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 ( dopt, DynFlag(Opt_D_dump_inlinings),
15 import SimplUtils ( mkCase, mkLam, newId, prepareAlts,
16 simplBinder, simplBinders, simplLamBndrs, simplRecBndrs, simplLetBndr,
17 SimplCont(..), DupFlag(..), LetRhsFlag(..),
18 mkStop, mkBoringStop, pushContArgs,
19 contResultType, countArgs, contIsDupable, contIsRhsOrArg,
20 getContArgs, interestingCallContext, interestingArg, isStrictType
22 import Var ( mustHaveLocalBinding )
24 import Id ( Id, idType, idInfo, idArity, isDataConWorkId,
25 setIdUnfolding, isDeadBinder,
26 idNewDemandInfo, setIdInfo,
27 setIdOccInfo, zapLamIdInfo, setOneShotLambda,
29 import OccName ( encodeFS )
30 import IdInfo ( OccInfo(..), isLoopBreaker,
31 setArityInfo, zapDemandInfo,
35 import NewDemand ( isStrictDmd )
36 import DataCon ( dataConNumInstArgs, dataConRepStrictness )
38 import PprCore ( pprParendExpr, pprCoreExpr )
39 import CoreUnfold ( mkOtherCon, mkUnfolding, callSiteInline )
40 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
41 exprIsConApp_maybe, mkPiTypes, findAlt,
42 exprType, exprIsValue,
43 exprOkForSpeculation, exprArity,
44 mkCoerce, mkCoerce2, mkSCC, mkInlineMe, mkAltExpr, applyTypeToArg
46 import Rules ( lookupRule )
47 import BasicTypes ( isMarkedStrict )
48 import CostCentre ( currentCCS )
49 import Type ( isUnLiftedType, seqType, tyConAppArgs, funArgTy,
50 splitFunTy_maybe, splitFunTy, eqType
52 import Subst ( mkSubst, substTy, substExpr,
53 isInScope, lookupIdSubst, simplIdInfo
55 import TysPrim ( realWorldStatePrimTy )
56 import PrelInfo ( realWorldPrimId )
57 import BasicTypes ( TopLevelFlag(..), isTopLevel,
61 import Maybe ( Maybe )
62 import Maybes ( orElse )
64 import Util ( notNull )
68 The guts of the simplifier is in this module, but the driver loop for
69 the simplifier is in SimplCore.lhs.
72 -----------------------------------------
73 *** IMPORTANT NOTE ***
74 -----------------------------------------
75 The simplifier used to guarantee that the output had no shadowing, but
76 it does not do so any more. (Actually, it never did!) The reason is
77 documented with simplifyArgs.
80 -----------------------------------------
81 *** IMPORTANT NOTE ***
82 -----------------------------------------
83 Many parts of the simplifier return a bunch of "floats" as well as an
84 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
86 All "floats" are let-binds, not case-binds, but some non-rec lets may
87 be unlifted (with RHS ok-for-speculation).
91 -----------------------------------------
92 ORGANISATION OF FUNCTIONS
93 -----------------------------------------
95 - simplify all top-level binders
96 - for NonRec, call simplRecOrTopPair
97 - for Rec, call simplRecBind
100 ------------------------------
101 simplExpr (applied lambda) ==> simplNonRecBind
102 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
103 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
105 ------------------------------
106 simplRecBind [binders already simplfied]
107 - use simplRecOrTopPair on each pair in turn
109 simplRecOrTopPair [binder already simplified]
110 Used for: recursive bindings (top level and nested)
111 top-level non-recursive bindings
113 - check for PreInlineUnconditionally
117 Used for: non-top-level non-recursive bindings
118 beta reductions (which amount to the same thing)
119 Because it can deal with strict arts, it takes a
120 "thing-inside" and returns an expression
122 - check for PreInlineUnconditionally
123 - simplify binder, including its IdInfo
132 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
133 Used for: binding case-binder and constr args in a known-constructor case
134 - check for PreInLineUnconditionally
138 ------------------------------
139 simplLazyBind: [binder already simplified, RHS not]
140 Used for: recursive bindings (top level and nested)
141 top-level non-recursive bindings
142 non-top-level, but *lazy* non-recursive bindings
143 [must not be strict or unboxed]
144 Returns floats + an augmented environment, not an expression
145 - substituteIdInfo and add result to in-scope
146 [so that rules are available in rec rhs]
149 - float if exposes constructor or PAP
153 completeNonRecX: [binder and rhs both simplified]
154 - if the the thing needs case binding (unlifted and not ok-for-spec)
160 completeLazyBind: [given a simplified RHS]
161 [used for both rec and non-rec bindings, top level and not]
162 - try PostInlineUnconditionally
163 - add unfolding [this is the only place we add an unfolding]
168 Right hand sides and arguments
169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
170 In many ways we want to treat
171 (a) the right hand side of a let(rec), and
172 (b) a function argument
173 in the same way. But not always! In particular, we would
174 like to leave these arguments exactly as they are, so they
175 will match a RULE more easily.
180 It's harder to make the rule match if we ANF-ise the constructor,
181 or eta-expand the PAP:
183 f (let { a = g x; b = h x } in (a,b))
186 On the other hand if we see the let-defns
191 then we *do* want to ANF-ise and eta-expand, so that p and q
192 can be safely inlined.
194 Even floating lets out is a bit dubious. For let RHS's we float lets
195 out if that exposes a value, so that the value can be inlined more vigorously.
198 r = let x = e in (x,x)
200 Here, if we float the let out we'll expose a nice constructor. We did experiments
201 that showed this to be a generally good thing. But it was a bad thing to float
202 lets out unconditionally, because that meant they got allocated more often.
204 For function arguments, there's less reason to expose a constructor (it won't
205 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
206 So for the moment we don't float lets out of function arguments either.
211 For eta expansion, we want to catch things like
213 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
215 If the \x was on the RHS of a let, we'd eta expand to bring the two
216 lambdas together. And in general that's a good thing to do. Perhaps
217 we should eta expand wherever we find a (value) lambda? Then the eta
218 expansion at a let RHS can concentrate solely on the PAP case.
221 %************************************************************************
223 \subsection{Bindings}
225 %************************************************************************
228 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
230 simplTopBinds env binds
231 = -- Put all the top-level binders into scope at the start
232 -- so that if a transformation rule has unexpectedly brought
233 -- anything into scope, then we don't get a complaint about that.
234 -- It's rather as if the top-level binders were imported.
235 simplRecBndrs env (bindersOfBinds binds) `thenSmpl` \ (env, bndrs') ->
236 simpl_binds env binds bndrs' `thenSmpl` \ (floats, _) ->
237 freeTick SimplifierDone `thenSmpl_`
238 returnSmpl (floatBinds floats)
240 -- We need to track the zapped top-level binders, because
241 -- they should have their fragile IdInfo zapped (notably occurrence info)
242 -- That's why we run down binds and bndrs' simultaneously.
