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(..),
28 DynFlags, DynFlag(..), dopt )
29 import StaticFlags ( opt_UF_UpdateInPlace, opt_SimplNoPreInlining,
32 import CoreFVs ( exprFreeVars )
33 import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial,
34 etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce,
35 findDefault, exprOkForSpeculation, exprIsHNF, mergeAlts,
38 import Literal ( mkStringLit )
39 import CoreUnfold ( smallEnoughToInline )
40 import MkId ( eRROR_ID, wrapNewTypeBody )
41 import Id ( Id, idType, isDataConWorkId, idOccInfo, isDictId,
42 isDeadBinder, idNewDemandInfo, isExportedId, mkSysLocal,
43 idUnfolding, idNewStrictness, idInlinePragma, idHasRules
45 import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
47 import Var ( tyVarKind, mkTyVar )
48 import Name ( mkSysTvName )
49 import Type ( Type, splitFunTys, dropForAlls, isStrictType,
50 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys )
51 import Coercion ( isEqPredTy
53 import Coercion ( Coercion, mkUnsafeCoercion, coercionKind )
54 import TyCon ( tyConDataCons_maybe, isClosedNewTyCon )
55 import DataCon ( DataCon, dataConRepArity, dataConExTyVars,
56 dataConInstArgTys, dataConTyCon )
58 import BasicTypes ( TopLevelFlag(..), isNotTopLevel, OccInfo(..), isLoopBreaker, isOneOcc,
59 Activation, isAlwaysActive, isActive )
60 import Util ( lengthExceeds )
65 %************************************************************************
67 \subsection{The continuation data type}
69 %************************************************************************
72 data SimplCont -- Strict contexts
73 = Stop OutType -- Type of the result
75 Bool -- True <=> There is something interesting about
76 -- the context, and hence the inliner
77 -- should be a bit keener (see interestingCallContext)
79 -- (a) This is the RHS of a thunk whose type suggests
80 -- that update-in-place would be possible
81 -- (b) This is an argument of a function that has RULES
82 -- Inlining the call might allow the rule to fire
84 | CoerceIt OutCoercion -- The coercion simplified
88 CoreExpr -- The argument
89 (Maybe SimplEnv) -- (Just se) => the arg is un-simplified and this is its subst-env
90 -- Nothing => the arg is already simplified; don't repeatedly simplify it!
91 SimplCont -- and its environment
94 InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
97 | ArgOf LetRhsFlag -- An arbitrary strict context: the argument
98 -- of a strict function, or a primitive-arg fn
100 -- No DupFlag, because we never duplicate it
101 OutType -- arg_ty: type of the argument itself
102 OutType -- cont_ty: the type of the expression being sought by the context
103 -- f (error "foo") ==> coerce t (error "foo")
105 -- We need to know the type t, to which to coerce.
107 (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
108 -- The result expression in the OutExprStuff has type cont_ty
110 data LetRhsFlag = AnArg -- It's just an argument not a let RHS
111 | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
113 instance Outputable LetRhsFlag where
114 ppr AnArg = ptext SLIT("arg")
115 ppr AnRhs = ptext SLIT("rhs")
117 instance Outputable SimplCont where
118 ppr (Stop ty is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs) <+> ppr ty
119 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
120 ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...")
121 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
122 (nest 4 (ppr alts)) $$ ppr cont
123 ppr (CoerceIt co cont) = (ptext SLIT("CoerceIt") <+> ppr co) $$ ppr cont
125 data DupFlag = OkToDup | NoDup
127 instance Outputable DupFlag where
128 ppr OkToDup = ptext SLIT("ok")
129 ppr NoDup = ptext SLIT("nodup")
134 mkBoringStop :: OutType -> SimplCont
135 mkBoringStop ty = Stop ty AnArg False
137 mkLazyArgStop :: OutType -> Bool -> SimplCont
138 mkLazyArgStop ty has_rules = Stop ty AnArg (canUpdateInPlace ty || has_rules)
140 mkRhsStop :: OutType -> SimplCont
141 mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty)
143 contIsRhs :: SimplCont -> Bool
144 contIsRhs (Stop _ AnRhs _) = True
145 contIsRhs (ArgOf AnRhs _ _ _) = True
146 contIsRhs other = False
148 contIsRhsOrArg (Stop _ _ _) = True
149 contIsRhsOrArg (ArgOf _ _ _ _) = True
150 contIsRhsOrArg other = False
153 contIsDupable :: SimplCont -> Bool
154 contIsDupable (Stop _ _ _) = True
155 contIsDupable (ApplyTo OkToDup _ _ _) = True
156 contIsDupable (Select OkToDup _ _ _ _) = True
157 contIsDupable (CoerceIt _ cont) = contIsDupable cont
158 contIsDupable other = False
161 discardableCont :: SimplCont -> Bool
162 discardableCont (Stop _ _ _) = False
163 discardableCont (CoerceIt _ cont) = discardableCont cont
164 discardableCont other = True
166 discardCont :: Type -- The type expected
167 -> SimplCont -- A continuation, expecting the previous type
168 -> SimplCont -- Replace the continuation with a suitable coerce
169 discardCont from_ty cont = case cont of
170 Stop to_ty is_rhs _ -> cont
171 other -> CoerceIt co (mkBoringStop to_ty)
173 co = mkUnsafeCoercion from_ty to_ty
174 to_ty = contResultType cont
177 contResultType :: SimplCont -> OutType
178 contResultType (Stop to_ty _ _) = to_ty
179 contResultType (ArgOf _ _ to_ty _) = to_ty
180 contResultType (ApplyTo _ _ _ cont) = contResultType cont
181 contResultType (CoerceIt _ cont) = contResultType cont
182 contResultType (Select _ _ _ _ cont) = contResultType cont
185 countValArgs :: SimplCont -> Int
186 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
187 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
188 countValArgs other = 0
190 countArgs :: SimplCont -> Int
191 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
195 pushContArgs ::[OutArg] -> SimplCont -> SimplCont
196 -- Pushes args with the specified environment
197 pushContArgs [] cont = cont
198 pushContArgs (arg : args) cont = ApplyTo NoDup arg Nothing (pushContArgs args cont)
203 getContArgs :: SwitchChecker
204 -> OutId -> SimplCont
205 -> ([(InExpr, Maybe SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
206 SimplCont) -- Remaining continuation
207 -- getContArgs id k = (args, k', inl)
208 -- args are the leading ApplyTo items in k
209 -- (i.e. outermost comes first)
210 -- augmented with demand info from the functionn
211 getContArgs chkr fun orig_cont
213 -- Ignore strictness info if the no-case-of-case
214 -- flag is on. Strictness changes evaluation order
215 -- and that can change full laziness
216 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
217 | otherwise = computed_stricts
219 go [] stricts orig_cont
221 ----------------------------
224 go acc ss (ApplyTo _ arg@(Type _) se cont)
225 = go ((arg,se,False) : acc) ss cont
226 -- NB: don't bother to instantiate the function type
229 go acc (s:ss) (ApplyTo _ arg se cont)
230 = go ((arg,se,s) : acc) ss cont
232 -- We're run out of arguments, or else we've run out of demands
233 -- The latter only happens if the result is guaranteed bottom
234 -- This is the case for
235 -- * case (error "hello") of { ... }
236 -- * (error "Hello") arg
237 -- * f (error "Hello") where f is strict
239 -- Then, especially in the first of these cases, we'd like to discard
240 -- the continuation, leaving just the bottoming expression. But the
241 -- type might not be right, so we may have to add a coerce.
