2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
8 mkLam, prepareAlts, mkCase,
11 preInlineUnconditionally, postInlineUnconditionally, activeInline, activeRule,
14 -- The continuation type
15 SimplCont(..), DupFlag(..), LetRhsFlag(..),
16 contIsDupable, contResultType,
17 countValArgs, countArgs, pushContArgs,
18 mkBoringStop, mkRhsStop, contIsRhs, contIsRhsOrArg,
19 getContArgs, interestingCallContext, interestingArg, isStrictType
23 #include "HsVersions.h"
26 import DynFlags ( SimplifierSwitch(..), SimplifierMode(..),
28 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, exprIsValue
37 import Id ( idType, isDataConWorkId, idOccInfo, isDictId, idArity,
38 mkSysLocal, isDeadBinder, idNewDemandInfo, isExportedId,
39 idUnfolding, idNewStrictness, idInlinePragma,
41 import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
43 import Type ( Type, splitFunTys, dropForAlls, isStrictType,
44 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys
46 import Name ( mkSysTvName )
47 import TyCon ( tyConDataCons_maybe, isAlgTyCon, isNewTyCon )
48 import DataCon ( dataConRepArity, dataConTyVars, dataConArgTys, isVanillaDataCon )
49 import Var ( tyVarKind, mkTyVar )
51 import BasicTypes ( TopLevelFlag(..), isTopLevel, isNotTopLevel, OccInfo(..), isLoopBreaker, isOneOcc,
52 Activation, isAlwaysActive, isActive )
53 import Util ( lengthExceeds )
58 %************************************************************************
60 \subsection{The continuation data type}
62 %************************************************************************
65 data SimplCont -- Strict contexts
66 = Stop OutType -- Type of the result
68 Bool -- True <=> This is the RHS of a thunk whose type suggests
69 -- that update-in-place would be possible
70 -- (This makes the inliner a little keener.)
72 | CoerceIt OutType -- The To-type, simplified
75 | InlinePlease -- This continuation makes a function very
76 SimplCont -- keen to inline itelf
79 InExpr SimplEnv -- The argument, as yet unsimplified,
80 SimplCont -- and its environment
83 InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
86 | ArgOf LetRhsFlag -- An arbitrary strict context: the argument
87 -- of a strict function, or a primitive-arg fn
89 -- No DupFlag because we never duplicate it
90 OutType -- arg_ty: type of the argument itself
91 OutType -- cont_ty: the type of the expression being sought by the context
92 -- f (error "foo") ==> coerce t (error "foo")
94 -- We need to know the type t, to which to coerce.
96 (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
97 -- The result expression in the OutExprStuff has type cont_ty
99 data LetRhsFlag = AnArg -- It's just an argument not a let RHS
100 | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
102 instance Outputable LetRhsFlag where
103 ppr AnArg = ptext SLIT("arg")
104 ppr AnRhs = ptext SLIT("rhs")
106 instance Outputable SimplCont where
107 ppr (Stop _ is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs)
108 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
109 ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...")
110 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
111 (nest 4 (ppr alts)) $$ ppr cont
112 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
113 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
115 data DupFlag = OkToDup | NoDup
117 instance Outputable DupFlag where
118 ppr OkToDup = ptext SLIT("ok")
119 ppr NoDup = ptext SLIT("nodup")
123 mkBoringStop, mkRhsStop :: OutType -> SimplCont
124 mkBoringStop ty = Stop ty AnArg (canUpdateInPlace ty)
125 mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty)
127 contIsRhs :: SimplCont -> Bool
128 contIsRhs (Stop _ AnRhs _) = True
129 contIsRhs (ArgOf AnRhs _ _ _) = True
130 contIsRhs other = False
132 contIsRhsOrArg (Stop _ _ _) = True
133 contIsRhsOrArg (ArgOf _ _ _ _) = True
134 contIsRhsOrArg other = False
137 contIsDupable :: SimplCont -> Bool
138 contIsDupable (Stop _ _ _) = True
139 contIsDupable (ApplyTo OkToDup _ _ _) = True
140 contIsDupable (Select OkToDup _ _ _ _) = True
141 contIsDupable (CoerceIt _ cont) = contIsDupable cont
142 contIsDupable (InlinePlease cont) = contIsDupable cont
143 contIsDupable other = False
146 discardableCont :: SimplCont -> Bool
147 discardableCont (Stop _ _ _) = False
148 discardableCont (CoerceIt _ cont) = discardableCont cont
149 discardableCont (InlinePlease cont) = discardableCont cont
150 discardableCont other = True
152 discardCont :: SimplCont -- A continuation, expecting
153 -> SimplCont -- Replace the continuation with a suitable coerce
154 discardCont cont = case cont of
155 Stop to_ty is_rhs _ -> cont
156 other -> CoerceIt to_ty (mkBoringStop to_ty)
158 to_ty = contResultType cont
161 contResultType :: SimplCont -> OutType
162 contResultType (Stop to_ty _ _) = to_ty
163 contResultType (ArgOf _ _ to_ty _) = to_ty
164 contResultType (ApplyTo _ _ _ cont) = contResultType cont
165 contResultType (CoerceIt _ cont) = contResultType cont
166 contResultType (InlinePlease cont) = contResultType cont
167 contResultType (Select _ _ _ _ cont) = contResultType cont
170 countValArgs :: SimplCont -> Int
171 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
172 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
173 countValArgs other = 0
175 countArgs :: SimplCont -> Int
176 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
180 pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont
181 -- Pushes args with the specified environment
182 pushContArgs env [] cont = cont
183 pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont)
188 getContArgs :: SwitchChecker
189 -> OutId -> SimplCont
190 -> ([(InExpr, SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
191 SimplCont, -- Remaining continuation
192 Bool) -- Whether we came across an InlineCall
193 -- getContArgs id k = (args, k', inl)
194 -- args are the leading ApplyTo items in k
195 -- (i.e. outermost comes first)
196 -- augmented with demand info from the functionn
197 getContArgs chkr fun orig_cont
199 -- Ignore strictness info if the no-case-of-case
200 -- flag is on. Strictness changes evaluation order
201 -- and that can change full laziness
202 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
203 | otherwise = computed_stricts
205 go [] stricts False orig_cont
207 ----------------------------
210 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
211 = go ((arg,se,False) : acc) ss inl cont
212 -- NB: don't bother to instantiate the function type
215 go acc (s:ss) inl (ApplyTo _ arg se cont)
216 = go ((arg,se,s) : acc) ss inl cont
218 -- An Inline continuation
219 go acc ss inl (InlinePlease cont)
220 = go acc ss True cont
222 -- We're run out of arguments, or else we've run out of demands
223 -- The latter only happens if the result is guaranteed bottom
224 -- This is the case for
225 -- * case (error "hello") of { ... }
226 -- * (error "Hello") arg
227 -- * f (error "Hello") where f is strict
229 -- Then, especially in the first of these cases, we'd like to discard
230 -- the continuation, leaving just the bottoming expression. But the
231 -- type might not be right, so we may have to add a coerce.
