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 DynFlags ( DynFlags, DynFlag(..) )
12 import Id ( Id, idName, idType, mkUserLocal,
13 idInlinePragma, setInlinePragma )
14 import TcType ( Type, mkTyVarTy, tcSplitSigmaTy,
15 tyVarsOfTypes, tyVarsOfTheta, isClassPred,
16 tcCmpType, isUnLiftedType
18 import CoreSubst ( Subst, mkEmptySubst, extendTvSubstList, lookupIdSubst,
19 substBndr, substBndrs, substTy, substInScope,
20 cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs
25 import CoreUtils ( applyTypeToArgs, mkPiTypes )
26 import CoreFVs ( exprFreeVars, exprsFreeVars, idFreeVars )
27 import CoreTidy ( tidyRules )
28 import CoreLint ( showPass, endPass )
29 import Rules ( addIdSpecialisations, mkLocalRule, lookupRule, emptyRuleBase, rulesOfBinds )
30 import PprCore ( pprRules )
31 import UniqSupply ( UniqSupply,
32 UniqSM, initUs_, thenUs, returnUs, getUniqueUs,
36 import MkId ( voidArgId, realWorldPrimId )
38 import Maybes ( catMaybes, maybeToBool )
39 import ErrUtils ( dumpIfSet_dyn )
40 import BasicTypes ( Activation( AlwaysActive ) )
42 import List ( partition )
43 import Util ( zipEqual, zipWithEqual, cmpList, lengthIs,
44 equalLength, lengthAtLeast, notNull )
51 %************************************************************************
53 \subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
55 %************************************************************************
57 These notes describe how we implement specialisation to eliminate
60 The specialisation pass works on Core
61 syntax, complete with all the explicit dictionary application,
62 abstraction and construction as added by the type checker. The
63 existing type checker remains largely as it is.
65 One important thought: the {\em types} passed to an overloaded
66 function, and the {\em dictionaries} passed are mutually redundant.
67 If the same function is applied to the same type(s) then it is sure to
68 be applied to the same dictionary(s)---or rather to the same {\em
69 values}. (The arguments might look different but they will evaluate
72 Second important thought: we know that we can make progress by
73 treating dictionary arguments as static and worth specialising on. So
74 we can do without binding-time analysis, and instead specialise on
75 dictionary arguments and no others.
84 and suppose f is overloaded.
86 STEP 1: CALL-INSTANCE COLLECTION
88 We traverse <body>, accumulating all applications of f to types and
91 (Might there be partial applications, to just some of its types and
92 dictionaries? In principle yes, but in practice the type checker only
93 builds applications of f to all its types and dictionaries, so partial
94 applications could only arise as a result of transformation, and even
95 then I think it's unlikely. In any case, we simply don't accumulate such
96 partial applications.)
101 So now we have a collection of calls to f:
105 Notice that f may take several type arguments. To avoid ambiguity, we
106 say that f is called at type t1/t2 and t3/t4.
108 We take equivalence classes using equality of the *types* (ignoring
109 the dictionary args, which as mentioned previously are redundant).
111 STEP 3: SPECIALISATION
113 For each equivalence class, choose a representative (f t1 t2 d1 d2),
114 and create a local instance of f, defined thus:
116 f@t1/t2 = <f_rhs> t1 t2 d1 d2
118 f_rhs presumably has some big lambdas and dictionary lambdas, so lots
119 of simplification will now result. However we don't actually *do* that
120 simplification. Rather, we leave it for the simplifier to do. If we
121 *did* do it, though, we'd get more call instances from the specialised
122 RHS. We can work out what they are by instantiating the call-instance
123 set from f's RHS with the types t1, t2.
125 Add this new id to f's IdInfo, to record that f has a specialised version.
127 Before doing any of this, check that f's IdInfo doesn't already
128 tell us about an existing instance of f at the required type/s.
129 (This might happen if specialisation was applied more than once, or
130 it might arise from user SPECIALIZE pragmas.)
134 Wait a minute! What if f is recursive? Then we can't just plug in
135 its right-hand side, can we?
137 But it's ok. The type checker *always* creates non-recursive definitions
138 for overloaded recursive functions. For example:
140 f x = f (x+x) -- Yes I know its silly
144 f a (d::Num a) = let p = +.sel a d
146 letrec fl (y::a) = fl (p y y)
150 We still have recusion for non-overloaded functions which we
151 speciailise, but the recursive call should get specialised to the
152 same recursive version.
158 All this is crystal clear when the function is applied to *constant
159 types*; that is, types which have no type variables inside. But what if
160 it is applied to non-constant types? Suppose we find a call of f at type
161 t1/t2. There are two possibilities:
163 (a) The free type variables of t1, t2 are in scope at the definition point
164 of f. In this case there's no problem, we proceed just as before. A common
165 example is as follows. Here's the Haskell:
170 After typechecking we have
172 g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
173 in +.sel a d (f a d y) (f a d y)
175 Notice that the call to f is at type type "a"; a non-constant type.
176 Both calls to f are at the same type, so we can specialise to give:
178 g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
179 in +.sel a d (f@a y) (f@a y)
182 (b) The other case is when the type variables in the instance types
183 are *not* in scope at the definition point of f. The example we are
184 working with above is a good case. There are two instances of (+.sel a d),
185 but "a" is not in scope at the definition of +.sel. Can we do anything?
