
460 it means that we can replace the call
462 f Int t ===> (\d. f' t)
464 This chucks one dictionary away and proceeds with the
465 specialised version of f, namely f'.
468 What can't be done this way?
469 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
470 There is no way, post-typechecker, to get a dictionary for (say)
471 Eq a from a dictionary for Eq [a]. So if we find
475 we can't transform to
480 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
482 Of course, we currently have no way to automatically derive
483 eqList, nor to connect it to the Eq [a] instance decl, but you
484 can imagine that it might somehow be possible. Taking advantage
485 of this is permanently ruled out.
487 Still, this is no great hardship, because we intend to eliminate
488 overloading altogether anyway!
492 A note about non-tyvar dictionaries
493 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
494 Some Ids have types like
496 forall a,b,c. Eq a -> Ord [a] -> tau
498 This seems curious at first, because we usually only have dictionary
499 args whose types are of the form (C a) where a is a type variable.
500 But this doesn't hold for the functions arising from instance decls,
501 which sometimes get arguements with types of form (C (T a)) for some
504 Should we specialise wrt this compound-type dictionary? We used to say
506 "This is a heuristic judgement, as indeed is the fact that we
507 specialise wrt only dictionaries. We choose *not* to specialise
508 wrt compound dictionaries because at the moment the only place
509 they show up is in instance decls, where they are simply plugged
510 into a returned dictionary. So nothing is gained by specialising
513 But it is simpler and more uniform to specialise wrt these dicts too;
514 and in future GHC is likely to support full fledged type signatures
516 f ;: Eq [(a,b)] => ...
519 %************************************************************************
521 \subsubsection{The new specialiser}
523 %************************************************************************
525 Our basic game plan is this. For let(rec) bound function
526 f :: (C a, D c) => (a,b,c,d) -> Bool
528 * Find any specialised calls of f, (f ts ds), where
529 ts are the type arguments t1 .. t4, and
530 ds are the dictionary arguments d1 .. d2.
532 * Add a new definition for f1 (say):
534 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
536 Note that we abstract over the unconstrained type arguments.
540 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
542 to the specialisations of f. This will be used by the
543 simplifier to replace calls
544 (f t1 t2 t3 t4) da db
546 (\d1 d1 -> f1 t2 t4) da db
548 All the stuff about how many dictionaries to discard, and what types
549 to apply the specialised function to, are handled by the fact that the
550 SpecEnv contains a template for the result of the specialisation.
552 We don't build *partial* specialisations for f. For example:
554 f :: Eq a => a -> a -> Bool
555 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
557 Here, little is gained by making a specialised copy of f.
558 There's a distinct danger that the specialised version would
559 first build a dictionary for (Eq b, Eq c), and then select the (==)
560 method from it! Even if it didn't, not a great deal is saved.
562 We do, however, generate polymorphic, but not overloaded, specialisations:
564 f :: Eq a => [a] -> b -> b -> b
565 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
567 Hence, the invariant is this:
569 *** no specialised version is overloaded ***
572 %************************************************************************
574 \subsubsection{The exported function}
576 %************************************************************************
579 specProgram :: DynFlags -> UniqSupply -> [CoreBind] -> IO [CoreBind]
580 specProgram dflags us binds
582 showPass dflags "Specialise"
584 let binds' = initSM us (go binds `thenSM` \ (binds', uds') ->
585 returnSM (dumpAllDictBinds uds' binds'))
587 endPass dflags "Specialise" Opt_D_dump_spec binds'
589 dumpIfSet_dyn dflags Opt_D_dump_rules "Top-level specialisations"
590 (vcat (map dump_specs (concat (map bindersOf binds'))))
594 -- We need to start with a Subst that knows all the things
595 -- that are in scope, so that the substitution engine doesn't
596 -- accidentally re-use a unique that's already in use
597 -- Easiest thing is to do it all at once, as if all the top-level
598 -- decls were mutually recursive
599 top_subst = mkSubst (mkInScopeSet (mkVarSet (bindersOfBinds binds))) emptySubstEnv
601 go [] = returnSM ([], emptyUDs)
602 go (bind:binds) = go binds `thenSM` \ (binds', uds) ->
603 specBind top_subst bind uds `thenSM` \ (bind', uds') ->
604 returnSM (bind' ++ binds', uds')
606 dump_specs var = pprCoreRules var (idSpecialisation var)
609 %************************************************************************
611 \subsubsection{@specExpr@: the main function}
613 %************************************************************************
616 specVar :: Subst -> Id -> CoreExpr
617 specVar subst v = case lookupIdSubst subst v of
621 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
622 -- We carry a substitution down:
623 -- a) we must clone any binding that might flaot outwards,
624 -- to avoid name clashes
625 -- b) we carry a type substitution to use when analysing
626 -- the RHS of specialised bindings (no type-let!)
