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 idSpecialisation, setIdSpecialisation,
28 setIdOccInfo, zapLamIdInfo, setOneShotLambda,
30 import OccName ( encodeFS )
31 import IdInfo ( OccInfo(..), isLoopBreaker,
36 import NewDemand ( isStrictDmd )
37 import DataCon ( dataConNumInstArgs, dataConRepStrictness )
39 import PprCore ( pprParendExpr, pprCoreExpr )
40 import CoreUnfold ( mkOtherCon, mkUnfolding, callSiteInline )
41 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
42 exprIsConApp_maybe, mkPiTypes, findAlt,
43 exprType, exprIsValue,
44 exprOkForSpeculation, exprArity,
45 mkCoerce, mkCoerce2, mkSCC, mkInlineMe, mkAltExpr, applyTypeToArg
47 import Rules ( lookupRule )
48 import BasicTypes ( isMarkedStrict )
49 import CostCentre ( currentCCS )
50 import Type ( isUnLiftedType, seqType, tyConAppArgs, funArgTy,
51 splitFunTy_maybe, splitFunTy, eqType
53 import Subst ( mkSubst, substTy, substExpr, substRules,
54 isInScope, lookupIdSubst, simplIdInfo
56 import TysPrim ( realWorldStatePrimTy )
57 import PrelInfo ( realWorldPrimId )
58 import BasicTypes ( TopLevelFlag(..), isTopLevel,
62 import Maybe ( Maybe )
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 = -- Substitute the rules for this binder in the light
457 -- of earlier substitutions in this very letrec group,
458 -- add the substituted rules to the IdInfo, and
459 -- extend the in-scope env, so that the IdInfo for this
460 -- binder extends over the RHS for the binder itself.
462 -- This is important. Manuel found cases where he really, really
463 -- wanted a RULE for a recursive function to apply in that function's
464 -- own right-hand side.
466 -- NB: does no harm for non-recursive bindings
468 -- NB2: just rules! In particular, the arity of an Id is not visible
469 -- in its own RHS, else we eta-reduce
473 -- which isn't sound. And it makes the arity in f's IdInfo greater than
474 -- the manifest arity, which isn't good.
476 bndr2 = bndr1 `setIdInfo` simplIdInfo (getSubst env) (idInfo bndr)
477 env1 = modifyInScope env bndr2 bndr2
478 rhs_env = setInScope rhs_se env1
479 is_top_level = isTopLevel top_lvl
480 ok_float_unlifted = not is_top_level && isNonRec is_rec
481 rhs_cont = mkStop (idType bndr1) AnRhs
483 -- Simplify the RHS; note the mkStop, which tells
484 -- the simplifier that this is the RHS of a let.
485 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
487 -- If any of the floats can't be floated, give up now
488 -- (The allLifted predicate says True for empty floats.)
489 if (not ok_float_unlifted && not (allLifted floats)) then
490 completeLazyBind env1 top_lvl bndr bndr2
491 (wrapFloats floats rhs1)
494 -- ANF-ise a constructor or PAP rhs
495 mkAtomicArgs False {- Not strict -}
496 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
498 -- If the result is a PAP, float the floats out, else wrap them
499 -- By this time it's already been ANF-ised (if necessary)
500 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
501 completeLazyBind env1 top_lvl bndr bndr2 rhs2
503 -- We use exprIsTrivial here because we want to reveal lone variables.
504 -- E.g. let { x = letrec { y = E } in y } in ...
505 -- Here we definitely want to float the y=E defn.
506 -- exprIsValue definitely isn't right for that.
508 -- BUT we can't use "exprIsCheap", because that causes a strictness bug.
509 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
510 -- The case expression is 'cheap', but it's wrong to transform to
511 -- y* = E; x = case (scc y) of {...}
512 -- Either we must be careful not to float demanded non-values, or
513 -- we must use exprIsValue for the test, which ensures that the
514 -- thing is non-strict. I think. The WARN below tests for this.
515 else if is_top_level || exprIsTrivial rhs2 || exprIsValue rhs2 then
517 -- There's a subtlety here. There may be a binding (x* = e) in the
518 -- floats, where the '*' means 'will be demanded'. So is it safe
519 -- to float it out? Answer no, but it won't matter because
520 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
521 -- and so there can't be any 'will be demanded' bindings in the floats.
523 WARN( not is_top_level && any demanded_float (floatBinds floats),
524 ppr (filter demanded_float (floatBinds floats)) )
526 tick LetFloatFromLet `thenSmpl_` (
527 addFloats env1 floats $ \ env2 ->
528 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
529 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
532 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
535 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
536 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
537 demanded_float (Rec _) = False
542 %************************************************************************
544 \subsection{Completing a lazy binding}
546 %************************************************************************
549 * deals only with Ids, not TyVars
550 * takes an already-simplified binder and RHS
551 * is used for both recursive and non-recursive bindings
552 * is used for both top-level and non-top-level bindings
554 It does the following:
555 - tries discarding a dead binding
556 - tries PostInlineUnconditionally
557 - add unfolding [this is the only place we add an unfolding]
560 It does *not* attempt to do let-to-case. Why? Because it is used for
561 - top-level bindings (when let-to-case is impossible)
562 - many situations where the "rhs" is known to be a WHNF
563 (so let-to-case is inappropriate).
566 completeLazyBind :: SimplEnv
567 -> TopLevelFlag -- Flag stuck into unfolding
568 -> InId -- Old binder
569 -> OutId -- New binder
570 -> OutExpr -- Simplified RHS
571 -> SimplM (FloatsWith SimplEnv)
572 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
573 -- by extending the substitution (e.g. let x = y in ...)