233 | null ss && discardableCont cont = (reverse acc, discardCont cont, inl)
234 | otherwise = (reverse acc, cont, inl)
236 ----------------------------
237 vanilla_stricts, computed_stricts :: [Bool]
238 vanilla_stricts = repeat False
239 computed_stricts = zipWith (||) fun_stricts arg_stricts
241 ----------------------------
242 (val_arg_tys, _) = splitFunTys (dropForAlls (idType fun))
243 arg_stricts = map isStrictType val_arg_tys ++ repeat False
244 -- These argument types are used as a cheap and cheerful way to find
245 -- unboxed arguments, which must be strict. But it's an InType
246 -- and so there might be a type variable where we expect a function
247 -- type (the substitution hasn't happened yet). And we don't bother
248 -- doing the type applications for a polymorphic function.
249 -- Hence the splitFunTys*IgnoringForAlls*
251 ----------------------------
252 -- If fun_stricts is finite, it means the function returns bottom
253 -- after that number of value args have been consumed
254 -- Otherwise it's infinite, extended with False
256 = case splitStrictSig (idNewStrictness fun) of
257 (demands, result_info)
258 | not (demands `lengthExceeds` countValArgs orig_cont)
259 -> -- Enough args, use the strictness given.
260 -- For bottoming functions we used to pretend that the arg
261 -- is lazy, so that we don't treat the arg as an
262 -- interesting context. This avoids substituting
263 -- top-level bindings for (say) strings into
264 -- calls to error. But now we are more careful about
265 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
266 if isBotRes result_info then
267 map isStrictDmd demands -- Finite => result is bottom
269 map isStrictDmd demands ++ vanilla_stricts
271 other -> vanilla_stricts -- Not enough args, or no strictness
274 interestingArg :: OutExpr -> Bool
275 -- An argument is interesting if it has *some* structure
276 -- We are here trying to avoid unfolding a function that
277 -- is applied only to variables that have no unfolding
278 -- (i.e. they are probably lambda bound): f x y z
279 -- There is little point in inlining f here.
280 interestingArg (Var v) = hasSomeUnfolding (idUnfolding v)
281 -- Was: isValueUnfolding (idUnfolding v')
282 -- But that seems over-pessimistic
284 -- This accounts for an argument like
285 -- () or [], which is definitely interesting
286 interestingArg (Type _) = False
287 interestingArg (App fn (Type _)) = interestingArg fn
288 interestingArg (Note _ a) = interestingArg a
289 interestingArg other = True
290 -- Consider let x = 3 in f x
291 -- The substitution will contain (x -> ContEx 3), and we want to
292 -- to say that x is an interesting argument.
293 -- But consider also (\x. f x y) y
294 -- The substitution will contain (x -> ContEx y), and we want to say
295 -- that x is not interesting (assuming y has no unfolding)
298 Comment about interestingCallContext
299 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
300 We want to avoid inlining an expression where there can't possibly be
301 any gain, such as in an argument position. Hence, if the continuation
302 is interesting (eg. a case scrutinee, application etc.) then we
303 inline, otherwise we don't.
305 Previously some_benefit used to return True only if the variable was
306 applied to some value arguments. This didn't work:
308 let x = _coerce_ (T Int) Int (I# 3) in
309 case _coerce_ Int (T Int) x of
312 we want to inline x, but can't see that it's a constructor in a case
313 scrutinee position, and some_benefit is False.
317 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
319 .... case dMonadST _@_ x0 of (a,b,c) -> ....
321 we'd really like to inline dMonadST here, but we *don't* want to
322 inline if the case expression is just
324 case x of y { DEFAULT -> ... }
326 since we can just eliminate this case instead (x is in WHNF). Similar
327 applies when x is bound to a lambda expression. Hence
328 contIsInteresting looks for case expressions with just a single
332 interestingCallContext :: Bool -- False <=> no args at all
333 -> Bool -- False <=> no value args
335 -- The "lone-variable" case is important. I spent ages
336 -- messing about with unsatisfactory varaints, but this is nice.
337 -- The idea is that if a variable appear all alone
338 -- as an arg of lazy fn, or rhs Stop
339 -- as scrutinee of a case Select
340 -- as arg of a strict fn ArgOf
341 -- then we should not inline it (unless there is some other reason,
342 -- e.g. is is the sole occurrence). We achieve this by making
343 -- interestingCallContext return False for a lone variable.
345 -- Why? At least in the case-scrutinee situation, turning
346 -- let x = (a,b) in case x of y -> ...
348 -- let x = (a,b) in case (a,b) of y -> ...
350 -- let x = (a,b) in let y = (a,b) in ...
351 -- is bad if the binding for x will remain.
353 -- Another example: I discovered that strings
354 -- were getting inlined straight back into applications of 'error'
355 -- because the latter is strict.
357 -- f = \x -> ...(error s)...
359 -- Fundamentally such contexts should not ecourage inlining because
360 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
361 -- so there's no gain.
363 -- However, even a type application or coercion isn't a lone variable.
365 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
366 -- We had better inline that sucker! The case won't see through it.
