2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[Simplify]{The main module of the simplifier}
7 module Simplify ( simplTopBinds, simplExpr ) where
9 #include "HsVersions.h"
11 import CmdLineOpts ( intSwitchSet, switchIsOn,
12 opt_SccProfilingOn, opt_PprStyle_Debug, opt_SimplDoEtaReduction,
13 opt_SimplNoPreInlining, opt_DictsStrict, opt_SimplPedanticBottoms,
17 import SimplUtils ( mkCase, transformRhs, findAlt,
18 simplBinder, simplBinders, simplIds, findDefault,
19 SimplCont(..), DupFlag(..), contResultType, analyseCont,
20 discardInline, countArgs, countValArgs, discardCont, contIsDupable
22 import Var ( TyVar, mkSysTyVar, tyVarKind, maybeModifyIdInfo )
25 import Id ( Id, idType, idInfo, idUnique, isDataConId, isDataConId_maybe,
26 idUnfolding, setIdUnfolding, isExportedId, isDeadBinder,
27 idSpecialisation, setIdSpecialisation,
28 idDemandInfo, setIdDemandInfo,
30 idOccInfo, setIdOccInfo,
31 zapLamIdInfo, zapFragileIdInfo,
32 idStrictness, isBottomingId,
33 setInlinePragma, mayHaveNoBinding,
34 setOneShotLambda, maybeModifyIdInfo
36 import IdInfo ( InlinePragInfo(..), OccInfo(..), StrictnessInfo(..),
37 ArityInfo(..), atLeastArity, arityLowerBound, unknownArity,
38 specInfo, inlinePragInfo, setArityInfo, setInlinePragInfo, setUnfoldingInfo,
39 CprInfo(..), cprInfo, occInfo
41 import Demand ( Demand, isStrict, wwLazy )
42 import DataCon ( DataCon, dataConNumInstArgs, dataConRepStrictness, dataConRepArity,
43 dataConSig, dataConArgTys
45 import Name ( isLocallyDefined )
47 import CoreFVs ( exprFreeVars )
48 import CoreUnfold ( Unfolding, mkOtherCon, mkUnfolding, otherCons, maybeUnfoldingTemplate,
49 callSiteInline, hasSomeUnfolding, noUnfolding
51 import CoreUtils ( cheapEqExpr, exprIsDupable, exprIsCheap, exprIsTrivial, exprIsConApp_maybe,
52 exprType, coreAltsType, exprArity, exprIsValue, idAppIsCheap,
53 exprOkForSpeculation, etaReduceExpr,
54 mkCoerce, mkSCC, mkInlineMe, mkAltExpr
56 import Rules ( lookupRule )
57 import CostCentre ( isSubsumedCCS, currentCCS, isEmptyCC )
58 import Type ( Type, mkTyVarTy, mkTyVarTys, isUnLiftedType, seqType,
59 mkFunTy, splitFunTy, splitFunTys, splitFunTy_maybe,
61 funResultTy, isDictTy, isDataType, applyTy, applyTys, mkFunTys
63 import Subst ( Subst, mkSubst, emptySubst, substTy, substExpr,
64 substEnv, isInScope, lookupIdSubst, substIdInfo
66 import TyCon ( isDataTyCon, tyConDataCons, tyConClass_maybe, tyConArity, isDataTyCon )
67 import TysPrim ( realWorldStatePrimTy )
68 import PrelInfo ( realWorldPrimId )
69 import BasicTypes ( TopLevelFlag(..), isTopLevel, isLoopBreaker )
70 import Maybes ( maybeToBool )
71 import Util ( zipWithEqual, lengthExceeds )
74 import Unique ( foldrIdKey ) -- Temp
78 The guts of the simplifier is in this module, but the driver
79 loop for the simplifier is in SimplCore.lhs.
82 %************************************************************************
86 %************************************************************************
89 simplTopBinds :: [InBind] -> SimplM [OutBind]
92 = -- Put all the top-level binders into scope at the start
93 -- so that if a transformation rule has unexpectedly brought
94 -- anything into scope, then we don't get a complaint about that.
95 -- It's rather as if the top-level binders were imported.
96 simplIds (bindersOfBinds binds) $ \ bndrs' ->
97 simpl_binds binds bndrs' `thenSmpl` \ (binds', _) ->
98 freeTick SimplifierDone `thenSmpl_`
102 -- We need to track the zapped top-level binders, because
103 -- they should have their fragile IdInfo zapped (notably occurrence info)
104 simpl_binds [] bs = ASSERT( null bs ) returnSmpl ([], panic "simplTopBinds corner")
105 simpl_binds (NonRec bndr rhs : binds) (b:bs) = simplLazyBind True bndr b rhs (simpl_binds binds bs)
106 simpl_binds (Rec pairs : binds) bs = simplRecBind True pairs (take n bs) (simpl_binds binds (drop n bs))
110 simplRecBind :: Bool -> [(InId, InExpr)] -> [OutId]
111 -> SimplM (OutStuff a) -> SimplM (OutStuff a)
112 simplRecBind top_lvl pairs bndrs' thing_inside
113 = go pairs bndrs' `thenSmpl` \ (binds', (binds'', res)) ->
114 returnSmpl (Rec (flattenBinds binds') : binds'', res)
116 go [] _ = thing_inside `thenSmpl` \ stuff ->
117 returnSmpl ([], stuff)
119 go ((bndr, rhs) : pairs) (bndr' : bndrs')
120 = simplLazyBind top_lvl bndr bndr' rhs (go pairs bndrs')
121 -- Don't float unboxed bindings out,
122 -- because we can't "rec" them
126 %************************************************************************
128 \subsection[Simplify-simplExpr]{The main function: simplExpr}
130 %************************************************************************
133 addLetBind :: OutId -> OutExpr -> SimplM (OutStuff a) -> SimplM (OutStuff a)
134 addLetBind bndr rhs thing_inside
135 = thing_inside `thenSmpl` \ (binds, res) ->
136 returnSmpl (NonRec bndr rhs : binds, res)
138 addLetBinds :: [CoreBind] -> SimplM (OutStuff a) -> SimplM (OutStuff a)
139 addLetBinds binds1 thing_inside
140 = thing_inside `thenSmpl` \ (binds2, res) ->
141 returnSmpl (binds1 ++ binds2, res)
143 needsCaseBinding ty rhs = isUnLiftedType ty && not (exprOkForSpeculation rhs)
144 -- Make a case expression instead of a let
145 -- These can arise either from the desugarer,
146 -- or from beta reductions: (\x.e) (x +# y)
148 addCaseBind bndr rhs thing_inside
149 = getInScope `thenSmpl` \ in_scope ->
150 thing_inside `thenSmpl` \ (floats, (_, body)) ->
151 returnSmpl ([], (in_scope, Case rhs bndr [(DEFAULT, [], mkLets floats body)]))
153 addNonRecBind bndr rhs thing_inside
154 -- Checks for needing a case binding
155 | needsCaseBinding (idType bndr) rhs = addCaseBind bndr rhs thing_inside
156 | otherwise = addLetBind bndr rhs thing_inside
159 The reason for this OutExprStuff stuff is that we want to float *after*
160 simplifying a RHS, not before. If we do so naively we get quadratic
161 behaviour as things float out.
163 To see why it's important to do it after, consider this (real) example:
177 a -- Can't inline a this round, cos it appears twice
181 Each of the ==> steps is a round of simplification. We'd save a
182 whole round if we float first. This can cascade. Consider
187 let f = let d1 = ..d.. in \y -> e
191 in \x -> ...(\y ->e)...
