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 ( opt_D_verbose_core2core, opt_D_dump_spec, opt_D_dump_rules )
12 import Id ( Id, idName, idType, mkTemplateLocals, mkUserLocal,
13 idSpecialisation, setIdNoDiscard, isExportedId,
14 modifyIdInfo, idUnfolding
16 import IdInfo ( zapSpecPragInfo )
20 import Type ( Type, mkTyVarTy, splitSigmaTy, splitFunTysN,
21 tyVarsOfType, tyVarsOfTypes, tyVarsOfTheta, applyTys,
22 mkForAllTys, boxedTypeKind
24 import PprType ( {- instance Outputable Type -} )
25 import Subst ( Subst, mkSubst, substTy, mkSubst, substBndrs, extendSubstList,
26 substId, substAndCloneId, substAndCloneIds, lookupIdSubst
28 import Var ( TyVar, mkSysTyVar, setVarUnique )
32 import CoreUtils ( applyTypeToArgs )
33 import CoreUnfold ( certainlyWillInline )
34 import CoreFVs ( exprFreeVars, exprsFreeVars )
35 import CoreLint ( beginPass, endPass )
36 import PprCore ( pprCoreRules )
37 import Rules ( addIdSpecialisations )
39 import UniqSupply ( UniqSupply,
40 UniqSM, initUs_, thenUs, thenUs_, returnUs, getUniqueUs,
41 getUs, setUs, uniqFromSupply, splitUniqSupply, mapUs
43 import Name ( nameOccName, mkSpecOcc, getSrcLoc )
45 import Maybes ( MaybeErr(..), catMaybes )
46 import ErrUtils ( dumpIfSet )
48 import List ( partition )
49 import Util ( zipEqual, zipWithEqual, mapAccumL )
56 %************************************************************************
58 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
60 %************************************************************************
62 These notes describe how we implement specialisation to eliminate
65 The specialisation pass works on Core
66 syntax, complete with all the explicit dictionary application,
67 abstraction and construction as added by the type checker. The
68 existing type checker remains largely as it is.
70 One important thought: the {\em types} passed to an overloaded
71 function, and the {\em dictionaries} passed are mutually redundant.
72 If the same function is applied to the same type(s) then it is sure to
73 be applied to the same dictionary(s)---or rather to the same {\em
74 values}. (The arguments might look different but they will evaluate
77 Second important thought: we know that we can make progress by
78 treating dictionary arguments as static and worth specialising on. So
79 we can do without binding-time analysis, and instead specialise on
80 dictionary arguments and no others.
89 and suppose f is overloaded.
91 STEP 1: CALL-INSTANCE COLLECTION
93 We traverse <body>, accumulating all applications of f to types and
96 (Might there be partial applications, to just some of its types and
97 dictionaries? In principle yes, but in practice the type checker only
98 builds applications of f to all its types and dictionaries, so partial
99 applications could only arise as a result of transformation, and even
100 then I think it's unlikely. In any case, we simply don't accumulate such
101 partial applications.)
106 So now we have a collection of calls to f:
110 Notice that f may take several type arguments. To avoid ambiguity, we
111 say that f is called at type t1/t2 and t3/t4.
113 We take equivalence classes using equality of the *types* (ignoring
114 the dictionary args, which as mentioned previously are redundant).
116 STEP 3: SPECIALISATION
118 For each equivalence class, choose a representative (f t1 t2 d1 d2),
119 and create a local instance of f, defined thus:
121 f@t1/t2 = <f_rhs> t1 t2 d1 d2
123 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
124 of simplification will now result. However we don't actually *do* that
125 simplification. Rather, we leave it for the simplifier to do. If we
126 *did* do it, though, we'd get more call instances from the specialised
127 RHS. We can work out what they are by instantiating the call-instance
128 set from f's RHS with the types t1, t2.
130 Add this new id to f's IdInfo, to record that f has a specialised version.
132 Before doing any of this, check that f's IdInfo doesn't already
133 tell us about an existing instance of f at the required type/s.
134 (This might happen if specialisation was applied more than once, or
135 it might arise from user SPECIALIZE pragmas.)
139 Wait a minute! What if f is recursive? Then we can't just plug in
140 its right-hand side, can we?
142 But it's ok. The type checker *always* creates non-recursive definitions
143 for overloaded recursive functions. For example:
145 f x = f (x+x) -- Yes I know its silly
149 f a (d::Num a) = let p = +.sel a d
151 letrec fl (y::a) = fl (p y y)
155 We still have recusion for non-overloaded functions which we
156 speciailise, but the recursive call should get specialised to the
157 same recursive version.
