2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[Specialise]{Stamping out overloading, and (optionally) polymorphism}
7 -- The above warning supression flag is a temporary kludge.
8 -- While working on this module you are encouraged to remove it and fix
9 -- any warnings in the module. See
10 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
13 module Specialise ( specProgram ) where
15 #include "HsVersions.h"
17 import DynFlags ( DynFlags, DynFlag(..) )
18 import Id ( Id, idName, idType, mkUserLocal, idCoreRules,
19 idInlinePragma, setInlinePragma )
20 import TcType ( Type, mkTyVarTy, tcSplitSigmaTy,
21 tyVarsOfTypes, tyVarsOfTheta, isClassPred,
22 tcCmpType, isUnLiftedType
24 import CoreSubst ( Subst, mkEmptySubst, extendTvSubstList, lookupIdSubst,
25 substBndr, substBndrs, substTy, substInScope,
26 cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs
28 import SimplUtils ( interestingArg )
33 import CoreUtils ( applyTypeToArgs, mkPiTypes )
34 import CoreFVs ( exprFreeVars, exprsFreeVars, idFreeVars )
35 import CoreLint ( showPass, endPass )
36 import UniqSupply ( UniqSupply,
41 import MkId ( voidArgId, realWorldPrimId )
43 import Maybes ( catMaybes, isJust )
44 import ErrUtils ( dumpIfSet_dyn )
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!
490 A note about non-tyvar dictionaries
491 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
492 Some Ids have types like
494 forall a,b,c. Eq a -> Ord [a] -> tau
496 This seems curious at first, because we usually only have dictionary
497 args whose types are of the form (C a) where a is a type variable.
498 But this doesn't hold for the functions arising from instance decls,
499 which sometimes get arguements with types of form (C (T a)) for some
502 Should we specialise wrt this compound-type dictionary? We used to say
504 "This is a heuristic judgement, as indeed is the fact that we
505 specialise wrt only dictionaries. We choose *not* to specialise
506 wrt compound dictionaries because at the moment the only place
507 they show up is in instance decls, where they are simply plugged
508 into a returned dictionary. So nothing is gained by specialising
511 But it is simpler and more uniform to specialise wrt these dicts too;
512 and in future GHC is likely to support full fledged type signatures
514 f :: Eq [(a,b)] => ...
517 %************************************************************************
519 \subsubsection{The new specialiser}
521 %************************************************************************
523 Our basic game plan is this. For let(rec) bound function
524 f :: (C a, D c) => (a,b,c,d) -> Bool
526 * Find any specialised calls of f, (f ts ds), where
527 ts are the type arguments t1 .. t4, and
528 ds are the dictionary arguments d1 .. d2.
530 * Add a new definition for f1 (say):
532 f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
534 Note that we abstract over the unconstrained type arguments.
538 [t1,b,t3,d] |-> \d1 d2 -> f1 b d
540 to the specialisations of f. This will be used by the
541 simplifier to replace calls
542 (f t1 t2 t3 t4) da db
544 (\d1 d1 -> f1 t2 t4) da db
546 All the stuff about how many dictionaries to discard, and what types
547 to apply the specialised function to, are handled by the fact that the
548 SpecEnv contains a template for the result of the specialisation.
550 We don't build *partial* specialisations for f. For example:
552 f :: Eq a => a -> a -> Bool
553 {-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
555 Here, little is gained by making a specialised copy of f.
556 There's a distinct danger that the specialised version would
557 first build a dictionary for (Eq b, Eq c), and then select the (==)
558 method from it! Even if it didn't, not a great deal is saved.
560 We do, however, generate polymorphic, but not overloaded, specialisations:
562 f :: Eq a => [a] -> b -> b -> b
563 {#- SPECIALISE f :: [Int] -> b -> b -> b #-}
565 Hence, the invariant is this:
567 *** no specialised version is overloaded ***
570 %************************************************************************
572 \subsubsection{The exported function}
574 %************************************************************************
577 specProgram :: DynFlags -> UniqSupply -> [CoreBind] -> IO [CoreBind]
578 specProgram dflags us binds = do
580 showPass dflags "Specialise"
582 let binds' = initSM us (do (binds', uds') <- go binds
583 return (dumpAllDictBinds uds' binds'))
585 endPass dflags "Specialise" Opt_D_dump_spec binds'
587 dumpIfSet_dyn dflags Opt_D_dump_rules "Top-level specialisations"
588 (pprRulesForUser (rulesOfBinds binds'))
592 -- We need to start with a Subst that knows all the things
593 -- that are in scope, so that the substitution engine doesn't
594 -- accidentally re-use a unique that's already in use
595 -- Easiest thing is to do it all at once, as if all the top-level
596 -- decls were mutually recursive
597 top_subst = mkEmptySubst (mkInScopeSet (mkVarSet (bindersOfBinds binds)))
599 go [] = return ([], emptyUDs)
600 go (bind:binds) = do (binds', uds) <- go binds
601 (bind', uds') <- specBind top_subst bind uds
602 return (bind' ++ binds', uds')
605 %************************************************************************
607 \subsubsection{@specExpr@: the main function}
609 %************************************************************************
612 specVar :: Subst -> Id -> CoreExpr
613 specVar subst v = lookupIdSubst subst v
615 specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
616 -- We carry a substitution down:
617 -- a) we must clone any binding that might flaot outwards,
618 -- to avoid name clashes
619 -- b) we carry a type substitution to use when analysing
620 -- the RHS of specialised bindings (no type-let!)