193 Only in this second round can the \y be applied, and it
194 might do the same again.
198 simplExpr :: CoreExpr -> SimplM CoreExpr
199 simplExpr expr = getSubst `thenSmpl` \ subst ->
200 simplExprC expr (Stop (substTy subst (exprType expr)))
201 -- The type in the Stop continuation is usually not used
202 -- It's only needed when discarding continuations after finding
203 -- a function that returns bottom.
204 -- Hence the lazy substitution
206 simplExprC :: CoreExpr -> SimplCont -> SimplM CoreExpr
207 -- Simplify an expression, given a continuation
209 simplExprC expr cont = simplExprF expr cont `thenSmpl` \ (floats, (_, body)) ->
210 returnSmpl (mkLets floats body)
212 simplExprF :: InExpr -> SimplCont -> SimplM OutExprStuff
213 -- Simplify an expression, returning floated binds
215 simplExprF (Var v) cont
218 simplExprF (Lit lit) (Select _ bndr alts se cont)
219 = knownCon (Lit lit) (LitAlt lit) [] bndr alts se cont
221 simplExprF (Lit lit) cont
222 = rebuild (Lit lit) cont
224 simplExprF (App fun arg) cont
225 = getSubstEnv `thenSmpl` \ se ->
226 simplExprF fun (ApplyTo NoDup arg se cont)
228 simplExprF (Case scrut bndr alts) cont
229 = getSubstEnv `thenSmpl` \ subst_env ->
230 getSwitchChecker `thenSmpl` \ chkr ->
231 if not (switchIsOn chkr NoCaseOfCase) then
232 -- Simplify the scrutinee with a Select continuation
233 simplExprF scrut (Select NoDup bndr alts subst_env cont)
236 -- If case-of-case is off, simply simplify the case expression
237 -- in a vanilla Stop context, and rebuild the result around it
238 simplExprC scrut (Select NoDup bndr alts subst_env
239 (Stop (contResultType cont))) `thenSmpl` \ case_expr' ->
240 rebuild case_expr' cont
243 simplExprF (Let (Rec pairs) body) cont
244 = simplIds (map fst pairs) $ \ bndrs' ->
245 -- NB: bndrs' don't have unfoldings or spec-envs
246 -- We add them as we go down, using simplPrags
248 simplRecBind False pairs bndrs' (simplExprF body cont)
250 simplExprF expr@(Lam _ _) cont = simplLam expr cont
252 simplExprF (Type ty) cont
253 = ASSERT( case cont of { Stop _ -> True; ArgOf _ _ _ -> True; other -> False } )
254 simplType ty `thenSmpl` \ ty' ->
255 rebuild (Type ty') cont
257 -- Comments about the Coerce case
258 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
259 -- It's worth checking for a coerce in the continuation,
260 -- in case we can cancel them. For example, in the initial form of a worker
261 -- we may find (coerce T (coerce S (\x.e))) y
262 -- and we'd like it to simplify to e[y/x] in one round of simplification
264 simplExprF (Note (Coerce to from) e) (CoerceIt outer_to cont)
265 = simplType from `thenSmpl` \ from' ->
266 if outer_to == from' then
267 -- The coerces cancel out
270 -- They don't cancel, but the inner one is redundant
271 simplExprF e (CoerceIt outer_to cont)
273 simplExprF (Note (Coerce to from) e) cont
274 = simplType to `thenSmpl` \ to' ->
275 simplExprF e (CoerceIt to' cont)
277 -- hack: we only distinguish subsumed cost centre stacks for the purposes of
278 -- inlining. All other CCCSs are mapped to currentCCS.
279 simplExprF (Note (SCC cc) e) cont
280 = setEnclosingCC currentCCS $
281 simplExpr e `thenSmpl` \ e ->
282 rebuild (mkSCC cc e) cont
284 simplExprF (Note InlineCall e) cont
285 = simplExprF e (InlinePlease cont)
287 -- Comments about the InlineMe case
288 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
289 -- Don't inline in the RHS of something that has an
290 -- inline pragma. But be careful that the InScopeEnv that
291 -- we return does still have inlinings on!
293 -- It really is important to switch off inlinings. This function
294 -- may be inlinined in other modules, so we don't want to remove
295 -- (by inlining) calls to functions that have specialisations, or
296 -- that may have transformation rules in an importing scope.
297 -- E.g. {-# INLINE f #-}
299 -- and suppose that g is strict *and* has specialisations.
300 -- If we inline g's wrapper, we deny f the chance of getting
301 -- the specialised version of g when f is inlined at some call site
302 -- (perhaps in some other module).
304 simplExprF (Note InlineMe e) cont
306 Stop _ -> -- Totally boring continuation
307 -- Don't inline inside an INLINE expression
308 switchOffInlining (simplExpr e) `thenSmpl` \ e' ->
309 rebuild (mkInlineMe e') cont
311 other -> -- Dissolve the InlineMe note if there's
312 -- an interesting context of any kind to combine with
313 -- (even a type application -- anything except Stop)
316 -- A non-recursive let is dealt with by simplBeta
317 simplExprF (Let (NonRec bndr rhs) body) cont
318 = getSubstEnv `thenSmpl` \ se ->
319 simplBeta bndr rhs se (contResultType cont) $
324 ---------------------------------
330 zap_it = mkLamBndrZapper fun cont
331 cont_ty = contResultType cont
333 -- Type-beta reduction
334 go (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
335 = ASSERT( isTyVar bndr )
336 tick (BetaReduction bndr) `thenSmpl_`
337 simplTyArg ty_arg arg_se `thenSmpl` \ ty_arg' ->
338 extendSubst bndr (DoneTy ty_arg')
341 -- Ordinary beta reduction
342 go (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
343 = tick (BetaReduction bndr) `thenSmpl_`
344 simplBeta zapped_bndr arg arg_se cont_ty
347 zapped_bndr = zap_it bndr
350 go lam@(Lam _ _) cont = completeLam [] lam cont
352 -- Exactly enough args
353 go expr cont = simplExprF expr cont
355 -- completeLam deals with the case where a lambda doesn't have an ApplyTo
357 -- We used to try for eta reduction here, but I found that this was
358 -- eta reducing things like
359 -- f = \x -> (coerce (\x -> e))
360 -- This made f's arity reduce, which is a bad thing, so I removed the
361 -- eta reduction at this point, and now do it only when binding
362 -- (at the call to postInlineUnconditionally)
364 completeLam acc (Lam bndr body) cont
365 = simplBinder bndr $ \ bndr' ->
366 completeLam (bndr':acc) body cont
368 completeLam acc body cont
369 = simplExpr body `thenSmpl` \ body' ->
370 rebuild (foldl (flip Lam) body' acc) cont
371 -- Remember, acc is the *reversed* binders
373 mkLamBndrZapper :: CoreExpr -- Function
374 -> SimplCont -- The context
375 -> Id -> Id -- Use this to zap the binders
376 mkLamBndrZapper fun cont
377 | n_args >= n_params fun = \b -> b -- Enough args
378 | otherwise = \b -> zapLamIdInfo b
380 -- NB: we count all the args incl type args
381 -- so we must count all the binders (incl type lambdas)
382 n_args = countArgs cont
384 n_params (Note _ e) = n_params e
385 n_params (Lam b e) = 1 + n_params e
386 n_params other = 0::Int
390 ---------------------------------
392 simplType :: InType -> SimplM OutType
394 = getSubst `thenSmpl` \ subst ->
396 new_ty = substTy subst ty
403 %************************************************************************
407 %************************************************************************
409 @simplBeta@ is used for non-recursive lets in expressions,
410 as well as true beta reduction.