368 -- For now, I'm treating treating a variable applied to types
369 -- in a *lazy* context "lone". The motivating example was
371 -- g = /\a. \y. h (f a)
372 -- There's no advantage in inlining f here, and perhaps
373 -- a significant disadvantage. Hence some_val_args in the Stop case
375 interestingCallContext some_args some_val_args cont
378 interesting (InlinePlease _) = True
379 interesting (Select _ _ _ _ _) = some_args
380 interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
381 -- Perhaps True is a bit over-keen, but I've
382 -- seen (coerce f) x, where f has an INLINE prag,
383 -- So we have to give some motivaiton for inlining it
384 interesting (ArgOf _ _ _ _) = some_val_args
385 interesting (Stop ty _ upd_in_place) = some_val_args && upd_in_place
386 interesting (CoerceIt _ cont) = interesting cont
387 -- If this call is the arg of a strict function, the context
388 -- is a bit interesting. If we inline here, we may get useful
389 -- evaluation information to avoid repeated evals: e.g.
391 -- Here the contIsInteresting makes the '*' keener to inline,
392 -- which in turn exposes a constructor which makes the '+' inline.
393 -- Assuming that +,* aren't small enough to inline regardless.
395 -- It's also very important to inline in a strict context for things
398 -- Here, the context of (f x) is strict, and if f's unfolding is
399 -- a build it's *great* to inline it here. So we must ensure that
400 -- the context for (f x) is not totally uninteresting.
404 canUpdateInPlace :: Type -> Bool
405 -- Consider let x = <wurble> in ...
406 -- If <wurble> returns an explicit constructor, we might be able
407 -- to do update in place. So we treat even a thunk RHS context
408 -- as interesting if update in place is possible. We approximate
409 -- this by seeing if the type has a single constructor with a
410 -- small arity. But arity zero isn't good -- we share the single copy
411 -- for that case, so no point in sharing.
414 | not opt_UF_UpdateInPlace = False
416 = case splitTyConApp_maybe ty of
418 Just (tycon, _) -> case tyConDataCons_maybe tycon of
419 Just [dc] -> arity == 1 || arity == 2
421 arity = dataConRepArity dc
427 %************************************************************************
429 \subsection{Decisions about inlining}
431 %************************************************************************
433 Inlining is controlled partly by the SimplifierMode switch. This has two
436 SimplGently (a) Simplifying before specialiser/full laziness
437 (b) Simplifiying inside INLINE pragma
438 (c) Simplifying the LHS of a rule
439 (d) Simplifying a GHCi expression or Template
442 SimplPhase n Used at all other times
444 The key thing about SimplGently is that it does no call-site inlining.
445 Before full laziness we must be careful not to inline wrappers,
446 because doing so inhibits floating
447 e.g. ...(case f x of ...)...
448 ==> ...(case (case x of I# x# -> fw x#) of ...)...
449 ==> ...(case x of I# x# -> case fw x# of ...)...
450 and now the redex (f x) isn't floatable any more.
452 The no-inling thing is also important for Template Haskell. You might be
453 compiling in one-shot mode with -O2; but when TH compiles a splice before
454 running it, we don't want to use -O2. Indeed, we don't want to inline
455 anything, because the byte-code interpreter might get confused about
456 unboxed tuples and suchlike.
460 SimplGently is also used as the mode to simplify inside an InlineMe note.
463 inlineMode :: SimplifierMode
464 inlineMode = SimplGently
467 It really is important to switch off inlinings inside such
468 expressions. Consider the following example
474 in ...g...g...g...g...g...
476 Now, if that's the ONLY occurrence of f, it will be inlined inside g,
477 and thence copied multiple times when g is inlined.
480 This function may be inlinined in other modules, so we
481 don't want to remove (by inlining) calls to functions that have
482 specialisations, or that may have transformation rules in an importing
485 E.g. {-# INLINE f #-}
488 and suppose that g is strict *and* has specialisations. If we inline
489 g's wrapper, we deny f the chance of getting the specialised version
490 of g when f is inlined at some call site (perhaps in some other
493 It's also important not to inline a worker back into a wrapper.
495 wraper = inline_me (\x -> ...worker... )
496 Normally, the inline_me prevents the worker getting inlined into
497 the wrapper (initially, the worker's only call site!). But,
498 if the wrapper is sure to be called, the strictness analyser will
499 mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
500 continuation. That's why the keep_inline predicate returns True for
501 ArgOf continuations. It shouldn't do any harm not to dissolve the
502 inline-me note under these circumstances.
504 Note that the result is that we do very little simplification
507 all xs = foldr (&&) True xs
508 any p = all . map p {-# INLINE any #-}
510 Problem: any won't get deforested, and so if it's exported and the
511 importer doesn't use the inlining, (eg passes it as an arg) then we
512 won't get deforestation at all. We havn't solved this problem yet!
515 preInlineUnconditionally
516 ~~~~~~~~~~~~~~~~~~~~~~~~
517 @preInlineUnconditionally@ examines a bndr to see if it is used just
518 once in a completely safe way, so that it is safe to discard the
519 binding inline its RHS at the (unique) usage site, REGARDLESS of how
520 big the RHS might be. If this is the case we don't simplify the RHS
521 first, but just inline it un-simplified.
523 This is much better than first simplifying a perhaps-huge RHS and then
524 inlining and re-simplifying it. Indeed, it can be at least quadratically
533 We may end up simplifying e1 N times, e2 N-1 times, e3 N-3 times etc.
534 This can happen with cascades of functions too:
541 THE MAIN INVARIANT is this:
543 ---- preInlineUnconditionally invariant -----
544 IF preInlineUnconditionally chooses to inline x = <rhs>
545 THEN doing the inlining should not change the occurrence
546 info for the free vars of <rhs>
547 ----------------------------------------------
549 For example, it's tempting to look at trivial binding like
551 and inline it unconditionally. But suppose x is used many times,
552 but this is the unique occurrence of y. Then inlining x would change
553 y's occurrence info, which breaks the invariant. It matters: y
554 might have a BIG rhs, which will now be dup'd at every occurrenc of x.
557 Evne RHSs labelled InlineMe aren't caught here, because there might be
558 no benefit from inlining at the call site.
