2 % (c) The University of Glasgow 2006
3 % (c) The AQUA Project, Glasgow University, 1994-1998
8 Unfoldings (which can travel across module boundaries) are in Core
9 syntax (namely @CoreExpr@s).
11 The type @Unfolding@ sits ``above'' simply-Core-expressions
12 unfoldings, capturing ``higher-level'' things we know about a binding,
13 usually things that the simplifier found out (e.g., ``it's a
14 literal''). In the corner of a @CoreUnfolding@ unfolding, you will
15 find, unsurprisingly, a Core expression.
19 Unfolding, UnfoldingGuidance, -- Abstract types
21 noUnfolding, mkTopUnfolding, mkImplicitUnfolding, mkUnfolding,
22 mkCompulsoryUnfolding, seqUnfolding,
23 evaldUnfolding, mkOtherCon, otherCons,
24 unfoldingTemplate, maybeUnfoldingTemplate,
25 isEvaldUnfolding, isValueUnfolding, isCheapUnfolding, isCompulsoryUnfolding,
26 hasUnfolding, hasSomeUnfolding, neverUnfold,
28 couldBeSmallEnoughToInline,
29 certainlyWillInline, smallEnoughToInline,
31 callSiteInline, CallCtxt(..)
38 import PprCore () -- Instances
40 import CoreSubst ( Subst, emptySubst, substTy, extendIdSubst, extendTvSubst
41 , lookupIdSubst, substBndr, substBndrs, substRecBndrs )
48 import Type hiding( substTy, extendTvSubst )
58 %************************************************************************
60 \subsection{Making unfoldings}
62 %************************************************************************
65 mkTopUnfolding :: CoreExpr -> Unfolding
66 mkTopUnfolding expr = mkUnfolding True {- Top level -} expr
68 mkImplicitUnfolding :: CoreExpr -> Unfolding
69 -- For implicit Ids, do a tiny bit of optimising first
70 mkImplicitUnfolding expr
71 = CoreUnfolding (simpleOptExpr emptySubst expr)
75 (calcUnfoldingGuidance opt_UF_CreationThreshold expr)
77 mkUnfolding :: Bool -> CoreExpr -> Unfolding
78 mkUnfolding top_lvl expr
79 = CoreUnfolding (occurAnalyseExpr expr)
86 -- OK to inline inside a lambda
88 (calcUnfoldingGuidance opt_UF_CreationThreshold expr)
89 -- Sometimes during simplification, there's a large let-bound thing
90 -- which has been substituted, and so is now dead; so 'expr' contains
91 -- two copies of the thing while the occurrence-analysed expression doesn't
92 -- Nevertheless, we don't occ-analyse before computing the size because the
93 -- size computation bales out after a while, whereas occurrence analysis does not.
95 -- This can occasionally mean that the guidance is very pessimistic;
96 -- it gets fixed up next round
98 instance Outputable Unfolding where
99 ppr NoUnfolding = ptext (sLit "No unfolding")
100 ppr (OtherCon cs) = ptext (sLit "OtherCon") <+> ppr cs
101 ppr (CompulsoryUnfolding e) = ptext (sLit "Compulsory") <+> ppr e
102 ppr (CoreUnfolding e top hnf cheap g)
103 = ptext (sLit "Unf") <+> sep [ppr top <+> ppr hnf <+> ppr cheap <+> ppr g,
106 mkCompulsoryUnfolding :: CoreExpr -> Unfolding
107 mkCompulsoryUnfolding expr -- Used for things that absolutely must be unfolded
108 = CompulsoryUnfolding (occurAnalyseExpr expr)
112 %************************************************************************
114 \subsection{The UnfoldingGuidance type}
116 %************************************************************************
119 instance Outputable UnfoldingGuidance where
120 ppr UnfoldNever = ptext (sLit "NEVER")
121 ppr (UnfoldIfGoodArgs v cs size discount)
122 = hsep [ ptext (sLit "IF_ARGS"), int v,
123 brackets (hsep (map int cs)),
130 calcUnfoldingGuidance
131 :: Int -- bomb out if size gets bigger than this