412 Very similar to @simplLazyBind@, but not quite the same.
415 simplBeta :: InId -- Binder
416 -> InExpr -> SubstEnv -- Arg, with its subst-env
417 -> OutType -- Type of thing computed by the context
418 -> SimplM OutExprStuff -- The body
419 -> SimplM OutExprStuff
421 simplBeta bndr rhs rhs_se cont_ty thing_inside
423 = pprPanic "simplBeta" (ppr bndr <+> ppr rhs)
426 simplBeta bndr rhs rhs_se cont_ty thing_inside
427 | preInlineUnconditionally False {- not black listed -} bndr
428 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
429 extendSubst bndr (ContEx rhs_se rhs) thing_inside
432 = -- Simplify the RHS
433 simplBinder bndr $ \ bndr' ->
434 simplValArg (idType bndr') (idDemandInfo bndr)
435 rhs rhs_se cont_ty $ \ rhs' ->
437 -- Now complete the binding and simplify the body
438 if needsCaseBinding (idType bndr') rhs' then
439 addCaseBind bndr' rhs' thing_inside
441 completeBinding bndr bndr' False False rhs' thing_inside
446 simplTyArg :: InType -> SubstEnv -> SimplM OutType
448 = getInScope `thenSmpl` \ in_scope ->
450 ty_arg' = substTy (mkSubst in_scope se) ty_arg
452 seqType ty_arg' `seq`
455 simplValArg :: OutType -- Type of arg
456 -> Demand -- Demand on the argument
457 -> InExpr -> SubstEnv
458 -> OutType -- Type of thing computed by the context
459 -> (OutExpr -> SimplM OutExprStuff)
460 -> SimplM OutExprStuff
462 simplValArg arg_ty demand arg arg_se cont_ty thing_inside
464 isUnLiftedType arg_ty ||
465 (opt_DictsStrict && isDictTy arg_ty && isDataType arg_ty)
466 -- Return true only for dictionary types where the dictionary
467 -- has more than one component (else we risk poking on the component
468 -- of a newtype dictionary)
469 = transformRhs arg `thenSmpl` \ t_arg ->
470 getEnv `thenSmpl` \ env ->
472 simplExprF t_arg (ArgOf NoDup cont_ty $ \ rhs' ->
473 setAllExceptInScope env $
474 etaFirst thing_inside rhs')
477 = simplRhs False {- Not top level -}
478 True {- OK to float unboxed -}
482 -- Do eta-reduction on the simplified RHS, if eta reduction is on
483 -- NB: etaFirst only eta-reduces if that results in something trivial
484 etaFirst | opt_SimplDoEtaReduction = \ thing_inside rhs -> thing_inside (etaCoreExprToTrivial rhs)
485 | otherwise = \ thing_inside rhs -> thing_inside rhs
487 -- Try for eta reduction, but *only* if we get all
488 -- the way to an exprIsTrivial expression. We don't want to remove
489 -- extra lambdas unless we are going to avoid allocating this thing altogether
490 etaCoreExprToTrivial rhs | exprIsTrivial rhs' = rhs'
493 rhs' = etaReduceExpr rhs
498 - deals only with Ids, not TyVars
499 - take an already-simplified RHS
501 It does *not* attempt to do let-to-case. Why? Because they are used for
504 (when let-to-case is impossible)
506 - many situations where the "rhs" is known to be a WHNF
507 (so let-to-case is inappropriate).
510 completeBinding :: InId -- Binder
511 -> OutId -- New binder
512 -> Bool -- True <=> top level
513 -> Bool -- True <=> black-listed; don't inline
514 -> OutExpr -- Simplified RHS
515 -> SimplM (OutStuff a) -- Thing inside
516 -> SimplM (OutStuff a)
518 completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
519 | (case occ_info of -- This happens; for example, the case_bndr during case of
520 IAmDead -> True -- known constructor: case (a,b) of x { (p,q) -> ... }
521 other -> False) -- Here x isn't mentioned in the RHS, so we don't want to
522 -- create the (dead) let-binding let x = (a,b) in ...
525 | postInlineUnconditionally black_listed occ_info old_bndr new_rhs
526 -- Maybe we don't need a let-binding! Maybe we can just
527 -- inline it right away. Unlike the preInlineUnconditionally case
528 -- we are allowed to look at the RHS.
530 -- NB: a loop breaker never has postInlineUnconditionally True
531 -- and non-loop-breakers only have *forward* references
532 -- Hence, it's safe to discard the binding
534 -- NB: You might think that postInlineUnconditionally is an optimisation,
536 -- let x = f Bool in (x, y)
537 -- then because of the constructor, x will not be *inlined* in the pair,
538 -- so the trivial binding will stay. But in this postInlineUnconditionally
539 -- gag we use the *substitution* to substitute (f Bool) for x, and that *will*
541 = tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
542 extendSubst old_bndr (DoneEx new_rhs)
546 = getSubst `thenSmpl` \ subst ->
548 -- We make new IdInfo for the new binder by starting from the old binder,
549 -- doing appropriate substitutions.
550 -- Then we add arity and unfolding info to get the new binder
551 old_info = idInfo old_bndr
552 new_bndr_info = substIdInfo subst old_info (idInfo new_bndr)
553 `setArityInfo` ArityAtLeast (exprArity new_rhs)
555 -- Add the unfolding *only* for non-loop-breakers
556 -- Making loop breakers not have an unfolding at all
557 -- means that we can avoid tests in exprIsConApp, for example.
558 -- This is important: if exprIsConApp says 'yes' for a recursive
559 -- thing we can get into an infinite loop
560 info_w_unf | isLoopBreaker (occInfo old_info) = new_bndr_info
561 | otherwise = new_bndr_info `setUnfoldingInfo` mkUnfolding top_lvl new_rhs
563 final_id = new_bndr `setIdInfo` info_w_unf
565 -- These seqs forces the Id, and hence its IdInfo,
566 -- and hence any inner substitutions
568 addLetBind final_id new_rhs $
569 modifyInScope new_bndr final_id thing_inside
572 occ_info = idOccInfo old_bndr
576 %************************************************************************
578 \subsection{simplLazyBind}
580 %************************************************************************
582 simplLazyBind basically just simplifies the RHS of a let(rec).
583 It does two important optimisations though:
585 * It floats let(rec)s out of the RHS, even if they
586 are hidden by big lambdas
588 * It does eta expansion
591 simplLazyBind :: Bool -- True <=> top level
594 -> SimplM (OutStuff a) -- The body of the binding
595 -> SimplM (OutStuff a)
596 -- When called, the subst env is correct for the entire let-binding
597 -- and hence right for the RHS.
