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, simplRecIds, simplLetId, simplLamBinder,
19 SimplCont(..), DupFlag(..), mkStop, mkRhsStop,
20 contResultType, discardInline, countArgs, contIsDupable,
21 getContArgs, interestingCallContext, interestingArg, isStrictType
23 import Var ( mkSysTyVar, tyVarKind, mustHaveLocalBinding )
25 import Literal ( Literal )
26 import Id ( Id, idType, idInfo, isDataConId, hasNoBinding,
27 idUnfolding, setIdUnfolding, isExportedId, isDeadBinder,
28 idNewDemandInfo, setIdInfo,
29 idOccInfo, setIdOccInfo,
30 zapLamIdInfo, setOneShotLambda,
32 import IdInfo ( OccInfo(..), isDeadOcc, isLoopBreaker,
37 import NewDemand ( isStrictDmd )
38 import DataCon ( dataConNumInstArgs, dataConRepStrictness,
39 dataConSig, dataConArgTys
42 import PprCore ( pprParendExpr, pprCoreExpr )
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,
50 mkCoerce, mkSCC, mkInlineMe, mkAltExpr
52 import Rules ( lookupRule )
53 import BasicTypes ( isMarkedStrict )
54 import CostCentre ( currentCCS )
55 import Type ( mkTyVarTys, isUnLiftedType, seqType,
56 mkFunTy, splitTyConApp_maybe, tyConAppArgs,
57 funResultTy, splitFunTy_maybe, splitFunTy, eqType
59 import Subst ( mkSubst, substTy, substEnv, substExpr,
60 isInScope, lookupIdSubst, simplIdInfo
62 import TyCon ( isDataTyCon, tyConDataConsIfAvailable )
63 import TysPrim ( realWorldStatePrimTy )
64 import PrelInfo ( realWorldPrimId )
66 import Maybes ( maybeToBool )
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 simplRecIds (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 = simplVar v cont
192 simplExprF (Lit lit) cont = simplLit lit cont
193 simplExprF expr@(Lam _ _) cont = simplLam expr cont
194 simplExprF (Note note expr) cont = simplNote note expr cont
196 simplExprF (App fun arg) cont
197 = getSubstEnv `thenSmpl` \ se ->
198 simplExprF fun (ApplyTo NoDup arg se cont)
200 simplExprF (Type ty) cont
201 = ASSERT( case cont of { Stop _ _ -> True; ArgOf _ _ _ -> True; other -> False } )
202 simplType ty `thenSmpl` \ ty' ->
203 rebuild (Type ty') cont
205 simplExprF (Case scrut bndr alts) cont
206 = getSubstEnv `thenSmpl` \ subst_env ->
207 getSwitchChecker `thenSmpl` \ chkr ->
208 if not (switchIsOn chkr NoCaseOfCase) then
209 -- Simplify the scrutinee with a Select continuation
210 simplExprF scrut (Select NoDup bndr alts subst_env cont)
213 -- If case-of-case is off, simply simplify the case expression
214 -- in a vanilla Stop context, and rebuild the result around it
215 simplExprC scrut (Select NoDup bndr alts subst_env
216 (mkStop (contResultType cont))) `thenSmpl` \ case_expr' ->
217 rebuild case_expr' cont
219 simplExprF (Let (Rec pairs) body) cont
220 = simplRecIds (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 -- A non-recursive let is dealt with by simplNonRecBind
227 simplExprF (Let (NonRec bndr rhs) body) cont
228 = getSubstEnv `thenSmpl` \ se ->
229 simplNonRecBind bndr rhs se (contResultType cont) $
233 ---------------------------------
234 simplType :: InType -> SimplM OutType
236 = getSubst `thenSmpl` \ subst ->
238 new_ty = substTy subst ty
243 ---------------------------------
244 simplLit :: Literal -> SimplCont -> SimplM OutExprStuff
246 simplLit lit (Select _ bndr alts se cont)
247 = knownCon (Lit lit) (LitAlt lit) [] bndr alts se cont
249 simplLit lit cont = rebuild (Lit lit) cont
253 %************************************************************************
257 %************************************************************************
263 zap_it = mkLamBndrZapper fun cont
264 cont_ty = contResultType cont
266 -- Type-beta reduction
267 go (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
268 = ASSERT( isTyVar bndr )
269 tick (BetaReduction bndr) `thenSmpl_`
270 simplTyArg ty_arg arg_se `thenSmpl` \ ty_arg' ->
271 extendSubst bndr (DoneTy ty_arg')
274 -- Ordinary beta reduction
275 go (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
276 = tick (BetaReduction bndr) `thenSmpl_`
277 simplNonRecBind zapped_bndr arg arg_se cont_ty
280 zapped_bndr = zap_it bndr
283 go lam@(Lam _ _) cont = completeLam [] lam cont
285 -- Exactly enough args
286 go expr cont = simplExprF expr cont
288 -- completeLam deals with the case where a lambda doesn't have an ApplyTo
289 -- continuation, so there are real lambdas left to put in the result
291 -- We try for eta reduction here, but *only* if we get all the
292 -- way to an exprIsTrivial expression.
293 -- We don't want to remove extra lambdas unless we are going
294 -- to avoid allocating this thing altogether
296 completeLam rev_bndrs (Lam bndr body) cont
297 = simplLamBinder bndr $ \ bndr' ->
298 completeLam (bndr':rev_bndrs) body cont
300 completeLam rev_bndrs body cont
301 = simplExpr body `thenSmpl` \ body' ->
302 case try_eta body' of
303 Just etad_lam -> tick (EtaReduction (head rev_bndrs)) `thenSmpl_`
304 rebuild etad_lam cont
306 Nothing -> rebuild (foldl (flip Lam) body' rev_bndrs) cont
308 -- We don't use CoreUtils.etaReduce, because we can be more
310 -- (a) we already have the binders,
311 -- (b) we can do the triviality test before computing the free vars
312 -- [in fact I take the simple path and look for just a variable]
313 -- (c) we don't want to eta-reduce a data con worker or primop
314 -- because we only have to eta-expand them later when we saturate
315 try_eta body | not opt_SimplDoEtaReduction = Nothing
316 | otherwise = go rev_bndrs body
318 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
319 go [] body | ok_body body = Just body -- Success!
320 go _ _ = Nothing -- Failure!