186 Yes, we can "common them up", a sort of limited common sub-expression deal.
189 g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
190 f@a (x::a) = +.sel@a x x
191 in +.sel@a (f@a y) (f@a y)
193 This can save work, and can't be spotted by the type checker, because
194 the two instances of +.sel weren't originally at the same type.
198 * There are quite a few variations here. For example, the defn of
199 +.sel could be floated ouside the \y, to attempt to gain laziness.
200 It certainly mustn't be floated outside the \d because the d has to
203 * We don't want to inline f_rhs in this case, because
204 that will duplicate code. Just commoning up the call is the point.
206 * Nothing gets added to +.sel's IdInfo.
208 * Don't bother unless the equivalence class has more than one item!
210 Not clear whether this is all worth it. It is of course OK to
211 simply discard call-instances when passing a big lambda.
213 Polymorphism 2 -- Overloading
215 Consider a function whose most general type is
217 f :: forall a b. Ord a => [a] -> b -> b
219 There is really no point in making a version of g at Int/Int and another
220 at Int/Bool, because it's only instancing the type variable "a" which
221 buys us any efficiency. Since g is completely polymorphic in b there
222 ain't much point in making separate versions of g for the different
225 That suggests that we should identify which of g's type variables
226 are constrained (like "a") and which are unconstrained (like "b").
227 Then when taking equivalence classes in STEP 2, we ignore the type args
228 corresponding to unconstrained type variable. In STEP 3 we make
229 polymorphic versions. Thus:
231 f@t1/ = /\b -> <f_rhs> t1 b d1 d2
240 f a (d::Num a) = let g = ...
242 ...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
244 Here, g is only called at one type, but the dictionary isn't in scope at the
245 definition point for g. Usually the type checker would build a
246 definition for d1 which enclosed g, but the transformation system
247 might have moved d1's defn inward. Solution: float dictionary bindings
248 outwards along with call instances.
252 f x = let g p q = p==q
258 Before specialisation, leaving out type abstractions we have
260 f df x = let g :: Eq a => a -> a -> Bool
262 h :: Num a => a -> a -> (a, Bool)
263 h dh r s = let deq = eqFromNum dh
264 in (+ dh r s, g deq r s)
268 After specialising h we get a specialised version of h, like this:
270 h' r s = let deq = eqFromNum df
271 in (+ df r s, g deq r s)
273 But we can't naively make an instance for g from this, because deq is not in scope
274 at the defn of g. Instead, we have to float out the (new) defn of deq
275 to widen its scope. Notice that this floating can't be done in advance -- it only
276 shows up when specialisation is done.
278 User SPECIALIZE pragmas
279 ~~~~~~~~~~~~~~~~~~~~~~~
280 Specialisation pragmas can be digested by the type checker, and implemented
281 by adding extra definitions along with that of f, in the same way as before
283 f@t1/t2 = <f_rhs> t1 t2 d1 d2
285 Indeed the pragmas *have* to be dealt with by the type checker, because
286 only it knows how to build the dictionaries d1 and d2! For example
288 g :: Ord a => [a] -> [a]
289 {-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
291 Here, the specialised version of g is an application of g's rhs to the
292 Ord dictionary for (Tree Int), which only the type checker can conjure
293 up. There might not even *be* one, if (Tree Int) is not an instance of
294 Ord! (All the other specialision has suitable dictionaries to hand
297 Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
298 it is buried in a complex (as-yet-un-desugared) binding group.
301 f@t1/t2 = f* t1 t2 d1 d2
303 where f* is the Id f with an IdInfo which says "inline me regardless!".
304 Indeed all the specialisation could be done in this way.
305 That in turn means that the simplifier has to be prepared to inline absolutely
306 any in-scope let-bound thing.
309 Again, the pragma should permit polymorphism in unconstrained variables:
311 h :: Ord a => [a] -> b -> b
312 {-# SPECIALIZE h :: [Int] -> b -> b #-}
314 We *insist* that all overloaded type variables are specialised to ground types,
315 (and hence there can be no context inside a SPECIALIZE pragma).
316 We *permit* unconstrained type variables to be specialised to
318 - or left as a polymorphic type variable
319 but nothing in between. So
321 {-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
323 is *illegal*. (It can be handled, but it adds complication, and gains the
327 SPECIALISING INSTANCE DECLARATIONS
328 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
331 instance Foo a => Foo [a] where
333 {-# SPECIALIZE instance Foo [Int] #-}
335 The original instance decl creates a dictionary-function
338 dfun.Foo.List :: forall a. Foo a -> Foo [a]
340 The SPECIALIZE pragma just makes a specialised copy, just as for
341 ordinary function definitions:
343 dfun.Foo.List@Int :: Foo [Int]
344 dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
346 The information about what instance of the dfun exist gets added to
347 the dfun's IdInfo in the same way as a user-defined function too.