132 -> CoreExpr -- expression to look at
134 calcUnfoldingGuidance bOMB_OUT_SIZE expr
135 = case collect_val_bndrs expr of { (inline, val_binders, body) ->
137 n_val_binders = length val_binders
139 max_inline_size = n_val_binders+2
140 -- The idea is that if there is an INLINE pragma (inline is True)
141 -- and there's a big body, we give a size of n_val_binders+2. This
142 -- This is just enough to fail the no-size-increase test in callSiteInline,
143 -- so that INLINE things don't get inlined into entirely boring contexts,
147 case (sizeExpr (iUnbox bOMB_OUT_SIZE) val_binders body) of
150 | not inline -> UnfoldNever
151 -- A big function with an INLINE pragma must
152 -- have an UnfoldIfGoodArgs guidance
153 | otherwise -> UnfoldIfGoodArgs n_val_binders
154 (map (const 0) val_binders)
157 SizeIs size cased_args scrut_discount
160 (map discount_for val_binders)
162 (iBox scrut_discount)
164 boxed_size = iBox size
166 final_size | inline = boxed_size `min` max_inline_size
167 | otherwise = boxed_size
169 -- Sometimes an INLINE thing is smaller than n_val_binders+2.
170 -- A particular case in point is a constructor, which has size 1.
171 -- We want to inline this regardless, hence the `min`
173 discount_for b = foldlBag (\acc (b',n) -> if b==b' then acc+n else acc)
177 collect_val_bndrs e = go False [] e
178 -- We need to be a bit careful about how we collect the
179 -- value binders. In ptic, if we see
180 -- __inline_me (\x y -> e)
181 -- We want to say "2 value binders". Why? So that
182 -- we take account of information given for the arguments
184 go _ rev_vbs (Note InlineMe e) = go True rev_vbs e
185 go inline rev_vbs (Lam b e) | isId b = go inline (b:rev_vbs) e
186 | otherwise = go inline rev_vbs e
187 go inline rev_vbs e = (inline, reverse rev_vbs, e)
191 sizeExpr :: FastInt -- Bomb out if it gets bigger than this
192 -> [Id] -- Arguments; we're interested in which of these
197 sizeExpr bOMB_OUT_SIZE top_args expr
200 size_up (Type _) = sizeZero -- Types cost nothing
201 size_up (Var _) = sizeOne
203 size_up (Note InlineMe _) = sizeOne -- Inline notes make it look very small
204 -- This can be important. If you have an instance decl like this:
205 -- instance Foo a => Foo [a] where
206 -- {-# INLINE op1, op2 #-}
209 -- then we'll get a dfun which is a pair of two INLINE lambdas
211 size_up (Note _ body) = size_up body -- Other notes cost nothing
213 size_up (Cast e _) = size_up e
215 size_up (App fun (Type _)) = size_up fun
216 size_up (App fun arg) = size_up_app fun [arg]
218 size_up (Lit lit) = sizeN (litSize lit)
220 size_up (Lam b e) | isId b = lamScrutDiscount (size_up e `addSizeN` 1)
221 | otherwise = size_up e
223 size_up (Let (NonRec binder rhs) body)
224 = nukeScrutDiscount (size_up rhs) `addSize`
225 size_up body `addSizeN`
226 (if isUnLiftedType (idType binder) then 0 else 1)
227 -- For the allocation
228 -- If the binder has an unlifted type there is no allocation
230 size_up (Let (Rec pairs) body)
231 = nukeScrutDiscount rhs_size `addSize`
232 size_up body `addSizeN`
233 length pairs -- For the allocation
235 rhs_size = foldr (addSize . size_up . snd) sizeZero pairs
237 size_up (Case (Var v) _ _ alts)
238 | v `elem` top_args -- We are scrutinising an argument variable
240 {- I'm nuking this special case; BUT see the comment with case alternatives.
242 (a) It's too eager. We don't want to inline a wrapper into a
243 context with no benefit.