622 ---------------- First the easy cases --------------------
623 specExpr subst (Type ty) = return (Type (substTy subst ty), emptyUDs)
624 specExpr subst (Var v) = return (specVar subst v, emptyUDs)
625 specExpr _ (Lit lit) = return (Lit lit, emptyUDs)
626 specExpr subst (Cast e co) = do
627 (e', uds) <- specExpr subst e
628 return ((Cast e' (substTy subst co)), uds)
629 specExpr subst (Note note body) = do
630 (body', uds) <- specExpr subst body
631 return (Note (specNote subst note) body', uds)
634 ---------------- Applications might generate a call instance --------------------
635 specExpr subst expr@(App {})
638 go (App fun arg) args = do (arg', uds_arg) <- specExpr subst arg
639 (fun', uds_app) <- go fun (arg':args)
640 return (App fun' arg', uds_arg `plusUDs` uds_app)
642 go (Var f) args = case specVar subst f of
643 Var f' -> return (Var f', mkCallUDs f' args)
644 e' -> return (e', emptyUDs) -- I don't expect this!
645 go other _ = specExpr subst other
647 ---------------- Lambda/case require dumping of usage details --------------------
648 specExpr subst e@(Lam _ _) = do
649 (body', uds) <- specExpr subst' body
650 let (filtered_uds, body'') = dumpUDs bndrs' uds body'
651 return (mkLams bndrs' body'', filtered_uds)
653 (bndrs, body) = collectBinders e
654 (subst', bndrs') = substBndrs subst bndrs
655 -- More efficient to collect a group of binders together all at once
656 -- and we don't want to split a lambda group with dumped bindings
658 specExpr subst (Case scrut case_bndr ty alts) = do
659 (scrut', uds_scrut) <- specExpr subst scrut
660 (alts', uds_alts) <- mapAndCombineSM spec_alt alts
661 return (Case scrut' case_bndr' (substTy subst ty) alts', uds_scrut `plusUDs` uds_alts)
663 (subst_alt, case_bndr') = substBndr subst case_bndr
664 -- No need to clone case binder; it can't float like a let(rec)
666 spec_alt (con, args, rhs) = do
667 (rhs', uds) <- specExpr subst_rhs rhs
668 let (uds', rhs'') = dumpUDs args uds rhs'
669 return ((con, args', rhs''), uds')
671 (subst_rhs, args') = substBndrs subst_alt args
673 ---------------- Finally, let is the interesting case --------------------
674 specExpr subst (Let bind body) = do
676 (rhs_subst, body_subst, bind') <- cloneBindSM subst bind
678 -- Deal with the body
679 (body', body_uds) <- specExpr body_subst body
681 -- Deal with the bindings
682 (binds', uds) <- specBind rhs_subst bind' body_uds
685 return (foldr Let body' binds', uds)
687 -- Must apply the type substitution to coerceions
688 specNote :: Subst -> Note -> Note
689 specNote _ note = note
692 %************************************************************************
694 \subsubsection{Dealing with a binding}
696 %************************************************************************
699 specBind :: Subst -- Use this for RHSs
701 -> UsageDetails -- Info on how the scope of the binding
702 -> SpecM ([CoreBind], -- New bindings
703 UsageDetails) -- And info to pass upstream
705 specBind rhs_subst bind body_uds
706 = do { (bind', bind_uds) <- specBindItself rhs_subst bind (calls body_uds)
707 ; return (finishSpecBind bind' bind_uds body_uds) }
709 finishSpecBind :: CoreBind -> UsageDetails -> UsageDetails -> ([CoreBind], UsageDetails)
711 (MkUD { dict_binds = rhs_dbs, calls = rhs_calls, ud_fvs = rhs_fvs })
712 (MkUD { dict_binds = body_dbs, calls = body_calls, ud_fvs = body_fvs })
713 | not (mkVarSet bndrs `intersectsVarSet` all_fvs)
714 -- Common case 1: the bound variables are not
715 -- mentioned in the dictionary bindings
716 = ([bind], MkUD { dict_binds = body_dbs `unionBags` rhs_dbs
717 -- It's important that the `unionBags` is this way round,
718 -- because body_uds may bind dictionaries that are
719 -- used in the calls passed to specDefn. So the
720 -- dictionary bindings in rhs_uds may mention
721 -- dictionaries bound in body_uds.
