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,
16 import SimplUtils ( mkCase, transformRhs, findAlt,
17 simplBinder, simplBinders, simplIds, findDefault,
18 SimplCont(..), DupFlag(..), mkStop, mkRhsStop,
19 contResultType, discardInline, countArgs, contIsDupable,
20 getContArgs, interestingCallContext, interestingArg, isStrictType
22 import Var ( mkSysTyVar, tyVarKind )
24 import VarSet ( elemVarSet )
25 import Id ( Id, idType, idInfo, isDataConId,
26 idUnfolding, setIdUnfolding, isExportedId, isDeadBinder,
27 idDemandInfo, setIdInfo,
28 idOccInfo, setIdOccInfo,
29 zapLamIdInfo, setOneShotLambda,
31 import IdInfo ( OccInfo(..), isDeadOcc, isLoopBreaker,
32 ArityInfo, setArityInfo, unknownArity,
36 import Demand ( Demand, isStrict )
37 import DataCon ( dataConNumInstArgs, dataConRepStrictness,
38 dataConSig, dataConArgTys
41 import CoreFVs ( mustHaveLocalBinding, exprFreeVars )
42 import CoreUnfold ( mkOtherCon, mkUnfolding, otherCons,
45 import CoreUtils ( cheapEqExpr, exprIsDupable, exprIsTrivial, exprIsConApp_maybe,
46 exprType, coreAltsType, exprIsValue, idAppIsCheap,
47 exprOkForSpeculation, etaReduceExpr,
48 mkCoerce, mkSCC, mkInlineMe, mkAltExpr
50 import Rules ( lookupRule )
51 import CostCentre ( currentCCS )
52 import Type ( mkTyVarTys, isUnLiftedType, seqType,
53 mkFunTy, splitFunTy, splitTyConApp_maybe,
56 import Subst ( mkSubst, substTy, substExpr,
57 isInScope, lookupIdSubst, substIdInfo
59 import TyCon ( isDataTyCon, tyConDataConsIfAvailable )
60 import TysPrim ( realWorldStatePrimTy )
61 import PrelInfo ( realWorldPrimId )
62 import Maybes ( maybeToBool )
63 import Util ( zipWithEqual )
68 The guts of the simplifier is in this module, but the driver
69 loop for the simplifier is in SimplCore.lhs.
72 -----------------------------------------
73 *** IMPORTANT NOTE ***
74 -----------------------------------------
75 The simplifier used to guarantee that the output had no shadowing, but
76 it does not do so any more. (Actually, it never did!) The reason is
77 documented with simplifyArgs.
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 %************************************************************************
132 The reason for this OutExprStuff stuff is that we want to float *after*
133 simplifying a RHS, not before. If we do so naively we get quadratic
134 behaviour as things float out.
136 To see why it's important to do it after, consider this (real) example:
150 a -- Can't inline a this round, cos it appears twice
154 Each of the ==> steps is a round of simplification. We'd save a
155 whole round if we float first. This can cascade. Consider
160 let f = let d1 = ..d.. in \y -> e
164 in \x -> ...(\y ->e)...
166 Only in this second round can the \y be applied, and it
167 might do the same again.
171 simplExpr :: CoreExpr -> SimplM CoreExpr
172 simplExpr expr = getSubst `thenSmpl` \ subst ->
173 simplExprC expr (mkStop (substTy subst (exprType expr)))
174 -- The type in the Stop continuation is usually not used
175 -- It's only needed when discarding continuations after finding
176 -- a function that returns bottom.
177 -- Hence the lazy substitution
179 simplExprC :: CoreExpr -> SimplCont -> SimplM CoreExpr
180 -- Simplify an expression, given a continuation
182 simplExprC expr cont = simplExprF expr cont `thenSmpl` \ (floats, (_, body)) ->
183 returnSmpl (mkLets floats body)
185 simplExprF :: InExpr -> SimplCont -> SimplM OutExprStuff
186 -- Simplify an expression, returning floated binds
188 simplExprF (Var v) cont
191 simplExprF (Lit lit) (Select _ bndr alts se cont)
192 = knownCon (Lit lit) (LitAlt lit) [] bndr alts se cont
194 simplExprF (Lit lit) cont
195 = rebuild (Lit lit) cont
197 simplExprF (App fun arg) cont
198 = getSubstEnv `thenSmpl` \ se ->
199 simplExprF fun (ApplyTo NoDup arg se cont)
201 simplExprF (Case scrut bndr alts) cont
202 = getSubstEnv `thenSmpl` \ subst_env ->
203 getSwitchChecker `thenSmpl` \ chkr ->
204 if not (switchIsOn chkr NoCaseOfCase) then
205 -- Simplify the scrutinee with a Select continuation
206 simplExprF scrut (Select NoDup bndr alts subst_env cont)
209 -- If case-of-case is off, simply simplify the case expression
210 -- in a vanilla Stop context, and rebuild the result around it
211 simplExprC scrut (Select NoDup bndr alts subst_env
212 (mkStop (contResultType cont))) `thenSmpl` \ case_expr' ->
213 rebuild case_expr' cont
216 simplExprF (Let (Rec pairs) body) cont
217 = simplIds (map fst pairs) $ \ bndrs' ->
218 -- NB: bndrs' don't have unfoldings or spec-envs
219 -- We add them as we go down, using simplPrags
221 simplRecBind False pairs bndrs' (simplExprF body cont)
223 simplExprF expr@(Lam _ _) cont = simplLam expr cont
225 simplExprF (Type ty) cont
226 = ASSERT( case cont of { Stop _ _ -> True; ArgOf _ _ _ -> True; other -> False } )
227 simplType ty `thenSmpl` \ ty' ->
228 rebuild (Type ty') cont
230 -- Comments about the Coerce case
231 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
232 -- It's worth checking for a coerce in the continuation,
233 -- in case we can cancel them. For example, in the initial form of a worker
234 -- we may find (coerce T (coerce S (\x.e))) y
235 -- and we'd like it to simplify to e[y/x] in one round of simplification
237 simplExprF (Note (Coerce to from) e) (CoerceIt outer_to cont)
238 = simplType from `thenSmpl` \ from' ->
239 if outer_to == from' then
240 -- The coerces cancel out
243 -- They don't cancel, but the inner one is redundant
244 simplExprF e (CoerceIt outer_to cont)
246 simplExprF (Note (Coerce to from) e) cont
247 = simplType to `thenSmpl` \ to' ->
248 simplExprF e (CoerceIt to' cont)
250 -- hack: we only distinguish subsumed cost centre stacks for the purposes of
251 -- inlining. All other CCCSs are mapped to currentCCS.