322 ok_body (Var v) = not (v `elem` rev_bndrs) && not (hasNoBinding v)
323 ok_body other = False
324 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
326 mkLamBndrZapper :: CoreExpr -- Function
327 -> SimplCont -- The context
328 -> Id -> Id -- Use this to zap the binders
329 mkLamBndrZapper fun cont
330 | n_args >= n_params fun = \b -> b -- Enough args
331 | otherwise = \b -> zapLamIdInfo b
333 -- NB: we count all the args incl type args
334 -- so we must count all the binders (incl type lambdas)
335 n_args = countArgs cont
337 n_params (Note _ e) = n_params e
338 n_params (Lam b e) = 1 + n_params e
339 n_params other = 0::Int
343 %************************************************************************
347 %************************************************************************
350 simplNote (Coerce to from) body cont
351 = getInScope `thenSmpl` \ in_scope ->
353 addCoerce s1 k1 (CoerceIt t1 cont)
354 -- coerce T1 S1 (coerce S1 K1 e)
357 -- coerce T1 K1 e, otherwise
359 -- For example, in the initial form of a worker
360 -- we may find (coerce T (coerce S (\x.e))) y
361 -- and we'd like it to simplify to e[y/x] in one round
363 | t1 `eqType` k1 = cont -- The coerces cancel out
364 | otherwise = CoerceIt t1 cont -- They don't cancel, but
365 -- the inner one is redundant
367 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
368 | Just (s1, s2) <- splitFunTy_maybe s1s2
369 -- (coerce (T1->T2) (S1->S2) F) E
371 -- coerce T2 S2 (F (coerce S1 T1 E))
373 -- t1t2 must be a function type, T1->T2
374 -- but s1s2 might conceivably not be
376 -- When we build the ApplyTo we can't mix the out-types
377 -- with the InExpr in the argument, so we simply substitute
378 -- to make it all consistent. This isn't a common case.
380 (t1,t2) = splitFunTy t1t2
381 new_arg = mkCoerce s1 t1 (substExpr (mkSubst in_scope arg_se) arg)
383 ApplyTo dup new_arg emptySubstEnv (addCoerce t2 s2 cont)
385 addCoerce to' _ cont = CoerceIt to' cont
387 simplType to `thenSmpl` \ to' ->
388 simplType from `thenSmpl` \ from' ->
389 simplExprF body (addCoerce to' from' cont)
392 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
393 -- inlining. All other CCCSs are mapped to currentCCS.
394 simplNote (SCC cc) e cont
395 = setEnclosingCC currentCCS $
396 simplExpr e `thenSmpl` \ e ->
397 rebuild (mkSCC cc e) cont
399 simplNote InlineCall e cont
400 = simplExprF e (InlinePlease cont)
402 -- Comments about the InlineMe case
403 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
404 -- Don't inline in the RHS of something that has an
405 -- inline pragma. But be careful that the InScopeEnv that
406 -- we return does still have inlinings on!
408 -- It really is important to switch off inlinings. This function
409 -- may be inlinined in other modules, so we don't want to remove
410 -- (by inlining) calls to functions that have specialisations, or
411 -- that may have transformation rules in an importing scope.
412 -- E.g. {-# INLINE f #-}
414 -- and suppose that g is strict *and* has specialisations.
415 -- If we inline g's wrapper, we deny f the chance of getting
416 -- the specialised version of g when f is inlined at some call site
417 -- (perhaps in some other module).
419 -- It's also important not to inline a worker back into a wrapper.
420 -- A wrapper looks like
421 -- wraper = inline_me (\x -> ...worker... )
422 -- Normally, the inline_me prevents the worker getting inlined into
423 -- the wrapper (initially, the worker's only call site!). But,
424 -- if the wrapper is sure to be called, the strictness analyser will
425 -- mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
426 -- continuation. That's why the keep_inline predicate returns True for
427 -- ArgOf continuations. It shouldn't do any harm not to dissolve the
428 -- inline-me note under these circumstances
430 simplNote InlineMe e cont
431 | keep_inline cont -- Totally boring continuation
432 = -- Don't inline inside an INLINE expression
433 noInlineBlackList `thenSmpl` \ bl ->
434 setBlackList bl (simplExpr e) `thenSmpl` \ e' ->
435 rebuild (mkInlineMe e') cont
437 | otherwise -- Dissolve the InlineMe note if there's
438 -- an interesting context of any kind to combine with
439 -- (even a type application -- anything except Stop)
442 keep_inline (Stop _ _) = True -- See notes above
443 keep_inline (ArgOf _ _ _) = True -- about this predicate
444 keep_inline other = False
448 %************************************************************************
452 %************************************************************************
454 @simplNonRecBind@ is used for non-recursive lets in expressions,
455 as well as true beta reduction.
457 Very similar to @simplLazyBind@, but not quite the same.
460 simplNonRecBind :: InId -- Binder
461 -> InExpr -> SubstEnv -- Arg, with its subst-env
462 -> OutType -- Type of thing computed by the context
463 -> SimplM OutExprStuff -- The body
464 -> SimplM OutExprStuff
466 simplNonRecBind bndr rhs rhs_se cont_ty thing_inside
468 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
471 simplNonRecBind bndr rhs rhs_se cont_ty thing_inside
472 | preInlineUnconditionally False {- not black listed -} bndr
473 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
474 extendSubst bndr (ContEx rhs_se rhs) thing_inside
477 = -- Simplify the binder.
478 -- Don't use simplBinder because that doesn't keep
479 -- fragile occurrence in the substitution
480 simplLetId bndr $ \ bndr' ->
481 getSubst `thenSmpl` \ bndr_subst ->
483 -- Substitute its IdInfo (which simplLetId does not)
484 -- The appropriate substitution env is the one right here,
485 -- not rhs_se. Often they are the same, when all this
486 -- has arisen from an application (\x. E) RHS, perhaps they aren't
487 bndr'' = simplIdInfo bndr_subst (idInfo bndr) bndr'
488 bndr_ty' = idType bndr'
489 is_strict = isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty'
491 modifyInScope bndr'' bndr'' $
493 -- Simplify the argument
494 simplValArg bndr_ty' is_strict rhs rhs_se cont_ty $ \ rhs' ->
496 -- Now complete the binding and simplify the body
497 if needsCaseBinding bndr_ty' rhs' then
498 addCaseBind bndr'' rhs' thing_inside
500 completeBinding bndr bndr'' False False rhs' thing_inside
505 simplTyArg :: InType -> SubstEnv -> SimplM OutType
507 = getInScope `thenSmpl` \ in_scope ->
509 ty_arg' = substTy (mkSubst in_scope se) ty_arg
511 seqType ty_arg' `seq`
514 simplValArg :: OutType -- rhs_ty: Type of arg; used only occasionally
515 -> Bool -- True <=> evaluate eagerly
516 -> InExpr -> SubstEnv
517 -> OutType -- cont_ty: Type of thing computed by the context
518 -> (OutExpr -> SimplM OutExprStuff)
519 -- Takes an expression of type rhs_ty,
520 -- returns an expression of type cont_ty
521 -> SimplM OutExprStuff -- An expression of type cont_ty
523 simplValArg arg_ty is_strict arg arg_se cont_ty thing_inside
525 = getEnv `thenSmpl` \ env ->
527 simplExprF arg (ArgOf NoDup cont_ty $ \ rhs' ->
528 setAllExceptInScope env $
532 = simplRhs False {- Not top level -}
533 True {- OK to float unboxed -}
540 - deals only with Ids, not TyVars
541 - take an already-simplified RHS
543 It does *not* attempt to do let-to-case. Why? Because they are used for
546 (when let-to-case is impossible)
548 - many situations where the "rhs" is known to be a WHNF
549 (so let-to-case is inappropriate).