560 [Sept 01] Don't unconditionally inline a top-level thing, because that
561 can simply make a static thing into something built dynamically. E.g.
565 [Remember that we treat \s as a one-shot lambda.] No point in
566 inlining x unless there is something interesting about the call site.
568 But watch out: if you aren't careful, some useful foldr/build fusion
569 can be lost (most notably in spectral/hartel/parstof) because the
570 foldr didn't see the build. Doing the dynamic allocation isn't a big
571 deal, in fact, but losing the fusion can be. But the right thing here
572 seems to be to do a callSiteInline based on the fact that there is
573 something interesting about the call site (it's strict). Hmm. That
576 Conclusion: inline top level things gaily until Phase 0 (the last
577 phase), at which point don't.
580 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
581 preInlineUnconditionally env top_lvl bndr rhs
583 | opt_SimplNoPreInlining = False
584 | otherwise = case idOccInfo bndr of
585 IAmDead -> True -- Happens in ((\x.1) v)
586 OneOcc in_lam True int_cxt -> try_once in_lam int_cxt
590 active = case phase of
591 SimplGently -> isAlwaysActive prag
592 SimplPhase n -> isActive n prag
593 prag = idInlinePragma bndr
595 try_once in_lam int_cxt -- There's one textual occurrence
596 = not in_lam && (isNotTopLevel top_lvl || early_phase)
597 || (exprIsValue rhs && int_cxt)
598 -- exprIsValue => free vars of rhs are (Once in_lam) or Many,
599 -- so substituting rhs inside a lambda doesn't change the occ info
600 -- Caveat: except the fn of a PAP, but since it has arity > 0, it
601 -- must be a HNF, so it doesn't matter if we push it inside
604 -- int_cxt The context isn't totally boring
605 -- E.g. let f = \ab.BIG in \y. map f xs
606 -- Don't want to substitute for f, because then we allocate
607 -- its closure every time the \y is called
608 -- But: let f = \ab.BIG in \y. map (f y) xs
609 -- Now we do want to substitute for f, even though it's not
610 -- saturated, because we're going to allocate a closure for
611 -- (f y) every time round the loop anyhow.
613 early_phase = case phase of
614 SimplPhase 0 -> False
616 -- If we don't have this early_phase test, consider
617 -- x = length [1,2,3]
618 -- The full laziness pass carefully floats all the cons cells to
619 -- top level, and preInlineUnconditionally floats them all back in.
620 -- Result is (a) static allocation replaced by dynamic allocation
621 -- (b) many simplifier iterations because this tickles
622 -- a related problem; only one inlining per pass
624 -- On the other hand, I have seen cases where top-level fusion is
625 -- lost if we don't inline top level thing (e.g. string constants)
626 -- Hence the test for phase zero (which is the phase for all the final
627 -- simplifications). Until phase zero we take no special notice of
628 -- top level things, but then we become more leery about inlining
633 postInlineUnconditionally
634 ~~~~~~~~~~~~~~~~~~~~~~~~~
635 @postInlineUnconditionally@ decides whether to unconditionally inline
636 a thing based on the form of its RHS; in particular if it has a
637 trivial RHS. If so, we can inline and discard the binding altogether.
639 NB: a loop breaker has must_keep_binding = True and non-loop-breakers
640 only have *forward* references Hence, it's safe to discard the binding
642 NOTE: This isn't our last opportunity to inline. We're at the binding
643 site right now, and we'll get another opportunity when we get to the
646 Note that we do this unconditional inlining only for trival RHSs.
647 Don't inline even WHNFs inside lambdas; doing so may simply increase
648 allocation when the function is called. This isn't the last chance; see
651 NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here Why?
652 Because we don't even want to inline them into the RHS of constructor
653 arguments. See NOTE above
655 NB: At one time even NOINLINE was ignored here: if the rhs is trivial
656 it's best to inline it anyway. We often get a=E; b=a from desugaring,
657 with both a and b marked NOINLINE. But that seems incompatible with
658 our new view that inlining is like a RULE, so I'm sticking to the 'active'
662 postInlineUnconditionally :: SimplEnv -> OutId -> OccInfo -> OutExpr -> Bool
663 postInlineUnconditionally env bndr occ_info rhs
665 | isLoopBreaker occ_info = False
666 | isExportedId bndr = False
667 | exprIsTrivial rhs = True
670 active = case getMode env of
671 SimplGently -> isAlwaysActive prag
672 SimplPhase n -> isActive n prag
673 prag = idInlinePragma bndr
675 activeInline :: SimplEnv -> OutId -> OccInfo -> Bool
676 activeInline env id occ
677 = case getMode env of
678 SimplGently -> isOneOcc occ && isAlwaysActive prag
679 -- No inlining at all when doing gentle stuff,
680 -- except for local things that occur once
681 -- The reason is that too little clean-up happens if you
682 -- don't inline use-once things. Also a bit of inlining is *good* for
683 -- full laziness; it can expose constant sub-expressions.
684 -- Example in spectral/mandel/Mandel.hs, where the mandelset
685 -- function gets a useful let-float if you inline windowToViewport
687 -- NB: we used to have a second exception, for data con wrappers.
688 -- On the grounds that we use gentle mode for rule LHSs, and
689 -- they match better when data con wrappers are inlined.
690 -- But that only really applies to the trivial wrappers (like (:)),
691 -- and they are now constructed as Compulsory unfoldings (in MkId)
692 -- so they'll happen anyway.
694 SimplPhase n -> isActive n prag
696 prag = idInlinePragma id
698 activeRule :: SimplEnv -> Maybe (Activation -> Bool)
699 -- Nothing => No rules at all
701 | opt_RulesOff = Nothing
703 = case getMode env of
704 SimplGently -> Just isAlwaysActive
705 -- Used to be Nothing (no rules in gentle mode)
706 -- Main motivation for changing is that I wanted
707 -- lift String ===> ...