252 simplExprF (Note (SCC cc) e) cont
253 = setEnclosingCC currentCCS $
254 simplExpr e `thenSmpl` \ e ->
255 rebuild (mkSCC cc e) cont
257 simplExprF (Note InlineCall e) cont
258 = simplExprF e (InlinePlease cont)
260 -- Comments about the InlineMe case
261 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
262 -- Don't inline in the RHS of something that has an
263 -- inline pragma. But be careful that the InScopeEnv that
264 -- we return does still have inlinings on!
266 -- It really is important to switch off inlinings. This function
267 -- may be inlinined in other modules, so we don't want to remove
268 -- (by inlining) calls to functions that have specialisations, or
269 -- that may have transformation rules in an importing scope.
270 -- E.g. {-# INLINE f #-}
272 -- and suppose that g is strict *and* has specialisations.
273 -- If we inline g's wrapper, we deny f the chance of getting
274 -- the specialised version of g when f is inlined at some call site
275 -- (perhaps in some other module).
277 simplExprF (Note InlineMe e) cont
279 Stop _ _ -> -- Totally boring continuation
280 -- Don't inline inside an INLINE expression
281 setBlackList noInlineBlackList (simplExpr e) `thenSmpl` \ e' ->
282 rebuild (mkInlineMe e') cont
284 other -> -- Dissolve the InlineMe note if there's
285 -- an interesting context of any kind to combine with
286 -- (even a type application -- anything except Stop)
289 -- A non-recursive let is dealt with by simplBeta
290 simplExprF (Let (NonRec bndr rhs) body) cont
291 = getSubstEnv `thenSmpl` \ se ->
292 simplBeta bndr rhs se (contResultType cont) $
297 ---------------------------------
303 zap_it = mkLamBndrZapper fun cont
304 cont_ty = contResultType cont
306 -- Type-beta reduction
307 go (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
308 = ASSERT( isTyVar bndr )
309 tick (BetaReduction bndr) `thenSmpl_`
310 simplTyArg ty_arg arg_se `thenSmpl` \ ty_arg' ->
311 extendSubst bndr (DoneTy ty_arg')
314 -- Ordinary beta reduction
315 go (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
316 = tick (BetaReduction bndr) `thenSmpl_`
317 simplBeta zapped_bndr arg arg_se cont_ty
320 zapped_bndr = zap_it bndr
323 go lam@(Lam _ _) cont = completeLam [] lam cont
325 -- Exactly enough args
326 go expr cont = simplExprF expr cont
328 -- completeLam deals with the case where a lambda doesn't have an ApplyTo
329 -- continuation, so there are real lambdas left to put in the result
331 -- We try for eta reduction here, but *only* if we get all the
332 -- way to an exprIsTrivial expression.
333 -- We don't want to remove extra lambdas unless we are going
334 -- to avoid allocating this thing altogether
336 completeLam rev_bndrs (Lam bndr body) cont
337 = simplBinder bndr $ \ bndr' ->
338 completeLam (bndr':rev_bndrs) body cont
340 completeLam rev_bndrs body cont
341 = simplExpr body `thenSmpl` \ body' ->
342 case try_eta body' of
343 Just etad_lam -> tick (EtaReduction (head rev_bndrs)) `thenSmpl_`
344 rebuild etad_lam cont
346 Nothing -> rebuild (foldl (flip Lam) body' rev_bndrs) cont
348 -- We don't use CoreUtils.etaReduceExpr, because we can be more
349 -- efficient here: (a) we already have the binders, (b) we can do
350 -- the triviality test before computing the free vars
351 try_eta body | not opt_SimplDoEtaReduction = Nothing
352 | otherwise = go rev_bndrs body
354 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
355 go [] body | ok_body body = Just body -- Success!
356 go _ _ = Nothing -- Failure!
358 ok_body body = exprIsTrivial body && not (any (`elemVarSet` exprFreeVars body) rev_bndrs)
359 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
361 mkLamBndrZapper :: CoreExpr -- Function
362 -> SimplCont -- The context
363 -> Id -> Id -- Use this to zap the binders
364 mkLamBndrZapper fun cont
365 | n_args >= n_params fun = \b -> b -- Enough args
366 | otherwise = \b -> zapLamIdInfo b
368 -- NB: we count all the args incl type args
369 -- so we must count all the binders (incl type lambdas)
370 n_args = countArgs cont
372 n_params (Note _ e) = n_params e
373 n_params (Lam b e) = 1 + n_params e
374 n_params other = 0::Int
378 ---------------------------------
380 simplType :: InType -> SimplM OutType
382 = getSubst `thenSmpl` \ subst ->
384 new_ty = substTy subst ty
391 %************************************************************************
395 %************************************************************************
397 @simplBeta@ is used for non-recursive lets in expressions,
398 as well as true beta reduction.
400 Very similar to @simplLazyBind@, but not quite the same.
403 simplBeta :: InId -- Binder
404 -> InExpr -> SubstEnv -- Arg, with its subst-env
405 -> OutType -- Type of thing computed by the context
406 -> SimplM OutExprStuff -- The body
407 -> SimplM OutExprStuff
409 simplBeta bndr rhs rhs_se cont_ty thing_inside
411 = pprPanic "simplBeta" (ppr bndr <+> ppr rhs)
414 simplBeta bndr rhs rhs_se cont_ty thing_inside
415 | preInlineUnconditionally False {- not black listed -} bndr
416 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
417 extendSubst bndr (ContEx rhs_se rhs) thing_inside
420 = -- Simplify the RHS
421 simplBinder bndr $ \ bndr' ->
423 bndr_ty' = idType bndr'
424 is_strict = isStrict (idDemandInfo bndr) || isStrictType bndr_ty'
426 simplValArg bndr_ty' is_strict rhs rhs_se cont_ty $ \ rhs' ->
428 -- Now complete the binding and simplify the body
429 if needsCaseBinding bndr_ty' rhs' then
430 addCaseBind bndr' rhs' thing_inside
432 completeBinding bndr bndr' False False rhs' thing_inside
437 simplTyArg :: InType -> SubstEnv -> SimplM OutType
439 = getInScope `thenSmpl` \ in_scope ->
441 ty_arg' = substTy (mkSubst in_scope se) ty_arg
443 seqType ty_arg' `seq`
446 simplValArg :: OutType -- rhs_ty: Type of arg; used only occasionally
447 -> Bool -- True <=> evaluate eagerly
448 -> InExpr -> SubstEnv
449 -> OutType -- cont_ty: Type of thing computed by the context
450 -> (OutExpr -> SimplM OutExprStuff)
451 -- Takes an expression of type rhs_ty,
452 -- returns an expression of type cont_ty
453 -> SimplM OutExprStuff -- An expression of type cont_ty
455 simplValArg arg_ty is_strict arg arg_se cont_ty thing_inside
457 = getEnv `thenSmpl` \ env ->
459 simplExprF arg (ArgOf NoDup cont_ty $ \ rhs' ->
460 setAllExceptInScope env $
464 = simplRhs False {- Not top level -}
465 True {- OK to float unboxed -}
472 - deals only with Ids, not TyVars
473 - take an already-simplified RHS
475 It does *not* attempt to do let-to-case. Why? Because they are used for
478 (when let-to-case is impossible)
480 - many situations where the "rhs" is known to be a WHNF
481 (so let-to-case is inappropriate).