723 , ud_fvs = all_fvs })
725 | case bind of { NonRec {} -> True; Rec {} -> False }
726 -- Common case 2: no specialisation happened, and binding
727 -- is non-recursive. But the binding may be
728 -- mentioned in body_dbs, so we should put it first
729 = ([], MkUD { dict_binds = rhs_dbs `unionBags` ((bind, b_fvs) `consBag` body_dbs)
731 , ud_fvs = all_fvs `unionVarSet` b_fvs })
733 | otherwise -- General case: make a huge Rec (sigh)
734 = ([], MkUD { dict_binds = unitBag (Rec all_db_prs, all_db_fvs)
736 , ud_fvs = all_fvs `unionVarSet` b_fvs })
738 all_fvs = rhs_fvs `unionVarSet` body_fvs
739 all_calls = zapCalls bndrs (rhs_calls `unionCalls` body_calls)
741 bndrs = bindersOf bind
742 b_fvs = bind_fvs bind
744 (all_db_prs, all_db_fvs) = add (bind, b_fvs) $
745 foldrBag add ([], emptyVarSet) $
746 rhs_dbs `unionBags` body_dbs
747 add (NonRec b r, b_fvs) (prs, fvs) = ((b,r) : prs, b_fvs `unionVarSet` fvs)
748 add (Rec b_prs, b_fvs) (prs, fvs) = (b_prs ++ prs, b_fvs `unionVarSet` fvs)
750 ---------------------------
751 specBindItself :: Subst -> CoreBind -> CallDetails -> SpecM (CoreBind, UsageDetails)
753 -- specBindItself deals with the RHS, specialising it according
754 -- to the calls found in the body (if any)
755 specBindItself rhs_subst (NonRec fn rhs) call_info
756 = do { (rhs', rhs_uds) <- specExpr rhs_subst rhs -- Do RHS of original fn
757 ; (fn', spec_defns, spec_uds) <- specDefn rhs_subst call_info fn rhs
758 ; if null spec_defns then
759 return (NonRec fn rhs', rhs_uds)
761 return (Rec ((fn',rhs') : spec_defns), rhs_uds `plusUDs` spec_uds) }
762 -- bndr' mentions the spec_defns in its SpecEnv
763 -- Not sure why we couln't just put the spec_defns first
765 specBindItself rhs_subst (Rec pairs) call_info
766 -- Note [Specialising a recursive group]
767 = do { let (bndrs,rhss) = unzip pairs
768 ; (rhss', rhs_uds) <- mapAndCombineSM (specExpr rhs_subst) rhss
769 ; let all_calls = call_info `unionCalls` calls rhs_uds
770 ; (bndrs1, spec_defns1, spec_uds1) <- specDefns rhs_subst all_calls pairs
772 ; if null spec_defns1 then -- Common case: no specialisation
773 return (Rec (bndrs `zip` rhss'), rhs_uds)
774 else do -- Specialisation occurred; do it again
775 { (bndrs2, spec_defns2, spec_uds2) <- specDefns rhs_subst
776 (calls spec_uds1) (bndrs1 `zip` rhss)
778 ; let all_defns = spec_defns1 ++ spec_defns2 ++ zip bndrs2 rhss'
780 ; return (Rec all_defns, rhs_uds `plusUDs` spec_uds1 `plusUDs` spec_uds2) } }
783 ---------------------------
785 -> CallDetails -- Info on how it is used in its scope
786 -> [(Id,CoreExpr)] -- The things being bound and their un-processed RHS
787 -> SpecM ([Id], -- Original Ids with RULES added
788 [(Id,CoreExpr)], -- Extra, specialised bindings
789 UsageDetails) -- Stuff to fling upwards from the specialised versions
791 -- Specialise a list of bindings (the contents of a Rec), but flowing usages
792 -- upwards binding by binding. Example: { f = ...g ...; g = ...f .... }
793 -- Then if the input CallDetails has a specialised call for 'g', whose specialisation
794 -- in turn generates a specialised call for 'f', we catch that in this one sweep.
795 -- But not vice versa (it's a fixpoint problem).