163 All this is crystal clear when the function is applied to *constant
164 types*; that is, types which have no type variables inside. But what if
165 it is applied to non-constant types? Suppose we find a call of f at type
166 t1/t2. There are two possibilities:
168 (a) The free type variables of t1, t2 are in scope at the definition point
169 of f. In this case there's no problem, we proceed just as before. A common
170 example is as follows. Here's the Haskell:
175 After typechecking we have
177 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
178 in +.sel a d (f a d y) (f a d y)
180 Notice that the call to f is at type type "a"; a non-constant type.
181 Both calls to f are at the same type, so we can specialise to give:
183 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
184 in +.sel a d (f@a y) (f@a y)
187 (b) The other case is when the type variables in the instance types
188 are *not* in scope at the definition point of f. The example we are
189 working with above is a good case. There are two instances of (+.sel a d),
190 but "a" is not in scope at the definition of +.sel. Can we do anything?
191 Yes, we can "common them up", a sort of limited common sub-expression deal.
194 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
195 f@a (x::a) = +.sel@a x x
196 in +.sel@a (f@a y) (f@a y)
198 This can save work, and can't be spotted by the type checker, because
199 the two instances of +.sel weren't originally at the same type.
203 * There are quite a few variations here. For example, the defn of
204 +.sel could be floated ouside the \y, to attempt to gain laziness.
205 It certainly mustn't be floated outside the \d because the d has to
208 * We don't want to inline f_rhs in this case, because
209 that will duplicate code. Just commoning up the call is the point.
211 * Nothing gets added to +.sel's IdInfo.
213 * Don't bother unless the equivalence class has more than one item!
215 Not clear whether this is all worth it. It is of course OK to
216 simply discard call-instances when passing a big lambda.
218 Polymorphism 2 -- Overloading
220 Consider a function whose most general type is
222 f :: forall a b. Ord a => [a] -> b -> b
224 There is really no point in making a version of g at Int/Int and another
225 at Int/Bool, because it's only instancing the type variable "a" which
226 buys us any efficiency. Since g is completely polymorphic in b there
227 ain't much point in making separate versions of g for the different
230 That suggests that we should identify which of g's type variables
231 are constrained (like "a") and which are unconstrained (like "b").
232 Then when taking equivalence classes in STEP 2, we ignore the type args
233 corresponding to unconstrained type variable. In STEP 3 we make
234 polymorphic versions. Thus:
236 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
245 f a (d::Num a) = let g = ...
247 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
249 Here, g is only called at one type, but the dictionary isn't in scope at the
250 definition point for g. Usually the type checker would build a
251 definition for d1 which enclosed g, but the transformation system
252 might have moved d1's defn inward. Solution: float dictionary bindings
253 outwards along with call instances.
257 f x = let g p q = p==q
263 Before specialisation, leaving out type abstractions we have
265 f df x = let g :: Eq a => a -> a -> Bool
267 h :: Num a => a -> a -> (a, Bool)
268 h dh r s = let deq = eqFromNum dh
269 in (+ dh r s, g deq r s)
273 After specialising h we get a specialised version of h, like this:
275 h' r s = let deq = eqFromNum df
276 in (+ df r s, g deq r s)
278 But we can't naively make an instance for g from this, because deq is not in scope
279 at the defn of g. Instead, we have to float out the (new) defn of deq
280 to widen its scope. Notice that this floating can't be done in advance -- it only
281 shows up when specialisation is done.
283 User SPECIALIZE pragmas
284 ~~~~~~~~~~~~~~~~~~~~~~~
285 Specialisation pragmas can be digested by the type checker, and implemented
286 by adding extra definitions along with that of f, in the same way as before
288 f@t1/t2 = <f_rhs> t1 t2 d1 d2
290 Indeed the pragmas *have* to be dealt with by the type checker, because
291 only it knows how to build the dictionaries d1 and d2! For example
293 g :: Ord a => [a] -> [a]
294 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
296 Here, the specialised version of g is an application of g's rhs to the
297 Ord dictionary for (Tree Int), which only the type checker can conjure
298 up. There might not even *be* one, if (Tree Int) is not an instance of
299 Ord! (All the other specialision has suitable dictionaries to hand
302 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
303 it is buried in a complex (as-yet-un-desugared) binding group.
306 f@t1/t2 = f* t1 t2 d1 d2
308 where f* is the Id f with an IdInfo which says "inline me regardless!".
309 Indeed all the specialisation could be done in this way.
310 That in turn means that the simplifier has to be prepared to inline absolutely
311 any in-scope let-bound thing.
314 Again, the pragma should permit polymorphism in unconstrained variables:
316 h :: Ord a => [a] -> b -> b
317 {-# SPECIALIZE h :: [Int] -> b -> b #-}
319 We *insist* that all overloaded type variables are specialised to ground types,
320 (and hence there can be no context inside a SPECIALIZE pragma).
321 We *permit* unconstrained type variables to be specialised to
323 - or left as a polymorphic type variable
324 but nothing in between. So
326 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
328 is *illegal*. (It can be handled, but it adds complication, and gains the
332 SPECIALISING INSTANCE DECLARATIONS
333 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
336 instance Foo a => Foo [a] where
338 {-# SPECIALIZE instance Foo [Int] #-}
340 The original instance decl creates a dictionary-function
343 dfun.Foo.List :: forall a. Foo a -> Foo [a]
345 The SPECIALIZE pragma just makes a specialised copy, just as for
346 ordinary function definitions:
348 dfun.Foo.List@Int :: Foo [Int]
349 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
351 The information about what instance of the dfun exist gets added to
352 the dfun's IdInfo in the same way as a user-defined function too.