244 E.g. \ x. f (x+x) no point in inlining (+) here!
246 (b) It's ineffective. Once g's wrapper is inlined, its case-expressions
247 aren't scrutinising arguments any more
251 [alt] -> size_up_alt alt `addSize` SizeIs (_ILIT(0)) (unitBag (v, 1)) (_ILIT(0))
252 -- We want to make wrapper-style evaluation look cheap, so that
253 -- when we inline a wrapper it doesn't make call site (much) bigger
254 -- Otherwise we get nasty phase ordering stuff:
257 -- If we inline g's wrapper, f looks big, and doesn't get inlined
258 -- into h; if we inline f first, while it looks small, then g's
259 -- wrapper will get inlined later anyway. To avoid this nasty
260 -- ordering difference, we make (case a of (x,y) -> ...),
261 -- *where a is one of the arguments* look free.
265 alts_size (foldr addSize sizeOne alt_sizes) -- The 1 is for the scrutinee
266 (foldr1 maxSize alt_sizes)
268 -- Good to inline if an arg is scrutinised, because
269 -- that may eliminate allocation in the caller
270 -- And it eliminates the case itself
273 alt_sizes = map size_up_alt alts
275 -- alts_size tries to compute a good discount for
276 -- the case when we are scrutinising an argument variable
277 alts_size (SizeIs tot _tot_disc _tot_scrut) -- Size of all alternatives
278 (SizeIs max max_disc max_scrut) -- Size of biggest alternative
279 = SizeIs tot (unitBag (v, iBox (_ILIT(1) +# tot -# max)) `unionBags` max_disc) max_scrut
280 -- If the variable is known, we produce a discount that
281 -- will take us back to 'max', the size of rh largest alternative
282 -- The 1+ is a little discount for reduced allocation in the caller
283 alts_size tot_size _ = tot_size
285 size_up (Case e _ _ alts) = nukeScrutDiscount (size_up e) `addSize`
286 foldr (addSize . size_up_alt) sizeZero alts
287 -- We don't charge for the case itself
288 -- It's a strict thing, and the price of the call
289 -- is paid by scrut. Also consider
290 -- case f x of DEFAULT -> e
291 -- This is just ';'! Don't charge for it.
294 size_up_app (App fun arg) args
295 | isTypeArg arg = size_up_app fun args
296 | otherwise = size_up_app fun (arg:args)
297 size_up_app fun args = foldr (addSize . nukeScrutDiscount . size_up)
298 (size_up_fun fun args)
301 -- A function application with at least one value argument
302 -- so if the function is an argument give it an arg-discount
304 -- Also behave specially if the function is a build
306 -- Also if the function is a constant Id (constr or primop)
307 -- compute discounts specially
308 size_up_fun (Var fun) args
309 | fun `hasKey` buildIdKey = buildSize
310 | fun `hasKey` augmentIdKey = augmentSize
312 = case idDetails fun of
313 DataConWorkId dc -> conSizeN dc (valArgCount args)
315 FCallId _ -> sizeN opt_UF_DearOp
316 PrimOpId op -> primOpSize op (valArgCount args)
317 -- foldr addSize (primOpSize op) (map arg_discount args)
318 -- At one time I tried giving an arg-discount if a primop
319 -- is applied to one of the function's arguments, but it's
320 -- not good. At the moment, any unlifted-type arg gets a
321 -- 'True' for 'yes I'm evald', so we collect the discount even
322 -- if we know nothing about it. And just having it in a primop
323 -- doesn't help at all if we don't know something more.