484 completeBinding :: InId -- Binder
485 -> OutId -- New binder
486 -> Bool -- True <=> top level
487 -> Bool -- True <=> black-listed; don't inline
488 -> OutExpr -- Simplified RHS
489 -> SimplM (OutStuff a) -- Thing inside
490 -> SimplM (OutStuff a)
492 completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
493 | isDeadOcc occ_info -- This happens; for example, the case_bndr during case of
494 -- known constructor: case (a,b) of x { (p,q) -> ... }
495 -- Here x isn't mentioned in the RHS, so we don't want to
496 -- create the (dead) let-binding let x = (a,b) in ...
499 | exprIsTrivial new_rhs
500 -- We're looking at a binding with a trivial RHS, so
501 -- perhaps we can discard it altogether!
503 -- NB: a loop breaker never has postInlineUnconditionally True
504 -- and non-loop-breakers only have *forward* references
505 -- Hence, it's safe to discard the binding
507 -- NOTE: This isn't our last opportunity to inline.
508 -- We're at the binding site right now, and
509 -- we'll get another opportunity when we get to the ocurrence(s)
511 -- Note that we do this unconditional inlining only for trival RHSs.
512 -- Don't inline even WHNFs inside lambdas; doing so may
513 -- simply increase allocation when the function is called
514 -- This isn't the last chance; see NOTE above.
516 -- NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here
517 -- Why? Because we don't even want to inline them into the
518 -- RHS of constructor arguments. See NOTE above
520 -- NB: Even NOINLINEis ignored here: if the rhs is trivial
521 -- it's best to inline it anyway. We often get a=E; b=a
522 -- from desugaring, with both a and b marked NOINLINE.
523 = if must_keep_binding then -- Keep the binding
524 finally_bind_it unknownArity new_rhs
525 -- Arity doesn't really matter because for a trivial RHS
526 -- we will inline like crazy at call sites
527 -- If this turns out be false, we can easily compute arity
528 else -- Drop the binding
529 extendSubst old_bndr (DoneEx new_rhs) $
530 -- Use the substitution to make quite, quite sure that the substitution
531 -- will happen, since we are going to discard the binding
532 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
535 | Note coercion@(Coerce _ inner_ty) inner_rhs <- new_rhs
536 -- [NB inner_rhs is guaranteed non-trivial by now]
537 -- x = coerce t e ==> c = e; x = inline_me (coerce t c)
538 -- Now x can get inlined, which moves the coercion
539 -- to the usage site. This is a bit like worker/wrapper stuff,
540 -- but it's useful to do it very promptly, so that
541 -- x = coerce T (I# 3)
545 -- This in turn means that
546 -- case (coerce Int x) of ...
548 -- Also the full-blown w/w thing isn't set up for non-functions
550 -- The inline_me note is so that the simplifier doesn't
551 -- just substitute c back inside x's rhs! (Typically, x will
552 -- get substituted away, but not if it's exported.)
553 = newId SLIT("c") inner_ty $ \ c_id ->
554 completeBinding c_id c_id top_lvl False inner_rhs $
555 completeBinding old_bndr new_bndr top_lvl black_listed
556 (Note InlineMe (Note coercion (Var c_id))) $
561 = transformRhs new_rhs finally_bind_it
564 old_info = idInfo old_bndr
565 occ_info = occInfo old_info
566 loop_breaker = isLoopBreaker occ_info
567 trivial_rhs = exprIsTrivial new_rhs
568 must_keep_binding = black_listed || loop_breaker || isExportedId old_bndr
570 finally_bind_it arity_info new_rhs
571 = getSubst `thenSmpl` \ subst ->
573 -- We make new IdInfo for the new binder by starting from the old binder,
574 -- doing appropriate substitutions.
575 -- Then we add arity and unfolding info to get the new binder
576 new_bndr_info = substIdInfo subst old_info (idInfo new_bndr)
577 `setArityInfo` arity_info
579 -- Add the unfolding *only* for non-loop-breakers
580 -- Making loop breakers not have an unfolding at all
581 -- means that we can avoid tests in exprIsConApp, for example.
582 -- This is important: if exprIsConApp says 'yes' for a recursive
583 -- thing, then we can get into an infinite loop
584 info_w_unf | loop_breaker = new_bndr_info
585 | otherwise = new_bndr_info `setUnfoldingInfo` mkUnfolding top_lvl new_rhs
587 final_id = new_bndr `setIdInfo` info_w_unf
589 -- These seqs forces the Id, and hence its IdInfo,
590 -- and hence any inner substitutions
592 addLetBind (NonRec final_id new_rhs) $
593 modifyInScope new_bndr final_id thing_inside
598 %************************************************************************
600 \subsection{simplLazyBind}
602 %************************************************************************
604 simplLazyBind basically just simplifies the RHS of a let(rec).
605 It does two important optimisations though:
607 * It floats let(rec)s out of the RHS, even if they
608 are hidden by big lambdas
610 * It does eta expansion
613 simplLazyBind :: Bool -- True <=> top level
616 -> SimplM (OutStuff a) -- The body of the binding
617 -> SimplM (OutStuff a)
618 -- When called, the subst env is correct for the entire let-binding
619 -- and hence right for the RHS.