243 simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
244 simpl_binds env [] bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
245 simpl_binds env (bind:binds) bs = simpl_bind env bind bs `thenSmpl` \ (floats,env) ->
246 addFloats env floats $ \env ->
247 simpl_binds env binds (drop_bs bind bs)
249 drop_bs (NonRec _ _) (_ : bs) = bs
250 drop_bs (Rec prs) bs = drop (length prs) bs
252 simpl_bind env bind bs
253 = getDOptsSmpl `thenSmpl` \ dflags ->
254 if dopt Opt_D_dump_inlinings dflags then
255 pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
257 simpl_bind1 env bind bs
259 simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
260 simpl_bind1 env (Rec pairs) bs' = simplRecBind env TopLevel pairs bs'
264 %************************************************************************
266 \subsection{simplNonRec}
268 %************************************************************************
270 simplNonRecBind is used for
271 * non-top-level non-recursive lets in expressions
275 * An unsimplified (binder, rhs) pair
276 * The env for the RHS. It may not be the same as the
277 current env because the bind might occur via (\x.E) arg
279 It uses the CPS form because the binding might be strict, in which
280 case we might discard the continuation:
281 let x* = error "foo" in (...x...)
283 It needs to turn unlifted bindings into a @case@. They can arise
284 from, say: (\x -> e) (4# + 3#)
287 simplNonRecBind :: SimplEnv
289 -> InExpr -> SimplEnv -- Arg, with its subst-env
290 -> OutType -- Type of thing computed by the context
291 -> (SimplEnv -> SimplM FloatsWithExpr) -- The body
292 -> SimplM FloatsWithExpr
294 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
296 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
299 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
300 | preInlineUnconditionally env NotTopLevel bndr
301 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
302 thing_inside (extendSubst env bndr (ContEx (getSubstEnv rhs_se) rhs))
305 | isStrictDmd (idNewDemandInfo bndr) || isStrictType (idType bndr) -- A strict let
306 = -- Don't use simplBinder because that doesn't keep
307 -- fragile occurrence info in the substitution
308 simplLetBndr env bndr `thenSmpl` \ (env, bndr1) ->
309 simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty $ \ env1 rhs1 ->
311 -- Now complete the binding and simplify the body
313 -- simplLetBndr doesn't deal with the IdInfo, so we must
314 -- do so here (c.f. simplLazyBind)
315 bndr2 = bndr1 `setIdInfo` simplIdInfo (getSubst env) (idInfo bndr)
316 env2 = modifyInScope env1 bndr2 bndr2
318 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
320 | otherwise -- Normal, lazy case
321 = -- Don't use simplBinder because that doesn't keep
322 -- fragile occurrence info in the substitution
323 simplLetBndr env bndr `thenSmpl` \ (env, bndr') ->
324 simplLazyBind env NotTopLevel NonRecursive
325 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
326 addFloats env floats thing_inside
329 A specialised variant of simplNonRec used when the RHS is already simplified, notably
330 in knownCon. It uses case-binding where necessary.
333 simplNonRecX :: SimplEnv
334 -> InId -- Old binder
335 -> OutExpr -- Simplified RHS
336 -> (SimplEnv -> SimplM FloatsWithExpr)
337 -> SimplM FloatsWithExpr
339 simplNonRecX env bndr new_rhs thing_inside
340 | needsCaseBinding (idType bndr) new_rhs
341 -- Make this test *before* the preInlineUnconditionally
342 -- Consider case I# (quotInt# x y) of
343 -- I# v -> let w = J# v in ...
344 -- If we gaily inline (quotInt# x y) for v, we end up building an
346 -- let w = J# (quotInt# x y) in ...
347 -- because quotInt# can fail.
348 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
349 thing_inside env `thenSmpl` \ (floats, body) ->
350 returnSmpl (emptyFloats env, Case new_rhs bndr' [(DEFAULT, [], wrapFloats floats body)])
352 | preInlineUnconditionally env NotTopLevel bndr
353 -- This happens; for example, the case_bndr during case of
354 -- known constructor: case (a,b) of x { (p,q) -> ... }
355 -- Here x isn't mentioned in the RHS, so we don't want to
356 -- create the (dead) let-binding let x = (a,b) in ...
358 -- Similarly, single occurrences can be inlined vigourously
359 -- e.g. case (f x, g y) of (a,b) -> ....
360 -- If a,b occur once we can avoid constructing the let binding for them.
361 = thing_inside (extendSubst env bndr (ContEx emptySubstEnv new_rhs))
364 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
365 completeNonRecX env False {- Non-strict; pessimistic -}
366 bndr bndr' new_rhs thing_inside
368 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
369 = mkAtomicArgs is_strict
370 True {- OK to float unlifted -}
371 new_rhs `thenSmpl` \ (aux_binds, rhs2) ->
373 -- Make the arguments atomic if necessary,
374 -- adding suitable bindings
375 addAtomicBindsE env (fromOL aux_binds) $ \ env ->
376 completeLazyBind env NotTopLevel
377 old_bndr new_bndr rhs2 `thenSmpl` \ (floats, env) ->
378 addFloats env floats thing_inside
382 %************************************************************************
384 \subsection{Lazy bindings}
386 %************************************************************************
388 simplRecBind is used for
389 * recursive bindings only
392 simplRecBind :: SimplEnv -> TopLevelFlag
393 -> [(InId, InExpr)] -> [OutId]
394 -> SimplM (FloatsWith SimplEnv)
395 simplRecBind env top_lvl pairs bndrs'
396 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
397 returnSmpl (flattenFloats floats, env)
399 go env [] _ = returnSmpl (emptyFloats env, env)
401 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
402 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
403 addFloats env floats (\env -> go env pairs bndrs')
407 simplRecOrTopPair is used for
408 * recursive bindings (whether top level or not)
409 * top-level non-recursive bindings
411 It assumes the binder has already been simplified, but not its IdInfo.
414 simplRecOrTopPair :: SimplEnv
416 -> InId -> OutId -- Binder, both pre-and post simpl
417 -> InExpr -- The RHS and its environment
418 -> SimplM (FloatsWith SimplEnv)
420 simplRecOrTopPair env top_lvl bndr bndr' rhs
421 | preInlineUnconditionally env top_lvl bndr -- Check for unconditional inline
422 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
423 returnSmpl (emptyFloats env, extendSubst env bndr (ContEx (getSubstEnv env) rhs))
426 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
427 -- May not actually be recursive, but it doesn't matter
431 simplLazyBind is used for
432 * recursive bindings (whether top level or not)
433 * top-level non-recursive bindings
434 * non-top-level *lazy* non-recursive bindings
436 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
437 from SimplRecOrTopBind]
440 1. It assumes that the binder is *already* simplified,
441 and is in scope, but not its IdInfo
443 2. It assumes that the binder type is lifted.
445 3. It does not check for pre-inline-unconditionallly;
446 that should have been done already.
449 simplLazyBind :: SimplEnv
450 -> TopLevelFlag -> RecFlag
451 -> InId -> OutId -- Binder, both pre-and post simpl
452 -> InExpr -> SimplEnv -- The RHS and its environment
453 -> SimplM (FloatsWith SimplEnv)
455 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
456 = let -- Transfer the IdInfo of the original binder to the new binder
457 -- This is crucial: we must preserve
461 -- etc. To do this we must apply the current substitution,
462 -- which incorporates earlier substitutions in this very letrec group.
464 -- NB 1. We do this *before* processing the RHS of the binder, so that
465 -- its substituted rules are visible in its own RHS.
466 -- This is important. Manuel found cases where he really, really
467 -- wanted a RULE for a recursive function to apply in that function's
468 -- own right-hand side.
