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 CostCentre ( currentCCS )
54 import Type ( mkTyVarTys, isUnLiftedType, seqType,
55 mkFunTy, splitTyConApp_maybe, tyConAppArgs,
56 funResultTy, splitFunTy_maybe, splitFunTy, eqType
58 import Subst ( mkSubst, substTy, substEnv, substExpr,
59 isInScope, lookupIdSubst, simplIdInfo
61 import TyCon ( isDataTyCon, tyConDataConsIfAvailable )
62 import TysPrim ( realWorldStatePrimTy )
63 import PrelInfo ( realWorldPrimId )
65 import Maybes ( maybeToBool )
70 The guts of the simplifier is in this module, but the driver
71 loop for the simplifier is in SimplCore.lhs.
74 -----------------------------------------
75 *** IMPORTANT NOTE ***
76 -----------------------------------------
77 The simplifier used to guarantee that the output had no shadowing, but
78 it does not do so any more. (Actually, it never did!) The reason is
79 documented with simplifyArgs.
84 %************************************************************************
88 %************************************************************************
91 simplTopBinds :: [InBind] -> SimplM [OutBind]
94 = -- Put all the top-level binders into scope at the start
95 -- so that if a transformation rule has unexpectedly brought
96 -- anything into scope, then we don't get a complaint about that.
97 -- It's rather as if the top-level binders were imported.
98 simplRecIds (bindersOfBinds binds) $ \ bndrs' ->
99 simpl_binds binds bndrs' `thenSmpl` \ (binds', _) ->
100 freeTick SimplifierDone `thenSmpl_`
101 returnSmpl (fromOL binds')
104 -- We need to track the zapped top-level binders, because
105 -- they should have their fragile IdInfo zapped (notably occurrence info)
106 simpl_binds [] bs = ASSERT( null bs ) returnSmpl (nilOL, panic "simplTopBinds corner")
107 simpl_binds (NonRec bndr rhs : binds) (b:bs) = simplLazyBind True bndr b rhs (simpl_binds binds bs)
108 simpl_binds (Rec pairs : binds) bs = simplRecBind True pairs (take n bs) (simpl_binds binds (drop n bs))
112 simplRecBind :: Bool -> [(InId, InExpr)] -> [OutId]
113 -> SimplM (OutStuff a) -> SimplM (OutStuff a)
114 simplRecBind top_lvl pairs bndrs' thing_inside
115 = go pairs bndrs' `thenSmpl` \ (binds', (_, (binds'', res))) ->
116 returnSmpl (unitOL (Rec (flattenBinds (fromOL binds'))) `appOL` binds'', res)
118 go [] _ = thing_inside `thenSmpl` \ stuff ->
121 go ((bndr, rhs) : pairs) (bndr' : bndrs')
122 = simplLazyBind top_lvl bndr bndr' rhs (go pairs bndrs')
123 -- Don't float unboxed bindings out,
124 -- because we can't "rec" them
128 %************************************************************************
130 \subsection[Simplify-simplExpr]{The main function: simplExpr}
132 %************************************************************************
134 The reason for this OutExprStuff stuff is that we want to float *after*
135 simplifying a RHS, not before. If we do so naively we get quadratic
136 behaviour as things float out.
138 To see why it's important to do it after, consider this (real) example:
152 a -- Can't inline a this round, cos it appears twice
156 Each of the ==> steps is a round of simplification. We'd save a
157 whole round if we float first. This can cascade. Consider
162 let f = let d1 = ..d.. in \y -> e
166 in \x -> ...(\y ->e)...
168 Only in this second round can the \y be applied, and it
169 might do the same again.
173 simplExpr :: CoreExpr -> SimplM CoreExpr
174 simplExpr expr = getSubst `thenSmpl` \ subst ->
175 simplExprC expr (mkStop (substTy subst (exprType expr)))
176 -- The type in the Stop continuation is usually not used
177 -- It's only needed when discarding continuations after finding
178 -- a function that returns bottom.
179 -- Hence the lazy substitution
181 simplExprC :: CoreExpr -> SimplCont -> SimplM CoreExpr
182 -- Simplify an expression, given a continuation
184 simplExprC expr cont = simplExprF expr cont `thenSmpl` \ (floats, (_, body)) ->
185 returnSmpl (wrapFloats floats body)
187 simplExprF :: InExpr -> SimplCont -> SimplM OutExprStuff
188 -- Simplify an expression, returning floated binds
190 simplExprF (Var v) cont = simplVar v cont
191 simplExprF (Lit lit) cont = simplLit lit cont
192 simplExprF expr@(Lam _ _) cont = simplLam expr cont
193 simplExprF (Note note expr) cont = simplNote note expr cont
195 simplExprF (App fun arg) cont
196 = getSubstEnv `thenSmpl` \ se ->
197 simplExprF fun (ApplyTo NoDup arg se cont)
199 simplExprF (Type ty) cont
200 = ASSERT( case cont of { Stop _ _ -> True; ArgOf _ _ _ -> True; other -> False } )
201 simplType ty `thenSmpl` \ ty' ->
202 rebuild (Type ty') cont
204 simplExprF (Case scrut bndr alts) cont
205 = getSubstEnv `thenSmpl` \ subst_env ->
206 getSwitchChecker `thenSmpl` \ chkr ->
207 if not (switchIsOn chkr NoCaseOfCase) then
208 -- Simplify the scrutinee with a Select continuation
209 simplExprF scrut (Select NoDup bndr alts subst_env cont)
212 -- If case-of-case is off, simply simplify the case expression
213 -- in a vanilla Stop context, and rebuild the result around it
214 simplExprC scrut (Select NoDup bndr alts subst_env
215 (mkStop (contResultType cont))) `thenSmpl` \ case_expr' ->
216 rebuild case_expr' cont
218 simplExprF (Let (Rec pairs) body) cont
219 = simplRecIds (map fst pairs) $ \ bndrs' ->
220 -- NB: bndrs' don't have unfoldings or spec-envs
221 -- We add them as we go down, using simplPrags
223 simplRecBind False pairs bndrs' (simplExprF body cont)
225 -- A non-recursive let is dealt with by simplNonRecBind
226 simplExprF (Let (NonRec bndr rhs) body) cont
227 = getSubstEnv `thenSmpl` \ se ->
228 simplNonRecBind bndr rhs se (contResultType cont) $
232 ---------------------------------
233 simplType :: InType -> SimplM OutType
235 = getSubst `thenSmpl` \ subst ->
237 new_ty = substTy subst ty
242 ---------------------------------
243 simplLit :: Literal -> SimplCont -> SimplM OutExprStuff
245 simplLit lit (Select _ bndr alts se cont)
246 = knownCon (Lit lit) (LitAlt lit) [] bndr alts se cont
248 simplLit lit cont = rebuild (Lit lit) cont
252 %************************************************************************
256 %************************************************************************
262 zap_it = mkLamBndrZapper fun cont
263 cont_ty = contResultType cont
265 -- Type-beta reduction
266 go (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
267 = ASSERT( isTyVar bndr )
268 tick (BetaReduction bndr) `thenSmpl_`
269 simplTyArg ty_arg arg_se `thenSmpl` \ ty_arg' ->
270 extendSubst bndr (DoneTy ty_arg')
273 -- Ordinary beta reduction
274 go (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
275 = tick (BetaReduction bndr) `thenSmpl_`
276 simplNonRecBind zapped_bndr arg arg_se cont_ty
279 zapped_bndr = zap_it bndr
282 go lam@(Lam _ _) cont = completeLam [] lam cont
284 -- Exactly enough args
285 go expr cont = simplExprF expr cont
287 -- completeLam deals with the case where a lambda doesn't have an ApplyTo
288 -- continuation, so there are real lambdas left to put in the result
290 -- We try for eta reduction here, but *only* if we get all the
291 -- way to an exprIsTrivial expression.