325 _ -> fun_discount fun `addSizeN`
326 (1 + length (filter (not . exprIsTrivial) args))
327 -- The 1+ is for the function itself
328 -- Add 1 for each non-trivial arg;
329 -- the allocation cost, as in let(rec)
330 -- Slight hack here: for constructors the args are almost always
331 -- trivial; and for primops they are almost always prim typed
332 -- We should really only count for non-prim-typed args in the
333 -- general case, but that seems too much like hard work
335 size_up_fun other _ = size_up other
338 size_up_alt (_con, _bndrs, rhs) = size_up rhs
339 -- Don't charge for args, so that wrappers look cheap
340 -- (See comments about wrappers with Case)
343 -- We want to record if we're case'ing, or applying, an argument
344 fun_discount v | v `elem` top_args = SizeIs (_ILIT(0)) (unitBag (v, opt_UF_FunAppDiscount)) (_ILIT(0))
345 fun_discount _ = sizeZero
348 -- These addSize things have to be here because
349 -- I don't want to give them bOMB_OUT_SIZE as an argument
351 addSizeN TooBig _ = TooBig
352 addSizeN (SizeIs n xs d) m = mkSizeIs bOMB_OUT_SIZE (n +# iUnbox m) xs d
354 addSize TooBig _ = TooBig
355 addSize _ TooBig = TooBig
356 addSize (SizeIs n1 xs d1) (SizeIs n2 ys d2)
357 = mkSizeIs bOMB_OUT_SIZE (n1 +# n2) (xs `unionBags` ys) (d1 +# d2)
360 Code for manipulating sizes
363 data ExprSize = TooBig
364 | SizeIs FastInt -- Size found
365 (Bag (Id,Int)) -- Arguments cased herein, and discount for each such
366 FastInt -- Size to subtract if result is scrutinised
367 -- by a case expression
369 -- subtract the discount before deciding whether to bale out. eg. we
370 -- want to inline a large constructor application into a selector:
371 -- tup = (a_1, ..., a_99)
372 -- x = case tup of ...
374 mkSizeIs :: FastInt -> FastInt -> Bag (Id, Int) -> FastInt -> ExprSize
375 mkSizeIs max n xs d | (n -# d) ># max = TooBig
376 | otherwise = SizeIs n xs d
378 maxSize :: ExprSize -> ExprSize -> ExprSize
379 maxSize TooBig _ = TooBig
380 maxSize _ TooBig = TooBig
381 maxSize s1@(SizeIs n1 _ _) s2@(SizeIs n2 _ _) | n1 ># n2 = s1
384 sizeZero, sizeOne :: ExprSize
385 sizeN :: Int -> ExprSize
386 conSizeN :: DataCon ->Int -> ExprSize
388 sizeZero = SizeIs (_ILIT(0)) emptyBag (_ILIT(0))
389 sizeOne = SizeIs (_ILIT(1)) emptyBag (_ILIT(0))
390 sizeN n = SizeIs (iUnbox n) emptyBag (_ILIT(0))
392 | isUnboxedTupleCon dc = SizeIs (_ILIT(0)) emptyBag (iUnbox n +# _ILIT(1))
393 | otherwise = SizeIs (_ILIT(1)) emptyBag (iUnbox n +# _ILIT(1))
394 -- Treat constructors as size 1; we are keen to expose them
395 -- (and we charge separately for their args). We can't treat
396 -- them as size zero, else we find that (iBox x) has size 1,
397 -- which is the same as a lone variable; and hence 'v' will
398 -- always be replaced by (iBox x), where v is bound to iBox x.
400 -- However, unboxed tuples count as size zero
401 -- I found occasions where we had
402 -- f x y z = case op# x y z of { s -> (# s, () #) }
403 -- and f wasn't getting inlined
405 primOpSize :: PrimOp -> Int -> ExprSize
407 | not (primOpIsDupable op) = sizeN opt_UF_DearOp
408 | not (primOpOutOfLine op) = sizeN (2 - n_args)
409 -- Be very keen to inline simple primops.
410 -- We give a discount of 1 for each arg so that (op# x y z) costs 2.
411 -- We can't make it cost 1, else we'll inline let v = (op# x y z)
412 -- at every use of v, which is excessive.
414 -- A good example is:
415 -- let x = +# p q in C {x}
416 -- Even though x get's an occurrence of 'many', its RHS looks cheap,
417 -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
418 | otherwise = sizeOne
420 buildSize :: ExprSize
421 buildSize = SizeIs (_ILIT(-2)) emptyBag (_ILIT(4))
422 -- We really want to inline applications of build
423 -- build t (\cn -> e) should cost only the cost of e (because build will be inlined later)
424 -- Indeed, we should add a result_discount becuause build is
425 -- very like a constructor. We don't bother to check that the
426 -- build is saturated (it usually is). The "-2" discounts for the \c n,
427 -- The "4" is rather arbitrary.