470 -- NB 2: We do not transfer the arity (see Subst.substIdInfo)
471 -- The arity of an Id should not be visible
472 -- in its own RHS, else we eta-reduce
476 -- which isn't sound. And it makes the arity in f's IdInfo greater than
477 -- the manifest arity, which isn't good.
478 -- The arity will get added later.
480 -- NB 3: It's important that we *do* transer the loop-breaker OccInfo,
481 -- because that's what stops the Id getting inlined infinitely, in the body
484 -- NB 4: does no harm for non-recursive bindings
486 bndr2 = bndr1 `setIdInfo` simplIdInfo (getSubst env) (idInfo bndr)
487 env1 = modifyInScope env bndr2 bndr2
488 rhs_env = setInScope rhs_se env1
489 is_top_level = isTopLevel top_lvl
490 ok_float_unlifted = not is_top_level && isNonRec is_rec
491 rhs_cont = mkStop (idType bndr1) AnRhs
493 -- Simplify the RHS; note the mkStop, which tells
494 -- the simplifier that this is the RHS of a let.
495 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
497 -- If any of the floats can't be floated, give up now
498 -- (The allLifted predicate says True for empty floats.)
499 if (not ok_float_unlifted && not (allLifted floats)) then
500 completeLazyBind env1 top_lvl bndr bndr2
501 (wrapFloats floats rhs1)
504 -- ANF-ise a constructor or PAP rhs
505 mkAtomicArgs False {- Not strict -}
506 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
508 -- If the result is a PAP, float the floats out, else wrap them
509 -- By this time it's already been ANF-ised (if necessary)
510 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
511 completeLazyBind env1 top_lvl bndr bndr2 rhs2
513 else if is_top_level || exprIsTrivial rhs2 || exprIsValue rhs2 then
514 -- WARNING: long dodgy argument coming up
515 -- WANTED: a better way to do this
517 -- We can't use "exprIsCheap" instead of exprIsValue,
518 -- because that causes a strictness bug.
519 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
520 -- The case expression is 'cheap', but it's wrong to transform to
521 -- y* = E; x = case (scc y) of {...}
522 -- Either we must be careful not to float demanded non-values, or
523 -- we must use exprIsValue for the test, which ensures that the
524 -- thing is non-strict. So exprIsValue => bindings are non-strict
525 -- I think. The WARN below tests for this.
527 -- We use exprIsTrivial here because we want to reveal lone variables.
528 -- E.g. let { x = letrec { y = E } in y } in ...
529 -- Here we definitely want to float the y=E defn.
530 -- exprIsValue definitely isn't right for that.
532 -- Again, the floated binding can't be strict; if it's recursive it'll
533 -- be non-strict; if it's non-recursive it'd be inlined.
535 -- Note [SCC-and-exprIsTrivial]
537 -- y = let { x* = E } in scc "foo" x
538 -- then we do *not* want to float out the x binding, because
539 -- it's strict! Fortunately, exprIsTrivial replies False to
542 -- There's a subtlety here. There may be a binding (x* = e) in the
543 -- floats, where the '*' means 'will be demanded'. So is it safe
544 -- to float it out? Answer no, but it won't matter because
545 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
546 -- and so there can't be any 'will be demanded' bindings in the floats.
548 ASSERT2( is_top_level || not (any demanded_float (floatBinds floats)),
549 ppr (filter demanded_float (floatBinds floats)) )
551 tick LetFloatFromLet `thenSmpl_` (
552 addFloats env1 floats $ \ env2 ->
553 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
554 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
557 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
560 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
561 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
562 demanded_float (Rec _) = False
567 %************************************************************************
569 \subsection{Completing a lazy binding}
571 %************************************************************************
574 * deals only with Ids, not TyVars
575 * takes an already-simplified binder and RHS
576 * is used for both recursive and non-recursive bindings
577 * is used for both top-level and non-top-level bindings
579 It does the following:
580 - tries discarding a dead binding
581 - tries PostInlineUnconditionally
582 - add unfolding [this is the only place we add an unfolding]
585 It does *not* attempt to do let-to-case. Why? Because it is used for
586 - top-level bindings (when let-to-case is impossible)
587 - many situations where the "rhs" is known to be a WHNF
588 (so let-to-case is inappropriate).
591 completeLazyBind :: SimplEnv
592 -> TopLevelFlag -- Flag stuck into unfolding
593 -> InId -- Old binder
594 -> OutId -- New binder
595 -> OutExpr -- Simplified RHS
596 -> SimplM (FloatsWith SimplEnv)
597 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
598 -- by extending the substitution (e.g. let x = y in ...)
599 -- The new binding (if any) is returned as part of the floats.
600 -- NB: the returned SimplEnv has the right SubstEnv, but you should
601 -- (as usual) use the in-scope-env from the floats
603 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
604 | postInlineUnconditionally env new_bndr occ_info new_rhs
605 = -- Drop the binding
606 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
607 returnSmpl (emptyFloats env, extendSubst env old_bndr (DoneEx new_rhs))
608 -- Use the substitution to make quite, quite sure that the substitution
609 -- will happen, since we are going to discard the binding
614 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
616 -- Add the unfolding *only* for non-loop-breakers
617 -- Making loop breakers not have an unfolding at all
618 -- means that we can avoid tests in exprIsConApp, for example.
619 -- This is important: if exprIsConApp says 'yes' for a recursive
620 -- thing, then we can get into an infinite loop
622 -- If the unfolding is a value, the demand info may
623 -- go pear-shaped, so we nuke it. Example:
625 -- case x of (p,q) -> h p q x
626 -- Here x is certainly demanded. But after we've nuked
627 -- the case, we'll get just
628 -- let x = (a,b) in h a b x
629 -- and now x is not demanded (I'm assuming h is lazy)
630 -- This really happens. Similarly
631 -- let f = \x -> e in ...f..f...
632 -- After inling f at some of its call sites the original binding may
633 -- (for example) be no longer strictly demanded.
634 -- The solution here is a bit ad hoc...
635 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
636 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
637 final_info | loop_breaker = new_bndr_info
638 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
639 | otherwise = info_w_unf
641 final_id = new_bndr `setIdInfo` final_info
643 -- These seqs forces the Id, and hence its IdInfo,
644 -- and hence any inner substitutions
646 returnSmpl (unitFloat env final_id new_rhs, env)
649 loop_breaker = isLoopBreaker occ_info
650 old_info = idInfo old_bndr
651 occ_info = occInfo old_info
656 %************************************************************************
658 \subsection[Simplify-simplExpr]{The main function: simplExpr}
660 %************************************************************************
662 The reason for this OutExprStuff stuff is that we want to float *after*
663 simplifying a RHS, not before. If we do so naively we get quadratic
664 behaviour as things float out.
666 To see why it's important to do it after, consider this (real) example:
680 a -- Can't inline a this round, cos it appears twice
684 Each of the ==> steps is a round of simplification. We'd save a
685 whole round if we float first. This can cascade. Consider
690 let f = let d1 = ..d.. in \y -> e
694 in \x -> ...(\y ->e)...
696 Only in this second round can the \y be applied, and it
697 might do the same again.
701 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
702 simplExpr env expr = simplExprC env expr (mkStop expr_ty' AnArg)
704 expr_ty' = substTy (getSubst env) (exprType expr)
705 -- The type in the Stop continuation, expr_ty', is usually not used
706 -- It's only needed when discarding continuations after finding
707 -- a function that returns bottom.
