2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
9 mkLam, mkCase, prepareAlts, bindCaseBndr,
12 preInlineUnconditionally, postInlineUnconditionally,
13 activeInline, activeRule,
15 -- The continuation type
16 SimplCont(..), DupFlag(..), ArgInfo(..),
17 contIsDupable, contResultType, contIsTrivial, contArgs, dropArgs,
18 countValArgs, countArgs,
19 mkBoringStop, mkLazyArgStop, contIsRhsOrArg,
20 interestingCallContext,
22 interestingArg, mkArgInfo,
27 #include "HsVersions.h"
33 import qualified CoreSubst
37 import CoreArity ( etaExpand, exprEtaExpandArity )
41 import Var ( isCoVar )
44 import Type hiding( substTy )
45 import Coercion ( coercionKind )
47 import Unify ( dataConCannotMatch )
59 %************************************************************************
63 %************************************************************************
65 A SimplCont allows the simplifier to traverse the expression in a
66 zipper-like fashion. The SimplCont represents the rest of the expression,
67 "above" the point of interest.
69 You can also think of a SimplCont as an "evaluation context", using
70 that term in the way it is used for operational semantics. This is the
71 way I usually think of it, For example you'll often see a syntax for
72 evaluation context looking like
73 C ::= [] | C e | case C of alts | C `cast` co
74 That's the kind of thing we are doing here, and I use that syntax in
79 * A SimplCont describes a *strict* context (just like
80 evaluation contexts do). E.g. Just [] is not a SimplCont
82 * A SimplCont describes a context that *does not* bind
83 any variables. E.g. \x. [] is not a SimplCont
87 = Stop -- An empty context, or hole, []
88 CallCtxt -- True <=> There is something interesting about
89 -- the context, and hence the inliner
90 -- should be a bit keener (see interestingCallContext)
92 -- This is an argument of a function that has RULES
93 -- Inlining the call might allow the rule to fire
95 | CoerceIt -- C `cast` co
96 OutCoercion -- The coercion simplified
101 InExpr SimplEnv -- The argument and its static env
104 | Select -- case C of alts
106 InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
109 -- The two strict forms have no DupFlag, because we never duplicate them
110 | StrictBind -- (\x* \xs. e) C
111 InId [InBndr] -- let x* = [] in e
112 InExpr SimplEnv -- is a special case
116 OutExpr -- e; *always* of form (Var v `App1` e1 .. `App` en)
117 CallCtxt -- Whether *this* argument position is interesting
118 ArgInfo -- Whether the function at the head of e has rules, etc
119 SimplCont -- plus strictness flags for *further* args
123 ai_rules :: Bool, -- Function has rules (recursively)
124 -- => be keener to inline in all args
125 ai_strs :: [Bool], -- Strictness of arguments
126 -- Usually infinite, but if it is finite it guarantees
127 -- that the function diverges after being given
128 -- that number of args
129 ai_discs :: [Int] -- Discounts for arguments; non-zero => be keener to inline
133 instance Outputable SimplCont where
134 ppr (Stop interesting) = ptext (sLit "Stop") <> brackets (ppr interesting)
135 ppr (ApplyTo dup arg _ cont) = ((ptext (sLit "ApplyTo") <+> ppr dup <+> pprParendExpr arg)
136 {- $$ nest 2 (pprSimplEnv se) -}) $$ ppr cont
137 ppr (StrictBind b _ _ _ cont) = (ptext (sLit "StrictBind") <+> ppr b) $$ ppr cont
138 ppr (StrictArg f _ _ cont) = (ptext (sLit "StrictArg") <+> ppr f) $$ ppr cont
139 ppr (Select dup bndr alts _ cont) = (ptext (sLit "Select") <+> ppr dup <+> ppr bndr) $$
140 (nest 4 (ppr alts)) $$ ppr cont
141 ppr (CoerceIt co cont) = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont
143 data DupFlag = OkToDup | NoDup
145 instance Outputable DupFlag where
146 ppr OkToDup = ptext (sLit "ok")
147 ppr NoDup = ptext (sLit "nodup")
152 mkBoringStop :: SimplCont
153 mkBoringStop = Stop BoringCtxt
155 mkLazyArgStop :: CallCtxt -> SimplCont
156 mkLazyArgStop cci = Stop cci
159 contIsRhsOrArg :: SimplCont -> Bool
160 contIsRhsOrArg (Stop {}) = True
161 contIsRhsOrArg (StrictBind {}) = True
162 contIsRhsOrArg (StrictArg {}) = True
163 contIsRhsOrArg _ = False
166 contIsDupable :: SimplCont -> Bool
167 contIsDupable (Stop {}) = True
168 contIsDupable (ApplyTo OkToDup _ _ _) = True
169 contIsDupable (Select OkToDup _ _ _ _) = True
170 contIsDupable (CoerceIt _ cont) = contIsDupable cont
171 contIsDupable _ = False
174 contIsTrivial :: SimplCont -> Bool
175 contIsTrivial (Stop {}) = True
176 contIsTrivial (ApplyTo _ (Type _) _ cont) = contIsTrivial cont
177 contIsTrivial (CoerceIt _ cont) = contIsTrivial cont
178 contIsTrivial _ = False
181 contResultType :: SimplEnv -> OutType -> SimplCont -> OutType
182 contResultType env ty cont
185 subst_ty se ty = substTy (se `setInScope` env) ty
188 go (CoerceIt co cont) _ = go cont (snd (coercionKind co))
189 go (StrictBind _ bs body se cont) _ = go cont (subst_ty se (exprType (mkLams bs body)))
190 go (StrictArg fn _ _ cont) _ = go cont (funResultTy (exprType fn))
191 go (Select _ _ alts se cont) _ = go cont (subst_ty se (coreAltsType alts))
192 go (ApplyTo _ arg se cont) ty = go cont (apply_to_arg ty arg se)
194 apply_to_arg ty (Type ty_arg) se = applyTy ty (subst_ty se ty_arg)
195 apply_to_arg ty _ _ = funResultTy ty
198 countValArgs :: SimplCont -> Int
199 countValArgs (ApplyTo _ (Type _) _ cont) = countValArgs cont
200 countValArgs (ApplyTo _ _ _ cont) = 1 + countValArgs cont
203 countArgs :: SimplCont -> Int
204 countArgs (ApplyTo _ _ _ cont) = 1 + countArgs cont
207 contArgs :: SimplCont -> ([OutExpr], SimplCont)
208 -- Uses substitution to turn each arg into an OutExpr
209 contArgs cont = go [] cont
211 go args (ApplyTo _ arg se cont) = go (substExpr se arg : args) cont
212 go args cont = (reverse args, cont)
214 dropArgs :: Int -> SimplCont -> SimplCont
215 dropArgs 0 cont = cont
216 dropArgs n (ApplyTo _ _ _ cont) = dropArgs (n-1) cont
217 dropArgs n other = pprPanic "dropArgs" (ppr n <+> ppr other)
221 Note [Interesting call context]
222 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
223 We want to avoid inlining an expression where there can't possibly be
224 any gain, such as in an argument position. Hence, if the continuation
225 is interesting (eg. a case scrutinee, application etc.) then we
226 inline, otherwise we don't.