797 specDefns _subst _call_info []
798 = return ([], [], emptyUDs)
799 specDefns subst call_info ((bndr,rhs):pairs)
800 = do { (bndrs', spec_defns, spec_uds) <- specDefns subst call_info pairs
801 ; let all_calls = call_info `unionCalls` calls spec_uds
802 ; (bndr', spec_defns1, spec_uds1) <- specDefn subst all_calls bndr rhs
803 ; return (bndr' : bndrs',
804 spec_defns1 ++ spec_defns,
805 spec_uds1 `plusUDs` spec_uds) }
807 ---------------------------
809 -> CallDetails -- Info on how it is used in its scope
810 -> Id -> CoreExpr -- The thing being bound and its un-processed RHS
811 -> SpecM (Id, -- Original Id with added RULES
812 [(Id,CoreExpr)], -- Extra, specialised bindings
813 UsageDetails) -- Stuff to fling upwards from the specialised versions
815 specDefn subst calls fn rhs
816 -- The first case is the interesting one
817 | rhs_tyvars `lengthIs` n_tyvars -- Rhs of fn's defn has right number of big lambdas
818 && rhs_ids `lengthAtLeast` n_dicts -- and enough dict args
819 && notNull calls_for_me -- And there are some calls to specialise
821 -- && not (certainlyWillInline (idUnfolding fn)) -- And it's not small
822 -- See Note [Inline specialisation] for why we do not
823 -- switch off specialisation for inline functions
825 = do { -- Make a specialised version for each call in calls_for_me
826 stuff <- mapM spec_call calls_for_me
827 ; let (spec_defns, spec_uds, spec_rules) = unzip3 (catMaybes stuff)
828 fn' = addIdSpecialisations fn spec_rules
829 ; return (fn', spec_defns, plusUDList spec_uds) }
831 | otherwise -- No calls or RHS doesn't fit our preconceptions
832 = WARN( notNull calls_for_me, ptext (sLit "Missed specialisation opportunity for") <+> ppr fn )
833 -- Note [Specialisation shape]
834 return (fn, [], emptyUDs)
838 (tyvars, theta, _) = tcSplitSigmaTy fn_type
839 n_tyvars = length tyvars
840 n_dicts = length theta
841 inline_prag = idInlinePragma fn
843 -- It's important that we "see past" any INLINE pragma
844 -- else we'll fail to specialise an INLINE thing
845 (inline_rhs, rhs_inside) = dropInline rhs
846 (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs_inside
848 rhs_dicts = take n_dicts rhs_ids
849 body = mkLams (drop n_dicts rhs_ids) rhs_body
850 -- Glue back on the non-dict lambdas
852 calls_for_me = case lookupFM calls fn of
854 Just cs -> fmToList cs
856 already_covered :: [CoreExpr] -> Bool
857 already_covered args -- Note [Specialisations already covered]
858 = isJust (lookupRule (const True) (substInScope subst)
859 fn args (idCoreRules fn))
861 mk_ty_args :: [Maybe Type] -> [CoreExpr]
862 mk_ty_args call_ts = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
864 mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
865 mk_ty_arg _ (Just ty) = Type ty
867 ----------------------------------------------------------
868 -- Specialise to one particular call pattern
869 spec_call :: (CallKey, ([DictExpr], VarSet)) -- Call instance
870 -> SpecM (Maybe ((Id,CoreExpr), -- Specialised definition
871 UsageDetails, -- Usage details from specialised body
872 CoreRule)) -- Info for the Id's SpecEnv
873 spec_call (CallKey call_ts, (call_ds, _))
874 = ASSERT( call_ts `lengthIs` n_tyvars && call_ds `lengthIs` n_dicts )
876 -- Suppose f's defn is f = /\ a b c -> \ d1 d2 -> rhs
877 -- Supppose the call is for f [Just t1, Nothing, Just t3] [dx1, dx2]
879 -- Construct the new binding
880 -- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b d -> rhs)
881 -- PLUS the usage-details
882 -- { d1' = dx1; d2' = dx2 }
883 -- where d1', d2' are cloned versions of d1,d2, with the type substitution
884 -- applied. These auxiliary bindings just avoid duplication of dx1, dx2
886 -- Note that the substitution is applied to the whole thing.