574 -- The new binding (if any) is returned as part of the floats.
575 -- NB: the returned SimplEnv has the right SubstEnv, but you should
576 -- (as usual) use the in-scope-env from the floats
578 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
579 | postInlineUnconditionally env new_bndr occ_info new_rhs
580 = -- Drop the binding
581 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
582 returnSmpl (emptyFloats env, extendSubst env old_bndr (DoneEx new_rhs))
583 -- Use the substitution to make quite, quite sure that the substitution
584 -- will happen, since we are going to discard the binding
589 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
591 -- Add the unfolding *only* for non-loop-breakers
592 -- Making loop breakers not have an unfolding at all
593 -- means that we can avoid tests in exprIsConApp, for example.
594 -- This is important: if exprIsConApp says 'yes' for a recursive
595 -- thing, then we can get into an infinite loop
596 info_w_unf | loop_breaker = new_bndr_info
597 | otherwise = new_bndr_info `setUnfoldingInfo` unfolding
598 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
600 final_id = new_bndr `setIdInfo` info_w_unf
602 -- These seqs forces the Id, and hence its IdInfo,
603 -- and hence any inner substitutions
605 returnSmpl (unitFloat env final_id new_rhs, env)
608 loop_breaker = isLoopBreaker occ_info
609 old_info = idInfo old_bndr
610 occ_info = occInfo old_info
615 %************************************************************************
617 \subsection[Simplify-simplExpr]{The main function: simplExpr}
619 %************************************************************************
621 The reason for this OutExprStuff stuff is that we want to float *after*
622 simplifying a RHS, not before. If we do so naively we get quadratic
623 behaviour as things float out.
625 To see why it's important to do it after, consider this (real) example:
639 a -- Can't inline a this round, cos it appears twice
643 Each of the ==> steps is a round of simplification. We'd save a
644 whole round if we float first. This can cascade. Consider
649 let f = let d1 = ..d.. in \y -> e
653 in \x -> ...(\y ->e)...
655 Only in this second round can the \y be applied, and it
656 might do the same again.
660 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
661 simplExpr env expr = simplExprC env expr (mkStop expr_ty' AnArg)
663 expr_ty' = substTy (getSubst env) (exprType expr)
664 -- The type in the Stop continuation, expr_ty', is usually not used
665 -- It's only needed when discarding continuations after finding
666 -- a function that returns bottom.
667 -- Hence the lazy substitution
670 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
671 -- Simplify an expression, given a continuation
672 simplExprC env expr cont
673 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
674 returnSmpl (wrapFloats floats expr)
676 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
677 -- Simplify an expression, returning floated binds
679 simplExprF env (Var v) cont = simplVar env v cont
680 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
681 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
682 simplExprF env (Note note expr) cont = simplNote env note expr cont
683 simplExprF env (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg env cont)
685 simplExprF env (Type ty) cont
686 = ASSERT( contIsRhsOrArg cont )
687 simplType env ty `thenSmpl` \ ty' ->
688 rebuild env (Type ty') cont
690 simplExprF env (Case scrut bndr alts) cont
691 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
692 = -- Simplify the scrutinee with a Select continuation
693 simplExprF env scrut (Select NoDup bndr alts env cont)
696 = -- If case-of-case is off, simply simplify the case expression
697 -- in a vanilla Stop context, and rebuild the result around it
698 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
699 rebuild env case_expr' cont
701 case_cont = Select NoDup bndr alts env (mkBoringStop (contResultType cont))
703 simplExprF env (Let (Rec pairs) body) cont
704 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
705 -- NB: bndrs' don't have unfoldings or rules
706 -- We add them as we go down
708 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
709 addFloats env floats $ \ env ->
710 simplExprF env body cont
712 -- A non-recursive let is dealt with by simplNonRecBind
713 simplExprF env (Let (NonRec bndr rhs) body) cont
714 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
715 simplExprF env body cont
718 ---------------------------------
719 simplType :: SimplEnv -> InType -> SimplM OutType
720 -- Kept monadic just so we can do the seqType
722 = seqType new_ty `seq` returnSmpl new_ty
724 new_ty = substTy (getSubst env) ty
728 %************************************************************************
732 %************************************************************************
735 simplLam env fun cont
738 zap_it = mkLamBndrZapper fun (countArgs cont)
739 cont_ty = contResultType cont
741 -- Type-beta reduction
742 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
743 = ASSERT( isTyVar bndr )
744 tick (BetaReduction bndr) `thenSmpl_`
745 simplType (setInScope arg_se env) ty_arg `thenSmpl` \ ty_arg' ->
746 go (extendSubst env bndr (DoneTy ty_arg')) body body_cont
748 -- Ordinary beta reduction
749 go env (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
750 = tick (BetaReduction bndr) `thenSmpl_`
751 simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
752 go env body body_cont
754 -- Not enough args, so there are real lambdas left to put in the result
755 go env lam@(Lam _ _) cont