552 completeBinding :: InId -- Binder
553 -> OutId -- New binder
554 -> Bool -- True <=> top level
555 -> Bool -- True <=> black-listed; don't inline
556 -> OutExpr -- Simplified RHS
557 -> SimplM (OutStuff a) -- Thing inside
558 -> SimplM (OutStuff a)
560 completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
561 | isDeadOcc occ_info -- This happens; for example, the case_bndr during case of
562 -- known constructor: case (a,b) of x { (p,q) -> ... }
563 -- Here x isn't mentioned in the RHS, so we don't want to
564 -- create the (dead) let-binding let x = (a,b) in ...
567 | trivial_rhs && not must_keep_binding
568 -- We're looking at a binding with a trivial RHS, so
569 -- perhaps we can discard it altogether!
571 -- NB: a loop breaker has must_keep_binding = True
572 -- and non-loop-breakers only have *forward* references
573 -- Hence, it's safe to discard the binding
575 -- NOTE: This isn't our last opportunity to inline.
576 -- We're at the binding site right now, and
577 -- we'll get another opportunity when we get to the ocurrence(s)
579 -- Note that we do this unconditional inlining only for trival RHSs.
580 -- Don't inline even WHNFs inside lambdas; doing so may
581 -- simply increase allocation when the function is called
582 -- This isn't the last chance; see NOTE above.
584 -- NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here
585 -- Why? Because we don't even want to inline them into the
586 -- RHS of constructor arguments. See NOTE above
588 -- NB: Even NOINLINEis ignored here: if the rhs is trivial
589 -- it's best to inline it anyway. We often get a=E; b=a
590 -- from desugaring, with both a and b marked NOINLINE.
591 = -- Drop the binding
592 extendSubst old_bndr (DoneEx new_rhs) $
593 -- Use the substitution to make quite, quite sure that the substitution
594 -- will happen, since we are going to discard the binding
595 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
598 | Note coercion@(Coerce _ inner_ty) inner_rhs <- new_rhs,
599 not trivial_rhs && not (isUnLiftedType inner_ty)
600 -- x = coerce t e ==> c = e; x = inline_me (coerce t c)
601 -- Now x can get inlined, which moves the coercion
602 -- to the usage site. This is a bit like worker/wrapper stuff,
603 -- but it's useful to do it very promptly, so that
604 -- x = coerce T (I# 3)
608 -- This in turn means that
609 -- case (coerce Int x) of ...
611 -- Also the full-blown w/w thing isn't set up for non-functions
613 -- The (not (isUnLiftedType inner_ty)) avoids the nasty case of
614 -- x::Int = coerce Int Int# (foo y)
617 -- x::Int = coerce Int Int# v
618 -- which would be bogus because then v will be evaluated strictly.
619 -- How can this arise? Via
620 -- x::Int = case (foo y) of { ... }
621 -- followed by case elimination.
623 -- The inline_me note is so that the simplifier doesn't
624 -- just substitute c back inside x's rhs! (Typically, x will
625 -- get substituted away, but not if it's exported.)
626 = newId SLIT("c") inner_ty $ \ c_id ->
627 completeBinding c_id c_id top_lvl False inner_rhs $
628 completeBinding old_bndr new_bndr top_lvl black_listed
629 (Note InlineMe (Note coercion (Var c_id))) $
634 -- We make new IdInfo for the new binder by starting from the old binder,
635 -- doing appropriate substitutions.
636 -- Then we add arity and unfolding info to get the new binder
637 new_bndr_info = idInfo new_bndr `setArityInfo` arity
639 -- Add the unfolding *only* for non-loop-breakers
640 -- Making loop breakers not have an unfolding at all
641 -- means that we can avoid tests in exprIsConApp, for example.
642 -- This is important: if exprIsConApp says 'yes' for a recursive
643 -- thing, then we can get into an infinite loop
644 info_w_unf | loop_breaker = new_bndr_info
645 | otherwise = new_bndr_info `setUnfoldingInfo` mkUnfolding top_lvl new_rhs
647 final_id = new_bndr `setIdInfo` info_w_unf
649 -- These seqs forces the Id, and hence its IdInfo,
650 -- and hence any inner substitutions
652 addLetBind (NonRec final_id new_rhs) $
653 modifyInScope new_bndr final_id thing_inside
656 old_info = idInfo old_bndr
657 occ_info = occInfo old_info
658 loop_breaker = isLoopBreaker occ_info
659 trivial_rhs = exprIsTrivial new_rhs
660 must_keep_binding = black_listed || loop_breaker || isExportedId old_bndr
661 arity = exprArity new_rhs
666 %************************************************************************
668 \subsection{simplLazyBind}
670 %************************************************************************
672 simplLazyBind basically just simplifies the RHS of a let(rec).
673 It does two important optimisations though:
675 * It floats let(rec)s out of the RHS, even if they
676 are hidden by big lambdas
678 * It does eta expansion
681 simplLazyBind :: Bool -- True <=> top level
684 -> SimplM (OutStuff a) -- The body of the binding
685 -> SimplM (OutStuff a)
686 -- When called, the subst env is correct for the entire let-binding
687 -- and hence right for the RHS.
688 -- Also the binder has already been simplified, and hence is in scope
690 simplLazyBind top_lvl bndr bndr' rhs thing_inside
691 = getBlackList `thenSmpl` \ black_list_fn ->
693 black_listed = black_list_fn bndr
696 if preInlineUnconditionally black_listed bndr then
697 -- Inline unconditionally
698 tick (PreInlineUnconditionally bndr) `thenSmpl_`
699 getSubstEnv `thenSmpl` \ rhs_se ->
700 (extendSubst bndr (ContEx rhs_se rhs) thing_inside)
704 getSubst `thenSmpl` \ rhs_subst ->
706 -- Substitute IdInfo on binder, in the light of earlier
707 -- substitutions in this very letrec, and extend the in-scope
708 -- env so that it can see the new thing
709 bndr'' = simplIdInfo rhs_subst (idInfo bndr) bndr'
711 modifyInScope bndr'' bndr'' $
713 simplRhs top_lvl False {- Not ok to float unboxed (conservative) -}
715 rhs (substEnv rhs_subst) $ \ rhs' ->
717 -- Now compete the binding and simplify the body
718 completeBinding bndr bndr'' top_lvl black_listed rhs' thing_inside
724 simplRhs :: Bool -- True <=> Top level
725 -> Bool -- True <=> OK to float unboxed (speculative) bindings
726 -- False for (a) recursive and (b) top-level bindings
727 -> OutType -- Type of RHS; used only occasionally
728 -> InExpr -> SubstEnv
729 -> (OutExpr -> SimplM (OutStuff a))
730 -> SimplM (OutStuff a)
731 simplRhs top_lvl float_ubx rhs_ty rhs rhs_se thing_inside
733 setSubstEnv rhs_se (simplExprF rhs (mkRhsStop rhs_ty)) `thenSmpl` \ (floats1, (rhs_in_scope, rhs1)) ->
735 (floats2, rhs2) = splitFloats float_ubx floats1 rhs1
738 -- It's important that we do eta expansion on function *arguments* (which are
739 -- simplified with simplRhs), as well as let-bound right-hand sides.
740 -- Otherwise we find that things like
741 -- f (\x -> case x of I# x' -> coerce T (\ y -> ...))