350 Automatic instance decl specialisation?
351 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
352 Can instance decls be specialised automatically? It's tricky.
353 We could collect call-instance information for each dfun, but
354 then when we specialised their bodies we'd get new call-instances
355 for ordinary functions; and when we specialised their bodies, we might get
356 new call-instances of the dfuns, and so on. This all arises because of
357 the unrestricted mutual recursion between instance decls and value decls.
359 Still, there's no actual problem; it just means that we may not do all
360 the specialisation we could theoretically do.
362 Furthermore, instance decls are usually exported and used non-locally,
363 so we'll want to compile enough to get those specialisations done.
365 Lastly, there's no such thing as a local instance decl, so we can
366 survive solely by spitting out *usage* information, and then reading that
367 back in as a pragma when next compiling the file. So for now,
368 we only specialise instance decls in response to pragmas.
371 SPITTING OUT USAGE INFORMATION
372 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
374 To spit out usage information we need to traverse the code collecting
375 call-instance information for all imported (non-prelude?) functions
376 and data types. Then we equivalence-class it and spit it out.
378 This is done at the top-level when all the call instances which escape
379 must be for imported functions and data types.
381 *** Not currently done ***
384 Partial specialisation by pragmas
385 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
386 What about partial specialisation:
388 k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
389 {-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
393 {-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
395 Seems quite reasonable. Similar things could be done with instance decls:
397 instance (Foo a, Foo b) => Foo (a,b) where
399 {-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
400 {-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
402 Ho hum. Things are complex enough without this. I pass.
405 Requirements for the simplifer
406 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
407 The simplifier has to be able to take advantage of the specialisation.
409 * When the simplifier finds an application of a polymorphic f, it looks in
410 f's IdInfo in case there is a suitable instance to call instead. This converts
412 f t1 t2 d1 d2 ===> f_t1_t2
414 Note that the dictionaries get eaten up too!
416 * Dictionary selection operations on constant dictionaries must be
419 +.sel Int d ===> +Int
421 The obvious way to do this is in the same way as other specialised
422 calls: +.sel has inside it some IdInfo which tells that if it's applied
423 to the type Int then it should eat a dictionary and transform to +Int.
425 In short, dictionary selectors need IdInfo inside them for constant
428 * Exactly the same applies if a superclass dictionary is being
431 Eq.sel Int d ===> dEqInt
433 * Something similar applies to dictionary construction too. Suppose
434 dfun.Eq.List is the function taking a dictionary for (Eq a) to
435 one for (Eq [a]). Then we want
437 dfun.Eq.List Int d ===> dEq.List_Int
439 Where does the Eq [Int] dictionary come from? It is built in
440 response to a SPECIALIZE pragma on the Eq [a] instance decl.
442 In short, dfun Ids need IdInfo with a specialisation for each
443 constant instance of their instance declaration.
445 All this uses a single mechanism: the SpecEnv inside an Id
448 What does the specialisation IdInfo look like?
449 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
451 The SpecEnv of an Id maps a list of types (the template) to an expression
455 For example, if f has this SpecInfo:
457 [Int, a] -> \d:Ord Int. f' a
459 it means that we can replace the call
461 f Int t ===> (\d. f' t)
463 This chucks one dictionary away and proceeds with the
464 specialised version of f, namely f'.
467 What can't be done this way?
468 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
469 There is no way, post-typechecker, to get a dictionary for (say)
470 Eq a from a dictionary for Eq [a]. So if we find
474 we can't transform to
479 eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
481 Of course, we currently have no way to automatically derive
482 eqList, nor to connect it to the Eq [a] instance decl, but you
483 can imagine that it might somehow be possible. Taking advantage
484 of this is permanently ruled out.
486 Still, this is no great hardship, because we intend to eliminate
487 overloading altogether anyway!
491 A note about non-tyvar dictionaries
492 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
493 Some Ids have types like
495 forall a,b,c. Eq a -> Ord [a] -> tau
497 This seems curious at first, because we usually only have dictionary
498 args whose types are of the form (C a) where a is a type variable.
499 But this doesn't hold for the functions arising from instance decls,
500 which sometimes get arguements with types of form (C (T a)) for some
503 Should we specialise wrt this compound-type dictionary? We used to say
505 "This is a heuristic judgement, as indeed is the fact that we
506 specialise wrt only dictionaries. We choose *not* to specialise
507 wrt compound dictionaries because at the moment the only place
508 they show up is in instance decls, where they are simply plugged
509 into a returned dictionary. So nothing is gained by specialising
512 But it is simpler and more uniform to specialise wrt these dicts too;
513 and in future GHC is likely to support full fledged type signatures
515 f ;: Eq [(a,b)] => ...
518 %************************************************************************
520 \subsubsection{The new specialiser}
522 %************************************************************************
524 Our basic game plan is this. For let(rec) bound function
525 f :: (C a, D c) => (a,b,c,d) -> Bool
527 * Find any specialised calls of f, (f ts ds), where
528 ts are the type arguments t1 .. t4, and
529 ds are the dictionary arguments d1 .. d2.