292 -- We don't want to remove extra lambdas unless we are going
293 -- to avoid allocating this thing altogether
295 completeLam rev_bndrs (Lam bndr body) cont
296 = simplLamBinder bndr $ \ bndr' ->
297 completeLam (bndr':rev_bndrs) body cont
299 completeLam rev_bndrs body cont
300 = simplExpr body `thenSmpl` \ body' ->
301 case try_eta body' of
302 Just etad_lam -> tick (EtaReduction (head rev_bndrs)) `thenSmpl_`
303 rebuild etad_lam cont
305 Nothing -> rebuild (foldl (flip Lam) body' rev_bndrs) cont
307 -- We don't use CoreUtils.etaReduce, because we can be more
309 -- (a) we already have the binders,
310 -- (b) we can do the triviality test before computing the free vars
311 -- [in fact I take the simple path and look for just a variable]
312 -- (c) we don't want to eta-reduce a data con worker or primop
313 -- because we only have to eta-expand them later when we saturate
314 try_eta body | not opt_SimplDoEtaReduction = Nothing
315 | otherwise = go rev_bndrs body
317 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
318 go [] body | ok_body body = Just body -- Success!
319 go _ _ = Nothing -- Failure!
321 ok_body (Var v) = not (v `elem` rev_bndrs) && not (hasNoBinding v)
322 ok_body other = False
323 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
325 mkLamBndrZapper :: CoreExpr -- Function
326 -> SimplCont -- The context
327 -> Id -> Id -- Use this to zap the binders
328 mkLamBndrZapper fun cont
329 | n_args >= n_params fun = \b -> b -- Enough args
330 | otherwise = \b -> zapLamIdInfo b
332 -- NB: we count all the args incl type args
333 -- so we must count all the binders (incl type lambdas)
334 n_args = countArgs cont
336 n_params (Note _ e) = n_params e
337 n_params (Lam b e) = 1 + n_params e
338 n_params other = 0::Int
342 %************************************************************************
346 %************************************************************************
349 simplNote (Coerce to from) body cont
350 = getInScope `thenSmpl` \ in_scope ->
352 addCoerce s1 k1 (CoerceIt t1 cont)
353 -- coerce T1 S1 (coerce S1 K1 e)
356 -- coerce T1 K1 e, otherwise
358 -- For example, in the initial form of a worker
359 -- we may find (coerce T (coerce S (\x.e))) y
360 -- and we'd like it to simplify to e[y/x] in one round
362 | t1 `eqType` k1 = cont -- The coerces cancel out
363 | otherwise = CoerceIt t1 cont -- They don't cancel, but
364 -- the inner one is redundant
366 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
367 | Just (s1, s2) <- splitFunTy_maybe s1s2
368 -- (coerce (T1->T2) (S1->S2) F) E
370 -- coerce T2 S2 (F (coerce S1 T1 E))
372 -- t1t2 must be a function type, T1->T2
373 -- but s1s2 might conceivably not be
375 -- When we build the ApplyTo we can't mix the out-types
376 -- with the InExpr in the argument, so we simply substitute
377 -- to make it all consistent. This isn't a common case.
379 (t1,t2) = splitFunTy t1t2
380 new_arg = mkCoerce s1 t1 (substExpr (mkSubst in_scope arg_se) arg)
382 ApplyTo dup new_arg emptySubstEnv (addCoerce t2 s2 cont)
384 addCoerce to' _ cont = CoerceIt to' cont
386 simplType to `thenSmpl` \ to' ->
387 simplType from `thenSmpl` \ from' ->
388 simplExprF body (addCoerce to' from' cont)
391 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
392 -- inlining. All other CCCSs are mapped to currentCCS.
393 simplNote (SCC cc) e cont
394 = setEnclosingCC currentCCS $
395 simplExpr e `thenSmpl` \ e ->
396 rebuild (mkSCC cc e) cont
398 simplNote InlineCall e cont
399 = simplExprF e (InlinePlease cont)
401 -- Comments about the InlineMe case
402 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
403 -- Don't inline in the RHS of something that has an
404 -- inline pragma. But be careful that the InScopeEnv that
405 -- we return does still have inlinings on!
407 -- It really is important to switch off inlinings. This function
408 -- may be inlinined in other modules, so we don't want to remove
409 -- (by inlining) calls to functions that have specialisations, or
410 -- that may have transformation rules in an importing scope.
411 -- E.g. {-# INLINE f #-}
413 -- and suppose that g is strict *and* has specialisations.
414 -- If we inline g's wrapper, we deny f the chance of getting
415 -- the specialised version of g when f is inlined at some call site
416 -- (perhaps in some other module).
418 -- It's also important not to inline a worker back into a wrapper.
419 -- A wrapper looks like
420 -- wraper = inline_me (\x -> ...worker... )
421 -- Normally, the inline_me prevents the worker getting inlined into
422 -- the wrapper (initially, the worker's only call site!). But,
423 -- if the wrapper is sure to be called, the strictness analyser will
424 -- mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
425 -- continuation. That's why the keep_inline predicate returns True for
426 -- ArgOf continuations. It shouldn't do any harm not to dissolve the
427 -- inline-me note under these circumstances
429 simplNote InlineMe e cont
430 | keep_inline cont -- Totally boring continuation
431 = -- Don't inline inside an INLINE expression
432 noInlineBlackList `thenSmpl` \ bl ->
433 setBlackList bl (simplExpr e) `thenSmpl` \ e' ->
434 rebuild (mkInlineMe e') cont
436 | otherwise -- Dissolve the InlineMe note if there's
437 -- an interesting context of any kind to combine with
438 -- (even a type application -- anything except Stop)
441 keep_inline (Stop _ _) = True -- See notes above
442 keep_inline (ArgOf _ _ _) = True -- about this predicate
443 keep_inline other = False
447 %************************************************************************
451 %************************************************************************
453 @simplNonRecBind@ is used for non-recursive lets in expressions,
454 as well as true beta reduction.
456 Very similar to @simplLazyBind@, but not quite the same.
459 simplNonRecBind :: InId -- Binder
460 -> InExpr -> SubstEnv -- Arg, with its subst-env
461 -> OutType -- Type of thing computed by the context
462 -> SimplM OutExprStuff -- The body
463 -> SimplM OutExprStuff
465 simplNonRecBind bndr rhs rhs_se cont_ty thing_inside
467 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
470 simplNonRecBind bndr rhs rhs_se cont_ty thing_inside
471 | preInlineUnconditionally False {- not black listed -} bndr
472 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
473 extendSubst bndr (ContEx rhs_se rhs) thing_inside
476 = -- Simplify the binder.