742 -- get right through to the code generator as two separate lambdas,
743 -- which is a Bad Thing
744 tryRhsTyLam rhs2 `thenSmpl` \ (floats3, rhs3) ->
745 tryEtaExpansion rhs3 rhs_ty `thenSmpl` \ (floats4, rhs4) ->
747 -- Float lets if (a) we're at the top level
748 -- or (b) the resulting RHS is one we'd like to expose
750 -- NB: the test used to say "exprIsCheap", but that caused a strictness bug.
751 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
752 -- The case expression is 'cheap', but it's wrong to transform to
753 -- y* = E; x = case (scc y) of {...}
754 -- Either we must be careful not to float demanded non-values, or
755 -- we must use exprIsValue for the test, which ensures that the
756 -- thing is non-strict. I think. The WARN below tests for this
757 if (top_lvl || exprIsValue rhs4) then
759 -- There's a subtlety here. There may be a binding (x* = e) in the
760 -- floats, where the '*' means 'will be demanded'. So is it safe
761 -- to float it out? Answer no, but it won't matter because
762 -- we only float if arg' is a WHNF,
763 -- and so there can't be any 'will be demanded' bindings in the floats.
765 WARN( any demanded_float (fromOL floats2),
766 ppr (filter demanded_float (fromOL floats2)) )
768 (if (isNilOL floats2 && null floats3 && null floats4) then
771 tick LetFloatFromLet) `thenSmpl_`
773 addFloats floats2 rhs_in_scope $
774 addAuxiliaryBinds floats3 $
775 addAuxiliaryBinds floats4 $
778 -- Don't do the float
779 thing_inside (wrapFloats floats1 rhs1)
781 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
782 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
783 demanded_float (Rec _) = False
785 -- If float_ubx is true we float all the bindings, otherwise
786 -- we just float until we come across an unlifted one.
787 -- Remember that the unlifted bindings in the floats are all for
788 -- guaranteed-terminating non-exception-raising unlifted things,
789 -- which we are happy to do speculatively. However, we may still
790 -- not be able to float them out, because the context
791 -- is either a Rec group, or the top level, neither of which
792 -- can tolerate them.
793 splitFloats float_ubx floats rhs
794 | float_ubx = (floats, rhs) -- Float them all
795 | otherwise = go (fromOL floats)
798 go (f:fs) | must_stay f = (nilOL, mkLets (f:fs) rhs)
799 | otherwise = case go fs of
800 (out, rhs') -> (f `consOL` out, rhs')
802 must_stay (Rec prs) = False -- No unlifted bindings in here
803 must_stay (NonRec b r) = isUnLiftedType (idType b)
808 %************************************************************************
810 \subsection{Variables}
812 %************************************************************************
816 = getSubst `thenSmpl` \ subst ->
817 case lookupIdSubst subst var of
818 DoneEx e -> zapSubstEnv (simplExprF e cont)
819 ContEx env1 e -> setSubstEnv env1 (simplExprF e cont)
820 DoneId var1 occ -> WARN( not (isInScope var1 subst) && mustHaveLocalBinding var1,
821 text "simplVar:" <+> ppr var )
822 zapSubstEnv (completeCall var1 occ cont)
823 -- The template is already simplified, so don't re-substitute.
824 -- This is VITAL. Consider
826 -- let y = \z -> ...x... in
828 -- We'll clone the inner \x, adding x->x' in the id_subst
829 -- Then when we inline y, we must *not* replace x by x' in
830 -- the inlined copy!!
832 ---------------------------------------------------------
833 -- Dealing with a call
835 completeCall var occ_info cont
836 = getBlackList `thenSmpl` \ black_list_fn ->
837 getInScope `thenSmpl` \ in_scope ->
838 getContArgs var cont `thenSmpl` \ (args, call_cont, inline_call) ->
839 getDOptsSmpl `thenSmpl` \ dflags ->
841 black_listed = black_list_fn var
842 arg_infos = [ interestingArg in_scope arg subst
843 | (arg, subst, _) <- args, isValArg arg]
845 interesting_cont = interestingCallContext (not (null args))
846 (not (null arg_infos))
849 inline_cont | inline_call = discardInline cont
852 maybe_inline = callSiteInline dflags black_listed inline_call occ_info
853 var arg_infos interesting_cont
855 -- First, look for an inlining
856 case maybe_inline of {
857 Just unfolding -- There is an inlining!
858 -> tick (UnfoldingDone var) `thenSmpl_`
859 simplExprF unfolding inline_cont
862 Nothing -> -- No inlining!
865 simplifyArgs (isDataConId var) args (contResultType call_cont) $ \ args' ->
867 -- Next, look for rules or specialisations that match
869 -- It's important to simplify the args first, because the rule-matcher
870 -- doesn't do substitution as it goes. We don't want to use subst_args
871 -- (defined in the 'where') because that throws away useful occurrence info,
872 -- and perhaps-very-important specialisations.
874 -- Some functions have specialisations *and* are strict; in this case,
875 -- we don't want to inline the wrapper of the non-specialised thing; better
876 -- to call the specialised thing instead.
877 -- But the black-listing mechanism means that inlining of the wrapper
878 -- won't occur for things that have specialisations till a later phase, so
879 -- it's ok to try for inlining first.
881 -- You might think that we shouldn't apply rules for a loop breaker:
882 -- doing so might give rise to an infinite loop, because a RULE is
883 -- rather like an extra equation for the function:
884 -- RULE: f (g x) y = x+y
887 -- But it's too drastic to disable rules for loop breakers.
888 -- Even the foldr/build rule would be disabled, because foldr
889 -- is recursive, and hence a loop breaker:
890 -- foldr k z (build g) = g k z
891 -- So it's up to the programmer: rules can cause divergence
893 getSwitchChecker `thenSmpl` \ chkr ->
895 maybe_rule | switchIsOn chkr DontApplyRules = Nothing
896 | otherwise = lookupRule in_scope var args'
899 Just (rule_name, rule_rhs) ->
900 tick (RuleFired rule_name) `thenSmpl_`
902 (if dopt Opt_D_dump_inlinings dflags then
903 pprTrace "Rule fired" (vcat [
904 text "Rule:" <+> ptext rule_name,
905 text "Before:" <+> ppr var <+> sep (map pprParendExpr args'),
906 text "After: " <+> pprCoreExpr rule_rhs])
910 simplExprF rule_rhs call_cont ;
912 Nothing -> -- No rules
915 rebuild (mkApps (Var var) args') call_cont
919 ---------------------------------------------------------
920 -- Simplifying the arguments of a call
922 simplifyArgs :: Bool -- It's a data constructor
923 -> [(InExpr, SubstEnv, Bool)] -- Details of the arguments
924 -> OutType -- Type of the continuation
925 -> ([OutExpr] -> SimplM OutExprStuff)
926 -> SimplM OutExprStuff
928 -- Simplify the arguments to a call.
929 -- This part of the simplifier may break the no-shadowing invariant
931 -- f (...(\a -> e)...) (case y of (a,b) -> e')
932 -- where f is strict in its second arg
933 -- If we simplify the innermost one first we get (...(\a -> e)...)