228 Previously some_benefit used to return True only if the variable was
229 applied to some value arguments. This didn't work:
231 let x = _coerce_ (T Int) Int (I# 3) in
232 case _coerce_ Int (T Int) x of
235 we want to inline x, but can't see that it's a constructor in a case
236 scrutinee position, and some_benefit is False.
240 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
242 .... case dMonadST _@_ x0 of (a,b,c) -> ....
244 we'd really like to inline dMonadST here, but we *don't* want to
245 inline if the case expression is just
247 case x of y { DEFAULT -> ... }
249 since we can just eliminate this case instead (x is in WHNF). Similar
250 applies when x is bound to a lambda expression. Hence
251 contIsInteresting looks for case expressions with just a single
256 interestingCallContext :: SimplCont -> CallCtxt
257 -- See Note [Interesting call context]
258 interestingCallContext cont
261 interesting (Select _ bndr _ _ _)
262 | isDeadBinder bndr = CaseCtxt
263 | otherwise = ArgCtxt False 2 -- If the binder is used, this
264 -- is like a strict let
266 interesting (ApplyTo _ arg _ cont)
267 | isTypeArg arg = interesting cont
268 | otherwise = ValAppCtxt -- Can happen if we have (f Int |> co) y
269 -- If f has an INLINE prag we need to give it some
270 -- motivation to inline. See Note [Cast then apply]
273 interesting (StrictArg _ cci _ _) = cci
274 interesting (StrictBind {}) = BoringCtxt
275 interesting (Stop cci) = cci
276 interesting (CoerceIt _ cont) = interesting cont
277 -- If this call is the arg of a strict function, the context
278 -- is a bit interesting. If we inline here, we may get useful
279 -- evaluation information to avoid repeated evals: e.g.
281 -- Here the contIsInteresting makes the '*' keener to inline,
282 -- which in turn exposes a constructor which makes the '+' inline.
283 -- Assuming that +,* aren't small enough to inline regardless.
285 -- It's also very important to inline in a strict context for things
288 -- Here, the context of (f x) is strict, and if f's unfolding is
289 -- a build it's *great* to inline it here. So we must ensure that
290 -- the context for (f x) is not totally uninteresting.
295 -> [CoreRule] -- Rules for function
296 -> Int -- Number of value args
297 -> SimplCont -- Context of the call
300 mkArgInfo fun rules n_val_args call_cont
301 | n_val_args < idArity fun -- Note [Unsaturated functions]
302 = ArgInfo { ai_rules = False
303 , ai_strs = vanilla_stricts
304 , ai_discs = vanilla_discounts }
306 = ArgInfo { ai_rules = interestingArgContext rules call_cont
307 , ai_strs = add_type_str (idType fun) arg_stricts
308 , ai_discs = arg_discounts }
310 vanilla_discounts, arg_discounts :: [Int]
311 vanilla_discounts = repeat 0
312 arg_discounts = case idUnfolding fun of
313 CoreUnfolding {uf_guidance = UnfoldIfGoodArgs {ug_args = discounts}}
314 -> discounts ++ vanilla_discounts
315 _ -> vanilla_discounts
317 vanilla_stricts, arg_stricts :: [Bool]
318 vanilla_stricts = repeat False
321 = case splitStrictSig (idNewStrictness fun) of
322 (demands, result_info)
323 | not (demands `lengthExceeds` n_val_args)
324 -> -- Enough args, use the strictness given.
325 -- For bottoming functions we used to pretend that the arg
326 -- is lazy, so that we don't treat the arg as an
327 -- interesting context. This avoids substituting
328 -- top-level bindings for (say) strings into
329 -- calls to error. But now we are more careful about
330 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
331 if isBotRes result_info then
332 map isStrictDmd demands -- Finite => result is bottom
334 map isStrictDmd demands ++ vanilla_stricts
336 -> WARN( True, text "More demands than arity" <+> ppr fun <+> ppr (idArity fun)
337 <+> ppr n_val_args <+> ppr demands )
338 vanilla_stricts -- Not enough args, or no strictness
340 add_type_str :: Type -> [Bool] -> [Bool]
341 -- If the function arg types are strict, record that in the 'strictness bits'
342 -- No need to instantiate because unboxed types (which dominate the strict
343 -- types) can't instantiate type variables.
344 -- add_type_str is done repeatedly (for each call); might be better
345 -- once-for-all in the function
346 -- But beware primops/datacons with no strictness
347 add_type_str _ [] = []
348 add_type_str fun_ty strs -- Look through foralls
349 | Just (_, fun_ty') <- splitForAllTy_maybe fun_ty -- Includes coercions
350 = add_type_str fun_ty' strs
351 add_type_str fun_ty (str:strs) -- Add strict-type info
352 | Just (arg_ty, fun_ty') <- splitFunTy_maybe fun_ty
353 = (str || isStrictType arg_ty) : add_type_str fun_ty' strs
357 {- Note [Unsaturated functions]
358 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
359 Consider (test eyeball/inline4)
362 where f has arity 2. Then we do not want to inline 'x', because
363 it'll just be floated out again. Even if f has lots of discounts
364 on its first argument -- it must be saturated for these to kick in
367 interestingArgContext :: [CoreRule] -> SimplCont -> Bool
368 -- If the argument has form (f x y), where x,y are boring,
369 -- and f is marked INLINE, then we don't want to inline f.
370 -- But if the context of the argument is
372 -- where g has rules, then we *do* want to inline f, in case it
373 -- exposes a rule that might fire. Similarly, if the context is
375 -- where h has rules, then we do want to inline f; hence the
376 -- call_cont argument to interestingArgContext
378 -- The ai-rules flag makes this happen; if it's
379 -- set, the inliner gets just enough keener to inline f
380 -- regardless of how boring f's arguments are, if it's marked INLINE
382 -- The alternative would be to *always* inline an INLINE function,
383 -- regardless of how boring its context is; but that seems overkill
384 -- For example, it'd mean that wrapper functions were always inlined
385 interestingArgContext rules call_cont
386 = notNull rules || enclosing_fn_has_rules
388 enclosing_fn_has_rules = go call_cont
390 go (Select {}) = False
391 go (ApplyTo {}) = False
392 go (StrictArg _ cci _ _) = interesting cci
393 go (StrictBind {}) = False -- ??