429 augmentSize :: ExprSize
430 augmentSize = SizeIs (_ILIT(-2)) emptyBag (_ILIT(4))
431 -- Ditto (augment t (\cn -> e) ys) should cost only the cost of
432 -- e plus ys. The -2 accounts for the \cn
434 nukeScrutDiscount :: ExprSize -> ExprSize
435 nukeScrutDiscount (SizeIs n vs _) = SizeIs n vs (_ILIT(0))
436 nukeScrutDiscount TooBig = TooBig
438 -- When we return a lambda, give a discount if it's used (applied)
439 lamScrutDiscount :: ExprSize -> ExprSize
440 lamScrutDiscount (SizeIs n vs _) = case opt_UF_FunAppDiscount of { d -> SizeIs n vs (iUnbox d) }
441 lamScrutDiscount TooBig = TooBig
445 %************************************************************************
447 \subsection[considerUnfolding]{Given all the info, do (not) do the unfolding}
449 %************************************************************************
451 We have very limited information about an unfolding expression: (1)~so
452 many type arguments and so many value arguments expected---for our
453 purposes here, we assume we've got those. (2)~A ``size'' or ``cost,''
454 a single integer. (3)~An ``argument info'' vector. For this, what we
455 have at the moment is a Boolean per argument position that says, ``I
456 will look with great favour on an explicit constructor in this
457 position.'' (4)~The ``discount'' to subtract if the expression
458 is being scrutinised.
460 Assuming we have enough type- and value arguments (if not, we give up
461 immediately), then we see if the ``discounted size'' is below some
462 (semi-arbitrary) threshold. It works like this: for every argument
463 position where we're looking for a constructor AND WE HAVE ONE in our
464 hands, we get a (again, semi-arbitrary) discount [proportion to the
465 number of constructors in the type being scrutinized].
467 If we're in the context of a scrutinee ( \tr{(case <expr > of A .. -> ...;.. )})
468 and the expression in question will evaluate to a constructor, we use
469 the computed discount size *for the result only* rather than
470 computing the argument discounts. Since we know the result of
471 the expression is going to be taken apart, discounting its size
472 is more accurate (see @sizeExpr@ above for how this discount size
475 We use this one to avoid exporting inlinings that we ``couldn't possibly
476 use'' on the other side. Can be overridden w/ flaggery.
477 Just the same as smallEnoughToInline, except that it has no actual arguments.
480 couldBeSmallEnoughToInline :: Int -> CoreExpr -> Bool
481 couldBeSmallEnoughToInline threshold rhs = case calcUnfoldingGuidance threshold rhs of
485 certainlyWillInline :: Unfolding -> Bool
486 -- Sees if the unfolding is pretty certain to inline
487 certainlyWillInline (CoreUnfolding _ _ _ is_cheap (UnfoldIfGoodArgs n_vals _ size _))
488 = is_cheap && size - (n_vals +1) <= opt_UF_UseThreshold
489 certainlyWillInline _
492 smallEnoughToInline :: Unfolding -> Bool
493 smallEnoughToInline (CoreUnfolding _ _ _ _ (UnfoldIfGoodArgs _ _ size _))
494 = size <= opt_UF_UseThreshold
495 smallEnoughToInline _
499 %************************************************************************
501 \subsection{callSiteInline}
503 %************************************************************************
505 This is the key function. It decides whether to inline a variable at a call site
507 callSiteInline is used at call sites, so it is a bit more generous.