598 -- Also the binder has already been simplified, and hence is in scope
600 simplLazyBind top_lvl bndr bndr' rhs thing_inside
601 = getBlackList `thenSmpl` \ black_list_fn ->
603 black_listed = black_list_fn bndr
606 if preInlineUnconditionally black_listed bndr then
607 -- Inline unconditionally
608 tick (PreInlineUnconditionally bndr) `thenSmpl_`
609 getSubstEnv `thenSmpl` \ rhs_se ->
610 (extendSubst bndr (ContEx rhs_se rhs) thing_inside)
614 getSubstEnv `thenSmpl` \ rhs_se ->
615 simplRhs top_lvl False {- Not ok to float unboxed -}
617 rhs rhs_se $ \ rhs' ->
619 -- Now compete the binding and simplify the body
620 completeBinding bndr bndr' top_lvl black_listed rhs' thing_inside
626 simplRhs :: Bool -- True <=> Top level
627 -> Bool -- True <=> OK to float unboxed (speculative) bindings
628 -> OutType -> InExpr -> SubstEnv
629 -> (OutExpr -> SimplM (OutStuff a))
630 -> SimplM (OutStuff a)
631 simplRhs top_lvl float_ubx rhs_ty rhs rhs_se thing_inside
632 = -- Swizzle the inner lets past the big lambda (if any)
633 -- and try eta expansion
634 transformRhs rhs `thenSmpl` \ t_rhs ->
637 setSubstEnv rhs_se (simplExprF t_rhs (Stop rhs_ty)) `thenSmpl` \ (floats, (in_scope', rhs')) ->
639 -- Float lets out of RHS
641 (floats_out, rhs'') | float_ubx = (floats, rhs')
642 | otherwise = splitFloats floats rhs'
644 if (top_lvl || wantToExpose 0 rhs') && -- Float lets if (a) we're at the top level
645 not (null floats_out) -- or (b) the resulting RHS is one we'd like to expose
647 tickLetFloat floats_out `thenSmpl_`
650 -- There's a subtlety here. There may be a binding (x* = e) in the
651 -- floats, where the '*' means 'will be demanded'. So is it safe
652 -- to float it out? Answer no, but it won't matter because
653 -- we only float if arg' is a WHNF,
654 -- and so there can't be any 'will be demanded' bindings in the floats.
656 WARN( any demanded_float floats_out, ppr floats_out )
657 addLetBinds floats_out $
658 setInScope in_scope' $
659 etaFirst thing_inside rhs''
660 -- in_scope' may be excessive, but that's OK;
661 -- it's a superset of what's in scope
663 -- Don't do the float
664 etaFirst thing_inside (mkLets floats rhs')
666 -- In a let-from-let float, we just tick once, arbitrarily
667 -- choosing the first floated binder to identify it
668 tickLetFloat (NonRec b r : fs) = tick (LetFloatFromLet b)
669 tickLetFloat (Rec ((b,r):prs) : fs) = tick (LetFloatFromLet b)
671 demanded_float (NonRec b r) = isStrict (idDemandInfo b) && not (isUnLiftedType (idType b))
672 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
673 demanded_float (Rec _) = False
675 -- Don't float any unlifted bindings out, because the context
676 -- is either a Rec group, or the top level, neither of which
677 -- can tolerate them.
678 splitFloats floats rhs
682 go (f:fs) | must_stay f = ([], mkLets (f:fs) rhs)
683 | otherwise = case go fs of
684 (out, rhs') -> (f:out, rhs')
686 must_stay (Rec prs) = False -- No unlifted bindings in here
687 must_stay (NonRec b r) = isUnLiftedType (idType b)
689 wantToExpose :: Int -> CoreExpr -> Bool
690 -- True for expressions that we'd like to expose at the
691 -- top level of an RHS. This includes partial applications
692 -- even if the args aren't cheap; the next pass will let-bind the
693 -- args and eta expand the partial application. So exprIsCheap won't do.
694 -- Here's the motivating example:
695 -- z = letrec g = \x y -> ...g... in g E
696 -- Even though E is a redex we'd like to float the letrec to give
697 -- g = \x y -> ...g...
699 -- Now the next use of SimplUtils.tryEtaExpansion will give
700 -- g = \x y -> ...g...
701 -- z = let v = E in \w -> g v w
702 -- And now we'll float the v to give
703 -- g = \x y -> ...g...
706 -- Which is what we want; chances are z will be inlined now.
708 -- This defn isn't quite like
709 -- exprIsCheap (it ignores non-cheap args)
710 -- exprIsValue (may not say True for a lone variable)
711 -- which is slightly weird
712 wantToExpose n (Var v) = idAppIsCheap v n
713 wantToExpose n (Lit l) = True
714 wantToExpose n (Lam _ e) = True
715 wantToExpose n (Note _ e) = wantToExpose n e
716 wantToExpose n (App f (Type _)) = wantToExpose n f
717 wantToExpose n (App f a) = wantToExpose (n+1) f
718 wantToExpose n other = False -- There won't be any lets
723 %************************************************************************
725 \subsection{Variables}
727 %************************************************************************
731 = getSubst `thenSmpl` \ subst ->
732 case lookupIdSubst subst var of
733 DoneEx e -> zapSubstEnv (simplExprF e cont)
734 ContEx env1 e -> setSubstEnv env1 (simplExprF e cont)
735 DoneId var1 occ -> WARN( not (isInScope var1 subst) && isLocallyDefined var1 && not (mayHaveNoBinding var1),
736 text "simplVar:" <+> ppr var )
737 -- The mayHaveNoBinding test accouunts for the fact
738 -- that class dictionary constructors dont have top level
739 -- bindings and hence aren't in scope.
740 zapSubstEnv (completeCall var1 occ cont)
741 -- The template is already simplified, so don't re-substitute.
742 -- This is VITAL. Consider
744 -- let y = \z -> ...x... in
746 -- We'll clone the inner \x, adding x->x' in the id_subst
747 -- Then when we inline y, we must *not* replace x by x' in
748 -- the inlined copy!!
750 ---------------------------------------------------------
751 -- Dealing with a call
753 completeCall var occ cont
754 = getBlackList `thenSmpl` \ black_list_fn ->
755 getInScope `thenSmpl` \ in_scope ->
756 getSwitchChecker `thenSmpl` \ chkr ->
758 dont_use_rules = switchIsOn chkr DontApplyRules
759 no_case_of_case = switchIsOn chkr NoCaseOfCase
760 black_listed = black_list_fn var
762 (arg_infos, interesting_cont, inline_call) = analyseCont in_scope cont
763 discard_inline_cont | inline_call = discardInline cont
766 maybe_inline = callSiteInline black_listed inline_call occ
767 var arg_infos interesting_cont
769 -- First, look for an inlining
771 case maybe_inline of {
772 Just unfolding -- There is an inlining!
773 -> tick (UnfoldingDone var) `thenSmpl_`
774 simplExprF unfolding discard_inline_cont
777 Nothing -> -- No inlining!
779 -- Next, look for rules or specialisations that match
781 -- It's important to simplify the args first, because the rule-matcher
782 -- doesn't do substitution as it goes. We don't want to use subst_args
783 -- (defined in the 'where') because that throws away useful occurrence info,
784 -- and perhaps-very-important specialisations.
786 -- Some functions have specialisations *and* are strict; in this case,
787 -- we don't want to inline the wrapper of the non-specialised thing; better
788 -- to call the specialised thing instead.
789 -- But the black-listing mechanism means that inlining of the wrapper
790 -- won't occur for things that have specialisations till a later phase, so
791 -- it's ok to try for inlining first.
