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 ( switchIsOn, opt_SimplDoEtaReduction,
12 opt_SimplNoPreInlining,
13 dopt, DynFlag(Opt_D_dump_inlinings),
17 import SimplUtils ( mkCase, tryRhsTyLam, tryEtaExpansion,
18 simplBinder, simplBinders, simplIds,
19 SimplCont(..), DupFlag(..), mkStop, mkRhsStop,
20 contResultType, discardInline, countArgs, contIsDupable,
21 getContArgs, interestingCallContext, interestingArg, isStrictType
23 import Var ( mkSysTyVar, tyVarKind )
25 import Id ( Id, idType, idInfo, isDataConId, hasNoBinding,
26 idUnfolding, setIdUnfolding, isExportedId, isDeadBinder,
27 idDemandInfo, setIdInfo,
28 idOccInfo, setIdOccInfo,
29 zapLamIdInfo, setOneShotLambda,
31 import IdInfo ( OccInfo(..), isDeadOcc, isLoopBreaker,
33 setUnfoldingInfo, atLeastArity,
36 import Demand ( isStrict )
37 import DataCon ( dataConNumInstArgs, dataConRepStrictness,
38 dataConSig, dataConArgTys
41 import PprCore ( pprParendExpr, pprCoreExpr )
42 import CoreFVs ( mustHaveLocalBinding )
43 import CoreUnfold ( mkOtherCon, mkUnfolding, otherCons,
46 import CoreUtils ( cheapEqExpr, exprIsDupable, exprIsTrivial,
47 exprIsConApp_maybe, mkPiType, findAlt, findDefault,
48 exprType, coreAltsType, exprIsValue,
49 exprOkForSpeculation, exprArity, exprIsCheap,
50 mkCoerce, mkSCC, mkInlineMe, mkAltExpr
52 import Rules ( lookupRule )
53 import CostCentre ( currentCCS )
54 import Type ( mkTyVarTys, isUnLiftedType, seqType,
55 mkFunTy, splitTyConApp_maybe, tyConAppArgs,
58 import Subst ( mkSubst, substTy,
59 isInScope, lookupIdSubst, substIdInfo
61 import TyCon ( isDataTyCon, tyConDataConsIfAvailable )
62 import TysPrim ( realWorldStatePrimTy )
63 import PrelInfo ( realWorldPrimId )
65 import Maybes ( maybeToBool )
66 import Util ( zipWithEqual )
71 The guts of the simplifier is in this module, but the driver
72 loop for the simplifier is in SimplCore.lhs.
75 -----------------------------------------
76 *** IMPORTANT NOTE ***
77 -----------------------------------------
78 The simplifier used to guarantee that the output had no shadowing, but
79 it does not do so any more. (Actually, it never did!) The reason is
80 documented with simplifyArgs.
85 %************************************************************************
89 %************************************************************************
92 simplTopBinds :: [InBind] -> SimplM [OutBind]
95 = -- Put all the top-level binders into scope at the start
96 -- so that if a transformation rule has unexpectedly brought
97 -- anything into scope, then we don't get a complaint about that.
98 -- It's rather as if the top-level binders were imported.
99 simplIds (bindersOfBinds binds) $ \ bndrs' ->
100 simpl_binds binds bndrs' `thenSmpl` \ (binds', _) ->
101 freeTick SimplifierDone `thenSmpl_`
102 returnSmpl (fromOL binds')
105 -- We need to track the zapped top-level binders, because
106 -- they should have their fragile IdInfo zapped (notably occurrence info)
107 simpl_binds [] bs = ASSERT( null bs ) returnSmpl (nilOL, panic "simplTopBinds corner")
108 simpl_binds (NonRec bndr rhs : binds) (b:bs) = simplLazyBind True bndr b rhs (simpl_binds binds bs)
109 simpl_binds (Rec pairs : binds) bs = simplRecBind True pairs (take n bs) (simpl_binds binds (drop n bs))
113 simplRecBind :: Bool -> [(InId, InExpr)] -> [OutId]
114 -> SimplM (OutStuff a) -> SimplM (OutStuff a)
115 simplRecBind top_lvl pairs bndrs' thing_inside
116 = go pairs bndrs' `thenSmpl` \ (binds', (_, (binds'', res))) ->
117 returnSmpl (unitOL (Rec (flattenBinds (fromOL binds'))) `appOL` binds'', res)
119 go [] _ = thing_inside `thenSmpl` \ stuff ->
122 go ((bndr, rhs) : pairs) (bndr' : bndrs')
123 = simplLazyBind top_lvl bndr bndr' rhs (go pairs bndrs')
124 -- Don't float unboxed bindings out,
125 -- because we can't "rec" them
129 %************************************************************************
131 \subsection[Simplify-simplExpr]{The main function: simplExpr}
133 %************************************************************************
135 The reason for this OutExprStuff stuff is that we want to float *after*
136 simplifying a RHS, not before. If we do so naively we get quadratic
137 behaviour as things float out.
139 To see why it's important to do it after, consider this (real) example:
153 a -- Can't inline a this round, cos it appears twice
157 Each of the ==> steps is a round of simplification. We'd save a
158 whole round if we float first. This can cascade. Consider
163 let f = let d1 = ..d.. in \y -> e
167 in \x -> ...(\y ->e)...
169 Only in this second round can the \y be applied, and it
170 might do the same again.
174 simplExpr :: CoreExpr -> SimplM CoreExpr
175 simplExpr expr = getSubst `thenSmpl` \ subst ->
176 simplExprC expr (mkStop (substTy subst (exprType expr)))
177 -- The type in the Stop continuation is usually not used
178 -- It's only needed when discarding continuations after finding
179 -- a function that returns bottom.
180 -- Hence the lazy substitution
182 simplExprC :: CoreExpr -> SimplCont -> SimplM CoreExpr
183 -- Simplify an expression, given a continuation
185 simplExprC expr cont = simplExprF expr cont `thenSmpl` \ (floats, (_, body)) ->
186 returnSmpl (wrapFloats floats body)
188 simplExprF :: InExpr -> SimplCont -> SimplM OutExprStuff
189 -- Simplify an expression, returning floated binds
191 simplExprF (Var v) cont
194 simplExprF (Lit lit) (Select _ bndr alts se cont)
195 = knownCon (Lit lit) (LitAlt lit) [] bndr alts se cont
197 simplExprF (Lit lit) cont
198 = rebuild (Lit lit) cont
200 simplExprF (App fun arg) cont
201 = getSubstEnv `thenSmpl` \ se ->
202 simplExprF fun (ApplyTo NoDup arg se cont)
204 simplExprF (Case scrut bndr alts) cont
205 = getSubstEnv `thenSmpl` \ subst_env ->
206 getSwitchChecker `thenSmpl` \ chkr ->
207 if not (switchIsOn chkr NoCaseOfCase) then
208 -- Simplify the scrutinee with a Select continuation
209 simplExprF scrut (Select NoDup bndr alts subst_env cont)
212 -- If case-of-case is off, simply simplify the case expression
213 -- in a vanilla Stop context, and rebuild the result around it
214 simplExprC scrut (Select NoDup bndr alts subst_env
215 (mkStop (contResultType cont))) `thenSmpl` \ case_expr' ->
216 rebuild case_expr' cont
219 simplExprF (Let (Rec pairs) body) cont
220 = simplIds (map fst pairs) $ \ bndrs' ->
221 -- NB: bndrs' don't have unfoldings or spec-envs
222 -- We add them as we go down, using simplPrags
224 simplRecBind False pairs bndrs' (simplExprF body cont)
226 simplExprF expr@(Lam _ _) cont = simplLam expr cont
228 simplExprF (Type ty) cont
229 = ASSERT( case cont of { Stop _ _ -> True; ArgOf _ _ _ -> True; other -> False } )
230 simplType ty `thenSmpl` \ ty' ->
231 rebuild (Type ty') cont
233 -- Comments about the Coerce case
234 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
235 -- It's worth checking for a coerce in the continuation,
236 -- in case we can cancel them. For example, in the initial form of a worker
237 -- we may find (coerce T (coerce S (\x.e))) y
238 -- and we'd like it to simplify to e[y/x] in one round of simplification
240 simplExprF (Note (Coerce to from) e) (CoerceIt outer_to cont)
241 = simplType from `thenSmpl` \ from' ->
242 if outer_to == from' then
243 -- The coerces cancel out
246 -- They don't cancel, but the inner one is redundant
247 simplExprF e (CoerceIt outer_to cont)
249 simplExprF (Note (Coerce to from) e) cont
250 = simplType to `thenSmpl` \ to' ->
251 simplExprF e (CoerceIt to' cont)
253 -- hack: we only distinguish subsumed cost centre stacks for the purposes of
254 -- inlining. All other CCCSs are mapped to currentCCS.