508 It's a very important function that embodies lots of heuristics.
509 A non-WHNF can be inlined if it doesn't occur inside a lambda,
510 and occurs exactly once or
511 occurs once in each branch of a case and is small
513 If the thing is in WHNF, there's no danger of duplicating work,
514 so we can inline if it occurs once, or is small
516 NOTE: we don't want to inline top-level functions that always diverge.
517 It just makes the code bigger. Tt turns out that the convenient way to prevent
518 them inlining is to give them a NOINLINE pragma, which we do in
519 StrictAnal.addStrictnessInfoToTopId
522 callSiteInline :: DynFlags
523 -> Bool -- True <=> the Id can be inlined
525 -> Bool -- True if there are are no arguments at all (incl type args)
526 -> [Bool] -- One for each value arg; True if it is interesting
527 -> CallCtxt -- True <=> continuation is interesting
528 -> Maybe CoreExpr -- Unfolding, if any
531 data CallCtxt = BoringCtxt
533 | ArgCtxt Bool -- We're somewhere in the RHS of function with rules
534 -- => be keener to inline
535 Int -- We *are* the argument of a function with this arg discount
536 -- => be keener to inline
537 -- INVARIANT: ArgCtxt False 0 ==> BoringCtxt
539 | ValAppCtxt -- We're applied to at least one value arg
540 -- This arises when we have ((f x |> co) y)
541 -- Then the (f x) has argument 'x' but in a ValAppCtxt
543 | CaseCtxt -- We're the scrutinee of a case
544 -- that decomposes its scrutinee
546 instance Outputable CallCtxt where
547 ppr BoringCtxt = ptext (sLit "BoringCtxt")
548 ppr (ArgCtxt _ _) = ptext (sLit "ArgCtxt")
549 ppr CaseCtxt = ptext (sLit "CaseCtxt")
550 ppr ValAppCtxt = ptext (sLit "ValAppCtxt")
552 callSiteInline dflags active_inline id lone_variable arg_infos cont_info
553 = case idUnfolding id of {
554 NoUnfolding -> Nothing ;
555 OtherCon _ -> Nothing ;
557 CompulsoryUnfolding unf_template -> Just unf_template ;
558 -- CompulsoryUnfolding => there is no top-level binding
559 -- for these things, so we must inline it.
560 -- Only a couple of primop-like things have
561 -- compulsory unfoldings (see MkId.lhs).
562 -- We don't allow them to be inactive
564 CoreUnfolding unf_template is_top is_value is_cheap guidance ->
567 result | yes_or_no = Just unf_template
568 | otherwise = Nothing
570 n_val_args = length arg_infos
572 yes_or_no = active_inline && is_cheap && consider_safe
573 -- We consider even the once-in-one-branch
574 -- occurrences, because they won't all have been
575 -- caught by preInlineUnconditionally. In particular,
576 -- if the occurrence is once inside a lambda, and the
577 -- rhs is cheap but not a manifest lambda, then
578 -- pre-inline will not have inlined it for fear of
579 -- invalidating the occurrence info in the rhs.
582 -- consider_safe decides whether it's a good idea to
583 -- inline something, given that there's no
584 -- work-duplication issue (the caller checks that).
587 UnfoldIfGoodArgs n_vals_wanted arg_discounts size res_discount
588 | enough_args && size <= (n_vals_wanted + 1)
589 -- Inline unconditionally if there no size increase
590 -- Size of call is n_vals_wanted (+1 for the function)
594 -> some_benefit && small_enough && inline_enough_args
597 enough_args = n_val_args >= n_vals_wanted
599 not (dopt Opt_InlineIfEnoughArgs dflags) || enough_args
602 some_benefit = or arg_infos || really_interesting_cont
603 -- There must be something interesting
604 -- about some argument, or the result
605 -- context, to make it worth inlining
607 really_interesting_cont
608 | n_val_args < n_vals_wanted = False -- Too few args
609 | n_val_args == n_vals_wanted = interesting_saturated_call
610 | otherwise = True -- Extra args
611 -- really_interesting_cont tells if the result of the
612 -- call is in an interesting context.
614 interesting_saturated_call
616 BoringCtxt -> not is_top && n_vals_wanted > 0 -- Note [Nested functions]
617 CaseCtxt -> not lone_variable || not is_value -- Note [Lone variables]
618 ArgCtxt {} -> n_vals_wanted > 0 -- Note [Inlining in ArgCtxt]
619 ValAppCtxt -> True -- Note [Cast then apply]
621 small_enough = (size - discount) <= opt_UF_UseThreshold
622 discount = computeDiscount n_vals_wanted arg_discounts
623 res_discount' arg_infos
624 res_discount' = case cont_info of
626 CaseCtxt -> res_discount
627 _other -> 4 `min` res_discount
628 -- res_discount can be very large when a function returns
629 -- construtors; but we only want to invoke that large discount
630 -- when there's a case continuation.