531 * Add a new definition for f1 (say):
533 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
535 Note that we abstract over the unconstrained type arguments.
539 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
541 to the specialisations of f. This will be used by the
542 simplifier to replace calls
543 (f t1 t2 t3 t4) da db
545 (\d1 d1 -> f1 t2 t4) da db
547 All the stuff about how many dictionaries to discard, and what types
548 to apply the specialised function to, are handled by the fact that the
549 SpecEnv contains a template for the result of the specialisation.
551 We don't build *partial* specialisations for f. For example:
553 f :: Eq a => a -> a -> Bool
554 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
556 Here, little is gained by making a specialised copy of f.
557 There's a distinct danger that the specialised version would
558 first build a dictionary for (Eq b, Eq c), and then select the (==)
559 method from it! Even if it didn't, not a great deal is saved.
561 We do, however, generate polymorphic, but not overloaded, specialisations:
563 f :: Eq a => [a] -> b -> b -> b
564 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
566 Hence, the invariant is this:
568 *** no specialised version is overloaded ***
571 %************************************************************************
573 \subsubsection{The exported function}
575 %************************************************************************
578 specProgram :: DynFlags -> UniqSupply -> [CoreBind] -> IO [CoreBind]
579 specProgram dflags us binds
581 showPass dflags "Specialise"
583 let binds' = initSM us (go binds `thenSM` \ (binds', uds') ->
584 returnSM (dumpAllDictBinds uds' binds'))
586 endPass dflags "Specialise" Opt_D_dump_spec binds'
588 dumpIfSet_dyn dflags Opt_D_dump_rules "Top-level specialisations"
589 (pprRules (tidyRules emptyTidyEnv (rulesOfBinds binds')))
593 -- We need to start with a Subst that knows all the things
594 -- that are in scope, so that the substitution engine doesn't
595 -- accidentally re-use a unique that's already in use
596 -- Easiest thing is to do it all at once, as if all the top-level
597 -- decls were mutually recursive
598 top_subst = mkEmptySubst (mkInScopeSet (mkVarSet (bindersOfBinds binds)))
600 go [] = returnSM ([], emptyUDs)
601 go (bind:binds) = go binds `thenSM` \ (binds', uds) ->
602 specBind top_subst bind uds `thenSM` \ (bind', uds') ->
603 returnSM (bind' ++ binds', uds')
606 %************************************************************************
608 \subsubsection{@specExpr@: the main function}
610 %************************************************************************
613 specVar :: Subst -> Id -> CoreExpr
614 specVar subst v = lookupIdSubst subst v
616 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
617 -- We carry a substitution down:
618 -- a) we must clone any binding that might flaot outwards,
619 -- to avoid name clashes
620 -- b) we carry a type substitution to use when analysing
621 -- the RHS of specialised bindings (no type-let!)
623 ---------------- First the easy cases --------------------
624 specExpr subst (Type ty) = returnSM (Type (substTy subst ty), emptyUDs)
625 specExpr subst (Var v) = returnSM (specVar subst v, emptyUDs)
626 specExpr subst (Lit lit) = returnSM (Lit lit, emptyUDs)
627 specExpr subst (Cast e co) =
628 specExpr subst e `thenSM` \ (e', uds) ->
629 returnSM ((Cast e' (substTy subst co)), uds)
630 specExpr subst (Note note body)
631 = specExpr subst body `thenSM` \ (body', uds) ->
632 returnSM (Note (specNote subst note) body', uds)
635 ---------------- Applications might generate a call instance --------------------
636 specExpr subst expr@(App fun arg)
639 go (App fun arg) args = specExpr subst arg `thenSM` \ (arg', uds_arg) ->
640 go fun (arg':args) `thenSM` \ (fun', uds_app) ->
641 returnSM (App fun' arg', uds_arg `plusUDs` uds_app)
643 go (Var f) args = case specVar subst f of
644 Var f' -> returnSM (Var f', mkCallUDs subst f' args)
645 e' -> returnSM (e', emptyUDs) -- I don't expect this!