244 | null ss && discardableCont cont = (args, discardCont hole_ty cont)
245 | otherwise = (args, cont)
248 hole_ty = applyTypeToArgs (Var fun) (idType fun)
249 [substExpr_mb se arg | (arg,se,_) <- args]
250 substExpr_mb Nothing arg = arg
251 substExpr_mb (Just se) arg = substExpr se arg
253 ----------------------------
254 vanilla_stricts, computed_stricts :: [Bool]
255 vanilla_stricts = repeat False
256 computed_stricts = zipWith (||) fun_stricts arg_stricts
258 ----------------------------
259 (val_arg_tys, res_ty) = splitFunTys (dropForAlls (idType fun))
260 arg_stricts = map isStrictType val_arg_tys ++ repeat False
261 -- These argument types are used as a cheap and cheerful way to find
262 -- unboxed arguments, which must be strict. But it's an InType
263 -- and so there might be a type variable where we expect a function
264 -- type (the substitution hasn't happened yet). And we don't bother
265 -- doing the type applications for a polymorphic function.
266 -- Hence the splitFunTys*IgnoringForAlls*
268 ----------------------------
269 -- If fun_stricts is finite, it means the function returns bottom
270 -- after that number of value args have been consumed
271 -- Otherwise it's infinite, extended with False
273 = case splitStrictSig (idNewStrictness fun) of
274 (demands, result_info)
275 | not (demands `lengthExceeds` countValArgs orig_cont)
276 -> -- Enough args, use the strictness given.
277 -- For bottoming functions we used to pretend that the arg
278 -- is lazy, so that we don't treat the arg as an
279 -- interesting context. This avoids substituting
280 -- top-level bindings for (say) strings into
281 -- calls to error. But now we are more careful about
282 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
283 if isBotRes result_info then
284 map isStrictDmd demands -- Finite => result is bottom
286 map isStrictDmd demands ++ vanilla_stricts
288 other -> vanilla_stricts -- Not enough args, or no strictness
291 interestingArg :: OutExpr -> Bool
292 -- An argument is interesting if it has *some* structure
293 -- We are here trying to avoid unfolding a function that
294 -- is applied only to variables that have no unfolding
295 -- (i.e. they are probably lambda bound): f x y z
296 -- There is little point in inlining f here.
297 interestingArg (Var v) = hasSomeUnfolding (idUnfolding v)
298 -- Was: isValueUnfolding (idUnfolding v')
299 -- But that seems over-pessimistic
301 -- This accounts for an argument like
302 -- () or [], which is definitely interesting
303 interestingArg (Type _) = False
304 interestingArg (App fn (Type _)) = interestingArg fn
305 interestingArg (Note _ a) = interestingArg a
306 interestingArg other = True
307 -- Consider let x = 3 in f x
308 -- The substitution will contain (x -> ContEx 3), and we want to
309 -- to say that x is an interesting argument.
310 -- But consider also (\x. f x y) y
311 -- The substitution will contain (x -> ContEx y), and we want to say
312 -- that x is not interesting (assuming y has no unfolding)
315 Comment about interestingCallContext
316 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
317 We want to avoid inlining an expression where there can't possibly be
318 any gain, such as in an argument position. Hence, if the continuation
319 is interesting (eg. a case scrutinee, application etc.) then we
320 inline, otherwise we don't.
322 Previously some_benefit used to return True only if the variable was
323 applied to some value arguments. This didn't work:
325 let x = _coerce_ (T Int) Int (I# 3) in
326 case _coerce_ Int (T Int) x of
329 we want to inline x, but can't see that it's a constructor in a case
330 scrutinee position, and some_benefit is False.
334 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
336 .... case dMonadST _@_ x0 of (a,b,c) -> ....
338 we'd really like to inline dMonadST here, but we *don't* want to
339 inline if the case expression is just
341 case x of y { DEFAULT -> ... }
343 since we can just eliminate this case instead (x is in WHNF). Similar
344 applies when x is bound to a lambda expression. Hence
345 contIsInteresting looks for case expressions with just a single
349 interestingCallContext :: Bool -- False <=> no args at all
350 -> Bool -- False <=> no value args
352 -- The "lone-variable" case is important. I spent ages
353 -- messing about with unsatisfactory varaints, but this is nice.
354 -- The idea is that if a variable appear all alone
355 -- as an arg of lazy fn, or rhs Stop
356 -- as scrutinee of a case Select
357 -- as arg of a strict fn ArgOf
358 -- then we should not inline it (unless there is some other reason,
359 -- e.g. is is the sole occurrence). We achieve this by making
360 -- interestingCallContext return False for a lone variable.
362 -- Why? At least in the case-scrutinee situation, turning
363 -- let x = (a,b) in case x of y -> ...
