2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
11 preInlineUnconditionally, postInlineUnconditionally, activeInline, activeRule,
14 -- The continuation type
15 SimplCont(..), DupFlag(..), LetRhsFlag(..),
16 contIsDupable, contResultType,
17 countValArgs, countArgs, pushContArgs,
18 mkBoringStop, mkLazyArgStop, mkRhsStop, contIsRhs, contIsRhsOrArg,
19 getContArgs, interestingCallContext, interestingArgContext,
20 interestingArg, isStrictType
24 #include "HsVersions.h"
27 import DynFlags ( SimplifierSwitch(..), SimplifierMode(..),
29 import StaticFlags ( opt_UF_UpdateInPlace, opt_SimplNoPreInlining,
32 import CoreFVs ( exprFreeVars )
33 import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial,
34 etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2,
35 findDefault, exprOkForSpeculation, exprIsHNF, mergeAlts
37 import Literal ( mkStringLit )
38 import CoreUnfold ( smallEnoughToInline )
39 import MkId ( eRROR_ID )
40 import Id ( Id, idType, isDataConWorkId, idOccInfo, isDictId,
41 isDeadBinder, idNewDemandInfo, isExportedId,
42 idUnfolding, idNewStrictness, idInlinePragma, idHasRules
44 import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
46 import Type ( Type, splitFunTys, dropForAlls, isStrictType,
47 splitTyConApp_maybe, tyConAppArgs
49 import TyCon ( tyConDataCons_maybe )
50 import DataCon ( dataConRepArity )
52 import BasicTypes ( TopLevelFlag(..), isNotTopLevel, OccInfo(..), isLoopBreaker, isOneOcc,
53 Activation, isAlwaysActive, isActive )
54 import Util ( lengthExceeds )
59 %************************************************************************
61 \subsection{The continuation data type}
63 %************************************************************************
66 data SimplCont -- Strict contexts
67 = Stop OutType -- Type of the result
69 Bool -- True <=> There is something interesting about
70 -- the context, and hence the inliner
71 -- should be a bit keener (see interestingCallContext)
73 -- (a) This is the RHS of a thunk whose type suggests
74 -- that update-in-place would be possible
75 -- (b) This is an argument of a function that has RULES
76 -- Inlining the call might allow the rule to fire
78 | CoerceIt OutType -- The To-type, simplified
81 | InlinePlease -- This continuation makes a function very
82 SimplCont -- keen to inline itelf
85 InExpr SimplEnv -- The argument, as yet unsimplified,
86 SimplCont -- and its environment
89 InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
92 | ArgOf LetRhsFlag -- An arbitrary strict context: the argument
93 -- of a strict function, or a primitive-arg fn
95 -- No DupFlag, because we never duplicate it
96 OutType -- arg_ty: type of the argument itself
97 OutType -- cont_ty: the type of the expression being sought by the context
98 -- f (error "foo") ==> coerce t (error "foo")
100 -- We need to know the type t, to which to coerce.
102 (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
103 -- The result expression in the OutExprStuff has type cont_ty
105 data LetRhsFlag = AnArg -- It's just an argument not a let RHS
106 | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
108 instance Outputable LetRhsFlag where
109 ppr AnArg = ptext SLIT("arg")
110 ppr AnRhs = ptext SLIT("rhs")
112 instance Outputable SimplCont where
113 ppr (Stop ty is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs) <+> ppr ty
114 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
115 ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...")
116 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
117 (nest 4 (ppr alts)) $$ ppr cont
118 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
119 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
121 data DupFlag = OkToDup | NoDup
123 instance Outputable DupFlag where
124 ppr OkToDup = ptext SLIT("ok")
125 ppr NoDup = ptext SLIT("nodup")
129 mkBoringStop :: OutType -> SimplCont
130 mkBoringStop ty = Stop ty AnArg False
132 mkLazyArgStop :: OutType -> Bool -> SimplCont
133 mkLazyArgStop ty has_rules = Stop ty AnArg (canUpdateInPlace ty || has_rules)
135 mkRhsStop :: OutType -> SimplCont
136 mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty)
138 contIsRhs :: SimplCont -> Bool
139 contIsRhs (Stop _ AnRhs _) = True
140 contIsRhs (ArgOf AnRhs _ _ _) = True
141 contIsRhs other = False
143 contIsRhsOrArg (Stop _ _ _) = True
144 contIsRhsOrArg (ArgOf _ _ _ _) = True
145 contIsRhsOrArg other = False
148 contIsDupable :: SimplCont -> Bool
149 contIsDupable (Stop _ _ _) = True
150 contIsDupable (ApplyTo OkToDup _ _ _) = True
151 contIsDupable (Select OkToDup _ _ _ _) = True
152 contIsDupable (CoerceIt _ cont) = contIsDupable cont
153 contIsDupable (InlinePlease cont) = contIsDupable cont
154 contIsDupable other = False
157 discardableCont :: SimplCont -> Bool
158 discardableCont (Stop _ _ _) = False
159 discardableCont (CoerceIt _ cont) = discardableCont cont
160 discardableCont (InlinePlease cont) = discardableCont cont
161 discardableCont other = True
163 discardCont :: SimplCont -- A continuation, expecting
164 -> SimplCont -- Replace the continuation with a suitable coerce
165 discardCont cont = case cont of
166 Stop to_ty is_rhs _ -> cont
167 other -> CoerceIt to_ty (mkBoringStop to_ty)
169 to_ty = contResultType cont
172 contResultType :: SimplCont -> OutType
173 contResultType (Stop to_ty _ _) = to_ty
174 contResultType (ArgOf _ _ to_ty _) = to_ty
175 contResultType (ApplyTo _ _ _ cont) = contResultType cont
176 contResultType (CoerceIt _ cont) = contResultType cont
177 contResultType (InlinePlease cont) = contResultType cont
178 contResultType (Select _ _ _ _ cont) = contResultType cont
181 countValArgs :: SimplCont -> Int
182 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
183 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
184 countValArgs other = 0
186 countArgs :: SimplCont -> Int
187 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
191 pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont
192 -- Pushes args with the specified environment
193 pushContArgs env [] cont = cont
194 pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont)
199 getContArgs :: SwitchChecker
200 -> OutId -> SimplCont
201 -> ([(InExpr, SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
202 SimplCont, -- Remaining continuation
203 Bool) -- Whether we came across an InlineCall
204 -- getContArgs id k = (args, k', inl)
205 -- args are the leading ApplyTo items in k
206 -- (i.e. outermost comes first)
207 -- augmented with demand info from the functionn
208 getContArgs chkr fun orig_cont
210 -- Ignore strictness info if the no-case-of-case
211 -- flag is on. Strictness changes evaluation order
212 -- and that can change full laziness
213 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
214 | otherwise = computed_stricts
216 go [] stricts False orig_cont
218 ----------------------------
221 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
222 = go ((arg,se,False) : acc) ss inl cont
223 -- NB: don't bother to instantiate the function type
226 go acc (s:ss) inl (ApplyTo _ arg se cont)
227 = go ((arg,se,s) : acc) ss inl cont
229 -- An Inline continuation
230 go acc ss inl (InlinePlease cont)
231 = go acc ss True cont
233 -- We're run out of arguments, or else we've run out of demands
234 -- The latter only happens if the result is guaranteed bottom
235 -- This is the case for
236 -- * case (error "hello") of { ... }
237 -- * (error "Hello") arg
238 -- * f (error "Hello") where f is strict
240 -- Then, especially in the first of these cases, we'd like to discard
241 -- the continuation, leaving just the bottoming expression. But the
242 -- type might not be right, so we may have to add a coerce.
