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,
17 SimplCont(..), DupFlag(..), LetRhsFlag(..),
18 mkRhsStop, mkBoringStop, mkLazyArgStop, pushContArgs,
19 contResultType, countArgs, contIsDupable, contIsRhsOrArg,
20 getContArgs, interestingCallContext, interestingArg, isStrictType,
21 preInlineUnconditionally, postInlineUnconditionally,
22 interestingArgContext, inlineMode, activeInline, activeRule
24 import Id ( Id, idType, idInfo, idArity, isDataConWorkId,
25 idUnfolding, setIdUnfolding, isDeadBinder,
26 idNewDemandInfo, setIdInfo,
27 setIdOccInfo, zapLamIdInfo, setOneShotLambda
29 import MkId ( eRROR_ID )
30 import Literal ( mkStringLit )
31 import IdInfo ( OccInfo(..), isLoopBreaker,
32 setArityInfo, zapDemandInfo,
36 import NewDemand ( isStrictDmd )
37 import TcGadt ( dataConCanMatch )
38 import DataCon ( DataCon, dataConTyCon, dataConRepStrictness )
39 import TyCon ( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
41 import PprCore ( pprParendExpr, pprCoreExpr )
42 import CoreUnfold ( mkUnfolding, callSiteInline )
43 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
44 exprIsConApp_maybe, mkPiTypes, findAlt,
45 exprType, exprIsHNF, findDefault, mergeAlts,
46 exprOkForSpeculation, exprArity,
47 mkCoerce, mkSCC, mkInlineMe, applyTypeToArg
49 import Rules ( lookupRule )
50 import BasicTypes ( isMarkedStrict )
51 import CostCentre ( currentCCS )
52 import Type ( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
53 splitFunTy_maybe, splitFunTy, coreEqType, splitTyConApp_maybe,
54 isTyVarTy, mkTyVarTys, isFunTy, tcEqType
56 import Coercion ( Coercion, coercionKind,
57 mkTransCoercion, mkLeftCoercion, mkRightCoercion,
58 mkSymCoercion, splitCoercionKind_maybe, decomposeCo )
59 import Var ( tyVarKind, mkTyVar )
60 import VarEnv ( elemVarEnv, emptyVarEnv )
61 import TysPrim ( realWorldStatePrimTy )
62 import PrelInfo ( realWorldPrimId )
63 import BasicTypes ( TopLevelFlag(..), isTopLevel,
68 import Maybes ( orElse )
70 import Util ( notNull, filterOut )
74 The guts of the simplifier is in this module, but the driver loop for
75 the simplifier is in SimplCore.lhs.
78 -----------------------------------------
79 *** IMPORTANT NOTE ***
80 -----------------------------------------
81 The simplifier used to guarantee that the output had no shadowing, but
82 it does not do so any more. (Actually, it never did!) The reason is
83 documented with simplifyArgs.
86 -----------------------------------------
87 *** IMPORTANT NOTE ***
88 -----------------------------------------
89 Many parts of the simplifier return a bunch of "floats" as well as an
90 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
92 All "floats" are let-binds, not case-binds, but some non-rec lets may
93 be unlifted (with RHS ok-for-speculation).