365 -- let x = (a,b) in case (a,b) of y -> ...
367 -- let x = (a,b) in let y = (a,b) in ...
368 -- is bad if the binding for x will remain.
370 -- Another example: I discovered that strings
371 -- were getting inlined straight back into applications of 'error'
372 -- because the latter is strict.
374 -- f = \x -> ...(error s)...
376 -- Fundamentally such contexts should not ecourage inlining because
377 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
378 -- so there's no gain.
380 -- However, even a type application or coercion isn't a lone variable.
382 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
383 -- We had better inline that sucker! The case won't see through it.
385 -- For now, I'm treating treating a variable applied to types
386 -- in a *lazy* context "lone". The motivating example was
388 -- g = /\a. \y. h (f a)
389 -- There's no advantage in inlining f here, and perhaps
390 -- a significant disadvantage. Hence some_val_args in the Stop case
392 interestingCallContext some_args some_val_args cont
395 interesting (Select {}) = some_args
396 interesting (ApplyTo {}) = True -- Can happen if we have (coerce t (f x)) y
397 -- Perhaps True is a bit over-keen, but I've
398 -- seen (coerce f) x, where f has an INLINE prag,
399 -- So we have to give some motivaiton for inlining it
400 interesting (ArgOf {}) = some_val_args
401 interesting (Stop ty _ interesting) = some_val_args && interesting
402 interesting (CoerceIt _ cont) = interesting cont
403 -- If this call is the arg of a strict function, the context
404 -- is a bit interesting. If we inline here, we may get useful
405 -- evaluation information to avoid repeated evals: e.g.
407 -- Here the contIsInteresting makes the '*' keener to inline,
408 -- which in turn exposes a constructor which makes the '+' inline.
409 -- Assuming that +,* aren't small enough to inline regardless.
411 -- It's also very important to inline in a strict context for things
414 -- Here, the context of (f x) is strict, and if f's unfolding is
415 -- a build it's *great* to inline it here. So we must ensure that
416 -- the context for (f x) is not totally uninteresting.
420 interestingArgContext :: Id -> SimplCont -> Bool
421 -- If the argument has form (f x y), where x,y are boring,
422 -- and f is marked INLINE, then we don't want to inline f.
423 -- But if the context of the argument is
425 -- where g has rules, then we *do* want to inline f, in case it
426 -- exposes a rule that might fire. Similarly, if the context is
428 -- where h has rules, then we do want to inline f.
429 -- The interesting_arg_ctxt flag makes this happen; if it's
430 -- set, the inliner gets just enough keener to inline f
431 -- regardless of how boring f's arguments are, if it's marked INLINE
433 -- The alternative would be to *always* inline an INLINE function,
434 -- regardless of how boring its context is; but that seems overkill
435 -- For example, it'd mean that wrapper functions were always inlined
436 interestingArgContext fn cont
437 = idHasRules fn || go cont
439 go (Select {}) = False
440 go (ApplyTo {}) = False
442 go (CoerceIt _ c) = go c
443 go (Stop _ _ interesting) = interesting
446 canUpdateInPlace :: Type -> Bool
447 -- Consider let x = <wurble> in ...
448 -- If <wurble> returns an explicit constructor, we might be able
449 -- to do update in place. So we treat even a thunk RHS context
450 -- as interesting if update in place is possible. We approximate
451 -- this by seeing if the type has a single constructor with a
452 -- small arity. But arity zero isn't good -- we share the single copy
453 -- for that case, so no point in sharing.
456 | not opt_UF_UpdateInPlace = False
458 = case splitTyConApp_maybe ty of
460 Just (tycon, _) -> case tyConDataCons_maybe tycon of
461 Just [dc] -> arity == 1 || arity == 2
463 arity = dataConRepArity dc
469 %************************************************************************
471 \subsection{Decisions about inlining}
473 %************************************************************************
475 Inlining is controlled partly by the SimplifierMode switch. This has two
478 SimplGently (a) Simplifying before specialiser/full laziness
479 (b) Simplifiying inside INLINE pragma
480 (c) Simplifying the LHS of a rule
481 (d) Simplifying a GHCi expression or Template
484 SimplPhase n Used at all other times
486 The key thing about SimplGently is that it does no call-site inlining.
487 Before full laziness we must be careful not to inline wrappers,
488 because doing so inhibits floating
489 e.g. ...(case f x of ...)...
490 ==> ...(case (case x of I# x# -> fw x#) of ...)...
491 ==> ...(case x of I# x# -> case fw x# of ...)...
492 and now the redex (f x) isn't floatable any more.
494 The no-inlining thing is also important for Template Haskell. You might be
495 compiling in one-shot mode with -O2; but when TH compiles a splice before
496 running it, we don't want to use -O2. Indeed, we don't want to inline
497 anything, because the byte-code interpreter might get confused about
498 unboxed tuples and suchlike.
502 SimplGently is also used as the mode to simplify inside an InlineMe note.
505 inlineMode :: SimplifierMode
506 inlineMode = SimplGently
509 It really is important to switch off inlinings inside such
510 expressions. Consider the following example
516 in ...g...g...g...g...g...
518 Now, if that's the ONLY occurrence of f, it will be inlined inside g,
519 and thence copied multiple times when g is inlined.
522 This function may be inlinined in other modules, so we
523 don't want to remove (by inlining) calls to functions that have
524 specialisations, or that may have transformation rules in an importing
527 E.g. {-# INLINE f #-}
530 and suppose that g is strict *and* has specialisations. If we inline
531 g's wrapper, we deny f the chance of getting the specialised version
532 of g when f is inlined at some call site (perhaps in some other
535 It's also important not to inline a worker back into a wrapper.
537 wraper = inline_me (\x -> ...worker... )
538 Normally, the inline_me prevents the worker getting inlined into
539 the wrapper (initially, the worker's only call site!). But,
540 if the wrapper is sure to be called, the strictness analyser will
541 mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
542 continuation. That's why the keep_inline predicate returns True for
543 ArgOf continuations. It shouldn't do any harm not to dissolve the
544 inline-me note under these circumstances.