255 simplExprF (Note (SCC cc) e) cont
256 = setEnclosingCC currentCCS $
257 simplExpr e `thenSmpl` \ e ->
258 rebuild (mkSCC cc e) cont
260 simplExprF (Note InlineCall e) cont
261 = simplExprF e (InlinePlease cont)
263 -- Comments about the InlineMe case
264 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
265 -- Don't inline in the RHS of something that has an
266 -- inline pragma. But be careful that the InScopeEnv that
267 -- we return does still have inlinings on!
269 -- It really is important to switch off inlinings. This function
270 -- may be inlinined in other modules, so we don't want to remove
271 -- (by inlining) calls to functions that have specialisations, or
272 -- that may have transformation rules in an importing scope.
273 -- E.g. {-# INLINE f #-}
275 -- and suppose that g is strict *and* has specialisations.
276 -- If we inline g's wrapper, we deny f the chance of getting
277 -- the specialised version of g when f is inlined at some call site
278 -- (perhaps in some other module).
280 -- It's also important not to inline a worker back into a wrapper.
281 -- A wrapper looks like
282 -- wraper = inline_me (\x -> ...worker... )
283 -- Normally, the inline_me prevents the worker getting inlined into
284 -- the wrapper (initially, the worker's only call site!). But,
285 -- if the wrapper is sure to be called, the strictness analyser will
286 -- mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
287 -- continuation. That's why the keep_inline predicate returns True for
288 -- ArgOf continuations. It shouldn't do any harm not to dissolve the
289 -- inline-me note under these circumstances
291 simplExprF (Note InlineMe e) cont
292 | keep_inline cont -- Totally boring continuation
293 = -- Don't inline inside an INLINE expression
294 setBlackList noInlineBlackList (simplExpr e) `thenSmpl` \ e' ->
295 rebuild (mkInlineMe e') cont
297 | otherwise -- Dissolve the InlineMe note if there's
298 -- an interesting context of any kind to combine with
299 -- (even a type application -- anything except Stop)
302 keep_inline (Stop _ _) = True -- See notes above
303 keep_inline (ArgOf _ _ _) = True -- about this predicate
304 keep_inline other = False
306 -- A non-recursive let is dealt with by simplBeta
307 simplExprF (Let (NonRec bndr rhs) body) cont
308 = getSubstEnv `thenSmpl` \ se ->
309 simplBeta bndr rhs se (contResultType cont) $
314 ---------------------------------
320 zap_it = mkLamBndrZapper fun cont
321 cont_ty = contResultType cont
323 -- Type-beta reduction
324 go (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
325 = ASSERT( isTyVar bndr )
326 tick (BetaReduction bndr) `thenSmpl_`
327 simplTyArg ty_arg arg_se `thenSmpl` \ ty_arg' ->
328 extendSubst bndr (DoneTy ty_arg')
331 -- Ordinary beta reduction
332 go (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
333 = tick (BetaReduction bndr) `thenSmpl_`
334 simplBeta zapped_bndr arg arg_se cont_ty
337 zapped_bndr = zap_it bndr
340 go lam@(Lam _ _) cont = completeLam [] lam cont
342 -- Exactly enough args
343 go expr cont = simplExprF expr cont
345 -- completeLam deals with the case where a lambda doesn't have an ApplyTo
346 -- continuation, so there are real lambdas left to put in the result
348 -- We try for eta reduction here, but *only* if we get all the
349 -- way to an exprIsTrivial expression.
350 -- We don't want to remove extra lambdas unless we are going
351 -- to avoid allocating this thing altogether
353 completeLam rev_bndrs (Lam bndr body) cont
354 = simplBinder bndr $ \ bndr' ->
355 completeLam (bndr':rev_bndrs) body cont
357 completeLam rev_bndrs body cont
358 = simplExpr body `thenSmpl` \ body' ->
359 case try_eta body' of
360 Just etad_lam -> tick (EtaReduction (head rev_bndrs)) `thenSmpl_`
361 rebuild etad_lam cont
363 Nothing -> rebuild (foldl (flip Lam) body' rev_bndrs) cont
365 -- We don't use CoreUtils.etaReduce, because we can be more
367 -- (a) we already have the binders,
368 -- (b) we can do the triviality test before computing the free vars
369 -- [in fact I take the simple path and look for just a variable]
370 -- (c) we don't want to eta-reduce a data con worker or primop
371 -- because we only have to eta-expand them later when we saturate
372 try_eta body | not opt_SimplDoEtaReduction = Nothing
373 | otherwise = go rev_bndrs body
375 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
376 go [] body | ok_body body = Just body -- Success!
377 go _ _ = Nothing -- Failure!
379 ok_body (Var v) = not (v `elem` rev_bndrs) && not (hasNoBinding v)
380 ok_body other = False
381 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
383 mkLamBndrZapper :: CoreExpr -- Function
384 -> SimplCont -- The context
385 -> Id -> Id -- Use this to zap the binders
386 mkLamBndrZapper fun cont
387 | n_args >= n_params fun = \b -> b -- Enough args
388 | otherwise = \b -> zapLamIdInfo b
390 -- NB: we count all the args incl type args
391 -- so we must count all the binders (incl type lambdas)
392 n_args = countArgs cont
394 n_params (Note _ e) = n_params e
395 n_params (Lam b e) = 1 + n_params e
396 n_params other = 0::Int
400 ---------------------------------
402 simplType :: InType -> SimplM OutType
404 = getSubst `thenSmpl` \ subst ->
406 new_ty = substTy subst ty
413 %************************************************************************
417 %************************************************************************
419 @simplBeta@ is used for non-recursive lets in expressions,
420 as well as true beta reduction.