756 = simplLamBndrs env bndrs `thenSmpl` \ (env, bndrs') ->
757 simplExpr env body `thenSmpl` \ body' ->
758 mkLam env bndrs' body' cont `thenSmpl` \ (floats, new_lam) ->
759 addFloats env floats $ \ env ->
760 rebuild env new_lam cont
762 (bndrs,body) = collectBinders lam
764 -- Exactly enough args
765 go env expr cont = simplExprF env expr cont
767 mkLamBndrZapper :: CoreExpr -- Function
768 -> Int -- Number of args supplied, *including* type args
769 -> Id -> Id -- Use this to zap the binders
770 mkLamBndrZapper fun n_args
771 | n_args >= n_params fun = \b -> b -- Enough args
772 | otherwise = \b -> zapLamIdInfo b
774 -- NB: we count all the args incl type args
775 -- so we must count all the binders (incl type lambdas)
776 n_params (Note _ e) = n_params e
777 n_params (Lam b e) = 1 + n_params e
778 n_params other = 0::Int
782 %************************************************************************
786 %************************************************************************
789 simplNote env (Coerce to from) body cont
791 in_scope = getInScope env
793 addCoerce s1 k1 (CoerceIt t1 cont)
794 -- coerce T1 S1 (coerce S1 K1 e)
797 -- coerce T1 K1 e, otherwise
799 -- For example, in the initial form of a worker
800 -- we may find (coerce T (coerce S (\x.e))) y
801 -- and we'd like it to simplify to e[y/x] in one round
803 | t1 `eqType` k1 = cont -- The coerces cancel out
804 | otherwise = CoerceIt t1 cont -- They don't cancel, but
805 -- the inner one is redundant
807 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
808 | not (isTypeArg arg), -- This whole case only works for value args
809 -- Could upgrade to have equiv thing for type apps too
810 Just (s1, s2) <- splitFunTy_maybe s1s2
811 -- (coerce (T1->T2) (S1->S2) F) E
813 -- coerce T2 S2 (F (coerce S1 T1 E))
815 -- t1t2 must be a function type, T1->T2, because it's applied to something
816 -- but s1s2 might conceivably not be
818 -- When we build the ApplyTo we can't mix the out-types
819 -- with the InExpr in the argument, so we simply substitute
820 -- to make it all consistent. It's a bit messy.
821 -- But it isn't a common case.
823 (t1,t2) = splitFunTy t1t2
824 new_arg = mkCoerce2 s1 t1 (substExpr (mkSubst in_scope (getSubstEnv arg_se)) arg)
826 ApplyTo dup new_arg (zapSubstEnv env) (addCoerce t2 s2 cont)
828 addCoerce to' _ cont = CoerceIt to' cont
830 simplType env to `thenSmpl` \ to' ->
831 simplType env from `thenSmpl` \ from' ->
832 simplExprF env body (addCoerce to' from' cont)
835 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
836 -- inlining. All other CCCSs are mapped to currentCCS.
837 simplNote env (SCC cc) e cont
838 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
839 rebuild env (mkSCC cc e') cont
841 simplNote env InlineCall e cont
842 = simplExprF env e (InlinePlease cont)
844 -- See notes with SimplMonad.inlineMode
845 simplNote env InlineMe e cont
846 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
847 = -- Don't inline inside an INLINE expression
848 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
849 rebuild env (mkInlineMe e') cont
851 | otherwise -- Dissolve the InlineMe note if there's
852 -- an interesting context of any kind to combine with
853 -- (even a type application -- anything except Stop)
854 = simplExprF env e cont
856 simplNote env (CoreNote s) e cont
857 = simplExpr env e `thenSmpl` \ e' ->
858 rebuild env (Note (CoreNote s) e') cont
862 %************************************************************************
864 \subsection{Dealing with calls}
866 %************************************************************************
869 simplVar env var cont
870 = case lookupIdSubst (getSubst env) var of
871 DoneEx e -> simplExprF (zapSubstEnv env) e cont
872 ContEx se e -> simplExprF (setSubstEnv env se) e cont
873 DoneId var1 occ -> WARN( not (isInScope var1 (getSubst env)) && mustHaveLocalBinding var1,
874 text "simplVar:" <+> ppr var )
875 completeCall (zapSubstEnv env) var1 occ cont
876 -- The template is already simplified, so don't re-substitute.
877 -- This is VITAL. Consider
879 -- let y = \z -> ...x... in
881 -- We'll clone the inner \x, adding x->x' in the id_subst
882 -- Then when we inline y, we must *not* replace x by x' in
883 -- the inlined copy!!
885 ---------------------------------------------------------
886 -- Dealing with a call site
888 completeCall env var occ_info cont
889 = -- Simplify the arguments
890 getDOptsSmpl `thenSmpl` \ dflags ->
892 chkr = getSwitchChecker env
893 (args, call_cont, inline_call) = getContArgs chkr var cont
896 simplifyArgs env fn_ty args (contResultType call_cont) $ \ env args ->
898 -- Next, look for rules or specialisations that match
900 -- It's important to simplify the args first, because the rule-matcher
901 -- doesn't do substitution as it goes. We don't want to use subst_args
902 -- (defined in the 'where') because that throws away useful occurrence info,
903 -- and perhaps-very-important specialisations.
905 -- Some functions have specialisations *and* are strict; in this case,
906 -- we don't want to inline the wrapper of the non-specialised thing; better
907 -- to call the specialised thing instead.
908 -- We used to use the black-listing mechanism to ensure that inlining of
909 -- the wrapper didn't occur for things that have specialisations till a
910 -- later phase, so but now we just try RULES first
912 -- You might think that we shouldn't apply rules for a loop breaker:
913 -- doing so might give rise to an infinite loop, because a RULE is
914 -- rather like an extra equation for the function:
915 -- RULE: f (g x) y = x+y
918 -- But it's too drastic to disable rules for loop breakers.