793 prepareArgs no_case_of_case var cont $ \ args' cont' ->
795 maybe_rule | dont_use_rules = Nothing
796 | otherwise = lookupRule in_scope var args'
799 Just (rule_name, rule_rhs) ->
800 tick (RuleFired rule_name) `thenSmpl_`
801 simplExprF rule_rhs cont' ;
803 Nothing -> -- No rules
806 rebuild (mkApps (Var var) args') cont'
812 ---------------------------------------------------------
813 -- Preparing arguments for a call
815 prepareArgs :: Bool -- True if the no-case-of-case switch is on
816 -> OutId -> SimplCont
817 -> ([OutExpr] -> SimplCont -> SimplM OutExprStuff)
818 -> SimplM OutExprStuff
819 prepareArgs no_case_of_case fun orig_cont thing_inside
820 = go [] demands orig_fun_ty orig_cont
822 orig_fun_ty = idType fun
823 is_data_con = isDataConId fun
825 (demands, result_bot)
826 | no_case_of_case = ([], False) -- Ignore strictness info if the no-case-of-case
827 -- flag is on. Strictness changes evaluation order
828 -- and that can change full laziness
830 = case idStrictness fun of
831 StrictnessInfo demands result_bot
832 | not (demands `lengthExceeds` countValArgs orig_cont)
833 -> -- Enough args, use the strictness given.
834 -- For bottoming functions we used to pretend that the arg
835 -- is lazy, so that we don't treat the arg as an
836 -- interesting context. This avoids substituting
837 -- top-level bindings for (say) strings into
838 -- calls to error. But now we are more careful about
839 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
840 (demands, result_bot)
842 other -> ([], False) -- Not enough args, or no strictness
844 -- Main game plan: loop through the arguments, simplifying
845 -- each of them in turn. We carry with us a list of demands,
846 -- and the type of the function-applied-to-earlier-args
848 -- We've run out of demands, and the result is now bottom
850 -- * case (error "hello") of { ... }
851 -- * (error "Hello") arg
852 -- * f (error "Hello") where f is strict
854 go acc [] fun_ty cont
856 = tick_case_of_error cont `thenSmpl_`
857 thing_inside (reverse acc) (discardCont cont)
860 go acc ds fun_ty (ApplyTo _ arg@(Type ty_arg) se cont)
861 = simplTyArg ty_arg se `thenSmpl` \ new_ty_arg ->
862 go (Type new_ty_arg : acc) ds (applyTy fun_ty new_ty_arg) cont
865 go acc ds fun_ty (ApplyTo _ val_arg se cont)
866 | not is_data_con -- Function isn't a data constructor
867 = simplValArg arg_ty dem val_arg se (contResultType cont) $ \ new_arg ->
868 go (new_arg : acc) ds' res_ty cont
870 | exprIsTrivial val_arg -- Function is a data contstructor, arg is trivial
871 = getInScope `thenSmpl` \ in_scope ->
873 new_arg = substExpr (mkSubst in_scope se) val_arg
874 -- Simplify the RHS with inlining switched off, so that
875 -- only absolutely essential things will happen.
876 -- If we don't do this, consider:
877 -- let x = +# p q in C {x}
878 -- Even though x get's an occurrence of 'many', its RHS looks cheap,
879 -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
881 -- It's important that the substitution *does* deal with case-binder synonyms:
882 -- case x of y { True -> (x,1) }
883 -- Here we must be sure to substitute y for x when simplifying the args of the pair,
884 -- to increase the chances of being able to inline x. The substituter will do
885 -- that because the x->y mapping is held in the in-scope set.
887 -- It's not always the case that the new arg will be trivial
889 -- where, in one pass, f gets substituted by a constructor,
890 -- but x gets substituted by an expression (assume this is the
891 -- unique occurrence of x). It doesn't really matter -- it'll get
892 -- fixed up next pass. And it happens for dictionary construction,
893 -- which mentions the wrapper constructor to start with.
895 go (new_arg : acc) ds' res_ty cont
898 = simplValArg arg_ty dem val_arg se (contResultType cont) $ \ new_arg ->
899 -- A data constructor whose argument is now non-trivial;
900 -- so let/case bind it.
901 newId SLIT("a") arg_ty $ \ arg_id ->
902 addNonRecBind arg_id new_arg $
903 go (Var arg_id : acc) ds' res_ty cont
906 (arg_ty, res_ty) = splitFunTy fun_ty
907 (dem, ds') = case ds of
911 -- We're run out of arguments and the result ain't bottom
912 go acc ds fun_ty cont = thing_inside (reverse acc) cont
914 -- Boring: we must only record a tick if there was an interesting
915 -- continuation to discard. If not, we tick forever.
916 tick_case_of_error (Stop _) = returnSmpl ()
917 tick_case_of_error (CoerceIt _ (Stop _)) = returnSmpl ()
918 tick_case_of_error other = tick BottomFound
922 %************************************************************************
924 \subsection{Decisions about inlining}
926 %************************************************************************
928 NB: At one time I tried not pre/post-inlining top-level things,
929 even if they occur exactly once. Reason:
930 (a) some might appear as a function argument, so we simply
931 replace static allocation with dynamic allocation:
937 (b) some top level things might be black listed
939 HOWEVER, I found that some useful foldr/build fusion was lost (most
940 notably in spectral/hartel/parstof) because the foldr didn't see the build.
942 Doing the dynamic allocation isn't a big deal, in fact, but losing the
946 preInlineUnconditionally :: Bool {- Black listed -} -> InId -> Bool
947 -- Examines a bndr to see if it is used just once in a
948 -- completely safe way, so that it is safe to discard the binding
949 -- inline its RHS at the (unique) usage site, REGARDLESS of how
950 -- big the RHS might be. If this is the case we don't simplify
951 -- the RHS first, but just inline it un-simplified.
953 -- This is much better than first simplifying a perhaps-huge RHS
954 -- and then inlining and re-simplifying it.
956 -- NB: we don't even look at the RHS to see if it's trivial
959 -- where x is used many times, but this is the unique occurrence
960 -- of y. We should NOT inline x at all its uses, because then
961 -- we'd do the same for y -- aargh! So we must base this
962 -- pre-rhs-simplification decision solely on x's occurrences, not
965 -- Evne RHSs labelled InlineMe aren't caught here, because
966 -- there might be no benefit from inlining at the call site.
968 preInlineUnconditionally black_listed bndr
969 | black_listed || opt_SimplNoPreInlining = False
970 | otherwise = case idOccInfo bndr of
971 OneOcc in_lam once -> not in_lam && once
972 -- Not inside a lambda, one occurrence ==> safe!
976 postInlineUnconditionally :: Bool -- Black listed
978 -> InId -> OutExpr -> Bool
979 -- Examines a (bndr = rhs) binding, AFTER the rhs has been simplified
980 -- It returns True if it's ok to discard the binding and inline the
981 -- RHS at every use site.
983 -- NOTE: This isn't our last opportunity to inline.
984 -- We're at the binding site right now, and
985 -- we'll get another opportunity when we get to the ocurrence(s)
987 postInlineUnconditionally black_listed occ_info bndr rhs
988 | isExportedId bndr ||
990 isLoopBreaker occ_info = False -- Don't inline these
991 | otherwise = exprIsTrivial rhs -- Duplicating is free
992 -- Don't inline even WHNFs inside lambdas; doing so may
993 -- simply increase allocation when the function is called
994 -- This isn't the last chance; see NOTE above.
996 -- NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here
997 -- Why? Because we don't even want to inline them into the
998 -- RHS of constructor arguments. See NOTE above
1000 -- NB: Even NOINLINEis ignored here: if the rhs is trivial
1001 -- it's best to inline it anyway. We often get a=E; b=a
1002 -- from desugaring, with both a and b marked NOINLINE.