546 Note that the result is that we do very little simplification
549 all xs = foldr (&&) True xs
550 any p = all . map p {-# INLINE any #-}
552 Problem: any won't get deforested, and so if it's exported and the
553 importer doesn't use the inlining, (eg passes it as an arg) then we
554 won't get deforestation at all. We havn't solved this problem yet!
557 preInlineUnconditionally
558 ~~~~~~~~~~~~~~~~~~~~~~~~
559 @preInlineUnconditionally@ examines a bndr to see if it is used just
560 once in a completely safe way, so that it is safe to discard the
561 binding inline its RHS at the (unique) usage site, REGARDLESS of how
562 big the RHS might be. If this is the case we don't simplify the RHS
563 first, but just inline it un-simplified.
565 This is much better than first simplifying a perhaps-huge RHS and then
566 inlining and re-simplifying it. Indeed, it can be at least quadratically
575 We may end up simplifying e1 N times, e2 N-1 times, e3 N-3 times etc.
576 This can happen with cascades of functions too:
583 THE MAIN INVARIANT is this:
585 ---- preInlineUnconditionally invariant -----
586 IF preInlineUnconditionally chooses to inline x = <rhs>
587 THEN doing the inlining should not change the occurrence
588 info for the free vars of <rhs>
589 ----------------------------------------------
591 For example, it's tempting to look at trivial binding like
593 and inline it unconditionally. But suppose x is used many times,
594 but this is the unique occurrence of y. Then inlining x would change
595 y's occurrence info, which breaks the invariant. It matters: y
596 might have a BIG rhs, which will now be dup'd at every occurrenc of x.
599 Evne RHSs labelled InlineMe aren't caught here, because there might be
600 no benefit from inlining at the call site.
602 [Sept 01] Don't unconditionally inline a top-level thing, because that
603 can simply make a static thing into something built dynamically. E.g.
607 [Remember that we treat \s as a one-shot lambda.] No point in
608 inlining x unless there is something interesting about the call site.
610 But watch out: if you aren't careful, some useful foldr/build fusion
611 can be lost (most notably in spectral/hartel/parstof) because the
612 foldr didn't see the build. Doing the dynamic allocation isn't a big
613 deal, in fact, but losing the fusion can be. But the right thing here
614 seems to be to do a callSiteInline based on the fact that there is
615 something interesting about the call site (it's strict). Hmm. That
618 Conclusion: inline top level things gaily until Phase 0 (the last
619 phase), at which point don't.
622 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
623 preInlineUnconditionally env top_lvl bndr rhs
625 | opt_SimplNoPreInlining = False
626 | otherwise = case idOccInfo bndr of
627 IAmDead -> True -- Happens in ((\x.1) v)
628 OneOcc in_lam True int_cxt -> try_once in_lam int_cxt
632 active = case phase of
633 SimplGently -> isAlwaysActive prag
634 SimplPhase n -> isActive n prag
635 prag = idInlinePragma bndr
637 try_once in_lam int_cxt -- There's one textual occurrence
638 | not in_lam = isNotTopLevel top_lvl || early_phase
639 | otherwise = int_cxt && canInlineInLam rhs
641 -- Be very careful before inlining inside a lambda, becuase (a) we must not
642 -- invalidate occurrence information, and (b) we want to avoid pushing a
643 -- single allocation (here) into multiple allocations (inside lambda).
644 -- Inlining a *function* with a single *saturated* call would be ok, mind you.
645 -- || (if is_cheap && not (canInlineInLam rhs) then pprTrace "preinline" (ppr bndr <+> ppr rhs) ok else ok)
647 -- is_cheap = exprIsCheap rhs
648 -- ok = is_cheap && int_cxt
650 -- int_cxt The context isn't totally boring
651 -- E.g. let f = \ab.BIG in \y. map f xs
652 -- Don't want to substitute for f, because then we allocate
653 -- its closure every time the \y is called
654 -- But: let f = \ab.BIG in \y. map (f y) xs
655 -- Now we do want to substitute for f, even though it's not
656 -- saturated, because we're going to allocate a closure for
657 -- (f y) every time round the loop anyhow.
659 -- canInlineInLam => free vars of rhs are (Once in_lam) or Many,
660 -- so substituting rhs inside a lambda doesn't change the occ info.
661 -- Sadly, not quite the same as exprIsHNF.
662 canInlineInLam (Lit l) = True
663 canInlineInLam (Lam b e) = isRuntimeVar b || canInlineInLam e
664 canInlineInLam (Note _ e) = canInlineInLam e
665 canInlineInLam _ = False
667 early_phase = case phase of
668 SimplPhase 0 -> False
670 -- If we don't have this early_phase test, consider
671 -- x = length [1,2,3]
672 -- The full laziness pass carefully floats all the cons cells to
673 -- top level, and preInlineUnconditionally floats them all back in.
674 -- Result is (a) static allocation replaced by dynamic allocation
675 -- (b) many simplifier iterations because this tickles
676 -- a related problem; only one inlining per pass
678 -- On the other hand, I have seen cases where top-level fusion is
679 -- lost if we don't inline top level thing (e.g. string constants)
680 -- Hence the test for phase zero (which is the phase for all the final
681 -- simplifications). Until phase zero we take no special notice of
682 -- top level things, but then we become more leery about inlining
687 postInlineUnconditionally
688 ~~~~~~~~~~~~~~~~~~~~~~~~~
689 @postInlineUnconditionally@ decides whether to unconditionally inline
690 a thing based on the form of its RHS; in particular if it has a
691 trivial RHS. If so, we can inline and discard the binding altogether.