919 -- Even the foldr/build rule would be disabled, because foldr
920 -- is recursive, and hence a loop breaker:
921 -- foldr k z (build g) = g k z
922 -- So it's up to the programmer: rules can cause divergence
925 in_scope = getInScope env
926 maybe_rule = case activeRule env of
927 Nothing -> Nothing -- No rules apply
928 Just act_fn -> lookupRule act_fn in_scope var args
931 Just (rule_name, rule_rhs) ->
932 tick (RuleFired rule_name) `thenSmpl_`
933 (if dopt Opt_D_dump_inlinings dflags then
934 pprTrace "Rule fired" (vcat [
935 text "Rule:" <+> ftext rule_name,
936 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
937 text "After: " <+> pprCoreExpr rule_rhs,
938 text "Cont: " <+> ppr call_cont])
941 simplExprF env rule_rhs call_cont ;
943 Nothing -> -- No rules
945 -- Next, look for an inlining
947 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
949 interesting_cont = interestingCallContext (notNull args)
953 active_inline = activeInline env var occ_info
954 maybe_inline = callSiteInline dflags active_inline inline_call occ_info
955 var arg_infos interesting_cont
957 case maybe_inline of {
958 Just unfolding -- There is an inlining!
959 -> tick (UnfoldingDone var) `thenSmpl_`
960 makeThatCall env var unfolding args call_cont
963 Nothing -> -- No inlining!
966 rebuild env (mkApps (Var var) args) call_cont
969 makeThatCall :: SimplEnv
971 -> InExpr -- Inlined function rhs
972 -> [OutExpr] -- Arguments, already simplified
973 -> SimplCont -- After the call
974 -> SimplM FloatsWithExpr
975 -- Similar to simplLam, but this time
976 -- the arguments are already simplified
977 makeThatCall orig_env var fun@(Lam _ _) args cont
978 = go orig_env fun args
980 zap_it = mkLamBndrZapper fun (length args)
982 -- Type-beta reduction
983 go env (Lam bndr body) (Type ty_arg : args)
984 = ASSERT( isTyVar bndr )
985 tick (BetaReduction bndr) `thenSmpl_`
986 go (extendSubst env bndr (DoneTy ty_arg)) body args
988 -- Ordinary beta reduction
989 go env (Lam bndr body) (arg : args)
990 = tick (BetaReduction bndr) `thenSmpl_`
991 simplNonRecX env (zap_it bndr) arg $ \ env ->
994 -- Not enough args, so there are real lambdas left to put in the result
996 = simplExprF env fun (pushContArgs orig_env args cont)
997 -- NB: orig_env; the correct environment to capture with
998 -- the arguments.... env has been augmented with substitutions
999 -- from the beta reductions.
1001 makeThatCall env var fun args cont
1002 = simplExprF env fun (pushContArgs env args cont)
1006 %************************************************************************
1008 \subsection{Arguments}
1010 %************************************************************************
1013 ---------------------------------------------------------
1014 -- Simplifying the arguments of a call
1016 simplifyArgs :: SimplEnv
1017 -> OutType -- Type of the function
1018 -> [(InExpr, SimplEnv, Bool)] -- Details of the arguments
1019 -> OutType -- Type of the continuation
1020 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1021 -> SimplM FloatsWithExpr
1023 -- [CPS-like because of strict arguments]
1025 -- Simplify the arguments to a call.
1026 -- This part of the simplifier may break the no-shadowing invariant
1028 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1029 -- where f is strict in its second arg
1030 -- If we simplify the innermost one first we get (...(\a -> e)...)
1031 -- Simplifying the second arg makes us float the case out, so we end up with
1032 -- case y of (a,b) -> f (...(\a -> e)...) e'
1033 -- So the output does not have the no-shadowing invariant. However, there is
1034 -- no danger of getting name-capture, because when the first arg was simplified
1035 -- we used an in-scope set that at least mentioned all the variables free in its
1036 -- static environment, and that is enough.
1038 -- We can't just do innermost first, or we'd end up with a dual problem:
1039 -- case x of (a,b) -> f e (...(\a -> e')...)
1041 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1042 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1043 -- continuations. We have to keep the right in-scope set around; AND we have
1044 -- to get the effect that finding (error "foo") in a strict arg position will
1045 -- discard the entire application and replace it with (error "foo"). Getting
1046 -- all this at once is TOO HARD!
1048 simplifyArgs env fn_ty args cont_ty thing_inside
1049 = go env fn_ty args thing_inside
1051 go env fn_ty [] thing_inside = thing_inside env []
1052 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty arg cont_ty $ \ env arg' ->
1053 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1054 thing_inside env (arg':args')
1056 simplifyArg env fn_ty (Type ty_arg, se, _) cont_ty thing_inside
1057 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1058 thing_inside env (Type new_ty_arg)
1060 simplifyArg env fn_ty (val_arg, arg_se, is_strict) cont_ty thing_inside
1062 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1064 | otherwise -- Lazy argument
1065 -- DO NOT float anything outside, hence simplExprC
1066 -- There is no benefit (unlike in a let-binding), and we'd
1067 -- have to be very careful about bogus strictness through
1068 -- floating a demanded let.