628 ---------------- First the easy cases --------------------
629 specExpr subst (Type ty) = returnSM (Type (substTy subst ty), emptyUDs)
630 specExpr subst (Var v) = returnSM (specVar subst v, emptyUDs)
631 specExpr subst (Lit lit) = returnSM (Lit lit, emptyUDs)
633 specExpr subst (Note note body)
634 = specExpr subst body `thenSM` \ (body', uds) ->
635 returnSM (Note (specNote subst note) body', uds)
638 ---------------- Applications might generate a call instance --------------------
639 specExpr subst expr@(App fun arg)
642 go (App fun arg) args = specExpr subst arg `thenSM` \ (arg', uds_arg) ->
643 go fun (arg':args) `thenSM` \ (fun', uds_app) ->
644 returnSM (App fun' arg', uds_arg `plusUDs` uds_app)
646 go (Var f) args = case specVar subst f of
647 Var f' -> returnSM (Var f', mkCallUDs subst f' args)
648 e' -> returnSM (e', emptyUDs) -- I don't expect this!
649 go other args = specExpr subst other
651 ---------------- Lambda/case require dumping of usage details --------------------
652 specExpr subst e@(Lam _ _)
653 = specExpr subst' body `thenSM` \ (body', uds) ->
655 (filtered_uds, body'') = dumpUDs bndrs' uds body'
657 returnSM (mkLams bndrs' body'', filtered_uds)
659 (bndrs, body) = collectBinders e
660 (subst', bndrs') = simplBndrs subst bndrs
661 -- More efficient to collect a group of binders together all at once
662 -- and we don't want to split a lambda group with dumped bindings
664 specExpr subst (Case scrut case_bndr alts)
665 = specExpr subst scrut `thenSM` \ (scrut', uds_scrut) ->
666 mapAndCombineSM spec_alt alts `thenSM` \ (alts', uds_alts) ->
667 returnSM (Case scrut' case_bndr' alts', uds_scrut `plusUDs` uds_alts)
669 (subst_alt, case_bndr') = simplBndr subst case_bndr
670 -- No need to clone case binder; it can't float like a let(rec)
672 spec_alt (con, args, rhs)
673 = specExpr subst_rhs rhs `thenSM` \ (rhs', uds) ->
675 (uds', rhs'') = dumpUDs args uds rhs'
677 returnSM ((con, args', rhs''), uds')
679 (subst_rhs, args') = simplBndrs subst_alt args
681 ---------------- Finally, let is the interesting case --------------------
682 specExpr subst (Let bind body)
684 cloneBindSM subst bind `thenSM` \ (rhs_subst, body_subst, bind') ->
686 -- Deal with the body
687 specExpr body_subst body `thenSM` \ (body', body_uds) ->
689 -- Deal with the bindings
690 specBind rhs_subst bind' body_uds `thenSM` \ (binds', uds) ->
693 returnSM (foldr Let body' binds', uds)
695 -- Must apply the type substitution to coerceions
696 specNote subst (Coerce t1 t2) = Coerce (substTy subst t1) (substTy subst t2)
697 specNote subst note = note
700 %************************************************************************
702 \subsubsection{Dealing with a binding}
704 %************************************************************************
707 specBind :: Subst -- Use this for RHSs
709 -> UsageDetails -- Info on how the scope of the binding
710 -> SpecM ([CoreBind], -- New bindings
711 UsageDetails) -- And info to pass upstream
713 specBind rhs_subst bind body_uds
714 = specBindItself rhs_subst bind (calls body_uds) `thenSM` \ (bind', bind_uds) ->
716 bndrs = bindersOf bind
717 all_uds = zapCalls bndrs (body_uds `plusUDs` bind_uds)
718 -- It's important that the `plusUDs` is this way round,
719 -- because body_uds may bind dictionaries that are
720 -- used in the calls passed to specDefn. So the
721 -- dictionary bindings in bind_uds may mention
722 -- dictionaries bound in body_uds.