887 -- This is convenient, but just slightly fragile. Notably:
888 -- * There had better be no name clashes in a/b/c
890 -- poly_tyvars = [b] in the example above
891 -- spec_tyvars = [a,c]
892 -- ty_args = [t1,b,t3]
893 poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
894 spec_tv_binds = [(tv,ty) | (tv, Just ty) <- rhs_tyvars `zip` call_ts]
895 spec_ty_args = map snd spec_tv_binds
896 ty_args = mk_ty_args call_ts
897 rhs_subst = extendTvSubstList subst spec_tv_binds
899 ; (rhs_subst', rhs_dicts') <- cloneBinders rhs_subst rhs_dicts
900 ; let inst_args = ty_args ++ map Var rhs_dicts'
902 ; if already_covered inst_args then
905 { -- Figure out the type of the specialised function
906 let body_ty = applyTypeToArgs rhs fn_type inst_args
907 (lam_args, app_args) -- Add a dummy argument if body_ty is unlifted
908 | isUnLiftedType body_ty -- C.f. WwLib.mkWorkerArgs
909 = (poly_tyvars ++ [voidArgId], poly_tyvars ++ [realWorldPrimId])
910 | otherwise = (poly_tyvars, poly_tyvars)
911 spec_id_ty = mkPiTypes lam_args body_ty
913 ; spec_f <- newIdSM fn spec_id_ty
914 ; (spec_rhs, rhs_uds) <- specExpr rhs_subst' (mkLams lam_args body)
916 -- The rule to put in the function's specialisation is:
917 -- forall b, d1',d2'. f t1 b t3 d1' d2' = f1 b
918 rule_name = mkFastString ("SPEC " ++ showSDoc (ppr fn <+> ppr spec_ty_args))
919 spec_env_rule = mkLocalRule
921 inline_prag -- Note [Auto-specialisation and RULES]
923 (poly_tyvars ++ rhs_dicts')
925 (mkVarApps (Var spec_f) app_args)
927 -- Add the { d1' = dx1; d2' = dx2 } usage stuff
928 final_uds = foldr addDictBind rhs_uds (my_zipEqual "spec_call" rhs_dicts' call_ds)
930 spec_pr | inline_rhs = (spec_f `setInlinePragma` inline_prag, Note InlineMe spec_rhs)
931 | otherwise = (spec_f, spec_rhs)
933 ; return (Just (spec_pr, final_uds, spec_env_rule)) } }
935 my_zipEqual doc xs ys
936 | debugIsOn && not (equalLength xs ys)
937 = pprPanic "my_zipEqual" (vcat
939 , ppr fn <+> ppr call_ts
940 , ppr (idType fn), ppr theta
941 , ppr n_dicts, ppr rhs_dicts
943 | otherwise = zipEqual doc xs ys
946 Note [Specialising a recursive group]
947 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
949 let rec { f x = ...g x'...
950 ; g y = ...f y'.... }
952 Here we specialise 'f' at Char; but that is very likely to lead to
953 a specialisation of 'g' at Char. We must do the latter, else the
954 whole point of specialisation is lost.
956 But we do not want to keep iterating to a fixpoint, because in the
957 presence of polymorphic recursion we might generate an infinite number
960 So we use the following heuristic:
961 * Arrange the rec block in dependency order, so far as possible
962 (the occurrence analyser already does this)
964 * Specialise it much like a sequence of lets
966 * Then go through the block a second time, feeding call-info from
967 the RHSs back in the bottom, as it were
969 In effect, the ordering maxmimises the effectiveness of each sweep,
970 and we do just two sweeps. This should catch almost every case of
971 monomorphic recursion -- the exception could be a very knotted-up
972 recursion with multiple cycles tied up together.
974 This plan is implemented in the Rec case of specBindItself.
976 Note [Specialisations already covered]
977 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
978 We obviously don't want to generate two specialisations for the same
979 argument pattern. There are two wrinkles
981 1. We do the already-covered test in specDefn, not when we generate
982 the CallInfo in mkCallUDs. We used to test in the latter place, but
983 we now iterate the specialiser somewhat, and the Id at the call site
984 might therefore not have all the RULES that we can see in specDefn
986 2. What about two specialisations where the second is an *instance*
987 of the first? If the more specific one shows up first, we'll generate
988 specialisations for both. If the *less* specific one shows up first,
989 we *don't* currently generate a specialisation for the more specific
990 one. (See the call to lookupRule in already_covered.) Reasons:
991 (a) lookupRule doesn't say which matches are exact (bad reason)
992 (b) if the earlier specialisation is user-provided, it's
993 far from clear that we should auto-specialise further
995 Note [Auto-specialisation and RULES]
996 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1001 f :: (Int -> Int) -> Int
1003 {-# RULE f g = 0 #-}
1005 Suppose that auto-specialisation makes a specialised version of
1006 g::Int->Int That version won't appear in the LHS of the RULE for f.
1007 So if the specialisation rule fires too early, the rule for f may
1010 It might be possible to add new rules, to "complete" the rewrite system.
1012 RULE forall d. g Int d = g_spec
1016 But that's a bit complicated. For now we ask the programmer's help,
1017 by *copying the INLINE activation pragma* to the auto-specialised rule.
1018 So if g says {-# NOINLINE[2] g #-}, then the auto-spec rule will also
1019 not be active until phase 2.
1022 Note [Specialisation shape]
1023 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1024 We only specialise a function if it has visible top-level lambdas
1025 corresponding to its overloading. E.g. if
1026 f :: forall a. Eq a => ....