97 -----------------------------------------
98 ORGANISATION OF FUNCTIONS
99 -----------------------------------------
101 - simplify all top-level binders
102 - for NonRec, call simplRecOrTopPair
103 - for Rec, call simplRecBind
106 ------------------------------
107 simplExpr (applied lambda) ==> simplNonRecBind
108 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
109 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
111 ------------------------------
112 simplRecBind [binders already simplfied]
113 - use simplRecOrTopPair on each pair in turn
115 simplRecOrTopPair [binder already simplified]
116 Used for: recursive bindings (top level and nested)
117 top-level non-recursive bindings
119 - check for PreInlineUnconditionally
123 Used for: non-top-level non-recursive bindings
124 beta reductions (which amount to the same thing)
125 Because it can deal with strict arts, it takes a
126 "thing-inside" and returns an expression
128 - check for PreInlineUnconditionally
129 - simplify binder, including its IdInfo
138 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
139 Used for: binding case-binder and constr args in a known-constructor case
140 - check for PreInLineUnconditionally
144 ------------------------------
145 simplLazyBind: [binder already simplified, RHS not]
146 Used for: recursive bindings (top level and nested)
147 top-level non-recursive bindings
148 non-top-level, but *lazy* non-recursive bindings
149 [must not be strict or unboxed]
150 Returns floats + an augmented environment, not an expression
151 - substituteIdInfo and add result to in-scope
152 [so that rules are available in rec rhs]
155 - float if exposes constructor or PAP
159 completeNonRecX: [binder and rhs both simplified]
160 - if the the thing needs case binding (unlifted and not ok-for-spec)
166 completeLazyBind: [given a simplified RHS]
167 [used for both rec and non-rec bindings, top level and not]
168 - try PostInlineUnconditionally
169 - add unfolding [this is the only place we add an unfolding]
174 Right hand sides and arguments
175 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
176 In many ways we want to treat
177 (a) the right hand side of a let(rec), and
178 (b) a function argument
179 in the same way. But not always! In particular, we would
180 like to leave these arguments exactly as they are, so they
181 will match a RULE more easily.
186 It's harder to make the rule match if we ANF-ise the constructor,
187 or eta-expand the PAP:
189 f (let { a = g x; b = h x } in (a,b))
192 On the other hand if we see the let-defns
197 then we *do* want to ANF-ise and eta-expand, so that p and q
198 can be safely inlined.
200 Even floating lets out is a bit dubious. For let RHS's we float lets
201 out if that exposes a value, so that the value can be inlined more vigorously.
204 r = let x = e in (x,x)
206 Here, if we float the let out we'll expose a nice constructor. We did experiments
207 that showed this to be a generally good thing. But it was a bad thing to float
208 lets out unconditionally, because that meant they got allocated more often.
210 For function arguments, there's less reason to expose a constructor (it won't
211 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
212 So for the moment we don't float lets out of function arguments either.
217 For eta expansion, we want to catch things like
219 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
221 If the \x was on the RHS of a let, we'd eta expand to bring the two
222 lambdas together. And in general that's a good thing to do. Perhaps
223 we should eta expand wherever we find a (value) lambda? Then the eta
224 expansion at a let RHS can concentrate solely on the PAP case.
227 %************************************************************************
229 \subsection{Bindings}
231 %************************************************************************
234 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
236 simplTopBinds env binds
237 = -- Put all the top-level binders into scope at the start
238 -- so that if a transformation rule has unexpectedly brought
239 -- anything into scope, then we don't get a complaint about that.
240 -- It's rather as if the top-level binders were imported.
241 simplRecBndrs env (bindersOfBinds binds) `thenSmpl` \ (env, bndrs') ->
242 simpl_binds env binds bndrs' `thenSmpl` \ (floats, _) ->
243 freeTick SimplifierDone `thenSmpl_`
244 returnSmpl (floatBinds floats)
246 -- We need to track the zapped top-level binders, because
247 -- they should have their fragile IdInfo zapped (notably occurrence info)
248 -- That's why we run down binds and bndrs' simultaneously.
249 simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
250 simpl_binds env [] bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
251 simpl_binds env (bind:binds) bs = simpl_bind env bind bs `thenSmpl` \ (floats,env) ->
252 addFloats env floats $ \env ->
253 simpl_binds env binds (drop_bs bind bs)
255 drop_bs (NonRec _ _) (_ : bs) = bs
256 drop_bs (Rec prs) bs = drop (length prs) bs
258 simpl_bind env bind bs
259 = getDOptsSmpl `thenSmpl` \ dflags ->
260 if dopt Opt_D_dump_inlinings dflags then
261 pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
263 simpl_bind1 env bind bs
265 simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
266 simpl_bind1 env (Rec pairs) bs' = simplRecBind env TopLevel pairs bs'
270 %************************************************************************
272 \subsection{simplNonRec}
274 %************************************************************************
276 simplNonRecBind is used for
277 * non-top-level non-recursive lets in expressions
281 * An unsimplified (binder, rhs) pair
282 * The env for the RHS. It may not be the same as the
283 current env because the bind might occur via (\x.E) arg
285 It uses the CPS form because the binding might be strict, in which
286 case we might discard the continuation:
287 let x* = error "foo" in (...x...)