477 -- Don't use simplBinder because that doesn't keep
478 -- fragile occurrence in the substitution
479 simplLetId bndr $ \ bndr' ->
480 getSubst `thenSmpl` \ bndr_subst ->
482 -- Substitute its IdInfo (which simplLetId does not)
483 -- The appropriate substitution env is the one right here,
484 -- not rhs_se. Often they are the same, when all this
485 -- has arisen from an application (\x. E) RHS, perhaps they aren't
486 bndr'' = simplIdInfo bndr_subst (idInfo bndr) bndr'
487 bndr_ty' = idType bndr'
488 is_strict = isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty'
490 modifyInScope bndr'' bndr'' $
492 -- Simplify the argument
493 simplValArg bndr_ty' is_strict rhs rhs_se cont_ty $ \ rhs' ->
495 -- Now complete the binding and simplify the body
496 if needsCaseBinding bndr_ty' rhs' then
497 addCaseBind bndr'' rhs' thing_inside
499 completeBinding bndr bndr'' False False rhs' thing_inside
504 simplTyArg :: InType -> SubstEnv -> SimplM OutType
506 = getInScope `thenSmpl` \ in_scope ->
508 ty_arg' = substTy (mkSubst in_scope se) ty_arg
510 seqType ty_arg' `seq`
513 simplValArg :: OutType -- rhs_ty: Type of arg; used only occasionally
514 -> Bool -- True <=> evaluate eagerly
515 -> InExpr -> SubstEnv
516 -> OutType -- cont_ty: Type of thing computed by the context
517 -> (OutExpr -> SimplM OutExprStuff)
518 -- Takes an expression of type rhs_ty,
519 -- returns an expression of type cont_ty
520 -> SimplM OutExprStuff -- An expression of type cont_ty
522 simplValArg arg_ty is_strict arg arg_se cont_ty thing_inside
524 = getEnv `thenSmpl` \ env ->
526 simplExprF arg (ArgOf NoDup cont_ty $ \ rhs' ->
527 setAllExceptInScope env $
531 = simplRhs False {- Not top level -}
532 True {- OK to float unboxed -}
539 - deals only with Ids, not TyVars
540 - take an already-simplified RHS
542 It does *not* attempt to do let-to-case. Why? Because they are used for
545 (when let-to-case is impossible)
547 - many situations where the "rhs" is known to be a WHNF
548 (so let-to-case is inappropriate).
551 completeBinding :: InId -- Binder
552 -> OutId -- New binder
553 -> Bool -- True <=> top level
554 -> Bool -- True <=> black-listed; don't inline
555 -> OutExpr -- Simplified RHS
556 -> SimplM (OutStuff a) -- Thing inside
557 -> SimplM (OutStuff a)
559 completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
560 | isDeadOcc occ_info -- This happens; for example, the case_bndr during case of
561 -- known constructor: case (a,b) of x { (p,q) -> ... }
562 -- Here x isn't mentioned in the RHS, so we don't want to
563 -- create the (dead) let-binding let x = (a,b) in ...
566 | trivial_rhs && not must_keep_binding
567 -- We're looking at a binding with a trivial RHS, so
568 -- perhaps we can discard it altogether!
570 -- NB: a loop breaker has must_keep_binding = True
571 -- and non-loop-breakers only have *forward* references
572 -- Hence, it's safe to discard the binding
574 -- NOTE: This isn't our last opportunity to inline.
575 -- We're at the binding site right now, and
576 -- we'll get another opportunity when we get to the ocurrence(s)
578 -- Note that we do this unconditional inlining only for trival RHSs.
579 -- Don't inline even WHNFs inside lambdas; doing so may
580 -- simply increase allocation when the function is called
581 -- This isn't the last chance; see NOTE above.
583 -- NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here
584 -- Why? Because we don't even want to inline them into the
585 -- RHS of constructor arguments. See NOTE above
587 -- NB: Even NOINLINEis ignored here: if the rhs is trivial
588 -- it's best to inline it anyway. We often get a=E; b=a
589 -- from desugaring, with both a and b marked NOINLINE.
590 = -- Drop the binding
591 extendSubst old_bndr (DoneEx new_rhs) $
592 -- Use the substitution to make quite, quite sure that the substitution
593 -- will happen, since we are going to discard the binding
594 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
597 | Note coercion@(Coerce _ inner_ty) inner_rhs <- new_rhs,
598 not trivial_rhs && not (isUnLiftedType inner_ty)
599 -- x = coerce t e ==> c = e; x = inline_me (coerce t c)
600 -- Now x can get inlined, which moves the coercion
601 -- to the usage site. This is a bit like worker/wrapper stuff,
602 -- but it's useful to do it very promptly, so that
603 -- x = coerce T (I# 3)
607 -- This in turn means that
608 -- case (coerce Int x) of ...
610 -- Also the full-blown w/w thing isn't set up for non-functions
612 -- The (not (isUnLiftedType inner_ty)) avoids the nasty case of
613 -- x::Int = coerce Int Int# (foo y)
616 -- x::Int = coerce Int Int# v
617 -- which would be bogus because then v will be evaluated strictly.
618 -- How can this arise? Via
619 -- x::Int = case (foo y) of { ... }
620 -- followed by case elimination.
622 -- The inline_me note is so that the simplifier doesn't
623 -- just substitute c back inside x's rhs! (Typically, x will
624 -- get substituted away, but not if it's exported.)
625 = newId SLIT("c") inner_ty $ \ c_id ->
626 completeBinding c_id c_id top_lvl False inner_rhs $
627 completeBinding old_bndr new_bndr top_lvl black_listed
628 (Note InlineMe (Note coercion (Var c_id))) $
633 -- We make new IdInfo for the new binder by starting from the old binder,
634 -- doing appropriate substitutions.
635 -- Then we add arity and unfolding info to get the new binder
636 new_bndr_info = idInfo new_bndr `setArityInfo` arity
638 -- Add the unfolding *only* for non-loop-breakers
639 -- Making loop breakers not have an unfolding at all
640 -- means that we can avoid tests in exprIsConApp, for example.
641 -- This is important: if exprIsConApp says 'yes' for a recursive
642 -- thing, then we can get into an infinite loop
643 info_w_unf | loop_breaker = new_bndr_info
644 | otherwise = new_bndr_info `setUnfoldingInfo` mkUnfolding top_lvl new_rhs
646 final_id = new_bndr `setIdInfo` info_w_unf
648 -- These seqs forces the Id, and hence its IdInfo,
649 -- and hence any inner substitutions
651 addLetBind (NonRec final_id new_rhs) $
652 modifyInScope new_bndr final_id thing_inside
655 old_info = idInfo old_bndr
656 occ_info = occInfo old_info
657 loop_breaker = isLoopBreaker occ_info
658 trivial_rhs = exprIsTrivial new_rhs
659 must_keep_binding = black_listed || loop_breaker || isExportedId old_bndr
660 arity = exprArity new_rhs
665 %************************************************************************
667 \subsection{simplLazyBind}
669 %************************************************************************
671 simplLazyBind basically just simplifies the RHS of a let(rec).
672 It does two important optimisations though:
674 * It floats let(rec)s out of the RHS, even if they
675 are hidden by big lambdas
677 * It does eta expansion
680 simplLazyBind :: Bool -- True <=> top level
683 -> SimplM (OutStuff a) -- The body of the binding
684 -> SimplM (OutStuff a)
685 -- When called, the subst env is correct for the entire let-binding
686 -- and hence right for the RHS.