708 -- Hence the lazy substitution
711 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
712 -- Simplify an expression, given a continuation
713 simplExprC env expr cont
714 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
715 returnSmpl (wrapFloats floats expr)
717 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
718 -- Simplify an expression, returning floated binds
720 simplExprF env (Var v) cont = simplVar env v cont
721 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
722 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
723 simplExprF env (Note note expr) cont = simplNote env note expr cont
724 simplExprF env (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg env cont)
726 simplExprF env (Type ty) cont
727 = ASSERT( contIsRhsOrArg cont )
728 simplType env ty `thenSmpl` \ ty' ->
729 rebuild env (Type ty') cont
731 simplExprF env (Case scrut bndr alts) cont
732 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
733 = -- Simplify the scrutinee with a Select continuation
734 simplExprF env scrut (Select NoDup bndr alts env cont)
737 = -- If case-of-case is off, simply simplify the case expression
738 -- in a vanilla Stop context, and rebuild the result around it
739 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
740 rebuild env case_expr' cont
742 case_cont = Select NoDup bndr alts env (mkBoringStop (contResultType cont))
744 simplExprF env (Let (Rec pairs) body) cont
745 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
746 -- NB: bndrs' don't have unfoldings or rules
747 -- We add them as we go down
749 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
750 addFloats env floats $ \ env ->
751 simplExprF env body cont
753 -- A non-recursive let is dealt with by simplNonRecBind
754 simplExprF env (Let (NonRec bndr rhs) body) cont
755 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
756 simplExprF env body cont
759 ---------------------------------
760 simplType :: SimplEnv -> InType -> SimplM OutType
761 -- Kept monadic just so we can do the seqType
763 = seqType new_ty `seq` returnSmpl new_ty
765 new_ty = substTy (getSubst env) ty
769 %************************************************************************
773 %************************************************************************
776 simplLam env fun cont
779 zap_it = mkLamBndrZapper fun (countArgs cont)
780 cont_ty = contResultType cont
782 -- Type-beta reduction
783 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
784 = ASSERT( isTyVar bndr )
785 tick (BetaReduction bndr) `thenSmpl_`
786 simplType (setInScope arg_se env) ty_arg `thenSmpl` \ ty_arg' ->
787 go (extendSubst env bndr (DoneTy ty_arg')) body body_cont
789 -- Ordinary beta reduction
790 go env (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
791 = tick (BetaReduction bndr) `thenSmpl_`
792 simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
793 go env body body_cont
795 -- Not enough args, so there are real lambdas left to put in the result
796 go env lam@(Lam _ _) cont
797 = simplLamBndrs env bndrs `thenSmpl` \ (env, bndrs') ->
798 simplExpr env body `thenSmpl` \ body' ->
799 mkLam env bndrs' body' cont `thenSmpl` \ (floats, new_lam) ->
800 addFloats env floats $ \ env ->
801 rebuild env new_lam cont
803 (bndrs,body) = collectBinders lam
805 -- Exactly enough args
806 go env expr cont = simplExprF env expr cont
808 mkLamBndrZapper :: CoreExpr -- Function
809 -> Int -- Number of args supplied, *including* type args
810 -> Id -> Id -- Use this to zap the binders
811 mkLamBndrZapper fun n_args
812 | n_args >= n_params fun = \b -> b -- Enough args
813 | otherwise = \b -> zapLamIdInfo b
815 -- NB: we count all the args incl type args
816 -- so we must count all the binders (incl type lambdas)
817 n_params (Note _ e) = n_params e
818 n_params (Lam b e) = 1 + n_params e
819 n_params other = 0::Int
823 %************************************************************************
827 %************************************************************************
830 simplNote env (Coerce to from) body cont
832 in_scope = getInScope env
834 addCoerce s1 k1 (CoerceIt t1 cont)
835 -- coerce T1 S1 (coerce S1 K1 e)
838 -- coerce T1 K1 e, otherwise
840 -- For example, in the initial form of a worker
841 -- we may find (coerce T (coerce S (\x.e))) y
842 -- and we'd like it to simplify to e[y/x] in one round
844 | t1 `eqType` k1 = cont -- The coerces cancel out
845 | otherwise = CoerceIt t1 cont -- They don't cancel, but
846 -- the inner one is redundant
848 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
849 | not (isTypeArg arg), -- This whole case only works for value args
850 -- Could upgrade to have equiv thing for type apps too
851 Just (s1, s2) <- splitFunTy_maybe s1s2
852 -- (coerce (T1->T2) (S1->S2) F) E
854 -- coerce T2 S2 (F (coerce S1 T1 E))
856 -- t1t2 must be a function type, T1->T2, because it's applied to something
857 -- but s1s2 might conceivably not be
859 -- When we build the ApplyTo we can't mix the out-types
860 -- with the InExpr in the argument, so we simply substitute
861 -- to make it all consistent. It's a bit messy.
862 -- But it isn't a common case.
864 (t1,t2) = splitFunTy t1t2
865 new_arg = mkCoerce2 s1 t1 (substExpr (mkSubst in_scope (getSubstEnv arg_se)) arg)
867 ApplyTo dup new_arg (zapSubstEnv env) (addCoerce t2 s2 cont)
869 addCoerce to' _ cont = CoerceIt to' cont
871 simplType env to `thenSmpl` \ to' ->
872 simplType env from `thenSmpl` \ from' ->
873 simplExprF env body (addCoerce to' from' cont)
876 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
877 -- inlining. All other CCCSs are mapped to currentCCS.
878 simplNote env (SCC cc) e cont
879 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
880 rebuild env (mkSCC cc e') cont
882 simplNote env InlineCall e cont
883 = simplExprF env e (InlinePlease cont)
885 -- See notes with SimplMonad.inlineMode
886 simplNote env InlineMe e cont
887 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
888 = -- Don't inline inside an INLINE expression
889 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
890 rebuild env (mkInlineMe e') cont
892 | otherwise -- Dissolve the InlineMe note if there's
893 -- an interesting context of any kind to combine with
894 -- (even a type application -- anything except Stop)
895 = simplExprF env e cont
897 simplNote env (CoreNote s) e cont
898 = simplExpr env e `thenSmpl` \ e' ->
899 rebuild env (Note (CoreNote s) e') cont
903 %************************************************************************
905 \subsection{Dealing with calls}
907 %************************************************************************
910 simplVar env var cont
911 = case lookupIdSubst (getSubst env) var of
912 DoneEx e -> simplExprF (zapSubstEnv env) e cont
913 ContEx se e -> simplExprF (setSubstEnv env se) e cont
914 DoneId var1 occ -> WARN( not (isInScope var1 (getSubst env)) && mustHaveLocalBinding var1,
915 text "simplVar:" <+> ppr var )
916 completeCall (zapSubstEnv env) var1 occ cont
917 -- The template is already simplified, so don't re-substitute.
918 -- This is VITAL. Consider
920 -- let y = \z -> ...x... in
922 -- We'll clone the inner \x, adding x->x' in the id_subst
923 -- Then when we inline y, we must *not* replace x by x' in
924 -- the inlined copy!!