244 | null ss && discardableCont cont = (reverse acc, discardCont cont, inl)
245 | otherwise = (reverse acc, cont, inl)
247 ----------------------------
248 vanilla_stricts, computed_stricts :: [Bool]
249 vanilla_stricts = repeat False
250 computed_stricts = zipWith (||) fun_stricts arg_stricts
252 ----------------------------
253 (val_arg_tys, _) = splitFunTys (dropForAlls (idType fun))
254 arg_stricts = map isStrictType val_arg_tys ++ repeat False
255 -- These argument types are used as a cheap and cheerful way to find
256 -- unboxed arguments, which must be strict. But it's an InType
257 -- and so there might be a type variable where we expect a function
258 -- type (the substitution hasn't happened yet). And we don't bother
259 -- doing the type applications for a polymorphic function.
260 -- Hence the splitFunTys*IgnoringForAlls*
262 ----------------------------
263 -- If fun_stricts is finite, it means the function returns bottom
264 -- after that number of value args have been consumed
265 -- Otherwise it's infinite, extended with False
267 = case splitStrictSig (idNewStrictness fun) of
268 (demands, result_info)
269 | not (demands `lengthExceeds` countValArgs orig_cont)
270 -> -- Enough args, use the strictness given.
271 -- For bottoming functions we used to pretend that the arg
272 -- is lazy, so that we don't treat the arg as an
273 -- interesting context. This avoids substituting
274 -- top-level bindings for (say) strings into
275 -- calls to error. But now we are more careful about
276 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
277 if isBotRes result_info then
278 map isStrictDmd demands -- Finite => result is bottom
280 map isStrictDmd demands ++ vanilla_stricts
282 other -> vanilla_stricts -- Not enough args, or no strictness
285 interestingArg :: OutExpr -> Bool
286 -- An argument is interesting if it has *some* structure
287 -- We are here trying to avoid unfolding a function that
288 -- is applied only to variables that have no unfolding
289 -- (i.e. they are probably lambda bound): f x y z
290 -- There is little point in inlining f here.
291 interestingArg (Var v) = hasSomeUnfolding (idUnfolding v)
292 -- Was: isValueUnfolding (idUnfolding v')
293 -- But that seems over-pessimistic
295 -- This accounts for an argument like
296 -- () or [], which is definitely interesting
297 interestingArg (Type _) = False
298 interestingArg (App fn (Type _)) = interestingArg fn
299 interestingArg (Note _ a) = interestingArg a
300 interestingArg other = True
301 -- Consider let x = 3 in f x
302 -- The substitution will contain (x -> ContEx 3), and we want to
303 -- to say that x is an interesting argument.
304 -- But consider also (\x. f x y) y
305 -- The substitution will contain (x -> ContEx y), and we want to say
306 -- that x is not interesting (assuming y has no unfolding)
309 Comment about interestingCallContext
310 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
311 We want to avoid inlining an expression where there can't possibly be
312 any gain, such as in an argument position. Hence, if the continuation
313 is interesting (eg. a case scrutinee, application etc.) then we
314 inline, otherwise we don't.
316 Previously some_benefit used to return True only if the variable was
317 applied to some value arguments. This didn't work:
319 let x = _coerce_ (T Int) Int (I# 3) in
320 case _coerce_ Int (T Int) x of
323 we want to inline x, but can't see that it's a constructor in a case
324 scrutinee position, and some_benefit is False.
328 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
330 .... case dMonadST _@_ x0 of (a,b,c) -> ....
332 we'd really like to inline dMonadST here, but we *don't* want to
333 inline if the case expression is just
335 case x of y { DEFAULT -> ... }
337 since we can just eliminate this case instead (x is in WHNF). Similar
338 applies when x is bound to a lambda expression. Hence
339 contIsInteresting looks for case expressions with just a single
343 interestingCallContext :: Bool -- False <=> no args at all
344 -> Bool -- False <=> no value args
346 -- The "lone-variable" case is important. I spent ages
347 -- messing about with unsatisfactory varaints, but this is nice.
348 -- The idea is that if a variable appear all alone
349 -- as an arg of lazy fn, or rhs Stop
350 -- as scrutinee of a case Select
351 -- as arg of a strict fn ArgOf
352 -- then we should not inline it (unless there is some other reason,
353 -- e.g. is is the sole occurrence). We achieve this by making
354 -- interestingCallContext return False for a lone variable.