355 Automatic instance decl specialisation?
356 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
357 Can instance decls be specialised automatically? It's tricky.
358 We could collect call-instance information for each dfun, but
359 then when we specialised their bodies we'd get new call-instances
360 for ordinary functions; and when we specialised their bodies, we might get
361 new call-instances of the dfuns, and so on. This all arises because of
362 the unrestricted mutual recursion between instance decls and value decls.
364 Still, there's no actual problem; it just means that we may not do all
365 the specialisation we could theoretically do.
367 Furthermore, instance decls are usually exported and used non-locally,
368 so we'll want to compile enough to get those specialisations done.
370 Lastly, there's no such thing as a local instance decl, so we can
371 survive solely by spitting out *usage* information, and then reading that
372 back in as a pragma when next compiling the file. So for now,
373 we only specialise instance decls in response to pragmas.
376 SPITTING OUT USAGE INFORMATION
377 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
379 To spit out usage information we need to traverse the code collecting
380 call-instance information for all imported (non-prelude?) functions
381 and data types. Then we equivalence-class it and spit it out.
383 This is done at the top-level when all the call instances which escape
384 must be for imported functions and data types.
386 *** Not currently done ***
389 Partial specialisation by pragmas
390 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
391 What about partial specialisation:
393 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
394 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
398 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
400 Seems quite reasonable. Similar things could be done with instance decls:
402 instance (Foo a, Foo b) => Foo (a,b) where
404 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
405 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
407 Ho hum. Things are complex enough without this. I pass.
410 Requirements for the simplifer
411 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
412 The simplifier has to be able to take advantage of the specialisation.
414 * When the simplifier finds an application of a polymorphic f, it looks in
415 f's IdInfo in case there is a suitable instance to call instead. This converts
417 f t1 t2 d1 d2 ===> f_t1_t2
419 Note that the dictionaries get eaten up too!
421 * Dictionary selection operations on constant dictionaries must be
424 +.sel Int d ===> +Int
426 The obvious way to do this is in the same way as other specialised
427 calls: +.sel has inside it some IdInfo which tells that if it's applied
428 to the type Int then it should eat a dictionary and transform to +Int.
430 In short, dictionary selectors need IdInfo inside them for constant
433 * Exactly the same applies if a superclass dictionary is being
436 Eq.sel Int d ===> dEqInt
438 * Something similar applies to dictionary construction too. Suppose
439 dfun.Eq.List is the function taking a dictionary for (Eq a) to
440 one for (Eq [a]). Then we want
442 dfun.Eq.List Int d ===> dEq.List_Int
444 Where does the Eq [Int] dictionary come from? It is built in
445 response to a SPECIALIZE pragma on the Eq [a] instance decl.
447 In short, dfun Ids need IdInfo with a specialisation for each
448 constant instance of their instance declaration.
450 All this uses a single mechanism: the SpecEnv inside an Id
453 What does the specialisation IdInfo look like?
454 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
456 The SpecEnv of an Id maps a list of types (the template) to an expression
460 For example, if f has this SpecInfo:
462 [Int, a] -> \d:Ord Int. f' a
464 it means that we can replace the call
466 f Int t ===> (\d. f' t)
468 This chucks one dictionary away and proceeds with the
469 specialised version of f, namely f'.
472 What can't be done this way?
473 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
474 There is no way, post-typechecker, to get a dictionary for (say)
475 Eq a from a dictionary for Eq [a]. So if we find
479 we can't transform to
484 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
486 Of course, we currently have no way to automatically derive
487 eqList, nor to connect it to the Eq [a] instance decl, but you
488 can imagine that it might somehow be possible. Taking advantage
489 of this is permanently ruled out.
491 Still, this is no great hardship, because we intend to eliminate
492 overloading altogether anyway!
496 A note about non-tyvar dictionaries
497 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
498 Some Ids have types like
500 forall a,b,c. Eq a -> Ord [a] -> tau
502 This seems curious at first, because we usually only have dictionary
503 args whose types are of the form (C a) where a is a type variable.
504 But this doesn't hold for the functions arising from instance decls,
505 which sometimes get arguements with types of form (C (T a)) for some
508 Should we specialise wrt this compound-type dictionary? We used to say
510 "This is a heuristic judgement, as indeed is the fact that we
511 specialise wrt only dictionaries. We choose *not* to specialise
512 wrt compound dictionaries because at the moment the only place
513 they show up is in instance decls, where they are simply plugged
514 into a returned dictionary. So nothing is gained by specialising
517 But it is simpler and more uniform to specialise wrt these dicts too;
518 and in future GHC is likely to support full fledged type signatures
520 f ;: Eq [(a,b)] => ...