724 case splitUDs bndrs all_uds of
726 (_, ([],[])) -- This binding doesn't bind anything needed
727 -- in the UDs, so put the binding here
728 -- This is the case for most non-dict bindings, except
729 -- for the few that are mentioned in a dict binding
730 -- that is floating upwards in body_uds
731 -> returnSM ([bind'], all_uds)
733 (float_uds, (dict_binds, calls)) -- This binding is needed in the UDs, so float it out
734 -> returnSM ([], float_uds `plusUDs` mkBigUD bind' dict_binds calls)
737 -- A truly gruesome function
738 mkBigUD bind@(NonRec _ _) dbs calls
739 = -- Common case: non-recursive and no specialisations
740 -- (if there were any specialistions it would have been made recursive)
741 MkUD { dict_binds = listToBag (mkDB bind : dbs),
742 calls = listToCallDetails calls }
744 mkBigUD bind dbs calls
746 MkUD { dict_binds = unitBag (mkDB (Rec (bind_prs bind ++ dbsToPairs dbs))),
748 calls = listToCallDetails calls }
750 bind_prs (NonRec b r) = [(b,r)]
751 bind_prs (Rec prs) = prs
754 dbsToPairs ((bind,_):dbs) = bind_prs bind ++ dbsToPairs dbs
756 -- specBindItself deals with the RHS, specialising it according
757 -- to the calls found in the body (if any)
758 specBindItself rhs_subst (NonRec bndr rhs) call_info
759 = specDefn rhs_subst call_info (bndr,rhs) `thenSM` \ ((bndr',rhs'), spec_defns, spec_uds) ->
761 new_bind | null spec_defns = NonRec bndr' rhs'
762 | otherwise = Rec ((bndr',rhs'):spec_defns)
763 -- bndr' mentions the spec_defns in its SpecEnv
764 -- Not sure why we couln't just put the spec_defns first
766 returnSM (new_bind, spec_uds)
768 specBindItself rhs_subst (Rec pairs) call_info
769 = mapSM (specDefn rhs_subst call_info) pairs `thenSM` \ stuff ->
771 (pairs', spec_defns_s, spec_uds_s) = unzip3 stuff
772 spec_defns = concat spec_defns_s
773 spec_uds = plusUDList spec_uds_s
774 new_bind = Rec (spec_defns ++ pairs')
776 returnSM (new_bind, spec_uds)
779 specDefn :: Subst -- Subst to use for RHS
780 -> CallDetails -- Info on how it is used in its scope
781 -> (Id, CoreExpr) -- The thing being bound and its un-processed RHS
782 -> SpecM ((Id, CoreExpr), -- The thing and its processed RHS
783 -- the Id may now have specialisations attached
784 [(Id,CoreExpr)], -- Extra, specialised bindings
785 UsageDetails -- Stuff to fling upwards from the RHS and its
786 ) -- specialised versions
788 specDefn subst calls (fn, rhs)
789 -- The first case is the interesting one
790 | n_tyvars == length rhs_tyvars -- Rhs of fn's defn has right number of big lambdas
791 && n_dicts <= length rhs_bndrs -- and enough dict args
792 && not (null calls_for_me) -- And there are some calls to specialise
793 && not (certainlyWillInline fn) -- And it's not small
794 -- If it's small, it's better just to inline
795 -- it than to construct lots of specialisations
796 = -- Specialise the body of the function
797 specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
799 -- Make a specialised version for each call in calls_for_me
800 mapSM spec_call calls_for_me `thenSM` \ stuff ->
802 (spec_defns, spec_uds, spec_rules) = unzip3 stuff
804 fn' = addIdSpecialisations zapped_fn spec_rules
806 returnSM ((fn',rhs'),
808 rhs_uds `plusUDs` plusUDList spec_uds)
810 | otherwise -- No calls or RHS doesn't fit our preconceptions
811 = specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
812 returnSM ((zapped_fn, rhs'), [], rhs_uds)
815 zapped_fn = zapSpecPragmaId fn
816 -- If the fn is a SpecPragmaId, make it discardable
817 -- It's role as a holder for a call instance is o'er
818 -- But it might be alive for some other reason by now.