356 -- Why? At least in the case-scrutinee situation, turning
357 -- let x = (a,b) in case x of y -> ...
359 -- let x = (a,b) in case (a,b) of y -> ...
361 -- let x = (a,b) in let y = (a,b) in ...
362 -- is bad if the binding for x will remain.
364 -- Another example: I discovered that strings
365 -- were getting inlined straight back into applications of 'error'
366 -- because the latter is strict.
368 -- f = \x -> ...(error s)...
370 -- Fundamentally such contexts should not ecourage inlining because
371 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
372 -- so there's no gain.
374 -- However, even a type application or coercion isn't a lone variable.
376 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
377 -- We had better inline that sucker! The case won't see through it.
379 -- For now, I'm treating treating a variable applied to types
380 -- in a *lazy* context "lone". The motivating example was
382 -- g = /\a. \y. h (f a)
383 -- There's no advantage in inlining f here, and perhaps
384 -- a significant disadvantage. Hence some_val_args in the Stop case
386 interestingCallContext some_args some_val_args cont
389 interesting (InlinePlease _) = True
390 interesting (Select _ _ _ _ _) = some_args
391 interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
392 -- Perhaps True is a bit over-keen, but I've
393 -- seen (coerce f) x, where f has an INLINE prag,
394 -- So we have to give some motivaiton for inlining it
395 interesting (ArgOf _ _ _ _) = some_val_args
396 interesting (Stop ty _ interesting) = some_val_args && interesting
397 interesting (CoerceIt _ cont) = interesting cont
398 -- If this call is the arg of a strict function, the context
399 -- is a bit interesting. If we inline here, we may get useful
400 -- evaluation information to avoid repeated evals: e.g.
402 -- Here the contIsInteresting makes the '*' keener to inline,
403 -- which in turn exposes a constructor which makes the '+' inline.
404 -- Assuming that +,* aren't small enough to inline regardless.
406 -- It's also very important to inline in a strict context for things
409 -- Here, the context of (f x) is strict, and if f's unfolding is
410 -- a build it's *great* to inline it here. So we must ensure that
411 -- the context for (f x) is not totally uninteresting.
415 interestingArgContext :: Id -> SimplCont -> Bool
416 -- If the argument has form (f x y), where x,y are boring,
417 -- and f is marked INLINE, then we don't want to inline f.
418 -- But if the context of the argument is
420 -- where g has rules, then we *do* want to inline f, in case it
421 -- exposes a rule that might fire. Similarly, if the context is
423 -- where h has rules, then we do want to inline f.
424 -- The interesting_arg_ctxt flag makes this happen; if it's
425 -- set, the inliner gets just enough keener to inline f
426 -- regardless of how boring f's arguments are, if it's marked INLINE
428 -- The alternative would be to *always* inline an INLINE function,
429 -- regardless of how boring its context is; but that seems overkill
430 -- For example, it'd mean that wrapper functions were always inlined
431 interestingArgContext fn cont
432 = idHasRules fn || go cont
434 go (InlinePlease c) = go c
435 go (Select {}) = False
436 go (ApplyTo {}) = False
438 go (CoerceIt _ c) = go c
439 go (Stop _ _ interesting) = interesting
442 canUpdateInPlace :: Type -> Bool
443 -- Consider let x = <wurble> in ...
444 -- If <wurble> returns an explicit constructor, we might be able
445 -- to do update in place. So we treat even a thunk RHS context
446 -- as interesting if update in place is possible. We approximate
447 -- this by seeing if the type has a single constructor with a
448 -- small arity. But arity zero isn't good -- we share the single copy
449 -- for that case, so no point in sharing.
452 | not opt_UF_UpdateInPlace = False
454 = case splitTyConApp_maybe ty of
456 Just (tycon, _) -> case tyConDataCons_maybe tycon of
457 Just [dc] -> arity == 1 || arity == 2
459 arity = dataConRepArity dc
465 %************************************************************************
467 \subsection{Decisions about inlining}
469 %************************************************************************
471 Inlining is controlled partly by the SimplifierMode switch. This has two
474 SimplGently (a) Simplifying before specialiser/full laziness
475 (b) Simplifiying inside INLINE pragma
476 (c) Simplifying the LHS of a rule
477 (d) Simplifying a GHCi expression or Template
480 SimplPhase n Used at all other times
482 The key thing about SimplGently is that it does no call-site inlining.
483 Before full laziness we must be careful not to inline wrappers,
484 because doing so inhibits floating
485 e.g. ...(case f x of ...)...
486 ==> ...(case (case x of I# x# -> fw x#) of ...)...
487 ==> ...(case x of I# x# -> case fw x# of ...)...
488 and now the redex (f x) isn't floatable any more.
490 The no-inlining thing is also important for Template Haskell. You might be
491 compiling in one-shot mode with -O2; but when TH compiles a splice before
492 running it, we don't want to use -O2. Indeed, we don't want to inline
493 anything, because the byte-code interpreter might get confused about
494 unboxed tuples and suchlike.
498 SimplGently is also used as the mode to simplify inside an InlineMe note.
501 inlineMode :: SimplifierMode
502 inlineMode = SimplGently
505 It really is important to switch off inlinings inside such
506 expressions. Consider the following example
512 in ...g...g...g...g...g...
514 Now, if that's the ONLY occurrence of f, it will be inlined inside g,
515 and thence copied multiple times when g is inlined.
518 This function may be inlinined in other modules, so we
519 don't want to remove (by inlining) calls to functions that have
520 specialisations, or that may have transformation rules in an importing
523 E.g. {-# INLINE f #-}
526 and suppose that g is strict *and* has specialisations. If we inline
527 g's wrapper, we deny f the chance of getting the specialised version
528 of g when f is inlined at some call site (perhaps in some other
531 It's also important not to inline a worker back into a wrapper.
533 wraper = inline_me (\x -> ...worker... )
534 Normally, the inline_me prevents the worker getting inlined into
535 the wrapper (initially, the worker's only call site!). But,
536 if the wrapper is sure to be called, the strictness analyser will
537 mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
538 continuation. That's why the keep_inline predicate returns True for
539 ArgOf continuations. It shouldn't do any harm not to dissolve the
540 inline-me note under these circumstances.