687 -- Also the binder has already been simplified, and hence is in scope
689 simplLazyBind top_lvl bndr bndr' rhs thing_inside
690 = getBlackList `thenSmpl` \ black_list_fn ->
692 black_listed = black_list_fn bndr
695 if preInlineUnconditionally black_listed bndr then
696 -- Inline unconditionally
697 tick (PreInlineUnconditionally bndr) `thenSmpl_`
698 getSubstEnv `thenSmpl` \ rhs_se ->
699 (extendSubst bndr (ContEx rhs_se rhs) thing_inside)
703 getSubst `thenSmpl` \ rhs_subst ->
705 -- Substitute IdInfo on binder, in the light of earlier
706 -- substitutions in this very letrec, and extend the in-scope
707 -- env so that it can see the new thing
708 bndr'' = simplIdInfo rhs_subst (idInfo bndr) bndr'
710 modifyInScope bndr'' bndr'' $
712 simplRhs top_lvl False {- Not ok to float unboxed (conservative) -}
714 rhs (substEnv rhs_subst) $ \ rhs' ->
716 -- Now compete the binding and simplify the body
717 completeBinding bndr bndr'' top_lvl black_listed rhs' thing_inside
723 simplRhs :: Bool -- True <=> Top level
724 -> Bool -- True <=> OK to float unboxed (speculative) bindings
725 -- False for (a) recursive and (b) top-level bindings
726 -> OutType -- Type of RHS; used only occasionally
727 -> InExpr -> SubstEnv
728 -> (OutExpr -> SimplM (OutStuff a))
729 -> SimplM (OutStuff a)
730 simplRhs top_lvl float_ubx rhs_ty rhs rhs_se thing_inside
732 setSubstEnv rhs_se (simplExprF rhs (mkRhsStop rhs_ty)) `thenSmpl` \ (floats1, (rhs_in_scope, rhs1)) ->
734 (floats2, rhs2) = splitFloats float_ubx floats1 rhs1
737 -- It's important that we do eta expansion on function *arguments* (which are
738 -- simplified with simplRhs), as well as let-bound right-hand sides.
739 -- Otherwise we find that things like
740 -- f (\x -> case x of I# x' -> coerce T (\ y -> ...))
741 -- get right through to the code generator as two separate lambdas,
742 -- which is a Bad Thing
743 tryRhsTyLam rhs2 `thenSmpl` \ (floats3, rhs3) ->
744 tryEtaExpansion rhs3 rhs_ty `thenSmpl` \ (floats4, rhs4) ->
746 -- Float lets if (a) we're at the top level
747 -- or (b) the resulting RHS is one we'd like to expose
749 -- NB: the test used to say "exprIsCheap", but that caused a strictness bug.
750 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
751 -- The case expression is 'cheap', but it's wrong to transform to
752 -- y* = E; x = case (scc y) of {...}
753 -- Either we must be careful not to float demanded non-values, or
754 -- we must use exprIsValue for the test, which ensures that the
755 -- thing is non-strict. I think. The WARN below tests for this
756 if (top_lvl || exprIsValue rhs4) then
758 -- There's a subtlety here. There may be a binding (x* = e) in the
759 -- floats, where the '*' means 'will be demanded'. So is it safe
760 -- to float it out? Answer no, but it won't matter because
761 -- we only float if arg' is a WHNF,
762 -- and so there can't be any 'will be demanded' bindings in the floats.
764 WARN( any demanded_float (fromOL floats2),
765 ppr (filter demanded_float (fromOL floats2)) )
767 (if (isNilOL floats2 && null floats3 && null floats4) then
770 tick LetFloatFromLet) `thenSmpl_`
772 addFloats floats2 rhs_in_scope $
773 addAuxiliaryBinds floats3 $
774 addAuxiliaryBinds floats4 $
777 -- Don't do the float
778 thing_inside (wrapFloats floats1 rhs1)
780 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
781 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
782 demanded_float (Rec _) = False
784 -- If float_ubx is true we float all the bindings, otherwise
785 -- we just float until we come across an unlifted one.
786 -- Remember that the unlifted bindings in the floats are all for
787 -- guaranteed-terminating non-exception-raising unlifted things,
788 -- which we are happy to do speculatively. However, we may still
789 -- not be able to float them out, because the context
790 -- is either a Rec group, or the top level, neither of which
791 -- can tolerate them.
792 splitFloats float_ubx floats rhs
793 | float_ubx = (floats, rhs) -- Float them all
794 | otherwise = go (fromOL floats)
797 go (f:fs) | must_stay f = (nilOL, mkLets (f:fs) rhs)
798 | otherwise = case go fs of
799 (out, rhs') -> (f `consOL` out, rhs')
801 must_stay (Rec prs) = False -- No unlifted bindings in here
802 must_stay (NonRec b r) = isUnLiftedType (idType b)
807 %************************************************************************
809 \subsection{Variables}
811 %************************************************************************
815 = getSubst `thenSmpl` \ subst ->
816 case lookupIdSubst subst var of
817 DoneEx e -> zapSubstEnv (simplExprF e cont)
818 ContEx env1 e -> setSubstEnv env1 (simplExprF e cont)
819 DoneId var1 occ -> WARN( not (isInScope var1 subst) && mustHaveLocalBinding var1,
820 text "simplVar:" <+> ppr var )
821 zapSubstEnv (completeCall var1 occ cont)
822 -- The template is already simplified, so don't re-substitute.
823 -- This is VITAL. Consider
825 -- let y = \z -> ...x... in
827 -- We'll clone the inner \x, adding x->x' in the id_subst
828 -- Then when we inline y, we must *not* replace x by x' in
829 -- the inlined copy!!
831 ---------------------------------------------------------
832 -- Dealing with a call
834 completeCall var occ_info cont
835 = getBlackList `thenSmpl` \ black_list_fn ->
836 getInScope `thenSmpl` \ in_scope ->
837 getContArgs var cont `thenSmpl` \ (args, call_cont, inline_call) ->
838 getDOptsSmpl `thenSmpl` \ dflags ->
840 black_listed = black_list_fn var
841 arg_infos = [ interestingArg in_scope arg subst
842 | (arg, subst, _) <- args, isValArg arg]
844 interesting_cont = interestingCallContext (not (null args))
845 (not (null arg_infos))
848 inline_cont | inline_call = discardInline cont
851 maybe_inline = callSiteInline dflags black_listed inline_call occ_info
852 var arg_infos interesting_cont
854 -- First, look for an inlining
855 case maybe_inline of {
856 Just unfolding -- There is an inlining!
857 -> tick (UnfoldingDone var) `thenSmpl_`
858 simplExprF unfolding inline_cont
861 Nothing -> -- No inlining!
864 simplifyArgs (isDataConId var) args (contResultType call_cont) $ \ args' ->
866 -- Next, look for rules or specialisations that match
868 -- It's important to simplify the args first, because the rule-matcher
869 -- doesn't do substitution as it goes. We don't want to use subst_args
870 -- (defined in the 'where') because that throws away useful occurrence info,
871 -- and perhaps-very-important specialisations.
873 -- Some functions have specialisations *and* are strict; in this case,
874 -- we don't want to inline the wrapper of the non-specialised thing; better
875 -- to call the specialised thing instead.
876 -- But the black-listing mechanism means that inlining of the wrapper
877 -- won't occur for things that have specialisations till a later phase, so
878 -- it's ok to try for inlining first.
880 -- You might think that we shouldn't apply rules for a loop breaker:
881 -- doing so might give rise to an infinite loop, because a RULE is
882 -- rather like an extra equation for the function:
883 -- RULE: f (g x) y = x+y
886 -- But it's too drastic to disable rules for loop breakers.