1069 = simplExprC (setInScope arg_se env) val_arg
1070 (mkStop arg_ty AnArg) `thenSmpl` \ arg1 ->
1071 thing_inside env arg1
1073 arg_ty = funArgTy fn_ty
1076 simplStrictArg :: LetRhsFlag
1077 -> SimplEnv -- The env of the call
1078 -> InExpr -> SimplEnv -- The arg plus its env
1079 -> OutType -- arg_ty: type of the argument
1080 -> OutType -- cont_ty: Type of thing computed by the context
1081 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1082 -- Takes an expression of type rhs_ty,
1083 -- returns an expression of type cont_ty
1084 -- The env passed to this continuation is the
1085 -- env of the call, plus any new in-scope variables
1086 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1088 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1089 = simplExprF (setInScope arg_env call_env) arg
1090 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1091 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1092 -- to simplify the argument
1093 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1097 %************************************************************************
1099 \subsection{mkAtomicArgs}
1101 %************************************************************************
1103 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1104 constructor application and, if so, converts it to ANF, so that the
1105 resulting thing can be inlined more easily. Thus
1112 There are three sorts of binding context, specified by the two
1118 N N Top-level or recursive Only bind args of lifted type
1120 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1121 but lazy unlifted-and-ok-for-speculation
1123 Y Y Non-top-level, non-recursive, Bind all args
1124 and strict (demanded)
1131 there is no point in transforming to
1133 x = case (y div# z) of r -> MkC r
1135 because the (y div# z) can't float out of the let. But if it was
1136 a *strict* let, then it would be a good thing to do. Hence the
1137 context information.
1140 mkAtomicArgs :: Bool -- A strict binding
1141 -> Bool -- OK to float unlifted args
1143 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1144 OutExpr) -- things that need case-binding,
1145 -- if the strict-binding flag is on
1147 mkAtomicArgs is_strict ok_float_unlifted rhs
1148 | (Var fun, args) <- collectArgs rhs, -- It's an application
1149 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1150 = go fun nilOL [] args -- Have a go
1152 | otherwise = bale_out -- Give up
1155 bale_out = returnSmpl (nilOL, rhs)
1157 go fun binds rev_args []
1158 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1160 go fun binds rev_args (arg : args)
1161 | exprIsTrivial arg -- Easy case
1162 = go fun binds (arg:rev_args) args
1164 | not can_float_arg -- Can't make this arg atomic
1165 = bale_out -- ... so give up
1167 | otherwise -- Don't forget to do it recursively
1168 -- E.g. x = a:b:c:[]
1169 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1170 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1171 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1172 (Var arg_id : rev_args) args
1174 arg_ty = exprType arg
1175 can_float_arg = is_strict
1176 || not (isUnLiftedType arg_ty)
1177 || (ok_float_unlifted && exprOkForSpeculation arg)
1180 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1181 -> (SimplEnv -> SimplM (FloatsWith a))
1182 -> SimplM (FloatsWith a)
1183 addAtomicBinds env [] thing_inside = thing_inside env
1184 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1185 addAtomicBinds env bs thing_inside
1187 addAtomicBindsE :: SimplEnv -> [(OutId,OutExpr)]
1188 -> (SimplEnv -> SimplM FloatsWithExpr)
1189 -> SimplM FloatsWithExpr
1190 -- Same again, but this time we're in an expression context,
1191 -- and may need to do some case bindings
1193 addAtomicBindsE env [] thing_inside
1195 addAtomicBindsE env ((v,r):bs) thing_inside
1196 | needsCaseBinding (idType v) r
1197 = addAtomicBindsE (addNewInScopeIds env [v]) bs thing_inside `thenSmpl` \ (floats, expr) ->
1198 WARN( exprIsTrivial expr, ppr v <+> pprCoreExpr expr )
1199 returnSmpl (emptyFloats env, Case r v [(DEFAULT,[], wrapFloats floats expr)])
1202 = addAuxiliaryBind env (NonRec v r) $ \ env ->
1203 addAtomicBindsE env bs thing_inside
1207 %************************************************************************
1209 \subsection{The main rebuilder}
1211 %************************************************************************
1214 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1216 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1217 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1218 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1219 rebuild env expr (InlinePlease cont) = rebuild env (Note InlineCall expr) cont
1220 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1221 rebuild env expr (ApplyTo _ arg se cont) = rebuildApp (setInScope se env) expr arg cont
1223 rebuildApp env fun arg cont
1224 = simplExpr env arg `thenSmpl` \ arg' ->
1225 rebuild env (App fun arg') cont
1227 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1231 %************************************************************************
1233 \subsection{Functions dealing with a case}
1235 %************************************************************************
1237 Blob of helper functions for the "case-of-something-else" situation.
1240 ---------------------------------------------------------
1241 -- Eliminate the case if possible
1243 rebuildCase :: SimplEnv
1244 -> OutExpr -- Scrutinee
1245 -> InId -- Case binder
1246 -> [InAlt] -- Alternatives
1248 -> SimplM FloatsWithExpr
1250 rebuildCase env scrut case_bndr alts cont
1251 | Just (con,args) <- exprIsConApp_maybe scrut
1252 -- Works when the scrutinee is a variable with a known unfolding
1253 -- as well as when it's an explicit constructor application
1254 = knownCon env (DataAlt con) args case_bndr alts cont
1256 | Lit lit <- scrut -- No need for same treatment as constructors
1257 -- because literals are inlined more vigorously
1258 = knownCon env (LitAlt lit) [] case_bndr alts cont
1261 = prepareAlts scrut case_bndr alts `thenSmpl` \ (better_alts, handled_cons) ->
1263 -- Deal with the case binder, and prepare the continuation;
1264 -- The new subst_env is in place
1265 prepareCaseCont env better_alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1266 addFloats env floats $ \ env ->
1268 -- Deal with variable scrutinee
1269 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr', zap_occ_info) ->
1271 -- Deal with the case alternatives
1272 simplAlts alt_env zap_occ_info handled_cons
1273 case_bndr' better_alts dup_cont `thenSmpl` \ alts' ->
1275 -- Put the case back together
1276 mkCase scrut case_bndr' alts' `thenSmpl` \ case_expr ->
1278 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1279 -- The case binder *not* scope over the whole returned case-expression
1280 rebuild env case_expr nondup_cont
1283 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1284 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1285 way, there's a chance that v will now only be used once, and hence
1290 There is a time we *don't* want to do that, namely when
1291 -fno-case-of-case is on. This happens in the first simplifier pass,
1292 and enhances full laziness. Here's the bad case:
1293 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1294 If we eliminate the inner case, we trap it inside the I# v -> arm,
1295 which might prevent some full laziness happening. I've seen this
1296 in action in spectral/cichelli/Prog.hs:
1297 [(m,n) | m <- [1..max], n <- [1..max]]