394 go (CoerceIt _ c) = go c
395 go (Stop cci) = interesting cci
397 interesting (ArgCtxt rules _) = rules
398 interesting _ = False
403 %************************************************************************
405 \subsection{Decisions about inlining}
407 %************************************************************************
409 Inlining is controlled partly by the SimplifierMode switch. This has two
412 SimplGently (a) Simplifying before specialiser/full laziness
413 (b) Simplifiying inside INLINE pragma
414 (c) Simplifying the LHS of a rule
415 (d) Simplifying a GHCi expression or Template
418 SimplPhase n _ Used at all other times
420 The key thing about SimplGently is that it does no call-site inlining.
421 Before full laziness we must be careful not to inline wrappers,
422 because doing so inhibits floating
423 e.g. ...(case f x of ...)...
424 ==> ...(case (case x of I# x# -> fw x#) of ...)...
425 ==> ...(case x of I# x# -> case fw x# of ...)...
426 and now the redex (f x) isn't floatable any more.
428 The no-inlining thing is also important for Template Haskell. You might be
429 compiling in one-shot mode with -O2; but when TH compiles a splice before
430 running it, we don't want to use -O2. Indeed, we don't want to inline
431 anything, because the byte-code interpreter might get confused about
432 unboxed tuples and suchlike.
436 We don't simplify inside InlineRules (which come from INLINE pragmas).
437 It really is important to switch off inlinings inside such
438 expressions. Consider the following example
444 in ...g...g...g...g...g...
446 Now, if that's the ONLY occurrence of f, it will be inlined inside g,
447 and thence copied multiple times when g is inlined.
450 This function may be inlinined in other modules, so we
451 don't want to remove (by inlining) calls to functions that have
452 specialisations, or that may have transformation rules in an importing
455 E.g. {-# INLINE f #-}
458 and suppose that g is strict *and* has specialisations. If we inline
459 g's wrapper, we deny f the chance of getting the specialised version
460 of g when f is inlined at some call site (perhaps in some other
463 It's also important not to inline a worker back into a wrapper.
465 wraper = inline_me (\x -> ...worker... )
466 Normally, the inline_me prevents the worker getting inlined into
467 the wrapper (initially, the worker's only call site!). But,
468 if the wrapper is sure to be called, the strictness analyser will
469 mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
470 continuation. That's why the keep_inline predicate returns True for
471 ArgOf continuations. It shouldn't do any harm not to dissolve the
472 inline-me note under these circumstances.
474 Note that the result is that we do very little simplification
477 all xs = foldr (&&) True xs
478 any p = all . map p {-# INLINE any #-}
480 Problem: any won't get deforested, and so if it's exported and the
481 importer doesn't use the inlining, (eg passes it as an arg) then we
482 won't get deforestation at all. We havn't solved this problem yet!
485 preInlineUnconditionally
486 ~~~~~~~~~~~~~~~~~~~~~~~~
487 @preInlineUnconditionally@ examines a bndr to see if it is used just
488 once in a completely safe way, so that it is safe to discard the
489 binding inline its RHS at the (unique) usage site, REGARDLESS of how
490 big the RHS might be. If this is the case we don't simplify the RHS
491 first, but just inline it un-simplified.
493 This is much better than first simplifying a perhaps-huge RHS and then
494 inlining and re-simplifying it. Indeed, it can be at least quadratically
503 We may end up simplifying e1 N times, e2 N-1 times, e3 N-3 times etc.
504 This can happen with cascades of functions too:
511 THE MAIN INVARIANT is this:
513 ---- preInlineUnconditionally invariant -----
514 IF preInlineUnconditionally chooses to inline x = <rhs>
515 THEN doing the inlining should not change the occurrence
516 info for the free vars of <rhs>
517 ----------------------------------------------
519 For example, it's tempting to look at trivial binding like
521 and inline it unconditionally. But suppose x is used many times,
522 but this is the unique occurrence of y. Then inlining x would change
523 y's occurrence info, which breaks the invariant. It matters: y
524 might have a BIG rhs, which will now be dup'd at every occurrenc of x.
527 Even RHSs labelled InlineMe aren't caught here, because there might be
528 no benefit from inlining at the call site.
530 [Sept 01] Don't unconditionally inline a top-level thing, because that
531 can simply make a static thing into something built dynamically. E.g.
535 [Remember that we treat \s as a one-shot lambda.] No point in
536 inlining x unless there is something interesting about the call site.
538 But watch out: if you aren't careful, some useful foldr/build fusion
539 can be lost (most notably in spectral/hartel/parstof) because the
540 foldr didn't see the build. Doing the dynamic allocation isn't a big
541 deal, in fact, but losing the fusion can be. But the right thing here
542 seems to be to do a callSiteInline based on the fact that there is
543 something interesting about the call site (it's strict). Hmm. That
546 Conclusion: inline top level things gaily until Phase 0 (the last
547 phase), at which point don't.
550 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
551 preInlineUnconditionally env top_lvl bndr rhs
553 | opt_SimplNoPreInlining = False
554 | otherwise = case idOccInfo bndr of
555 IAmDead -> True -- Happens in ((\x.1) v)
556 OneOcc in_lam True int_cxt -> try_once in_lam int_cxt
560 active = case phase of
561 SimplGently -> isEarlyActive act
562 SimplPhase n _ -> isActive n act
563 act = idInlineActivation bndr
565 try_once in_lam int_cxt -- There's one textual occurrence
566 | not in_lam = isNotTopLevel top_lvl || early_phase
567 | otherwise = int_cxt && canInlineInLam rhs
569 -- Be very careful before inlining inside a lambda, becuase (a) we must not
570 -- invalidate occurrence information, and (b) we want to avoid pushing a
571 -- single allocation (here) into multiple allocations (inside lambda).
572 -- Inlining a *function* with a single *saturated* call would be ok, mind you.
573 -- || (if is_cheap && not (canInlineInLam rhs) then pprTrace "preinline" (ppr bndr <+> ppr rhs) ok else ok)
575 -- is_cheap = exprIsCheap rhs
576 -- ok = is_cheap && int_cxt
578 -- int_cxt The context isn't totally boring
579 -- E.g. let f = \ab.BIG in \y. map f xs
580 -- Don't want to substitute for f, because then we allocate
581 -- its closure every time the \y is called
582 -- But: let f = \ab.BIG in \y. map (f y) xs
583 -- Now we do want to substitute for f, even though it's not
584 -- saturated, because we're going to allocate a closure for
585 -- (f y) every time round the loop anyhow.