887 -- Even the foldr/build rule would be disabled, because foldr
888 -- is recursive, and hence a loop breaker:
889 -- foldr k z (build g) = g k z
890 -- So it's up to the programmer: rules can cause divergence
892 getSwitchChecker `thenSmpl` \ chkr ->
894 maybe_rule | switchIsOn chkr DontApplyRules = Nothing
895 | otherwise = lookupRule in_scope var args'
898 Just (rule_name, rule_rhs) ->
899 tick (RuleFired rule_name) `thenSmpl_`
901 (if dopt Opt_D_dump_inlinings dflags then
902 pprTrace "Rule fired" (vcat [
903 text "Rule:" <+> ptext rule_name,
904 text "Before:" <+> ppr var <+> sep (map pprParendExpr args'),
905 text "After: " <+> pprCoreExpr rule_rhs])
909 simplExprF rule_rhs call_cont ;
911 Nothing -> -- No rules
914 rebuild (mkApps (Var var) args') call_cont
918 ---------------------------------------------------------
919 -- Simplifying the arguments of a call
921 simplifyArgs :: Bool -- It's a data constructor
922 -> [(InExpr, SubstEnv, Bool)] -- Details of the arguments
923 -> OutType -- Type of the continuation
924 -> ([OutExpr] -> SimplM OutExprStuff)
925 -> SimplM OutExprStuff
927 -- Simplify the arguments to a call.
928 -- This part of the simplifier may break the no-shadowing invariant
930 -- f (...(\a -> e)...) (case y of (a,b) -> e')
931 -- where f is strict in its second arg
932 -- If we simplify the innermost one first we get (...(\a -> e)...)
933 -- Simplifying the second arg makes us float the case out, so we end up with
934 -- case y of (a,b) -> f (...(\a -> e)...) e'
935 -- So the output does not have the no-shadowing invariant. However, there is
936 -- no danger of getting name-capture, because when the first arg was simplified
937 -- we used an in-scope set that at least mentioned all the variables free in its
938 -- static environment, and that is enough.
940 -- We can't just do innermost first, or we'd end up with a dual problem:
941 -- case x of (a,b) -> f e (...(\a -> e')...)
943 -- I spent hours trying to recover the no-shadowing invariant, but I just could
944 -- not think of an elegant way to do it. The simplifier is already knee-deep in
945 -- continuations. We have to keep the right in-scope set around; AND we have
946 -- to get the effect that finding (error "foo") in a strict arg position will
947 -- discard the entire application and replace it with (error "foo"). Getting
948 -- all this at once is TOO HARD!
950 simplifyArgs is_data_con args cont_ty thing_inside
952 = go args thing_inside
954 | otherwise -- It's a data constructor, so we want
955 -- to switch off inlining in the arguments
956 -- If we don't do this, consider:
957 -- let x = +# p q in C {x}
958 -- Even though x get's an occurrence of 'many', its RHS looks cheap,
959 -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
960 = getBlackList `thenSmpl` \ old_bl ->
961 noInlineBlackList `thenSmpl` \ ni_bl ->
964 setBlackList old_bl $
968 go [] thing_inside = thing_inside []
969 go (arg:args) thing_inside = simplifyArg is_data_con arg cont_ty $ \ arg' ->
971 thing_inside (arg':args')
973 simplifyArg is_data_con (Type ty_arg, se, _) cont_ty thing_inside
974 = simplTyArg ty_arg se `thenSmpl` \ new_ty_arg ->
975 thing_inside (Type new_ty_arg)
977 simplifyArg is_data_con (val_arg, se, is_strict) cont_ty thing_inside
978 = getInScope `thenSmpl` \ in_scope ->
980 arg_ty = substTy (mkSubst in_scope se) (exprType val_arg)
982 if not is_data_con then
983 -- An ordinary function
984 simplValArg arg_ty is_strict val_arg se cont_ty thing_inside
986 -- A data constructor
987 -- simplifyArgs has already switched off inlining, so
988 -- all we have to do here is to let-bind any non-trivial argument
990 -- It's not always the case that new_arg will be trivial
992 -- where, in one pass, f gets substituted by a constructor,
993 -- but x gets substituted by an expression (assume this is the
994 -- unique occurrence of x). It doesn't really matter -- it'll get
995 -- fixed up next pass. And it happens for dictionary construction,
996 -- which mentions the wrapper constructor to start with.
997 simplValArg arg_ty is_strict val_arg se cont_ty $ \ arg' ->
999 if exprIsTrivial arg' then
1002 newId SLIT("a") (exprType arg') $ \ arg_id ->
1003 addNonRecBind arg_id arg' $
1004 thing_inside (Var arg_id)
1008 %************************************************************************
1010 \subsection{Decisions about inlining}
1012 %************************************************************************
1014 NB: At one time I tried not pre/post-inlining top-level things,
1015 even if they occur exactly once. Reason:
1016 (a) some might appear as a function argument, so we simply
1017 replace static allocation with dynamic allocation:
1023 (b) some top level things might be black listed
1025 HOWEVER, I found that some useful foldr/build fusion was lost (most
1026 notably in spectral/hartel/parstof) because the foldr didn't see the build.
1028 Doing the dynamic allocation isn't a big deal, in fact, but losing the
1032 preInlineUnconditionally :: Bool {- Black listed -} -> InId -> Bool
1033 -- Examines a bndr to see if it is used just once in a
1034 -- completely safe way, so that it is safe to discard the binding
1035 -- inline its RHS at the (unique) usage site, REGARDLESS of how
1036 -- big the RHS might be. If this is the case we don't simplify
1037 -- the RHS first, but just inline it un-simplified.
1039 -- This is much better than first simplifying a perhaps-huge RHS
1040 -- and then inlining and re-simplifying it.
1042 -- NB: we don't even look at the RHS to see if it's trivial
1045 -- where x is used many times, but this is the unique occurrence
1046 -- of y. We should NOT inline x at all its uses, because then
1047 -- we'd do the same for y -- aargh! So we must base this
1048 -- pre-rhs-simplification decision solely on x's occurrences, not
1051 -- Evne RHSs labelled InlineMe aren't caught here, because
1052 -- there might be no benefit from inlining at the call site.
1054 preInlineUnconditionally black_listed bndr
1055 | black_listed || opt_SimplNoPreInlining = False
1056 | otherwise = case idOccInfo bndr of
1057 OneOcc in_lam once -> not in_lam && once
1058 -- Not inside a lambda, one occurrence ==> safe!
1064 %************************************************************************
1066 \subsection{The main rebuilder}
1068 %************************************************************************
1071 -------------------------------------------------------------------
1072 -- Finish rebuilding
1073 rebuild_done expr = returnOutStuff expr
1075 ---------------------------------------------------------
1076 rebuild :: OutExpr -> SimplCont -> SimplM OutExprStuff
1078 -- Stop continuation
1079 rebuild expr (Stop _ _) = rebuild_done expr
1081 -- ArgOf continuation
1082 rebuild expr (ArgOf _ _ cont_fn) = cont_fn expr
1084 -- ApplyTo continuation
1085 rebuild expr cont@(ApplyTo _ arg se cont')
1086 = setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1087 rebuild (App expr arg') cont'
1089 -- Coerce continuation
1090 rebuild expr (CoerceIt to_ty cont)
1091 = rebuild (mkCoerce to_ty (exprType expr) expr) cont
1093 -- Inline continuation
1094 rebuild expr (InlinePlease cont)
1095 = rebuild (Note InlineCall expr) cont
1097 rebuild scrut (Select _ bndr alts se cont)
1098 = rebuild_case scrut bndr alts se cont
1101 Case elimination [see the code above]
1103 Start with a simple situation:
1105 case x# of ===> e[x#/y#]
1108 (when x#, y# are of primitive type, of course). We can't (in general)
1109 do this for algebraic cases, because we might turn bottom into
1112 Actually, we generalise this idea to look for a case where we're
1113 scrutinising a variable, and we know that only the default case can
1118 other -> ...(case x of
1122 Here the inner case can be eliminated. This really only shows up in
1123 eliminating error-checking code.