693 NB: a loop breaker has must_keep_binding = True and non-loop-breakers
694 only have *forward* references Hence, it's safe to discard the binding
696 NOTE: This isn't our last opportunity to inline. We're at the binding
697 site right now, and we'll get another opportunity when we get to the
700 Note that we do this unconditional inlining only for trival RHSs.
701 Don't inline even WHNFs inside lambdas; doing so may simply increase
702 allocation when the function is called. This isn't the last chance; see
705 NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here Why?
706 Because we don't even want to inline them into the RHS of constructor
707 arguments. See NOTE above
709 NB: At one time even NOINLINE was ignored here: if the rhs is trivial
710 it's best to inline it anyway. We often get a=E; b=a from desugaring,
711 with both a and b marked NOINLINE. But that seems incompatible with
712 our new view that inlining is like a RULE, so I'm sticking to the 'active'
716 postInlineUnconditionally
717 :: SimplEnv -> TopLevelFlag
718 -> InId -- The binder (an OutId would be fine too)
719 -> OccInfo -- From the InId
723 postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding
725 | isLoopBreaker occ_info = False
726 | isExportedId bndr = False
727 | exprIsTrivial rhs = True
730 -- The point of examining occ_info here is that for *non-values*
731 -- that occur outside a lambda, the call-site inliner won't have
732 -- a chance (becuase it doesn't know that the thing
733 -- only occurs once). The pre-inliner won't have gotten
734 -- it either, if the thing occurs in more than one branch
735 -- So the main target is things like
738 -- True -> case x of ...
739 -- False -> case x of ...
740 -- I'm not sure how important this is in practice
741 OneOcc in_lam one_br int_cxt -- OneOcc => no work-duplication issue
742 -> smallEnoughToInline unfolding -- Small enough to dup
743 -- ToDo: consider discount on smallEnoughToInline if int_cxt is true
745 -- NB: Do NOT inline arbitrarily big things, even if one_br is True
746 -- Reason: doing so risks exponential behaviour. We simplify a big
747 -- expression, inline it, and simplify it again. But if the
748 -- very same thing happens in the big expression, we get
750 -- PRINCIPLE: when we've already simplified an expression once,
751 -- make sure that we only inline it if it's reasonably small.
753 && ((isNotTopLevel top_lvl && not in_lam) ||
754 -- But outside a lambda, we want to be reasonably aggressive
755 -- about inlining into multiple branches of case
756 -- e.g. let x = <non-value>
757 -- in case y of { C1 -> ..x..; C2 -> ..x..; C3 -> ... }
758 -- Inlining can be a big win if C3 is the hot-spot, even if
759 -- the uses in C1, C2 are not 'interesting'
760 -- An example that gets worse if you add int_cxt here is 'clausify'
762 (isCheapUnfolding unfolding && int_cxt))
763 -- isCheap => acceptable work duplication; in_lam may be true
764 -- int_cxt to prevent us inlining inside a lambda without some
765 -- good reason. See the notes on int_cxt in preInlineUnconditionally
767 IAmDead -> True -- This happens; for example, the case_bndr during case of
768 -- known constructor: case (a,b) of x { (p,q) -> ... }
769 -- Here x isn't mentioned in the RHS, so we don't want to
770 -- create the (dead) let-binding let x = (a,b) in ...
774 -- Here's an example that we don't handle well:
775 -- let f = if b then Left (\x.BIG) else Right (\y.BIG)
776 -- in \y. ....case f of {...} ....
777 -- Here f is used just once, and duplicating the case work is fine (exprIsCheap).
779 -- * We can't preInlineUnconditionally because that woud invalidate
780 -- the occ info for b.
781 -- * We can't postInlineUnconditionally because the RHS is big, and
782 -- that risks exponential behaviour
783 -- * We can't call-site inline, because the rhs is big
787 active = case getMode env of
788 SimplGently -> isAlwaysActive prag
789 SimplPhase n -> isActive n prag
790 prag = idInlinePragma bndr
792 activeInline :: SimplEnv -> OutId -> OccInfo -> Bool
793 activeInline env id occ
794 = case getMode env of
795 SimplGently -> isOneOcc occ && isAlwaysActive prag
796 -- No inlining at all when doing gentle stuff,
797 -- except for local things that occur once
798 -- The reason is that too little clean-up happens if you
799 -- don't inline use-once things. Also a bit of inlining is *good* for
800 -- full laziness; it can expose constant sub-expressions.
801 -- Example in spectral/mandel/Mandel.hs, where the mandelset
802 -- function gets a useful let-float if you inline windowToViewport
804 -- NB: we used to have a second exception, for data con wrappers.
805 -- On the grounds that we use gentle mode for rule LHSs, and
806 -- they match better when data con wrappers are inlined.
807 -- But that only really applies to the trivial wrappers (like (:)),
808 -- and they are now constructed as Compulsory unfoldings (in MkId)
809 -- so they'll happen anyway.
811 SimplPhase n -> isActive n prag
813 prag = idInlinePragma id
815 activeRule :: SimplEnv -> Maybe (Activation -> Bool)
816 -- Nothing => No rules at all
818 | opt_RulesOff = Nothing
820 = case getMode env of
821 SimplGently -> Just isAlwaysActive
822 -- Used to be Nothing (no rules in gentle mode)
823 -- Main motivation for changing is that I wanted
824 -- lift String ===> ...
825 -- to work in Template Haskell when simplifying
826 -- splices, so we get simpler code for literal strings
827 SimplPhase n -> Just (isActive n)
831 %************************************************************************
833 \subsection{Rebuilding a lambda}
835 %************************************************************************
838 mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
842 a) eta reduction, if that gives a trivial expression
843 b) eta expansion [only if there are some value lambdas]
844 c) floating lets out through big lambdas
845 [only if all tyvar lambdas, and only if this lambda
849 mkLam env bndrs body cont
850 = getDOptsSmpl `thenSmpl` \dflags ->
851 mkLam' dflags env bndrs body cont
853 mkLam' dflags env bndrs body cont
854 | dopt Opt_DoEtaReduction dflags,
855 Just etad_lam <- tryEtaReduce bndrs body
856 = tick (EtaReduction (head bndrs)) `thenSmpl_`
857 returnSmpl (emptyFloats env, etad_lam)
859 | dopt Opt_DoLambdaEtaExpansion dflags,
860 any isRuntimeVar bndrs
861 = tryEtaExpansion dflags body `thenSmpl` \ body' ->
862 returnSmpl (emptyFloats env, mkLams bndrs body')
864 {- Sept 01: I'm experimenting with getting the
865 full laziness pass to float out past big lambdsa
866 | all isTyVar bndrs, -- Only for big lambdas
867 contIsRhs cont -- Only try the rhs type-lambda floating
868 -- if this is indeed a right-hand side; otherwise
869 -- we end up floating the thing out, only for float-in
870 -- to float it right back in again!