1298 Hence the check for NoCaseOfCase.
1302 There is another situation when we don't want to do it. If we have
1304 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1305 ...other cases .... }
1307 We'll perform the binder-swap for the outer case, giving
1309 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1310 ...other cases .... }
1312 But there is no point in doing it for the inner case, because w1 can't
1313 be inlined anyway. Furthermore, doing the case-swapping involves
1314 zapping w2's occurrence info (see paragraphs that follow), and that
1315 forces us to bind w2 when doing case merging. So we get
1317 case x of w1 { A -> let w2 = w1 in e1
1318 B -> let w2 = w1 in e2
1319 ...other cases .... }
1321 This is plain silly in the common case where w2 is dead.
1323 Even so, I can't see a good way to implement this idea. I tried
1324 not doing the binder-swap if the scrutinee was already evaluated
1325 but that failed big-time:
1329 case v of w { MkT x ->
1330 case x of x1 { I# y1 ->
1331 case x of x2 { I# y2 -> ...
1333 Notice that because MkT is strict, x is marked "evaluated". But to
1334 eliminate the last case, we must either make sure that x (as well as
1335 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1336 the binder-swap. So this whole note is a no-op.
1340 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1341 any occurrence info (eg IAmDead) in the case binder, because the
1342 case-binder now effectively occurs whenever v does. AND we have to do
1343 the same for the pattern-bound variables! Example:
1345 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1347 Here, b and p are dead. But when we move the argment inside the first
1348 case RHS, and eliminate the second case, we get
1350 case x or { (a,b) -> a b }
1352 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1353 happened. Hence the zap_occ_info function returned by simplCaseBinder
1356 simplCaseBinder env (Var v) case_bndr
1357 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1359 -- Failed try [see Note 2 above]
1360 -- not (isEvaldUnfolding (idUnfolding v))
1362 = simplBinder env (zap case_bndr) `thenSmpl` \ (env, case_bndr') ->
1363 returnSmpl (modifyInScope env v case_bndr', case_bndr', zap)
1364 -- We could extend the substitution instead, but it would be
1365 -- a hack because then the substitution wouldn't be idempotent
1366 -- any more (v is an OutId). And this just just as well.
1368 zap b = b `setIdOccInfo` NoOccInfo
1370 simplCaseBinder env other_scrut case_bndr
1371 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1372 returnSmpl (env, case_bndr', \ bndr -> bndr) -- NoOp on bndr
1378 simplAlts :: SimplEnv
1379 -> (InId -> InId) -- Occ-info zapper
1380 -> [AltCon] -- Alternatives the scrutinee can't be
1381 -- in the default case
1382 -> OutId -- Case binder
1383 -> [InAlt] -> SimplCont
1384 -> SimplM [OutAlt] -- Includes the continuation
1386 simplAlts env zap_occ_info handled_cons case_bndr' alts cont'
1387 = mapSmpl simpl_alt alts
1389 inst_tys' = tyConAppArgs (idType case_bndr')
1391 simpl_alt (DEFAULT, _, rhs)
1393 -- In the default case we record the constructors that the
1394 -- case-binder *can't* be.
1395 -- We take advantage of any OtherCon info in the case scrutinee
1396 case_bndr_w_unf = case_bndr' `setIdUnfolding` mkOtherCon handled_cons
1397 env_with_unf = modifyInScope env case_bndr' case_bndr_w_unf
1399 simplExprC env_with_unf rhs cont' `thenSmpl` \ rhs' ->
1400 returnSmpl (DEFAULT, [], rhs')
1402 simpl_alt (con, vs, rhs)
1403 = -- Deal with the pattern-bound variables
1404 -- Mark the ones that are in ! positions in the data constructor
1405 -- as certainly-evaluated.
1406 -- NB: it happens that simplBinders does *not* erase the OtherCon
1407 -- form of unfolding, so it's ok to add this info before
1408 -- doing simplBinders
1409 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1411 -- Bind the case-binder to (con args)
1413 unfolding = mkUnfolding False (mkAltExpr con vs' inst_tys')
1414 env_with_unf = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` unfolding)
1416 simplExprC env_with_unf rhs cont' `thenSmpl` \ rhs' ->
1417 returnSmpl (con, vs', rhs')
1420 -- add_evals records the evaluated-ness of the bound variables of
1421 -- a case pattern. This is *important*. Consider
1422 -- data T = T !Int !Int
1424 -- case x of { T a b -> T (a+1) b }
1426 -- We really must record that b is already evaluated so that we don't
1427 -- go and re-evaluate it when constructing the result.