926 ---------------------------------------------------------
927 -- Dealing with a call site
929 completeCall env var occ_info cont
930 = -- Simplify the arguments
931 getDOptsSmpl `thenSmpl` \ dflags ->
933 chkr = getSwitchChecker env
934 (args, call_cont, inline_call) = getContArgs chkr var cont
937 simplifyArgs env fn_ty args (contResultType call_cont) $ \ env args ->
939 -- Next, look for rules or specialisations that match
941 -- It's important to simplify the args first, because the rule-matcher
942 -- doesn't do substitution as it goes. We don't want to use subst_args
943 -- (defined in the 'where') because that throws away useful occurrence info,
944 -- and perhaps-very-important specialisations.
946 -- Some functions have specialisations *and* are strict; in this case,
947 -- we don't want to inline the wrapper of the non-specialised thing; better
948 -- to call the specialised thing instead.
949 -- We used to use the black-listing mechanism to ensure that inlining of
950 -- the wrapper didn't occur for things that have specialisations till a
951 -- later phase, so but now we just try RULES first
953 -- You might think that we shouldn't apply rules for a loop breaker:
954 -- doing so might give rise to an infinite loop, because a RULE is
955 -- rather like an extra equation for the function:
956 -- RULE: f (g x) y = x+y
959 -- But it's too drastic to disable rules for loop breakers.
960 -- Even the foldr/build rule would be disabled, because foldr
961 -- is recursive, and hence a loop breaker:
962 -- foldr k z (build g) = g k z
963 -- So it's up to the programmer: rules can cause divergence
966 in_scope = getInScope env
967 maybe_rule = case activeRule env of
968 Nothing -> Nothing -- No rules apply
969 Just act_fn -> lookupRule act_fn in_scope var args
972 Just (rule_name, rule_rhs) ->
973 tick (RuleFired rule_name) `thenSmpl_`
974 (if dopt Opt_D_dump_inlinings dflags then
975 pprTrace "Rule fired" (vcat [
976 text "Rule:" <+> ftext rule_name,
977 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
978 text "After: " <+> pprCoreExpr rule_rhs,
979 text "Cont: " <+> ppr call_cont])
982 simplExprF env rule_rhs call_cont ;
984 Nothing -> -- No rules
986 -- Next, look for an inlining
988 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
990 interesting_cont = interestingCallContext (notNull args)
994 active_inline = activeInline env var occ_info
995 maybe_inline = callSiteInline dflags active_inline inline_call occ_info
996 var arg_infos interesting_cont
998 case maybe_inline of {
999 Just unfolding -- There is an inlining!
1000 -> tick (UnfoldingDone var) `thenSmpl_`
1001 makeThatCall env var unfolding args call_cont
1004 Nothing -> -- No inlining!
1007 rebuild env (mkApps (Var var) args) call_cont
1010 makeThatCall :: SimplEnv
1012 -> InExpr -- Inlined function rhs
1013 -> [OutExpr] -- Arguments, already simplified
1014 -> SimplCont -- After the call
1015 -> SimplM FloatsWithExpr
1016 -- Similar to simplLam, but this time
1017 -- the arguments are already simplified
1018 makeThatCall orig_env var fun@(Lam _ _) args cont
1019 = go orig_env fun args
1021 zap_it = mkLamBndrZapper fun (length args)
1023 -- Type-beta reduction
1024 go env (Lam bndr body) (Type ty_arg : args)
1025 = ASSERT( isTyVar bndr )
1026 tick (BetaReduction bndr) `thenSmpl_`
1027 go (extendSubst env bndr (DoneTy ty_arg)) body args
1029 -- Ordinary beta reduction
1030 go env (Lam bndr body) (arg : args)
1031 = tick (BetaReduction bndr) `thenSmpl_`
1032 simplNonRecX env (zap_it bndr) arg $ \ env ->
1035 -- Not enough args, so there are real lambdas left to put in the result
1037 = simplExprF env fun (pushContArgs orig_env args cont)
1038 -- NB: orig_env; the correct environment to capture with
1039 -- the arguments.... env has been augmented with substitutions
1040 -- from the beta reductions.
1042 makeThatCall env var fun args cont
1043 = simplExprF env fun (pushContArgs env args cont)
1047 %************************************************************************
1049 \subsection{Arguments}
1051 %************************************************************************
1054 ---------------------------------------------------------
1055 -- Simplifying the arguments of a call
1057 simplifyArgs :: SimplEnv
1058 -> OutType -- Type of the function
1059 -> [(InExpr, SimplEnv, Bool)] -- Details of the arguments
1060 -> OutType -- Type of the continuation
1061 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1062 -> SimplM FloatsWithExpr
1064 -- [CPS-like because of strict arguments]
1066 -- Simplify the arguments to a call.
1067 -- This part of the simplifier may break the no-shadowing invariant
1069 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1070 -- where f is strict in its second arg
1071 -- If we simplify the innermost one first we get (...(\a -> e)...)
1072 -- Simplifying the second arg makes us float the case out, so we end up with
1073 -- case y of (a,b) -> f (...(\a -> e)...) e'
1074 -- So the output does not have the no-shadowing invariant. However, there is
1075 -- no danger of getting name-capture, because when the first arg was simplified
1076 -- we used an in-scope set that at least mentioned all the variables free in its
1077 -- static environment, and that is enough.
1079 -- We can't just do innermost first, or we'd end up with a dual problem:
1080 -- case x of (a,b) -> f e (...(\a -> e')...)
1082 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1083 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1084 -- continuations. We have to keep the right in-scope set around; AND we have
1085 -- to get the effect that finding (error "foo") in a strict arg position will
1086 -- discard the entire application and replace it with (error "foo"). Getting
1087 -- all this at once is TOO HARD!
1089 simplifyArgs env fn_ty args cont_ty thing_inside
1090 = go env fn_ty args thing_inside
1092 go env fn_ty [] thing_inside = thing_inside env []
1093 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty arg cont_ty $ \ env arg' ->
1094 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1095 thing_inside env (arg':args')
1097 simplifyArg env fn_ty (Type ty_arg, se, _) cont_ty thing_inside
1098 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1099 thing_inside env (Type new_ty_arg)
1101 simplifyArg env fn_ty (val_arg, arg_se, is_strict) cont_ty thing_inside
1103 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1105 | otherwise -- Lazy argument
1106 -- DO NOT float anything outside, hence simplExprC
1107 -- There is no benefit (unlike in a let-binding), and we'd
1108 -- have to be very careful about bogus strictness through
1109 -- floating a demanded let.
1110 = simplExprC (setInScope arg_se env) val_arg
1111 (mkStop arg_ty AnArg) `thenSmpl` \ arg1 ->
1112 thing_inside env arg1
1114 arg_ty = funArgTy fn_ty
1117 simplStrictArg :: LetRhsFlag
1118 -> SimplEnv -- The env of the call
1119 -> InExpr -> SimplEnv -- The arg plus its env
1120 -> OutType -- arg_ty: type of the argument
1121 -> OutType -- cont_ty: Type of thing computed by the context
1122 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1123 -- Takes an expression of type rhs_ty,
1124 -- returns an expression of type cont_ty
1125 -- The env passed to this continuation is the
1126 -- env of the call, plus any new in-scope variables
1127 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1129 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1130 = simplExprF (setInScope arg_env call_env) arg
1131 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1132 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1133 -- to simplify the argument
1134 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1138 %************************************************************************
1140 \subsection{mkAtomicArgs}
1142 %************************************************************************
1144 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1145 constructor application and, if so, converts it to ANF, so that the
1146 resulting thing can be inlined more easily. Thus
1153 There are three sorts of binding context, specified by the two
1159 N N Top-level or recursive Only bind args of lifted type
1161 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1162 but lazy unlifted-and-ok-for-speculation
1164 Y Y Non-top-level, non-recursive, Bind all args
1165 and strict (demanded)
1172 there is no point in transforming to
1174 x = case (y div# z) of r -> MkC r
1176 because the (y div# z) can't float out of the let. But if it was
1177 a *strict* let, then it would be a good thing to do. Hence the
1178 context information.