1125 We also make sure that we deal with this very common case:
1130 Here we are using the case as a strict let; if x is used only once
1131 then we want to inline it. We have to be careful that this doesn't
1132 make the program terminate when it would have diverged before, so we
1134 - x is used strictly, or
1135 - e is already evaluated (it may so if e is a variable)
1137 Lastly, we generalise the transformation to handle this:
1143 We only do this for very cheaply compared r's (constructors, literals
1144 and variables). If pedantic bottoms is on, we only do it when the
1145 scrutinee is a PrimOp which can't fail.
1147 We do it *here*, looking at un-simplified alternatives, because we
1148 have to check that r doesn't mention the variables bound by the
1149 pattern in each alternative, so the binder-info is rather useful.
1151 So the case-elimination algorithm is:
1153 1. Eliminate alternatives which can't match
1155 2. Check whether all the remaining alternatives
1156 (a) do not mention in their rhs any of the variables bound in their pattern
1157 and (b) have equal rhss
1159 3. Check we can safely ditch the case:
1160 * PedanticBottoms is off,
1161 or * the scrutinee is an already-evaluated variable
1162 or * the scrutinee is a primop which is ok for speculation
1163 -- ie we want to preserve divide-by-zero errors, and
1164 -- calls to error itself!
1166 or * [Prim cases] the scrutinee is a primitive variable
1168 or * [Alg cases] the scrutinee is a variable and
1169 either * the rhs is the same variable
1170 (eg case x of C a b -> x ===> x)
1171 or * there is only one alternative, the default alternative,
1172 and the binder is used strictly in its scope.
1173 [NB this is helped by the "use default binder where
1174 possible" transformation; see below.]
1177 If so, then we can replace the case with one of the rhss.
1180 Blob of helper functions for the "case-of-something-else" situation.
1183 ---------------------------------------------------------
1184 -- Eliminate the case if possible
1186 rebuild_case scrut bndr alts se cont
1187 | maybeToBool maybe_con_app
1188 = knownCon scrut (DataAlt con) args bndr alts se cont
1190 | canEliminateCase scrut bndr alts
1191 = tick (CaseElim bndr) `thenSmpl_` (
1193 simplBinder bndr $ \ bndr' ->
1194 -- Remember to bind the case binder!
1195 completeBinding bndr bndr' False False scrut $
1196 simplExprF (head (rhssOfAlts alts)) cont)
1199 = complete_case scrut bndr alts se cont
1202 maybe_con_app = exprIsConApp_maybe scrut
1203 Just (con, args) = maybe_con_app
1205 -- See if we can get rid of the case altogether
1206 -- See the extensive notes on case-elimination above
1207 canEliminateCase scrut bndr alts
1208 = -- Check that the RHSs are all the same, and
1209 -- don't use the binders in the alternatives
1210 -- This test succeeds rapidly in the common case of
1211 -- a single DEFAULT alternative
1212 all (cheapEqExpr rhs1) other_rhss && all binders_unused alts
1214 -- Check that the scrutinee can be let-bound instead of case-bound
1215 && ( exprOkForSpeculation scrut
1216 -- OK not to evaluate it
1217 -- This includes things like (==# a# b#)::Bool
1218 -- so that we simplify
1219 -- case ==# a# b# of { True -> x; False -> x }
1222 -- This particular example shows up in default methods for
1223 -- comparision operations (e.g. in (>=) for Int.Int32)
1224 || exprIsValue scrut -- It's already evaluated
1225 || var_demanded_later scrut -- It'll be demanded later
1227 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1228 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1229 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1230 -- its argument: case x of { y -> dataToTag# y }
1231 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1232 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1237 (rhs1:other_rhss) = rhssOfAlts alts
1238 binders_unused (_, bndrs, _) = all isDeadBinder bndrs
1240 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo bndr) -- It's going to be evaluated later
1241 var_demanded_later other = False
1244 ---------------------------------------------------------
1245 -- Case of something else
1247 complete_case scrut case_bndr alts se cont
1248 = -- Prepare case alternatives
1249 prepareCaseAlts case_bndr (splitTyConApp_maybe (idType case_bndr))
1250 impossible_cons alts `thenSmpl` \ better_alts ->
1252 -- Set the new subst-env in place (before dealing with the case binder)
1255 -- Deal with the case binder, and prepare the continuation;
1256 -- The new subst_env is in place
1257 prepareCaseCont better_alts cont $ \ cont' ->
1260 -- Deal with variable scrutinee
1262 getSwitchChecker `thenSmpl` \ chkr ->
1263 simplCaseBinder (switchIsOn chkr NoCaseOfCase)
1264 scrut case_bndr $ \ case_bndr' zap_occ_info ->
1266 -- Deal with the case alternatives
1267 simplAlts zap_occ_info impossible_cons
1268 case_bndr' better_alts cont' `thenSmpl` \ alts' ->
1270 mkCase scrut case_bndr' alts'
1271 ) `thenSmpl` \ case_expr ->
1273 -- Notice that the simplBinder, prepareCaseCont, etc, do *not* scope
1274 -- over the rebuild_done; rebuild_done returns the in-scope set, and
1275 -- that should not include these chaps!
1276 rebuild_done case_expr
1278 impossible_cons = case scrut of
1279 Var v -> otherCons (idUnfolding v)
1283 knownCon :: OutExpr -> AltCon -> [OutExpr]
1284 -> InId -> [InAlt] -> SubstEnv -> SimplCont
1285 -> SimplM OutExprStuff
1287 knownCon expr con args bndr alts se cont
1288 = -- Arguments should be atomic;
1290 WARN( not (all exprIsTrivial args),
1291 text "knownCon" <+> ppr expr )
1292 tick (KnownBranch bndr) `thenSmpl_`
1294 simplBinder bndr $ \ bndr' ->
1295 completeBinding bndr bndr' False False expr $
1296 -- Don't use completeBeta here. The expr might be
1297 -- an unboxed literal, like 3, or a variable
1298 -- whose unfolding is an unboxed literal... and
1299 -- completeBeta will just construct another case
1301 case findAlt con alts of
1302 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1305 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1308 (DataAlt dc, bs, rhs) -> ASSERT( length bs == length real_args )
1309 extendSubstList bs (map mk real_args) $
1312 real_args = drop (dataConNumInstArgs dc) args
1313 mk (Type ty) = DoneTy ty
1314 mk other = DoneEx other
1319 prepareCaseCont :: [InAlt] -> SimplCont
1320 -> (SimplCont -> SimplM (OutStuff a))
1321 -> SimplM (OutStuff a)
1322 -- Polymorphic recursion here!
1324 prepareCaseCont [alt] cont thing_inside = thing_inside cont
1325 prepareCaseCont alts cont thing_inside = simplType (coreAltsType alts) `thenSmpl` \ alts_ty ->
1326 mkDupableCont alts_ty cont thing_inside
1327 -- At one time I passed in the un-simplified type, and simplified
1328 -- it only if we needed to construct a join binder, but that
1329 -- didn't work because we have to decompse function types
1330 -- (using funResultTy) in mkDupableCont.