422 Very similar to @simplLazyBind@, but not quite the same.
425 simplBeta :: InId -- Binder
426 -> InExpr -> SubstEnv -- Arg, with its subst-env
427 -> OutType -- Type of thing computed by the context
428 -> SimplM OutExprStuff -- The body
429 -> SimplM OutExprStuff
431 simplBeta bndr rhs rhs_se cont_ty thing_inside
433 = pprPanic "simplBeta" (ppr bndr <+> ppr rhs)
436 simplBeta bndr rhs rhs_se cont_ty thing_inside
437 | preInlineUnconditionally False {- not black listed -} bndr
438 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
439 extendSubst bndr (ContEx rhs_se rhs) thing_inside
442 = -- Simplify the RHS
443 simplBinder bndr $ \ bndr' ->
445 bndr_ty' = idType bndr'
446 is_strict = isStrict (idDemandInfo bndr) || isStrictType bndr_ty'
448 simplValArg bndr_ty' is_strict rhs rhs_se cont_ty $ \ rhs' ->
450 -- Now complete the binding and simplify the body
451 if needsCaseBinding bndr_ty' rhs' then
452 addCaseBind bndr' rhs' thing_inside
454 completeBinding bndr bndr' False False rhs' thing_inside
459 simplTyArg :: InType -> SubstEnv -> SimplM OutType
461 = getInScope `thenSmpl` \ in_scope ->
463 ty_arg' = substTy (mkSubst in_scope se) ty_arg
465 seqType ty_arg' `seq`
468 simplValArg :: OutType -- rhs_ty: Type of arg; used only occasionally
469 -> Bool -- True <=> evaluate eagerly
470 -> InExpr -> SubstEnv
471 -> OutType -- cont_ty: Type of thing computed by the context
472 -> (OutExpr -> SimplM OutExprStuff)
473 -- Takes an expression of type rhs_ty,
474 -- returns an expression of type cont_ty
475 -> SimplM OutExprStuff -- An expression of type cont_ty
477 simplValArg arg_ty is_strict arg arg_se cont_ty thing_inside
479 = getEnv `thenSmpl` \ env ->
481 simplExprF arg (ArgOf NoDup cont_ty $ \ rhs' ->
482 setAllExceptInScope env $
486 = simplRhs False {- Not top level -}
487 True {- OK to float unboxed -}
494 - deals only with Ids, not TyVars
495 - take an already-simplified RHS
497 It does *not* attempt to do let-to-case. Why? Because they are used for
500 (when let-to-case is impossible)
502 - many situations where the "rhs" is known to be a WHNF
503 (so let-to-case is inappropriate).
506 completeBinding :: InId -- Binder
507 -> OutId -- New binder
508 -> Bool -- True <=> top level
509 -> Bool -- True <=> black-listed; don't inline
510 -> OutExpr -- Simplified RHS
511 -> SimplM (OutStuff a) -- Thing inside
512 -> SimplM (OutStuff a)
514 completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
515 | isDeadOcc occ_info -- This happens; for example, the case_bndr during case of
516 -- known constructor: case (a,b) of x { (p,q) -> ... }
517 -- Here x isn't mentioned in the RHS, so we don't want to
518 -- create the (dead) let-binding let x = (a,b) in ...
521 | trivial_rhs && not must_keep_binding
522 -- We're looking at a binding with a trivial RHS, so
523 -- perhaps we can discard it altogether!
525 -- NB: a loop breaker has must_keep_binding = True
526 -- and non-loop-breakers only have *forward* references
527 -- Hence, it's safe to discard the binding
529 -- NOTE: This isn't our last opportunity to inline.
530 -- We're at the binding site right now, and
531 -- we'll get another opportunity when we get to the ocurrence(s)
533 -- Note that we do this unconditional inlining only for trival RHSs.
534 -- Don't inline even WHNFs inside lambdas; doing so may
535 -- simply increase allocation when the function is called
536 -- This isn't the last chance; see NOTE above.
538 -- NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here
539 -- Why? Because we don't even want to inline them into the
540 -- RHS of constructor arguments. See NOTE above
542 -- NB: Even NOINLINEis ignored here: if the rhs is trivial
543 -- it's best to inline it anyway. We often get a=E; b=a
544 -- from desugaring, with both a and b marked NOINLINE.
545 = -- Drop the binding
546 extendSubst old_bndr (DoneEx new_rhs) $
547 -- Use the substitution to make quite, quite sure that the substitution
548 -- will happen, since we are going to discard the binding
549 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
552 | Note coercion@(Coerce _ inner_ty) inner_rhs <- new_rhs,
553 not trivial_rhs && not (isUnLiftedType inner_ty)
554 -- x = coerce t e ==> c = e; x = inline_me (coerce t c)
555 -- Now x can get inlined, which moves the coercion
556 -- to the usage site. This is a bit like worker/wrapper stuff,
557 -- but it's useful to do it very promptly, so that
558 -- x = coerce T (I# 3)
562 -- This in turn means that
563 -- case (coerce Int x) of ...
565 -- Also the full-blown w/w thing isn't set up for non-functions
567 -- The (not (isUnLiftedType inner_ty)) avoids the nasty case of
568 -- x::Int = coerce Int Int# (foo y)
571 -- x::Int = coerce Int Int# v
572 -- which would be bogus because then v will be evaluated strictly.
573 -- How can this arise? Via
574 -- x::Int = case (foo y) of { ... }
575 -- followed by case elimination.
577 -- The inline_me note is so that the simplifier doesn't
578 -- just substitute c back inside x's rhs! (Typically, x will
579 -- get substituted away, but not if it's exported.)
580 = newId SLIT("c") inner_ty $ \ c_id ->
581 completeBinding c_id c_id top_lvl False inner_rhs $
582 completeBinding old_bndr new_bndr top_lvl black_listed
583 (Note InlineMe (Note coercion (Var c_id))) $
587 = getSubst `thenSmpl` \ subst ->
589 -- We make new IdInfo for the new binder by starting from the old binder,
590 -- doing appropriate substitutions.
591 -- Then we add arity and unfolding info to get the new binder
592 new_bndr_info = substIdInfo subst old_info (idInfo new_bndr)
593 `setArityInfo` arity_info
595 -- Add the unfolding *only* for non-loop-breakers
596 -- Making loop breakers not have an unfolding at all
597 -- means that we can avoid tests in exprIsConApp, for example.
598 -- This is important: if exprIsConApp says 'yes' for a recursive
599 -- thing, then we can get into an infinite loop
600 info_w_unf | loop_breaker = new_bndr_info
601 | otherwise = new_bndr_info `setUnfoldingInfo` mkUnfolding top_lvl new_rhs
603 final_id = new_bndr `setIdInfo` info_w_unf
605 -- These seqs forces the Id, and hence its IdInfo,
606 -- and hence any inner substitutions
608 addLetBind (NonRec final_id new_rhs) $
609 modifyInScope new_bndr final_id thing_inside
612 old_info = idInfo old_bndr
613 occ_info = occInfo old_info
614 loop_breaker = isLoopBreaker occ_info
615 trivial_rhs = exprIsTrivial new_rhs
616 must_keep_binding = black_listed || loop_breaker || isExportedId old_bndr
617 arity_info = atLeastArity (exprArity new_rhs)
622 %************************************************************************
624 \subsection{simplLazyBind}
626 %************************************************************************
628 simplLazyBind basically just simplifies the RHS of a let(rec).
629 It does two important optimisations though:
631 * It floats let(rec)s out of the RHS, even if they
632 are hidden by big lambdas
634 * It does eta expansion
637 simplLazyBind :: Bool -- True <=> top level
640 -> SimplM (OutStuff a) -- The body of the binding
641 -> SimplM (OutStuff a)
642 -- When called, the subst env is correct for the entire let-binding
643 -- and hence right for the RHS.