1181 mkAtomicArgs :: Bool -- A strict binding
1182 -> Bool -- OK to float unlifted args
1184 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1185 OutExpr) -- things that need case-binding,
1186 -- if the strict-binding flag is on
1188 mkAtomicArgs is_strict ok_float_unlifted rhs
1189 | (Var fun, args) <- collectArgs rhs, -- It's an application
1190 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1191 = go fun nilOL [] args -- Have a go
1193 | otherwise = bale_out -- Give up
1196 bale_out = returnSmpl (nilOL, rhs)
1198 go fun binds rev_args []
1199 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1201 go fun binds rev_args (arg : args)
1202 | exprIsTrivial arg -- Easy case
1203 = go fun binds (arg:rev_args) args
1205 | not can_float_arg -- Can't make this arg atomic
1206 = bale_out -- ... so give up
1208 | otherwise -- Don't forget to do it recursively
1209 -- E.g. x = a:b:c:[]
1210 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1211 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1212 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1213 (Var arg_id : rev_args) args
1215 arg_ty = exprType arg
1216 can_float_arg = is_strict
1217 || not (isUnLiftedType arg_ty)
1218 || (ok_float_unlifted && exprOkForSpeculation arg)
1221 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1222 -> (SimplEnv -> SimplM (FloatsWith a))
1223 -> SimplM (FloatsWith a)
1224 addAtomicBinds env [] thing_inside = thing_inside env
1225 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1226 addAtomicBinds env bs thing_inside
1228 addAtomicBindsE :: SimplEnv -> [(OutId,OutExpr)]
1229 -> (SimplEnv -> SimplM FloatsWithExpr)
1230 -> SimplM FloatsWithExpr
1231 -- Same again, but this time we're in an expression context,
1232 -- and may need to do some case bindings
1234 addAtomicBindsE env [] thing_inside
1236 addAtomicBindsE env ((v,r):bs) thing_inside
1237 | needsCaseBinding (idType v) r
1238 = addAtomicBindsE (addNewInScopeIds env [v]) bs thing_inside `thenSmpl` \ (floats, expr) ->
1239 WARN( exprIsTrivial expr, ppr v <+> pprCoreExpr expr )
1240 returnSmpl (emptyFloats env, Case r v [(DEFAULT,[], wrapFloats floats expr)])
1243 = addAuxiliaryBind env (NonRec v r) $ \ env ->
1244 addAtomicBindsE env bs thing_inside
1248 %************************************************************************
1250 \subsection{The main rebuilder}
1252 %************************************************************************
1255 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1257 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1258 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1259 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1260 rebuild env expr (InlinePlease cont) = rebuild env (Note InlineCall expr) cont
1261 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1262 rebuild env expr (ApplyTo _ arg se cont) = rebuildApp (setInScope se env) expr arg cont
1264 rebuildApp env fun arg cont
1265 = simplExpr env arg `thenSmpl` \ arg' ->
1266 rebuild env (App fun arg') cont
1268 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1272 %************************************************************************
1274 \subsection{Functions dealing with a case}
1276 %************************************************************************
1278 Blob of helper functions for the "case-of-something-else" situation.
1281 ---------------------------------------------------------
1282 -- Eliminate the case if possible
1284 rebuildCase :: SimplEnv
1285 -> OutExpr -- Scrutinee
1286 -> InId -- Case binder
1287 -> [InAlt] -- Alternatives
1289 -> SimplM FloatsWithExpr
1291 rebuildCase env scrut case_bndr alts cont
1292 | Just (con,args) <- exprIsConApp_maybe scrut
1293 -- Works when the scrutinee is a variable with a known unfolding
1294 -- as well as when it's an explicit constructor application
1295 = knownCon env (DataAlt con) args case_bndr alts cont
1297 | Lit lit <- scrut -- No need for same treatment as constructors
1298 -- because literals are inlined more vigorously
1299 = knownCon env (LitAlt lit) [] case_bndr alts cont
1302 = prepareAlts scrut case_bndr alts `thenSmpl` \ (better_alts, handled_cons) ->
1304 -- Deal with the case binder, and prepare the continuation;
1305 -- The new subst_env is in place
1306 prepareCaseCont env better_alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1307 addFloats env floats $ \ env ->
1309 -- Deal with variable scrutinee
1310 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr', zap_occ_info) ->
1312 -- Deal with the case alternatives
1313 simplAlts alt_env zap_occ_info handled_cons
1314 case_bndr' better_alts dup_cont `thenSmpl` \ alts' ->
1316 -- Put the case back together
1317 mkCase scrut case_bndr' alts' `thenSmpl` \ case_expr ->
1319 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1320 -- The case binder *not* scope over the whole returned case-expression
1321 rebuild env case_expr nondup_cont
1324 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1325 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1326 way, there's a chance that v will now only be used once, and hence
1331 There is a time we *don't* want to do that, namely when
1332 -fno-case-of-case is on. This happens in the first simplifier pass,
1333 and enhances full laziness. Here's the bad case:
1334 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1335 If we eliminate the inner case, we trap it inside the I# v -> arm,
1336 which might prevent some full laziness happening. I've seen this
1337 in action in spectral/cichelli/Prog.hs:
1338 [(m,n) | m <- [1..max], n <- [1..max]]
1339 Hence the check for NoCaseOfCase.
1343 There is another situation when we don't want to do it. If we have
1345 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1346 ...other cases .... }
1348 We'll perform the binder-swap for the outer case, giving
1350 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1351 ...other cases .... }
1353 But there is no point in doing it for the inner case, because w1 can't
1354 be inlined anyway. Furthermore, doing the case-swapping involves
1355 zapping w2's occurrence info (see paragraphs that follow), and that
1356 forces us to bind w2 when doing case merging. So we get
1358 case x of w1 { A -> let w2 = w1 in e1
1359 B -> let w2 = w1 in e2
1360 ...other cases .... }
1362 This is plain silly in the common case where w2 is dead.
1364 Even so, I can't see a good way to implement this idea. I tried
1365 not doing the binder-swap if the scrutinee was already evaluated
1366 but that failed big-time:
1370 case v of w { MkT x ->
1371 case x of x1 { I# y1 ->
1372 case x of x2 { I# y2 -> ...
1374 Notice that because MkT is strict, x is marked "evaluated". But to
1375 eliminate the last case, we must either make sure that x (as well as
1376 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1377 the binder-swap. So this whole note is a no-op.