587 -- canInlineInLam => free vars of rhs are (Once in_lam) or Many,
588 -- so substituting rhs inside a lambda doesn't change the occ info.
589 -- Sadly, not quite the same as exprIsHNF.
590 canInlineInLam (Lit _) = True
591 canInlineInLam (Lam b e) = isRuntimeVar b || canInlineInLam e
592 canInlineInLam (Note _ e) = canInlineInLam e
593 canInlineInLam _ = False
595 early_phase = case phase of
596 SimplPhase 0 _ -> False
598 -- If we don't have this early_phase test, consider
599 -- x = length [1,2,3]
600 -- The full laziness pass carefully floats all the cons cells to
601 -- top level, and preInlineUnconditionally floats them all back in.
602 -- Result is (a) static allocation replaced by dynamic allocation
603 -- (b) many simplifier iterations because this tickles
604 -- a related problem; only one inlining per pass
606 -- On the other hand, I have seen cases where top-level fusion is
607 -- lost if we don't inline top level thing (e.g. string constants)
608 -- Hence the test for phase zero (which is the phase for all the final
609 -- simplifications). Until phase zero we take no special notice of
610 -- top level things, but then we become more leery about inlining
615 postInlineUnconditionally
616 ~~~~~~~~~~~~~~~~~~~~~~~~~
617 @postInlineUnconditionally@ decides whether to unconditionally inline
618 a thing based on the form of its RHS; in particular if it has a
619 trivial RHS. If so, we can inline and discard the binding altogether.
621 NB: a loop breaker has must_keep_binding = True and non-loop-breakers
622 only have *forward* references Hence, it's safe to discard the binding
624 NOTE: This isn't our last opportunity to inline. We're at the binding
625 site right now, and we'll get another opportunity when we get to the
628 Note that we do this unconditional inlining only for trival RHSs.
629 Don't inline even WHNFs inside lambdas; doing so may simply increase
630 allocation when the function is called. This isn't the last chance; see
633 NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here Why?
634 Because we don't even want to inline them into the RHS of constructor
635 arguments. See NOTE above
637 NB: At one time even NOINLINE was ignored here: if the rhs is trivial
638 it's best to inline it anyway. We often get a=E; b=a from desugaring,
639 with both a and b marked NOINLINE. But that seems incompatible with
640 our new view that inlining is like a RULE, so I'm sticking to the 'active'
644 postInlineUnconditionally
645 :: SimplEnv -> TopLevelFlag
646 -> OutId -- The binder (an InId would be fine too)
647 -> OccInfo -- From the InId
651 postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding
653 | isLoopBreaker occ_info = False -- If it's a loop-breaker of any kind, don't inline
654 -- because it might be referred to "earlier"
655 | isExportedId bndr = False
656 | isInlineRule unfolding = False -- Note [InlineRule and postInlineUnconditionally]
657 | exprIsTrivial rhs = True
660 -- The point of examining occ_info here is that for *non-values*
661 -- that occur outside a lambda, the call-site inliner won't have
662 -- a chance (becuase it doesn't know that the thing
663 -- only occurs once). The pre-inliner won't have gotten
664 -- it either, if the thing occurs in more than one branch
665 -- So the main target is things like
668 -- True -> case x of ...
669 -- False -> case x of ...
670 -- I'm not sure how important this is in practice
671 OneOcc in_lam _one_br int_cxt -- OneOcc => no code-duplication issue
672 -> smallEnoughToInline unfolding -- Small enough to dup
673 -- ToDo: consider discount on smallEnoughToInline if int_cxt is true
675 -- NB: Do NOT inline arbitrarily big things, even if one_br is True
676 -- Reason: doing so risks exponential behaviour. We simplify a big
677 -- expression, inline it, and simplify it again. But if the
678 -- very same thing happens in the big expression, we get
680 -- PRINCIPLE: when we've already simplified an expression once,
681 -- make sure that we only inline it if it's reasonably small.
683 && ((isNotTopLevel top_lvl && not in_lam) ||
684 -- But outside a lambda, we want to be reasonably aggressive
685 -- about inlining into multiple branches of case
686 -- e.g. let x = <non-value>
687 -- in case y of { C1 -> ..x..; C2 -> ..x..; C3 -> ... }
688 -- Inlining can be a big win if C3 is the hot-spot, even if
689 -- the uses in C1, C2 are not 'interesting'
690 -- An example that gets worse if you add int_cxt here is 'clausify'
692 (isCheapUnfolding unfolding && int_cxt))
693 -- isCheap => acceptable work duplication; in_lam may be true
694 -- int_cxt to prevent us inlining inside a lambda without some
695 -- good reason. See the notes on int_cxt in preInlineUnconditionally
697 IAmDead -> True -- This happens; for example, the case_bndr during case of
698 -- known constructor: case (a,b) of x { (p,q) -> ... }
699 -- Here x isn't mentioned in the RHS, so we don't want to
700 -- create the (dead) let-binding let x = (a,b) in ...
704 -- Here's an example that we don't handle well:
705 -- let f = if b then Left (\x.BIG) else Right (\y.BIG)
706 -- in \y. ....case f of {...} ....
707 -- Here f is used just once, and duplicating the case work is fine (exprIsCheap).
709 -- - We can't preInlineUnconditionally because that woud invalidate
710 -- the occ info for b.
711 -- - We can't postInlineUnconditionally because the RHS is big, and
712 -- that risks exponential behaviour
713 -- - We can't call-site inline, because the rhs is big
717 active = case getMode env of
718 SimplGently -> isAlwaysActive act
719 SimplPhase n _ -> isActive n act
720 act = idInlineActivation bndr
722 activeInline :: SimplEnv -> OutId -> Bool
724 = case getMode env of
726 -- No inlining at all when doing gentle stuff,
727 -- except for local things that occur once (pre/postInlineUnconditionally)
728 -- The reason is that too little clean-up happens if you
729 -- don't inline use-once things. Also a bit of inlining is *good* for
730 -- full laziness; it can expose constant sub-expressions.
731 -- Example in spectral/mandel/Mandel.hs, where the mandelset
732 -- function gets a useful let-float if you inline windowToViewport
734 -- NB: we used to have a second exception, for data con wrappers.
735 -- On the grounds that we use gentle mode for rule LHSs, and
736 -- they match better when data con wrappers are inlined.