644 -- Also the binder has already been simplified, and hence is in scope
646 simplLazyBind top_lvl bndr bndr' rhs thing_inside
647 = getBlackList `thenSmpl` \ black_list_fn ->
649 black_listed = black_list_fn bndr
652 if preInlineUnconditionally black_listed bndr then
653 -- Inline unconditionally
654 tick (PreInlineUnconditionally bndr) `thenSmpl_`
655 getSubstEnv `thenSmpl` \ rhs_se ->
656 (extendSubst bndr (ContEx rhs_se rhs) thing_inside)
660 getSubstEnv `thenSmpl` \ rhs_se ->
661 simplRhs top_lvl False {- Not ok to float unboxed (conservative) -}
663 rhs rhs_se $ \ rhs' ->
665 -- Now compete the binding and simplify the body
666 completeBinding bndr bndr' top_lvl black_listed rhs' thing_inside
672 simplRhs :: Bool -- True <=> Top level
673 -> Bool -- True <=> OK to float unboxed (speculative) bindings
674 -- False for (a) recursive and (b) top-level bindings
675 -> OutType -- Type of RHS; used only occasionally
676 -> InExpr -> SubstEnv
677 -> (OutExpr -> SimplM (OutStuff a))
678 -> SimplM (OutStuff a)
679 simplRhs top_lvl float_ubx rhs_ty rhs rhs_se thing_inside
681 setSubstEnv rhs_se (simplExprF rhs (mkRhsStop rhs_ty)) `thenSmpl` \ (floats1, (rhs_in_scope, rhs1)) ->
683 (floats2, rhs2) = splitFloats float_ubx floats1 rhs1
685 -- There's a subtlety here. There may be a binding (x* = e) in the
686 -- floats, where the '*' means 'will be demanded'. So is it safe
687 -- to float it out? Answer no, but it won't matter because
688 -- we only float if arg' is a WHNF,
689 -- and so there can't be any 'will be demanded' bindings in the floats.
691 WARN( any demanded_float (fromOL floats2), ppr (fromOL floats2) )
694 -- It's important that we do eta expansion on function *arguments* (which are
695 -- simplified with simplRhs), as well as let-bound right-hand sides.
696 -- Otherwise we find that things like
697 -- f (\x -> case x of I# x' -> coerce T (\ y -> ...))
698 -- get right through to the code generator as two separate lambdas,
699 -- which is a Bad Thing
700 tryRhsTyLam rhs2 `thenSmpl` \ (floats3, rhs3) ->
701 tryEtaExpansion rhs3 rhs_ty `thenSmpl` \ (floats4, rhs4) ->
703 -- Float lets if (a) we're at the top level
704 -- or (b) the resulting RHS is one we'd like to expose
705 if (top_lvl || exprIsCheap rhs4) then
706 (if (isNilOL floats2 && null floats3 && null floats4) then
709 tick LetFloatFromLet) `thenSmpl_`
711 addFloats floats2 rhs_in_scope $
712 addAuxiliaryBinds floats3 $
713 addAuxiliaryBinds floats4 $
716 -- Don't do the float
717 thing_inside (wrapFloats floats1 rhs1)
719 demanded_float (NonRec b r) = isStrict (idDemandInfo b) && not (isUnLiftedType (idType b))
720 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
721 demanded_float (Rec _) = False
723 -- If float_ubx is true we float all the bindings, otherwise
724 -- we just float until we come across an unlifted one.
725 -- Remember that the unlifted bindings in the floats are all for
726 -- guaranteed-terminating non-exception-raising unlifted things,
727 -- which we are happy to do speculatively. However, we may still
728 -- not be able to float them out, because the context
729 -- is either a Rec group, or the top level, neither of which
730 -- can tolerate them.
731 splitFloats float_ubx floats rhs
732 | float_ubx = (floats, rhs) -- Float them all
733 | otherwise = go (fromOL floats)
736 go (f:fs) | must_stay f = (nilOL, mkLets (f:fs) rhs)
737 | otherwise = case go fs of
738 (out, rhs') -> (f `consOL` out, rhs')
740 must_stay (Rec prs) = False -- No unlifted bindings in here
741 must_stay (NonRec b r) = isUnLiftedType (idType b)
746 %************************************************************************
748 \subsection{Variables}
750 %************************************************************************
754 = getSubst `thenSmpl` \ subst ->
755 case lookupIdSubst subst var of
756 DoneEx e -> zapSubstEnv (simplExprF e cont)
757 ContEx env1 e -> setSubstEnv env1 (simplExprF e cont)
758 DoneId var1 occ -> WARN( not (isInScope var1 subst) && mustHaveLocalBinding var1,
759 text "simplVar:" <+> ppr var )
760 zapSubstEnv (completeCall var1 occ cont)
761 -- The template is already simplified, so don't re-substitute.
762 -- This is VITAL. Consider
764 -- let y = \z -> ...x... in
766 -- We'll clone the inner \x, adding x->x' in the id_subst
767 -- Then when we inline y, we must *not* replace x by x' in
768 -- the inlined copy!!
770 ---------------------------------------------------------
771 -- Dealing with a call
773 completeCall var occ_info cont
774 = getBlackList `thenSmpl` \ black_list_fn ->
775 getInScope `thenSmpl` \ in_scope ->
776 getContArgs var cont `thenSmpl` \ (args, call_cont, inline_call) ->
777 getDOptsSmpl `thenSmpl` \ dflags ->
779 black_listed = black_list_fn var
780 arg_infos = [ interestingArg in_scope arg subst
781 | (arg, subst, _) <- args, isValArg arg]
783 interesting_cont = interestingCallContext (not (null args))
784 (not (null arg_infos))
787 inline_cont | inline_call = discardInline cont
790 maybe_inline = callSiteInline dflags black_listed inline_call occ_info
791 var arg_infos interesting_cont
793 -- First, look for an inlining
794 case maybe_inline of {
795 Just unfolding -- There is an inlining!
796 -> tick (UnfoldingDone var) `thenSmpl_`
797 simplExprF unfolding inline_cont
800 Nothing -> -- No inlining!
803 simplifyArgs (isDataConId var) args (contResultType call_cont) $ \ args' ->
805 -- Next, look for rules or specialisations that match
807 -- It's important to simplify the args first, because the rule-matcher
808 -- doesn't do substitution as it goes. We don't want to use subst_args
809 -- (defined in the 'where') because that throws away useful occurrence info,
810 -- and perhaps-very-important specialisations.
812 -- Some functions have specialisations *and* are strict; in this case,
813 -- we don't want to inline the wrapper of the non-specialised thing; better
814 -- to call the specialised thing instead.
815 -- But the black-listing mechanism means that inlining of the wrapper
816 -- won't occur for things that have specialisations till a later phase, so
817 -- it's ok to try for inlining first.
819 -- You might think that we shouldn't apply rules for a loop breaker:
820 -- doing so might give rise to an infinite loop, because a RULE is
821 -- rather like an extra equation for the function:
822 -- RULE: f (g x) y = x+y
825 -- But it's too drastic to disable rules for loop breakers.