523 %************************************************************************
525 \subsubsection{The new specialiser}
527 %************************************************************************
529 Our basic game plan is this. For let(rec) bound function
530 f :: (C a, D c) => (a,b,c,d) -> Bool
532 * Find any specialised calls of f, (f ts ds), where
533 ts are the type arguments t1 .. t4, and
534 ds are the dictionary arguments d1 .. d2.
536 * Add a new definition for f1 (say):
538 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
540 Note that we abstract over the unconstrained type arguments.
544 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
546 to the specialisations of f. This will be used by the
547 simplifier to replace calls
548 (f t1 t2 t3 t4) da db
550 (\d1 d1 -> f1 t2 t4) da db
552 All the stuff about how many dictionaries to discard, and what types
553 to apply the specialised function to, are handled by the fact that the
554 SpecEnv contains a template for the result of the specialisation.
556 We don't build *partial* specialisations for f. For example:
558 f :: Eq a => a -> a -> Bool
559 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
561 Here, little is gained by making a specialised copy of f.
562 There's a distinct danger that the specialised version would
563 first build a dictionary for (Eq b, Eq c), and then select the (==)
564 method from it! Even if it didn't, not a great deal is saved.
566 We do, however, generate polymorphic, but not overloaded, specialisations:
568 f :: Eq a => [a] -> b -> b -> b
569 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
571 Hence, the invariant is this:
573 *** no specialised version is overloaded ***
576 %************************************************************************
578 \subsubsection{The exported function}
580 %************************************************************************
583 specProgram :: UniqSupply -> [CoreBind] -> IO [CoreBind]
586 beginPass "Specialise"
588 let binds' = initSM us (go binds `thenSM` \ (binds', uds') ->
589 returnSM (dumpAllDictBinds uds' binds'))
591 endPass "Specialise" (opt_D_dump_spec || opt_D_verbose_core2core) binds'
593 dumpIfSet opt_D_dump_rules "Top-level specialisations"
594 (vcat (map dump_specs (concat (map bindersOf binds'))))
598 -- We need to start with a Subst that knows all the things
599 -- that are in scope, so that the substitution engine doesn't
600 -- accidentally re-use a unique that's already in use
601 -- Easiest thing is to do it all at once, as if all the top-level
602 -- decls were mutually recursive
603 top_subst = mkSubst (mkVarSet (bindersOfBinds binds)) emptySubstEnv
605 go [] = returnSM ([], emptyUDs)
606 go (bind:binds) = go binds `thenSM` \ (binds', uds) ->
607 specBind top_subst bind uds `thenSM` \ (bind', uds') ->
608 returnSM (bind' ++ binds', uds')
610 dump_specs var = pprCoreRules var (idSpecialisation var)
613 %************************************************************************
615 \subsubsection{@specExpr@: the main function}
617 %************************************************************************
620 specVar :: Subst -> Id -> CoreExpr
621 specVar subst v = case lookupIdSubst subst v of
625 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
626 -- We carry a substitution down:
627 -- a) we must clone any binding that might flaot outwards,
628 -- to avoid name clashes
629 -- b) we carry a type substitution to use when analysing
630 -- the RHS of specialised bindings (no type-let!)
632 ---------------- First the easy cases --------------------
633 specExpr subst (Type ty) = returnSM (Type (substTy subst ty), emptyUDs)
634 specExpr subst (Var v) = returnSM (specVar subst v, emptyUDs)
635 specExpr subst (Lit lit) = returnSM (Lit lit, emptyUDs)
637 specExpr subst (Note note body)
638 = specExpr subst body `thenSM` \ (body', uds) ->
639 returnSM (Note (specNote subst note) body', uds)
642 ---------------- Applications might generate a call instance --------------------
643 specExpr subst expr@(App fun arg)
646 go (App fun arg) args = specExpr subst arg `thenSM` \ (arg', uds_arg) ->
647 go fun (arg':args) `thenSM` \ (fun', uds_app) ->
648 returnSM (App fun' arg', uds_arg `plusUDs` uds_app)
650 go (Var f) args = case specVar subst f of
651 Var f' -> returnSM (Var f', mkCallUDs f' args)
652 e' -> returnSM (e', emptyUDs) -- I don't expect this!