871 = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
872 returnSmpl (floats, mkLams bndrs body')
876 = returnSmpl (emptyFloats env, mkLams bndrs body)
880 %************************************************************************
882 \subsection{Eta expansion and reduction}
884 %************************************************************************
886 We try for eta reduction here, but *only* if we get all the
887 way to an exprIsTrivial expression.
888 We don't want to remove extra lambdas unless we are going
889 to avoid allocating this thing altogether
892 tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr
893 tryEtaReduce bndrs body
894 -- We don't use CoreUtils.etaReduce, because we can be more
896 -- (a) we already have the binders
897 -- (b) we can do the triviality test before computing the free vars
898 = go (reverse bndrs) body
900 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
901 go [] fun | ok_fun fun = Just fun -- Success!
902 go _ _ = Nothing -- Failure!
904 ok_fun fun = exprIsTrivial fun
905 && not (any (`elemVarSet` (exprFreeVars fun)) bndrs)
906 && (exprIsHNF fun || all ok_lam bndrs)
907 ok_lam v = isTyVar v || isDictId v
908 -- The exprIsHNF is because eta reduction is not
909 -- valid in general: \x. bot /= bot
910 -- So we need to be sure that the "fun" is a value.
912 -- However, we always want to reduce (/\a -> f a) to f
913 -- This came up in a RULE: foldr (build (/\a -> g a))
914 -- did not match foldr (build (/\b -> ...something complex...))
915 -- The type checker can insert these eta-expanded versions,
916 -- with both type and dictionary lambdas; hence the slightly
919 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
923 Try eta expansion for RHSs
926 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
929 where (in both cases)
931 * The xi can include type variables
933 * The yi are all value variables
935 * N is a NORMAL FORM (i.e. no redexes anywhere)
936 wanting a suitable number of extra args.
938 We may have to sandwich some coerces between the lambdas
939 to make the types work. exprEtaExpandArity looks through coerces
940 when computing arity; and etaExpand adds the coerces as necessary when
941 actually computing the expansion.
944 tryEtaExpansion :: DynFlags -> OutExpr -> SimplM OutExpr
945 -- There is at least one runtime binder in the binders
946 tryEtaExpansion dflags body
947 = getUniquesSmpl `thenSmpl` \ us ->
948 returnSmpl (etaExpand fun_arity us body (exprType body))
950 fun_arity = exprEtaExpandArity dflags body
954 %************************************************************************
956 \subsection{Floating lets out of big lambdas}
958 %************************************************************************
960 tryRhsTyLam tries this transformation, when the big lambda appears as
961 the RHS of a let(rec) binding:
963 /\abc -> let(rec) x = e in b
965 let(rec) x' = /\abc -> let x = x' a b c in e
967 /\abc -> let x = x' a b c in b
969 This is good because it can turn things like:
971 let f = /\a -> letrec g = ... g ... in g
973 letrec g' = /\a -> ... g' a ...
977 which is better. In effect, it means that big lambdas don't impede
980 This optimisation is CRUCIAL in eliminating the junk introduced by
981 desugaring mutually recursive definitions. Don't eliminate it lightly!
983 So far as the implementation is concerned:
985 Invariant: go F e = /\tvs -> F e
989 = Let x' = /\tvs -> F e
993 G = F . Let x = x' tvs
995 go F (Letrec xi=ei in b)
996 = Letrec {xi' = /\tvs -> G ei}
1000 G = F . Let {xi = xi' tvs}
1002 [May 1999] If we do this transformation *regardless* then we can
1003 end up with some pretty silly stuff. For example,
1006 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
1011 st = /\s -> ...[y1 s/x1, y2 s/x2]
1014 Unless the "..." is a WHNF there is really no point in doing this.
1015 Indeed it can make things worse. Suppose x1 is used strictly,
1018 x1* = case f y of { (a,b) -> e }
1020 If we abstract this wrt the tyvar we then can't do the case inline
1021 as we would normally do.
1025 {- Trying to do this in full laziness
1027 tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
1028 -- Call ensures that all the binders are type variables
1030 tryRhsTyLam env tyvars body -- Only does something if there's a let
1031 | not (all isTyVar tyvars)
1032 || not (worth_it body) -- inside a type lambda,
1033 = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
1036 = go env (\x -> x) body
1039 worth_it e@(Let _ _) = whnf_in_middle e
1042 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
1043 whnf_in_middle (Let _ e) = whnf_in_middle e
1044 whnf_in_middle e = exprIsCheap e
1046 main_tyvar_set = mkVarSet tyvars
1048 go env fn (Let bind@(NonRec var rhs) body)
1050 = go env (fn . Let bind) body
1052 go env fn (Let (NonRec var rhs) body)
1053 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
1054 addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
1055 go env (fn . Let (mk_silly_bind var rhs')) body
1059 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
1060 -- Abstract only over the type variables free in the rhs
1061 -- wrt which the new binding is abstracted. But the naive
1062 -- approach of abstract wrt the tyvars free in the Id's type
1064 -- /\ a b -> let t :: (a,b) = (e1, e2)
1067 -- Here, b isn't free in x's type, but we must nevertheless
1068 -- abstract wrt b as well, because t's type mentions b.
1069 -- Since t is floated too, we'd end up with the bogus:
1070 -- poly_t = /\ a b -> (e1, e2)
1071 -- poly_x = /\ a -> fst (poly_t a *b*)
1072 -- So for now we adopt the even more naive approach of
1073 -- abstracting wrt *all* the tyvars. We'll see if that
1074 -- gives rise to problems. SLPJ June 98
1076 go env fn (Let (Rec prs) body)
1077 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
1079 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
1080 pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
1082 addAuxiliaryBind env (Rec pairs) $ \ env ->
1085 (vars,rhss) = unzip prs
1086 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
1087 -- See notes with tyvars_here above
1089 go env fn body = returnSmpl (emptyFloats env, fn body)
1091 mk_poly tyvars_here var
1092 = getUniqueSmpl `thenSmpl` \ uniq ->
1094 poly_name = setNameUnique (idName var) uniq -- Keep same name
1095 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
1096 poly_id = mkLocalId poly_name poly_ty
1098 -- In the olden days, it was crucial to copy the occInfo of the original var,
1099 -- because we were looking at occurrence-analysed but as yet unsimplified code!