826 -- Even the foldr/build rule would be disabled, because foldr
827 -- is recursive, and hence a loop breaker:
828 -- foldr k z (build g) = g k z
829 -- So it's up to the programmer: rules can cause divergence
831 getSwitchChecker `thenSmpl` \ chkr ->
833 maybe_rule | switchIsOn chkr DontApplyRules = Nothing
834 | otherwise = lookupRule in_scope var args'
837 Just (rule_name, rule_rhs) ->
838 tick (RuleFired rule_name) `thenSmpl_`
840 (if dopt Opt_D_dump_inlinings dflags then
841 pprTrace "Rule fired" (vcat [
842 text "Rule:" <+> ptext rule_name,
843 text "Before:" <+> ppr var <+> sep (map pprParendExpr args'),
844 text "After: " <+> pprCoreExpr rule_rhs])
848 simplExprF rule_rhs call_cont ;
850 Nothing -> -- No rules
853 rebuild (mkApps (Var var) args') call_cont
857 ---------------------------------------------------------
858 -- Simplifying the arguments of a call
860 simplifyArgs :: Bool -- It's a data constructor
861 -> [(InExpr, SubstEnv, Bool)] -- Details of the arguments
862 -> OutType -- Type of the continuation
863 -> ([OutExpr] -> SimplM OutExprStuff)
864 -> SimplM OutExprStuff
866 -- Simplify the arguments to a call.
867 -- This part of the simplifier may break the no-shadowing invariant
869 -- f (...(\a -> e)...) (case y of (a,b) -> e')
870 -- where f is strict in its second arg
871 -- If we simplify the innermost one first we get (...(\a -> e)...)
872 -- Simplifying the second arg makes us float the case out, so we end up with
873 -- case y of (a,b) -> f (...(\a -> e)...) e'
874 -- So the output does not have the no-shadowing invariant. However, there is
875 -- no danger of getting name-capture, because when the first arg was simplified
876 -- we used an in-scope set that at least mentioned all the variables free in its
877 -- static environment, and that is enough.
879 -- We can't just do innermost first, or we'd end up with a dual problem:
880 -- case x of (a,b) -> f e (...(\a -> e')...)
882 -- I spent hours trying to recover the no-shadowing invariant, but I just could
883 -- not think of an elegant way to do it. The simplifier is already knee-deep in
884 -- continuations. We have to keep the right in-scope set around; AND we have
885 -- to get the effect that finding (error "foo") in a strict arg position will
886 -- discard the entire application and replace it with (error "foo"). Getting
887 -- all this at once is TOO HARD!
889 simplifyArgs is_data_con args cont_ty thing_inside
891 = go args thing_inside
893 | otherwise -- It's a data constructor, so we want
894 -- to switch off inlining in the arguments
895 -- If we don't do this, consider:
896 -- let x = +# p q in C {x}
897 -- Even though x get's an occurrence of 'many', its RHS looks cheap,
898 -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
899 = getBlackList `thenSmpl` \ old_bl ->
900 setBlackList noInlineBlackList $
902 setBlackList old_bl $
906 go [] thing_inside = thing_inside []
907 go (arg:args) thing_inside = simplifyArg is_data_con arg cont_ty $ \ arg' ->
909 thing_inside (arg':args')
911 simplifyArg is_data_con (Type ty_arg, se, _) cont_ty thing_inside
912 = simplTyArg ty_arg se `thenSmpl` \ new_ty_arg ->
913 thing_inside (Type new_ty_arg)
915 simplifyArg is_data_con (val_arg, se, is_strict) cont_ty thing_inside
916 = getInScope `thenSmpl` \ in_scope ->
918 arg_ty = substTy (mkSubst in_scope se) (exprType val_arg)
920 if not is_data_con then
921 -- An ordinary function
922 simplValArg arg_ty is_strict val_arg se cont_ty thing_inside
924 -- A data constructor
925 -- simplifyArgs has already switched off inlining, so
926 -- all we have to do here is to let-bind any non-trivial argument
928 -- It's not always the case that new_arg will be trivial
930 -- where, in one pass, f gets substituted by a constructor,
931 -- but x gets substituted by an expression (assume this is the
932 -- unique occurrence of x). It doesn't really matter -- it'll get
933 -- fixed up next pass. And it happens for dictionary construction,
934 -- which mentions the wrapper constructor to start with.
935 simplValArg arg_ty is_strict val_arg se cont_ty $ \ arg' ->
937 if exprIsTrivial arg' then
940 newId SLIT("a") (exprType arg') $ \ arg_id ->
941 addNonRecBind arg_id arg' $
942 thing_inside (Var arg_id)
946 %************************************************************************
948 \subsection{Decisions about inlining}
950 %************************************************************************
952 NB: At one time I tried not pre/post-inlining top-level things,
953 even if they occur exactly once. Reason:
954 (a) some might appear as a function argument, so we simply
955 replace static allocation with dynamic allocation:
961 (b) some top level things might be black listed
963 HOWEVER, I found that some useful foldr/build fusion was lost (most
964 notably in spectral/hartel/parstof) because the foldr didn't see the build.
966 Doing the dynamic allocation isn't a big deal, in fact, but losing the
970 preInlineUnconditionally :: Bool {- Black listed -} -> InId -> Bool
971 -- Examines a bndr to see if it is used just once in a
972 -- completely safe way, so that it is safe to discard the binding
973 -- inline its RHS at the (unique) usage site, REGARDLESS of how
974 -- big the RHS might be. If this is the case we don't simplify
975 -- the RHS first, but just inline it un-simplified.
977 -- This is much better than first simplifying a perhaps-huge RHS
978 -- and then inlining and re-simplifying it.
980 -- NB: we don't even look at the RHS to see if it's trivial
983 -- where x is used many times, but this is the unique occurrence
984 -- of y. We should NOT inline x at all its uses, because then
985 -- we'd do the same for y -- aargh! So we must base this
986 -- pre-rhs-simplification decision solely on x's occurrences, not
989 -- Evne RHSs labelled InlineMe aren't caught here, because
990 -- there might be no benefit from inlining at the call site.
992 preInlineUnconditionally black_listed bndr
993 | black_listed || opt_SimplNoPreInlining = False
994 | otherwise = case idOccInfo bndr of
995 OneOcc in_lam once -> not in_lam && once
996 -- Not inside a lambda, one occurrence ==> safe!
1002 %************************************************************************
1004 \subsection{The main rebuilder}
1006 %************************************************************************
1009 -------------------------------------------------------------------
1010 -- Finish rebuilding
1011 rebuild_done expr = returnOutStuff expr
1013 ---------------------------------------------------------
1014 rebuild :: OutExpr -> SimplCont -> SimplM OutExprStuff
1016 -- Stop continuation
1017 rebuild expr (Stop _ _) = rebuild_done expr
1019 -- ArgOf continuation
1020 rebuild expr (ArgOf _ _ cont_fn) = cont_fn expr
1022 -- ApplyTo continuation
1023 rebuild expr cont@(ApplyTo _ arg se cont')
1024 = setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1025 rebuild (App expr arg') cont'
1027 -- Coerce continuation
1028 rebuild expr (CoerceIt to_ty cont)
1029 = rebuild (mkCoerce to_ty (exprType expr) expr) cont
1031 -- Inline continuation
1032 rebuild expr (InlinePlease cont)
1033 = rebuild (Note InlineCall expr) cont
1035 rebuild scrut (Select _ bndr alts se cont)
1036 = rebuild_case scrut bndr alts se cont
1039 Case elimination [see the code above]
1041 Start with a simple situation:
1043 case x# of ===> e[x#/y#]
1046 (when x#, y# are of primitive type, of course). We can't (in general)
1047 do this for algebraic cases, because we might turn bottom into
1050 Actually, we generalise this idea to look for a case where we're
1051 scrutinising a variable, and we know that only the default case can
1056 other -> ...(case x of
1060 Here the inner case can be eliminated. This really only shows up in
1061 eliminating error-checking code.