620 -- Also the binder has already been simplified, and hence is in scope
622 simplLazyBind top_lvl bndr bndr' rhs thing_inside
623 = getBlackList `thenSmpl` \ black_list_fn ->
625 black_listed = black_list_fn bndr
628 if preInlineUnconditionally black_listed bndr then
629 -- Inline unconditionally
630 tick (PreInlineUnconditionally bndr) `thenSmpl_`
631 getSubstEnv `thenSmpl` \ rhs_se ->
632 (extendSubst bndr (ContEx rhs_se rhs) thing_inside)
636 getSubstEnv `thenSmpl` \ rhs_se ->
637 simplRhs top_lvl False {- Not ok to float unboxed (conservative) -}
639 rhs rhs_se $ \ rhs' ->
641 -- Now compete the binding and simplify the body
642 completeBinding bndr bndr' top_lvl black_listed rhs' thing_inside
648 simplRhs :: Bool -- True <=> Top level
649 -> Bool -- True <=> OK to float unboxed (speculative) bindings
650 -- False for (a) recursive and (b) top-level bindings
651 -> OutType -- Type of RHS; used only occasionally
652 -> InExpr -> SubstEnv
653 -> (OutExpr -> SimplM (OutStuff a))
654 -> SimplM (OutStuff a)
655 simplRhs top_lvl float_ubx rhs_ty rhs rhs_se thing_inside
657 setSubstEnv rhs_se (simplExprF rhs (mkRhsStop rhs_ty)) `thenSmpl` \ (floats, (in_scope', rhs')) ->
659 -- Float lets out of RHS
661 (floats_out, rhs'') = splitFloats float_ubx floats rhs'
663 if (top_lvl || wantToExpose 0 rhs') && -- Float lets if (a) we're at the top level
664 not (null floats_out) -- or (b) the resulting RHS is one we'd like to expose
666 tickLetFloat floats_out `thenSmpl_`
669 -- There's a subtlety here. There may be a binding (x* = e) in the
670 -- floats, where the '*' means 'will be demanded'. So is it safe
671 -- to float it out? Answer no, but it won't matter because
672 -- we only float if arg' is a WHNF,
673 -- and so there can't be any 'will be demanded' bindings in the floats.
675 WARN( any demanded_float floats_out, ppr floats_out )
676 addLetBinds floats_out $
677 setInScope in_scope' $
679 -- in_scope' may be excessive, but that's OK;
680 -- it's a superset of what's in scope
682 -- Don't do the float
683 thing_inside (mkLets floats rhs')
685 -- In a let-from-let float, we just tick once, arbitrarily
686 -- choosing the first floated binder to identify it
687 tickLetFloat (NonRec b r : fs) = tick (LetFloatFromLet b)
688 tickLetFloat (Rec ((b,r):prs) : fs) = tick (LetFloatFromLet b)
690 demanded_float (NonRec b r) = isStrict (idDemandInfo b) && not (isUnLiftedType (idType b))
691 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
692 demanded_float (Rec _) = False
694 -- If float_ubx is true we float all the bindings, otherwise
695 -- we just float until we come across an unlifted one.
696 -- Remember that the unlifted bindings in the floats are all for
697 -- guaranteed-terminating non-exception-raising unlifted things,
698 -- which we are happy to do speculatively. However, we may still
699 -- not be able to float them out, because the context
700 -- is either a Rec group, or the top level, neither of which
701 -- can tolerate them.
702 splitFloats float_ubx floats rhs
703 | float_ubx = (floats, rhs) -- Float them all
704 | otherwise = go floats
707 go (f:fs) | must_stay f = ([], mkLets (f:fs) rhs)
708 | otherwise = case go fs of
709 (out, rhs') -> (f:out, rhs')
711 must_stay (Rec prs) = False -- No unlifted bindings in here
712 must_stay (NonRec b r) = isUnLiftedType (idType b)
714 wantToExpose :: Int -> CoreExpr -> Bool
715 -- True for expressions that we'd like to expose at the
716 -- top level of an RHS. This includes partial applications
717 -- even if the args aren't cheap; the next pass will let-bind the
718 -- args and eta expand the partial application. So exprIsCheap won't do.
719 -- Here's the motivating example:
720 -- z = letrec g = \x y -> ...g... in g E
721 -- Even though E is a redex we'd like to float the letrec to give
722 -- g = \x y -> ...g...
724 -- Now the next use of SimplUtils.tryEtaExpansion will give
725 -- g = \x y -> ...g...
726 -- z = let v = E in \w -> g v w
727 -- And now we'll float the v to give
728 -- g = \x y -> ...g...
731 -- Which is what we want; chances are z will be inlined now.
733 wantToExpose n (Var v) = idAppIsCheap v n
734 wantToExpose n (Lit l) = True
735 wantToExpose n (Lam _ e) = True
736 wantToExpose n (Note _ e) = wantToExpose n e
737 wantToExpose n (App f (Type _)) = wantToExpose n f
738 wantToExpose n (App f a) = wantToExpose (n+1) f
739 wantToExpose n other = False -- There won't be any lets
744 %************************************************************************
746 \subsection{Variables}
748 %************************************************************************
752 = getSubst `thenSmpl` \ subst ->
753 case lookupIdSubst subst var of
754 DoneEx e -> zapSubstEnv (simplExprF e cont)
755 ContEx env1 e -> setSubstEnv env1 (simplExprF e cont)
756 DoneId var1 occ -> WARN( not (isInScope var1 subst) && mustHaveLocalBinding var1,
757 text "simplVar:" <+> ppr var )
758 zapSubstEnv (completeCall var1 occ cont)
759 -- The template is already simplified, so don't re-substitute.
760 -- This is VITAL. Consider
762 -- let y = \z -> ...x... in
764 -- We'll clone the inner \x, adding x->x' in the id_subst
765 -- Then when we inline y, we must *not* replace x by x' in
766 -- the inlined copy!!
768 ---------------------------------------------------------
769 -- Dealing with a call
771 completeCall var occ cont
772 = getBlackList `thenSmpl` \ black_list_fn ->
773 getInScope `thenSmpl` \ in_scope ->
774 getContArgs var cont `thenSmpl` \ (args, call_cont, inline_call) ->
776 black_listed = black_list_fn var
777 arg_infos = [ interestingArg in_scope arg subst
778 | (arg, subst, _) <- args, isValArg arg]
780 interesting_cont = interestingCallContext (not (null args))
781 (not (null arg_infos))
784 inline_cont | inline_call = discardInline cont
787 maybe_inline = callSiteInline black_listed inline_call occ
788 var arg_infos interesting_cont
790 -- First, look for an inlining
791 case maybe_inline of {
792 Just unfolding -- There is an inlining!
793 -> tick (UnfoldingDone var) `thenSmpl_`
794 simplExprF unfolding inline_cont
797 Nothing -> -- No inlining!
800 simplifyArgs (isDataConId var) args (contResultType call_cont) $ \ args' ->
802 -- Next, look for rules or specialisations that match
804 -- It's important to simplify the args first, because the rule-matcher
805 -- doesn't do substitution as it goes. We don't want to use subst_args
806 -- (defined in the 'where') because that throws away useful occurrence info,
807 -- and perhaps-very-important specialisations.
809 -- Some functions have specialisations *and* are strict; in this case,
810 -- we don't want to inline the wrapper of the non-specialised thing; better
811 -- to call the specialised thing instead.