737 -- But that only really applies to the trivial wrappers (like (:)),
738 -- and they are now constructed as Compulsory unfoldings (in MkId)
739 -- so they'll happen anyway.
741 SimplPhase n _ -> isActive n act
743 act = idInlineActivation id
745 activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool)
746 -- Nothing => No rules at all
747 activeRule dflags env
748 | not (dopt Opt_EnableRewriteRules dflags)
749 = Nothing -- Rewriting is off
751 = case getMode env of
752 SimplGently -> Just isAlwaysActive
753 -- Used to be Nothing (no rules in gentle mode)
754 -- Main motivation for changing is that I wanted
755 -- lift String ===> ...
756 -- to work in Template Haskell when simplifying
757 -- splices, so we get simpler code for literal strings
758 SimplPhase n _ -> Just (isActive n)
761 Note [InlineRule and postInlineUnconditionally]
762 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
763 Do not do postInlineUnconditionally if the Id has an InlineRule, otherwise
764 we lose the unfolding. Example
766 -- f has InlineRule with rhs (e |> co)
770 Then there's a danger we'll optimise to
775 and now postInlineUnconditionally, losing the InlineRule on f. Now f'
776 won't inline because 'e' is too big.
779 %************************************************************************
783 %************************************************************************
786 mkLam :: SimplEnv -> [OutBndr] -> OutExpr -> SimplM OutExpr
787 -- mkLam tries three things
788 -- a) eta reduction, if that gives a trivial expression
789 -- b) eta expansion [only if there are some value lambdas]
794 = do { dflags <- getDOptsSmpl
795 ; mkLam' dflags bndrs body }
797 mkLam' :: DynFlags -> [OutBndr] -> OutExpr -> SimplM OutExpr
798 mkLam' dflags bndrs (Cast body co)
799 | not (any bad bndrs)
800 -- Note [Casts and lambdas]
801 = do { lam <- mkLam' dflags bndrs body
802 ; return (mkCoerce (mkPiTypes bndrs co) lam) }
804 co_vars = tyVarsOfType co
805 bad bndr = isCoVar bndr && bndr `elemVarSet` co_vars
807 mkLam' dflags bndrs body
808 | dopt Opt_DoEtaReduction dflags,
809 Just etad_lam <- tryEtaReduce bndrs body
810 = do { tick (EtaReduction (head bndrs))
813 | dopt Opt_DoLambdaEtaExpansion dflags,
814 not (inGentleMode env), -- In gentle mode don't eta-expansion
815 any isRuntimeVar bndrs -- because it can clutter up the code
816 -- with casts etc that may not be removed
817 = do { let body' = tryEtaExpansion dflags body
818 ; return (mkLams bndrs body') }
821 = return (mkLams bndrs body)
824 Note [Casts and lambdas]
825 ~~~~~~~~~~~~~~~~~~~~~~~~
827 (\x. (\y. e) `cast` g1) `cast` g2
828 There is a danger here that the two lambdas look separated, and the
829 full laziness pass might float an expression to between the two.
831 So this equation in mkLam' floats the g1 out, thus:
832 (\x. e `cast` g1) --> (\x.e) `cast` (tx -> g1)
835 In general, this floats casts outside lambdas, where (I hope) they
836 might meet and cancel with some other cast:
837 \x. e `cast` co ===> (\x. e) `cast` (tx -> co)
838 /\a. e `cast` co ===> (/\a. e) `cast` (/\a. co)
839 /\g. e `cast` co ===> (/\g. e) `cast` (/\g. co)
842 Notice that it works regardless of 'e'. Originally it worked only
843 if 'e' was itself a lambda, but in some cases that resulted in
844 fruitless iteration in the simplifier. A good example was when
845 compiling Text.ParserCombinators.ReadPrec, where we had a definition
846 like (\x. Get `cast` g)
847 where Get is a constructor with nonzero arity. Then mkLam eta-expanded
848 the Get, and the next iteration eta-reduced it, and then eta-expanded
851 Note also the side condition for the case of coercion binders.
852 It does not make sense to transform
853 /\g. e `cast` g ==> (/\g.e) `cast` (/\g.g)
854 because the latter is not well-kinded.
856 -- c) floating lets out through big lambdas
857 -- [only if all tyvar lambdas, and only if this lambda
858 -- is the RHS of a let]
860 {- Sept 01: I'm experimenting with getting the
861 full laziness pass to float out past big lambdsa
862 | all isTyVar bndrs, -- Only for big lambdas
863 contIsRhs cont -- Only try the rhs type-lambda floating
864 -- if this is indeed a right-hand side; otherwise
865 -- we end up floating the thing out, only for float-in
866 -- to float it right back in again!
867 = do (floats, body') <- tryRhsTyLam env bndrs body
868 return (floats, mkLams bndrs body')
872 %************************************************************************
876 %************************************************************************
878 Note [Eta reduction conditions]
879 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
880 We try for eta reduction here, but *only* if we get all the way to an
881 trivial expression. We don't want to remove extra lambdas unless we
882 are going to avoid allocating this thing altogether.
884 There are some particularly delicate points here:
886 * Eta reduction is not valid in general:
888 This matters, partly for old-fashioned correctness reasons but,
889 worse, getting it wrong can yield a seg fault. Consider
891 h y = case (case y of { True -> f `seq` True; False -> False }) of
892 True -> ...; False -> ...
894 If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
895 says f=bottom, and replaces the (f `seq` True) with just
896 (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
897 *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
898 the definition again, so that it does not termninate after all.
899 Result: seg-fault because the boolean case actually gets a function value.
902 So it's important to to the right thing.
904 * Note [Arity care]: we need to be careful if we just look at f's
905 arity. Currently (Dec07), f's arity is visible in its own RHS (see
906 Note [Arity robustness] in SimplEnv) so we must *not* trust the
907 arity when checking that 'f' is a value. Otherwise we will
912 Which might change a terminiating program (think (f `seq` e)) to a
913 non-terminating one. So we check for being a loop breaker first.
915 However for GlobalIds we can look at the arity; and for primops we
916 must, since they have no unfolding.
918 * Regardless of whether 'f' is a value, we always want to
919 reduce (/\a -> f a) to f
920 This came up in a RULE: foldr (build (/\a -> g a))
921 did not match foldr (build (/\b -> ...something complex...))