1063 We also make sure that we deal with this very common case:
1068 Here we are using the case as a strict let; if x is used only once
1069 then we want to inline it. We have to be careful that this doesn't
1070 make the program terminate when it would have diverged before, so we
1072 - x is used strictly, or
1073 - e is already evaluated (it may so if e is a variable)
1075 Lastly, we generalise the transformation to handle this:
1081 We only do this for very cheaply compared r's (constructors, literals
1082 and variables). If pedantic bottoms is on, we only do it when the
1083 scrutinee is a PrimOp which can't fail.
1085 We do it *here*, looking at un-simplified alternatives, because we
1086 have to check that r doesn't mention the variables bound by the
1087 pattern in each alternative, so the binder-info is rather useful.
1089 So the case-elimination algorithm is:
1091 1. Eliminate alternatives which can't match
1093 2. Check whether all the remaining alternatives
1094 (a) do not mention in their rhs any of the variables bound in their pattern
1095 and (b) have equal rhss
1097 3. Check we can safely ditch the case:
1098 * PedanticBottoms is off,
1099 or * the scrutinee is an already-evaluated variable
1100 or * the scrutinee is a primop which is ok for speculation
1101 -- ie we want to preserve divide-by-zero errors, and
1102 -- calls to error itself!
1104 or * [Prim cases] the scrutinee is a primitive variable
1106 or * [Alg cases] the scrutinee is a variable and
1107 either * the rhs is the same variable
1108 (eg case x of C a b -> x ===> x)
1109 or * there is only one alternative, the default alternative,
1110 and the binder is used strictly in its scope.
1111 [NB this is helped by the "use default binder where
1112 possible" transformation; see below.]
1115 If so, then we can replace the case with one of the rhss.
1118 Blob of helper functions for the "case-of-something-else" situation.
1121 ---------------------------------------------------------
1122 -- Eliminate the case if possible
1124 rebuild_case scrut bndr alts se cont
1125 | maybeToBool maybe_con_app
1126 = knownCon scrut (DataAlt con) args bndr alts se cont
1128 | canEliminateCase scrut bndr alts
1129 = tick (CaseElim bndr) `thenSmpl_` (
1131 simplBinder bndr $ \ bndr' ->
1132 -- Remember to bind the case binder!
1133 completeBinding bndr bndr' False False scrut $
1134 simplExprF (head (rhssOfAlts alts)) cont)
1137 = complete_case scrut bndr alts se cont
1140 maybe_con_app = exprIsConApp_maybe scrut
1141 Just (con, args) = maybe_con_app
1143 -- See if we can get rid of the case altogether
1144 -- See the extensive notes on case-elimination above
1145 canEliminateCase scrut bndr alts
1146 = -- Check that the RHSs are all the same, and
1147 -- don't use the binders in the alternatives
1148 -- This test succeeds rapidly in the common case of
1149 -- a single DEFAULT alternative
1150 all (cheapEqExpr rhs1) other_rhss && all binders_unused alts
1152 -- Check that the scrutinee can be let-bound instead of case-bound
1153 && ( exprOkForSpeculation scrut
1154 -- OK not to evaluate it
1155 -- This includes things like (==# a# b#)::Bool
1156 -- so that we simplify
1157 -- case ==# a# b# of { True -> x; False -> x }
1160 -- This particular example shows up in default methods for
1161 -- comparision operations (e.g. in (>=) for Int.Int32)
1162 || exprIsValue scrut -- It's already evaluated
1163 || var_demanded_later scrut -- It'll be demanded later
1165 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1166 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1167 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1168 -- its argument: case x of { y -> dataToTag# y }
1169 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1170 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1175 (rhs1:other_rhss) = rhssOfAlts alts
1176 binders_unused (_, bndrs, _) = all isDeadBinder bndrs
1178 var_demanded_later (Var v) = isStrict (idDemandInfo bndr) -- It's going to be evaluated later
1179 var_demanded_later other = False
1182 ---------------------------------------------------------
1183 -- Case of something else
1185 complete_case scrut case_bndr alts se cont
1186 = -- Prepare case alternatives
1187 prepareCaseAlts case_bndr (splitTyConApp_maybe (idType case_bndr))
1188 impossible_cons alts `thenSmpl` \ better_alts ->
1190 -- Set the new subst-env in place (before dealing with the case binder)
1193 -- Deal with the case binder, and prepare the continuation;
1194 -- The new subst_env is in place
1195 prepareCaseCont better_alts cont $ \ cont' ->
1198 -- Deal with variable scrutinee
1200 getSwitchChecker `thenSmpl` \ chkr ->
1201 simplCaseBinder (switchIsOn chkr NoCaseOfCase)
1202 scrut case_bndr $ \ case_bndr' zap_occ_info ->
1204 -- Deal with the case alternatives
1205 simplAlts zap_occ_info impossible_cons
1206 case_bndr' better_alts cont' `thenSmpl` \ alts' ->
1208 mkCase scrut case_bndr' alts'
1209 ) `thenSmpl` \ case_expr ->
1211 -- Notice that the simplBinder, prepareCaseCont, etc, do *not* scope
1212 -- over the rebuild_done; rebuild_done returns the in-scope set, and
1213 -- that should not include these chaps!
1214 rebuild_done case_expr
1216 impossible_cons = case scrut of
1217 Var v -> otherCons (idUnfolding v)
1221 knownCon :: OutExpr -> AltCon -> [OutExpr]
1222 -> InId -> [InAlt] -> SubstEnv -> SimplCont
1223 -> SimplM OutExprStuff
1225 knownCon expr con args bndr alts se cont
1226 = -- Arguments should be atomic;
1228 WARN( not (all exprIsTrivial args),
1229 text "knownCon" <+> ppr expr )
1230 tick (KnownBranch bndr) `thenSmpl_`
1232 simplBinder bndr $ \ bndr' ->
1233 completeBinding bndr bndr' False False expr $
1234 -- Don't use completeBeta here. The expr might be
1235 -- an unboxed literal, like 3, or a variable
1236 -- whose unfolding is an unboxed literal... and
1237 -- completeBeta will just construct another case
1239 case findAlt con alts of
1240 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1243 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1246 (DataAlt dc, bs, rhs) -> ASSERT( length bs == length real_args )
1247 extendSubstList bs (map mk real_args) $
1250 real_args = drop (dataConNumInstArgs dc) args
1251 mk (Type ty) = DoneTy ty
1252 mk other = DoneEx other
1257 prepareCaseCont :: [InAlt] -> SimplCont
1258 -> (SimplCont -> SimplM (OutStuff a))
1259 -> SimplM (OutStuff a)
1260 -- Polymorphic recursion here!
1262 prepareCaseCont [alt] cont thing_inside = thing_inside cont
1263 prepareCaseCont alts cont thing_inside = simplType (coreAltsType alts) `thenSmpl` \ alts_ty ->
1264 mkDupableCont alts_ty cont thing_inside
1265 -- At one time I passed in the un-simplified type, and simplified
1266 -- it only if we needed to construct a join binder, but that
1267 -- didn't work because we have to decompse function types
1268 -- (using funResultTy) in mkDupableCont.