646 go other args = specExpr subst other
648 ---------------- Lambda/case require dumping of usage details --------------------
649 specExpr subst e@(Lam _ _)
650 = specExpr subst' body `thenSM` \ (body', uds) ->
652 (filtered_uds, body'') = dumpUDs bndrs' uds body'
654 returnSM (mkLams bndrs' body'', filtered_uds)
656 (bndrs, body) = collectBinders e
657 (subst', bndrs') = substBndrs subst bndrs
658 -- More efficient to collect a group of binders together all at once
659 -- and we don't want to split a lambda group with dumped bindings
661 specExpr subst (Case scrut case_bndr ty alts)
662 = specExpr subst scrut `thenSM` \ (scrut', uds_scrut) ->
663 mapAndCombineSM spec_alt alts `thenSM` \ (alts', uds_alts) ->
664 returnSM (Case scrut' case_bndr' (substTy subst ty) alts', uds_scrut `plusUDs` uds_alts)
666 (subst_alt, case_bndr') = substBndr subst case_bndr
667 -- No need to clone case binder; it can't float like a let(rec)
669 spec_alt (con, args, rhs)
670 = specExpr subst_rhs rhs `thenSM` \ (rhs', uds) ->
672 (uds', rhs'') = dumpUDs args uds rhs'
674 returnSM ((con, args', rhs''), uds')
676 (subst_rhs, args') = substBndrs subst_alt args
678 ---------------- Finally, let is the interesting case --------------------
679 specExpr subst (Let bind body)
681 cloneBindSM subst bind `thenSM` \ (rhs_subst, body_subst, bind') ->
683 -- Deal with the body
684 specExpr body_subst body `thenSM` \ (body', body_uds) ->
686 -- Deal with the bindings
687 specBind rhs_subst bind' body_uds `thenSM` \ (binds', uds) ->
690 returnSM (foldr Let body' binds', uds)
692 -- Must apply the type substitution to coerceions
693 specNote subst note = note
696 %************************************************************************
698 \subsubsection{Dealing with a binding}
700 %************************************************************************
703 specBind :: Subst -- Use this for RHSs
705 -> UsageDetails -- Info on how the scope of the binding
706 -> SpecM ([CoreBind], -- New bindings
707 UsageDetails) -- And info to pass upstream
709 specBind rhs_subst bind body_uds
710 = specBindItself rhs_subst bind (calls body_uds) `thenSM` \ (bind', bind_uds) ->
712 bndrs = bindersOf bind
713 all_uds = zapCalls bndrs (body_uds `plusUDs` bind_uds)
714 -- It's important that the `plusUDs` is this way round,
715 -- because body_uds may bind dictionaries that are
716 -- used in the calls passed to specDefn. So the
717 -- dictionary bindings in bind_uds may mention
718 -- dictionaries bound in body_uds.
720 case splitUDs bndrs all_uds of
722 (_, ([],[])) -- This binding doesn't bind anything needed
723 -- in the UDs, so put the binding here
724 -- This is the case for most non-dict bindings, except
725 -- for the few that are mentioned in a dict binding
726 -- that is floating upwards in body_uds
727 -> returnSM ([bind'], all_uds)
729 (float_uds, (dict_binds, calls)) -- This binding is needed in the UDs, so float it out
730 -> returnSM ([], float_uds `plusUDs` mkBigUD bind' dict_binds calls)
733 -- A truly gruesome function
734 mkBigUD bind@(NonRec _ _) dbs calls
735 = -- Common case: non-recursive and no specialisations
736 -- (if there were any specialistions it would have been made recursive)
737 MkUD { dict_binds = listToBag (mkDB bind : dbs),
738 calls = listToCallDetails calls }
740 mkBigUD bind dbs calls
742 MkUD { dict_binds = unitBag (mkDB (Rec (bind_prs bind ++ dbsToPairs dbs))),
744 calls = listToCallDetails calls }
746 bind_prs (NonRec b r) = [(b,r)]
747 bind_prs (Rec prs) = prs
750 dbsToPairs ((bind,_):dbs) = bind_prs bind ++ dbsToPairs dbs
752 -- specBindItself deals with the RHS, specialising it according
753 -- to the calls found in the body (if any)
754 specBindItself rhs_subst (NonRec bndr rhs) call_info
755 = specDefn rhs_subst call_info (bndr,rhs) `thenSM` \ ((bndr',rhs'), spec_defns, spec_uds) ->
757 new_bind | null spec_defns = NonRec bndr' rhs'
758 | otherwise = Rec ((bndr',rhs'):spec_defns)
759 -- bndr' mentions the spec_defns in its SpecEnv
760 -- Not sure why we couln't just put the spec_defns first
762 returnSM (new_bind, spec_uds)
764 specBindItself rhs_subst (Rec pairs) call_info
765 = mapSM (specDefn rhs_subst call_info) pairs `thenSM` \ stuff ->
767 (pairs', spec_defns_s, spec_uds_s) = unzip3 stuff
768 spec_defns = concat spec_defns_s
769 spec_uds = plusUDList spec_uds_s
770 new_bind = Rec (spec_defns ++ pairs')
772 returnSM (new_bind, spec_uds)
775 specDefn :: Subst -- Subst to use for RHS
776 -> CallDetails -- Info on how it is used in its scope
777 -> (Id, CoreExpr) -- The thing being bound and its un-processed RHS
778 -> SpecM ((Id, CoreExpr), -- The thing and its processed RHS
779 -- the Id may now have specialisations attached
780 [(Id,CoreExpr)], -- Extra, specialised bindings
781 UsageDetails -- Stuff to fling upwards from the RHS and its
782 ) -- specialised versions
784 specDefn subst calls (fn, rhs)
785 -- The first case is the interesting one
786 | rhs_tyvars `lengthIs` n_tyvars -- Rhs of fn's defn has right number of big lambdas
787 && rhs_ids `lengthAtLeast` n_dicts -- and enough dict args
788 && notNull calls_for_me -- And there are some calls to specialise
790 -- && not (certainlyWillInline (idUnfolding fn)) -- And it's not small
791 -- See Note [Inline specialisation] for why we do not