1027 then its body must look like
1030 Reason: when specialising the body for a call (f ty dexp), we want to
1031 substitute dexp for d, and pick up specialised calls in the body of f.
1033 This doesn't always work. One example I came across was htis:
1034 newtype Gen a = MkGen{ unGen :: Int -> a }
1036 choose :: Eq a => a -> Gen a
1037 choose n = MkGen (\r -> n)
1039 oneof = choose (1::Int)
1041 It's a silly exapmle, but we get
1042 choose = /\a. g `cast` co
1043 where choose doesn't have any dict arguments. Thus far I have not
1044 tried to fix this (wait till there's a real example).
1047 Note [Inline specialisations]
1048 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1049 We transfer to the specialised function any INLINE stuff from the
1050 original. This means (a) the Activation in the IdInfo, and (b) any
1051 InlineMe on the RHS.
1053 This is a change (Jun06). Previously the idea is that the point of
1054 inlining was precisely to specialise the function at its call site,
1055 and that's not so important for the specialised copies. But
1056 *pragma-directed* specialisation now takes place in the
1057 typechecker/desugarer, with manually specified INLINEs. The
1058 specialiation here is automatic. It'd be very odd if a function
1059 marked INLINE was specialised (because of some local use), and then
1060 forever after (including importing modules) the specialised version
1061 wasn't INLINEd. After all, the programmer said INLINE!
1063 You might wonder why we don't just not specialise INLINE functions.
1064 It's because even INLINE functions are sometimes not inlined, when
1065 they aren't applied to interesting arguments. But perhaps the type
1066 arguments alone are enough to specialise (even though the args are too
1067 boring to trigger inlining), and it's certainly better to call the
1068 specialised version.
1070 A case in point is dictionary functions, which are current marked
1071 INLINE, but which are worth specialising.
1074 dropInline :: CoreExpr -> (Bool, CoreExpr)
1075 dropInline (Note InlineMe rhs) = (True, rhs)
1076 dropInline rhs = (False, rhs)
1079 %************************************************************************
1081 \subsubsection{UsageDetails and suchlike}
1083 %************************************************************************
1088 dict_binds :: !(Bag DictBind),
1089 -- Floated dictionary bindings
1090 -- The order is important;
1091 -- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
1092 -- (Remember, Bags preserve order in GHC.)
1094 calls :: !CallDetails,
1096 ud_fvs :: !VarSet -- A superset of the variables mentioned in
1097 -- either dict_binds or calls
1100 instance Outputable UsageDetails where
1101 ppr (MkUD { dict_binds = dbs, calls = calls, ud_fvs = fvs })
1102 = ptext (sLit "MkUD") <+> braces (sep (punctuate comma
1103 [ptext (sLit "binds") <+> equals <+> ppr dbs,
1104 ptext (sLit "calls") <+> equals <+> ppr calls,
1105 ptext (sLit "fvs") <+> equals <+> ppr fvs]))
1107 type DictBind = (CoreBind, VarSet)
1108 -- The set is the free vars of the binding
1109 -- both tyvars and dicts
1111 type DictExpr = CoreExpr
1113 emptyUDs :: UsageDetails
1114 emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM, ud_fvs = emptyVarSet }
1116 ------------------------------------------------------------
1117 type CallDetails = FiniteMap Id CallInfo
1118 newtype CallKey = CallKey [Maybe Type] -- Nothing => unconstrained type argument
1120 -- CallInfo uses a FiniteMap, thereby ensuring that
1121 -- we record only one call instance for any key
1123 -- The list of types and dictionaries is guaranteed to
1124 -- match the type of f
1125 type CallInfo = FiniteMap CallKey ([DictExpr], VarSet)
1126 -- Range is dict args and the vars of the whole
1127 -- call (including tyvars)
1128 -- [*not* include the main id itself, of course]
1130 instance Outputable CallKey where
1131 ppr (CallKey ts) = ppr ts
1133 -- Type isn't an instance of Ord, so that we can control which
1134 -- instance we use. That's tiresome here. Oh well
1135 instance Eq CallKey where
1136 k1 == k2 = case k1 `compare` k2 of { EQ -> True; _ -> False }
1138 instance Ord CallKey where
1139 compare (CallKey k1) (CallKey k2) = cmpList cmp k1 k2
1141 cmp Nothing Nothing = EQ
1142 cmp Nothing (Just _) = LT
1143 cmp (Just _) Nothing = GT
1144 cmp (Just t1) (Just t2) = tcCmpType t1 t2
1146 unionCalls :: CallDetails -> CallDetails -> CallDetails
1147 unionCalls c1 c2 = plusFM_C plusFM c1 c2
1149 singleCall :: Id -> [Maybe Type] -> [DictExpr] -> UsageDetails
1150 singleCall id tys dicts
1151 = MkUD {dict_binds = emptyBag,
1152 calls = unitFM id (unitFM (CallKey tys) (dicts, call_fvs)),
1155 call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
1156 tys_fvs = tyVarsOfTypes (catMaybes tys)
1157 -- The type args (tys) are guaranteed to be part of the dictionary
1158 -- types, because they are just the constrained types,
1159 -- and the dictionary is therefore sure to be bound
1160 -- inside the binding for any type variables free in the type;
1161 -- hence it's safe to neglect tyvars free in tys when making
1162 -- the free-var set for this call
1163 -- BUT I don't trust this reasoning; play safe and include tys_fvs
1165 -- We don't include the 'id' itself.