812 -- But the black-listing mechanism means that inlining of the wrapper
813 -- won't occur for things that have specialisations till a later phase, so
814 -- it's ok to try for inlining first.
816 getSwitchChecker `thenSmpl` \ chkr ->
818 maybe_rule | switchIsOn chkr DontApplyRules = Nothing
819 | otherwise = lookupRule in_scope var args'
822 Just (rule_name, rule_rhs) ->
823 tick (RuleFired rule_name) `thenSmpl_`
824 simplExprF rule_rhs call_cont ;
826 Nothing -> -- No rules
829 rebuild (mkApps (Var var) args') call_cont
833 ---------------------------------------------------------
834 -- Simplifying the arguments of a call
836 simplifyArgs :: Bool -- It's a data constructor
837 -> [(InExpr, SubstEnv, Bool)] -- Details of the arguments
838 -> OutType -- Type of the continuation
839 -> ([OutExpr] -> SimplM OutExprStuff)
840 -> SimplM OutExprStuff
842 -- Simplify the arguments to a call.
843 -- This part of the simplifier may break the no-shadowing invariant
845 -- f (...(\a -> e)...) (case y of (a,b) -> e')
846 -- where f is strict in its second arg
847 -- If we simplify the innermost one first we get (...(\a -> e)...)
848 -- Simplifying the second arg makes us float the case out, so we end up with
849 -- case y of (a,b) -> f (...(\a -> e)...) e'
850 -- So the output does not have the no-shadowing invariant. However, there is
851 -- no danger of getting name-capture, because when the first arg was simplified
852 -- we used an in-scope set that at least mentioned all the variables free in its
853 -- static environment, and that is enough.
855 -- We can't just do innermost first, or we'd end up with a dual problem:
856 -- case x of (a,b) -> f e (...(\a -> e')...)
858 -- I spent hours trying to recover the no-shadowing invariant, but I just could
859 -- not think of an elegant way to do it. The simplifier is already knee-deep in
860 -- continuations. We have to keep the right in-scope set around; AND we have
861 -- to get the effect that finding (error "foo") in a strict arg position will
862 -- discard the entire application and replace it with (error "foo"). Getting
863 -- all this at once is TOO HARD!
865 simplifyArgs is_data_con args cont_ty thing_inside
867 = go args thing_inside
869 | otherwise -- It's a data constructor, so we want
870 -- to switch off inlining in the arguments
871 -- If we don't do this, consider:
872 -- let x = +# p q in C {x}
873 -- Even though x get's an occurrence of 'many', its RHS looks cheap,
874 -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
875 = getBlackList `thenSmpl` \ old_bl ->
876 setBlackList noInlineBlackList $
878 setBlackList old_bl $
882 go [] thing_inside = thing_inside []
883 go (arg:args) thing_inside = simplifyArg is_data_con arg cont_ty $ \ arg' ->
885 thing_inside (arg':args')
887 simplifyArg is_data_con (Type ty_arg, se, _) cont_ty thing_inside
888 = simplTyArg ty_arg se `thenSmpl` \ new_ty_arg ->
889 thing_inside (Type new_ty_arg)
891 simplifyArg is_data_con (val_arg, se, is_strict) cont_ty thing_inside
892 = getInScope `thenSmpl` \ in_scope ->
894 arg_ty = substTy (mkSubst in_scope se) (exprType val_arg)
896 if not is_data_con then
897 -- An ordinary function
898 simplValArg arg_ty is_strict val_arg se cont_ty thing_inside
900 -- A data constructor
901 -- simplifyArgs has already switched off inlining, so
902 -- all we have to do here is to let-bind any non-trivial argument
904 -- It's not always the case that new_arg will be trivial
906 -- where, in one pass, f gets substituted by a constructor,
907 -- but x gets substituted by an expression (assume this is the
908 -- unique occurrence of x). It doesn't really matter -- it'll get
909 -- fixed up next pass. And it happens for dictionary construction,
910 -- which mentions the wrapper constructor to start with.
911 simplValArg arg_ty is_strict val_arg se cont_ty $ \ arg' ->
913 if exprIsTrivial arg' then
916 newId SLIT("a") (exprType arg') $ \ arg_id ->
917 addNonRecBind arg_id arg' $
918 thing_inside (Var arg_id)
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!
978 %************************************************************************
980 \subsection{The main rebuilder}
982 %************************************************************************
985 -------------------------------------------------------------------
988 = getInScope `thenSmpl` \ in_scope ->
989 returnSmpl ([], (in_scope, expr))
991 ---------------------------------------------------------
992 rebuild :: OutExpr -> SimplCont -> SimplM OutExprStuff
995 rebuild expr (Stop _ _) = rebuild_done expr
997 -- ArgOf continuation
998 rebuild expr (ArgOf _ _ cont_fn) = cont_fn expr
1000 -- ApplyTo continuation
1001 rebuild expr cont@(ApplyTo _ arg se cont')
1002 = setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1003 rebuild (App expr arg') cont'
1005 -- Coerce continuation
1006 rebuild expr (CoerceIt to_ty cont)
1007 = rebuild (mkCoerce to_ty (exprType expr) expr) cont
1009 -- Inline continuation
1010 rebuild expr (InlinePlease cont)
1011 = rebuild (Note InlineCall expr) cont
1013 rebuild scrut (Select _ bndr alts se cont)
1014 = rebuild_case scrut bndr alts se cont
1017 Case elimination [see the code above]
1019 Start with a simple situation:
1021 case x# of ===> e[x#/y#]
1024 (when x#, y# are of primitive type, of course). We can't (in general)
1025 do this for algebraic cases, because we might turn bottom into
1028 Actually, we generalise this idea to look for a case where we're
1029 scrutinising a variable, and we know that only the default case can
1034 other -> ...(case x of
1038 Here the inner case can be eliminated. This really only shows up in
1039 eliminating error-checking code.
1041 We also make sure that we deal with this very common case:
1046 Here we are using the case as a strict let; if x is used only once
1047 then we want to inline it. We have to be careful that this doesn't
1048 make the program terminate when it would have diverged before, so we
1050 - x is used strictly, or
1051 - e is already evaluated (it may so if e is a variable)
1053 Lastly, we generalise the transformation to handle this:
1059 We only do this for very cheaply compared r's (constructors, literals
1060 and variables). If pedantic bottoms is on, we only do it when the
1061 scrutinee is a PrimOp which can't fail.
1063 We do it *here*, looking at un-simplified alternatives, because we
1064 have to check that r doesn't mention the variables bound by the
1065 pattern in each alternative, so the binder-info is rather useful.
