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 DynFlags ( dopt, DynFlag(Opt_D_dump_inlinings),
16 import SimplUtils ( mkCase, mkLam, prepareAlts,
17 SimplCont(..), DupFlag(..), LetRhsFlag(..),
18 mkRhsStop, mkBoringStop, pushContArgs,
19 contResultType, countArgs, contIsDupable, contIsRhsOrArg,
20 getContArgs, interestingCallContext, interestingArg, isStrictType,
21 preInlineUnconditionally, postInlineUnconditionally,
22 inlineMode, activeInline, activeRule
24 import Id ( Id, idType, idInfo, idArity, isDataConWorkId,
25 setIdUnfolding, isDeadBinder,
26 idNewDemandInfo, setIdInfo,
27 setIdOccInfo, zapLamIdInfo, setOneShotLambda,
29 import MkId ( eRROR_ID )
30 import Literal ( mkStringLit )
31 import OccName ( encodeFS )
32 import IdInfo ( OccInfo(..), isLoopBreaker,
33 setArityInfo, zapDemandInfo,
37 import NewDemand ( isStrictDmd )
38 import Unify ( coreRefineTys )
39 import DataCon ( dataConTyCon, dataConRepStrictness, isVanillaDataCon )
40 import TyCon ( tyConArity )
42 import PprCore ( pprParendExpr, pprCoreExpr )
43 import CoreUnfold ( mkOtherCon, mkUnfolding, evaldUnfolding, callSiteInline )
44 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
45 exprIsConApp_maybe, mkPiTypes, findAlt,
46 exprType, exprIsValue,
47 exprOkForSpeculation, exprArity,
48 mkCoerce, mkCoerce2, mkSCC, mkInlineMe, applyTypeToArg
50 import Rules ( lookupRule )
51 import BasicTypes ( isMarkedStrict )
52 import CostCentre ( currentCCS )
53 import Type ( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
54 splitFunTy_maybe, splitFunTy, coreEqType
56 import VarEnv ( elemVarEnv )
57 import TysPrim ( realWorldStatePrimTy )
58 import PrelInfo ( realWorldPrimId )
59 import BasicTypes ( TopLevelFlag(..), isTopLevel,
63 import Maybe ( Maybe )
64 import Maybes ( orElse )
66 import Util ( notNull )
70 The guts of the simplifier is in this module, but the driver loop for
71 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.
82 -----------------------------------------
83 *** IMPORTANT NOTE ***
84 -----------------------------------------
85 Many parts of the simplifier return a bunch of "floats" as well as an
86 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
88 All "floats" are let-binds, not case-binds, but some non-rec lets may
89 be unlifted (with RHS ok-for-speculation).
93 -----------------------------------------
94 ORGANISATION OF FUNCTIONS
95 -----------------------------------------
97 - simplify all top-level binders
98 - for NonRec, call simplRecOrTopPair
99 - for Rec, call simplRecBind
102 ------------------------------
103 simplExpr (applied lambda) ==> simplNonRecBind
104 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
105 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
107 ------------------------------
108 simplRecBind [binders already simplfied]
109 - use simplRecOrTopPair on each pair in turn
111 simplRecOrTopPair [binder already simplified]
112 Used for: recursive bindings (top level and nested)
113 top-level non-recursive bindings
115 - check for PreInlineUnconditionally
119 Used for: non-top-level non-recursive bindings
120 beta reductions (which amount to the same thing)
121 Because it can deal with strict arts, it takes a
122 "thing-inside" and returns an expression
124 - check for PreInlineUnconditionally
125 - simplify binder, including its IdInfo
134 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
135 Used for: binding case-binder and constr args in a known-constructor case
136 - check for PreInLineUnconditionally
140 ------------------------------
141 simplLazyBind: [binder already simplified, RHS not]
142 Used for: recursive bindings (top level and nested)
143 top-level non-recursive bindings
144 non-top-level, but *lazy* non-recursive bindings
145 [must not be strict or unboxed]
146 Returns floats + an augmented environment, not an expression
147 - substituteIdInfo and add result to in-scope
148 [so that rules are available in rec rhs]
151 - float if exposes constructor or PAP
155 completeNonRecX: [binder and rhs both simplified]
156 - if the the thing needs case binding (unlifted and not ok-for-spec)
162 completeLazyBind: [given a simplified RHS]
163 [used for both rec and non-rec bindings, top level and not]
164 - try PostInlineUnconditionally
165 - add unfolding [this is the only place we add an unfolding]
170 Right hand sides and arguments
171 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
172 In many ways we want to treat
173 (a) the right hand side of a let(rec), and
174 (b) a function argument
175 in the same way. But not always! In particular, we would
176 like to leave these arguments exactly as they are, so they
177 will match a RULE more easily.
182 It's harder to make the rule match if we ANF-ise the constructor,
183 or eta-expand the PAP:
185 f (let { a = g x; b = h x } in (a,b))
188 On the other hand if we see the let-defns
193 then we *do* want to ANF-ise and eta-expand, so that p and q
194 can be safely inlined.
196 Even floating lets out is a bit dubious. For let RHS's we float lets
197 out if that exposes a value, so that the value can be inlined more vigorously.
200 r = let x = e in (x,x)
202 Here, if we float the let out we'll expose a nice constructor. We did experiments
203 that showed this to be a generally good thing. But it was a bad thing to float
204 lets out unconditionally, because that meant they got allocated more often.
206 For function arguments, there's less reason to expose a constructor (it won't
207 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
208 So for the moment we don't float lets out of function arguments either.
213 For eta expansion, we want to catch things like
215 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
217 If the \x was on the RHS of a let, we'd eta expand to bring the two
218 lambdas together. And in general that's a good thing to do. Perhaps
219 we should eta expand wherever we find a (value) lambda? Then the eta
220 expansion at a let RHS can concentrate solely on the PAP case.
223 %************************************************************************
225 \subsection{Bindings}
227 %************************************************************************
230 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
232 simplTopBinds env binds
233 = -- Put all the top-level binders into scope at the start
234 -- so that if a transformation rule has unexpectedly brought
235 -- anything into scope, then we don't get a complaint about that.
236 -- It's rather as if the top-level binders were imported.
237 simplLetBndrs env (bindersOfBinds binds) `thenSmpl` \ (env, bndrs') ->
238 simpl_binds env binds bndrs' `thenSmpl` \ (floats, _) ->
239 freeTick SimplifierDone `thenSmpl_`
240 returnSmpl (floatBinds floats)
242 -- We need to track the zapped top-level binders, because
243 -- they should have their fragile IdInfo zapped (notably occurrence info)
244 -- That's why we run down binds and bndrs' simultaneously.
245 simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
246 simpl_binds env [] bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
247 simpl_binds env (bind:binds) bs = simpl_bind env bind bs `thenSmpl` \ (floats,env) ->
248 addFloats env floats $ \env ->
249 simpl_binds env binds (drop_bs bind bs)
251 drop_bs (NonRec _ _) (_ : bs) = bs
252 drop_bs (Rec prs) bs = drop (length prs) bs
254 simpl_bind env bind bs
255 = getDOptsSmpl `thenSmpl` \ dflags ->
256 if dopt Opt_D_dump_inlinings dflags then
257 pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
259 simpl_bind1 env bind bs
261 simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
262 simpl_bind1 env (Rec pairs) bs' = simplRecBind env TopLevel pairs bs'
266 %************************************************************************
268 \subsection{simplNonRec}
270 %************************************************************************
272 simplNonRecBind is used for
273 * non-top-level non-recursive lets in expressions
277 * An unsimplified (binder, rhs) pair
278 * The env for the RHS. It may not be the same as the
279 current env because the bind might occur via (\x.E) arg
281 It uses the CPS form because the binding might be strict, in which
282 case we might discard the continuation:
283 let x* = error "foo" in (...x...)