542 Note that the result is that we do very little simplification
545 all xs = foldr (&&) True xs
546 any p = all . map p {-# INLINE any #-}
548 Problem: any won't get deforested, and so if it's exported and the
549 importer doesn't use the inlining, (eg passes it as an arg) then we
550 won't get deforestation at all. We havn't solved this problem yet!
553 preInlineUnconditionally
554 ~~~~~~~~~~~~~~~~~~~~~~~~
555 @preInlineUnconditionally@ examines a bndr to see if it is used just
556 once in a completely safe way, so that it is safe to discard the
557 binding inline its RHS at the (unique) usage site, REGARDLESS of how
558 big the RHS might be. If this is the case we don't simplify the RHS
559 first, but just inline it un-simplified.
561 This is much better than first simplifying a perhaps-huge RHS and then
562 inlining and re-simplifying it. Indeed, it can be at least quadratically
571 We may end up simplifying e1 N times, e2 N-1 times, e3 N-3 times etc.
572 This can happen with cascades of functions too:
579 THE MAIN INVARIANT is this:
581 ---- preInlineUnconditionally invariant -----
582 IF preInlineUnconditionally chooses to inline x = <rhs>
583 THEN doing the inlining should not change the occurrence
584 info for the free vars of <rhs>
585 ----------------------------------------------
587 For example, it's tempting to look at trivial binding like
589 and inline it unconditionally. But suppose x is used many times,
590 but this is the unique occurrence of y. Then inlining x would change
591 y's occurrence info, which breaks the invariant. It matters: y
592 might have a BIG rhs, which will now be dup'd at every occurrenc of x.
595 Evne RHSs labelled InlineMe aren't caught here, because there might be
596 no benefit from inlining at the call site.
598 [Sept 01] Don't unconditionally inline a top-level thing, because that
599 can simply make a static thing into something built dynamically. E.g.
603 [Remember that we treat \s as a one-shot lambda.] No point in
604 inlining x unless there is something interesting about the call site.
606 But watch out: if you aren't careful, some useful foldr/build fusion
607 can be lost (most notably in spectral/hartel/parstof) because the
608 foldr didn't see the build. Doing the dynamic allocation isn't a big
609 deal, in fact, but losing the fusion can be. But the right thing here
610 seems to be to do a callSiteInline based on the fact that there is
611 something interesting about the call site (it's strict). Hmm. That
614 Conclusion: inline top level things gaily until Phase 0 (the last
615 phase), at which point don't.
618 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
619 preInlineUnconditionally env top_lvl bndr rhs
621 | opt_SimplNoPreInlining = False
622 | otherwise = case idOccInfo bndr of
623 IAmDead -> True -- Happens in ((\x.1) v)
624 OneOcc in_lam True int_cxt -> try_once in_lam int_cxt
628 active = case phase of
629 SimplGently -> isAlwaysActive prag
630 SimplPhase n -> isActive n prag
631 prag = idInlinePragma bndr
633 try_once in_lam int_cxt -- There's one textual occurrence
634 | not in_lam = isNotTopLevel top_lvl || early_phase
635 | otherwise = int_cxt && canInlineInLam rhs
637 -- Be very careful before inlining inside a lambda, becuase (a) we must not
638 -- invalidate occurrence information, and (b) we want to avoid pushing a
639 -- single allocation (here) into multiple allocations (inside lambda).
640 -- Inlining a *function* with a single *saturated* call would be ok, mind you.
641 -- || (if is_cheap && not (canInlineInLam rhs) then pprTrace "preinline" (ppr bndr <+> ppr rhs) ok else ok)
643 -- is_cheap = exprIsCheap rhs
644 -- ok = is_cheap && int_cxt
646 -- int_cxt The context isn't totally boring
647 -- E.g. let f = \ab.BIG in \y. map f xs
648 -- Don't want to substitute for f, because then we allocate
649 -- its closure every time the \y is called
650 -- But: let f = \ab.BIG in \y. map (f y) xs
651 -- Now we do want to substitute for f, even though it's not
652 -- saturated, because we're going to allocate a closure for
653 -- (f y) every time round the loop anyhow.
655 -- canInlineInLam => free vars of rhs are (Once in_lam) or Many,
656 -- so substituting rhs inside a lambda doesn't change the occ info.
657 -- Sadly, not quite the same as exprIsHNF.
658 canInlineInLam (Lit l) = True
659 canInlineInLam (Lam b e) = isRuntimeVar b || canInlineInLam e
660 canInlineInLam (Note _ e) = canInlineInLam e
661 canInlineInLam _ = False
663 early_phase = case phase of
664 SimplPhase 0 -> False
666 -- If we don't have this early_phase test, consider
667 -- x = length [1,2,3]
668 -- The full laziness pass carefully floats all the cons cells to
669 -- top level, and preInlineUnconditionally floats them all back in.
670 -- Result is (a) static allocation replaced by dynamic allocation
671 -- (b) many simplifier iterations because this tickles
672 -- a related problem; only one inlining per pass
674 -- On the other hand, I have seen cases where top-level fusion is
675 -- lost if we don't inline top level thing (e.g. string constants)
676 -- Hence the test for phase zero (which is the phase for all the final
677 -- simplifications). Until phase zero we take no special notice of
678 -- top level things, but then we become more leery about inlining
683 postInlineUnconditionally
684 ~~~~~~~~~~~~~~~~~~~~~~~~~
685 @postInlineUnconditionally@ decides whether to unconditionally inline
686 a thing based on the form of its RHS; in particular if it has a
687 trivial RHS. If so, we can inline and discard the binding altogether.