934 -- Simplifying the second arg makes us float the case out, so we end up with
935 -- case y of (a,b) -> f (...(\a -> e)...) e'
936 -- So the output does not have the no-shadowing invariant. However, there is
937 -- no danger of getting name-capture, because when the first arg was simplified
938 -- we used an in-scope set that at least mentioned all the variables free in its
939 -- static environment, and that is enough.
941 -- We can't just do innermost first, or we'd end up with a dual problem:
942 -- case x of (a,b) -> f e (...(\a -> e')...)
944 -- I spent hours trying to recover the no-shadowing invariant, but I just could
945 -- not think of an elegant way to do it. The simplifier is already knee-deep in
946 -- continuations. We have to keep the right in-scope set around; AND we have
947 -- to get the effect that finding (error "foo") in a strict arg position will
948 -- discard the entire application and replace it with (error "foo"). Getting
949 -- all this at once is TOO HARD!
951 simplifyArgs is_data_con args cont_ty thing_inside
953 = go args thing_inside
955 | otherwise -- It's a data constructor, so we want
956 -- to switch off inlining in the arguments
957 -- If we don't do this, consider:
958 -- let x = +# p q in C {x}
959 -- Even though x get's an occurrence of 'many', its RHS looks cheap,
960 -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
961 = getBlackList `thenSmpl` \ old_bl ->
962 noInlineBlackList `thenSmpl` \ ni_bl ->
965 setBlackList old_bl $
969 go [] thing_inside = thing_inside []
970 go (arg:args) thing_inside = simplifyArg is_data_con arg cont_ty $ \ arg' ->
972 thing_inside (arg':args')
974 simplifyArg is_data_con (Type ty_arg, se, _) cont_ty thing_inside
975 = simplTyArg ty_arg se `thenSmpl` \ new_ty_arg ->
976 thing_inside (Type new_ty_arg)
978 simplifyArg is_data_con (val_arg, se, is_strict) cont_ty thing_inside
979 = getInScope `thenSmpl` \ in_scope ->
981 arg_ty = substTy (mkSubst in_scope se) (exprType val_arg)
983 if not is_data_con then
984 -- An ordinary function
985 simplValArg arg_ty is_strict val_arg se cont_ty thing_inside
987 -- A data constructor
988 -- simplifyArgs has already switched off inlining, so
989 -- all we have to do here is to let-bind any non-trivial argument
991 -- It's not always the case that new_arg will be trivial
993 -- where, in one pass, f gets substituted by a constructor,
994 -- but x gets substituted by an expression (assume this is the
995 -- unique occurrence of x). It doesn't really matter -- it'll get
996 -- fixed up next pass. And it happens for dictionary construction,
997 -- which mentions the wrapper constructor to start with.
998 simplValArg arg_ty is_strict val_arg se cont_ty $ \ arg' ->
1000 if exprIsTrivial arg' then
1003 newId SLIT("a") (exprType arg') $ \ arg_id ->
1004 addNonRecBind arg_id arg' $
1005 thing_inside (Var arg_id)
1009 %************************************************************************
1011 \subsection{Decisions about inlining}
1013 %************************************************************************
1015 NB: At one time I tried not pre/post-inlining top-level things,
1016 even if they occur exactly once. Reason:
1017 (a) some might appear as a function argument, so we simply
1018 replace static allocation with dynamic allocation:
1024 (b) some top level things might be black listed
1026 HOWEVER, I found that some useful foldr/build fusion was lost (most
1027 notably in spectral/hartel/parstof) because the foldr didn't see the build.
1029 Doing the dynamic allocation isn't a big deal, in fact, but losing the
1033 preInlineUnconditionally :: Bool {- Black listed -} -> InId -> Bool
1034 -- Examines a bndr to see if it is used just once in a
1035 -- completely safe way, so that it is safe to discard the binding
1036 -- inline its RHS at the (unique) usage site, REGARDLESS of how
1037 -- big the RHS might be. If this is the case we don't simplify
1038 -- the RHS first, but just inline it un-simplified.
1040 -- This is much better than first simplifying a perhaps-huge RHS
1041 -- and then inlining and re-simplifying it.
1043 -- NB: we don't even look at the RHS to see if it's trivial
1046 -- where x is used many times, but this is the unique occurrence
1047 -- of y. We should NOT inline x at all its uses, because then
1048 -- we'd do the same for y -- aargh! So we must base this
1049 -- pre-rhs-simplification decision solely on x's occurrences, not
1052 -- Evne RHSs labelled InlineMe aren't caught here, because
1053 -- there might be no benefit from inlining at the call site.
1055 preInlineUnconditionally black_listed bndr
1056 | black_listed || opt_SimplNoPreInlining = False
1057 | otherwise = case idOccInfo bndr of
1058 OneOcc in_lam once -> not in_lam && once
1059 -- Not inside a lambda, one occurrence ==> safe!
1065 %************************************************************************
1067 \subsection{The main rebuilder}
1069 %************************************************************************
1072 -------------------------------------------------------------------
1073 -- Finish rebuilding
1074 rebuild_done expr = returnOutStuff expr
1076 ---------------------------------------------------------
1077 rebuild :: OutExpr -> SimplCont -> SimplM OutExprStuff
1079 -- Stop continuation
1080 rebuild expr (Stop _ _) = rebuild_done expr
1082 -- ArgOf continuation
1083 rebuild expr (ArgOf _ _ cont_fn) = cont_fn expr
1085 -- ApplyTo continuation
1086 rebuild expr cont@(ApplyTo _ arg se cont')
1087 = setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1088 rebuild (App expr arg') cont'
1090 -- Coerce continuation
1091 rebuild expr (CoerceIt to_ty cont)
1092 = rebuild (mkCoerce to_ty (exprType expr) expr) cont
1094 -- Inline continuation
1095 rebuild expr (InlinePlease cont)
1096 = rebuild (Note InlineCall expr) cont
1098 rebuild scrut (Select _ bndr alts se cont)
1099 = rebuild_case scrut bndr alts se cont
1102 Case elimination [see the code above]
1104 Start with a simple situation:
1106 case x# of ===> e[x#/y#]
1109 (when x#, y# are of primitive type, of course). We can't (in general)
1110 do this for algebraic cases, because we might turn bottom into
1113 Actually, we generalise this idea to look for a case where we're
1114 scrutinising a variable, and we know that only the default case can
1119 other -> ...(case x of
1123 Here the inner case can be eliminated. This really only shows up in
1124 eliminating error-checking code.
1126 We also make sure that we deal with this very common case:
1131 Here we are using the case as a strict let; if x is used only once
1132 then we want to inline it. We have to be careful that this doesn't
1133 make the program terminate when it would have diverged before, so we
1135 - x is used strictly, or
1136 - e is already evaluated (it may so if e is a variable)
1138 Lastly, we generalise the transformation to handle this:
1144 We only do this for very cheaply compared r's (constructors, literals
1145 and variables). If pedantic bottoms is on, we only do it when the
1146 scrutinee is a PrimOp which can't fail.
1148 We do it *here*, looking at un-simplified alternatives, because we
1149 have to check that r doesn't mention the variables bound by the
1150 pattern in each alternative, so the binder-info is rather useful.