1167 mkCallUDs :: Id -> [CoreExpr] -> UsageDetails
1170 || not (all isClassPred theta)
1171 -- Only specialise if all overloading is on class params.
1172 -- In ptic, with implicit params, the type args
1173 -- *don't* say what the value of the implicit param is!
1174 || not (spec_tys `lengthIs` n_tyvars)
1175 || not ( dicts `lengthIs` n_dicts)
1176 || not (any interestingArg dicts) -- Note [Interesting dictionary arguments]
1177 -- See also Note [Specialisations already covered]
1178 = emptyUDs -- Not overloaded, or no specialisation wanted
1181 = singleCall f spec_tys dicts
1183 (tyvars, theta, _) = tcSplitSigmaTy (idType f)
1184 constrained_tyvars = tyVarsOfTheta theta
1185 n_tyvars = length tyvars
1186 n_dicts = length theta
1188 spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
1189 dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
1192 | tyvar `elemVarSet` constrained_tyvars = Just ty
1193 | otherwise = Nothing
1196 Note [Interesting dictionary arguments]
1197 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1199 \a.\d:Eq a. let f = ... in ...(f d)...
1200 There really is not much point in specialising f wrt the dictionary d,
1201 because the code for the specialised f is not improved at all, because
1202 d is lambda-bound. We simply get junk specialisations.
1204 We re-use the function SimplUtils.interestingArg function to determine
1205 what sort of dictionary arguments have *some* information in them.
1209 plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
1210 plusUDs (MkUD {dict_binds = db1, calls = calls1, ud_fvs = fvs1})
1211 (MkUD {dict_binds = db2, calls = calls2, ud_fvs = fvs2})
1212 = MkUD {dict_binds = d, calls = c, ud_fvs = fvs1 `unionVarSet` fvs2}
1214 d = db1 `unionBags` db2
1215 c = calls1 `unionCalls` calls2
1217 plusUDList :: [UsageDetails] -> UsageDetails
1218 plusUDList = foldr plusUDs emptyUDs
1220 -- zapCalls deletes calls to ids from uds
1221 zapCalls :: [Id] -> CallDetails -> CallDetails
1222 zapCalls ids calls = delListFromFM calls ids
1224 mkDB :: CoreBind -> DictBind
1225 mkDB bind = (bind, bind_fvs bind)
1227 bind_fvs :: CoreBind -> VarSet
1228 bind_fvs (NonRec bndr rhs) = pair_fvs (bndr,rhs)
1229 bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
1232 rhs_fvs = unionVarSets (map pair_fvs prs)
1234 pair_fvs :: (Id, CoreExpr) -> VarSet
1235 pair_fvs (bndr, rhs) = exprFreeVars rhs `unionVarSet` idFreeVars bndr
1236 -- Don't forget variables mentioned in the
1237 -- rules of the bndr. C.f. OccAnal.addRuleUsage
1238 -- Also tyvars mentioned in its type; they may not appear in the RHS
1242 addDictBind :: (Id,CoreExpr) -> UsageDetails -> UsageDetails
1243 addDictBind (dict,rhs) uds
1244 = uds { dict_binds = db `consBag` dict_binds uds
1245 , ud_fvs = ud_fvs uds `unionVarSet` fvs }
1247 db@(_, fvs) = mkDB (NonRec dict rhs)
1249 dumpAllDictBinds :: UsageDetails -> [CoreBind] -> [CoreBind]
1250 dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
1251 = foldrBag add binds dbs
1253 add (bind,_) binds = bind : binds
1255 dumpUDs :: [CoreBndr]
1256 -> UsageDetails -> CoreExpr
1257 -> (UsageDetails, CoreExpr)
1258 dumpUDs bndrs (MkUD { dict_binds = orig_dbs
1259 , calls = orig_calls
1260 , ud_fvs = fvs}) body
1261 = (new_uds, foldrBag add_let body dump_dbs)
1262 -- This may delete fewer variables
1263 -- than in priciple possible
1266 MkUD { dict_binds = free_dbs
1267 , calls = free_calls
1268 , ud_fvs = fvs `minusVarSet` bndr_set}
1270 bndr_set = mkVarSet bndrs
1271 add_let (bind,_) body = Let bind body
1273 (free_dbs, dump_dbs, dump_set)
1274 = foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
1275 -- Important that it's foldl not foldr;
1276 -- we're accumulating the set of dumped ids in dump_set
1278 free_calls = filterCalls dump_set orig_calls
1280 dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
1281 | dump_idset `intersectsVarSet` fvs -- Dump it
1282 = (free_dbs, dump_dbs `snocBag` db,