1429 add_evals (DataAlt dc) vs = cat_evals vs (dataConRepStrictness dc)
1430 add_evals other_con vs = vs
1432 cat_evals [] [] = []
1433 cat_evals (v:vs) (str:strs)
1434 | isTyVar v = v : cat_evals vs (str:strs)
1435 | isMarkedStrict str = evald_v : cat_evals vs strs
1436 | otherwise = zapped_v : cat_evals vs strs
1438 zapped_v = zap_occ_info v
1439 evald_v = zapped_v `setIdUnfolding` mkOtherCon []
1443 %************************************************************************
1445 \subsection{Known constructor}
1447 %************************************************************************
1449 We are a bit careful with occurrence info. Here's an example
1451 (\x* -> case x of (a*, b) -> f a) (h v, e)
1453 where the * means "occurs once". This effectively becomes
1454 case (h v, e) of (a*, b) -> f a)
1456 let a* = h v; b = e in f a
1460 All this should happen in one sweep.
1463 knownCon :: SimplEnv -> AltCon -> [OutExpr]
1464 -> InId -> [InAlt] -> SimplCont
1465 -> SimplM FloatsWithExpr
1467 knownCon env con args bndr alts cont
1468 = tick (KnownBranch bndr) `thenSmpl_`
1469 case findAlt con alts of
1470 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1471 simplNonRecX env bndr scrut $ \ env ->
1472 -- This might give rise to a binding with non-atomic args
1473 -- like x = Node (f x) (g x)
1474 -- but no harm will be done
1475 simplExprF env rhs cont
1478 LitAlt lit -> Lit lit
1479 DataAlt dc -> mkConApp dc args
1481 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1482 simplNonRecX env bndr (Lit lit) $ \ env ->
1483 simplExprF env rhs cont
1485 (DataAlt dc, bs, rhs) -> ASSERT( length bs + n_tys == length args )
1486 bind_args env bs (drop n_tys args) $ \ env ->
1488 con_app = mkConApp dc (take n_tys args ++ con_args)
1489 con_args = [substExpr (getSubst env) (varToCoreExpr b) | b <- bs]
1490 -- args are aready OutExprs, but bs are InIds
1492 simplNonRecX env bndr con_app $ \ env ->
1493 simplExprF env rhs cont
1495 n_tys = dataConNumInstArgs dc -- Non-existential type args
1497 bind_args env [] _ thing_inside = thing_inside env
1499 bind_args env (b:bs) (Type ty : args) thing_inside
1500 = bind_args (extendSubst env b (DoneTy ty)) bs args thing_inside
1502 bind_args env (b:bs) (arg : args) thing_inside
1503 = simplNonRecX env b arg $ \ env ->
1504 bind_args env bs args thing_inside
1508 %************************************************************************
1510 \subsection{Duplicating continuations}
1512 %************************************************************************
1515 prepareCaseCont :: SimplEnv
1516 -> [InAlt] -> SimplCont
1517 -> SimplM (FloatsWith (SimplCont,SimplCont))
1518 -- Return a duplicatable continuation, a non-duplicable part
1519 -- plus some extra bindings
1521 -- No need to make it duplicatable if there's only one alternative
1522 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1523 prepareCaseCont env alts cont = mkDupableCont env cont
1527 mkDupableCont :: SimplEnv -> SimplCont
1528 -> SimplM (FloatsWith (SimplCont, SimplCont))
1530 mkDupableCont env cont
1531 | contIsDupable cont
1532 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1534 mkDupableCont env (CoerceIt ty cont)
1535 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1536 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1538 mkDupableCont env (InlinePlease cont)
1539 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1540 returnSmpl (floats, (InlinePlease dup_cont, nondup_cont))
1542 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1543 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1544 -- Do *not* duplicate an ArgOf continuation
1545 -- Because ArgOf continuations are opaque, we gain nothing by
1546 -- propagating them into the expressions, and we do lose a lot.
1547 -- Here's an example:
1548 -- && (case x of { T -> F; F -> T }) E
1549 -- Now, && is strict so we end up simplifying the case with
1550 -- an ArgOf continuation. If we let-bind it, we get
1552 -- let $j = \v -> && v E
1553 -- in simplExpr (case x of { T -> F; F -> T })
1554 -- (ArgOf (\r -> $j r)
1555 -- And after simplifying more we get
1557 -- let $j = \v -> && v E
1558 -- in case of { T -> $j F; F -> $j T }
1559 -- Which is a Very Bad Thing
1561 -- The desire not to duplicate is the entire reason that
1562 -- mkDupableCont returns a pair of continuations.
1564 -- The original plan had:
1565 -- e.g. (...strict-fn...) [...hole...]
1567 -- let $j = \a -> ...strict-fn...
1568 -- in $j [...hole...]
1570 mkDupableCont env (ApplyTo _ arg se cont)
1571 = -- e.g. [...hole...] (...arg...)
1573 -- let a = ...arg...
1574 -- in [...hole...] a
1575 simplExpr (setInScope se env) arg `thenSmpl` \ arg' ->
1577 mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1578 addFloats env floats $ \ env ->
1580 if exprIsDupable arg' then
1581 returnSmpl (emptyFloats env, (ApplyTo OkToDup arg' (zapSubstEnv se) dup_cont, nondup_cont))
1583 newId FSLIT("a") (exprType arg') `thenSmpl` \ arg_id ->
1585 tick (CaseOfCase arg_id) `thenSmpl_`
1586 -- Want to tick here so that we go round again,
1587 -- and maybe copy or inline the code.
1588 -- Not strictly CaseOfCase, but never mind
1590 returnSmpl (unitFloat env arg_id arg',
1591 (ApplyTo OkToDup (Var arg_id) (zapSubstEnv se) dup_cont,
1593 -- But what if the arg should be case-bound?
1594 -- This has been this way for a long time, so I'll leave it,
1595 -- but I can't convince myself that it's right.