792 -- switch off specialisation for inline functions
794 = -- Specialise the body of the function
795 specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
797 -- Make a specialised version for each call in calls_for_me
798 mapSM spec_call calls_for_me `thenSM` \ stuff ->
800 (spec_defns, spec_uds, spec_rules) = unzip3 stuff
802 fn' = addIdSpecialisations fn spec_rules
804 returnSM ((fn',rhs'),
806 rhs_uds `plusUDs` plusUDList spec_uds)
808 | otherwise -- No calls or RHS doesn't fit our preconceptions
809 = WARN( notNull calls_for_me, ptext SLIT("Missed specialisation opportunity for") <+> ppr fn )
810 -- Note [Specialisation shape]
811 specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
812 returnSM ((fn, rhs'), [], rhs_uds)
816 (tyvars, theta, _) = tcSplitSigmaTy fn_type
817 n_tyvars = length tyvars
818 n_dicts = length theta
819 inline_prag = idInlinePragma fn
821 -- It's important that we "see past" any INLINE pragma
822 -- else we'll fail to specialise an INLINE thing
823 (inline_rhs, rhs_inside) = dropInline rhs
824 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs_inside
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( call_ts `lengthIs` n_tyvars && call_ds `lengthIs` 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 = extendTvSubstList subst (spec_tyvars `zip` [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 body_ty = applyTypeToArgs rhs fn_type inst_args
876 (lam_args, app_args) -- Add a dummy argument if body_ty is unlifted
877 | isUnLiftedType body_ty -- C.f. WwLib.mkWorkerArgs
878 = (poly_tyvars ++ [voidArgId], poly_tyvars ++ [realWorldPrimId])
879 | otherwise = (poly_tyvars, poly_tyvars)
880 spec_id_ty = mkPiTypes lam_args body_ty
882 newIdSM fn spec_id_ty `thenSM` \ spec_f ->
883 specExpr rhs_subst' (mkLams lam_args body) `thenSM` \ (spec_rhs, rhs_uds) ->
885 -- The rule to put in the function's specialisation is:
886 -- forall b,d, d1',d2'. f t1 b t3 d d1' d2' = f1 b d
887 spec_env_rule = mkLocalRule (mkFastString ("SPEC " ++ showSDoc (ppr fn)))
888 AlwaysActive (idName fn)
889 (poly_tyvars ++ rhs_dicts')
891 (mkVarApps (Var spec_f) app_args)
893 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
894 final_uds = foldr addDictBind rhs_uds (my_zipEqual "spec_call" rhs_dicts' call_ds)
896 spec_pr | inline_rhs = (spec_f `setInlinePragma` inline_prag, Note InlineMe spec_rhs)
897 | otherwise = (spec_f, spec_rhs)
899 returnSM (spec_pr, final_uds, spec_env_rule)
902 my_zipEqual doc xs ys
904 | not (equalLength xs ys) = pprPanic "my_zipEqual" (vcat
906 , ppr fn <+> ppr call_ts
907 , ppr (idType fn), ppr theta
908 , ppr n_dicts, ppr rhs_dicts
911 | otherwise = zipEqual doc xs ys
914 Note [Specialisation shape]
915 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
916 We only specialise a function if it has visible top-level lambdas
917 corresponding to its overloading. E.g. if
918 f :: forall a. Eq a => ....
919 then its body must look like
922 Reason: when specialising the body for a call (f ty dexp), we want to
923 substitute dexp for d, and pick up specialised calls in the body of f.
925 This doesn't always work. One example I came across was htis:
926 newtype Gen a = MkGen{ unGen :: Int -> a }
928 choose :: Eq a => a -> Gen a
929 choose n = MkGen (\r -> n)
931 oneof = choose (1::Int)
933 It's a silly exapmle, but we get
934 choose = /\a. g `cast` co
935 where choose doesn't have any dict arguments. Thus far I have not
936 tried to fix this (wait till there's a real example).
939 Note [Inline specialisations]
940 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
941 We transfer to the specialised function any INLINE stuff from the
942 original. This means (a) the Activation in the IdInfo, and (b) any
945 This is a change (Jun06). Previously the idea is that the point of
946 inlining was precisely to specialise the function at its call site,
947 and that's not so important for the specialised copies. But
948 *pragma-directed* specialisation now takes place in the
949 typechecker/desugarer, with manually specified INLINEs. The
950 specialiation here is automatic. It'd be very odd if a function
951 marked INLINE was specialised (because of some local use), and then
952 forever after (including importing modules) the specialised version
953 wasn't INLINEd. After all, the programmer said INLINE!
955 You might wonder why we don't just not specialise INLINE functions.
956 It's because even INLINE functions are sometimes not inlined, when
957 they aren't applied to interesting arguments. But perhaps the type
958 arguments alone are enough to specialise (even though the args are too
959 boring to trigger inlining), and it's certainly better to call the
962 A case in point is dictionary functions, which are current marked
963 INLINE, but which are worth specialising.
966 dropInline :: CoreExpr -> (Bool, CoreExpr)
967 dropInline (Note InlineMe rhs) = (True, rhs)
968 dropInline rhs = (False, rhs)
971 %************************************************************************
973 \subsubsection{UsageDetails and suchlike}
975 %************************************************************************
980 dict_binds :: !(Bag DictBind),
981 -- Floated dictionary bindings
982 -- The order is important;
983 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
984 -- (Remember, Bags preserve order in GHC.)