653 go other args = specExpr subst other
655 ---------------- Lambda/case require dumping of usage details --------------------
656 specExpr subst e@(Lam _ _)
657 = specExpr subst' body `thenSM` \ (body', uds) ->
659 (filtered_uds, body'') = dumpUDs bndrs' uds body'
661 returnSM (mkLams bndrs' body'', filtered_uds)
663 (bndrs, body) = collectBinders e
664 (subst', bndrs') = substBndrs subst bndrs
665 -- More efficient to collect a group of binders together all at once
666 -- and we don't want to split a lambda group with dumped bindings
668 specExpr subst (Case scrut case_bndr alts)
669 = specExpr subst scrut `thenSM` \ (scrut', uds_scrut) ->
670 mapAndCombineSM spec_alt alts `thenSM` \ (alts', uds_alts) ->
671 returnSM (Case scrut' case_bndr' alts', uds_scrut `plusUDs` uds_alts)
673 (subst_alt, case_bndr') = substId subst case_bndr
674 -- No need to clone case binder; it can't float like a let(rec)
676 spec_alt (con, args, rhs)
677 = specExpr subst_rhs rhs `thenSM` \ (rhs', uds) ->
679 (uds', rhs'') = dumpUDs args uds rhs'
681 returnSM ((con, args', rhs''), uds')
683 (subst_rhs, args') = substBndrs subst_alt args
685 ---------------- Finally, let is the interesting case --------------------
686 specExpr subst (Let bind body)
688 cloneBindSM subst bind `thenSM` \ (rhs_subst, body_subst, bind') ->
690 -- Deal with the body
691 specExpr body_subst body `thenSM` \ (body', body_uds) ->
693 -- Deal with the bindings
694 specBind rhs_subst bind' body_uds `thenSM` \ (binds', uds) ->
697 returnSM (foldr Let body' binds', uds)
699 -- Must apply the type substitution to coerceions
700 specNote subst (Coerce t1 t2) = Coerce (substTy subst t1) (substTy subst t2)
701 specNote subst note = note
704 %************************************************************************
706 \subsubsection{Dealing with a binding}
708 %************************************************************************
711 specBind :: Subst -- Use this for RHSs
713 -> UsageDetails -- Info on how the scope of the binding
714 -> SpecM ([CoreBind], -- New bindings
715 UsageDetails) -- And info to pass upstream
717 specBind rhs_subst bind body_uds
718 = specBindItself rhs_subst bind (calls body_uds) `thenSM` \ (bind', bind_uds) ->
720 bndrs = bindersOf bind
721 all_uds = zapCalls bndrs (body_uds `plusUDs` bind_uds)
722 -- It's important that the `plusUDs` is this way round,
723 -- because body_uds may bind dictionaries that are
724 -- used in the calls passed to specDefn. So the
725 -- dictionary bindings in bind_uds may mention
726 -- dictionaries bound in body_uds.
728 case splitUDs bndrs all_uds of
730 (_, ([],[])) -- This binding doesn't bind anything needed
731 -- in the UDs, so put the binding here
732 -- This is the case for most non-dict bindings, except
733 -- for the few that are mentioned in a dict binding
734 -- that is floating upwards in body_uds
735 -> returnSM ([bind'], all_uds)
737 (float_uds, (dict_binds, calls)) -- This binding is needed in the UDs, so float it out
738 -> returnSM ([], float_uds `plusUDs` mkBigUD bind' dict_binds calls)
741 -- A truly gruesome function
742 mkBigUD bind@(NonRec _ _) dbs calls
743 = -- Common case: non-recursive and no specialisations
744 -- (if there were any specialistions it would have been made recursive)
745 MkUD { dict_binds = listToBag (mkDB bind : dbs),
746 calls = listToCallDetails calls }
748 mkBigUD bind dbs calls
750 MkUD { dict_binds = unitBag (mkDB (Rec (bind_prs bind ++ dbsToPairs dbs))),
752 calls = listToCallDetails calls }
754 bind_prs (NonRec b r) = [(b,r)]
755 bind_prs (Rec prs) = prs
758 dbsToPairs ((bind,_):dbs) = bind_prs bind ++ dbsToPairs dbs
760 -- specBindItself deals with the RHS, specialising it according
761 -- to the calls found in the body (if any)
762 specBindItself rhs_subst (NonRec bndr rhs) call_info
763 = specDefn rhs_subst call_info (bndr,rhs) `thenSM` \ ((bndr',rhs'), spec_defns, spec_uds) ->
765 new_bind | null spec_defns = NonRec bndr' rhs'
766 | otherwise = Rec ((bndr',rhs'):spec_defns)
767 -- bndr' mentions the spec_defns in its SpecEnv
768 -- Not sure why we couln't just put the spec_defns first
770 returnSM (new_bind, spec_uds)
772 specBindItself rhs_subst (Rec pairs) call_info
773 = mapSM (specDefn rhs_subst call_info) pairs `thenSM` \ stuff ->
775 (pairs', spec_defns_s, spec_uds_s) = unzip3 stuff
776 spec_defns = concat spec_defns_s
777 spec_uds = plusUDList spec_uds_s
778 new_bind = Rec (spec_defns ++ pairs')
780 returnSM (new_bind, spec_uds)
783 specDefn :: Subst -- Subst to use for RHS
784 -> CallDetails -- Info on how it is used in its scope
785 -> (Id, CoreExpr) -- The thing being bound and its un-processed RHS
786 -> SpecM ((Id, CoreExpr), -- The thing and its processed RHS
787 -- the Id may now have specialisations attached
788 [(Id,CoreExpr)], -- Extra, specialised bindings
789 UsageDetails -- Stuff to fling upwards from the RHS and its
790 ) -- specialised versions
792 specDefn subst calls (fn, rhs)
793 -- The first case is the interesting one
794 | n_tyvars == length rhs_tyvars -- Rhs of fn's defn has right number of big lambdas
795 && n_dicts <= length rhs_bndrs -- and enough dict args
796 && not (null calls_for_me) -- And there are some calls to specialise
797 && not (certainlyWillInline fn) -- And it's not small
798 -- If it's small, it's better just to inline
799 -- it than to construct lots of specialisations
800 = -- Specialise the body of the function
801 specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
803 -- Make a specialised version for each call in calls_for_me
804 mapSM spec_call calls_for_me `thenSM` \ stuff ->
806 (spec_defns, spec_uds, spec_env_stuff) = unzip3 stuff
808 fn' = addIdSpecialisations zapped_fn spec_env_stuff
810 returnSM ((fn',rhs'),
812 rhs_uds `plusUDs` plusUDList spec_uds)
814 | otherwise -- No calls or RHS doesn't fit our preconceptions
815 = specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
816 returnSM ((zapped_fn, rhs'), [], rhs_uds)
819 zapped_fn = modifyIdInfo zapSpecPragInfo fn
820 -- If the fn is a SpecPragmaId, make it discardable
821 -- It's role as a holder for a call instance is o'er
822 -- But it might be alive for some other reason by now.