285 It needs to turn unlifted bindings into a @case@. They can arise
286 from, say: (\x -> e) (4# + 3#)
289 simplNonRecBind :: SimplEnv
291 -> InExpr -> SimplEnv -- Arg, with its subst-env
292 -> OutType -- Type of thing computed by the context
293 -> (SimplEnv -> SimplM FloatsWithExpr) -- The body
294 -> SimplM FloatsWithExpr
296 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
298 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
301 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
302 | preInlineUnconditionally env NotTopLevel bndr
303 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
304 thing_inside (extendIdSubst env bndr (mkContEx rhs_se rhs))
307 | isStrictDmd (idNewDemandInfo bndr) || isStrictType (idType bndr) -- A strict let
308 = -- Don't use simplBinder because that doesn't keep
309 -- fragile occurrence info in the substitution
310 simplLetBndr env bndr `thenSmpl` \ (env, bndr1) ->
311 simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty $ \ env1 rhs1 ->
313 -- Now complete the binding and simplify the body
315 -- simplLetBndr doesn't deal with the IdInfo, so we must
316 -- do so here (c.f. simplLazyBind)
317 bndr2 = bndr1 `setIdInfo` simplIdInfo env (idInfo bndr)
318 env2 = modifyInScope env1 bndr2 bndr2
320 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
322 | otherwise -- Normal, lazy case
323 = -- Don't use simplBinder because that doesn't keep
324 -- fragile occurrence info in the substitution
325 simplLetBndr env bndr `thenSmpl` \ (env, bndr') ->
326 simplLazyBind env NotTopLevel NonRecursive
327 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
328 addFloats env floats thing_inside
331 A specialised variant of simplNonRec used when the RHS is already simplified, notably
332 in knownCon. It uses case-binding where necessary.
335 simplNonRecX :: SimplEnv
336 -> InId -- Old binder
337 -> OutExpr -- Simplified RHS
338 -> (SimplEnv -> SimplM FloatsWithExpr)
339 -> SimplM FloatsWithExpr
341 simplNonRecX env bndr new_rhs thing_inside
342 | needsCaseBinding (idType bndr) new_rhs
343 -- Make this test *before* the preInlineUnconditionally
344 -- Consider case I# (quotInt# x y) of
345 -- I# v -> let w = J# v in ...
346 -- If we gaily inline (quotInt# x y) for v, we end up building an
348 -- let w = J# (quotInt# x y) in ...
349 -- because quotInt# can fail.
350 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
351 thing_inside env `thenSmpl` \ (floats, body) ->
352 let body' = wrapFloats floats body in
353 returnSmpl (emptyFloats env, Case new_rhs bndr' (exprType body') [(DEFAULT, [], body')])
355 | preInlineUnconditionally env NotTopLevel bndr
356 -- This happens; for example, the case_bndr during case of
357 -- known constructor: case (a,b) of x { (p,q) -> ... }
358 -- Here x isn't mentioned in the RHS, so we don't want to
359 -- create the (dead) let-binding let x = (a,b) in ...
361 -- Similarly, single occurrences can be inlined vigourously
362 -- e.g. case (f x, g y) of (a,b) -> ....
363 -- If a,b occur once we can avoid constructing the let binding for them.
364 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
367 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
368 completeNonRecX env False {- Non-strict; pessimistic -}
369 bndr bndr' new_rhs thing_inside
371 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
372 = mkAtomicArgs is_strict
373 True {- OK to float unlifted -}
374 new_rhs `thenSmpl` \ (aux_binds, rhs2) ->
376 -- Make the arguments atomic if necessary,
377 -- adding suitable bindings
378 addAtomicBindsE env (fromOL aux_binds) $ \ env ->
379 completeLazyBind env NotTopLevel
380 old_bndr new_bndr rhs2 `thenSmpl` \ (floats, env) ->
381 addFloats env floats thing_inside
385 %************************************************************************
387 \subsection{Lazy bindings}
389 %************************************************************************
391 simplRecBind is used for
392 * recursive bindings only
395 simplRecBind :: SimplEnv -> TopLevelFlag
396 -> [(InId, InExpr)] -> [OutId]
397 -> SimplM (FloatsWith SimplEnv)
398 simplRecBind env top_lvl pairs bndrs'
399 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
400 returnSmpl (flattenFloats floats, env)
402 go env [] _ = returnSmpl (emptyFloats env, env)
404 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
405 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
406 addFloats env floats (\env -> go env pairs bndrs')
410 simplRecOrTopPair is used for
411 * recursive bindings (whether top level or not)
412 * top-level non-recursive bindings
414 It assumes the binder has already been simplified, but not its IdInfo.
417 simplRecOrTopPair :: SimplEnv
419 -> InId -> OutId -- Binder, both pre-and post simpl
420 -> InExpr -- The RHS and its environment
421 -> SimplM (FloatsWith SimplEnv)
423 simplRecOrTopPair env top_lvl bndr bndr' rhs
424 | preInlineUnconditionally env top_lvl bndr -- Check for unconditional inline
425 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
426 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
429 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
430 -- May not actually be recursive, but it doesn't matter
434 simplLazyBind is used for
435 * recursive bindings (whether top level or not)
436 * top-level non-recursive bindings
437 * non-top-level *lazy* non-recursive bindings
439 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
440 from SimplRecOrTopBind]
443 1. It assumes that the binder is *already* simplified,
444 and is in scope, but not its IdInfo
446 2. It assumes that the binder type is lifted.
448 3. It does not check for pre-inline-unconditionallly;
449 that should have been done already.
452 simplLazyBind :: SimplEnv
453 -> TopLevelFlag -> RecFlag
454 -> InId -> OutId -- Binder, both pre-and post simpl
455 -> InExpr -> SimplEnv -- The RHS and its environment
456 -> SimplM (FloatsWith SimplEnv)
458 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
459 = let -- Transfer the IdInfo of the original binder to the new binder
460 -- This is crucial: we must preserve
464 -- etc. To do this we must apply the current substitution,
465 -- which incorporates earlier substitutions in this very letrec group.
467 -- NB 1. We do this *before* processing the RHS of the binder, so that
468 -- its substituted rules are visible in its own RHS.
469 -- This is important. Manuel found cases where he really, really
470 -- wanted a RULE for a recursive function to apply in that function's
471 -- own right-hand side.
473 -- NB 2: We do not transfer the arity (see Subst.substIdInfo)
474 -- The arity of an Id should not be visible
475 -- in its own RHS, else we eta-reduce
479 -- which isn't sound. And it makes the arity in f's IdInfo greater than
480 -- the manifest arity, which isn't good.
481 -- The arity will get added later.
483 -- NB 3: It's important that we *do* transer the loop-breaker OccInfo,
484 -- because that's what stops the Id getting inlined infinitely, in the body
487 -- NB 4: does no harm for non-recursive bindings
489 bndr2 = bndr1 `setIdInfo` simplIdInfo env (idInfo bndr)
490 env1 = modifyInScope env bndr2 bndr2
491 rhs_env = setInScope rhs_se env1
492 is_top_level = isTopLevel top_lvl
493 ok_float_unlifted = not is_top_level && isNonRec is_rec
494 rhs_cont = mkRhsStop (idType bndr1)
496 -- Simplify the RHS; note the mkRhsStop, which tells
497 -- the simplifier that this is the RHS of a let.