1007 %************************************************************************
1009 \subsection{The main rebuilder}
1011 %************************************************************************
1014 -------------------------------------------------------------------
1015 -- Finish rebuilding
1017 = getInScope `thenSmpl` \ in_scope ->
1018 returnSmpl ([], (in_scope, expr))
1020 ---------------------------------------------------------
1021 rebuild :: OutExpr -> SimplCont -> SimplM OutExprStuff
1023 -- Stop continuation
1024 rebuild expr (Stop _) = rebuild_done expr
1026 -- ArgOf continuation
1027 rebuild expr (ArgOf _ _ cont_fn) = cont_fn expr
1029 -- ApplyTo continuation
1030 rebuild expr cont@(ApplyTo _ arg se cont')
1031 = setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1032 rebuild (App expr arg') cont'
1034 -- Coerce continuation
1035 rebuild expr (CoerceIt to_ty cont)
1036 = rebuild (mkCoerce to_ty (exprType expr) expr) cont
1038 -- Inline continuation
1039 rebuild expr (InlinePlease cont)
1040 = rebuild (Note InlineCall expr) cont
1042 rebuild scrut (Select _ bndr alts se cont)
1043 = rebuild_case scrut bndr alts se cont
1046 Case elimination [see the code above]
1048 Start with a simple situation:
1050 case x# of ===> e[x#/y#]
1053 (when x#, y# are of primitive type, of course). We can't (in general)
1054 do this for algebraic cases, because we might turn bottom into
1057 Actually, we generalise this idea to look for a case where we're
1058 scrutinising a variable, and we know that only the default case can
1063 other -> ...(case x of
1067 Here the inner case can be eliminated. This really only shows up in
1068 eliminating error-checking code.
1070 We also make sure that we deal with this very common case:
1075 Here we are using the case as a strict let; if x is used only once
1076 then we want to inline it. We have to be careful that this doesn't
1077 make the program terminate when it would have diverged before, so we
1079 - x is used strictly, or
1080 - e is already evaluated (it may so if e is a variable)
1082 Lastly, we generalise the transformation to handle this:
1088 We only do this for very cheaply compared r's (constructors, literals
1089 and variables). If pedantic bottoms is on, we only do it when the
1090 scrutinee is a PrimOp which can't fail.
1092 We do it *here*, looking at un-simplified alternatives, because we
1093 have to check that r doesn't mention the variables bound by the
1094 pattern in each alternative, so the binder-info is rather useful.
1096 So the case-elimination algorithm is:
1098 1. Eliminate alternatives which can't match
1100 2. Check whether all the remaining alternatives
1101 (a) do not mention in their rhs any of the variables bound in their pattern
1102 and (b) have equal rhss
1104 3. Check we can safely ditch the case:
1105 * PedanticBottoms is off,
1106 or * the scrutinee is an already-evaluated variable
1107 or * the scrutinee is a primop which is ok for speculation
1108 -- ie we want to preserve divide-by-zero errors, and
1109 -- calls to error itself!
1111 or * [Prim cases] the scrutinee is a primitive variable
1113 or * [Alg cases] the scrutinee is a variable and
1114 either * the rhs is the same variable
1115 (eg case x of C a b -> x ===> x)
1116 or * there is only one alternative, the default alternative,
1117 and the binder is used strictly in its scope.
1118 [NB this is helped by the "use default binder where
1119 possible" transformation; see below.]
1122 If so, then we can replace the case with one of the rhss.
1125 Blob of helper functions for the "case-of-something-else" situation.
1128 ---------------------------------------------------------
1129 -- Eliminate the case if possible
1131 rebuild_case scrut bndr alts se cont
1132 | maybeToBool maybe_con_app
1133 = knownCon scrut (DataAlt con) args bndr alts se cont
1135 | canEliminateCase scrut bndr alts
1136 = tick (CaseElim bndr) `thenSmpl_` (
1138 simplBinder bndr $ \ bndr' ->
1139 -- Remember to bind the case binder!
1140 completeBinding bndr bndr' False False scrut $
1141 simplExprF (head (rhssOfAlts alts)) cont)
1144 = complete_case scrut bndr alts se cont
1147 maybe_con_app = exprIsConApp_maybe scrut
1148 Just (con, args) = maybe_con_app
1150 -- See if we can get rid of the case altogether
1151 -- See the extensive notes on case-elimination above
1152 canEliminateCase scrut bndr alts
1153 = -- Check that the RHSs are all the same, and
1154 -- don't use the binders in the alternatives
1155 -- This test succeeds rapidly in the common case of
1156 -- a single DEFAULT alternative
1157 all (cheapEqExpr rhs1) other_rhss && all binders_unused alts
1159 -- Check that the scrutinee can be let-bound instead of case-bound
1160 && ( exprOkForSpeculation scrut
1161 -- OK not to evaluate it
1162 -- This includes things like (==# a# b#)::Bool
1163 -- so that we simplify
1164 -- case ==# a# b# of { True -> x; False -> x }
1167 -- This particular example shows up in default methods for
1168 -- comparision operations (e.g. in (>=) for Int.Int32)
1169 || exprIsValue scrut -- It's already evaluated
1170 || var_demanded_later scrut -- It'll be demanded later
1172 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1173 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1174 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1175 -- its argument: case x of { y -> dataToTag# y }
1176 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1177 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1182 (rhs1:other_rhss) = rhssOfAlts alts
1183 binders_unused (_, bndrs, _) = all isDeadBinder bndrs
1185 var_demanded_later (Var v) = isStrict (idDemandInfo bndr) -- It's going to be evaluated later
1186 var_demanded_later other = False
1189 ---------------------------------------------------------
1190 -- Case of something else
1192 complete_case scrut case_bndr alts se cont
1193 = -- Prepare case alternatives
1194 prepareCaseAlts case_bndr (splitTyConApp_maybe (idType case_bndr))
1195 impossible_cons alts `thenSmpl` \ better_alts ->
1197 -- Set the new subst-env in place (before dealing with the case binder)
1200 -- Deal with the case binder, and prepare the continuation;
1201 -- The new subst_env is in place
1202 prepareCaseCont better_alts cont $ \ cont' ->
1205 -- Deal with variable scrutinee
1207 getSwitchChecker `thenSmpl` \ chkr ->
1208 simplCaseBinder (switchIsOn chkr NoCaseOfCase)
1209 scrut case_bndr $ \ case_bndr' zap_occ_info ->
1211 -- Deal with the case alternatives
1212 simplAlts zap_occ_info impossible_cons
1213 case_bndr' better_alts cont' `thenSmpl` \ alts' ->
1215 mkCase scrut case_bndr' alts'
1216 ) `thenSmpl` \ case_expr ->
1218 -- Notice that the simplBinder, prepareCaseCont, etc, do *not* scope
1219 -- over the rebuild_done; rebuild_done returns the in-scope set, and
1220 -- that should not include these chaps!
1221 rebuild_done case_expr
1223 impossible_cons = case scrut of
1224 Var v -> otherCons (idUnfolding v)
1228 knownCon :: OutExpr -> AltCon -> [OutExpr]
1229 -> InId -> [InAlt] -> SubstEnv -> SimplCont
1230 -> SimplM OutExprStuff
1232 knownCon expr con args bndr alts se cont
1233 = tick (KnownBranch bndr) `thenSmpl_`
1235 simplBinder bndr $ \ bndr' ->
1236 completeBinding bndr bndr' False False expr $
1237 -- Don't use completeBeta here. The expr might be
1238 -- an unboxed literal, like 3, or a variable
1239 -- whose unfolding is an unboxed literal... and
1240 -- completeBeta will just construct another case
1242 case findAlt con alts of
1243 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1246 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1249 (DataAlt dc, bs, rhs) -> ASSERT( length bs == length real_args )
1250 extendSubstList bs (map mk real_args) $
1253 real_args = drop (dataConNumInstArgs dc) args
1254 mk (Type ty) = DoneTy ty
1255 mk other = DoneEx other
1260 prepareCaseCont :: [InAlt] -> SimplCont
1261 -> (SimplCont -> SimplM (OutStuff a))
1262 -> SimplM (OutStuff a)
1263 -- Polymorphic recursion here!