825 (tyvars, theta, tau) = splitSigmaTy fn_type
826 n_tyvars = length tyvars
827 n_dicts = length theta
829 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs
830 rhs_dicts = take n_dicts rhs_ids
831 rhs_bndrs = rhs_tyvars ++ rhs_dicts
832 body = mkLams (drop n_dicts rhs_ids) rhs_body
833 -- Glue back on the non-dict lambdas
835 calls_for_me = case lookupFM calls fn of
837 Just cs -> fmToList cs
839 ----------------------------------------------------------
840 -- Specialise to one particular call pattern
841 spec_call :: ([Maybe Type], ([DictExpr], VarSet)) -- Call instance
842 -> SpecM ((Id,CoreExpr), -- Specialised definition
843 UsageDetails, -- Usage details from specialised body
844 ([CoreBndr], [CoreExpr], CoreExpr)) -- Info for the Id's SpecEnv
845 spec_call (call_ts, (call_ds, call_fvs))
846 = ASSERT( length call_ts == n_tyvars && length call_ds == n_dicts )
847 -- Calls are only recorded for properly-saturated applications
849 -- Suppose f's defn is f = /\ a b c d -> \ d1 d2 -> rhs
850 -- Supppose the call is for f [Just t1, Nothing, Just t3, Nothing] [dx1, dx2]
852 -- Construct the new binding
853 -- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b d -> rhs)
854 -- PLUS the usage-details
855 -- { d1' = dx1; d2' = dx2 }
856 -- where d1', d2' are cloned versions of d1,d2, with the type substitution applied.
858 -- Note that the substitution is applied to the whole thing.
859 -- This is convenient, but just slightly fragile. Notably:
860 -- * There had better be no name clashes in a/b/c/d
863 -- poly_tyvars = [b,d] in the example above
864 -- spec_tyvars = [a,c]
865 -- ty_args = [t1,b,t3,d]
866 poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
867 spec_tyvars = [tv | (tv, Just _) <- rhs_tyvars `zip` call_ts]
868 ty_args = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
870 mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
871 mk_ty_arg rhs_tyvar (Just ty) = Type ty
872 rhs_subst = extendSubstList subst spec_tyvars [DoneTy ty | Just ty <- call_ts]
874 cloneBinders rhs_subst rhs_dicts `thenSM` \ (rhs_subst', rhs_dicts') ->
876 inst_args = ty_args ++ map Var rhs_dicts'
878 -- Figure out the type of the specialised function
879 spec_id_ty = mkForAllTys poly_tyvars (applyTypeToArgs rhs fn_type inst_args)
881 newIdSM fn spec_id_ty `thenSM` \ spec_f ->
882 specExpr rhs_subst' (mkLams poly_tyvars body) `thenSM` \ (spec_rhs, rhs_uds) ->
884 -- The rule to put in the function's specialisation is:
885 -- forall b,d, d1',d2'. f t1 b t3 d d1' d2' = f1 b d
886 spec_env_rule = (poly_tyvars ++ rhs_dicts',
888 mkTyApps (Var spec_f) (map mkTyVarTy poly_tyvars))
890 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
891 final_uds = foldr addDictBind rhs_uds (my_zipEqual "spec_call" rhs_dicts' call_ds)
893 returnSM ((spec_f, spec_rhs),
898 my_zipEqual doc xs ys
899 | length xs /= length ys = pprPanic "my_zipEqual" (ppr xs $$ ppr ys $$ (ppr fn <+> ppr call_ts) $$ ppr rhs)
900 | otherwise = zipEqual doc xs ys
903 %************************************************************************
905 \subsubsection{UsageDetails and suchlike}
907 %************************************************************************
912 dict_binds :: !(Bag DictBind),
913 -- Floated dictionary bindings
914 -- The order is important;
915 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
916 -- (Remember, Bags preserve order in GHC.)