986 calls :: !CallDetails
989 type DictBind = (CoreBind, VarSet)
990 -- The set is the free vars of the binding
991 -- both tyvars and dicts
993 type DictExpr = CoreExpr
995 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM }
997 type ProtoUsageDetails = ([DictBind],
998 [(Id, CallKey, ([DictExpr], VarSet))]
1001 ------------------------------------------------------------
1002 type CallDetails = FiniteMap Id CallInfo
1003 newtype CallKey = CallKey [Maybe Type] -- Nothing => unconstrained type argument
1004 type CallInfo = FiniteMap CallKey
1005 ([DictExpr], VarSet) -- Dict args and the vars of the whole
1006 -- call (including tyvars)
1007 -- [*not* include the main id itself, of course]
1008 -- The finite maps eliminate duplicates
1009 -- The list of types and dictionaries is guaranteed to
1010 -- match the type of f
1012 -- Type isn't an instance of Ord, so that we can control which
1013 -- instance we use. That's tiresome here. Oh well
1014 instance Eq CallKey where
1015 k1 == k2 = case k1 `compare` k2 of { EQ -> True; other -> False }
1017 instance Ord CallKey where
1018 compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
1020 cmp Nothing Nothing = EQ
1021 cmp Nothing (Just t2) = LT
1022 cmp (Just t1) Nothing = GT
1023 cmp (Just t1) (Just t2) = tcCmpType t1 t2
1025 unionCalls :: CallDetails -> CallDetails -> CallDetails
1026 unionCalls c1 c2 = plusFM_C plusFM c1 c2
1028 singleCall :: Id -> [Maybe Type] -> [DictExpr] -> CallDetails
1029 singleCall id tys dicts
1030 = unitFM id (unitFM (CallKey tys) (dicts, call_fvs))
1032 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
1033 tys_fvs = tyVarsOfTypes (catMaybes tys)
1034 -- The type args (tys) are guaranteed to be part of the dictionary
1035 -- types, because they are just the constrained types,
1036 -- and the dictionary is therefore sure to be bound
1037 -- inside the binding for any type variables free in the type;
1038 -- hence it's safe to neglect tyvars free in tys when making
1039 -- the free-var set for this call
1040 -- BUT I don't trust this reasoning; play safe and include tys_fvs
1042 -- We don't include the 'id' itself.
1044 listToCallDetails calls
1045 = foldr (unionCalls . mk_call) emptyFM calls
1047 mk_call (id, tys, dicts_w_fvs) = unitFM id (unitFM tys dicts_w_fvs)
1048 -- NB: the free vars of the call are provided
1050 callDetailsToList calls = [ (id,tys,dicts)
1051 | (id,fm) <- fmToList calls,
1052 (tys, dicts) <- fmToList fm
1055 mkCallUDs subst f args
1057 || not (all isClassPred theta)
1058 -- Only specialise if all overloading is on class params.
1059 -- In ptic, with implicit params, the type args
1060 -- *don't* say what the value of the implicit param is!
1061 || not (spec_tys `lengthIs` n_tyvars)
1062 || not ( dicts `lengthIs` n_dicts)
1063 || maybeToBool (lookupRule (\act -> True) (substInScope subst) emptyRuleBase f args)
1064 -- There's already a rule covering this call. A typical case
1065 -- is where there's an explicit user-provided rule. Then
1066 -- we don't want to create a specialised version
1067 -- of the function that overlaps.
1068 = emptyUDs -- Not overloaded, or no specialisation wanted
1071 = MkUD {dict_binds = emptyBag,
1072 calls = singleCall f spec_tys dicts
1075 (tyvars, theta, _) = tcSplitSigmaTy (idType f)
1076 constrained_tyvars = tyVarsOfTheta theta
1077 n_tyvars = length tyvars
1078 n_dicts = length theta
1080 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
1081 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
1084 | tyvar `elemVarSet` constrained_tyvars = Just ty
1085 | otherwise = Nothing
1087 ------------------------------------------------------------
1088 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
1089 plusUDs (MkUD {dict_binds = db1, calls = calls1})
1090 (MkUD {dict_binds = db2, calls = calls2})
1091 = MkUD {dict_binds = d, calls = c}
1093 d = db1 `unionBags` db2
1094 c = calls1 `unionCalls` calls2
1096 plusUDList = foldr plusUDs emptyUDs
1098 -- zapCalls deletes calls to ids from uds
1099 zapCalls ids uds = uds {calls = delListFromFM (calls uds) ids}
1101 mkDB bind = (bind, bind_fvs bind)
1103 bind_fvs (NonRec bndr rhs) = pair_fvs (bndr,rhs)
1104 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1107 rhs_fvs = unionVarSets (map pair_fvs prs)
1109 pair_fvs (bndr, rhs) = exprFreeVars rhs `unionVarSet` idFreeVars bndr
1110 -- Don't forget variables mentioned in the
1111 -- rules of the bndr. C.f. OccAnal.addRuleUsage
1112 -- Also tyvars mentioned in its type; they may not appear in the RHS
1116 addDictBind (dict,rhs) uds = uds { dict_binds = mkDB (NonRec dict rhs) `consBag` dict_binds uds }
1118 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
1119 = foldrBag add binds dbs
1121 add (bind,_) binds = bind : binds
1123 dumpUDs :: [CoreBndr]