1271 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1272 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1273 way, there's a chance that v will now only be used once, and hence
1276 There is a time we *don't* want to do that, namely when
1277 -fno-case-of-case is on. This happens in the first simplifier pass,
1278 and enhances full laziness. Here's the bad case:
1279 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1280 If we eliminate the inner case, we trap it inside the I# v -> arm,
1281 which might prevent some full laziness happening. I've seen this
1282 in action in spectral/cichelli/Prog.hs:
1283 [(m,n) | m <- [1..max], n <- [1..max]]
1284 Hence the no_case_of_case argument
1287 If we do this, then we have to nuke any occurrence info (eg IAmDead)
1288 in the case binder, because the case-binder now effectively occurs
1289 whenever v does. AND we have to do the same for the pattern-bound
1292 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1294 Here, b and p are dead. But when we move the argment inside the first
1295 case RHS, and eliminate the second case, we get
1297 case x or { (a,b) -> a b }
1299 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1300 happened. Hence the zap_occ_info function returned by simplCaseBinder
1303 simplCaseBinder no_case_of_case (Var v) case_bndr thing_inside
1304 | not no_case_of_case
1305 = simplBinder (zap case_bndr) $ \ case_bndr' ->
1306 modifyInScope v case_bndr' $
1307 -- We could extend the substitution instead, but it would be
1308 -- a hack because then the substitution wouldn't be idempotent
1309 -- any more (v is an OutId). And this just just as well.
1310 thing_inside case_bndr' zap
1312 zap b = b `setIdOccInfo` NoOccInfo
1314 simplCaseBinder add_eval_info other_scrut case_bndr thing_inside
1315 = simplBinder case_bndr $ \ case_bndr' ->
1316 thing_inside case_bndr' (\ bndr -> bndr) -- NoOp on bndr
1319 prepareCaseAlts does two things:
1321 1. Remove impossible alternatives
1323 2. If the DEFAULT alternative can match only one possible constructor,
1324 then make that constructor explicit.
1326 case e of x { DEFAULT -> rhs }
1328 case e of x { (a,b) -> rhs }
1329 where the type is a single constructor type. This gives better code
1330 when rhs also scrutinises x or e.
1333 prepareCaseAlts bndr (Just (tycon, inst_tys)) scrut_cons alts
1335 = case (findDefault filtered_alts, missing_cons) of
1337 ((alts_no_deflt, Just rhs), [data_con]) -- Just one missing constructor!
1338 -> tick (FillInCaseDefault bndr) `thenSmpl_`
1340 (_,_,ex_tyvars,_,_,_) = dataConSig data_con
1342 getUniquesSmpl (length ex_tyvars) `thenSmpl` \ tv_uniqs ->
1344 ex_tyvars' = zipWithEqual "simpl_alt" mk tv_uniqs ex_tyvars
1345 mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
1346 arg_tys = dataConArgTys data_con
1347 (inst_tys ++ mkTyVarTys ex_tyvars')
1349 newIds SLIT("a") arg_tys $ \ bndrs ->
1350 returnSmpl ((DataAlt data_con, ex_tyvars' ++ bndrs, rhs) : alts_no_deflt)
1352 other -> returnSmpl filtered_alts
1354 -- Filter out alternatives that can't possibly match
1355 filtered_alts = case scrut_cons of
1357 other -> [alt | alt@(con,_,_) <- alts, not (con `elem` scrut_cons)]
1359 missing_cons = [data_con | data_con <- tyConDataConsIfAvailable tycon,
1360 not (data_con `elem` handled_data_cons)]
1361 handled_data_cons = [data_con | DataAlt data_con <- scrut_cons] ++
1362 [data_con | (DataAlt data_con, _, _) <- filtered_alts]
1365 prepareCaseAlts _ _ scrut_cons alts
1366 = returnSmpl alts -- Functions
1369 ----------------------
1370 simplAlts zap_occ_info scrut_cons case_bndr' alts cont'
1371 = mapSmpl simpl_alt alts
1373 inst_tys' = tyConAppArgs (idType case_bndr')
1375 -- handled_cons is all the constructors that are dealt
1376 -- with, either by being impossible, or by there being an alternative
1377 handled_cons = scrut_cons ++ [con | (con,_,_) <- alts, con /= DEFAULT]
1379 simpl_alt (DEFAULT, _, rhs)
1380 = -- In the default case we record the constructors that the
1381 -- case-binder *can't* be.
1382 -- We take advantage of any OtherCon info in the case scrutinee
1383 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` mkOtherCon handled_cons) $
1384 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1385 returnSmpl (DEFAULT, [], rhs')
1387 simpl_alt (con, vs, rhs)
1388 = -- Deal with the pattern-bound variables
1389 -- Mark the ones that are in ! positions in the data constructor
1390 -- as certainly-evaluated.
1391 -- NB: it happens that simplBinders does *not* erase the OtherCon
1392 -- form of unfolding, so it's ok to add this info before
1393 -- doing simplBinders
1394 simplBinders (add_evals con vs) $ \ vs' ->
1396 -- Bind the case-binder to (con args)
1398 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1400 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` unfolding) $
1401 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1402 returnSmpl (con, vs', rhs')
1405 -- add_evals records the evaluated-ness of the bound variables of
1406 -- a case pattern. This is *important*. Consider
1407 -- data T = T !Int !Int
1409 -- case x of { T a b -> T (a+1) b }
1411 -- We really must record that b is already evaluated so that we don't
1412 -- go and re-evaluate it when constructing the result.
1414 add_evals (DataAlt dc) vs = cat_evals vs (dataConRepStrictness dc)
1415 add_evals other_con vs = vs
1417 cat_evals [] [] = []
1418 cat_evals (v:vs) (str:strs)
1419 | isTyVar v = v : cat_evals vs (str:strs)
1420 | isStrict str = (v' `setIdUnfolding` mkOtherCon []) : cat_evals vs strs
1421 | otherwise = v' : cat_evals vs strs
1427 %************************************************************************
1429 \subsection{Duplicating continuations}
1431 %************************************************************************
1434 mkDupableCont :: OutType -- Type of the thing to be given to the continuation
1436 -> (SimplCont -> SimplM (OutStuff a))
1437 -> SimplM (OutStuff a)
1438 mkDupableCont ty cont thing_inside
1439 | contIsDupable cont
1442 mkDupableCont _ (CoerceIt ty cont) thing_inside
1443 = mkDupableCont ty cont $ \ cont' ->
1444 thing_inside (CoerceIt ty cont')
1446 mkDupableCont ty (InlinePlease cont) thing_inside
1447 = mkDupableCont ty cont $ \ cont' ->
1448 thing_inside (InlinePlease cont')
1450 mkDupableCont join_arg_ty (ArgOf _ cont_ty cont_fn) thing_inside
1451 = -- Build the RHS of the join point
1452 newId SLIT("a") join_arg_ty ( \ arg_id ->
1453 cont_fn (Var arg_id) `thenSmpl` \ (floats, (_, rhs)) ->
1454 returnSmpl (Lam (setOneShotLambda arg_id) (wrapFloats floats rhs))
1455 ) `thenSmpl` \ join_rhs ->
1457 -- Build the join Id and continuation
1458 -- We give it a "$j" name just so that for later amusement
1459 -- we can identify any join points that don't end up as let-no-escapes
1460 -- [NOTE: the type used to be exprType join_rhs, but this seems more elegant.]