918 calls :: !CallDetails
921 type DictBind = (CoreBind, VarSet)
922 -- The set is the free vars of the binding
923 -- both tyvars and dicts
925 type DictExpr = CoreExpr
927 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM }
929 type ProtoUsageDetails = ([DictBind],
930 [(Id, [Maybe Type], ([DictExpr], VarSet))]
933 ------------------------------------------------------------
934 type CallDetails = FiniteMap Id CallInfo
935 type CallInfo = FiniteMap [Maybe Type] -- Nothing => unconstrained type argument
936 ([DictExpr], VarSet) -- Dict args and the vars of the whole
937 -- call (including tyvars)
938 -- [*not* include the main id itself, of course]
939 -- The finite maps eliminate duplicates
940 -- The list of types and dictionaries is guaranteed to
941 -- match the type of f
943 unionCalls :: CallDetails -> CallDetails -> CallDetails
944 unionCalls c1 c2 = plusFM_C plusFM c1 c2
946 singleCall :: (Id, [Maybe Type], [DictExpr]) -> CallDetails
947 singleCall (id, tys, dicts)
948 = unitFM id (unitFM tys (dicts, call_fvs))
950 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
951 tys_fvs = tyVarsOfTypes (catMaybes tys)
952 -- The type args (tys) are guaranteed to be part of the dictionary
953 -- types, because they are just the constrained types,
954 -- and the dictionary is therefore sure to be bound
955 -- inside the binding for any type variables free in the type;
956 -- hence it's safe to neglect tyvars free in tys when making
957 -- the free-var set for this call
958 -- BUT I don't trust this reasoning; play safe and include tys_fvs
960 -- We don't include the 'id' itself.
962 listToCallDetails calls
963 = foldr (unionCalls . mk_call) emptyFM calls
965 mk_call (id, tys, dicts_w_fvs) = unitFM id (unitFM tys dicts_w_fvs)
966 -- NB: the free vars of the call are provided
968 callDetailsToList calls = [ (id,tys,dicts)
969 | (id,fm) <- fmToList calls,
970 (tys,dicts) <- fmToList fm
975 || length spec_tys /= n_tyvars
976 || length dicts /= n_dicts
977 = emptyUDs -- Not overloaded
980 = MkUD {dict_binds = emptyBag,
981 calls = singleCall (f, spec_tys, dicts)
984 (tyvars, theta, tau) = splitSigmaTy (idType f)
985 constrained_tyvars = tyVarsOfTheta theta
986 n_tyvars = length tyvars
987 n_dicts = length theta
989 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
990 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
992 mk_spec_ty tyvar ty | tyvar `elemVarSet` constrained_tyvars
997 ------------------------------------------------------------
998 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
999 plusUDs (MkUD {dict_binds = db1, calls = calls1})
1000 (MkUD {dict_binds = db2, calls = calls2})
1001 = MkUD {dict_binds = d, calls = c}
1003 d = db1 `unionBags` db2
1004 c = calls1 `unionCalls` calls2
1006 plusUDList = foldr plusUDs emptyUDs
1008 -- zapCalls deletes calls to ids from uds
1009 zapCalls ids uds = uds {calls = delListFromFM (calls uds) ids}
1011 mkDB bind = (bind, bind_fvs bind)
1013 bind_fvs (NonRec bndr rhs) = exprFreeVars rhs
1014 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1017 rhs_fvs = unionVarSets [exprFreeVars rhs | (bndr,rhs) <- prs]
1019 addDictBind (dict,rhs) uds = uds { dict_binds = mkDB (NonRec dict rhs) `consBag` dict_binds uds }
1021 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
1022 = foldrBag add binds dbs
1024 add (bind,_) binds = bind : binds
1026 dumpUDs :: [CoreBndr]
1027 -> UsageDetails -> CoreExpr
1028 -> (UsageDetails, CoreExpr)
1029 dumpUDs bndrs uds body
1030 = (free_uds, foldr add_let body dict_binds)
1032 (free_uds, (dict_binds, _)) = splitUDs bndrs uds
1033 add_let (bind,_) body = Let bind body
1035 splitUDs :: [CoreBndr]
1037 -> (UsageDetails, -- These don't mention the binders
1038 ProtoUsageDetails) -- These do
1040 splitUDs bndrs uds@(MkUD {dict_binds = orig_dbs,
1041 calls = orig_calls})
1043 = if isEmptyBag dump_dbs && null dump_calls then
1044 -- Common case: binder doesn't affect floats
1048 -- Binders bind some of the fvs of the floats
1049 (MkUD {dict_binds = free_dbs,
1050 calls = listToCallDetails free_calls},
1051 (bagToList dump_dbs, dump_calls)
1055 bndr_set = mkVarSet bndrs
1057 (free_dbs, dump_dbs, dump_idset)
1058 = foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
1059 -- Important that it's foldl not foldr;
1060 -- we're accumulating the set of dumped ids in dump_set
1062 -- Filter out any calls that mention things that are being dumped