708 -- to work in Template Haskell when simplifying
709 -- splices, so we get simpler code for literal strings
710 SimplPhase n -> Just (isActive n)
714 %************************************************************************
716 \subsection{Rebuilding a lambda}
718 %************************************************************************
721 mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
725 a) eta reduction, if that gives a trivial expression
726 b) eta expansion [only if there are some value lambdas]
727 c) floating lets out through big lambdas
728 [only if all tyvar lambdas, and only if this lambda
732 mkLam env bndrs body cont
733 = getDOptsSmpl `thenSmpl` \dflags ->
734 mkLam' dflags env bndrs body cont
736 mkLam' dflags env bndrs body cont
737 | dopt Opt_DoEtaReduction dflags,
738 Just etad_lam <- tryEtaReduce bndrs body
739 = tick (EtaReduction (head bndrs)) `thenSmpl_`
740 returnSmpl (emptyFloats env, etad_lam)
742 | dopt Opt_DoLambdaEtaExpansion dflags,
743 any isRuntimeVar bndrs
744 = tryEtaExpansion body `thenSmpl` \ body' ->
745 returnSmpl (emptyFloats env, mkLams bndrs body')
747 {- Sept 01: I'm experimenting with getting the
748 full laziness pass to float out past big lambdsa
749 | all isTyVar bndrs, -- Only for big lambdas
750 contIsRhs cont -- Only try the rhs type-lambda floating
751 -- if this is indeed a right-hand side; otherwise
752 -- we end up floating the thing out, only for float-in
753 -- to float it right back in again!
754 = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
755 returnSmpl (floats, mkLams bndrs body')
759 = returnSmpl (emptyFloats env, mkLams bndrs body)
763 %************************************************************************
765 \subsection{Eta expansion and reduction}
767 %************************************************************************
769 We try for eta reduction here, but *only* if we get all the
770 way to an exprIsTrivial expression.
771 We don't want to remove extra lambdas unless we are going
772 to avoid allocating this thing altogether
775 tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr
776 tryEtaReduce bndrs body
777 -- We don't use CoreUtils.etaReduce, because we can be more
779 -- (a) we already have the binders
780 -- (b) we can do the triviality test before computing the free vars
781 = go (reverse bndrs) body
783 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
784 go [] fun | ok_fun fun = Just fun -- Success!
785 go _ _ = Nothing -- Failure!
787 ok_fun fun = exprIsTrivial fun
788 && not (any (`elemVarSet` (exprFreeVars fun)) bndrs)
789 && (exprIsValue fun || all ok_lam bndrs)
790 ok_lam v = isTyVar v || isDictId v
791 -- The exprIsValue is because eta reduction is not
792 -- valid in general: \x. bot /= bot
793 -- So we need to be sure that the "fun" is a value.
795 -- However, we always want to reduce (/\a -> f a) to f
796 -- This came up in a RULE: foldr (build (/\a -> g a))
797 -- did not match foldr (build (/\b -> ...something complex...))
798 -- The type checker can insert these eta-expanded versions,
799 -- with both type and dictionary lambdas; hence the slightly
802 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
806 Try eta expansion for RHSs
809 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
812 where (in both cases)
814 * The xi can include type variables
816 * The yi are all value variables
818 * N is a NORMAL FORM (i.e. no redexes anywhere)
819 wanting a suitable number of extra args.
821 We may have to sandwich some coerces between the lambdas
822 to make the types work. exprEtaExpandArity looks through coerces
823 when computing arity; and etaExpand adds the coerces as necessary when
824 actually computing the expansion.
827 tryEtaExpansion :: OutExpr -> SimplM OutExpr
828 -- There is at least one runtime binder in the binders
830 = getUniquesSmpl `thenSmpl` \ us ->
831 returnSmpl (etaExpand fun_arity us body (exprType body))
833 fun_arity = exprEtaExpandArity body
837 %************************************************************************
839 \subsection{Floating lets out of big lambdas}
841 %************************************************************************
843 tryRhsTyLam tries this transformation, when the big lambda appears as
844 the RHS of a let(rec) binding:
846 /\abc -> let(rec) x = e in b
848 let(rec) x' = /\abc -> let x = x' a b c in e
850 /\abc -> let x = x' a b c in b
852 This is good because it can turn things like:
854 let f = /\a -> letrec g = ... g ... in g
856 letrec g' = /\a -> ... g' a ...
860 which is better. In effect, it means that big lambdas don't impede
863 This optimisation is CRUCIAL in eliminating the junk introduced by
864 desugaring mutually recursive definitions. Don't eliminate it lightly!
866 So far as the implementation is concerned:
868 Invariant: go F e = /\tvs -> F e
872 = Let x' = /\tvs -> F e
876 G = F . Let x = x' tvs
878 go F (Letrec xi=ei in b)
879 = Letrec {xi' = /\tvs -> G ei}
883 G = F . Let {xi = xi' tvs}
885 [May 1999] If we do this transformation *regardless* then we can
886 end up with some pretty silly stuff. For example,
889 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
894 st = /\s -> ...[y1 s/x1, y2 s/x2]
897 Unless the "..." is a WHNF there is really no point in doing this.
898 Indeed it can make things worse. Suppose x1 is used strictly,
901 x1* = case f y of { (a,b) -> e }
903 If we abstract this wrt the tyvar we then can't do the case inline
904 as we would normally do.
908 {- Trying to do this in full laziness
910 tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
911 -- Call ensures that all the binders are type variables
913 tryRhsTyLam env tyvars body -- Only does something if there's a let
914 | not (all isTyVar tyvars)
915 || not (worth_it body) -- inside a type lambda,
916 = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
919 = go env (\x -> x) body
922 worth_it e@(Let _ _) = whnf_in_middle e
925 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
926 whnf_in_middle (Let _ e) = whnf_in_middle e
927 whnf_in_middle e = exprIsCheap e
929 main_tyvar_set = mkVarSet tyvars
931 go env fn (Let bind@(NonRec var rhs) body)
933 = go env (fn . Let bind) body
935 go env fn (Let (NonRec var rhs) body)
936 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
937 addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
938 go env (fn . Let (mk_silly_bind var rhs')) body
942 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
943 -- Abstract only over the type variables free in the rhs
944 -- wrt which the new binding is abstracted. But the naive
945 -- approach of abstract wrt the tyvars free in the Id's type
947 -- /\ a b -> let t :: (a,b) = (e1, e2)
950 -- Here, b isn't free in x's type, but we must nevertheless
951 -- abstract wrt b as well, because t's type mentions b.