1152 So the case-elimination algorithm is:
1154 1. Eliminate alternatives which can't match
1156 2. Check whether all the remaining alternatives
1157 (a) do not mention in their rhs any of the variables bound in their pattern
1158 and (b) have equal rhss
1160 3. Check we can safely ditch the case:
1161 * PedanticBottoms is off,
1162 or * the scrutinee is an already-evaluated variable
1163 or * the scrutinee is a primop which is ok for speculation
1164 -- ie we want to preserve divide-by-zero errors, and
1165 -- calls to error itself!
1167 or * [Prim cases] the scrutinee is a primitive variable
1169 or * [Alg cases] the scrutinee is a variable and
1170 either * the rhs is the same variable
1171 (eg case x of C a b -> x ===> x)
1172 or * there is only one alternative, the default alternative,
1173 and the binder is used strictly in its scope.
1174 [NB this is helped by the "use default binder where
1175 possible" transformation; see below.]
1178 If so, then we can replace the case with one of the rhss.
1181 Blob of helper functions for the "case-of-something-else" situation.
1184 ---------------------------------------------------------
1185 -- Eliminate the case if possible
1187 rebuild_case scrut bndr alts se cont
1188 | maybeToBool maybe_con_app
1189 = knownCon scrut (DataAlt con) args bndr alts se cont
1191 | canEliminateCase scrut bndr alts
1192 = tick (CaseElim bndr) `thenSmpl_` (
1194 simplBinder bndr $ \ bndr' ->
1195 -- Remember to bind the case binder!
1196 completeBinding bndr bndr' False False scrut $
1197 simplExprF (head (rhssOfAlts alts)) cont)
1200 = complete_case scrut bndr alts se cont
1203 maybe_con_app = exprIsConApp_maybe scrut
1204 Just (con, args) = maybe_con_app
1206 -- See if we can get rid of the case altogether
1207 -- See the extensive notes on case-elimination above
1208 canEliminateCase scrut bndr alts
1209 = -- Check that the RHSs are all the same, and
1210 -- don't use the binders in the alternatives
1211 -- This test succeeds rapidly in the common case of
1212 -- a single DEFAULT alternative
1213 all (cheapEqExpr rhs1) other_rhss && all binders_unused alts
1215 -- Check that the scrutinee can be let-bound instead of case-bound
1216 && ( exprOkForSpeculation scrut
1217 -- OK not to evaluate it
1218 -- This includes things like (==# a# b#)::Bool
1219 -- so that we simplify
1220 -- case ==# a# b# of { True -> x; False -> x }
1223 -- This particular example shows up in default methods for
1224 -- comparision operations (e.g. in (>=) for Int.Int32)
1225 || exprIsValue scrut -- It's already evaluated
1226 || var_demanded_later scrut -- It'll be demanded later
1228 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1229 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1230 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1231 -- its argument: case x of { y -> dataToTag# y }
1232 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1233 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1238 (rhs1:other_rhss) = rhssOfAlts alts
1239 binders_unused (_, bndrs, _) = all isDeadBinder bndrs
1241 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo bndr) -- It's going to be evaluated later
1242 var_demanded_later other = False
1245 ---------------------------------------------------------
1246 -- Case of something else
1248 complete_case scrut case_bndr alts se cont
1249 = -- Prepare case alternatives
1250 prepareCaseAlts case_bndr (splitTyConApp_maybe (idType case_bndr))
1251 impossible_cons alts `thenSmpl` \ better_alts ->
1253 -- Set the new subst-env in place (before dealing with the case binder)
1256 -- Deal with the case binder, and prepare the continuation;
1257 -- The new subst_env is in place
1258 prepareCaseCont better_alts cont $ \ cont' ->
1261 -- Deal with variable scrutinee
1263 getSwitchChecker `thenSmpl` \ chkr ->
1264 simplCaseBinder (switchIsOn chkr NoCaseOfCase)
1265 scrut case_bndr $ \ case_bndr' zap_occ_info ->
1267 -- Deal with the case alternatives
1268 simplAlts zap_occ_info impossible_cons
1269 case_bndr' better_alts cont' `thenSmpl` \ alts' ->
1271 mkCase scrut case_bndr' alts'
1272 ) `thenSmpl` \ case_expr ->
1274 -- Notice that the simplBinder, prepareCaseCont, etc, do *not* scope
1275 -- over the rebuild_done; rebuild_done returns the in-scope set, and
1276 -- that should not include these chaps!
1277 rebuild_done case_expr
1279 impossible_cons = case scrut of
1280 Var v -> otherCons (idUnfolding v)
1284 knownCon :: OutExpr -> AltCon -> [OutExpr]
1285 -> InId -> [InAlt] -> SubstEnv -> SimplCont
1286 -> SimplM OutExprStuff
1288 knownCon expr con args bndr alts se cont
1289 = -- Arguments should be atomic;
1291 WARN( not (all exprIsTrivial args),
1292 text "knownCon" <+> ppr expr )
1293 tick (KnownBranch bndr) `thenSmpl_`
1295 simplBinder bndr $ \ bndr' ->
1296 completeBinding bndr bndr' False False expr $
1297 -- Don't use completeBeta here. The expr might be
1298 -- an unboxed literal, like 3, or a variable
1299 -- whose unfolding is an unboxed literal... and
1300 -- completeBeta will just construct another case
1302 case findAlt con alts of
1303 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1306 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1309 (DataAlt dc, bs, rhs) -> ASSERT( length bs == length real_args )
1310 extendSubstList bs (map mk real_args) $
1313 real_args = drop (dataConNumInstArgs dc) args
1314 mk (Type ty) = DoneTy ty
1315 mk other = DoneEx other
1320 prepareCaseCont :: [InAlt] -> SimplCont
1321 -> (SimplCont -> SimplM (OutStuff a))
1322 -> SimplM (OutStuff a)
1323 -- Polymorphic recursion here!
1325 prepareCaseCont [alt] cont thing_inside = thing_inside cont
1326 prepareCaseCont alts cont thing_inside = simplType (coreAltsType alts) `thenSmpl` \ alts_ty ->
1327 mkDupableCont alts_ty cont thing_inside
1328 -- At one time I passed in the un-simplified type, and simplified
1329 -- it only if we needed to construct a join binder, but that
1330 -- didn't work because we have to decompse function types
1331 -- (using funResultTy) in mkDupableCont.