1461 newId SLIT("$j") (mkFunTy join_arg_ty cont_ty) $ \ join_id ->
1463 new_cont = ArgOf OkToDup cont_ty
1464 (\arg' -> rebuild_done (App (Var join_id) arg'))
1467 tick (CaseOfCase join_id) `thenSmpl_`
1468 -- Want to tick here so that we go round again,
1469 -- and maybe copy or inline the code;
1470 -- not strictly CaseOf Case
1471 addLetBind (NonRec join_id join_rhs) $
1472 thing_inside new_cont
1474 mkDupableCont ty (ApplyTo _ arg se cont) thing_inside
1475 = mkDupableCont (funResultTy ty) cont $ \ cont' ->
1476 setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1477 if exprIsDupable arg' then
1478 thing_inside (ApplyTo OkToDup arg' emptySubstEnv cont')
1480 newId SLIT("a") (exprType arg') $ \ bndr ->
1482 tick (CaseOfCase bndr) `thenSmpl_`
1483 -- Want to tick here so that we go round again,
1484 -- and maybe copy or inline the code;
1485 -- not strictly CaseOf Case
1487 addLetBind (NonRec bndr arg') $
1488 -- But what if the arg should be case-bound? We can't use
1489 -- addNonRecBind here because its type is too specific.
1490 -- This has been this way for a long time, so I'll leave it,
1491 -- but I can't convince myself that it's right.
1493 thing_inside (ApplyTo OkToDup (Var bndr) emptySubstEnv cont')
1496 mkDupableCont ty (Select _ case_bndr alts se cont) thing_inside
1497 = tick (CaseOfCase case_bndr) `thenSmpl_`
1499 simplBinder case_bndr $ \ case_bndr' ->
1500 prepareCaseCont alts cont $ \ cont' ->
1501 mkDupableAlts case_bndr case_bndr' cont' alts $ \ alts' ->
1502 returnOutStuff alts'
1503 ) `thenSmpl` \ (alt_binds, (in_scope, alts')) ->
1505 addFloats alt_binds in_scope $
1507 -- NB that the new alternatives, alts', are still InAlts, using the original
1508 -- binders. That means we can keep the case_bndr intact. This is important
1509 -- because another case-of-case might strike, and so we want to keep the
1510 -- info that the case_bndr is dead (if it is, which is often the case).
1511 -- This is VITAL when the type of case_bndr is an unboxed pair (often the
1512 -- case in I/O rich code. We aren't allowed a lambda bound
1513 -- arg of unboxed tuple type, and indeed such a case_bndr is always dead
1514 thing_inside (Select OkToDup case_bndr alts' se (mkStop (contResultType cont)))
1516 mkDupableAlts :: InId -> OutId -> SimplCont -> [InAlt]
1517 -> ([InAlt] -> SimplM (OutStuff a))
1518 -> SimplM (OutStuff a)
1519 mkDupableAlts case_bndr case_bndr' cont [] thing_inside
1521 mkDupableAlts case_bndr case_bndr' cont (alt:alts) thing_inside
1522 = mkDupableAlt case_bndr case_bndr' cont alt $ \ alt' ->
1523 mkDupableAlts case_bndr case_bndr' cont alts $ \ alts' ->
1524 thing_inside (alt' : alts')
1526 mkDupableAlt case_bndr case_bndr' cont alt@(con, bndrs, rhs) thing_inside
1527 = simplBinders bndrs $ \ bndrs' ->
1528 simplExprC rhs cont `thenSmpl` \ rhs' ->
1530 if (case cont of { Stop _ _ -> exprIsDupable rhs'; other -> False}) then
1531 -- It is worth checking for a small RHS because otherwise we
1532 -- get extra let bindings that may cause an extra iteration of the simplifier to
1533 -- inline back in place. Quite often the rhs is just a variable or constructor.
1534 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1535 -- iterations because the version with the let bindings looked big, and so wasn't
1536 -- inlined, but after the join points had been inlined it looked smaller, and so
1539 -- But since the continuation is absorbed into the rhs, we only do this
1540 -- for a Stop continuation.
1542 -- NB: we have to check the size of rhs', not rhs.
1543 -- Duplicating a small InAlt might invalidate occurrence information
1544 -- However, if it *is* dupable, we return the *un* simplified alternative,
1545 -- because otherwise we'd need to pair it up with an empty subst-env.
1546 -- (Remember we must zap the subst-env before re-simplifying something).
1547 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1552 rhs_ty' = exprType rhs'
1553 (used_bndrs, used_bndrs')
1554 = unzip [pr | pr@(bndr,bndr') <- zip (case_bndr : bndrs)
1555 (case_bndr' : bndrs'),
1556 not (isDeadBinder bndr)]
1557 -- The new binders have lost their occurrence info,
1558 -- so we have to extract it from the old ones
1560 ( if null used_bndrs'
1561 -- If we try to lift a primitive-typed something out
1562 -- for let-binding-purposes, we will *caseify* it (!),
1563 -- with potentially-disastrous strictness results. So
1564 -- instead we turn it into a function: \v -> e
1565 -- where v::State# RealWorld#. The value passed to this function
1566 -- is realworld#, which generates (almost) no code.
1568 -- There's a slight infelicity here: we pass the overall
1569 -- case_bndr to all the join points if it's used in *any* RHS,
1570 -- because we don't know its usage in each RHS separately
1572 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1573 -- we make the join point into a function whenever used_bndrs'
1574 -- is empty. This makes the join-point more CPR friendly.
1575 -- Consider: let j = if .. then I# 3 else I# 4
1576 -- in case .. of { A -> j; B -> j; C -> ... }
1578 -- Now CPR should not w/w j because it's a thunk, so
1579 -- that means that the enclosing function can't w/w either,
1580 -- which is a lose. Here's the example that happened in practice:
1581 -- kgmod :: Int -> Int -> Int
1582 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1586 then newId SLIT("w") realWorldStatePrimTy $ \ rw_id ->
1587 returnSmpl ([rw_id], [Var realWorldPrimId])
1589 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs)
1591 `thenSmpl` \ (final_bndrs', final_args) ->
1593 -- See comment about "$j" name above
1594 newId SLIT("$j") (foldr mkPiType rhs_ty' final_bndrs') $ \ join_bndr ->
1595 -- Notice the funky mkPiType. If the contructor has existentials
1596 -- it's possible that the join point will be abstracted over
1597 -- type varaibles as well as term variables.
1598 -- Example: Suppose we have
1599 -- data T = forall t. C [t]
1601 -- case (case e of ...) of
1602 -- C t xs::[t] -> rhs
1603 -- We get the join point
1604 -- let j :: forall t. [t] -> ...
1605 -- j = /\t \xs::[t] -> rhs
1607 -- case (case e of ...) of
1608 -- C t xs::[t] -> j t xs
1611 -- We make the lambdas into one-shot-lambdas. The
1612 -- join point is sure to be applied at most once, and doing so
1613 -- prevents the body of the join point being floated out by
1614 -- the full laziness pass
1615 really_final_bndrs = map one_shot final_bndrs'
1616 one_shot v | isId v = setOneShotLambda v
1619 addLetBind (NonRec join_bndr (mkLams really_final_bndrs rhs')) $
1620 thing_inside (con, bndrs, mkApps (Var join_bndr) final_args)