821 (tyvars, theta, _) = tcSplitSigmaTy fn_type
822 n_tyvars = length tyvars
823 n_dicts = length theta
825 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs
826 rhs_dicts = take n_dicts rhs_ids
827 rhs_bndrs = rhs_tyvars ++ rhs_dicts
828 body = mkLams (drop n_dicts rhs_ids) rhs_body
829 -- Glue back on the non-dict lambdas
831 calls_for_me = case lookupFM calls fn of
833 Just cs -> fmToList cs
835 ----------------------------------------------------------
836 -- Specialise to one particular call pattern
837 spec_call :: (CallKey, ([DictExpr], VarSet)) -- Call instance
838 -> SpecM ((Id,CoreExpr), -- Specialised definition
839 UsageDetails, -- Usage details from specialised body
840 CoreRule) -- Info for the Id's SpecEnv
841 spec_call (CallKey call_ts, (call_ds, call_fvs))
842 = ASSERT( length call_ts == n_tyvars && length call_ds == n_dicts )
843 -- Calls are only recorded for properly-saturated applications
845 -- Suppose f's defn is f = /\ a b c d -> \ d1 d2 -> rhs
846 -- Supppose the call is for f [Just t1, Nothing, Just t3, Nothing] [dx1, dx2]
848 -- Construct the new binding
849 -- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b d -> rhs)
850 -- PLUS the usage-details
851 -- { d1' = dx1; d2' = dx2 }
852 -- where d1', d2' are cloned versions of d1,d2, with the type substitution applied.
854 -- Note that the substitution is applied to the whole thing.
855 -- This is convenient, but just slightly fragile. Notably:
856 -- * There had better be no name clashes in a/b/c/d
859 -- poly_tyvars = [b,d] in the example above
860 -- spec_tyvars = [a,c]
861 -- ty_args = [t1,b,t3,d]
862 poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
863 spec_tyvars = [tv | (tv, Just _) <- rhs_tyvars `zip` call_ts]
864 ty_args = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
866 mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
867 mk_ty_arg rhs_tyvar (Just ty) = Type ty
868 rhs_subst = extendSubstList subst spec_tyvars [DoneTy ty | Just ty <- call_ts]
870 cloneBinders rhs_subst rhs_dicts `thenSM` \ (rhs_subst', rhs_dicts') ->
872 inst_args = ty_args ++ map Var rhs_dicts'
874 -- Figure out the type of the specialised function
875 spec_id_ty = mkForAllTys poly_tyvars (applyTypeToArgs rhs fn_type inst_args)
877 newIdSM fn spec_id_ty `thenSM` \ spec_f ->
878 specExpr rhs_subst' (mkLams poly_tyvars body) `thenSM` \ (spec_rhs, rhs_uds) ->
880 -- The rule to put in the function's specialisation is:
881 -- forall b,d, d1',d2'. f t1 b t3 d d1' d2' = f1 b d
882 spec_env_rule = Rule (_PK_ ("SPEC " ++ showSDoc (ppr fn)))
883 (poly_tyvars ++ rhs_dicts')
885 (mkTyApps (Var spec_f) (map mkTyVarTy poly_tyvars))
887 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
888 final_uds = foldr addDictBind rhs_uds (my_zipEqual "spec_call" rhs_dicts' call_ds)
890 returnSM ((spec_f, spec_rhs),
895 my_zipEqual doc xs ys
896 | length xs /= length ys = pprPanic "my_zipEqual" (ppr xs $$ ppr ys $$ (ppr fn <+> ppr call_ts) $$ ppr rhs)
897 | otherwise = zipEqual doc xs ys
900 %************************************************************************
902 \subsubsection{UsageDetails and suchlike}
904 %************************************************************************
909 dict_binds :: !(Bag DictBind),
910 -- Floated dictionary bindings
911 -- The order is important;
912 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
913 -- (Remember, Bags preserve order in GHC.)