1124 -> UsageDetails -> CoreExpr
1125 -> (UsageDetails, CoreExpr)
1126 dumpUDs bndrs uds body
1127 = (free_uds, foldr add_let body dict_binds)
1129 (free_uds, (dict_binds, _)) = splitUDs bndrs uds
1130 add_let (bind,_) body = Let bind body
1132 splitUDs :: [CoreBndr]
1134 -> (UsageDetails, -- These don't mention the binders
1135 ProtoUsageDetails) -- These do
1137 splitUDs bndrs uds@(MkUD {dict_binds = orig_dbs,
1138 calls = orig_calls})
1140 = if isEmptyBag dump_dbs && null dump_calls then
1141 -- Common case: binder doesn't affect floats
1145 -- Binders bind some of the fvs of the floats
1146 (MkUD {dict_binds = free_dbs,
1147 calls = listToCallDetails free_calls},
1148 (bagToList dump_dbs, dump_calls)
1152 bndr_set = mkVarSet bndrs
1154 (free_dbs, dump_dbs, dump_idset)
1155 = foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
1156 -- Important that it's foldl not foldr;
1157 -- we're accumulating the set of dumped ids in dump_set
1159 -- Filter out any calls that mention things that are being dumped
1160 orig_call_list = callDetailsToList orig_calls
1161 (dump_calls, free_calls) = partition captured orig_call_list
1162 captured (id,tys,(dicts, fvs)) = fvs `intersectsVarSet` dump_idset
1163 || id `elemVarSet` dump_idset
1165 dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1166 | dump_idset `intersectsVarSet` fvs -- Dump it
1167 = (free_dbs, dump_dbs `snocBag` db,
1168 extendVarSetList dump_idset (bindersOf bind))
1170 | otherwise -- Don't dump it
1171 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1175 %************************************************************************
1177 \subsubsection{Boring helper functions}
1179 %************************************************************************
1182 type SpecM a = UniqSM a
1186 getUniqSM = getUniqueUs
1190 mapAndCombineSM f [] = returnSM ([], emptyUDs)
1191 mapAndCombineSM f (x:xs) = f x `thenSM` \ (y, uds1) ->
1192 mapAndCombineSM f xs `thenSM` \ (ys, uds2) ->
1193 returnSM (y:ys, uds1 `plusUDs` uds2)
1195 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1196 -- Clone the binders of the bind; return new bind with the cloned binders
1197 -- Return the substitution to use for RHSs, and the one to use for the body
1198 cloneBindSM subst (NonRec bndr rhs)
1199 = getUs `thenUs` \ us ->
1201 (subst', bndr') = cloneIdBndr subst us bndr
1203 returnUs (subst, subst', NonRec bndr' rhs)
1205 cloneBindSM subst (Rec pairs)
1206 = getUs `thenUs` \ us ->
1208 (subst', bndrs') = cloneRecIdBndrs subst us (map fst pairs)
1210 returnUs (subst', subst', Rec (bndrs' `zip` map snd pairs))
1212 cloneBinders subst bndrs
1213 = getUs `thenUs` \ us ->
1214 returnUs (cloneIdBndrs subst us bndrs)
1216 newIdSM old_id new_ty
1217 = getUniqSM `thenSM` \ uniq ->
1219 -- Give the new Id a similar occurrence name to the old one
1220 name = idName old_id
1221 new_id = mkUserLocal (mkSpecOcc (nameOccName name)) uniq new_ty (getSrcSpan name)
1227 Old (but interesting) stuff about unboxed bindings
1228 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1230 What should we do when a value is specialised to a *strict* unboxed value?
1232 map_*_* f (x:xs) = let h = f x
1236 Could convert let to case:
1238 map_*_Int# f (x:xs) = case f x of h# ->
1242 This may be undesirable since it forces evaluation here, but the value
1243 may not be used in all branches of the body. In the general case this
1244 transformation is impossible since the mutual recursion in a letrec
1245 cannot be expressed as a case.
1247 There is also a problem with top-level unboxed values, since our
1248 implementation cannot handle unboxed values at the top level.
1250 Solution: Lift the binding of the unboxed value and extract it when it
1253 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1258 Now give it to the simplifier and the _Lifting will be optimised away.
1260 The benfit is that we have given the specialised "unboxed" values a
1261 very simplep lifted semantics and then leave it up to the simplifier to
1262 optimise it --- knowing that the overheads will be removed in nearly
1265 In particular, the value will only be evaluted in the branches of the
1266 program which use it, rather than being forced at the point where the
1267 value is bound. For example:
1269 filtermap_*_* p f (x:xs)
1276 filtermap_*_Int# p f (x:xs)
1277 = let h = case (f x) of h# -> _Lift h#
1280 True -> case h of _Lift h#
1284 The binding for h can still be inlined in the one branch and the
1285 _Lifting eliminated.
1288 Question: When won't the _Lifting be eliminated?
1290 Answer: When they at the top-level (where it is necessary) or when
1291 inlining would duplicate work (or possibly code depending on
1292 options). However, the _Lifting will still be eliminated if the
1293 strictness analyser deems the lifted binding strict.