952 -- Since t is floated too, we'd end up with the bogus:
953 -- poly_t = /\ a b -> (e1, e2)
954 -- poly_x = /\ a -> fst (poly_t a *b*)
955 -- So for now we adopt the even more naive approach of
956 -- abstracting wrt *all* the tyvars. We'll see if that
957 -- gives rise to problems. SLPJ June 98
959 go env fn (Let (Rec prs) body)
960 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
962 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
963 pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
965 addAuxiliaryBind env (Rec pairs) $ \ env ->
968 (vars,rhss) = unzip prs
969 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
970 -- See notes with tyvars_here above
972 go env fn body = returnSmpl (emptyFloats env, fn body)
974 mk_poly tyvars_here var
975 = getUniqueSmpl `thenSmpl` \ uniq ->
977 poly_name = setNameUnique (idName var) uniq -- Keep same name
978 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
979 poly_id = mkLocalId poly_name poly_ty
981 -- In the olden days, it was crucial to copy the occInfo of the original var,
982 -- because we were looking at occurrence-analysed but as yet unsimplified code!
983 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
984 -- at already simplified code, so it doesn't matter
986 -- It's even right to retain single-occurrence or dead-var info:
987 -- Suppose we started with /\a -> let x = E in B
988 -- where x occurs once in B. Then we transform to:
989 -- let x' = /\a -> E in /\a -> let x* = x' a in B
990 -- where x* has an INLINE prag on it. Now, once x* is inlined,
991 -- the occurrences of x' will be just the occurrences originally
994 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
996 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
997 -- Suppose we start with:
999 -- x = /\ a -> let g = G in E
1001 -- Then we'll float to get
1003 -- x = let poly_g = /\ a -> G
1004 -- in /\ a -> let g = poly_g a in E
1006 -- But now the occurrence analyser will see just one occurrence
1007 -- of poly_g, not inside a lambda, so the simplifier will
1008 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
1009 -- (I used to think that the "don't inline lone occurrences" stuff
1010 -- would stop this happening, but since it's the *only* occurrence,
1011 -- PreInlineUnconditionally kicks in first!)
1013 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
1014 -- to appear many times. (NB: mkInlineMe eliminates
1015 -- such notes on trivial RHSs, so do it manually.)
1019 %************************************************************************
1021 \subsection{Case alternative filtering
1023 %************************************************************************
1025 prepareAlts does two things:
1027 1. Eliminate alternatives that cannot match, including the
1028 DEFAULT alternative.
1030 2. If the DEFAULT alternative can match only one possible constructor,
1031 then make that constructor explicit.
1033 case e of x { DEFAULT -> rhs }
1035 case e of x { (a,b) -> rhs }
1036 where the type is a single constructor type. This gives better code
1037 when rhs also scrutinises x or e.
1039 It's a good idea do do this stuff before simplifying the alternatives, to
1040 avoid simplifying alternatives we know can't happen, and to come up with
1041 the list of constructors that are handled, to put into the IdInfo of the
1042 case binder, for use when simplifying the alternatives.
1044 Eliminating the default alternative in (1) isn't so obvious, but it can
1047 data Colour = Red | Green | Blue
1056 DEFAULT -> [ case y of ... ]
1058 If we inline h into f, the default case of the inlined h can't happen.
1059 If we don't notice this, we may end up filtering out *all* the cases
1060 of the inner case y, which give us nowhere to go!
1064 prepareAlts :: OutExpr -- Scrutinee
1065 -> InId -- Case binder
1066 -> [InAlt] -- Increasing order
1067 -> SimplM ([InAlt], -- Better alternatives, still incresaing order
1068 [AltCon]) -- These cases are handled
1070 prepareAlts scrut case_bndr alts
1072 (alts_wo_default, maybe_deflt) = findDefault alts
1074 impossible_cons = case scrut of
1075 Var v -> otherCons (idUnfolding v)
1078 -- Filter out alternatives that can't possibly match
1079 better_alts | null impossible_cons = alts_wo_default
1080 | otherwise = [alt | alt@(con,_,_) <- alts_wo_default,
1081 not (con `elem` impossible_cons)]
1083 -- "handled_cons" are handled either by the context,
1084 -- or by a branch in this case expression
1085 -- (Don't add DEFAULT to the handled_cons!!)
1086 handled_cons = impossible_cons ++ [con | (con,_,_) <- better_alts]
1088 -- Filter out the default, if it can't happen,
1089 -- or replace it with "proper" alternative if there
1090 -- is only one constructor left
1091 prepareDefault case_bndr handled_cons maybe_deflt `thenSmpl` \ deflt_alt ->
1093 returnSmpl (mergeAlts better_alts deflt_alt, handled_cons)
1094 -- We need the mergeAlts in case the new default_alt
1095 -- has turned into a constructor alternative.
1097 prepareDefault case_bndr handled_cons (Just rhs)
1098 | Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr),
1099 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1100 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1101 -- case x of { DEFAULT -> e }
1102 -- and we don't want to fill in a default for them!
1103 Just all_cons <- tyConDataCons_maybe tycon,
1104 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1105 -- which GHC allows, then the case expression will have at most a default
1106 -- alternative. We don't want to eliminate that alternative, because the
1107 -- invariant is that there's always one alternative. It's more convenient
1109 -- case x of { DEFAULT -> e }
1110 -- as it is, rather than transform it to
1111 -- error "case cant match"
1112 -- which would be quite legitmate. But it's a really obscure corner, and
1113 -- not worth wasting code on.