1334 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1335 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1336 way, there's a chance that v will now only be used once, and hence
1339 There is a time we *don't* want to do that, namely when
1340 -fno-case-of-case is on. This happens in the first simplifier pass,
1341 and enhances full laziness. Here's the bad case:
1342 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1343 If we eliminate the inner case, we trap it inside the I# v -> arm,
1344 which might prevent some full laziness happening. I've seen this
1345 in action in spectral/cichelli/Prog.hs:
1346 [(m,n) | m <- [1..max], n <- [1..max]]
1347 Hence the no_case_of_case argument
1350 If we do this, then we have to nuke any occurrence info (eg IAmDead)
1351 in the case binder, because the case-binder now effectively occurs
1352 whenever v does. AND we have to do the same for the pattern-bound
1355 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1357 Here, b and p are dead. But when we move the argment inside the first
1358 case RHS, and eliminate the second case, we get
1360 case x or { (a,b) -> a b }
1362 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1363 happened. Hence the zap_occ_info function returned by simplCaseBinder
1366 simplCaseBinder no_case_of_case (Var v) case_bndr thing_inside
1367 | not no_case_of_case
1368 = simplBinder (zap case_bndr) $ \ case_bndr' ->
1369 modifyInScope v case_bndr' $
1370 -- We could extend the substitution instead, but it would be
1371 -- a hack because then the substitution wouldn't be idempotent
1372 -- any more (v is an OutId). And this just just as well.
1373 thing_inside case_bndr' zap
1375 zap b = b `setIdOccInfo` NoOccInfo
1377 simplCaseBinder add_eval_info other_scrut case_bndr thing_inside
1378 = simplBinder case_bndr $ \ case_bndr' ->
1379 thing_inside case_bndr' (\ bndr -> bndr) -- NoOp on bndr
1382 prepareCaseAlts does two things:
1384 1. Remove impossible alternatives
1386 2. If the DEFAULT alternative can match only one possible constructor,
1387 then make that constructor explicit.
1389 case e of x { DEFAULT -> rhs }
1391 case e of x { (a,b) -> rhs }
1392 where the type is a single constructor type. This gives better code
1393 when rhs also scrutinises x or e.
1396 prepareCaseAlts bndr (Just (tycon, inst_tys)) scrut_cons alts
1398 = case (findDefault filtered_alts, missing_cons) of
1400 ((alts_no_deflt, Just rhs), [data_con]) -- Just one missing constructor!
1401 -> tick (FillInCaseDefault bndr) `thenSmpl_`
1403 (_,_,ex_tyvars,_,_,_) = dataConSig data_con
1405 getUniquesSmpl `thenSmpl` \ tv_uniqs ->
1407 ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
1408 mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
1409 arg_tys = dataConArgTys data_con
1410 (inst_tys ++ mkTyVarTys ex_tyvars')
1412 newIds SLIT("a") arg_tys $ \ bndrs ->
1413 returnSmpl ((DataAlt data_con, ex_tyvars' ++ bndrs, rhs) : alts_no_deflt)
1415 other -> returnSmpl filtered_alts
1417 -- Filter out alternatives that can't possibly match
1418 filtered_alts = case scrut_cons of
1420 other -> [alt | alt@(con,_,_) <- alts, not (con `elem` scrut_cons)]
1422 missing_cons = [data_con | data_con <- tyConDataConsIfAvailable tycon,
1423 not (data_con `elem` handled_data_cons)]
1424 handled_data_cons = [data_con | DataAlt data_con <- scrut_cons] ++
1425 [data_con | (DataAlt data_con, _, _) <- filtered_alts]
1428 prepareCaseAlts _ _ scrut_cons alts
1429 = returnSmpl alts -- Functions
1432 ----------------------
1433 simplAlts zap_occ_info scrut_cons case_bndr' alts cont'
1434 = mapSmpl simpl_alt alts
1436 inst_tys' = tyConAppArgs (idType case_bndr')
1438 -- handled_cons is all the constructors that are dealt
1439 -- with, either by being impossible, or by there being an alternative
1440 (con_alts,_) = findDefault alts
1441 handled_cons = scrut_cons ++ [con | (con,_,_) <- con_alts]
1443 simpl_alt (DEFAULT, _, rhs)
1444 = -- In the default case we record the constructors that the
1445 -- case-binder *can't* be.
1446 -- We take advantage of any OtherCon info in the case scrutinee
1447 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` mkOtherCon handled_cons) $
1448 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1449 returnSmpl (DEFAULT, [], rhs')
1451 simpl_alt (con, vs, rhs)
1452 = -- Deal with the pattern-bound variables
1453 -- Mark the ones that are in ! positions in the data constructor
1454 -- as certainly-evaluated.
1455 -- NB: it happens that simplBinders does *not* erase the OtherCon
1456 -- form of unfolding, so it's ok to add this info before
1457 -- doing simplBinders
1458 simplBinders (add_evals con vs) $ \ vs' ->
1460 -- Bind the case-binder to (con args)
1462 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1464 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` unfolding) $
1465 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1466 returnSmpl (con, vs', rhs')
1469 -- add_evals records the evaluated-ness of the bound variables of
1470 -- a case pattern. This is *important*. Consider
1471 -- data T = T !Int !Int
1473 -- case x of { T a b -> T (a+1) b }
1475 -- We really must record that b is already evaluated so that we don't
1476 -- go and re-evaluate it when constructing the result.
1478 add_evals (DataAlt dc) vs = cat_evals vs (dataConRepStrictness dc)
1479 add_evals other_con vs = vs
1481 cat_evals [] [] = []
1482 cat_evals (v:vs) (str:strs)
1483 | isTyVar v = v : cat_evals vs (str:strs)
1484 | isMarkedStrict str = evald_v : cat_evals vs strs
1485 | otherwise = zapped_v : cat_evals vs strs
1487 zapped_v = zap_occ_info v
1488 evald_v = zapped_v `setIdUnfolding` mkOtherCon []
1492 %************************************************************************
1494 \subsection{Duplicating continuations}
1496 %************************************************************************
1499 mkDupableCont :: OutType -- Type of the thing to be given to the continuation
1501 -> (SimplCont -> SimplM (OutStuff a))
1502 -> SimplM (OutStuff a)
1503 mkDupableCont ty cont thing_inside
1504 | contIsDupable cont
1507 mkDupableCont _ (CoerceIt ty cont) thing_inside
1508 = mkDupableCont ty cont $ \ cont' ->
1509 thing_inside (CoerceIt ty cont')
1511 mkDupableCont ty (InlinePlease cont) thing_inside
1512 = mkDupableCont ty cont $ \ cont' ->
1513 thing_inside (InlinePlease cont')
1515 mkDupableCont join_arg_ty (ArgOf _ cont_ty cont_fn) thing_inside
1516 = -- Build the RHS of the join point
1517 newId SLIT("a") join_arg_ty ( \ arg_id ->
1518 cont_fn (Var arg_id) `thenSmpl` \ (floats, (_, rhs)) ->
1519 returnSmpl (Lam (setOneShotLambda arg_id) (wrapFloats floats rhs))
1520 ) `thenSmpl` \ join_rhs ->
1522 -- Build the join Id and continuation
1523 -- We give it a "$j" name just so that for later amusement
1524 -- we can identify any join points that don't end up as let-no-escapes
1525 -- [NOTE: the type used to be exprType join_rhs, but this seems more elegant.]