498 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
500 -- If any of the floats can't be floated, give up now
501 -- (The allLifted predicate says True for empty floats.)
502 if (not ok_float_unlifted && not (allLifted floats)) then
503 completeLazyBind env1 top_lvl bndr bndr2
504 (wrapFloats floats rhs1)
507 -- ANF-ise a constructor or PAP rhs
508 mkAtomicArgs False {- Not strict -}
509 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
511 -- If the result is a PAP, float the floats out, else wrap them
512 -- By this time it's already been ANF-ised (if necessary)
513 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
514 completeLazyBind env1 top_lvl bndr bndr2 rhs2
516 else if is_top_level || exprIsTrivial rhs2 || exprIsValue rhs2 then
517 -- WARNING: long dodgy argument coming up
518 -- WANTED: a better way to do this
520 -- We can't use "exprIsCheap" instead of exprIsValue,
521 -- because that causes a strictness bug.
522 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
523 -- The case expression is 'cheap', but it's wrong to transform to
524 -- y* = E; x = case (scc y) of {...}
525 -- Either we must be careful not to float demanded non-values, or
526 -- we must use exprIsValue for the test, which ensures that the
527 -- thing is non-strict. So exprIsValue => bindings are non-strict
528 -- I think. The WARN below tests for this.
530 -- We use exprIsTrivial here because we want to reveal lone variables.
531 -- E.g. let { x = letrec { y = E } in y } in ...
532 -- Here we definitely want to float the y=E defn.
533 -- exprIsValue definitely isn't right for that.
535 -- Again, the floated binding can't be strict; if it's recursive it'll
536 -- be non-strict; if it's non-recursive it'd be inlined.
538 -- Note [SCC-and-exprIsTrivial]
540 -- y = let { x* = E } in scc "foo" x
541 -- then we do *not* want to float out the x binding, because
542 -- it's strict! Fortunately, exprIsTrivial replies False to
545 -- There's a subtlety here. There may be a binding (x* = e) in the
546 -- floats, where the '*' means 'will be demanded'. So is it safe
547 -- to float it out? Answer no, but it won't matter because
548 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
549 -- and so there can't be any 'will be demanded' bindings in the floats.
551 ASSERT2( is_top_level || not (any demanded_float (floatBinds floats)),
552 ppr (filter demanded_float (floatBinds floats)) )
554 tick LetFloatFromLet `thenSmpl_` (
555 addFloats env1 floats $ \ env2 ->
556 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
557 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
560 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
563 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
564 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
565 demanded_float (Rec _) = False
570 %************************************************************************
572 \subsection{Completing a lazy binding}
574 %************************************************************************
577 * deals only with Ids, not TyVars
578 * takes an already-simplified binder and RHS
579 * is used for both recursive and non-recursive bindings
580 * is used for both top-level and non-top-level bindings
582 It does the following:
583 - tries discarding a dead binding
584 - tries PostInlineUnconditionally
585 - add unfolding [this is the only place we add an unfolding]
588 It does *not* attempt to do let-to-case. Why? Because it is used for
589 - top-level bindings (when let-to-case is impossible)
590 - many situations where the "rhs" is known to be a WHNF
591 (so let-to-case is inappropriate).
594 completeLazyBind :: SimplEnv
595 -> TopLevelFlag -- Flag stuck into unfolding
596 -> InId -- Old binder
597 -> OutId -- New binder
598 -> OutExpr -- Simplified RHS
599 -> SimplM (FloatsWith SimplEnv)
600 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
601 -- by extending the substitution (e.g. let x = y in ...)
602 -- The new binding (if any) is returned as part of the floats.
603 -- NB: the returned SimplEnv has the right SubstEnv, but you should
604 -- (as usual) use the in-scope-env from the floats
606 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
607 | postInlineUnconditionally env new_bndr occ_info new_rhs
608 = -- Drop the binding
609 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
610 returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
611 -- Use the substitution to make quite, quite sure that the substitution
612 -- will happen, since we are going to discard the binding
617 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
619 -- Add the unfolding *only* for non-loop-breakers
620 -- Making loop breakers not have an unfolding at all
621 -- means that we can avoid tests in exprIsConApp, for example.
622 -- This is important: if exprIsConApp says 'yes' for a recursive
623 -- thing, then we can get into an infinite loop
625 -- If the unfolding is a value, the demand info may
626 -- go pear-shaped, so we nuke it. Example:
628 -- case x of (p,q) -> h p q x
629 -- Here x is certainly demanded. But after we've nuked
630 -- the case, we'll get just
631 -- let x = (a,b) in h a b x
632 -- and now x is not demanded (I'm assuming h is lazy)
633 -- This really happens. Similarly
634 -- let f = \x -> e in ...f..f...
635 -- After inling f at some of its call sites the original binding may
636 -- (for example) be no longer strictly demanded.
637 -- The solution here is a bit ad hoc...
638 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
639 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
640 final_info | loop_breaker = new_bndr_info
641 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
642 | otherwise = info_w_unf
644 final_id = new_bndr `setIdInfo` final_info
646 -- These seqs forces the Id, and hence its IdInfo,
647 -- and hence any inner substitutions
649 returnSmpl (unitFloat env final_id new_rhs, env)
652 loop_breaker = isLoopBreaker occ_info
653 old_info = idInfo old_bndr
654 occ_info = occInfo old_info
659 %************************************************************************
661 \subsection[Simplify-simplExpr]{The main function: simplExpr}
663 %************************************************************************
665 The reason for this OutExprStuff stuff is that we want to float *after*
666 simplifying a RHS, not before. If we do so naively we get quadratic
667 behaviour as things float out.
669 To see why it's important to do it after, consider this (real) example:
683 a -- Can't inline a this round, cos it appears twice
687 Each of the ==> steps is a round of simplification. We'd save a
688 whole round if we float first. This can cascade. Consider
693 let f = let d1 = ..d.. in \y -> e
697 in \x -> ...(\y ->e)...
699 Only in this second round can the \y be applied, and it
700 might do the same again.
704 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
705 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
707 expr_ty' = substTy env (exprType expr)
708 -- The type in the Stop continuation, expr_ty', is usually not used
709 -- It's only needed when discarding continuations after finding
710 -- a function that returns bottom.