1333 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1334 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1335 way, there's a chance that v will now only be used once, and hence
1338 There is a time we *don't* want to do that, namely when
1339 -fno-case-of-case is on. This happens in the first simplifier pass,
1340 and enhances full laziness. Here's the bad case:
1341 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1342 If we eliminate the inner case, we trap it inside the I# v -> arm,
1343 which might prevent some full laziness happening. I've seen this
1344 in action in spectral/cichelli/Prog.hs:
1345 [(m,n) | m <- [1..max], n <- [1..max]]
1346 Hence the no_case_of_case argument
1349 If we do this, then we have to nuke any occurrence info (eg IAmDead)
1350 in the case binder, because the case-binder now effectively occurs
1351 whenever v does. AND we have to do the same for the pattern-bound
1354 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1356 Here, b and p are dead. But when we move the argment inside the first
1357 case RHS, and eliminate the second case, we get
1359 case x or { (a,b) -> a b }
1361 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1362 happened. Hence the zap_occ_info function returned by simplCaseBinder
1365 simplCaseBinder no_case_of_case (Var v) case_bndr thing_inside
1366 | not no_case_of_case
1367 = simplBinder (zap case_bndr) $ \ case_bndr' ->
1368 modifyInScope v case_bndr' $
1369 -- We could extend the substitution instead, but it would be
1370 -- a hack because then the substitution wouldn't be idempotent
1371 -- any more (v is an OutId). And this just just as well.
1372 thing_inside case_bndr' zap
1374 zap b = b `setIdOccInfo` NoOccInfo
1376 simplCaseBinder add_eval_info other_scrut case_bndr thing_inside
1377 = simplBinder case_bndr $ \ case_bndr' ->
1378 thing_inside case_bndr' (\ bndr -> bndr) -- NoOp on bndr
1381 prepareCaseAlts does two things:
1383 1. Remove impossible alternatives
1385 2. If the DEFAULT alternative can match only one possible constructor,
1386 then make that constructor explicit.
1388 case e of x { DEFAULT -> rhs }
1390 case e of x { (a,b) -> rhs }
1391 where the type is a single constructor type. This gives better code
1392 when rhs also scrutinises x or e.
1395 prepareCaseAlts bndr (Just (tycon, inst_tys)) scrut_cons alts
1397 = case (findDefault filtered_alts, missing_cons) of
1399 ((alts_no_deflt, Just rhs), [data_con]) -- Just one missing constructor!
1400 -> tick (FillInCaseDefault bndr) `thenSmpl_`
1402 (_,_,ex_tyvars,_,_,_) = dataConSig data_con
1404 getUniquesSmpl `thenSmpl` \ tv_uniqs ->
1406 ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
1407 mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
1408 arg_tys = dataConArgTys data_con
1409 (inst_tys ++ mkTyVarTys ex_tyvars')
1411 newIds SLIT("a") arg_tys $ \ bndrs ->
1412 returnSmpl ((DataAlt data_con, ex_tyvars' ++ bndrs, rhs) : alts_no_deflt)
1414 other -> returnSmpl filtered_alts
1416 -- Filter out alternatives that can't possibly match
1417 filtered_alts = case scrut_cons of
1419 other -> [alt | alt@(con,_,_) <- alts, not (con `elem` scrut_cons)]
1421 missing_cons = [data_con | data_con <- tyConDataConsIfAvailable tycon,
1422 not (data_con `elem` handled_data_cons)]
1423 handled_data_cons = [data_con | DataAlt data_con <- scrut_cons] ++
1424 [data_con | (DataAlt data_con, _, _) <- filtered_alts]
1427 prepareCaseAlts _ _ scrut_cons alts
1428 = returnSmpl alts -- Functions
1431 ----------------------
1432 simplAlts zap_occ_info scrut_cons case_bndr' alts cont'
1433 = mapSmpl simpl_alt alts
1435 inst_tys' = tyConAppArgs (idType case_bndr')
1437 -- handled_cons is all the constructors that are dealt
1438 -- with, either by being impossible, or by there being an alternative
1439 (con_alts,_) = findDefault alts
1440 handled_cons = scrut_cons ++ [con | (con,_,_) <- con_alts]
1442 simpl_alt (DEFAULT, _, rhs)
1443 = -- In the default case we record the constructors that the
1444 -- case-binder *can't* be.
1445 -- We take advantage of any OtherCon info in the case scrutinee
1446 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` mkOtherCon handled_cons) $
1447 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1448 returnSmpl (DEFAULT, [], rhs')
1450 simpl_alt (con, vs, rhs)
1451 = -- Deal with the pattern-bound variables
1452 -- Mark the ones that are in ! positions in the data constructor
1453 -- as certainly-evaluated.
1454 -- NB: it happens that simplBinders does *not* erase the OtherCon
1455 -- form of unfolding, so it's ok to add this info before
1456 -- doing simplBinders
1457 simplBinders (add_evals con vs) $ \ vs' ->
1459 -- Bind the case-binder to (con args)
1461 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1463 modifyInScope case_bndr' (case_bndr' `setIdUnfolding` unfolding) $
1464 simplExprC rhs cont' `thenSmpl` \ rhs' ->
1465 returnSmpl (con, vs', rhs')
1468 -- add_evals records the evaluated-ness of the bound variables of
1469 -- a case pattern. This is *important*. Consider
1470 -- data T = T !Int !Int
1472 -- case x of { T a b -> T (a+1) b }
1474 -- We really must record that b is already evaluated so that we don't
1475 -- go and re-evaluate it when constructing the result.
1477 add_evals (DataAlt dc) vs = cat_evals vs (dataConRepStrictness dc)
1478 add_evals other_con vs = vs
1480 cat_evals [] [] = []
1481 cat_evals (v:vs) (str:strs)
1482 | isTyVar v = v : cat_evals vs (str:strs)
1483 | isStrictDmd str = (v' `setIdUnfolding` mkOtherCon []) : cat_evals vs strs
1484 | otherwise = v' : cat_evals vs strs
1490 %************************************************************************
1492 \subsection{Duplicating continuations}
1494 %************************************************************************
1497 mkDupableCont :: OutType -- Type of the thing to be given to the continuation
1499 -> (SimplCont -> SimplM (OutStuff a))
1500 -> SimplM (OutStuff a)
1501 mkDupableCont ty cont thing_inside
1502 | contIsDupable cont
1505 mkDupableCont _ (CoerceIt ty cont) thing_inside
1506 = mkDupableCont ty cont $ \ cont' ->
1507 thing_inside (CoerceIt ty cont')
1509 mkDupableCont ty (InlinePlease cont) thing_inside
1510 = mkDupableCont ty cont $ \ cont' ->
1511 thing_inside (InlinePlease cont')
1513 mkDupableCont join_arg_ty (ArgOf _ cont_ty cont_fn) thing_inside
1514 = -- Build the RHS of the join point
1515 newId SLIT("a") join_arg_ty ( \ arg_id ->
1516 cont_fn (Var arg_id) `thenSmpl` \ (floats, (_, rhs)) ->
1517 returnSmpl (Lam (setOneShotLambda arg_id) (wrapFloats floats rhs))
1518 ) `thenSmpl` \ join_rhs ->
1520 -- Build the join Id and continuation
1521 -- We give it a "$j" name just so that for later amusement
1522 -- we can identify any join points that don't end up as let-no-escapes
1523 -- [NOTE: the type used to be exprType join_rhs, but this seems more elegant.]