922 The type checker can insert these eta-expanded versions,
923 with both type and dictionary lambdas; hence the slightly
926 * Never *reduce* arity. For example
928 Then if h has arity 1 we don't want to eta-reduce because then
929 f's arity would decrease, and that is bad
931 These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
935 tryEtaReduce :: [OutBndr] -> OutExpr -> Maybe OutExpr
936 tryEtaReduce bndrs body
937 = go (reverse bndrs) body
939 incoming_arity = count isId bndrs
941 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
942 go [] fun | ok_fun fun = Just fun -- Success!
943 go _ _ = Nothing -- Failure!
945 -- Note [Eta reduction conditions]
946 ok_fun (App fun (Type ty))
947 | not (any (`elemVarSet` tyVarsOfType ty) bndrs)
950 = not (fun_id `elem` bndrs)
951 && (ok_fun_id fun_id || all ok_lam bndrs)
954 ok_fun_id fun = fun_arity fun >= incoming_arity
956 fun_arity fun -- See Note [Arity care]
957 | isLocalId fun && isLoopBreaker (idOccInfo fun) = 0
958 | otherwise = idArity fun
960 ok_lam v = isTyVar v || isDictId v
962 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
966 %************************************************************************
970 %************************************************************************
974 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
977 where (in both cases)
979 * The xi can include type variables
981 * The yi are all value variables
983 * N is a NORMAL FORM (i.e. no redexes anywhere)
984 wanting a suitable number of extra args.
986 The biggest reason for doing this is for cases like
992 Here we want to get the lambdas together. A good exmaple is the nofib
993 program fibheaps, which gets 25% more allocation if you don't do this
996 We may have to sandwich some coerces between the lambdas
997 to make the types work. exprEtaExpandArity looks through coerces
998 when computing arity; and etaExpand adds the coerces as necessary when
999 actually computing the expansion.
1002 tryEtaExpansion :: DynFlags -> OutExpr -> OutExpr
1003 -- There is at least one runtime binder in the binders
1004 tryEtaExpansion dflags body
1005 = etaExpand fun_arity body
1007 fun_arity = exprEtaExpandArity dflags body
1011 %************************************************************************
1013 \subsection{Floating lets out of big lambdas}
1015 %************************************************************************
1017 Note [Floating and type abstraction]
1018 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1021 We'd like to float this to
1024 x = /\a. C (y1 a) (y2 a)
1025 for the usual reasons: we want to inline x rather vigorously.
1027 You may think that this kind of thing is rare. But in some programs it is
1028 common. For example, if you do closure conversion you might get:
1030 data a :-> b = forall e. (e -> a -> b) :$ e
1032 f_cc :: forall a. a :-> a
1033 f_cc = /\a. (\e. id a) :$ ()
1035 Now we really want to inline that f_cc thing so that the
1036 construction of the closure goes away.
1038 So I have elaborated simplLazyBind to understand right-hand sides that look
1042 and treat them specially. The real work is done in SimplUtils.abstractFloats,
1043 but there is quite a bit of plumbing in simplLazyBind as well.
1045 The same transformation is good when there are lets in the body:
1047 /\abc -> let(rec) x = e in b
1049 let(rec) x' = /\abc -> let x = x' a b c in e
1051 /\abc -> let x = x' a b c in b
1053 This is good because it can turn things like:
1055 let f = /\a -> letrec g = ... g ... in g
1057 letrec g' = /\a -> ... g' a ...
1059 let f = /\ a -> g' a
1061 which is better. In effect, it means that big lambdas don't impede
1064 This optimisation is CRUCIAL in eliminating the junk introduced by
1065 desugaring mutually recursive definitions. Don't eliminate it lightly!
1067 [May 1999] If we do this transformation *regardless* then we can
1068 end up with some pretty silly stuff. For example,
1071 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
1076 st = /\s -> ...[y1 s/x1, y2 s/x2]
1079 Unless the "..." is a WHNF there is really no point in doing this.
1080 Indeed it can make things worse. Suppose x1 is used strictly,
1083 x1* = case f y of { (a,b) -> e }
1085 If we abstract this wrt the tyvar we then can't do the case inline
1086 as we would normally do.
1088 That's why the whole transformation is part of the same process that
1089 floats let-bindings and constructor arguments out of RHSs. In particular,
1090 it is guarded by the doFloatFromRhs call in simplLazyBind.
1094 abstractFloats :: [OutTyVar] -> SimplEnv -> OutExpr -> SimplM ([OutBind], OutExpr)
1095 abstractFloats main_tvs body_env body
1096 = ASSERT( notNull body_floats )
1097 do { (subst, float_binds) <- mapAccumLM abstract empty_subst body_floats
1098 ; return (float_binds, CoreSubst.substExpr subst body) }
1100 main_tv_set = mkVarSet main_tvs
1101 body_floats = getFloats body_env
1102 empty_subst = CoreSubst.mkEmptySubst (seInScope body_env)
1104 abstract :: CoreSubst.Subst -> OutBind -> SimplM (CoreSubst.Subst, OutBind)
1105 abstract subst (NonRec id rhs)
1106 = do { (poly_id, poly_app) <- mk_poly tvs_here id
1107 ; let poly_rhs = mkLams tvs_here rhs'
1108 subst' = CoreSubst.extendIdSubst subst id poly_app
1109 ; return (subst', (NonRec poly_id poly_rhs)) }
1111 rhs' = CoreSubst.substExpr subst rhs
1112 tvs_here | any isCoVar main_tvs = main_tvs -- Note [Abstract over coercions]
1114 = varSetElems (main_tv_set `intersectVarSet` exprSomeFreeVars isTyVar rhs')
1116 -- Abstract only over the type variables free in the rhs
1117 -- wrt which the new binding is abstracted. But the naive
1118 -- approach of abstract wrt the tyvars free in the Id's type
1120 -- /\ a b -> let t :: (a,b) = (e1, e2)
1123 -- Here, b isn't free in x's type, but we must nevertheless
1124 -- abstract wrt b as well, because t's type mentions b.
1125 -- Since t is floated too, we'd end up with the bogus:
1126 -- poly_t = /\ a b -> (e1, e2)
1127 -- poly_x = /\ a -> fst (poly_t a *b*)
1128 -- So for now we adopt the even more naive approach of
1129 -- abstracting wrt *all* the tyvars. We'll see if that
1130 -- gives rise to problems. SLPJ June 98
1132 abstract subst (Rec prs)
1133 = do { (poly_ids, poly_apps) <- mapAndUnzipM (mk_poly tvs_here) ids
1134 ; let subst' = CoreSubst.extendSubstList subst (ids `zip` poly_apps)
1135 poly_rhss = [mkLams tvs_here (CoreSubst.substExpr subst' rhs) | rhs <- rhss]
1136 ; return (subst', Rec (poly_ids `zip` poly_rhss)) }
1138 (ids,rhss) = unzip prs
1139 -- For a recursive group, it's a bit of a pain to work out the minimal
1140 -- set of tyvars over which to abstract:
1141 -- /\ a b c. let x = ...a... in
1142 -- letrec { p = ...x...q...