1063 orig_call_list = callDetailsToList orig_calls
1064 (dump_calls, free_calls) = partition captured orig_call_list
1065 captured (id,tys,(dicts, fvs)) = fvs `intersectsVarSet` dump_idset
1066 || id `elemVarSet` dump_idset
1068 dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1069 | dump_idset `intersectsVarSet` fvs -- Dump it
1070 = (free_dbs, dump_dbs `snocBag` db,
1071 dump_idset `unionVarSet` mkVarSet (bindersOf bind))
1073 | otherwise -- Don't dump it
1074 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1078 %************************************************************************
1080 \subsubsection{Boring helper functions}
1082 %************************************************************************
1085 lookupId:: IdEnv Id -> Id -> Id
1086 lookupId env id = case lookupVarEnv env id of
1090 ----------------------------------------
1091 type SpecM a = UniqSM a
1096 getUniqSM = getUniqueUs
1097 getUniqSupplySM = getUs
1098 setUniqSupplySM = setUs
1102 mapAndCombineSM f [] = returnSM ([], emptyUDs)
1103 mapAndCombineSM f (x:xs) = f x `thenSM` \ (y, uds1) ->
1104 mapAndCombineSM f xs `thenSM` \ (ys, uds2) ->
1105 returnSM (y:ys, uds1 `plusUDs` uds2)
1107 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1108 -- Clone the binders of the bind; return new bind with the cloned binders
1109 -- Return the substitution to use for RHSs, and the one to use for the body
1110 cloneBindSM subst (NonRec bndr rhs)
1111 = getUs `thenUs` \ us ->
1113 (subst', us', bndr') = substAndCloneId subst us bndr
1116 returnUs (subst, subst', NonRec bndr' rhs)
1118 cloneBindSM subst (Rec pairs)
1119 = getUs `thenUs` \ us ->
1121 (subst', us', bndrs') = substAndCloneIds subst us (map fst pairs)
1124 returnUs (subst', subst', Rec (bndrs' `zip` map snd pairs))
1126 cloneBinders subst bndrs
1127 = getUs `thenUs` \ us ->
1129 (subst', us', bndrs') = substAndCloneIds subst us bndrs
1132 returnUs (subst', bndrs')
1135 newIdSM old_id new_ty
1136 = getUniqSM `thenSM` \ uniq ->
1138 -- Give the new Id a similar occurrence name to the old one
1139 name = idName old_id
1140 new_id = mkUserLocal (mkSpecOcc (nameOccName name)) uniq new_ty (getSrcLoc name)
1142 -- If the old Id was exported, make the new one non-discardable,
1143 -- else we will discard it since it doesn't seem to be called.
1144 new_id' | isExportedId old_id = setIdNoDiscard new_id
1145 | otherwise = new_id
1150 = getUniqSM `thenSM` \ uniq ->
1151 returnSM (mkSysTyVar uniq boxedTypeKind)
1155 Old (but interesting) stuff about unboxed bindings
1156 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1158 What should we do when a value is specialised to a *strict* unboxed value?
1160 map_*_* f (x:xs) = let h = f x
1164 Could convert let to case:
1166 map_*_Int# f (x:xs) = case f x of h# ->
1170 This may be undesirable since it forces evaluation here, but the value
1171 may not be used in all branches of the body. In the general case this
1172 transformation is impossible since the mutual recursion in a letrec
1173 cannot be expressed as a case.
1175 There is also a problem with top-level unboxed values, since our
1176 implementation cannot handle unboxed values at the top level.
1178 Solution: Lift the binding of the unboxed value and extract it when it
1181 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1186 Now give it to the simplifier and the _Lifting will be optimised away.
1188 The benfit is that we have given the specialised "unboxed" values a
1189 very simplep lifted semantics and then leave it up to the simplifier to
1190 optimise it --- knowing that the overheads will be removed in nearly
1193 In particular, the value will only be evaluted in the branches of the
1194 program which use it, rather than being forced at the point where the
1195 value is bound. For example:
1197 filtermap_*_* p f (x:xs)
1204 filtermap_*_Int# p f (x:xs)
1205 = let h = case (f x) of h# -> _Lift h#
1208 True -> case h of _Lift h#
1212 The binding for h can still be inlined in the one branch and the
1213 _Lifting eliminated.
1216 Question: When won't the _Lifting be eliminated?
1218 Answer: When they at the top-level (where it is necessary) or when
1219 inlining would duplicate work (or possibly code depending on
1220 options). However, the _Lifting will still be eliminated if the
1221 strictness analyser deems the lifted binding strict.