711 -- Hence the lazy substitution
714 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
715 -- Simplify an expression, given a continuation
716 simplExprC env expr cont
717 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
718 returnSmpl (wrapFloats floats expr)
720 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
721 -- Simplify an expression, returning floated binds
723 simplExprF env (Var v) cont = simplVar env v cont
724 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
725 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
726 simplExprF env (Note note expr) cont = simplNote env note expr cont
727 simplExprF env (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg env cont)
729 simplExprF env (Type ty) cont
730 = ASSERT( contIsRhsOrArg cont )
731 simplType env ty `thenSmpl` \ ty' ->
732 rebuild env (Type ty') cont
734 simplExprF env (Case scrut bndr case_ty alts) cont
735 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
736 = -- Simplify the scrutinee with a Select continuation
737 simplExprF env scrut (Select NoDup bndr alts env cont)
740 = -- If case-of-case is off, simply simplify the case expression
741 -- in a vanilla Stop context, and rebuild the result around it
742 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
743 rebuild env case_expr' cont
745 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
746 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
748 simplExprF env (Let (Rec pairs) body) cont
749 = simplLetBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
750 -- NB: bndrs' don't have unfoldings or rules
751 -- We add them as we go down
753 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
754 addFloats env floats $ \ env ->
755 simplExprF env body cont
757 -- A non-recursive let is dealt with by simplNonRecBind
758 simplExprF env (Let (NonRec bndr rhs) body) cont
759 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
760 simplExprF env body cont
763 ---------------------------------
764 simplType :: SimplEnv -> InType -> SimplM OutType
765 -- Kept monadic just so we can do the seqType
767 = seqType new_ty `seq` returnSmpl new_ty
769 new_ty = substTy env ty
773 %************************************************************************
777 %************************************************************************
780 simplLam env fun cont
783 zap_it = mkLamBndrZapper fun (countArgs cont)
784 cont_ty = contResultType cont
786 -- Type-beta reduction
787 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
788 = ASSERT( isTyVar bndr )
789 tick (BetaReduction bndr) `thenSmpl_`
790 simplType (setInScope arg_se env) ty_arg `thenSmpl` \ ty_arg' ->
791 go (extendTvSubst env bndr ty_arg') body body_cont
793 -- Ordinary beta reduction
794 go env (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
795 = tick (BetaReduction bndr) `thenSmpl_`
796 simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
797 go env body body_cont
799 -- Not enough args, so there are real lambdas left to put in the result
800 go env lam@(Lam _ _) cont
801 = simplLamBndrs env bndrs `thenSmpl` \ (env, bndrs') ->
802 simplExpr env body `thenSmpl` \ body' ->
803 mkLam env bndrs' body' cont `thenSmpl` \ (floats, new_lam) ->
804 addFloats env floats $ \ env ->
805 rebuild env new_lam cont
807 (bndrs,body) = collectBinders lam
809 -- Exactly enough args
810 go env expr cont = simplExprF env expr cont
812 mkLamBndrZapper :: CoreExpr -- Function
813 -> Int -- Number of args supplied, *including* type args
814 -> Id -> Id -- Use this to zap the binders
815 mkLamBndrZapper fun n_args
816 | n_args >= n_params fun = \b -> b -- Enough args
817 | otherwise = \b -> zapLamIdInfo b
819 -- NB: we count all the args incl type args
820 -- so we must count all the binders (incl type lambdas)
821 n_params (Note _ e) = n_params e
822 n_params (Lam b e) = 1 + n_params e
823 n_params other = 0::Int
827 %************************************************************************
831 %************************************************************************
834 simplNote env (Coerce to from) body cont
836 addCoerce s1 k1 (CoerceIt t1 cont)
837 -- coerce T1 S1 (coerce S1 K1 e)
840 -- coerce T1 K1 e, otherwise
842 -- For example, in the initial form of a worker
843 -- we may find (coerce T (coerce S (\x.e))) y
844 -- and we'd like it to simplify to e[y/x] in one round
846 | t1 `coreEqType` k1 = cont -- The coerces cancel out
847 | otherwise = CoerceIt t1 cont -- They don't cancel, but
848 -- the inner one is redundant
850 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
851 | not (isTypeArg arg), -- This whole case only works for value args
852 -- Could upgrade to have equiv thing for type apps too
853 Just (s1, s2) <- splitFunTy_maybe s1s2
854 -- (coerce (T1->T2) (S1->S2) F) E
856 -- coerce T2 S2 (F (coerce S1 T1 E))
858 -- t1t2 must be a function type, T1->T2, because it's applied to something
859 -- but s1s2 might conceivably not be
861 -- When we build the ApplyTo we can't mix the out-types
862 -- with the InExpr in the argument, so we simply substitute
863 -- to make it all consistent. It's a bit messy.
864 -- But it isn't a common case.
866 (t1,t2) = splitFunTy t1t2
867 new_arg = mkCoerce2 s1 t1 (substExpr arg_env arg)
868 arg_env = setInScope arg_se env
870 ApplyTo dup new_arg (zapSubstEnv env) (addCoerce t2 s2 cont)
872 addCoerce to' _ cont = CoerceIt to' cont
874 simplType env to `thenSmpl` \ to' ->
875 simplType env from `thenSmpl` \ from' ->
876 simplExprF env body (addCoerce to' from' cont)
879 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
880 -- inlining. All other CCCSs are mapped to currentCCS.
881 simplNote env (SCC cc) e cont
882 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
883 rebuild env (mkSCC cc e') cont
885 simplNote env InlineCall e cont
886 = simplExprF env e (InlinePlease cont)
888 -- See notes with SimplMonad.inlineMode
889 simplNote env InlineMe e cont
890 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
891 = -- Don't inline inside an INLINE expression
892 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
893 rebuild env (mkInlineMe e') cont
895 | otherwise -- Dissolve the InlineMe note if there's
896 -- an interesting context of any kind to combine with
897 -- (even a type application -- anything except Stop)
898 = simplExprF env e cont
900 simplNote env (CoreNote s) e cont
901 = simplExpr env e `thenSmpl` \ e' ->
902 rebuild env (Note (CoreNote s) e') cont
906 %************************************************************************
908 \subsection{Dealing with calls}
910 %************************************************************************
913 simplVar env var cont
914 = case substId env var of
915 DoneEx e -> simplExprF (zapSubstEnv env) e cont
916 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
917 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
918 -- Note [zapSubstEnv]
919 -- The template is already simplified, so don't re-substitute.
920 -- This is VITAL. Consider
922 -- let y = \z -> ...x... in
924 -- We'll clone the inner \x, adding x->x' in the id_subst
925 -- Then when we inline y, we must *not* replace x by x' in
926 -- the inlined copy!!
928 ---------------------------------------------------------
929 -- Dealing with a call site
931 completeCall env var occ_info cont
932 = -- Simplify the arguments
933 getDOptsSmpl `thenSmpl` \ dflags ->
935 chkr = getSwitchChecker env
936 (args, call_cont, inline_call) = getContArgs chkr var cont
939 simplifyArgs env fn_ty args (contResultType call_cont) $ \ env args ->
941 -- Next, look for rules or specialisations that match
943 -- It's important to simplify the args first, because the rule-matcher
944 -- doesn't do substitution as it goes. We don't want to use subst_args
945 -- (defined in the 'where') because that throws away useful occurrence info,
946 -- and perhaps-very-important specialisations.
948 -- Some functions have specialisations *and* are strict; in this case,
949 -- we don't want to inline the wrapper of the non-specialised thing; better
950 -- to call the specialised thing instead.
951 -- We used to use the black-listing mechanism to ensure that inlining of
952 -- the wrapper didn't occur for things that have specialisations till a
953 -- later phase, so but now we just try RULES first
955 -- You might think that we shouldn't apply rules for a loop breaker:
956 -- doing so might give rise to an infinite loop, because a RULE is
957 -- rather like an extra equation for the function:
958 -- RULE: f (g x) y = x+y
961 -- But it's too drastic to disable rules for loop breakers.