1114 let handled_data_cons = [data_con | DataAlt data_con <- handled_cons],
1115 let missing_cons = [con | con <- all_cons,
1116 not (con `elem` handled_data_cons)]
1117 = case missing_cons of
1118 [] -> returnSmpl [] -- Eliminate the default alternative
1119 -- if it can't match
1121 [con] -> -- It matches exactly one constructor, so fill it in
1122 tick (FillInCaseDefault case_bndr) `thenSmpl_`
1123 mk_args con inst_tys `thenSmpl` \ args ->
1124 returnSmpl [(DataAlt con, args, rhs)]
1126 two_or_more -> returnSmpl [(DEFAULT, [], rhs)]
1129 = returnSmpl [(DEFAULT, [], rhs)]
1131 prepareDefault case_bndr handled_cons Nothing
1134 mk_args missing_con inst_tys
1135 = mk_tv_bndrs missing_con inst_tys `thenSmpl` \ (tv_bndrs, inst_tys') ->
1136 getUniquesSmpl `thenSmpl` \ id_uniqs ->
1137 let arg_tys = dataConArgTys missing_con inst_tys'
1138 arg_ids = zipWith (mkSysLocal FSLIT("a")) id_uniqs arg_tys
1140 returnSmpl (tv_bndrs ++ arg_ids)
1142 mk_tv_bndrs missing_con inst_tys
1143 | isVanillaDataCon missing_con
1144 = returnSmpl ([], inst_tys)
1146 = getUniquesSmpl `thenSmpl` \ tv_uniqs ->
1147 let new_tvs = zipWith mk tv_uniqs (dataConTyVars missing_con)
1148 mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv)
1150 returnSmpl (new_tvs, mkTyVarTys new_tvs)
1154 %************************************************************************
1156 \subsection{Case absorption and identity-case elimination}
1158 %************************************************************************
1160 mkCase puts a case expression back together, trying various transformations first.
1163 mkCase :: OutExpr -> OutId -> OutType
1164 -> [OutAlt] -- Increasing order
1167 mkCase scrut case_bndr ty alts
1168 = getDOptsSmpl `thenSmpl` \dflags ->
1169 mkAlts dflags scrut case_bndr alts `thenSmpl` \ better_alts ->
1170 mkCase1 scrut case_bndr ty better_alts
1174 mkAlts tries these things:
1176 1. If several alternatives are identical, merge them into
1177 a single DEFAULT alternative. I've occasionally seen this
1178 making a big difference:
1180 case e of =====> case e of
1181 C _ -> f x D v -> ....v....
1182 D v -> ....v.... DEFAULT -> f x
1185 The point is that we merge common RHSs, at least for the DEFAULT case.
1186 [One could do something more elaborate but I've never seen it needed.]
1187 To avoid an expensive test, we just merge branches equal to the *first*
1188 alternative; this picks up the common cases
1189 a) all branches equal
1190 b) some branches equal to the DEFAULT (which occurs first)
1193 case e of b { ==> case e of b {
1194 p1 -> rhs1 p1 -> rhs1
1196 pm -> rhsm pm -> rhsm
1197 _ -> case b of b' { pn -> let b'=b in rhsn
1199 ... po -> let b'=b in rhso
1200 po -> rhso _ -> let b'=b in rhsd
1204 which merges two cases in one case when -- the default alternative of
1205 the outer case scrutises the same variable as the outer case This
1206 transformation is called Case Merging. It avoids that the same
1207 variable is scrutinised multiple times.
1210 The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
1216 where @is@ was something like
1218 p `is` n = p /= (-1) && p == n
1220 This gave rise to a horrible sequence of cases
1227 and similarly in cascade for all the join points!
1232 --------------------------------------------------
1233 -- 1. Merge identical branches
1234 --------------------------------------------------
1235 mkAlts dflags scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
1236 | all isDeadBinder bndrs1, -- Remember the default
1237 length filtered_alts < length con_alts -- alternative comes first
1238 = tick (AltMerge case_bndr) `thenSmpl_`
1239 returnSmpl better_alts
1241 filtered_alts = filter keep con_alts
1242 keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
1243 better_alts = (DEFAULT, [], rhs1) : filtered_alts
1246 --------------------------------------------------
1247 -- 2. Merge nested cases
1248 --------------------------------------------------
1250 mkAlts dflags scrut outer_bndr outer_alts
1251 | dopt Opt_CaseMerge dflags,
1252 (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
1253 Just (Case (Var scrut_var) inner_bndr _ inner_alts) <- maybe_outer_deflt,
1254 scruting_same_var scrut_var
1256 munged_inner_alts = [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts]
1257 munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
1259 new_alts = mergeAlts outer_alts_without_deflt munged_inner_alts
1260 -- The merge keeps the inner DEFAULT at the front, if there is one
1261 -- and eliminates any inner_alts that are shadowed by the outer_alts
1263 tick (CaseMerge outer_bndr) `thenSmpl_`
1265 -- Warning: don't call mkAlts recursively!
1266 -- Firstly, there's no point, because inner alts have already had
1267 -- mkCase applied to them, so they won't have a case in their default
1268 -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
1269 -- in munge_rhs may put a case into the DEFAULT branch!
1271 -- We are scrutinising the same variable if it's
1272 -- the outer case-binder, or if the outer case scrutinises a variable
1273 -- (and it's the same). Testing both allows us not to replace the
1274 -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
1275 scruting_same_var = case scrut of
1276 Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
1277 other -> \ v -> v == outer_bndr
1279 ------------------------------------------------
1281 ------------------------------------------------
1283 mkAlts dflags scrut case_bndr other_alts = returnSmpl other_alts
1286 ---------------------------------
1287 mergeAlts :: [OutAlt] -> [OutAlt] -> [OutAlt]
1288 -- Merge preserving order; alternatives in the first arg
1289 -- shadow ones in the second
1290 mergeAlts [] as2 = as2
1291 mergeAlts as1 [] = as1
1292 mergeAlts (a1:as1) (a2:as2)
1293 = case a1 `cmpAlt` a2 of
1294 LT -> a1 : mergeAlts as1 (a2:as2)
1295 EQ -> a1 : mergeAlts as1 as2 -- Discard a2
1296 GT -> a2 : mergeAlts (a1:as1) as2
1301 =================================================================================
1303 mkCase1 tries these things
1305 1. Eliminate the case altogether if possible
1313 and similar friends.