1283 extendVarSetList dump_idset (bindersOf bind))
1285 | otherwise -- Don't dump it
1286 = (free_dbs `snocBag` db, dump_dbs, dump_idset)
1288 filterCalls :: VarSet -> CallDetails -> CallDetails
1289 -- Remove any calls that mention the variables
1290 filterCalls bs calls
1291 = mapFM (\_ cs -> filter_calls cs) $
1292 filterFM (\k _ -> not (k `elemVarSet` bs)) calls
1294 filter_calls :: CallInfo -> CallInfo
1295 filter_calls = filterFM (\_ (_, fvs) -> not (fvs `intersectsVarSet` bs))
1299 %************************************************************************
1301 \subsubsection{Boring helper functions}
1303 %************************************************************************
1306 type SpecM a = UniqSM a
1308 initSM :: UniqSupply -> SpecM a -> a
1311 mapAndCombineSM :: (a -> SpecM (b, UsageDetails)) -> [a] -> SpecM ([b], UsageDetails)
1312 mapAndCombineSM _ [] = return ([], emptyUDs)
1313 mapAndCombineSM f (x:xs) = do (y, uds1) <- f x
1314 (ys, uds2) <- mapAndCombineSM f xs
1315 return (y:ys, uds1 `plusUDs` uds2)
1317 cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
1318 -- Clone the binders of the bind; return new bind with the cloned binders
1319 -- Return the substitution to use for RHSs, and the one to use for the body
1320 cloneBindSM subst (NonRec bndr rhs) = do
1321 us <- getUniqueSupplyM
1322 let (subst', bndr') = cloneIdBndr subst us bndr
1323 return (subst, subst', NonRec bndr' rhs)
1325 cloneBindSM subst (Rec pairs) = do
1326 us <- getUniqueSupplyM
1327 let (subst', bndrs') = cloneRecIdBndrs subst us (map fst pairs)
1328 return (subst', subst', Rec (bndrs' `zip` map snd pairs))
1330 cloneBinders :: Subst -> [CoreBndr] -> SpecM (Subst, [CoreBndr])
1331 cloneBinders subst bndrs = do
1332 us <- getUniqueSupplyM
1333 return (cloneIdBndrs subst us bndrs)
1335 newIdSM :: Id -> Type -> SpecM Id
1336 newIdSM old_id new_ty = do
1339 -- Give the new Id a similar occurrence name to the old one
1340 name = idName old_id
1341 new_id = mkUserLocal (mkSpecOcc (nameOccName name)) uniq new_ty (getSrcSpan name)
1346 Old (but interesting) stuff about unboxed bindings
1347 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1349 What should we do when a value is specialised to a *strict* unboxed value?
1351 map_*_* f (x:xs) = let h = f x
1355 Could convert let to case:
1357 map_*_Int# f (x:xs) = case f x of h# ->
1361 This may be undesirable since it forces evaluation here, but the value
1362 may not be used in all branches of the body. In the general case this
1363 transformation is impossible since the mutual recursion in a letrec
1364 cannot be expressed as a case.
1366 There is also a problem with top-level unboxed values, since our
1367 implementation cannot handle unboxed values at the top level.
1369 Solution: Lift the binding of the unboxed value and extract it when it
1372 map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
1377 Now give it to the simplifier and the _Lifting will be optimised away.
1379 The benfit is that we have given the specialised "unboxed" values a
1380 very simplep lifted semantics and then leave it up to the simplifier to
1381 optimise it --- knowing that the overheads will be removed in nearly
1384 In particular, the value will only be evaluted in the branches of the
1385 program which use it, rather than being forced at the point where the
1386 value is bound. For example:
1388 filtermap_*_* p f (x:xs)
1395 filtermap_*_Int# p f (x:xs)
1396 = let h = case (f x) of h# -> _Lift h#
1399 True -> case h of _Lift h#
1403 The binding for h can still be inlined in the one branch and the
1404 _Lifting eliminated.
1407 Question: When won't the _Lifting be eliminated?
1409 Answer: When they at the top-level (where it is necessary) or when
1410 inlining would duplicate work (or possibly code depending on
1411 options). However, the _Lifting will still be eliminated if the
1412 strictness analyser deems the lifted binding strict.