962 -- Even the foldr/build rule would be disabled, because foldr
963 -- is recursive, and hence a loop breaker:
964 -- foldr k z (build g) = g k z
965 -- So it's up to the programmer: rules can cause divergence
968 in_scope = getInScope env
970 maybe_rule = case activeRule env of
971 Nothing -> Nothing -- No rules apply
972 Just act_fn -> lookupRule act_fn in_scope rules var args
975 Just (rule_name, rule_rhs) ->
976 tick (RuleFired rule_name) `thenSmpl_`
977 (if dopt Opt_D_dump_inlinings dflags then
978 pprTrace "Rule fired" (vcat [
979 text "Rule:" <+> ftext rule_name,
980 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
981 text "After: " <+> pprCoreExpr rule_rhs,
982 text "Cont: " <+> ppr call_cont])
985 simplExprF env rule_rhs call_cont ;
987 Nothing -> -- No rules
989 -- Next, look for an inlining
991 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
993 interesting_cont = interestingCallContext (notNull args)
997 active_inline = activeInline env var occ_info
998 maybe_inline = callSiteInline dflags active_inline inline_call occ_info
999 var arg_infos interesting_cont
1001 case maybe_inline of {
1002 Just unfolding -- There is an inlining!
1003 -> tick (UnfoldingDone var) `thenSmpl_`
1004 makeThatCall env var unfolding args call_cont
1007 Nothing -> -- No inlining!
1010 rebuild env (mkApps (Var var) args) call_cont
1013 makeThatCall :: SimplEnv
1015 -> InExpr -- Inlined function rhs
1016 -> [OutExpr] -- Arguments, already simplified
1017 -> SimplCont -- After the call
1018 -> SimplM FloatsWithExpr
1019 -- Similar to simplLam, but this time
1020 -- the arguments are already simplified
1021 makeThatCall orig_env var fun@(Lam _ _) args cont
1022 = go orig_env fun args
1024 zap_it = mkLamBndrZapper fun (length args)
1026 -- Type-beta reduction
1027 go env (Lam bndr body) (Type ty_arg : args)
1028 = ASSERT( isTyVar bndr )
1029 tick (BetaReduction bndr) `thenSmpl_`
1030 go (extendTvSubst env bndr ty_arg) body args
1032 -- Ordinary beta reduction
1033 go env (Lam bndr body) (arg : args)
1034 = tick (BetaReduction bndr) `thenSmpl_`
1035 simplNonRecX env (zap_it bndr) arg $ \ env ->
1038 -- Not enough args, so there are real lambdas left to put in the result
1040 = simplExprF env fun (pushContArgs orig_env args cont)
1041 -- NB: orig_env; the correct environment to capture with
1042 -- the arguments.... env has been augmented with substitutions
1043 -- from the beta reductions.
1045 makeThatCall env var fun args cont
1046 = simplExprF env fun (pushContArgs env args cont)
1050 %************************************************************************
1052 \subsection{Arguments}
1054 %************************************************************************
1057 ---------------------------------------------------------
1058 -- Simplifying the arguments of a call
1060 simplifyArgs :: SimplEnv
1061 -> OutType -- Type of the function
1062 -> [(InExpr, SimplEnv, Bool)] -- Details of the arguments
1063 -> OutType -- Type of the continuation
1064 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1065 -> SimplM FloatsWithExpr
1067 -- [CPS-like because of strict arguments]
1069 -- Simplify the arguments to a call.
1070 -- This part of the simplifier may break the no-shadowing invariant
1072 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1073 -- where f is strict in its second arg
1074 -- If we simplify the innermost one first we get (...(\a -> e)...)
1075 -- Simplifying the second arg makes us float the case out, so we end up with
1076 -- case y of (a,b) -> f (...(\a -> e)...) e'
1077 -- So the output does not have the no-shadowing invariant. However, there is
1078 -- no danger of getting name-capture, because when the first arg was simplified
1079 -- we used an in-scope set that at least mentioned all the variables free in its
1080 -- static environment, and that is enough.
1082 -- We can't just do innermost first, or we'd end up with a dual problem:
1083 -- case x of (a,b) -> f e (...(\a -> e')...)
1085 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1086 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1087 -- continuations. We have to keep the right in-scope set around; AND we have
1088 -- to get the effect that finding (error "foo") in a strict arg position will
1089 -- discard the entire application and replace it with (error "foo"). Getting
1090 -- all this at once is TOO HARD!
1092 simplifyArgs env fn_ty args cont_ty thing_inside
1093 = go env fn_ty args thing_inside
1095 go env fn_ty [] thing_inside = thing_inside env []
1096 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty arg cont_ty $ \ env arg' ->
1097 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1098 thing_inside env (arg':args')
1100 simplifyArg env fn_ty (Type ty_arg, se, _) cont_ty thing_inside
1101 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1102 thing_inside env (Type new_ty_arg)
1104 simplifyArg env fn_ty (val_arg, arg_se, is_strict) cont_ty thing_inside
1106 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1108 | otherwise -- Lazy argument
1109 -- DO NOT float anything outside, hence simplExprC
1110 -- There is no benefit (unlike in a let-binding), and we'd
1111 -- have to be very careful about bogus strictness through
1112 -- floating a demanded let.
1113 = simplExprC (setInScope arg_se env) val_arg
1114 (mkBoringStop arg_ty) `thenSmpl` \ arg1 ->
1115 thing_inside env arg1
1117 arg_ty = funArgTy fn_ty
1120 simplStrictArg :: LetRhsFlag
1121 -> SimplEnv -- The env of the call
1122 -> InExpr -> SimplEnv -- The arg plus its env
1123 -> OutType -- arg_ty: type of the argument
1124 -> OutType -- cont_ty: Type of thing computed by the context
1125 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1126 -- Takes an expression of type rhs_ty,
1127 -- returns an expression of type cont_ty
1128 -- The env passed to this continuation is the
1129 -- env of the call, plus any new in-scope variables
1130 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1132 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1133 = simplExprF (setInScope arg_env call_env) arg
1134 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1135 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1136 -- to simplify the argument
1137 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1141 %************************************************************************
1143 \subsection{mkAtomicArgs}
1145 %************************************************************************
1147 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1148 constructor application and, if so, converts it to ANF, so that the
1149 resulting thing can be inlined more easily. Thus
1156 There are three sorts of binding context, specified by the two
1162 N N Top-level or recursive Only bind args of lifted type
1164 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1165 but lazy unlifted-and-ok-for-speculation
1167 Y Y Non-top-level, non-recursive, Bind all args
1168 and strict (demanded)
1175 there is no point in transforming to
1177 x = case (y div# z) of r -> MkC r
1179 because the (y div# z) can't float out of the let. But if it was
1180 a *strict* let, then it would be a good thing to do. Hence the
1181 context information.
1184 mkAtomicArgs :: Bool -- A strict binding
1185 -> Bool -- OK to float unlifted args
1187 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1188 OutExpr) -- things that need case-binding,
1189 -- if the strict-binding flag is on
1191 mkAtomicArgs is_strict ok_float_unlifted rhs
1192 | (Var fun, args) <- collectArgs rhs, -- It's an application
1193 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1194 = go fun nilOL [] args -- Have a go
1196 | otherwise = bale_out -- Give up
1199 bale_out = returnSmpl (nilOL, rhs)
1201 go fun binds rev_args []
1202 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1204 go fun binds rev_args (arg : args)
1205 | exprIsTrivial arg -- Easy case
1206 = go fun binds (arg:rev_args) args
1208 | not can_float_arg -- Can't make this arg atomic
1209 = bale_out -- ... so give up
1211 | otherwise -- Don't forget to do it recursively
1212 -- E.g. x = a:b:c:[]
1213 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1214 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1215 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1216 (Var arg_id : rev_args) args
1218 arg_ty = exprType arg
1219 can_float_arg = is_strict
1220 || not (isUnLiftedType arg_ty)
1221 || (ok_float_unlifted && exprOkForSpeculation arg)
1224 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1225 -> (SimplEnv -> SimplM (FloatsWith a))
1226 -> SimplM (FloatsWith a)
1227 addAtomicBinds env [] thing_inside = thing_inside env
1228 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1229 addAtomicBinds env bs thing_inside
1231 addAtomicBindsE :: SimplEnv -> [(OutId,OutExpr)]
1232 -> (SimplEnv -> SimplM FloatsWithExpr)
1233 -> SimplM FloatsWithExpr
1234 -- Same again, but this time we're in an expression context,
1235 -- and may need to do some case bindings
1237 addAtomicBindsE env [] thing_inside
1239 addAtomicBindsE env ((v,r):bs) thing_inside
1240 | needsCaseBinding (idType v) r
1241 = addAtomicBindsE (addNewInScopeIds env [v]) bs thing_inside `thenSmpl` \ (floats, expr) ->
1242 WARN( exprIsTrivial expr, ppr v <+> pprCoreExpr expr )
1243 (let body = wrapFloats floats expr in
1244 returnSmpl (emptyFloats env, Case r v (exprType body) [(DEFAULT,[],body)]))
1247 = addAuxiliaryBind env (NonRec v r) $ \ env ->
1248 addAtomicBindsE env bs thing_inside
1252 %************************************************************************
1254 \subsection{The main rebuilder}
1256 %************************************************************************
1259 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1261 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1262 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1263 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1264 rebuild env expr (InlinePlease cont) = rebuild env (Note InlineCall expr) cont
1265 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1266 rebuild env expr (ApplyTo _ arg se cont) = rebuildApp (setInScope se env) expr arg cont
1268 rebuildApp env fun arg cont
1269 = simplExpr env arg `thenSmpl` \ arg' ->
1270 rebuild env (App fun arg') cont
1272 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1276 %************************************************************************