1316 Start with a simple situation:
1318 case x# of ===> e[x#/y#]
1321 (when x#, y# are of primitive type, of course). We can't (in general)
1322 do this for algebraic cases, because we might turn bottom into
1325 Actually, we generalise this idea to look for a case where we're
1326 scrutinising a variable, and we know that only the default case can
1331 other -> ...(case x of
1335 Here the inner case can be eliminated. This really only shows up in
1336 eliminating error-checking code.
1338 We also make sure that we deal with this very common case:
1343 Here we are using the case as a strict let; if x is used only once
1344 then we want to inline it. We have to be careful that this doesn't
1345 make the program terminate when it would have diverged before, so we
1347 - x is used strictly, or
1348 - e is already evaluated (it may so if e is a variable)
1350 Lastly, we generalise the transformation to handle this:
1356 We only do this for very cheaply compared r's (constructors, literals
1357 and variables). If pedantic bottoms is on, we only do it when the
1358 scrutinee is a PrimOp which can't fail.
1360 We do it *here*, looking at un-simplified alternatives, because we
1361 have to check that r doesn't mention the variables bound by the
1362 pattern in each alternative, so the binder-info is rather useful.
1364 So the case-elimination algorithm is:
1366 1. Eliminate alternatives which can't match
1368 2. Check whether all the remaining alternatives
1369 (a) do not mention in their rhs any of the variables bound in their pattern
1370 and (b) have equal rhss
1372 3. Check we can safely ditch the case:
1373 * PedanticBottoms is off,
1374 or * the scrutinee is an already-evaluated variable
1375 or * the scrutinee is a primop which is ok for speculation
1376 -- ie we want to preserve divide-by-zero errors, and
1377 -- calls to error itself!
1379 or * [Prim cases] the scrutinee is a primitive variable
1381 or * [Alg cases] the scrutinee is a variable and
1382 either * the rhs is the same variable
1383 (eg case x of C a b -> x ===> x)
1384 or * there is only one alternative, the default alternative,
1385 and the binder is used strictly in its scope.
1386 [NB this is helped by the "use default binder where
1387 possible" transformation; see below.]
1390 If so, then we can replace the case with one of the rhss.
1392 Further notes about case elimination
1393 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1394 Consider: test :: Integer -> IO ()
1397 Turns out that this compiles to:
1400 eta1 :: State# RealWorld ->
1401 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1403 (PrelNum.jtos eta ($w[] @ Char))
1405 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1407 Notice the strange '<' which has no effect at all. This is a funny one.
1408 It started like this:
1410 f x y = if x < 0 then jtos x
1411 else if y==0 then "" else jtos x
1413 At a particular call site we have (f v 1). So we inline to get
1415 if v < 0 then jtos x
1416 else if 1==0 then "" else jtos x
1418 Now simplify the 1==0 conditional:
1420 if v<0 then jtos v else jtos v
1422 Now common-up the two branches of the case:
1424 case (v<0) of DEFAULT -> jtos v
1426 Why don't we drop the case? Because it's strict in v. It's technically
1427 wrong to drop even unnecessary evaluations, and in practice they
1428 may be a result of 'seq' so we *definitely* don't want to drop those.
1429 I don't really know how to improve this situation.
1433 --------------------------------------------------
1434 -- 0. Check for empty alternatives
1435 --------------------------------------------------
1438 mkCase1 scrut case_bndr ty []
1439 = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
1443 --------------------------------------------------
1444 -- 1. Eliminate the case altogether if poss
1445 --------------------------------------------------
1447 mkCase1 scrut case_bndr ty [(con,bndrs,rhs)]
1448 -- See if we can get rid of the case altogether
1449 -- See the extensive notes on case-elimination above
1450 -- mkCase made sure that if all the alternatives are equal,
1451 -- then there is now only one (DEFAULT) rhs
1452 | all isDeadBinder bndrs,
1454 -- Check that the scrutinee can be let-bound instead of case-bound
1455 exprOkForSpeculation scrut
1456 -- OK not to evaluate it
1457 -- This includes things like (==# a# b#)::Bool
1458 -- so that we simplify
1459 -- case ==# a# b# of { True -> x; False -> x }
1462 -- This particular example shows up in default methods for
1463 -- comparision operations (e.g. in (>=) for Int.Int32)
1464 || exprIsValue scrut -- It's already evaluated
1465 || var_demanded_later scrut -- It'll be demanded later
1467 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1468 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1469 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1470 -- its argument: case x of { y -> dataToTag# y }
1471 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1472 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1474 -- Also we don't want to discard 'seq's
1475 = tick (CaseElim case_bndr) `thenSmpl_`
1476 returnSmpl (bindCaseBndr case_bndr scrut rhs)
1479 -- The case binder is going to be evaluated later,
1480 -- and the scrutinee is a simple variable
1481 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1482 var_demanded_later other = False
1485 --------------------------------------------------
1487 --------------------------------------------------
1489 mkCase1 scrut case_bndr ty alts -- Identity case
1490 | all identity_alt alts
1491 = tick (CaseIdentity case_bndr) `thenSmpl_`
1492 returnSmpl (re_note scrut)
1494 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
1496 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
1497 identity_rhs (LitAlt lit) _ = Lit lit
1498 identity_rhs DEFAULT _ = Var case_bndr
1500 arg_tys = map Type (tyConAppArgs (idType case_bndr))
1503 -- case coerce T e of x { _ -> coerce T' x }
1504 -- And we definitely want to eliminate this case!
1505 -- So we throw away notes from the RHS, and reconstruct
1506 -- (at least an approximation) at the other end
1507 de_note (Note _ e) = de_note e
1510 -- re_note wraps a coerce if it might be necessary
1511 re_note scrut = case head alts of
1512 (_,_,rhs1@(Note _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut
1516 --------------------------------------------------
1518 --------------------------------------------------
1519 mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts)
1523 When adding auxiliary bindings for the case binder, it's worth checking if
1524 its dead, because it often is, and occasionally these mkCase transformations
1525 cascade rather nicely.
1528 bindCaseBndr bndr rhs body
1529 | isDeadBinder bndr = body
1530 | otherwise = bindNonRec bndr rhs body