915 calls :: !CallDetails
918 type DictBind = (CoreBind, VarSet)
919 -- The set is the free vars of the binding
920 -- both tyvars and dicts
922 type DictExpr = CoreExpr
924 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM }
926 type ProtoUsageDetails = ([DictBind],
927 [(Id, CallKey, ([DictExpr], VarSet))]
930 ------------------------------------------------------------
931 type CallDetails = FiniteMap Id CallInfo
932 newtype CallKey = CallKey [Maybe Type] -- Nothing => unconstrained type argument
933 type CallInfo = FiniteMap CallKey
934 ([DictExpr], VarSet) -- Dict args and the vars of the whole
935 -- call (including tyvars)
936 -- [*not* include the main id itself, of course]
937 -- The finite maps eliminate duplicates
938 -- The list of types and dictionaries is guaranteed to
939 -- match the type of f
941 -- Type isn't an instance of Ord, so that we can control which
942 -- instance we use. That's tiresome here. Oh well
943 instance Eq CallKey where
944 k1 == k2 = case k1 `compare` k2 of { EQ -> True; other -> False }
946 instance Ord CallKey where
947 compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
949 cmp Nothing Nothing = EQ
950 cmp Nothing (Just t2) = LT
951 cmp (Just t1) Nothing = GT
952 cmp (Just t1) (Just t2) = tcCmpType t1 t2
954 unionCalls :: CallDetails -> CallDetails -> CallDetails
955 unionCalls c1 c2 = plusFM_C plusFM c1 c2
957 singleCall :: Id -> [Maybe Type] -> [DictExpr] -> CallDetails
958 singleCall id tys dicts
959 = unitFM id (unitFM (CallKey tys) (dicts, call_fvs))
961 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
962 tys_fvs = tyVarsOfTypes (catMaybes tys)
963 -- The type args (tys) are guaranteed to be part of the dictionary
964 -- types, because they are just the constrained types,
965 -- and the dictionary is therefore sure to be bound
966 -- inside the binding for any type variables free in the type;
967 -- hence it's safe to neglect tyvars free in tys when making
968 -- the free-var set for this call
969 -- BUT I don't trust this reasoning; play safe and include tys_fvs
971 -- We don't include the 'id' itself.
973 listToCallDetails calls
974 = foldr (unionCalls . mk_call) emptyFM calls
976 mk_call (id, tys, dicts_w_fvs) = unitFM id (unitFM tys dicts_w_fvs)
977 -- NB: the free vars of the call are provided
979 callDetailsToList calls = [ (id,tys,dicts)
980 | (id,fm) <- fmToList calls,
981 (tys, dicts) <- fmToList fm
984 mkCallUDs subst f args
986 || length spec_tys /= n_tyvars
987 || length dicts /= n_dicts
988 || maybeToBool (lookupRule (substInScope subst) f args)
989 -- There's already a rule covering this call. A typical case
990 -- is where there's an explicit user-provided rule. Then
991 -- we don't want to create a specialised version
992 -- of the function that overlaps.
993 = emptyUDs -- Not overloaded, or no specialisation wanted
996 = MkUD {dict_binds = emptyBag,
997 calls = singleCall f spec_tys dicts
1000 (tyvars, theta, _) = tcSplitSigmaTy (idType f)
1001 constrained_tyvars = tyVarsOfTheta theta
1002 n_tyvars = length tyvars
1003 n_dicts = length theta
1005 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
1006 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
1008 mk_spec_ty tyvar ty | tyvar `elemVarSet` constrained_tyvars
1013 ------------------------------------------------------------
1014 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
1015 plusUDs (MkUD {dict_binds = db1, calls = calls1})
1016 (MkUD {dict_binds = db2, calls = calls2})
1017 = MkUD {dict_binds = d, calls = c}
1019 d = db1 `unionBags` db2
1020 c = calls1 `unionCalls` calls2
1022 plusUDList = foldr plusUDs emptyUDs
1024 -- zapCalls deletes calls to ids from uds
1025 zapCalls ids uds = uds {calls = delListFromFM (calls uds) ids}
1027 mkDB bind = (bind, bind_fvs bind)
1029 bind_fvs (NonRec bndr rhs) = exprFreeVars rhs
1030 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1033 rhs_fvs = unionVarSets [exprFreeVars rhs | (bndr,rhs) <- prs]
1035 addDictBind (dict,rhs) uds = uds { dict_binds = mkDB (NonRec dict rhs) `consBag` dict_binds uds }
1037 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
1038 = foldrBag add binds dbs
1040 add (bind,_) binds = bind : binds
1042 dumpUDs :: [CoreBndr]
1043 -> UsageDetails -> CoreExpr
1044 -> (UsageDetails, CoreExpr)
1045 dumpUDs bndrs uds body
1046 = (free_uds, foldr add_let body dict_binds)
1048 (free_uds, (dict_binds, _)) = splitUDs bndrs uds
1049 add_let (bind,_) body = Let bind body
1051 splitUDs :: [CoreBndr]
1053 -> (UsageDetails, -- These don't mention the binders
1054 ProtoUsageDetails) -- These do