1278 \subsection{Functions dealing with a case}
1280 %************************************************************************
1282 Blob of helper functions for the "case-of-something-else" situation.
1285 ---------------------------------------------------------
1286 -- Eliminate the case if possible
1288 rebuildCase :: SimplEnv
1289 -> OutExpr -- Scrutinee
1290 -> InId -- Case binder
1291 -> [InAlt] -- Alternatives (inceasing order)
1293 -> SimplM FloatsWithExpr
1295 rebuildCase env scrut case_bndr alts cont
1296 | Just (con,args) <- exprIsConApp_maybe scrut
1297 -- Works when the scrutinee is a variable with a known unfolding
1298 -- as well as when it's an explicit constructor application
1299 = knownCon env (DataAlt con) args case_bndr alts cont
1301 | Lit lit <- scrut -- No need for same treatment as constructors
1302 -- because literals are inlined more vigorously
1303 = knownCon env (LitAlt lit) [] case_bndr alts cont
1306 = prepareAlts scrut case_bndr alts `thenSmpl` \ (better_alts, handled_cons) ->
1308 -- Deal with the case binder, and prepare the continuation;
1309 -- The new subst_env is in place
1310 prepareCaseCont env better_alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1311 addFloats env floats $ \ env ->
1314 -- The case expression is annotated with the result type of the continuation
1315 -- This may differ from the type originally on the case. For example
1316 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1319 -- let j a# = <blob>
1320 -- in case(T) a of { True -> j 1#; False -> j 0# }
1321 -- Note that the case that scrutinises a now returns a T not an Int#
1322 res_ty' = contResultType dup_cont
1325 -- Deal with variable scrutinee
1326 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1328 -- Deal with the case alternatives
1329 simplAlts alt_env handled_cons
1330 case_bndr' better_alts dup_cont `thenSmpl` \ alts' ->
1332 -- Put the case back together
1333 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1335 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1336 -- The case binder *not* scope over the whole returned case-expression
1337 rebuild env case_expr nondup_cont
1340 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1341 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1342 way, there's a chance that v will now only be used once, and hence
1347 There is a time we *don't* want to do that, namely when
1348 -fno-case-of-case is on. This happens in the first simplifier pass,
1349 and enhances full laziness. Here's the bad case:
1350 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1351 If we eliminate the inner case, we trap it inside the I# v -> arm,
1352 which might prevent some full laziness happening. I've seen this
1353 in action in spectral/cichelli/Prog.hs:
1354 [(m,n) | m <- [1..max], n <- [1..max]]
1355 Hence the check for NoCaseOfCase.
1359 There is another situation when we don't want to do it. If we have
1361 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1362 ...other cases .... }
1364 We'll perform the binder-swap for the outer case, giving
1366 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1367 ...other cases .... }
1369 But there is no point in doing it for the inner case, because w1 can't
1370 be inlined anyway. Furthermore, doing the case-swapping involves
1371 zapping w2's occurrence info (see paragraphs that follow), and that
1372 forces us to bind w2 when doing case merging. So we get
1374 case x of w1 { A -> let w2 = w1 in e1
1375 B -> let w2 = w1 in e2
1376 ...other cases .... }
1378 This is plain silly in the common case where w2 is dead.
1380 Even so, I can't see a good way to implement this idea. I tried
1381 not doing the binder-swap if the scrutinee was already evaluated
1382 but that failed big-time:
1386 case v of w { MkT x ->
1387 case x of x1 { I# y1 ->
1388 case x of x2 { I# y2 -> ...
1390 Notice that because MkT is strict, x is marked "evaluated". But to
1391 eliminate the last case, we must either make sure that x (as well as
1392 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1393 the binder-swap. So this whole note is a no-op.
1397 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1398 any occurrence info (eg IAmDead) in the case binder, because the
1399 case-binder now effectively occurs whenever v does. AND we have to do
1400 the same for the pattern-bound variables! Example:
1402 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1404 Here, b and p are dead. But when we move the argment inside the first
1405 case RHS, and eliminate the second case, we get
1407 case x of { (a,b) -> a b }
1409 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1412 Indeed, this can happen anytime the case binder isn't dead:
1413 case <any> of x { (a,b) ->
1414 case x of { (p,q) -> p } }
1415 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1416 The point is that we bring into the envt a binding
1418 after the outer case, and that makes (a,b) alive. At least we do unless
1419 the case binder is guaranteed dead.
1422 simplCaseBinder env (Var v) case_bndr
1423 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1425 -- Failed try [see Note 2 above]
1426 -- not (isEvaldUnfolding (idUnfolding v))
1428 = simplBinder env (zap case_bndr) `thenSmpl` \ (env, case_bndr') ->
1429 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1430 -- We could extend the substitution instead, but it would be
1431 -- a hack because then the substitution wouldn't be idempotent
1432 -- any more (v is an OutId). And this does just as well.
1434 zap b = b `setIdOccInfo` NoOccInfo
1436 simplCaseBinder env other_scrut case_bndr
1437 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1438 returnSmpl (env, case_bndr')
1444 simplAlts :: SimplEnv
1445 -> [AltCon] -- Alternatives the scrutinee can't be
1446 -- in the default case
1447 -> OutId -- Case binder
1448 -> [InAlt] -> SimplCont
1449 -> SimplM [OutAlt] -- Includes the continuation
1451 simplAlts env handled_cons case_bndr' alts cont'
1452 = mapSmpl simpl_alt alts
1454 simpl_alt alt = simplAlt env handled_cons case_bndr' alt cont' `thenSmpl` \ (_, alt') ->
1457 simplAlt :: SimplEnv -> [AltCon] -> OutId -> InAlt -> SimplCont
1458 -> SimplM (Maybe TvSubstEnv, OutAlt)
1459 -- Simplify an alternative, returning the type refinement for the
1460 -- alternative, if the alternative does any refinement at all
1462 simplAlt env handled_cons case_bndr' (DEFAULT, bndrs, rhs) cont'
1463 = ASSERT( null bndrs )
1464 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1465 returnSmpl (Nothing, (DEFAULT, [], rhs'))
1467 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1468 -- Record the constructors that the case-binder *can't* be.
1470 simplAlt env handled_cons case_bndr' (LitAlt lit, bndrs, rhs) cont'
1471 = ASSERT( null bndrs )
1472 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1473 returnSmpl (Nothing, (LitAlt lit, [], rhs'))
1475 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1477 simplAlt env handled_cons case_bndr' (DataAlt con, vs, rhs) cont'
1478 | isVanillaDataCon con
1479 = -- Deal with the pattern-bound variables
1480 -- Mark the ones that are in ! positions in the data constructor
1481 -- as certainly-evaluated.