1524 newId SLIT("$j") (mkFunTy join_arg_ty cont_ty) $ \ join_id ->
1526 new_cont = ArgOf OkToDup cont_ty
1527 (\arg' -> rebuild_done (App (Var join_id) arg'))
1530 tick (CaseOfCase join_id) `thenSmpl_`
1531 -- Want to tick here so that we go round again,
1532 -- and maybe copy or inline the code;
1533 -- not strictly CaseOf Case
1534 addLetBind (NonRec join_id join_rhs) $
1535 thing_inside new_cont
1537 mkDupableCont ty (ApplyTo _ arg se cont) thing_inside
1538 = mkDupableCont (funResultTy ty) cont $ \ cont' ->
1539 setSubstEnv se (simplExpr arg) `thenSmpl` \ arg' ->
1540 if exprIsDupable arg' then
1541 thing_inside (ApplyTo OkToDup arg' emptySubstEnv cont')
1543 newId SLIT("a") (exprType arg') $ \ bndr ->
1545 tick (CaseOfCase bndr) `thenSmpl_`
1546 -- Want to tick here so that we go round again,
1547 -- and maybe copy or inline the code;
1548 -- not strictly CaseOf Case
1550 addLetBind (NonRec bndr arg') $
1551 -- But what if the arg should be case-bound? We can't use
1552 -- addNonRecBind here because its type is too specific.
1553 -- This has been this way for a long time, so I'll leave it,
1554 -- but I can't convince myself that it's right.
1556 thing_inside (ApplyTo OkToDup (Var bndr) emptySubstEnv cont')
1559 mkDupableCont ty (Select _ case_bndr alts se cont) thing_inside
1560 = tick (CaseOfCase case_bndr) `thenSmpl_`
1562 simplBinder case_bndr $ \ case_bndr' ->
1563 prepareCaseCont alts cont $ \ cont' ->
1564 mkDupableAlts case_bndr case_bndr' cont' alts $ \ alts' ->
1565 returnOutStuff alts'
1566 ) `thenSmpl` \ (alt_binds, (in_scope, alts')) ->
1568 addFloats alt_binds in_scope $
1570 -- NB that the new alternatives, alts', are still InAlts, using the original
1571 -- binders. That means we can keep the case_bndr intact. This is important
1572 -- because another case-of-case might strike, and so we want to keep the
1573 -- info that the case_bndr is dead (if it is, which is often the case).
1574 -- This is VITAL when the type of case_bndr is an unboxed pair (often the
1575 -- case in I/O rich code. We aren't allowed a lambda bound
1576 -- arg of unboxed tuple type, and indeed such a case_bndr is always dead
1577 thing_inside (Select OkToDup case_bndr alts' se (mkStop (contResultType cont)))
1579 mkDupableAlts :: InId -> OutId -> SimplCont -> [InAlt]
1580 -> ([InAlt] -> SimplM (OutStuff a))
1581 -> SimplM (OutStuff a)
1582 mkDupableAlts case_bndr case_bndr' cont [] thing_inside
1584 mkDupableAlts case_bndr case_bndr' cont (alt:alts) thing_inside
1585 = mkDupableAlt case_bndr case_bndr' cont alt $ \ alt' ->
1586 mkDupableAlts case_bndr case_bndr' cont alts $ \ alts' ->
1587 thing_inside (alt' : alts')
1589 mkDupableAlt case_bndr case_bndr' cont alt@(con, bndrs, rhs) thing_inside
1590 = simplBinders bndrs $ \ bndrs' ->
1591 simplExprC rhs cont `thenSmpl` \ rhs' ->
1593 if (case cont of { Stop _ _ -> exprIsDupable rhs'; other -> False}) then
1594 -- It is worth checking for a small RHS because otherwise we
1595 -- get extra let bindings that may cause an extra iteration of the simplifier to
1596 -- inline back in place. Quite often the rhs is just a variable or constructor.
1597 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1598 -- iterations because the version with the let bindings looked big, and so wasn't
1599 -- inlined, but after the join points had been inlined it looked smaller, and so
1602 -- But since the continuation is absorbed into the rhs, we only do this
1603 -- for a Stop continuation.
1605 -- NB: we have to check the size of rhs', not rhs.
1606 -- Duplicating a small InAlt might invalidate occurrence information
1607 -- However, if it *is* dupable, we return the *un* simplified alternative,
1608 -- because otherwise we'd need to pair it up with an empty subst-env.
1609 -- (Remember we must zap the subst-env before re-simplifying something).
1610 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1615 rhs_ty' = exprType rhs'
1616 (used_bndrs, used_bndrs')
1617 = unzip [pr | pr@(bndr,bndr') <- zip (case_bndr : bndrs)
1618 (case_bndr' : bndrs'),
1619 not (isDeadBinder bndr)]
1620 -- The new binders have lost their occurrence info,
1621 -- so we have to extract it from the old ones
1623 ( if null used_bndrs'
1624 -- If we try to lift a primitive-typed something out
1625 -- for let-binding-purposes, we will *caseify* it (!),
1626 -- with potentially-disastrous strictness results. So
1627 -- instead we turn it into a function: \v -> e
1628 -- where v::State# RealWorld#. The value passed to this function
1629 -- is realworld#, which generates (almost) no code.
1631 -- There's a slight infelicity here: we pass the overall
1632 -- case_bndr to all the join points if it's used in *any* RHS,
1633 -- because we don't know its usage in each RHS separately
1635 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1636 -- we make the join point into a function whenever used_bndrs'
1637 -- is empty. This makes the join-point more CPR friendly.
1638 -- Consider: let j = if .. then I# 3 else I# 4
1639 -- in case .. of { A -> j; B -> j; C -> ... }
1641 -- Now CPR doesn't w/w j because it's a thunk, so
1642 -- that means that the enclosing function can't w/w either,
1643 -- which is a lose. Here's the example that happened in practice:
1644 -- kgmod :: Int -> Int -> Int
1645 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1649 -- I have seen a case alternative like this:
1650 -- True -> \v -> ...
1651 -- It's a bit silly to add the realWorld dummy arg in this case, making
1654 -- (the \v alone is enough to make CPR happy) but I think it's rare
1656 then newId SLIT("w") realWorldStatePrimTy $ \ rw_id ->
1657 returnSmpl ([rw_id], [Var realWorldPrimId])
1659 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs)
1661 `thenSmpl` \ (final_bndrs', final_args) ->
1663 -- See comment about "$j" name above
1664 newId SLIT("$j") (foldr mkPiType rhs_ty' final_bndrs') $ \ join_bndr ->
1665 -- Notice the funky mkPiType. If the contructor has existentials
1666 -- it's possible that the join point will be abstracted over
1667 -- type varaibles as well as term variables.
1668 -- Example: Suppose we have
1669 -- data T = forall t. C [t]
1671 -- case (case e of ...) of
1672 -- C t xs::[t] -> rhs
1673 -- We get the join point
1674 -- let j :: forall t. [t] -> ...
1675 -- j = /\t \xs::[t] -> rhs
1677 -- case (case e of ...) of
1678 -- C t xs::[t] -> j t xs
1681 -- We make the lambdas into one-shot-lambdas. The
1682 -- join point is sure to be applied at most once, and doing so
1683 -- prevents the body of the join point being floated out by
1684 -- the full laziness pass
1685 really_final_bndrs = map one_shot final_bndrs'
1686 one_shot v | isId v = setOneShotLambda v
1689 addLetBind (NonRec join_bndr (mkLams really_final_bndrs rhs')) $
1690 thing_inside (con, bndrs, mkApps (Var join_bndr) final_args)