689 NB: a loop breaker has must_keep_binding = True and non-loop-breakers
690 only have *forward* references Hence, it's safe to discard the binding
692 NOTE: This isn't our last opportunity to inline. We're at the binding
693 site right now, and we'll get another opportunity when we get to the
696 Note that we do this unconditional inlining only for trival RHSs.
697 Don't inline even WHNFs inside lambdas; doing so may simply increase
698 allocation when the function is called. This isn't the last chance; see
701 NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here Why?
702 Because we don't even want to inline them into the RHS of constructor
703 arguments. See NOTE above
705 NB: At one time even NOINLINE was ignored here: if the rhs is trivial
706 it's best to inline it anyway. We often get a=E; b=a from desugaring,
707 with both a and b marked NOINLINE. But that seems incompatible with
708 our new view that inlining is like a RULE, so I'm sticking to the 'active'
712 postInlineUnconditionally :: SimplEnv -> TopLevelFlag -> OutId -> OccInfo -> OutExpr -> Unfolding -> Bool
713 postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding
715 | isLoopBreaker occ_info = False
716 | isExportedId bndr = False
717 | exprIsTrivial rhs = True
720 OneOcc in_lam one_br int_cxt
721 -> (one_br || smallEnoughToInline unfolding) -- Small enough to dup
722 -- ToDo: consider discount on smallEnoughToInline if int_cxt is true
724 -- NB: Do we want to inline arbitrarily big things becuase
725 -- one_br is True? that can lead to inline cascades. But
726 -- preInlineUnconditionlly has dealt with all the common cases
727 -- so perhaps it's worth the risk. Here's an example
728 -- let f = if b then Left (\x.BIG) else Right (\y.BIG)
730 -- We can't preInlineUnconditionally because that woud invalidate
731 -- the occ info for b. Yet f is used just once, and duplicating
732 -- the case work is fine (exprIsCheap).
734 && ((isNotTopLevel top_lvl && not in_lam) ||
735 -- But outside a lambda, we want to be reasonably aggressive
736 -- about inlining into multiple branches of case
737 -- e.g. let x = <non-value>
738 -- in case y of { C1 -> ..x..; C2 -> ..x..; C3 -> ... }
739 -- Inlining can be a big win if C3 is the hot-spot, even if
740 -- the uses in C1, C2 are not 'interesting'
741 -- An example that gets worse if you add int_cxt here is 'clausify'
743 (isCheapUnfolding unfolding && int_cxt))
744 -- isCheap => acceptable work duplication; in_lam may be true
745 -- int_cxt to prevent us inlining inside a lambda without some
746 -- good reason. See the notes on int_cxt in preInlineUnconditionally
749 -- The point here is that for *non-values* that occur
750 -- outside a lambda, the call-site inliner won't have
751 -- a chance (becuase it doesn't know that the thing
752 -- only occurs once). The pre-inliner won't have gotten
753 -- it either, if the thing occurs in more than one branch
754 -- So the main target is things like
757 -- True -> case x of ...
758 -- False -> case x of ...
759 -- I'm not sure how important this is in practice
761 active = case getMode env of
762 SimplGently -> isAlwaysActive prag
763 SimplPhase n -> isActive n prag
764 prag = idInlinePragma bndr
766 activeInline :: SimplEnv -> OutId -> OccInfo -> Bool
767 activeInline env id occ
768 = case getMode env of
769 SimplGently -> isOneOcc occ && isAlwaysActive prag
770 -- No inlining at all when doing gentle stuff,
771 -- except for local things that occur once
772 -- The reason is that too little clean-up happens if you
773 -- don't inline use-once things. Also a bit of inlining is *good* for
774 -- full laziness; it can expose constant sub-expressions.
775 -- Example in spectral/mandel/Mandel.hs, where the mandelset
776 -- function gets a useful let-float if you inline windowToViewport
778 -- NB: we used to have a second exception, for data con wrappers.
779 -- On the grounds that we use gentle mode for rule LHSs, and
780 -- they match better when data con wrappers are inlined.
781 -- But that only really applies to the trivial wrappers (like (:)),
782 -- and they are now constructed as Compulsory unfoldings (in MkId)
783 -- so they'll happen anyway.
785 SimplPhase n -> isActive n prag
787 prag = idInlinePragma id
789 activeRule :: SimplEnv -> Maybe (Activation -> Bool)
790 -- Nothing => No rules at all
792 | opt_RulesOff = Nothing
794 = case getMode env of
795 SimplGently -> Just isAlwaysActive
796 -- Used to be Nothing (no rules in gentle mode)
797 -- Main motivation for changing is that I wanted
798 -- lift String ===> ...
799 -- to work in Template Haskell when simplifying
800 -- splices, so we get simpler code for literal strings
801 SimplPhase n -> Just (isActive n)
805 %************************************************************************
807 \subsection{Rebuilding a lambda}
809 %************************************************************************
812 mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
816 a) eta reduction, if that gives a trivial expression
817 b) eta expansion [only if there are some value lambdas]
818 c) floating lets out through big lambdas
819 [only if all tyvar lambdas, and only if this lambda
823 mkLam env bndrs body cont
824 = getDOptsSmpl `thenSmpl` \dflags ->
825 mkLam' dflags env bndrs body cont
827 mkLam' dflags env bndrs body cont
828 | dopt Opt_DoEtaReduction dflags,
829 Just etad_lam <- tryEtaReduce bndrs body
830 = tick (EtaReduction (head bndrs)) `thenSmpl_`
831 returnSmpl (emptyFloats env, etad_lam)
833 | dopt Opt_DoLambdaEtaExpansion dflags,
834 any isRuntimeVar bndrs
835 = tryEtaExpansion body `thenSmpl` \ body' ->
836 returnSmpl (emptyFloats env, mkLams bndrs body')
838 {- Sept 01: I'm experimenting with getting the
839 full laziness pass to float out past big lambdsa
840 | all isTyVar bndrs, -- Only for big lambdas
841 contIsRhs cont -- Only try the rhs type-lambda floating
842 -- if this is indeed a right-hand side; otherwise
843 -- we end up floating the thing out, only for float-in
844 -- to float it right back in again!