1100 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
1101 -- at already simplified code, so it doesn't matter
1103 -- It's even right to retain single-occurrence or dead-var info:
1104 -- Suppose we started with /\a -> let x = E in B
1105 -- where x occurs once in B. Then we transform to:
1106 -- let x' = /\a -> E in /\a -> let x* = x' a in B
1107 -- where x* has an INLINE prag on it. Now, once x* is inlined,
1108 -- the occurrences of x' will be just the occurrences originally
1111 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
1113 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
1114 -- Suppose we start with:
1116 -- x = /\ a -> let g = G in E
1118 -- Then we'll float to get
1120 -- x = let poly_g = /\ a -> G
1121 -- in /\ a -> let g = poly_g a in E
1123 -- But now the occurrence analyser will see just one occurrence
1124 -- of poly_g, not inside a lambda, so the simplifier will
1125 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
1126 -- (I used to think that the "don't inline lone occurrences" stuff
1127 -- would stop this happening, but since it's the *only* occurrence,
1128 -- PreInlineUnconditionally kicks in first!)
1130 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
1131 -- to appear many times. (NB: mkInlineMe eliminates
1132 -- such notes on trivial RHSs, so do it manually.)
1136 %************************************************************************
1138 \subsection{Case absorption and identity-case elimination}
1140 %************************************************************************
1143 mkCase puts a case expression back together, trying various transformations first.
1146 mkCase :: OutExpr -> OutId -> OutType
1147 -> [OutAlt] -- Increasing order
1150 mkCase scrut case_bndr ty alts
1151 = getDOptsSmpl `thenSmpl` \dflags ->
1152 mkAlts dflags scrut case_bndr alts `thenSmpl` \ better_alts ->
1153 mkCase1 scrut case_bndr ty better_alts
1157 mkAlts tries these things:
1159 1. If several alternatives are identical, merge them into
1160 a single DEFAULT alternative. I've occasionally seen this
1161 making a big difference:
1163 case e of =====> case e of
1164 C _ -> f x D v -> ....v....
1165 D v -> ....v.... DEFAULT -> f x
1168 The point is that we merge common RHSs, at least for the DEFAULT case.
1169 [One could do something more elaborate but I've never seen it needed.]
1170 To avoid an expensive test, we just merge branches equal to the *first*
1171 alternative; this picks up the common cases
1172 a) all branches equal
1173 b) some branches equal to the DEFAULT (which occurs first)
1176 case e of b { ==> case e of b {
1177 p1 -> rhs1 p1 -> rhs1
1179 pm -> rhsm pm -> rhsm
1180 _ -> case b of b' { pn -> let b'=b in rhsn
1182 ... po -> let b'=b in rhso
1183 po -> rhso _ -> let b'=b in rhsd
1187 which merges two cases in one case when -- the default alternative of
1188 the outer case scrutises the same variable as the outer case This
1189 transformation is called Case Merging. It avoids that the same
1190 variable is scrutinised multiple times.
1193 The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
1199 where @is@ was something like
1201 p `is` n = p /= (-1) && p == n
1203 This gave rise to a horrible sequence of cases
1210 and similarly in cascade for all the join points!
1215 --------------------------------------------------
1216 -- 1. Merge identical branches
1217 --------------------------------------------------
1218 mkAlts dflags scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
1219 | all isDeadBinder bndrs1, -- Remember the default
1220 length filtered_alts < length con_alts -- alternative comes first
1221 = tick (AltMerge case_bndr) `thenSmpl_`
1222 returnSmpl better_alts
1224 filtered_alts = filter keep con_alts
1225 keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
1226 better_alts = (DEFAULT, [], rhs1) : filtered_alts
1229 --------------------------------------------------
1230 -- 2. Merge nested cases
1231 --------------------------------------------------
1233 mkAlts dflags scrut outer_bndr outer_alts
1234 | dopt Opt_CaseMerge dflags,
1235 (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
1236 Just (Case (Var scrut_var) inner_bndr _ inner_alts) <- maybe_outer_deflt,
1237 scruting_same_var scrut_var
1239 munged_inner_alts = [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts]
1240 munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
1242 new_alts = mergeAlts outer_alts_without_deflt munged_inner_alts
1243 -- The merge keeps the inner DEFAULT at the front, if there is one
1244 -- and eliminates any inner_alts that are shadowed by the outer_alts
1246 tick (CaseMerge outer_bndr) `thenSmpl_`
1248 -- Warning: don't call mkAlts recursively!
1249 -- Firstly, there's no point, because inner alts have already had
1250 -- mkCase applied to them, so they won't have a case in their default
1251 -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
1252 -- in munge_rhs may put a case into the DEFAULT branch!
1254 -- We are scrutinising the same variable if it's
1255 -- the outer case-binder, or if the outer case scrutinises a variable
1256 -- (and it's the same). Testing both allows us not to replace the
1257 -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
1258 scruting_same_var = case scrut of
1259 Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
1260 other -> \ v -> v == outer_bndr
1262 ------------------------------------------------
1264 ------------------------------------------------
1266 mkAlts dflags scrut case_bndr other_alts = returnSmpl other_alts
1271 =================================================================================
1273 mkCase1 tries these things
1275 1. Eliminate the case altogether if possible
1283 and similar friends.
1286 Start with a simple situation:
1288 case x# of ===> e[x#/y#]
1291 (when x#, y# are of primitive type, of course). We can't (in general)
1292 do this for algebraic cases, because we might turn bottom into
1295 Actually, we generalise this idea to look for a case where we're
1296 scrutinising a variable, and we know that only the default case can
1301 other -> ...(case x of
1305 Here the inner case can be eliminated. This really only shows up in
1306 eliminating error-checking code.