631 -- Otherwise we, rather arbitrarily, threshold it. Yuk.
632 -- But we want to aovid inlining large functions that return
633 -- constructors into contexts that are simply "interesting"
636 if dopt Opt_D_dump_inlinings dflags then
637 pprTrace ("Considering inlining: " ++ showSDoc (ppr id))
638 (vcat [text "active:" <+> ppr active_inline,
639 text "arg infos" <+> ppr arg_infos,
640 text "interesting continuation" <+> ppr cont_info,
641 text "is value:" <+> ppr is_value,
642 text "is cheap:" <+> ppr is_cheap,
643 text "guidance" <+> ppr guidance,
644 text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"])
651 Note [Nested functions]
652 ~~~~~~~~~~~~~~~~~~~~~~~
653 If a function has a nested defn we also record some-benefit, on the
654 grounds that we are often able to eliminate the binding, and hence the
655 allocation, for the function altogether; this is good for join points.
656 But this only makes sense for *functions*; inlining a constructor
657 doesn't help allocation unless the result is scrutinised. UNLESS the
658 constructor occurs just once, albeit possibly in multiple case
659 branches. Then inlining it doesn't increase allocation, but it does
660 increase the chance that the constructor won't be allocated at all in
661 the branches that don't use it.
663 Note [Cast then apply]
664 ~~~~~~~~~~~~~~~~~~~~~~
666 myIndex = __inline_me ( (/\a. <blah>) |> co )
667 co :: (forall a. a -> a) ~ (forall a. T a)
668 ... /\a.\x. case ((myIndex a) |> sym co) x of { ... } ...
670 We need to inline myIndex to unravel this; but the actual call (myIndex a) has
671 no value arguments. The ValAppCtxt gives it enough incentive to inline.
673 Note [Inlining in ArgCtxt]
674 ~~~~~~~~~~~~~~~~~~~~~~~~~~
675 The condition (n_vals_wanted > 0) here is very important, because otherwise
676 we end up inlining top-level stuff into useless places; eg
679 This can make a very big difference: it adds 16% to nofib 'integer' allocs,
682 At one stage I replaced this condition by 'True' (leading to the above
683 slow-down). The motivation was test eyeball/inline1.hs; but that seems
686 Note [Lone variables]
687 ~~~~~~~~~~~~~~~~~~~~~
688 The "lone-variable" case is important. I spent ages messing about
689 with unsatisfactory varaints, but this is nice. The idea is that if a
690 variable appears all alone
691 as an arg of lazy fn, or rhs Stop
692 as scrutinee of a case Select
693 as arg of a strict fn ArgOf
695 it is bound to a value
696 then we should not inline it (unless there is some other reason,
697 e.g. is is the sole occurrence). That is what is happening at
698 the use of 'lone_variable' in 'interesting_saturated_call'.
700 Why? At least in the case-scrutinee situation, turning
701 let x = (a,b) in case x of y -> ...
703 let x = (a,b) in case (a,b) of y -> ...
705 let x = (a,b) in let y = (a,b) in ...
706 is bad if the binding for x will remain.
708 Another example: I discovered that strings
709 were getting inlined straight back into applications of 'error'
710 because the latter is strict.
712 f = \x -> ...(error s)...
714 Fundamentally such contexts should not encourage inlining because the
715 context can ``see'' the unfolding of the variable (e.g. case or a
716 RULE) so there's no gain. If the thing is bound to a value.
721 foo = _inline_ (\n. [n])
722 bar = _inline_ (foo 20)
723 baz = \n. case bar of { (m:_) -> m + n }
724 Here we really want to inline 'bar' so that we can inline 'foo'
725 and the whole thing unravels as it should obviously do. This is
726 important: in the NDP project, 'bar' generates a closure data
727 structure rather than a list.