845 = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
846 returnSmpl (floats, mkLams bndrs body')
850 = returnSmpl (emptyFloats env, mkLams bndrs body)
854 %************************************************************************
856 \subsection{Eta expansion and reduction}
858 %************************************************************************
860 We try for eta reduction here, but *only* if we get all the
861 way to an exprIsTrivial expression.
862 We don't want to remove extra lambdas unless we are going
863 to avoid allocating this thing altogether
866 tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr
867 tryEtaReduce bndrs body
868 -- We don't use CoreUtils.etaReduce, because we can be more
870 -- (a) we already have the binders
871 -- (b) we can do the triviality test before computing the free vars
872 = go (reverse bndrs) body
874 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
875 go [] fun | ok_fun fun = Just fun -- Success!
876 go _ _ = Nothing -- Failure!
878 ok_fun fun = exprIsTrivial fun
879 && not (any (`elemVarSet` (exprFreeVars fun)) bndrs)
880 && (exprIsHNF fun || all ok_lam bndrs)
881 ok_lam v = isTyVar v || isDictId v
882 -- The exprIsHNF is because eta reduction is not
883 -- valid in general: \x. bot /= bot
884 -- So we need to be sure that the "fun" is a value.
886 -- However, we always want to reduce (/\a -> f a) to f
887 -- This came up in a RULE: foldr (build (/\a -> g a))
888 -- did not match foldr (build (/\b -> ...something complex...))
889 -- The type checker can insert these eta-expanded versions,
890 -- with both type and dictionary lambdas; hence the slightly
893 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
897 Try eta expansion for RHSs
900 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
903 where (in both cases)
905 * The xi can include type variables
907 * The yi are all value variables
909 * N is a NORMAL FORM (i.e. no redexes anywhere)
910 wanting a suitable number of extra args.
912 We may have to sandwich some coerces between the lambdas
913 to make the types work. exprEtaExpandArity looks through coerces
914 when computing arity; and etaExpand adds the coerces as necessary when
915 actually computing the expansion.
918 tryEtaExpansion :: OutExpr -> SimplM OutExpr
919 -- There is at least one runtime binder in the binders
921 = getUniquesSmpl `thenSmpl` \ us ->
922 returnSmpl (etaExpand fun_arity us body (exprType body))
924 fun_arity = exprEtaExpandArity body
928 %************************************************************************
930 \subsection{Floating lets out of big lambdas}
932 %************************************************************************
934 tryRhsTyLam tries this transformation, when the big lambda appears as
935 the RHS of a let(rec) binding:
937 /\abc -> let(rec) x = e in b
939 let(rec) x' = /\abc -> let x = x' a b c in e
941 /\abc -> let x = x' a b c in b
943 This is good because it can turn things like:
945 let f = /\a -> letrec g = ... g ... in g
947 letrec g' = /\a -> ... g' a ...
951 which is better. In effect, it means that big lambdas don't impede
954 This optimisation is CRUCIAL in eliminating the junk introduced by
955 desugaring mutually recursive definitions. Don't eliminate it lightly!
957 So far as the implementation is concerned:
959 Invariant: go F e = /\tvs -> F e
963 = Let x' = /\tvs -> F e
967 G = F . Let x = x' tvs
969 go F (Letrec xi=ei in b)
970 = Letrec {xi' = /\tvs -> G ei}
974 G = F . Let {xi = xi' tvs}
976 [May 1999] If we do this transformation *regardless* then we can
977 end up with some pretty silly stuff. For example,
980 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
985 st = /\s -> ...[y1 s/x1, y2 s/x2]
988 Unless the "..." is a WHNF there is really no point in doing this.
989 Indeed it can make things worse. Suppose x1 is used strictly,
992 x1* = case f y of { (a,b) -> e }
994 If we abstract this wrt the tyvar we then can't do the case inline
995 as we would normally do.
999 {- Trying to do this in full laziness
1001 tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
1002 -- Call ensures that all the binders are type variables
1004 tryRhsTyLam env tyvars body -- Only does something if there's a let
1005 | not (all isTyVar tyvars)
1006 || not (worth_it body) -- inside a type lambda,
1007 = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
1010 = go env (\x -> x) body
1013 worth_it e@(Let _ _) = whnf_in_middle e
1016 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
1017 whnf_in_middle (Let _ e) = whnf_in_middle e
1018 whnf_in_middle e = exprIsCheap e
1020 main_tyvar_set = mkVarSet tyvars
1022 go env fn (Let bind@(NonRec var rhs) body)
1024 = go env (fn . Let bind) body
1026 go env fn (Let (NonRec var rhs) body)
1027 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
1028 addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
1029 go env (fn . Let (mk_silly_bind var rhs')) body
1033 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
1034 -- Abstract only over the type variables free in the rhs
1035 -- wrt which the new binding is abstracted. But the naive
1036 -- approach of abstract wrt the tyvars free in the Id's type
1038 -- /\ a b -> let t :: (a,b) = (e1, e2)
1041 -- Here, b isn't free in x's type, but we must nevertheless
1042 -- abstract wrt b as well, because t's type mentions b.
1043 -- Since t is floated too, we'd end up with the bogus:
1044 -- poly_t = /\ a b -> (e1, e2)
1045 -- poly_x = /\ a -> fst (poly_t a *b*)
1046 -- So for now we adopt the even more naive approach of
1047 -- abstracting wrt *all* the tyvars. We'll see if that
1048 -- gives rise to problems. SLPJ June 98
1050 go env fn (Let (Rec prs) body)
1051 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
1053 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
1054 pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
1056 addAuxiliaryBind env (Rec pairs) $ \ env ->
1059 (vars,rhss) = unzip prs
1060 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
1061 -- See notes with tyvars_here above
1063 go env fn body = returnSmpl (emptyFloats env, fn body)
1065 mk_poly tyvars_here var
1066 = getUniqueSmpl `thenSmpl` \ uniq ->
1068 poly_name = setNameUnique (idName var) uniq -- Keep same name
1069 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
1070 poly_id = mkLocalId poly_name poly_ty
1072 -- In the olden days, it was crucial to copy the occInfo of the original var,
1073 -- because we were looking at occurrence-analysed but as yet unsimplified code!