1598 mkDupableCont env (Select _ case_bndr alts se cont)
1599 = -- e.g. (case [...hole...] of { pi -> ei })
1601 -- let ji = \xij -> ei
1602 -- in case [...hole...] of { pi -> ji xij }
1603 tick (CaseOfCase case_bndr) `thenSmpl_`
1605 alt_env = setInScope se env
1607 prepareCaseCont alt_env alts cont `thenSmpl` \ (floats1, (dup_cont, nondup_cont)) ->
1608 addFloats alt_env floats1 $ \ alt_env ->
1610 simplBinder alt_env case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1611 -- NB: simplBinder does not zap deadness occ-info, so
1612 -- a dead case_bndr' will still advertise its deadness
1613 -- This is really important because in
1614 -- case e of b { (# a,b #) -> ... }
1615 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1616 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1617 -- In the new alts we build, we have the new case binder, so it must retain
1620 mkDupableAlts alt_env case_bndr' alts dup_cont `thenSmpl` \ (floats2, alts') ->
1621 addFloats alt_env floats2 $ \ alt_env ->
1622 returnSmpl (emptyFloats alt_env,
1623 (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1624 (mkBoringStop (contResultType dup_cont)),
1627 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1628 -> SimplM (FloatsWith [InAlt])
1629 -- Absorbs the continuation into the new alternatives
1631 mkDupableAlts env case_bndr' alts dupable_cont
1634 go env [] = returnSmpl (emptyFloats env, [])
1636 = mkDupableAlt env case_bndr' dupable_cont alt `thenSmpl` \ (floats1, alt') ->
1637 addFloats env floats1 $ \ env ->
1638 go env alts `thenSmpl` \ (floats2, alts') ->
1639 returnSmpl (floats2, alt' : alts')
1641 mkDupableAlt env case_bndr' cont alt@(con, bndrs, rhs)
1642 = simplBinders env bndrs `thenSmpl` \ (env, bndrs') ->
1643 simplExprC env rhs cont `thenSmpl` \ rhs' ->
1645 if exprIsDupable rhs' then
1646 returnSmpl (emptyFloats env, (con, bndrs', rhs'))
1647 -- It is worth checking for a small RHS because otherwise we
1648 -- get extra let bindings that may cause an extra iteration of the simplifier to
1649 -- inline back in place. Quite often the rhs is just a variable or constructor.
1650 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1651 -- iterations because the version with the let bindings looked big, and so wasn't
1652 -- inlined, but after the join points had been inlined it looked smaller, and so
1655 -- NB: we have to check the size of rhs', not rhs.
1656 -- Duplicating a small InAlt might invalidate occurrence information
1657 -- However, if it *is* dupable, we return the *un* simplified alternative,
1658 -- because otherwise we'd need to pair it up with an empty subst-env....
1659 -- but we only have one env shared between all the alts.
1660 -- (Remember we must zap the subst-env before re-simplifying something).
1661 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1665 rhs_ty' = exprType rhs'
1666 used_bndrs' = filter (not . isDeadBinder) (case_bndr' : bndrs')
1667 -- The deadness info on the new binders is unscathed
1669 -- If we try to lift a primitive-typed something out
1670 -- for let-binding-purposes, we will *caseify* it (!),
1671 -- with potentially-disastrous strictness results. So
1672 -- instead we turn it into a function: \v -> e
1673 -- where v::State# RealWorld#. The value passed to this function
1674 -- is realworld#, which generates (almost) no code.
1676 -- There's a slight infelicity here: we pass the overall
1677 -- case_bndr to all the join points if it's used in *any* RHS,
1678 -- because we don't know its usage in each RHS separately
1680 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1681 -- we make the join point into a function whenever used_bndrs'
1682 -- is empty. This makes the join-point more CPR friendly.
1683 -- Consider: let j = if .. then I# 3 else I# 4
1684 -- in case .. of { A -> j; B -> j; C -> ... }
1686 -- Now CPR doesn't w/w j because it's a thunk, so
1687 -- that means that the enclosing function can't w/w either,
1688 -- which is a lose. Here's the example that happened in practice:
1689 -- kgmod :: Int -> Int -> Int
1690 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1694 -- I have seen a case alternative like this:
1695 -- True -> \v -> ...
1696 -- It's a bit silly to add the realWorld dummy arg in this case, making
1699 -- (the \v alone is enough to make CPR happy) but I think it's rare
1701 ( if null used_bndrs'
1702 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
1703 returnSmpl ([rw_id], [Var realWorldPrimId])
1705 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
1706 ) `thenSmpl` \ (final_bndrs', final_args) ->
1708 -- See comment about "$j" name above
1709 newId (encodeFS FSLIT("$j")) (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
1710 -- Notice the funky mkPiTypes. If the contructor has existentials
1711 -- it's possible that the join point will be abstracted over
1712 -- type varaibles as well as term variables.
1713 -- Example: Suppose we have
1714 -- data T = forall t. C [t]
1716 -- case (case e of ...) of
1717 -- C t xs::[t] -> rhs
1718 -- We get the join point
1719 -- let j :: forall t. [t] -> ...
1720 -- j = /\t \xs::[t] -> rhs
1722 -- case (case e of ...) of
1723 -- C t xs::[t] -> j t xs
1725 -- We make the lambdas into one-shot-lambdas. The
1726 -- join point is sure to be applied at most once, and doing so
1727 -- prevents the body of the join point being floated out by
1728 -- the full laziness pass
1729 really_final_bndrs = map one_shot final_bndrs'
1730 one_shot v | isId v = setOneShotLambda v
1732 join_rhs = mkLams really_final_bndrs rhs'
1733 join_call = mkApps (Var join_bndr) final_args
1735 returnSmpl (unitFloat env join_bndr join_rhs, (con, bndrs', join_call))