1381 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1382 any occurrence info (eg IAmDead) in the case binder, because the
1383 case-binder now effectively occurs whenever v does. AND we have to do
1384 the same for the pattern-bound variables! Example:
1386 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1388 Here, b and p are dead. But when we move the argment inside the first
1389 case RHS, and eliminate the second case, we get
1391 case x or { (a,b) -> a b }
1393 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1394 happened. Hence the zap_occ_info function returned by simplCaseBinder
1397 simplCaseBinder env (Var v) case_bndr
1398 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1400 -- Failed try [see Note 2 above]
1401 -- not (isEvaldUnfolding (idUnfolding v))
1403 = simplBinder env (zap case_bndr) `thenSmpl` \ (env, case_bndr') ->
1404 returnSmpl (modifyInScope env v case_bndr', case_bndr', zap)
1405 -- We could extend the substitution instead, but it would be
1406 -- a hack because then the substitution wouldn't be idempotent
1407 -- any more (v is an OutId). And this just just as well.
1409 zap b = b `setIdOccInfo` NoOccInfo
1411 simplCaseBinder env other_scrut case_bndr
1412 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1413 returnSmpl (env, case_bndr', \ bndr -> bndr) -- NoOp on bndr
1419 simplAlts :: SimplEnv
1420 -> (InId -> InId) -- Occ-info zapper
1421 -> [AltCon] -- Alternatives the scrutinee can't be
1422 -- in the default case
1423 -> OutId -- Case binder
1424 -> [InAlt] -> SimplCont
1425 -> SimplM [OutAlt] -- Includes the continuation
1427 simplAlts env zap_occ_info handled_cons case_bndr' alts cont'
1428 = mapSmpl simpl_alt alts
1430 inst_tys' = tyConAppArgs (idType case_bndr')
1432 simpl_alt (DEFAULT, _, rhs)
1434 -- In the default case we record the constructors that the
1435 -- case-binder *can't* be.
1436 -- We take advantage of any OtherCon info in the case scrutinee
1437 case_bndr_w_unf = case_bndr' `setIdUnfolding` mkOtherCon handled_cons
1438 env_with_unf = modifyInScope env case_bndr' case_bndr_w_unf
1440 simplExprC env_with_unf rhs cont' `thenSmpl` \ rhs' ->
1441 returnSmpl (DEFAULT, [], rhs')
1443 simpl_alt (con, vs, rhs)
1444 = -- Deal with the pattern-bound variables
1445 -- Mark the ones that are in ! positions in the data constructor
1446 -- as certainly-evaluated.
1447 -- NB: it happens that simplBinders does *not* erase the OtherCon
1448 -- form of unfolding, so it's ok to add this info before
1449 -- doing simplBinders
1450 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1452 -- Bind the case-binder to (con args)
1454 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1455 env_with_unf = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` unfolding)
1457 simplExprC env_with_unf rhs cont' `thenSmpl` \ rhs' ->
1458 returnSmpl (con, vs', rhs')
1461 -- add_evals records the evaluated-ness of the bound variables of
1462 -- a case pattern. This is *important*. Consider
1463 -- data T = T !Int !Int
1465 -- case x of { T a b -> T (a+1) b }
1467 -- We really must record that b is already evaluated so that we don't
1468 -- go and re-evaluate it when constructing the result.
1470 add_evals (DataAlt dc) vs = cat_evals dc vs (dataConRepStrictness dc)
1471 add_evals other_con vs = vs
1473 cat_evals dc vs strs
1477 go (v:vs) (str:strs)
1478 | isTyVar v = v : go vs (str:strs)
1479 | isMarkedStrict str = evald_v : go vs strs
1480 | otherwise = zapped_v : go vs strs
1482 zapped_v = zap_occ_info v
1483 evald_v = zapped_v `setIdUnfolding` mkOtherCon []
1484 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1488 %************************************************************************
1490 \subsection{Known constructor}
1492 %************************************************************************
1494 We are a bit careful with occurrence info. Here's an example
1496 (\x* -> case x of (a*, b) -> f a) (h v, e)
1498 where the * means "occurs once". This effectively becomes
1499 case (h v, e) of (a*, b) -> f a)
1501 let a* = h v; b = e in f a
1505 All this should happen in one sweep.
1508 knownCon :: SimplEnv -> AltCon -> [OutExpr]
1509 -> InId -> [InAlt] -> SimplCont
1510 -> SimplM FloatsWithExpr
1512 knownCon env con args bndr alts cont
1513 = tick (KnownBranch bndr) `thenSmpl_`
1514 case findAlt con alts of
1515 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1516 simplNonRecX env bndr scrut $ \ env ->
1517 -- This might give rise to a binding with non-atomic args
1518 -- like x = Node (f x) (g x)
1519 -- but no harm will be done
1520 simplExprF env rhs cont
1523 LitAlt lit -> Lit lit
1524 DataAlt dc -> mkConApp dc args
1526 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1527 simplNonRecX env bndr (Lit lit) $ \ env ->
1528 simplExprF env rhs cont
1530 (DataAlt dc, bs, rhs) -> ASSERT( length bs + n_tys == length args )
1531 bind_args env bs (drop n_tys args) $ \ env ->
1533 con_app = mkConApp dc (take n_tys args ++ con_args)
1534 con_args = [substExpr (getSubst env) (varToCoreExpr b) | b <- bs]
1535 -- args are aready OutExprs, but bs are InIds
1537 simplNonRecX env bndr con_app $ \ env ->
1538 simplExprF env rhs cont
1540 n_tys = dataConNumInstArgs dc -- Non-existential type args
1542 bind_args env [] _ thing_inside = thing_inside env
1544 bind_args env (b:bs) (Type ty : args) thing_inside
1545 = bind_args (extendSubst env b (DoneTy ty)) bs args thing_inside
1547 bind_args env (b:bs) (arg : args) thing_inside
1548 = simplNonRecX env b arg $ \ env ->
1549 bind_args env bs args thing_inside
1553 %************************************************************************
1555 \subsection{Duplicating continuations}
1557 %************************************************************************
1560 prepareCaseCont :: SimplEnv
1561 -> [InAlt] -> SimplCont
1562 -> SimplM (FloatsWith (SimplCont,SimplCont))
1563 -- Return a duplicatable continuation, a non-duplicable part
1564 -- plus some extra bindings
1566 -- No need to make it duplicatable if there's only one alternative
1567 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1568 prepareCaseCont env alts cont = mkDupableCont env cont
1572 mkDupableCont :: SimplEnv -> SimplCont
1573 -> SimplM (FloatsWith (SimplCont, SimplCont))
1575 mkDupableCont env cont
1576 | contIsDupable cont
1577 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1579 mkDupableCont env (CoerceIt ty cont)
1580 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1581 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1583 mkDupableCont env (InlinePlease cont)
1584 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1585 returnSmpl (floats, (InlinePlease dup_cont, nondup_cont))
1587 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1588 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1589 -- Do *not* duplicate an ArgOf continuation
1590 -- Because ArgOf continuations are opaque, we gain nothing by
1591 -- propagating them into the expressions, and we do lose a lot.
1592 -- Here's an example:
1593 -- && (case x of { T -> F; F -> T }) E
1594 -- Now, && is strict so we end up simplifying the case with
1595 -- an ArgOf continuation. If we let-bind it, we get
1597 -- let $j = \v -> && v E
1598 -- in simplExpr (case x of { T -> F; F -> T })
1599 -- (ArgOf (\r -> $j r)
1600 -- And after simplifying more we get
1602 -- let $j = \v -> && v E
1603 -- in case of { T -> $j F; F -> $j T }
1604 -- Which is a Very Bad Thing
1606 -- The desire not to duplicate is the entire reason that
1607 -- mkDupableCont returns a pair of continuations.