1308 We also make sure that we deal with this very common case:
1313 Here we are using the case as a strict let; if x is used only once
1314 then we want to inline it. We have to be careful that this doesn't
1315 make the program terminate when it would have diverged before, so we
1317 - x is used strictly, or
1318 - e is already evaluated (it may so if e is a variable)
1320 Lastly, we generalise the transformation to handle this:
1326 We only do this for very cheaply compared r's (constructors, literals
1327 and variables). If pedantic bottoms is on, we only do it when the
1328 scrutinee is a PrimOp which can't fail.
1330 We do it *here*, looking at un-simplified alternatives, because we
1331 have to check that r doesn't mention the variables bound by the
1332 pattern in each alternative, so the binder-info is rather useful.
1334 So the case-elimination algorithm is:
1336 1. Eliminate alternatives which can't match
1338 2. Check whether all the remaining alternatives
1339 (a) do not mention in their rhs any of the variables bound in their pattern
1340 and (b) have equal rhss
1342 3. Check we can safely ditch the case:
1343 * PedanticBottoms is off,
1344 or * the scrutinee is an already-evaluated variable
1345 or * the scrutinee is a primop which is ok for speculation
1346 -- ie we want to preserve divide-by-zero errors, and
1347 -- calls to error itself!
1349 or * [Prim cases] the scrutinee is a primitive variable
1351 or * [Alg cases] the scrutinee is a variable and
1352 either * the rhs is the same variable
1353 (eg case x of C a b -> x ===> x)
1354 or * there is only one alternative, the default alternative,
1355 and the binder is used strictly in its scope.
1356 [NB this is helped by the "use default binder where
1357 possible" transformation; see below.]
1360 If so, then we can replace the case with one of the rhss.
1362 Further notes about case elimination
1363 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1364 Consider: test :: Integer -> IO ()
1367 Turns out that this compiles to:
1370 eta1 :: State# RealWorld ->
1371 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1373 (PrelNum.jtos eta ($w[] @ Char))
1375 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1377 Notice the strange '<' which has no effect at all. This is a funny one.
1378 It started like this:
1380 f x y = if x < 0 then jtos x
1381 else if y==0 then "" else jtos x
1383 At a particular call site we have (f v 1). So we inline to get
1385 if v < 0 then jtos x
1386 else if 1==0 then "" else jtos x
1388 Now simplify the 1==0 conditional:
1390 if v<0 then jtos v else jtos v
1392 Now common-up the two branches of the case:
1394 case (v<0) of DEFAULT -> jtos v
1396 Why don't we drop the case? Because it's strict in v. It's technically
1397 wrong to drop even unnecessary evaluations, and in practice they
1398 may be a result of 'seq' so we *definitely* don't want to drop those.
1399 I don't really know how to improve this situation.
1403 --------------------------------------------------
1404 -- 0. Check for empty alternatives
1405 --------------------------------------------------
1407 -- This isn't strictly an error. It's possible that the simplifer might "see"
1408 -- that an inner case has no accessible alternatives before it "sees" that the
1409 -- entire branch of an outer case is inaccessible. So we simply
1410 -- put an error case here insteadd
1411 mkCase1 scrut case_bndr ty []
1412 = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
1413 return (mkApps (Var eRROR_ID)
1414 [Type ty, Lit (mkStringLit "Impossible alternative")])
1416 --------------------------------------------------
1417 -- 1. Eliminate the case altogether if poss
1418 --------------------------------------------------
1420 mkCase1 scrut case_bndr ty [(con,bndrs,rhs)]
1421 -- See if we can get rid of the case altogether
1422 -- See the extensive notes on case-elimination above
1423 -- mkCase made sure that if all the alternatives are equal,
1424 -- then there is now only one (DEFAULT) rhs
1425 | all isDeadBinder bndrs,
1427 -- Check that the scrutinee can be let-bound instead of case-bound
1428 exprOkForSpeculation scrut
1429 -- OK not to evaluate it
1430 -- This includes things like (==# a# b#)::Bool
1431 -- so that we simplify
1432 -- case ==# a# b# of { True -> x; False -> x }
1435 -- This particular example shows up in default methods for
1436 -- comparision operations (e.g. in (>=) for Int.Int32)
1437 || exprIsHNF scrut -- It's already evaluated
1438 || var_demanded_later scrut -- It'll be demanded later
1440 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1441 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1442 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1443 -- its argument: case x of { y -> dataToTag# y }
1444 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1445 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1447 -- Also we don't want to discard 'seq's
1448 = tick (CaseElim case_bndr) `thenSmpl_`
1449 returnSmpl (bindCaseBndr case_bndr scrut rhs)
1452 -- The case binder is going to be evaluated later,
1453 -- and the scrutinee is a simple variable
1454 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1455 var_demanded_later other = False
1458 --------------------------------------------------
1460 --------------------------------------------------
1462 mkCase1 scrut case_bndr ty alts -- Identity case
1463 | all identity_alt alts
1464 = tick (CaseIdentity case_bndr) `thenSmpl_`
1465 returnSmpl (re_cast scrut)
1467 identity_alt (con, args, rhs) = de_cast rhs `cheapEqExpr` mk_id_rhs con args
1469 mk_id_rhs (DataAlt con) args = mkConApp con (arg_tys ++ varsToCoreExprs args)
1470 mk_id_rhs (LitAlt lit) _ = Lit lit
1471 mk_id_rhs DEFAULT _ = Var case_bndr
1473 arg_tys = map Type (tyConAppArgs (idType case_bndr))
1476 -- case e of x { _ -> x `cast` c }
1477 -- And we definitely want to eliminate this case, to give
1479 -- So we throw away the cast from the RHS, and reconstruct
1480 -- it at the other end. All the RHS casts must be the same
1481 -- if (all identity_alt alts) holds.
1483 -- Don't worry about nested casts, because the simplifier combines them
1484 de_cast (Cast e _) = e
1487 re_cast scrut = case head alts of
1488 (_,_,Cast _ co) -> Cast scrut co
1493 --------------------------------------------------
1495 --------------------------------------------------
1496 mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts)
1500 When adding auxiliary bindings for the case binder, it's worth checking if
1501 its dead, because it often is, and occasionally these mkCase transformations
1502 cascade rather nicely.
1505 bindCaseBndr bndr rhs body
1506 | isDeadBinder bndr = body
1507 | otherwise = bindNonRec bndr rhs body