1526 newId SLIT("$j") (mkFunTy join_arg_ty cont_ty) $ \ join_id ->
1528 new_cont = ArgOf OkToDup cont_ty
1529 (\arg' -> rebuild_done (App (Var join_id) arg'))
1532 tick (CaseOfCase join_id) `thenSmpl_`
1533 -- Want to tick here so that we go round again,
1534 -- and maybe copy or inline the code;
1535 -- not strictly CaseOf Case
1536 addLetBind (NonRec join_id join_rhs) $
1537 thing_inside new_cont
1539 mkDupableCont ty (ApplyTo _ arg se cont) thing_inside
1540 = mkDupableCont (funResultTy ty) cont $ \ cont' ->
1541 setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1542 if exprIsDupable arg' then
1543 thing_inside (ApplyTo OkToDup arg' emptySubstEnv cont')
1545 newId SLIT("a") (exprType arg') $ \ bndr ->
1547 tick (CaseOfCase bndr) `thenSmpl_`
1548 -- Want to tick here so that we go round again,
1549 -- and maybe copy or inline the code;
1550 -- not strictly CaseOf Case
1552 addLetBind (NonRec bndr arg') $
1553 -- But what if the arg should be case-bound? We can't use
1554 -- addNonRecBind here because its type is too specific.
1555 -- This has been this way for a long time, so I'll leave it,
1556 -- but I can't convince myself that it's right.
1558 thing_inside (ApplyTo OkToDup (Var bndr) emptySubstEnv cont')
1561 mkDupableCont ty (Select _ case_bndr alts se cont) thing_inside
1562 = tick (CaseOfCase case_bndr) `thenSmpl_`
1564 simplBinder case_bndr $ \ case_bndr' ->
1565 prepareCaseCont alts cont $ \ cont' ->
1566 mkDupableAlts case_bndr case_bndr' cont' alts $ \ alts' ->
1567 returnOutStuff alts'
1568 ) `thenSmpl` \ (alt_binds, (in_scope, alts')) ->
1570 addFloats alt_binds in_scope $
1572 -- NB that the new alternatives, alts', are still InAlts, using the original
1573 -- binders. That means we can keep the case_bndr intact. This is important
1574 -- because another case-of-case might strike, and so we want to keep the
1575 -- info that the case_bndr is dead (if it is, which is often the case).
1576 -- This is VITAL when the type of case_bndr is an unboxed pair (often the
1577 -- case in I/O rich code. We aren't allowed a lambda bound
1578 -- arg of unboxed tuple type, and indeed such a case_bndr is always dead
1579 thing_inside (Select OkToDup case_bndr alts' se (mkStop (contResultType cont)))
1581 mkDupableAlts :: InId -> OutId -> SimplCont -> [InAlt]
1582 -> ([InAlt] -> SimplM (OutStuff a))
1583 -> SimplM (OutStuff a)
1584 mkDupableAlts case_bndr case_bndr' cont [] thing_inside
1586 mkDupableAlts case_bndr case_bndr' cont (alt:alts) thing_inside
1587 = mkDupableAlt case_bndr case_bndr' cont alt $ \ alt' ->
1588 mkDupableAlts case_bndr case_bndr' cont alts $ \ alts' ->
1589 thing_inside (alt' : alts')
1591 mkDupableAlt case_bndr case_bndr' cont alt@(con, bndrs, rhs) thing_inside
1592 = simplBinders bndrs $ \ bndrs' ->
1593 simplExprC rhs cont `thenSmpl` \ rhs' ->
1595 if (case cont of { Stop _ _ -> exprIsDupable rhs'; other -> False}) then
1596 -- It is worth checking for a small RHS because otherwise we
1597 -- get extra let bindings that may cause an extra iteration of the simplifier to
1598 -- inline back in place. Quite often the rhs is just a variable or constructor.
1599 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1600 -- iterations because the version with the let bindings looked big, and so wasn't
1601 -- inlined, but after the join points had been inlined it looked smaller, and so
1604 -- But since the continuation is absorbed into the rhs, we only do this
1605 -- for a Stop continuation.
1607 -- NB: we have to check the size of rhs', not rhs.
1608 -- Duplicating a small InAlt might invalidate occurrence information
1609 -- However, if it *is* dupable, we return the *un* simplified alternative,
1610 -- because otherwise we'd need to pair it up with an empty subst-env.
1611 -- (Remember we must zap the subst-env before re-simplifying something).
1612 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1617 rhs_ty' = exprType rhs'
1618 (used_bndrs, used_bndrs')
1619 = unzip [pr | pr@(bndr,bndr') <- zip (case_bndr : bndrs)
1620 (case_bndr' : bndrs'),
1621 not (isDeadBinder bndr)]
1622 -- The new binders have lost their occurrence info,
1623 -- so we have to extract it from the old ones
1625 ( if null used_bndrs'
1626 -- If we try to lift a primitive-typed something out
1627 -- for let-binding-purposes, we will *caseify* it (!),
1628 -- with potentially-disastrous strictness results. So
1629 -- instead we turn it into a function: \v -> e
1630 -- where v::State# RealWorld#. The value passed to this function
1631 -- is realworld#, which generates (almost) no code.
1633 -- There's a slight infelicity here: we pass the overall
1634 -- case_bndr to all the join points if it's used in *any* RHS,
1635 -- because we don't know its usage in each RHS separately
1637 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1638 -- we make the join point into a function whenever used_bndrs'
1639 -- is empty. This makes the join-point more CPR friendly.
1640 -- Consider: let j = if .. then I# 3 else I# 4
1641 -- in case .. of { A -> j; B -> j; C -> ... }
1643 -- Now CPR doesn't w/w j because it's a thunk, so
1644 -- that means that the enclosing function can't w/w either,
1645 -- which is a lose. Here's the example that happened in practice:
1646 -- kgmod :: Int -> Int -> Int
1647 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1651 -- I have seen a case alternative like this:
1652 -- True -> \v -> ...
1653 -- It's a bit silly to add the realWorld dummy arg in this case, making
1656 -- (the \v alone is enough to make CPR happy) but I think it's rare
1658 then newId SLIT("w") realWorldStatePrimTy $ \ rw_id ->
1659 returnSmpl ([rw_id], [Var realWorldPrimId])
1661 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs)
1663 `thenSmpl` \ (final_bndrs', final_args) ->
1665 -- See comment about "$j" name above
1666 newId SLIT("$j") (foldr mkPiType rhs_ty' final_bndrs') $ \ join_bndr ->
1667 -- Notice the funky mkPiType. If the contructor has existentials
1668 -- it's possible that the join point will be abstracted over
1669 -- type varaibles as well as term variables.
1670 -- Example: Suppose we have
1671 -- data T = forall t. C [t]
1673 -- case (case e of ...) of
1674 -- C t xs::[t] -> rhs
1675 -- We get the join point
1676 -- let j :: forall t. [t] -> ...
1677 -- j = /\t \xs::[t] -> rhs
1679 -- case (case e of ...) of
1680 -- C t xs::[t] -> j t xs
1683 -- We make the lambdas into one-shot-lambdas. The
1684 -- join point is sure to be applied at most once, and doing so
1685 -- prevents the body of the join point being floated out by
1686 -- the full laziness pass
1687 really_final_bndrs = map one_shot final_bndrs'
1688 one_shot v | isId v = setOneShotLambda v
1691 addLetBind (NonRec join_bndr (mkLams really_final_bndrs rhs')) $
1692 thing_inside (con, bndrs, mkApps (Var join_bndr) final_args)