1265 prepareCaseCont [alt] cont thing_inside = thing_inside cont
1266 prepareCaseCont alts cont thing_inside = simplType (coreAltsType alts) `thenSmpl` \ alts_ty ->
1267 mkDupableCont alts_ty cont thing_inside
1268 -- At one time I passed in the un-simplified type, and simplified
1269 -- it only if we needed to construct a join binder, but that
1270 -- didn't work because we have to decompse function types
1271 -- (using funResultTy) in mkDupableCont.
1274 simplCaseBinder checks whether the scrutinee is a variable, v.
1275 If so, try to eliminate uses of v in the RHSs in favour of case_bndr;
1276 that way, there's a chance that v will now only be used once, and hence inlined.
1278 There is a time we *don't* want to do that, namely when -fno-case-of-case
1279 is on. This happens in the first simplifier pass, and enhances full laziness.
1280 Here's the bad case:
1281 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1282 If we eliminate the inner case, we trap it inside the I# v -> arm,
1283 which might prevent some full laziness happening. I've seen this
1284 in action in spectral/cichelli/Prog.hs:
1285 [(m,n) | m <- [1..max], n <- [1..max]]
1286 Hence the no_case_of_case argument
1289 If we do this, then we have to nuke any occurrence info (eg IAmDead)
1290 in the case binder, because the case-binder now effectively occurs
1291 whenever v does. AND we have to do the same for the pattern-bound
1294 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1296 Here, b and p are dead. But when we move the argment inside the first
1297 case RHS, and eliminate the second case, we get
1299 case x or { (a,b) -> a b }
1301 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1302 happened. Hence the zap_occ_info function returned by simplCaseBinder
1305 simplCaseBinder no_case_of_case (Var v) case_bndr thing_inside
1306 | not no_case_of_case
1307 = simplBinder (zap case_bndr) $ \ case_bndr' ->
1308 modifyInScope v case_bndr' $
1309 -- We could extend the substitution instead, but it would be
1310 -- a hack because then the substitution wouldn't be idempotent
1311 -- any more (v is an OutId). And this just just as well.
1312 thing_inside case_bndr' zap
1314 zap b = b `setIdOccInfo` NoOccInfo
1316 simplCaseBinder add_eval_info other_scrut case_bndr thing_inside
1317 = simplBinder case_bndr $ \ case_bndr' ->
1318 thing_inside case_bndr' (\ bndr -> bndr) -- NoOp on bndr
1321 prepareCaseAlts does two things:
1323 1. Remove impossible alternatives
1325 2. If the DEFAULT alternative can match only one possible constructor,
1326 then make that constructor explicit.
1328 case e of x { DEFAULT -> rhs }
1330 case e of x { (a,b) -> rhs }
1331 where the type is a single constructor type. This gives better code
1332 when rhs also scrutinises x or e.
1335 prepareCaseAlts bndr (Just (tycon, inst_tys)) scrut_cons alts
1337 = case (findDefault filtered_alts, missing_cons) of
1339 ((alts_no_deflt, Just rhs), [data_con]) -- Just one missing constructor!
1340 -> tick (FillInCaseDefault bndr) `thenSmpl_`
1342 (_,_,ex_tyvars,_,_,_) = dataConSig data_con
1344 getUniquesSmpl (length ex_tyvars) `thenSmpl` \ tv_uniqs ->
1346 ex_tyvars' = zipWithEqual "simpl_alt" mk tv_uniqs ex_tyvars
1347 mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
1348 arg_tys = dataConArgTys data_con
1349 (inst_tys ++ mkTyVarTys ex_tyvars')
1351 newIds SLIT("a") arg_tys $ \ bndrs ->
1352 returnSmpl ((DataAlt data_con, ex_tyvars' ++ bndrs, rhs) : alts_no_deflt)
1354 other -> returnSmpl filtered_alts
1356 -- Filter out alternatives that can't possibly match
1357 filtered_alts = case scrut_cons of
1359 other -> [alt | alt@(con,_,_) <- alts, not (con `elem` scrut_cons)]
1361 missing_cons = [data_con | data_con <- tyConDataCons tycon,
1362 not (data_con `elem` handled_data_cons)]
1363 handled_data_cons = [data_con | DataAlt data_con <- scrut_cons] ++
1364 [data_con | (DataAlt data_con, _, _) <- filtered_alts]
1367 prepareCaseAlts _ _ scrut_cons alts
1368 = returnSmpl alts -- Functions
1371 ----------------------
1372 simplAlts zap_occ_info scrut_cons case_bndr' alts cont'
1373 = mapSmpl simpl_alt alts
1375 inst_tys' = case splitTyConApp_maybe (idType case_bndr') of
1376 Just (tycon, inst_tys) -> inst_tys
1378 -- handled_cons is all the constructors that are dealt
1379 -- with, either by being impossible, or by there being an alternative
1380 handled_cons = scrut_cons ++ [con | (con,_,_) <- alts, con /= DEFAULT]
1382 simpl_alt (DEFAULT, _, rhs)
1383 = -- In the default case we record the constructors that the
1384 -- case-binder *can't* be.
1385 -- We take advantage of any OtherCon info in the case scrutinee
1386 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` mkOtherCon handled_cons) $
1387 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1388 returnSmpl (DEFAULT, [], rhs')
1390 simpl_alt (con, vs, rhs)
1391 = -- Deal with the pattern-bound variables
1392 -- Mark the ones that are in ! positions in the data constructor
1393 -- as certainly-evaluated.
1394 -- NB: it happens that simplBinders does *not* erase the OtherCon
1395 -- form of unfolding, so it's ok to add this info before
1396 -- doing simplBinders
1397 simplBinders (add_evals con vs) $ \ vs' ->
1399 -- Bind the case-binder to (con args)
1401 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1403 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` unfolding) $
1404 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1405 returnSmpl (con, vs', rhs')
1408 -- add_evals records the evaluated-ness of the bound variables of
1409 -- a case pattern. This is *important*. Consider
1410 -- data T = T !Int !Int
1412 -- case x of { T a b -> T (a+1) b }
1414 -- We really must record that b is already evaluated so that we don't
1415 -- go and re-evaluate it when constructing the result.