1609 -- The original plan had:
1610 -- e.g. (...strict-fn...) [...hole...]
1612 -- let $j = \a -> ...strict-fn...
1613 -- in $j [...hole...]
1615 mkDupableCont env (ApplyTo _ arg se cont)
1616 = -- e.g. [...hole...] (...arg...)
1618 -- let a = ...arg...
1619 -- in [...hole...] a
1620 simplExpr (setInScope se env) arg `thenSmpl` \ arg' ->
1622 mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1623 addFloats env floats $ \ env ->
1625 if exprIsDupable arg' then
1626 returnSmpl (emptyFloats env, (ApplyTo OkToDup arg' (zapSubstEnv se) dup_cont, nondup_cont))
1628 newId FSLIT("a") (exprType arg') `thenSmpl` \ arg_id ->
1630 tick (CaseOfCase arg_id) `thenSmpl_`
1631 -- Want to tick here so that we go round again,
1632 -- and maybe copy or inline the code.
1633 -- Not strictly CaseOfCase, but never mind
1635 returnSmpl (unitFloat env arg_id arg',
1636 (ApplyTo OkToDup (Var arg_id) (zapSubstEnv se) dup_cont,
1638 -- But what if the arg should be case-bound?
1639 -- This has been this way for a long time, so I'll leave it,
1640 -- but I can't convince myself that it's right.
1643 mkDupableCont env (Select _ case_bndr alts se cont)
1644 = -- e.g. (case [...hole...] of { pi -> ei })
1646 -- let ji = \xij -> ei
1647 -- in case [...hole...] of { pi -> ji xij }
1648 tick (CaseOfCase case_bndr) `thenSmpl_`
1650 alt_env = setInScope se env
1652 prepareCaseCont alt_env alts cont `thenSmpl` \ (floats1, (dup_cont, nondup_cont)) ->
1653 addFloats alt_env floats1 $ \ alt_env ->
1655 simplBinder alt_env case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1656 -- NB: simplBinder does not zap deadness occ-info, so
1657 -- a dead case_bndr' will still advertise its deadness
1658 -- This is really important because in
1659 -- case e of b { (# a,b #) -> ... }
1660 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1661 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1662 -- In the new alts we build, we have the new case binder, so it must retain
1665 mkDupableAlts alt_env case_bndr' alts dup_cont `thenSmpl` \ (floats2, alts') ->
1666 addFloats alt_env floats2 $ \ alt_env ->
1667 returnSmpl (emptyFloats alt_env,
1668 (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1669 (mkBoringStop (contResultType dup_cont)),
1672 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1673 -> SimplM (FloatsWith [InAlt])
1674 -- Absorbs the continuation into the new alternatives
1676 mkDupableAlts env case_bndr' alts dupable_cont
1679 go env [] = returnSmpl (emptyFloats env, [])
1681 = mkDupableAlt env case_bndr' dupable_cont alt `thenSmpl` \ (floats1, alt') ->
1682 addFloats env floats1 $ \ env ->
1683 go env alts `thenSmpl` \ (floats2, alts') ->
1684 returnSmpl (floats2, alt' : alts')
1686 mkDupableAlt env case_bndr' cont alt@(con, bndrs, rhs)
1687 = simplBinders env bndrs `thenSmpl` \ (env, bndrs') ->
1688 simplExprC env rhs cont `thenSmpl` \ rhs' ->
1690 if exprIsDupable rhs' then
1691 returnSmpl (emptyFloats env, (con, bndrs', rhs'))
1692 -- It is worth checking for a small RHS because otherwise we
1693 -- get extra let bindings that may cause an extra iteration of the simplifier to
1694 -- inline back in place. Quite often the rhs is just a variable or constructor.
1695 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1696 -- iterations because the version with the let bindings looked big, and so wasn't
1697 -- inlined, but after the join points had been inlined it looked smaller, and so
1700 -- NB: we have to check the size of rhs', not rhs.
1701 -- Duplicating a small InAlt might invalidate occurrence information
1702 -- However, if it *is* dupable, we return the *un* simplified alternative,
1703 -- because otherwise we'd need to pair it up with an empty subst-env....
1704 -- but we only have one env shared between all the alts.
1705 -- (Remember we must zap the subst-env before re-simplifying something).
1706 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1710 rhs_ty' = exprType rhs'
1711 used_bndrs' = filter (not . isDeadBinder) (case_bndr' : bndrs')
1712 -- The deadness info on the new binders is unscathed
1714 -- If we try to lift a primitive-typed something out
1715 -- for let-binding-purposes, we will *caseify* it (!),
1716 -- with potentially-disastrous strictness results. So
1717 -- instead we turn it into a function: \v -> e
1718 -- where v::State# RealWorld#. The value passed to this function
1719 -- is realworld#, which generates (almost) no code.
1721 -- There's a slight infelicity here: we pass the overall
1722 -- case_bndr to all the join points if it's used in *any* RHS,
1723 -- because we don't know its usage in each RHS separately
1725 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1726 -- we make the join point into a function whenever used_bndrs'
1727 -- is empty. This makes the join-point more CPR friendly.
1728 -- Consider: let j = if .. then I# 3 else I# 4
1729 -- in case .. of { A -> j; B -> j; C -> ... }
1731 -- Now CPR doesn't w/w j because it's a thunk, so
1732 -- that means that the enclosing function can't w/w either,
1733 -- which is a lose. Here's the example that happened in practice:
1734 -- kgmod :: Int -> Int -> Int
1735 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1739 -- I have seen a case alternative like this:
1740 -- True -> \v -> ...
1741 -- It's a bit silly to add the realWorld dummy arg in this case, making
1744 -- (the \v alone is enough to make CPR happy) but I think it's rare
1746 ( if null used_bndrs'
1747 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
1748 returnSmpl ([rw_id], [Var realWorldPrimId])
1750 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
1751 ) `thenSmpl` \ (final_bndrs', final_args) ->
1753 -- See comment about "$j" name above
1754 newId (encodeFS FSLIT("$j")) (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
1755 -- Notice the funky mkPiTypes. If the contructor has existentials
1756 -- it's possible that the join point will be abstracted over
1757 -- type varaibles as well as term variables.
1758 -- Example: Suppose we have
1759 -- data T = forall t. C [t]
1761 -- case (case e of ...) of
1762 -- C t xs::[t] -> rhs
1763 -- We get the join point
1764 -- let j :: forall t. [t] -> ...
1765 -- j = /\t \xs::[t] -> rhs
1767 -- case (case e of ...) of
1768 -- C t xs::[t] -> j t xs
1770 -- We make the lambdas into one-shot-lambdas. The
1771 -- join point is sure to be applied at most once, and doing so
1772 -- prevents the body of the join point being floated out by
1773 -- the full laziness pass
1774 really_final_bndrs = map one_shot final_bndrs'
1775 one_shot v | isId v = setOneShotLambda v
1777 join_rhs = mkLams really_final_bndrs rhs'
1778 join_call = mkApps (Var join_bndr) final_args
1780 returnSmpl (unitFloat env join_bndr join_rhs, (con, bndrs', join_call))