1143 -- q = .....p...b... } in
1145 -- Since 'x' is abstracted over 'a', the {p,q} group must be abstracted
1146 -- over 'a' (because x is replaced by (poly_x a)) as well as 'b'.
1147 -- Since it's a pain, we just use the whole set, which is always safe
1149 -- If you ever want to be more selective, remember this bizarre case too:
1151 -- Here, we must abstract 'x' over 'a'.
1154 mk_poly tvs_here var
1155 = do { uniq <- getUniqueM
1156 ; let poly_name = setNameUnique (idName var) uniq -- Keep same name
1157 poly_ty = mkForAllTys tvs_here (idType var) -- But new type of course
1158 poly_id = transferPolyIdInfo var tvs_here $ -- Note [transferPolyIdInfo] in Id.lhs
1159 mkLocalId poly_name poly_ty
1160 ; return (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tvs_here)) }
1161 -- In the olden days, it was crucial to copy the occInfo of the original var,
1162 -- because we were looking at occurrence-analysed but as yet unsimplified code!
1163 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
1164 -- at already simplified code, so it doesn't matter
1166 -- It's even right to retain single-occurrence or dead-var info:
1167 -- Suppose we started with /\a -> let x = E in B
1168 -- where x occurs once in B. Then we transform to:
1169 -- let x' = /\a -> E in /\a -> let x* = x' a in B
1170 -- where x* has an INLINE prag on it. Now, once x* is inlined,
1171 -- the occurrences of x' will be just the occurrences originally
1175 Note [Abstract over coercions]
1176 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1177 If a coercion variable (g :: a ~ Int) is free in the RHS, then so is the
1178 type variable a. Rather than sort this mess out, we simply bale out and abstract
1179 wrt all the type variables if any of them are coercion variables.
1182 Historical note: if you use let-bindings instead of a substitution, beware of this:
1184 -- Suppose we start with:
1186 -- x = /\ a -> let g = G in E
1188 -- Then we'll float to get
1190 -- x = let poly_g = /\ a -> G
1191 -- in /\ a -> let g = poly_g a in E
1193 -- But now the occurrence analyser will see just one occurrence
1194 -- of poly_g, not inside a lambda, so the simplifier will
1195 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
1196 -- (I used to think that the "don't inline lone occurrences" stuff
1197 -- would stop this happening, but since it's the *only* occurrence,
1198 -- PreInlineUnconditionally kicks in first!)
1200 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
1201 -- to appear many times. (NB: mkInlineMe eliminates
1202 -- such notes on trivial RHSs, so do it manually.)
1204 %************************************************************************
1208 %************************************************************************
1210 prepareAlts tries these things:
1212 1. If several alternatives are identical, merge them into
1213 a single DEFAULT alternative. I've occasionally seen this
1214 making a big difference:
1216 case e of =====> case e of
1217 C _ -> f x D v -> ....v....
1218 D v -> ....v.... DEFAULT -> f x
1221 The point is that we merge common RHSs, at least for the DEFAULT case.
1222 [One could do something more elaborate but I've never seen it needed.]
1223 To avoid an expensive test, we just merge branches equal to the *first*
1224 alternative; this picks up the common cases
1225 a) all branches equal
1226 b) some branches equal to the DEFAULT (which occurs first)
1229 case e of b { ==> case e of b {
1230 p1 -> rhs1 p1 -> rhs1
1232 pm -> rhsm pm -> rhsm
1233 _ -> case b of b' { pn -> let b'=b in rhsn
1235 ... po -> let b'=b in rhso
1236 po -> rhso _ -> let b'=b in rhsd
1240 which merges two cases in one case when -- the default alternative of
1241 the outer case scrutises the same variable as the outer case This
1242 transformation is called Case Merging. It avoids that the same
1243 variable is scrutinised multiple times.
1246 The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
1252 where @is@ was something like
1254 p `is` n = p /= (-1) && p == n
1256 This gave rise to a horrible sequence of cases
1263 and similarly in cascade for all the join points!
1266 ~~~~~~~~~~~~~~~~~~~~
1267 We do this *here*, looking at un-simplified alternatives, because we
1268 have to check that r doesn't mention the variables bound by the
1269 pattern in each alternative, so the binder-info is rather useful.
1272 prepareAlts :: SimplEnv -> OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
1273 prepareAlts env scrut case_bndr' alts
1274 = do { dflags <- getDOptsSmpl
1275 ; alts <- combineIdenticalAlts case_bndr' alts
1277 ; let (alts_wo_default, maybe_deflt) = findDefault alts
1278 alt_cons = [con | (con,_,_) <- alts_wo_default]
1279 imposs_deflt_cons = nub (imposs_cons ++ alt_cons)
1280 -- "imposs_deflt_cons" are handled
1281 -- EITHER by the context,
1282 -- OR by a non-DEFAULT branch in this case expression.
1284 ; default_alts <- prepareDefault dflags env case_bndr' mb_tc_app
1285 imposs_deflt_cons maybe_deflt
1287 ; let trimmed_alts = filterOut impossible_alt alts_wo_default
1288 merged_alts = mergeAlts trimmed_alts default_alts
1289 -- We need the mergeAlts in case the new default_alt
1290 -- has turned into a constructor alternative.
1291 -- The merge keeps the inner DEFAULT at the front, if there is one
1292 -- and interleaves the alternatives in the right order
1294 ; return (imposs_deflt_cons, merged_alts) }
1296 mb_tc_app = splitTyConApp_maybe (idType case_bndr')
1297 Just (_, inst_tys) = mb_tc_app
1299 imposs_cons = case scrut of
1300 Var v -> otherCons (idUnfolding v)
1303 impossible_alt :: CoreAlt -> Bool
1304 impossible_alt (con, _, _) | con `elem` imposs_cons = True
1305 impossible_alt (DataAlt con, _, _) = dataConCannotMatch inst_tys con
1306 impossible_alt _ = False
1309 --------------------------------------------------
1310 -- 1. Merge identical branches
1311 --------------------------------------------------
1312 combineIdenticalAlts :: OutId -> [InAlt] -> SimplM [InAlt]
1314 combineIdenticalAlts case_bndr ((_con1,bndrs1,rhs1) : con_alts)
1315 | all isDeadBinder bndrs1, -- Remember the default
1316 length filtered_alts < length con_alts -- alternative comes first
1317 -- Also Note [Dead binders]
1318 = do { tick (AltMerge case_bndr)
1319 ; return ((DEFAULT, [], rhs1) : filtered_alts) }
1321 filtered_alts = filter keep con_alts
1322 keep (_con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
1324 combineIdenticalAlts _ alts = return alts
1326 -------------------------------------------------------------------------
1327 -- Prepare the default alternative
1328 -------------------------------------------------------------------------
1329 prepareDefault :: DynFlags
1331 -> OutId -- Case binder; need just for its type. Note that as an
1332 -- OutId, it has maximum information; this is important.