729 * Even a type application or coercion isn't a lone variable.
731 case $fMonadST @ RealWorld of { :DMonad a b c -> c }
732 We had better inline that sucker! The case won't see through it.
734 For now, I'm treating treating a variable applied to types
735 in a *lazy* context "lone". The motivating example was
738 There's no advantage in inlining f here, and perhaps
739 a significant disadvantage. Hence some_val_args in the Stop case
742 computeDiscount :: Int -> [Int] -> Int -> [Bool] -> Int
743 computeDiscount n_vals_wanted arg_discounts result_discount arg_infos
744 -- We multiple the raw discounts (args_discount and result_discount)
745 -- ty opt_UnfoldingKeenessFactor because the former have to do with
746 -- *size* whereas the discounts imply that there's some extra
747 -- *efficiency* to be gained (e.g. beta reductions, case reductions)
750 -- we also discount 1 for each argument passed, because these will
751 -- reduce with the lambdas in the function (we count 1 for a lambda
753 = 1 + -- Discount of 1 because the result replaces the call
754 -- so we count 1 for the function itself
755 length (take n_vals_wanted arg_infos) +
756 -- Discount of 1 for each arg supplied, because the
757 -- result replaces the call
758 round (opt_UF_KeenessFactor *
759 fromIntegral (arg_discount + result_discount))
761 arg_discount = sum (zipWith mk_arg_discount arg_discounts arg_infos)
763 mk_arg_discount discount is_evald | is_evald = discount
767 %************************************************************************
769 The Very Simple Optimiser
771 %************************************************************************
775 simpleOptExpr :: Subst -> CoreExpr -> CoreExpr
776 -- Return an occur-analysed and slightly optimised expression
777 -- The optimisation is very straightforward: just
778 -- inline non-recursive bindings that are used only once,
779 -- or wheere the RHS is trivial
781 simpleOptExpr subst expr
782 = go subst (occurAnalyseExpr expr)
784 go subst (Var v) = lookupIdSubst subst v
785 go subst (App e1 e2) = App (go subst e1) (go subst e2)
786 go subst (Type ty) = Type (substTy subst ty)
787 go _ (Lit lit) = Lit lit
788 go subst (Note note e) = Note note (go subst e)
789 go subst (Cast e co) = Cast (go subst e) (substTy subst co)
790 go subst (Let bind body) = go_bind subst bind body
791 go subst (Lam bndr body) = Lam bndr' (go subst' body)
793 (subst', bndr') = substBndr subst bndr
795 go subst (Case e b ty as) = Case (go subst e) b'
797 (map (go_alt subst') as)
799 (subst', b') = substBndr subst b
802 ----------------------
803 go_alt subst (con, bndrs, rhs) = (con, bndrs', go subst' rhs)
805 (subst', bndrs') = substBndrs subst bndrs
807 ----------------------
808 go_bind subst (Rec prs) body = Let (Rec (bndrs' `zip` rhss'))
811 (bndrs, rhss) = unzip prs
812 (subst', bndrs') = substRecBndrs subst bndrs
813 rhss' = map (go subst') rhss
815 go_bind subst (NonRec b r) body = go_nonrec subst b (go subst r) body
817 ----------------------
818 go_nonrec subst b (Type ty') body
819 | isTyVar b = go (extendTvSubst subst b ty') body
820 -- let a::* = TYPE ty in <body>
821 go_nonrec subst b r' body
822 | isId b -- let x = e in <body>
823 , exprIsTrivial r' || safe_to_inline (idOccInfo b)
824 = go (extendIdSubst subst b r') body
825 go_nonrec subst b r' body
826 = Let (NonRec b' r') (go subst' body)
828 (subst', b') = substBndr subst b
830 ----------------------
831 -- Unconditionally safe to inline
832 safe_to_inline :: OccInfo -> Bool
833 safe_to_inline IAmDead = True
834 safe_to_inline (OneOcc in_lam one_br _) = not in_lam && one_br
835 safe_to_inline (IAmALoopBreaker {}) = False
836 safe_to_inline NoOccInfo = False