1482 -- NB: it happens that simplBinders does *not* erase the OtherCon
1483 -- form of unfolding, so it's ok to add this info before
1484 -- doing simplBinders
1485 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1487 -- Bind the case-binder to (con args)
1488 let unf = mkUnfolding False (mkConApp con con_args)
1489 inst_tys' = tyConAppArgs (idType case_bndr')
1490 con_args = map Type inst_tys' ++ map varToCoreExpr vs'
1491 env' = mk_rhs_env env case_bndr' unf
1493 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1494 returnSmpl (Nothing, (DataAlt con, vs', rhs'))
1496 | otherwise -- GADT case
1498 (tvs,ids) = span isTyVar vs
1500 simplBinders env tvs `thenSmpl` \ (env1, tvs') ->
1501 case coreRefineTys (getInScope env1) con tvs' (idType case_bndr') of {
1502 Nothing -- Dead code; for now, I'm just going to put in an
1503 -- error case so I can see them
1504 -> let rhs' = mkApps (Var eRROR_ID)
1505 [Type (substTy env (exprType rhs)),
1506 Lit (mkStringLit "Impossible alternative (GADT)")]
1508 simplBinders env1 ids `thenSmpl` \ (env2, ids') ->
1509 returnSmpl (Nothing, (DataAlt con, tvs' ++ ids', rhs')) ;
1511 Just refine@(tv_subst_env, _) -> -- The normal case
1514 env2 = refineSimplEnv env1 refine
1515 -- Simplify the Ids in the refined environment, so their types
1516 -- reflect the refinement. Usually this doesn't matter, but it helps
1517 -- in mkDupableAlt, when we want to float a lambda that uses these binders
1518 -- Furthermore, it means the binders contain maximal type information
1520 simplBinders env2 (add_evals con ids) `thenSmpl` \ (env3, ids') ->
1521 let unf = mkUnfolding False con_app
1522 con_app = mkConApp con con_args
1523 con_args = map varToCoreExpr vs' -- NB: no inst_tys'
1524 env_w_unf = mk_rhs_env env3 case_bndr' unf
1527 simplExprC env_w_unf rhs cont' `thenSmpl` \ rhs' ->
1528 returnSmpl (Just tv_subst_env, (DataAlt con, vs', rhs')) }
1531 -- add_evals records the evaluated-ness of the bound variables of
1532 -- a case pattern. This is *important*. Consider
1533 -- data T = T !Int !Int
1535 -- case x of { T a b -> T (a+1) b }
1537 -- We really must record that b is already evaluated so that we don't
1538 -- go and re-evaluate it when constructing the result.
1539 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1541 cat_evals dc vs strs
1545 go (v:vs) strs | isTyVar v = v : go vs strs
1546 go (v:vs) (str:strs)
1547 | isMarkedStrict str = evald_v : go vs strs
1548 | otherwise = zapped_v : go vs strs
1550 zapped_v = zap_occ_info v
1551 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1552 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1554 -- If the case binder is alive, then we add the unfolding
1556 -- to the envt; so vs are now very much alive
1557 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1558 | otherwise = \id -> id `setIdOccInfo` NoOccInfo
1560 mk_rhs_env env case_bndr' case_bndr_unf
1561 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1565 %************************************************************************
1567 \subsection{Known constructor}
1569 %************************************************************************
1571 We are a bit careful with occurrence info. Here's an example
1573 (\x* -> case x of (a*, b) -> f a) (h v, e)
1575 where the * means "occurs once". This effectively becomes
1576 case (h v, e) of (a*, b) -> f a)
1578 let a* = h v; b = e in f a
1582 All this should happen in one sweep.
1585 knownCon :: SimplEnv -> AltCon -> [OutExpr]
1586 -> InId -> [InAlt] -> SimplCont
1587 -> SimplM FloatsWithExpr
1589 knownCon env con args bndr alts cont
1590 = tick (KnownBranch bndr) `thenSmpl_`
1591 case findAlt con alts of
1592 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1593 simplNonRecX env bndr scrut $ \ env ->
1594 -- This might give rise to a binding with non-atomic args
1595 -- like x = Node (f x) (g x)
1596 -- but no harm will be done
1597 simplExprF env rhs cont
1600 LitAlt lit -> Lit lit
1601 DataAlt dc -> mkConApp dc args
1603 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1604 simplNonRecX env bndr (Lit lit) $ \ env ->
1605 simplExprF env rhs cont
1607 (DataAlt dc, bs, rhs)
1608 -> ASSERT( n_drop_tys + length bs == length args )
1609 bind_args env bs (drop n_drop_tys args) $ \ env ->
1611 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1612 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1613 -- args are aready OutExprs, but bs are InIds
1615 simplNonRecX env bndr con_app $ \ env ->
1616 simplExprF env rhs cont
1618 n_drop_tys | isVanillaDataCon dc = tyConArity (dataConTyCon dc)
1620 -- Vanilla data constructors lack type arguments in the pattern
1623 bind_args env [] _ thing_inside = thing_inside env
1625 bind_args env (b:bs) (Type ty : args) thing_inside
1626 = ASSERT( isTyVar b )
1627 bind_args (extendTvSubst env b ty) bs args thing_inside
1629 bind_args env (b:bs) (arg : args) thing_inside
1631 simplNonRecX env b arg $ \ env ->
1632 bind_args env bs args thing_inside
1636 %************************************************************************
1638 \subsection{Duplicating continuations}
1640 %************************************************************************
1643 prepareCaseCont :: SimplEnv
1644 -> [InAlt] -> SimplCont
1645 -> SimplM (FloatsWith (SimplCont,SimplCont))
1646 -- Return a duplicatable continuation, a non-duplicable part
1647 -- plus some extra bindings
1649 -- No need to make it duplicatable if there's only one alternative
1650 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1651 prepareCaseCont env alts cont = mkDupableCont env cont
1655 mkDupableCont :: SimplEnv -> SimplCont
1656 -> SimplM (FloatsWith (SimplCont, SimplCont))
1658 mkDupableCont env cont
1659 | contIsDupable cont
1660 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1662 mkDupableCont env (CoerceIt ty cont)
1663 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1664 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1666 mkDupableCont env (InlinePlease cont)
1667 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1668 returnSmpl (floats, (InlinePlease dup_cont, nondup_cont))
1670 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1671 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1672 -- Do *not* duplicate an ArgOf continuation
1673 -- Because ArgOf continuations are opaque, we gain nothing by
1674 -- propagating them into the expressions, and we do lose a lot.
1675 -- Here's an example:
1676 -- && (case x of { T -> F; F -> T }) E
1677 -- Now, && is strict so we end up simplifying the case with
1678 -- an ArgOf continuation. If we let-bind it, we get
1680 -- let $j = \v -> && v E
1681 -- in simplExpr (case x of { T -> F; F -> T })
1682 -- (ArgOf (\r -> $j r)
1683 -- And after simplifying more we get
1685 -- let $j = \v -> && v E
1686 -- in case of { T -> $j F; F -> $j T }
1687 -- Which is a Very Bad Thing
1689 -- The desire not to duplicate is the entire reason that
1690 -- mkDupableCont returns a pair of continuations.
1692 -- The original plan had:
1693 -- e.g. (...strict-fn...) [...hole...]
1695 -- let $j = \a -> ...strict-fn...
1696 -- in $j [...hole...]