1067 So the case-elimination algorithm is:
1069 1. Eliminate alternatives which can't match
1071 2. Check whether all the remaining alternatives
1072 (a) do not mention in their rhs any of the variables bound in their pattern
1073 and (b) have equal rhss
1075 3. Check we can safely ditch the case:
1076 * PedanticBottoms is off,
1077 or * the scrutinee is an already-evaluated variable
1078 or * the scrutinee is a primop which is ok for speculation
1079 -- ie we want to preserve divide-by-zero errors, and
1080 -- calls to error itself!
1082 or * [Prim cases] the scrutinee is a primitive variable
1084 or * [Alg cases] the scrutinee is a variable and
1085 either * the rhs is the same variable
1086 (eg case x of C a b -> x ===> x)
1087 or * there is only one alternative, the default alternative,
1088 and the binder is used strictly in its scope.
1089 [NB this is helped by the "use default binder where
1090 possible" transformation; see below.]
1093 If so, then we can replace the case with one of the rhss.
1096 Blob of helper functions for the "case-of-something-else" situation.
1099 ---------------------------------------------------------
1100 -- Eliminate the case if possible
1102 rebuild_case scrut bndr alts se cont
1103 | maybeToBool maybe_con_app
1104 = knownCon scrut (DataAlt con) args bndr alts se cont
1106 | canEliminateCase scrut bndr alts
1107 = tick (CaseElim bndr) `thenSmpl_` (
1109 simplBinder bndr $ \ bndr' ->
1110 -- Remember to bind the case binder!
1111 completeBinding bndr bndr' False False scrut $
1112 simplExprF (head (rhssOfAlts alts)) cont)
1115 = complete_case scrut bndr alts se cont
1118 maybe_con_app = exprIsConApp_maybe scrut
1119 Just (con, args) = maybe_con_app
1121 -- See if we can get rid of the case altogether
1122 -- See the extensive notes on case-elimination above
1123 canEliminateCase scrut bndr alts
1124 = -- Check that the RHSs are all the same, and
1125 -- don't use the binders in the alternatives
1126 -- This test succeeds rapidly in the common case of
1127 -- a single DEFAULT alternative
1128 all (cheapEqExpr rhs1) other_rhss && all binders_unused alts
1130 -- Check that the scrutinee can be let-bound instead of case-bound
1131 && ( exprOkForSpeculation scrut
1132 -- OK not to evaluate it
1133 -- This includes things like (==# a# b#)::Bool
1134 -- so that we simplify
1135 -- case ==# a# b# of { True -> x; False -> x }
1138 -- This particular example shows up in default methods for
1139 -- comparision operations (e.g. in (>=) for Int.Int32)
1140 || exprIsValue scrut -- It's already evaluated
1141 || var_demanded_later scrut -- It'll be demanded later
1143 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1144 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1145 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1146 -- its argument: case x of { y -> dataToTag# y }
1147 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1148 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1153 (rhs1:other_rhss) = rhssOfAlts alts
1154 binders_unused (_, bndrs, _) = all isDeadBinder bndrs
1156 var_demanded_later (Var v) = isStrict (idDemandInfo bndr) -- It's going to be evaluated later
1157 var_demanded_later other = False
1160 ---------------------------------------------------------
1161 -- Case of something else
1163 complete_case scrut case_bndr alts se cont
1164 = -- Prepare case alternatives
1165 prepareCaseAlts case_bndr (splitTyConApp_maybe (idType case_bndr))
1166 impossible_cons alts `thenSmpl` \ better_alts ->
1168 -- Set the new subst-env in place (before dealing with the case binder)
1171 -- Deal with the case binder, and prepare the continuation;
1172 -- The new subst_env is in place
1173 prepareCaseCont better_alts cont $ \ cont' ->
1176 -- Deal with variable scrutinee
1178 getSwitchChecker `thenSmpl` \ chkr ->
1179 simplCaseBinder (switchIsOn chkr NoCaseOfCase)
1180 scrut case_bndr $ \ case_bndr' zap_occ_info ->
1182 -- Deal with the case alternatives
1183 simplAlts zap_occ_info impossible_cons
1184 case_bndr' better_alts cont' `thenSmpl` \ alts' ->
1186 mkCase scrut case_bndr' alts'
1187 ) `thenSmpl` \ case_expr ->
1189 -- Notice that the simplBinder, prepareCaseCont, etc, do *not* scope
1190 -- over the rebuild_done; rebuild_done returns the in-scope set, and
1191 -- that should not include these chaps!
1192 rebuild_done case_expr
1194 impossible_cons = case scrut of
1195 Var v -> otherCons (idUnfolding v)
1199 knownCon :: OutExpr -> AltCon -> [OutExpr]
1200 -> InId -> [InAlt] -> SubstEnv -> SimplCont
1201 -> SimplM OutExprStuff
1203 knownCon expr con args bndr alts se cont
1204 = tick (KnownBranch bndr) `thenSmpl_`
1206 simplBinder bndr $ \ bndr' ->
1207 completeBinding bndr bndr' False False expr $
1208 -- Don't use completeBeta here. The expr might be
1209 -- an unboxed literal, like 3, or a variable
1210 -- whose unfolding is an unboxed literal... and
1211 -- completeBeta will just construct another case
1213 case findAlt con alts of
1214 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1217 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1220 (DataAlt dc, bs, rhs) -> ASSERT( length bs == length real_args )
1221 extendSubstList bs (map mk real_args) $
1224 real_args = drop (dataConNumInstArgs dc) args
1225 mk (Type ty) = DoneTy ty
1226 mk other = DoneEx other
1231 prepareCaseCont :: [InAlt] -> SimplCont
1232 -> (SimplCont -> SimplM (OutStuff a))
1233 -> SimplM (OutStuff a)
1234 -- Polymorphic recursion here!
1236 prepareCaseCont [alt] cont thing_inside = thing_inside cont
1237 prepareCaseCont alts cont thing_inside = simplType (coreAltsType alts) `thenSmpl` \ alts_ty ->
1238 mkDupableCont alts_ty cont thing_inside
1239 -- At one time I passed in the un-simplified type, and simplified
1240 -- it only if we needed to construct a join binder, but that
1241 -- didn't work because we have to decompse function types
1242 -- (using funResultTy) in mkDupableCont.