1333 -- Test simpl013 is an example
1334 -> Maybe (TyCon, [Type]) -- Type of scrutinee, decomposed
1335 -> [AltCon] -- These cons can't happen when matching the default
1336 -> Maybe InExpr -- Rhs
1337 -> SimplM [InAlt] -- Still unsimplified
1338 -- We use a list because it's what mergeAlts expects,
1339 -- And becuase case-merging can cause many to show up
1341 ------- Merge nested cases ----------
1342 prepareDefault dflags env outer_bndr _bndr_ty imposs_cons (Just deflt_rhs)
1343 | dopt Opt_CaseMerge dflags
1344 , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs
1345 , DoneId inner_scrut_var' <- substId env inner_scrut_var
1346 -- Remember, inner_scrut_var is an InId, but outer_bndr is an OutId
1347 , inner_scrut_var' == outer_bndr
1348 -- NB: the substId means that if the outer scrutinee was a
1349 -- variable, and inner scrutinee is the same variable,
1350 -- then inner_scrut_var' will be outer_bndr
1351 -- via the magic of simplCaseBinder
1352 = do { tick (CaseMerge outer_bndr)
1354 ; let munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
1355 ; return [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts,
1356 not (con `elem` imposs_cons) ]
1357 -- NB: filter out any imposs_cons. Example:
1360 -- DEFAULT -> case x of
1363 -- When we merge, we must ensure that e1 takes
1364 -- precedence over e2 as the value for A!
1366 -- Warning: don't call prepareAlts recursively!
1367 -- Firstly, there's no point, because inner alts have already had
1368 -- mkCase applied to them, so they won't have a case in their default
1369 -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
1370 -- in munge_rhs may put a case into the DEFAULT branch!
1373 --------- Fill in known constructor -----------
1374 prepareDefault _ _ case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rhs)
1375 | -- This branch handles the case where we are
1376 -- scrutinisng an algebraic data type
1377 isAlgTyCon tycon -- It's a data type, tuple, or unboxed tuples.
1378 , not (isNewTyCon tycon) -- We can have a newtype, if we are just doing an eval:
1379 -- case x of { DEFAULT -> e }
1380 -- and we don't want to fill in a default for them!
1381 , Just all_cons <- tyConDataCons_maybe tycon
1382 , not (null all_cons) -- This is a tricky corner case. If the data type has no constructors,
1383 -- which GHC allows, then the case expression will have at most a default
1384 -- alternative. We don't want to eliminate that alternative, because the
1385 -- invariant is that there's always one alternative. It's more convenient
1387 -- case x of { DEFAULT -> e }
1388 -- as it is, rather than transform it to
1389 -- error "case cant match"
1390 -- which would be quite legitmate. But it's a really obscure corner, and
1391 -- not worth wasting code on.
1392 , let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1393 impossible con = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con
1394 = case filterOut impossible all_cons of
1395 [] -> return [] -- Eliminate the default alternative
1396 -- altogether if it can't match
1398 [con] -> -- It matches exactly one constructor, so fill it in
1399 do { tick (FillInCaseDefault case_bndr)
1401 ; let (ex_tvs, co_tvs, arg_ids) =
1402 dataConRepInstPat us con inst_tys
1403 ; return [(DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, deflt_rhs)] }
1405 _ -> return [(DEFAULT, [], deflt_rhs)]
1407 | debugIsOn, isAlgTyCon tycon, not (isOpenTyCon tycon), null (tyConDataCons tycon)
1408 -- This can legitimately happen for type families, so don't report that
1409 = pprTrace "prepareDefault" (ppr case_bndr <+> ppr tycon)
1410 $ return [(DEFAULT, [], deflt_rhs)]
1412 --------- Catch-all cases -----------
1413 prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons (Just deflt_rhs)
1414 = return [(DEFAULT, [], deflt_rhs)]
1416 prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons Nothing
1417 = return [] -- No default branch
1422 =================================================================================
1424 mkCase tries these things
1426 1. Eliminate the case altogether if possible
1434 and similar friends.
1438 mkCase :: OutExpr -> OutId -> [OutAlt] -- Increasing order
1441 --------------------------------------------------
1443 --------------------------------------------------
1445 mkCase scrut case_bndr alts -- Identity case
1446 | all identity_alt alts
1447 = do tick (CaseIdentity case_bndr)
1448 return (re_cast scrut)
1450 identity_alt (con, args, rhs) = check_eq con args (de_cast rhs)
1452 check_eq DEFAULT _ (Var v) = v == case_bndr
1453 check_eq (LitAlt lit') _ (Lit lit) = lit == lit'
1454 check_eq (DataAlt con) args rhs = rhs `cheapEqExpr` mkConApp con (arg_tys ++ varsToCoreExprs args)
1455 || rhs `cheapEqExpr` Var case_bndr
1456 check_eq _ _ _ = False
1458 arg_tys = map Type (tyConAppArgs (idType case_bndr))
1461 -- case e of x { _ -> x `cast` c }
1462 -- And we definitely want to eliminate this case, to give
1464 -- So we throw away the cast from the RHS, and reconstruct
1465 -- it at the other end. All the RHS casts must be the same
1466 -- if (all identity_alt alts) holds.
1468 -- Don't worry about nested casts, because the simplifier combines them
1469 de_cast (Cast e _) = e
1472 re_cast scrut = case head alts of
1473 (_,_,Cast _ co) -> Cast scrut co
1478 --------------------------------------------------
1480 --------------------------------------------------
1481 mkCase scrut bndr alts = return (Case scrut bndr (coreAltsType alts) alts)
1485 When adding auxiliary bindings for the case binder, it's worth checking if
1486 its dead, because it often is, and occasionally these mkCase transformations
1487 cascade rather nicely.
1490 bindCaseBndr :: Id -> CoreExpr -> CoreExpr -> CoreExpr
1491 bindCaseBndr bndr rhs body
1492 | isDeadBinder bndr = body
1493 | otherwise = bindNonRec bndr rhs body