1074 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
1075 -- at already simplified code, so it doesn't matter
1077 -- It's even right to retain single-occurrence or dead-var info:
1078 -- Suppose we started with /\a -> let x = E in B
1079 -- where x occurs once in B. Then we transform to:
1080 -- let x' = /\a -> E in /\a -> let x* = x' a in B
1081 -- where x* has an INLINE prag on it. Now, once x* is inlined,
1082 -- the occurrences of x' will be just the occurrences originally
1085 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
1087 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
1088 -- Suppose we start with:
1090 -- x = /\ a -> let g = G in E
1092 -- Then we'll float to get
1094 -- x = let poly_g = /\ a -> G
1095 -- in /\ a -> let g = poly_g a in E
1097 -- But now the occurrence analyser will see just one occurrence
1098 -- of poly_g, not inside a lambda, so the simplifier will
1099 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
1100 -- (I used to think that the "don't inline lone occurrences" stuff
1101 -- would stop this happening, but since it's the *only* occurrence,
1102 -- PreInlineUnconditionally kicks in first!)
1104 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
1105 -- to appear many times. (NB: mkInlineMe eliminates
1106 -- such notes on trivial RHSs, so do it manually.)
1110 %************************************************************************
1112 \subsection{Case absorption and identity-case elimination}
1114 %************************************************************************
1116 mkCase puts a case expression back together, trying various transformations first.
1119 mkCase :: OutExpr -> OutId -> OutType
1120 -> [OutAlt] -- Increasing order
1123 mkCase scrut case_bndr ty alts
1124 = getDOptsSmpl `thenSmpl` \dflags ->
1125 mkAlts dflags scrut case_bndr alts `thenSmpl` \ better_alts ->
1126 mkCase1 scrut case_bndr ty better_alts
1130 mkAlts tries these things:
1132 1. If several alternatives are identical, merge them into
1133 a single DEFAULT alternative. I've occasionally seen this
1134 making a big difference:
1136 case e of =====> case e of
1137 C _ -> f x D v -> ....v....
1138 D v -> ....v.... DEFAULT -> f x
1141 The point is that we merge common RHSs, at least for the DEFAULT case.
1142 [One could do something more elaborate but I've never seen it needed.]
1143 To avoid an expensive test, we just merge branches equal to the *first*
1144 alternative; this picks up the common cases
1145 a) all branches equal
1146 b) some branches equal to the DEFAULT (which occurs first)
1149 case e of b { ==> case e of b {
1150 p1 -> rhs1 p1 -> rhs1
1152 pm -> rhsm pm -> rhsm
1153 _ -> case b of b' { pn -> let b'=b in rhsn
1155 ... po -> let b'=b in rhso
1156 po -> rhso _ -> let b'=b in rhsd
1160 which merges two cases in one case when -- the default alternative of
1161 the outer case scrutises the same variable as the outer case This
1162 transformation is called Case Merging. It avoids that the same
1163 variable is scrutinised multiple times.
1166 The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
1172 where @is@ was something like
1174 p `is` n = p /= (-1) && p == n
1176 This gave rise to a horrible sequence of cases
1183 and similarly in cascade for all the join points!
1188 --------------------------------------------------
1189 -- 1. Merge identical branches
1190 --------------------------------------------------
1191 mkAlts dflags scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
1192 | all isDeadBinder bndrs1, -- Remember the default
1193 length filtered_alts < length con_alts -- alternative comes first
1194 = tick (AltMerge case_bndr) `thenSmpl_`
1195 returnSmpl better_alts
1197 filtered_alts = filter keep con_alts
1198 keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
1199 better_alts = (DEFAULT, [], rhs1) : filtered_alts
1202 --------------------------------------------------
1203 -- 2. Merge nested cases
1204 --------------------------------------------------
1206 mkAlts dflags scrut outer_bndr outer_alts
1207 | dopt Opt_CaseMerge dflags,
1208 (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
1209 Just (Case (Var scrut_var) inner_bndr _ inner_alts) <- maybe_outer_deflt,
1210 scruting_same_var scrut_var
1212 munged_inner_alts = [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts]
1213 munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
1215 new_alts = mergeAlts outer_alts_without_deflt munged_inner_alts
1216 -- The merge keeps the inner DEFAULT at the front, if there is one
1217 -- and eliminates any inner_alts that are shadowed by the outer_alts
1219 tick (CaseMerge outer_bndr) `thenSmpl_`
1221 -- Warning: don't call mkAlts recursively!
1222 -- Firstly, there's no point, because inner alts have already had
1223 -- mkCase applied to them, so they won't have a case in their default
1224 -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
1225 -- in munge_rhs may put a case into the DEFAULT branch!
1227 -- We are scrutinising the same variable if it's
1228 -- the outer case-binder, or if the outer case scrutinises a variable
1229 -- (and it's the same). Testing both allows us not to replace the
1230 -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
1231 scruting_same_var = case scrut of
1232 Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
1233 other -> \ v -> v == outer_bndr
1235 ------------------------------------------------
1237 ------------------------------------------------
1239 mkAlts dflags scrut case_bndr other_alts = returnSmpl other_alts
1244 =================================================================================
1246 mkCase1 tries these things
1248 1. Eliminate the case altogether if possible
1256 and similar friends.
1259 Start with a simple situation:
1261 case x# of ===> e[x#/y#]
1264 (when x#, y# are of primitive type, of course). We can't (in general)
1265 do this for algebraic cases, because we might turn bottom into
1268 Actually, we generalise this idea to look for a case where we're
1269 scrutinising a variable, and we know that only the default case can
1274 other -> ...(case x of
1278 Here the inner case can be eliminated. This really only shows up in
1279 eliminating error-checking code.