1245 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1246 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1247 way, there's a chance that v will now only be used once, and hence
1250 There is a time we *don't* want to do that, namely when
1251 -fno-case-of-case is on. This happens in the first simplifier pass,
1252 and enhances full laziness. Here's the bad case:
1253 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1254 If we eliminate the inner case, we trap it inside the I# v -> arm,
1255 which might prevent some full laziness happening. I've seen this
1256 in action in spectral/cichelli/Prog.hs:
1257 [(m,n) | m <- [1..max], n <- [1..max]]
1258 Hence the no_case_of_case argument
1261 If we do this, then we have to nuke any occurrence info (eg IAmDead)
1262 in the case binder, because the case-binder now effectively occurs
1263 whenever v does. AND we have to do the same for the pattern-bound
1266 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1268 Here, b and p are dead. But when we move the argment inside the first
1269 case RHS, and eliminate the second case, we get
1271 case x or { (a,b) -> a b }
1273 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1274 happened. Hence the zap_occ_info function returned by simplCaseBinder
1277 simplCaseBinder no_case_of_case (Var v) case_bndr thing_inside
1278 | not no_case_of_case
1279 = simplBinder (zap case_bndr) $ \ case_bndr' ->
1280 modifyInScope v case_bndr' $
1281 -- We could extend the substitution instead, but it would be
1282 -- a hack because then the substitution wouldn't be idempotent
1283 -- any more (v is an OutId). And this just just as well.
1284 thing_inside case_bndr' zap
1286 zap b = b `setIdOccInfo` NoOccInfo
1288 simplCaseBinder add_eval_info other_scrut case_bndr thing_inside
1289 = simplBinder case_bndr $ \ case_bndr' ->
1290 thing_inside case_bndr' (\ bndr -> bndr) -- NoOp on bndr
1293 prepareCaseAlts does two things:
1295 1. Remove impossible alternatives
1297 2. If the DEFAULT alternative can match only one possible constructor,
1298 then make that constructor explicit.
1300 case e of x { DEFAULT -> rhs }
1302 case e of x { (a,b) -> rhs }
1303 where the type is a single constructor type. This gives better code
1304 when rhs also scrutinises x or e.
1307 prepareCaseAlts bndr (Just (tycon, inst_tys)) scrut_cons alts
1309 = case (findDefault filtered_alts, missing_cons) of
1311 ((alts_no_deflt, Just rhs), [data_con]) -- Just one missing constructor!
1312 -> tick (FillInCaseDefault bndr) `thenSmpl_`
1314 (_,_,ex_tyvars,_,_,_) = dataConSig data_con
1316 getUniquesSmpl (length ex_tyvars) `thenSmpl` \ tv_uniqs ->
1318 ex_tyvars' = zipWithEqual "simpl_alt" mk tv_uniqs ex_tyvars
1319 mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
1320 arg_tys = dataConArgTys data_con
1321 (inst_tys ++ mkTyVarTys ex_tyvars')
1323 newIds SLIT("a") arg_tys $ \ bndrs ->
1324 returnSmpl ((DataAlt data_con, ex_tyvars' ++ bndrs, rhs) : alts_no_deflt)
1326 other -> returnSmpl filtered_alts
1328 -- Filter out alternatives that can't possibly match
1329 filtered_alts = case scrut_cons of
1331 other -> [alt | alt@(con,_,_) <- alts, not (con `elem` scrut_cons)]
1333 missing_cons = [data_con | data_con <- tyConDataConsIfAvailable tycon,
1334 not (data_con `elem` handled_data_cons)]
1335 handled_data_cons = [data_con | DataAlt data_con <- scrut_cons] ++
1336 [data_con | (DataAlt data_con, _, _) <- filtered_alts]
1339 prepareCaseAlts _ _ scrut_cons alts
1340 = returnSmpl alts -- Functions
1343 ----------------------
1344 simplAlts zap_occ_info scrut_cons case_bndr' alts cont'
1345 = mapSmpl simpl_alt alts
1347 inst_tys' = case splitTyConApp_maybe (idType case_bndr') of
1348 Just (tycon, inst_tys) -> inst_tys
1350 -- handled_cons is all the constructors that are dealt
1351 -- with, either by being impossible, or by there being an alternative
1352 handled_cons = scrut_cons ++ [con | (con,_,_) <- alts, con /= DEFAULT]
1354 simpl_alt (DEFAULT, _, rhs)
1355 = -- In the default case we record the constructors that the
1356 -- case-binder *can't* be.
1357 -- We take advantage of any OtherCon info in the case scrutinee
1358 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` mkOtherCon handled_cons) $
1359 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1360 returnSmpl (DEFAULT, [], rhs')
1362 simpl_alt (con, vs, rhs)
1363 = -- Deal with the pattern-bound variables
1364 -- Mark the ones that are in ! positions in the data constructor
1365 -- as certainly-evaluated.
1366 -- NB: it happens that simplBinders does *not* erase the OtherCon
1367 -- form of unfolding, so it's ok to add this info before
1368 -- doing simplBinders
1369 simplBinders (add_evals con vs) $ \ vs' ->
1371 -- Bind the case-binder to (con args)
1373 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1375 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` unfolding) $
1376 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1377 returnSmpl (con, vs', rhs')
1380 -- add_evals records the evaluated-ness of the bound variables of
1381 -- a case pattern. This is *important*. Consider
1382 -- data T = T !Int !Int
1384 -- case x of { T a b -> T (a+1) b }
1386 -- We really must record that b is already evaluated so that we don't
1387 -- go and re-evaluate it when constructing the result.
1389 add_evals (DataAlt dc) vs = cat_evals vs (dataConRepStrictness dc)
1390 add_evals other_con vs = vs
1392 cat_evals [] [] = []
1393 cat_evals (v:vs) (str:strs)
1394 | isTyVar v = v : cat_evals vs (str:strs)
1395 | isStrict str = (v' `setIdUnfolding` mkOtherCon []) : cat_evals vs strs
1396 | otherwise = v' : cat_evals vs strs
1402 %************************************************************************
1404 \subsection{Duplicating continuations}
1406 %************************************************************************
1409 mkDupableCont :: OutType -- Type of the thing to be given to the continuation
1411 -> (SimplCont -> SimplM (OutStuff a))
1412 -> SimplM (OutStuff a)
1413 mkDupableCont ty cont thing_inside
1414 | contIsDupable cont
1417 mkDupableCont _ (CoerceIt ty cont) thing_inside
1418 = mkDupableCont ty cont $ \ cont' ->
1419 thing_inside (CoerceIt ty cont')
1421 mkDupableCont ty (InlinePlease cont) thing_inside
1422 = mkDupableCont ty cont $ \ cont' ->
1423 thing_inside (InlinePlease cont')
1425 mkDupableCont join_arg_ty (ArgOf _ cont_ty cont_fn) thing_inside
1426 = -- Build the RHS of the join point
1427 newId SLIT("a") join_arg_ty ( \ arg_id ->
1428 cont_fn (Var arg_id) `thenSmpl` \ (binds, (_, rhs)) ->
1429 returnSmpl (Lam (setOneShotLambda arg_id) (mkLets binds rhs))
1430 ) `thenSmpl` \ join_rhs ->
1432 -- Build the join Id and continuation
1433 -- We give it a "$j" name just so that for later amusement
1434 -- we can identify any join points that don't end up as let-no-escapes
1435 -- [NOTE: the type used to be exprType join_rhs, but this seems more elegant.]