1417 add_evals (DataAlt dc) vs = cat_evals vs (dataConRepStrictness dc)
1418 add_evals other_con vs = vs
1420 cat_evals [] [] = []
1421 cat_evals (v:vs) (str:strs)
1422 | isTyVar v = v : cat_evals vs (str:strs)
1423 | isStrict str = (v' `setIdUnfolding` mkOtherCon []) : cat_evals vs strs
1424 | otherwise = v' : cat_evals vs strs
1430 %************************************************************************
1432 \subsection{Duplicating continuations}
1434 %************************************************************************
1437 mkDupableCont :: OutType -- Type of the thing to be given to the continuation
1439 -> (SimplCont -> SimplM (OutStuff a))
1440 -> SimplM (OutStuff a)
1441 mkDupableCont ty cont thing_inside
1442 | contIsDupable cont
1445 mkDupableCont _ (CoerceIt ty cont) thing_inside
1446 = mkDupableCont ty cont $ \ cont' ->
1447 thing_inside (CoerceIt ty cont')
1449 mkDupableCont ty (InlinePlease cont) thing_inside
1450 = mkDupableCont ty cont $ \ cont' ->
1451 thing_inside (InlinePlease cont')
1453 mkDupableCont join_arg_ty (ArgOf _ cont_ty cont_fn) thing_inside
1454 = -- Build the RHS of the join point
1455 newId SLIT("a") join_arg_ty ( \ arg_id ->
1456 cont_fn (Var arg_id) `thenSmpl` \ (binds, (_, rhs)) ->
1457 returnSmpl (Lam (setOneShotLambda arg_id) (mkLets binds rhs))
1458 ) `thenSmpl` \ join_rhs ->
1460 -- Build the join Id and continuation
1461 -- We give it a "$j" name just so that for later amusement
1462 -- we can identify any join points that don't end up as let-no-escapes
1463 newId SLIT("$j") (exprType join_rhs) $ \ join_id ->
1465 new_cont = ArgOf OkToDup cont_ty
1466 (\arg' -> rebuild_done (App (Var join_id) arg'))
1469 tick (CaseOfCase join_id) `thenSmpl_`
1470 -- Want to tick here so that we go round again,
1471 -- and maybe copy or inline the code;
1472 -- not strictly CaseOf Case
1473 addLetBind join_id join_rhs (thing_inside new_cont)
1475 mkDupableCont ty (ApplyTo _ arg se cont) thing_inside
1476 = mkDupableCont (funResultTy ty) cont $ \ cont' ->
1477 setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1478 if exprIsDupable arg' then
1479 thing_inside (ApplyTo OkToDup arg' emptySubstEnv cont')
1481 newId SLIT("a") (exprType arg') $ \ bndr ->
1483 tick (CaseOfCase bndr) `thenSmpl_`
1484 -- Want to tick here so that we go round again,
1485 -- and maybe copy or inline the code;
1486 -- not strictly CaseOf Case
1488 addLetBind bndr arg' $
1489 -- But what if the arg should be case-bound? We can't use
1490 -- addNonRecBind here because its type is too specific.
1491 -- This has been this way for a long time, so I'll leave it,
1492 -- but I can't convince myself that it's right.
1494 thing_inside (ApplyTo OkToDup (Var bndr) emptySubstEnv cont')
1497 mkDupableCont ty (Select _ case_bndr alts se cont) thing_inside
1498 = tick (CaseOfCase case_bndr) `thenSmpl_`
1500 simplBinder case_bndr $ \ case_bndr' ->
1501 prepareCaseCont alts cont $ \ cont' ->
1502 mapAndUnzipSmpl (mkDupableAlt case_bndr case_bndr' cont') alts `thenSmpl` \ (alt_binds_s, alts') ->
1503 returnSmpl (concat alt_binds_s, alts')
1504 ) `thenSmpl` \ (alt_binds, alts') ->
1506 extendInScopes [b | NonRec b _ <- alt_binds] $
1508 -- NB that the new alternatives, alts', are still InAlts, using the original
1509 -- binders. That means we can keep the case_bndr intact. This is important
1510 -- because another case-of-case might strike, and so we want to keep the
1511 -- info that the case_bndr is dead (if it is, which is often the case).
1512 -- This is VITAL when the type of case_bndr is an unboxed pair (often the
1513 -- case in I/O rich code. We aren't allowed a lambda bound
1514 -- arg of unboxed tuple type, and indeed such a case_bndr is always dead
1515 addLetBinds alt_binds $
1516 thing_inside (Select OkToDup case_bndr alts' se (Stop (contResultType cont)))
1518 mkDupableAlt :: InId -> OutId -> SimplCont -> InAlt -> SimplM (OutStuff InAlt)
1519 mkDupableAlt case_bndr case_bndr' cont alt@(con, bndrs, rhs)
1520 = simplBinders bndrs $ \ bndrs' ->
1521 simplExprC rhs cont `thenSmpl` \ rhs' ->
1523 if (case cont of { Stop _ -> exprIsDupable rhs'; other -> False}) then
1524 -- It is worth checking for a small RHS because otherwise we
1525 -- get extra let bindings that may cause an extra iteration of the simplifier to
1526 -- inline back in place. Quite often the rhs is just a variable or constructor.
1527 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1528 -- iterations because the version with the let bindings looked big, and so wasn't
1529 -- inlined, but after the join points had been inlined it looked smaller, and so
1532 -- But since the continuation is absorbed into the rhs, we only do this
1533 -- for a Stop continuation.
1535 -- NB: we have to check the size of rhs', not rhs.
1536 -- Duplicating a small InAlt might invalidate occurrence information
1537 -- However, if it *is* dupable, we return the *un* simplified alternative,
1538 -- because otherwise we'd need to pair it up with an empty subst-env.
1539 -- (Remember we must zap the subst-env before re-simplifying something).
1540 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1541 returnSmpl ([], alt)
1545 rhs_ty' = exprType rhs'
1546 (used_bndrs, used_bndrs')
1547 = unzip [pr | pr@(bndr,bndr') <- zip (case_bndr : bndrs)
1548 (case_bndr' : bndrs'),
1549 not (isDeadBinder bndr)]
1550 -- The new binders have lost their occurrence info,
1551 -- so we have to extract it from the old ones
1553 ( if null used_bndrs'
1554 -- If we try to lift a primitive-typed something out
1555 -- for let-binding-purposes, we will *caseify* it (!),
1556 -- with potentially-disastrous strictness results. So
1557 -- instead we turn it into a function: \v -> e
1558 -- where v::State# RealWorld#. The value passed to this function
1559 -- is realworld#, which generates (almost) no code.
1561 -- There's a slight infelicity here: we pass the overall
1562 -- case_bndr to all the join points if it's used in *any* RHS,
1563 -- because we don't know its usage in each RHS separately
1565 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1566 -- we make the join point into a function whenever used_bndrs'
1567 -- is empty. This makes the join-point more CPR friendly.
1568 -- Consider: let j = if .. then I# 3 else I# 4
1569 -- in case .. of { A -> j; B -> j; C -> ... }
1571 -- Now CPR should not w/w j because it's a thunk, so
1572 -- that means that the enclosing function can't w/w either,
1573 -- which is a lose. Here's the example that happened in practice:
1574 -- kgmod :: Int -> Int -> Int
1575 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1579 then newId SLIT("w") realWorldStatePrimTy $ \ rw_id ->
1580 returnSmpl ([rw_id], [Var realWorldPrimId])
1582 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs)
1584 `thenSmpl` \ (final_bndrs', final_args) ->
1586 -- See comment about "$j" name above
1587 newId SLIT("$j") (foldr (mkFunTy . idType) rhs_ty' final_bndrs') $ \ join_bndr ->
1589 -- Notice that we make the lambdas into one-shot-lambdas. The
1590 -- join point is sure to be applied at most once, and doing so
1591 -- prevents the body of the join point being floated out by
1592 -- the full laziness pass
1593 returnSmpl ([NonRec join_bndr (mkLams (map setOneShotLambda final_bndrs') rhs')],
1594 (con, bndrs, mkApps (Var join_bndr) final_args))