1056 splitUDs bndrs uds@(MkUD {dict_binds = orig_dbs,
1057 calls = orig_calls})
1059 = if isEmptyBag dump_dbs && null dump_calls then
1060 -- Common case: binder doesn't affect floats
1064 -- Binders bind some of the fvs of the floats
1065 (MkUD {dict_binds = free_dbs,
1066 calls = listToCallDetails free_calls},
1067 (bagToList dump_dbs, dump_calls)
1071 bndr_set = mkVarSet bndrs
1073 (free_dbs, dump_dbs, dump_idset)
1074 = foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
1075 -- Important that it's foldl not foldr;
1076 -- we're accumulating the set of dumped ids in dump_set
1078 -- Filter out any calls that mention things that are being dumped
1079 orig_call_list = callDetailsToList orig_calls
1080 (dump_calls, free_calls) = partition captured orig_call_list
1081 captured (id,tys,(dicts, fvs)) = fvs `intersectsVarSet` dump_idset
1082 || id `elemVarSet` dump_idset
1084 dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1085 | dump_idset `intersectsVarSet` fvs -- Dump it
1086 = (free_dbs, dump_dbs `snocBag` db,
1087 dump_idset `unionVarSet` mkVarSet (bindersOf bind))
1089 | otherwise -- Don't dump it
1090 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1094 %************************************************************************
1096 \subsubsection{Boring helper functions}
1098 %************************************************************************
1101 lookupId:: IdEnv Id -> Id -> Id
1102 lookupId env id = case lookupVarEnv env id of
1106 ----------------------------------------
1107 type SpecM a = UniqSM a
1111 getUniqSM = getUniqueUs
1115 mapAndCombineSM f [] = returnSM ([], emptyUDs)
1116 mapAndCombineSM f (x:xs) = f x `thenSM` \ (y, uds1) ->
1117 mapAndCombineSM f xs `thenSM` \ (ys, uds2) ->
1118 returnSM (y:ys, uds1 `plusUDs` uds2)
1120 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1121 -- Clone the binders of the bind; return new bind with the cloned binders
1122 -- Return the substitution to use for RHSs, and the one to use for the body
1123 cloneBindSM subst (NonRec bndr rhs)
1124 = getUs `thenUs` \ us ->
1126 (subst', bndr') = substAndCloneId subst us bndr
1128 returnUs (subst, subst', NonRec bndr' rhs)
1130 cloneBindSM subst (Rec pairs)
1131 = getUs `thenUs` \ us ->
1133 (subst', bndrs') = substAndCloneRecIds subst us (map fst pairs)
1135 returnUs (subst', subst', Rec (bndrs' `zip` map snd pairs))
1137 cloneBinders subst bndrs
1138 = getUs `thenUs` \ us ->
1139 returnUs (substAndCloneIds subst us bndrs)
1141 newIdSM old_id new_ty
1142 = getUniqSM `thenSM` \ uniq ->
1144 -- Give the new Id a similar occurrence name to the old one
1145 name = idName old_id
1146 new_id = mkUserLocal (mkSpecOcc (nameOccName name)) uniq new_ty (getSrcLoc name)
1152 Old (but interesting) stuff about unboxed bindings
1153 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1155 What should we do when a value is specialised to a *strict* unboxed value?
1157 map_*_* f (x:xs) = let h = f x
1161 Could convert let to case:
1163 map_*_Int# f (x:xs) = case f x of h# ->
1167 This may be undesirable since it forces evaluation here, but the value
1168 may not be used in all branches of the body. In the general case this
1169 transformation is impossible since the mutual recursion in a letrec
1170 cannot be expressed as a case.
1172 There is also a problem with top-level unboxed values, since our
1173 implementation cannot handle unboxed values at the top level.
1175 Solution: Lift the binding of the unboxed value and extract it when it
1178 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1183 Now give it to the simplifier and the _Lifting will be optimised away.
1185 The benfit is that we have given the specialised "unboxed" values a
1186 very simplep lifted semantics and then leave it up to the simplifier to
1187 optimise it --- knowing that the overheads will be removed in nearly
1190 In particular, the value will only be evaluted in the branches of the
1191 program which use it, rather than being forced at the point where the
1192 value is bound. For example:
1194 filtermap_*_* p f (x:xs)
1201 filtermap_*_Int# p f (x:xs)
1202 = let h = case (f x) of h# -> _Lift h#
1205 True -> case h of _Lift h#
1209 The binding for h can still be inlined in the one branch and the
1210 _Lifting eliminated.
1213 Question: When won't the _Lifting be eliminated?
1215 Answer: When they at the top-level (where it is necessary) or when
1216 inlining would duplicate work (or possibly code depending on
1217 options). However, the _Lifting will still be eliminated if the
1218 strictness analyser deems the lifted binding strict.