1436 newId SLIT("$j") (mkFunTy join_arg_ty cont_ty) $ \ join_id ->
1438 new_cont = ArgOf OkToDup cont_ty
1439 (\arg' -> rebuild_done (App (Var join_id) arg'))
1442 tick (CaseOfCase join_id) `thenSmpl_`
1443 -- Want to tick here so that we go round again,
1444 -- and maybe copy or inline the code;
1445 -- not strictly CaseOf Case
1446 addLetBind (NonRec join_id join_rhs) $
1447 thing_inside new_cont
1449 mkDupableCont ty (ApplyTo _ arg se cont) thing_inside
1450 = mkDupableCont (funResultTy ty) cont $ \ cont' ->
1451 setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1452 if exprIsDupable arg' then
1453 thing_inside (ApplyTo OkToDup arg' emptySubstEnv cont')
1455 newId SLIT("a") (exprType arg') $ \ bndr ->
1457 tick (CaseOfCase bndr) `thenSmpl_`
1458 -- Want to tick here so that we go round again,
1459 -- and maybe copy or inline the code;
1460 -- not strictly CaseOf Case
1462 addLetBind (NonRec bndr arg') $
1463 -- But what if the arg should be case-bound? We can't use
1464 -- addNonRecBind here because its type is too specific.
1465 -- This has been this way for a long time, so I'll leave it,
1466 -- but I can't convince myself that it's right.
1468 thing_inside (ApplyTo OkToDup (Var bndr) emptySubstEnv cont')
1471 mkDupableCont ty (Select _ case_bndr alts se cont) thing_inside
1472 = tick (CaseOfCase case_bndr) `thenSmpl_`
1474 simplBinder case_bndr $ \ case_bndr' ->
1475 prepareCaseCont alts cont $ \ cont' ->
1476 mapAndUnzipSmpl (mkDupableAlt case_bndr case_bndr' cont') alts `thenSmpl` \ (alt_binds_s, alts') ->
1477 returnSmpl (concat alt_binds_s, alts')
1478 ) `thenSmpl` \ (alt_binds, alts') ->
1480 addAuxiliaryBinds alt_binds $
1482 -- NB that the new alternatives, alts', are still InAlts, using the original
1483 -- binders. That means we can keep the case_bndr intact. This is important
1484 -- because another case-of-case might strike, and so we want to keep the
1485 -- info that the case_bndr is dead (if it is, which is often the case).
1486 -- This is VITAL when the type of case_bndr is an unboxed pair (often the
1487 -- case in I/O rich code. We aren't allowed a lambda bound
1488 -- arg of unboxed tuple type, and indeed such a case_bndr is always dead
1489 thing_inside (Select OkToDup case_bndr alts' se (mkStop (contResultType cont)))
1491 mkDupableAlt :: InId -> OutId -> SimplCont -> InAlt -> SimplM (OutStuff InAlt)
1492 mkDupableAlt case_bndr case_bndr' cont alt@(con, bndrs, rhs)
1493 = simplBinders bndrs $ \ bndrs' ->
1494 simplExprC rhs cont `thenSmpl` \ rhs' ->
1496 if (case cont of { Stop _ _ -> exprIsDupable rhs'; other -> False}) then
1497 -- It is worth checking for a small RHS because otherwise we
1498 -- get extra let bindings that may cause an extra iteration of the simplifier to
1499 -- inline back in place. Quite often the rhs is just a variable or constructor.
1500 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1501 -- iterations because the version with the let bindings looked big, and so wasn't
1502 -- inlined, but after the join points had been inlined it looked smaller, and so
1505 -- But since the continuation is absorbed into the rhs, we only do this
1506 -- for a Stop continuation.
1508 -- NB: we have to check the size of rhs', not rhs.
1509 -- Duplicating a small InAlt might invalidate occurrence information
1510 -- However, if it *is* dupable, we return the *un* simplified alternative,
1511 -- because otherwise we'd need to pair it up with an empty subst-env.
1512 -- (Remember we must zap the subst-env before re-simplifying something).
1513 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1514 returnSmpl ([], alt)
1518 rhs_ty' = exprType rhs'
1519 (used_bndrs, used_bndrs')
1520 = unzip [pr | pr@(bndr,bndr') <- zip (case_bndr : bndrs)
1521 (case_bndr' : bndrs'),
1522 not (isDeadBinder bndr)]
1523 -- The new binders have lost their occurrence info,
1524 -- so we have to extract it from the old ones
1526 ( if null used_bndrs'
1527 -- If we try to lift a primitive-typed something out
1528 -- for let-binding-purposes, we will *caseify* it (!),
1529 -- with potentially-disastrous strictness results. So
1530 -- instead we turn it into a function: \v -> e
1531 -- where v::State# RealWorld#. The value passed to this function
1532 -- is realworld#, which generates (almost) no code.
1534 -- There's a slight infelicity here: we pass the overall
1535 -- case_bndr to all the join points if it's used in *any* RHS,
1536 -- because we don't know its usage in each RHS separately
1538 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1539 -- we make the join point into a function whenever used_bndrs'
1540 -- is empty. This makes the join-point more CPR friendly.
1541 -- Consider: let j = if .. then I# 3 else I# 4
1542 -- in case .. of { A -> j; B -> j; C -> ... }
1544 -- Now CPR should not w/w j because it's a thunk, so
1545 -- that means that the enclosing function can't w/w either,
1546 -- which is a lose. Here's the example that happened in practice:
1547 -- kgmod :: Int -> Int -> Int
1548 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1552 then newId SLIT("w") realWorldStatePrimTy $ \ rw_id ->
1553 returnSmpl ([rw_id], [Var realWorldPrimId])
1555 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs)
1557 `thenSmpl` \ (final_bndrs', final_args) ->
1559 -- See comment about "$j" name above
1560 newId SLIT("$j") (foldr (mkFunTy . idType) rhs_ty' final_bndrs') $ \ join_bndr ->
1562 -- Notice that we make the lambdas into one-shot-lambdas. The
1563 -- join point is sure to be applied at most once, and doing so
1564 -- prevents the body of the join point being floated out by
1565 -- the full laziness pass
1566 returnSmpl ([NonRec join_bndr (mkLams (map setOneShotLambda final_bndrs') rhs')],
1567 (con, bndrs, mkApps (Var join_bndr) final_args))