1698 mkDupableCont env (ApplyTo _ arg se cont)
1699 = -- e.g. [...hole...] (...arg...)
1701 -- let a = ...arg...
1702 -- in [...hole...] a
1703 simplExpr (setInScope se env) arg `thenSmpl` \ arg' ->
1705 mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1706 addFloats env floats $ \ env ->
1708 if exprIsDupable arg' then
1709 returnSmpl (emptyFloats env, (ApplyTo OkToDup arg' (zapSubstEnv se) dup_cont, nondup_cont))
1711 newId FSLIT("a") (exprType arg') `thenSmpl` \ arg_id ->
1713 tick (CaseOfCase arg_id) `thenSmpl_`
1714 -- Want to tick here so that we go round again,
1715 -- and maybe copy or inline the code.
1716 -- Not strictly CaseOfCase, but never mind
1718 returnSmpl (unitFloat env arg_id arg',
1719 (ApplyTo OkToDup (Var arg_id) (zapSubstEnv se) dup_cont,
1721 -- But what if the arg should be case-bound?
1722 -- This has been this way for a long time, so I'll leave it,
1723 -- but I can't convince myself that it's right.
1726 mkDupableCont env (Select _ case_bndr alts se cont)
1727 = -- e.g. (case [...hole...] of { pi -> ei })
1729 -- let ji = \xij -> ei
1730 -- in case [...hole...] of { pi -> ji xij }
1731 tick (CaseOfCase case_bndr) `thenSmpl_`
1733 alt_env = setInScope se env
1735 prepareCaseCont alt_env alts cont `thenSmpl` \ (floats1, (dup_cont, nondup_cont)) ->
1736 addFloats alt_env floats1 $ \ alt_env ->
1738 simplBinder alt_env case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1739 -- NB: simplBinder does not zap deadness occ-info, so
1740 -- a dead case_bndr' will still advertise its deadness
1741 -- This is really important because in
1742 -- case e of b { (# a,b #) -> ... }
1743 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1744 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1745 -- In the new alts we build, we have the new case binder, so it must retain
1748 mkDupableAlts alt_env case_bndr' alts dup_cont `thenSmpl` \ (floats2, alts') ->
1749 addFloats alt_env floats2 $ \ alt_env ->
1750 returnSmpl (emptyFloats alt_env,
1751 (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1752 (mkBoringStop (contResultType dup_cont)),
1755 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1756 -> SimplM (FloatsWith [InAlt])
1757 -- Absorbs the continuation into the new alternatives
1759 mkDupableAlts env case_bndr' alts dupable_cont
1762 go env [] = returnSmpl (emptyFloats env, [])
1764 = mkDupableAlt env case_bndr' dupable_cont alt `thenSmpl` \ (floats1, alt') ->
1765 addFloats env floats1 $ \ env ->
1766 go env alts `thenSmpl` \ (floats2, alts') ->
1767 returnSmpl (floats2, alt' : alts')
1769 mkDupableAlt env case_bndr' cont alt
1770 = simplAlt env [] case_bndr' alt cont `thenSmpl` \ (mb_reft, (con, bndrs', rhs')) ->
1771 -- Safe to say that there are no handled-cons for the DEFAULT case
1773 if exprIsDupable rhs' then
1774 returnSmpl (emptyFloats env, (con, bndrs', rhs'))
1775 -- It is worth checking for a small RHS because otherwise we
1776 -- get extra let bindings that may cause an extra iteration of the simplifier to
1777 -- inline back in place. Quite often the rhs is just a variable or constructor.
1778 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1779 -- iterations because the version with the let bindings looked big, and so wasn't
1780 -- inlined, but after the join points had been inlined it looked smaller, and so
1783 -- NB: we have to check the size of rhs', not rhs.
1784 -- Duplicating a small InAlt might invalidate occurrence information
1785 -- However, if it *is* dupable, we return the *un* simplified alternative,
1786 -- because otherwise we'd need to pair it up with an empty subst-env....
1787 -- but we only have one env shared between all the alts.
1788 -- (Remember we must zap the subst-env before re-simplifying something).
1789 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1793 rhs_ty' = exprType rhs'
1794 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1796 | isTyVar bndr = not (mb_reft `refines` bndr)
1797 -- Don't abstract over tyvar binders which are refined away
1798 | otherwise = not (isDeadBinder bndr)
1799 -- The deadness info on the new Ids is preserved by simplBinders
1800 refines Nothing bndr = False
1801 refines (Just tv_subst) bndr = bndr `elemVarEnv` tv_subst
1802 -- See Note [Refinement] below
1804 -- If we try to lift a primitive-typed something out
1805 -- for let-binding-purposes, we will *caseify* it (!),
1806 -- with potentially-disastrous strictness results. So
1807 -- instead we turn it into a function: \v -> e
1808 -- where v::State# RealWorld#. The value passed to this function
1809 -- is realworld#, which generates (almost) no code.
1811 -- There's a slight infelicity here: we pass the overall
1812 -- case_bndr to all the join points if it's used in *any* RHS,
1813 -- because we don't know its usage in each RHS separately
1815 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1816 -- we make the join point into a function whenever used_bndrs'
1817 -- is empty. This makes the join-point more CPR friendly.
1818 -- Consider: let j = if .. then I# 3 else I# 4
1819 -- in case .. of { A -> j; B -> j; C -> ... }
1821 -- Now CPR doesn't w/w j because it's a thunk, so
1822 -- that means that the enclosing function can't w/w either,
1823 -- which is a lose. Here's the example that happened in practice:
1824 -- kgmod :: Int -> Int -> Int
1825 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1829 -- I have seen a case alternative like this:
1830 -- True -> \v -> ...
1831 -- It's a bit silly to add the realWorld dummy arg in this case, making
1834 -- (the \v alone is enough to make CPR happy) but I think it's rare
1836 ( if not (any isId used_bndrs')
1837 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
1838 returnSmpl ([rw_id], [Var realWorldPrimId])
1840 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
1841 ) `thenSmpl` \ (final_bndrs', final_args) ->
1843 -- See comment about "$j" name above
1844 newId (encodeFS FSLIT("$j")) (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
1845 -- Notice the funky mkPiTypes. If the contructor has existentials
1846 -- it's possible that the join point will be abstracted over
1847 -- type varaibles as well as term variables.
1848 -- Example: Suppose we have
1849 -- data T = forall t. C [t]
1851 -- case (case e of ...) of
1852 -- C t xs::[t] -> rhs
1853 -- We get the join point
1854 -- let j :: forall t. [t] -> ...
1855 -- j = /\t \xs::[t] -> rhs
1857 -- case (case e of ...) of
1858 -- C t xs::[t] -> j t xs
1860 -- We make the lambdas into one-shot-lambdas. The
1861 -- join point is sure to be applied at most once, and doing so
1862 -- prevents the body of the join point being floated out by
1863 -- the full laziness pass
1864 really_final_bndrs = map one_shot final_bndrs'
1865 one_shot v | isId v = setOneShotLambda v
1867 join_rhs = mkLams really_final_bndrs rhs'
1868 join_call = mkApps (Var join_bndr) final_args
1870 returnSmpl (unitFloat env join_bndr join_rhs, (con, bndrs', join_call))
1877 MkT :: a -> b -> T a
1881 MkT a' b (p::a') (q::b) -> [p,w]
1883 The danger is that we'll make a join point
1887 and that's ill-typed, because (p::a') but (w::a).
1889 Solution so far: don't abstract over a', because the type refinement
1890 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
1892 Then we must abstract over any refined free variables. Hmm. Maybe we
1893 could just abstract over *all* free variables, thereby lambda-lifting
1894 the join point? We should try this.