1281 We also make sure that we deal with this very common case:
1286 Here we are using the case as a strict let; if x is used only once
1287 then we want to inline it. We have to be careful that this doesn't
1288 make the program terminate when it would have diverged before, so we
1290 - x is used strictly, or
1291 - e is already evaluated (it may so if e is a variable)
1293 Lastly, we generalise the transformation to handle this:
1299 We only do this for very cheaply compared r's (constructors, literals
1300 and variables). If pedantic bottoms is on, we only do it when the
1301 scrutinee is a PrimOp which can't fail.
1303 We do it *here*, looking at un-simplified alternatives, because we
1304 have to check that r doesn't mention the variables bound by the
1305 pattern in each alternative, so the binder-info is rather useful.
1307 So the case-elimination algorithm is:
1309 1. Eliminate alternatives which can't match
1311 2. Check whether all the remaining alternatives
1312 (a) do not mention in their rhs any of the variables bound in their pattern
1313 and (b) have equal rhss
1315 3. Check we can safely ditch the case:
1316 * PedanticBottoms is off,
1317 or * the scrutinee is an already-evaluated variable
1318 or * the scrutinee is a primop which is ok for speculation
1319 -- ie we want to preserve divide-by-zero errors, and
1320 -- calls to error itself!
1322 or * [Prim cases] the scrutinee is a primitive variable
1324 or * [Alg cases] the scrutinee is a variable and
1325 either * the rhs is the same variable
1326 (eg case x of C a b -> x ===> x)
1327 or * there is only one alternative, the default alternative,
1328 and the binder is used strictly in its scope.
1329 [NB this is helped by the "use default binder where
1330 possible" transformation; see below.]
1333 If so, then we can replace the case with one of the rhss.
1335 Further notes about case elimination
1336 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1337 Consider: test :: Integer -> IO ()
1340 Turns out that this compiles to:
1343 eta1 :: State# RealWorld ->
1344 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1346 (PrelNum.jtos eta ($w[] @ Char))
1348 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1350 Notice the strange '<' which has no effect at all. This is a funny one.
1351 It started like this:
1353 f x y = if x < 0 then jtos x
1354 else if y==0 then "" else jtos x
1356 At a particular call site we have (f v 1). So we inline to get
1358 if v < 0 then jtos x
1359 else if 1==0 then "" else jtos x
1361 Now simplify the 1==0 conditional:
1363 if v<0 then jtos v else jtos v
1365 Now common-up the two branches of the case:
1367 case (v<0) of DEFAULT -> jtos v
1369 Why don't we drop the case? Because it's strict in v. It's technically
1370 wrong to drop even unnecessary evaluations, and in practice they
1371 may be a result of 'seq' so we *definitely* don't want to drop those.
1372 I don't really know how to improve this situation.
1376 --------------------------------------------------
1377 -- 0. Check for empty alternatives
1378 --------------------------------------------------
1380 -- This isn't strictly an error. It's possible that the simplifer might "see"
1381 -- that an inner case has no accessible alternatives before it "sees" that the
1382 -- entire branch of an outer case is inaccessible. So we simply
1383 -- put an error case here insteadd
1384 mkCase1 scrut case_bndr ty []
1385 = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
1386 return (mkApps (Var eRROR_ID)
1387 [Type ty, Lit (mkStringLit "Impossible alternative")])
1389 --------------------------------------------------
1390 -- 1. Eliminate the case altogether if poss
1391 --------------------------------------------------
1393 mkCase1 scrut case_bndr ty [(con,bndrs,rhs)]
1394 -- See if we can get rid of the case altogether
1395 -- See the extensive notes on case-elimination above
1396 -- mkCase made sure that if all the alternatives are equal,
1397 -- then there is now only one (DEFAULT) rhs
1398 | all isDeadBinder bndrs,
1400 -- Check that the scrutinee can be let-bound instead of case-bound
1401 exprOkForSpeculation scrut
1402 -- OK not to evaluate it
1403 -- This includes things like (==# a# b#)::Bool
1404 -- so that we simplify
1405 -- case ==# a# b# of { True -> x; False -> x }
1408 -- This particular example shows up in default methods for
1409 -- comparision operations (e.g. in (>=) for Int.Int32)
1410 || exprIsHNF scrut -- It's already evaluated
1411 || var_demanded_later scrut -- It'll be demanded later
1413 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1414 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1415 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1416 -- its argument: case x of { y -> dataToTag# y }
1417 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1418 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1420 -- Also we don't want to discard 'seq's
1421 = tick (CaseElim case_bndr) `thenSmpl_`
1422 returnSmpl (bindCaseBndr case_bndr scrut rhs)
1425 -- The case binder is going to be evaluated later,
1426 -- and the scrutinee is a simple variable
1427 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1428 var_demanded_later other = False
1431 --------------------------------------------------
1433 --------------------------------------------------
1435 mkCase1 scrut case_bndr ty alts -- Identity case
1436 | all identity_alt alts
1437 = tick (CaseIdentity case_bndr) `thenSmpl_`
1438 returnSmpl (re_note scrut)
1440 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
1442 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
1443 identity_rhs (LitAlt lit) _ = Lit lit
1444 identity_rhs DEFAULT _ = Var case_bndr
1446 arg_tys = map Type (tyConAppArgs (idType case_bndr))
1449 -- case coerce T e of x { _ -> coerce T' x }
1450 -- And we definitely want to eliminate this case!
1451 -- So we throw away notes from the RHS, and reconstruct
1452 -- (at least an approximation) at the other end
1453 de_note (Note _ e) = de_note e
1456 -- re_note wraps a coerce if it might be necessary
1457 re_note scrut = case head alts of
1458 (_,_,rhs1@(Note _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut
1462 --------------------------------------------------
1464 --------------------------------------------------
1465 mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts)
1469 When adding auxiliary bindings for the case binder, it's worth checking if
1470 its dead, because it often is, and occasionally these mkCase transformations
1471 cascade rather nicely.
1474 bindCaseBndr bndr rhs body
1475 | isDeadBinder bndr = body
1476 | otherwise = bindNonRec bndr rhs body