289 It needs to turn unlifted bindings into a @case@. They can arise
290 from, say: (\x -> e) (4# + 3#)
293 simplNonRecBind :: SimplEnv
295 -> InExpr -> SimplEnv -- Arg, with its subst-env
296 -> OutType -- Type of thing computed by the context
297 -> (SimplEnv -> SimplM FloatsWithExpr) -- The body
298 -> SimplM FloatsWithExpr
300 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
302 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
305 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
306 = simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
308 simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
309 | preInlineUnconditionally env NotTopLevel bndr rhs
310 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
311 thing_inside (extendIdSubst env bndr (mkContEx rhs_se rhs))
313 | isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty -- A strict let
314 = -- Don't use simplBinder because that doesn't keep
315 -- fragile occurrence info in the substitution
316 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr1) ->
317 simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty $ \ env1 rhs1 ->
319 -- Now complete the binding and simplify the body
321 (env2,bndr2) = addLetIdInfo env1 bndr bndr1
323 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
325 | otherwise -- Normal, lazy case
326 = -- Don't use simplBinder because that doesn't keep
327 -- fragile occurrence info in the substitution
328 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr') ->
329 simplLazyBind env NotTopLevel NonRecursive
330 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
331 addFloats env floats thing_inside
334 bndr_ty = idType bndr
337 A specialised variant of simplNonRec used when the RHS is already simplified, notably
338 in knownCon. It uses case-binding where necessary.
341 simplNonRecX :: SimplEnv
342 -> InId -- Old binder
343 -> OutExpr -- Simplified RHS
344 -> (SimplEnv -> SimplM FloatsWithExpr)
345 -> SimplM FloatsWithExpr
347 simplNonRecX env bndr new_rhs thing_inside
348 = do { (env, bndr') <- simplBinder env bndr
349 ; completeNonRecX env False {- Non-strict; pessimistic -}
350 bndr bndr' new_rhs thing_inside }
353 completeNonRecX :: SimplEnv
354 -> Bool -- Strict binding
355 -> InId -- Old binder
356 -> OutId -- New binder
357 -> OutExpr -- Simplified RHS
358 -> (SimplEnv -> SimplM FloatsWithExpr)
359 -> SimplM FloatsWithExpr
361 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
362 | needsCaseBinding (idType new_bndr) new_rhs
363 -- Make this test *before* the preInlineUnconditionally
364 -- Consider case I# (quotInt# x y) of
365 -- I# v -> let w = J# v in ...
366 -- If we gaily inline (quotInt# x y) for v, we end up building an
368 -- let w = J# (quotInt# x y) in ...
369 -- because quotInt# can fail.
370 = do { (floats, body) <- thing_inside env
371 ; let body' = wrapFloats floats body
372 ; return (emptyFloats env, Case new_rhs new_bndr (exprType body)
373 [(DEFAULT, [], body')]) }
376 = -- Make the arguments atomic if necessary,
377 -- adding suitable bindings
378 -- pprTrace "completeNonRecX" (ppr new_bndr <+> ppr new_rhs) $
379 mkAtomicArgsE env is_strict new_rhs $ \ env new_rhs ->
380 completeLazyBind env NotTopLevel
381 old_bndr new_bndr new_rhs `thenSmpl` \ (floats, env) ->
382 addFloats env floats thing_inside
384 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
385 Doing so risks exponential behaviour, because new_rhs has been simplified once already
386 In the cases described by the folowing commment, postInlineUnconditionally will
387 catch many of the relevant cases.
388 -- This happens; for example, the case_bndr during case of
389 -- known constructor: case (a,b) of x { (p,q) -> ... }
390 -- Here x isn't mentioned in the RHS, so we don't want to
391 -- create the (dead) let-binding let x = (a,b) in ...
393 -- Similarly, single occurrences can be inlined vigourously
394 -- e.g. case (f x, g y) of (a,b) -> ....
395 -- If a,b occur once we can avoid constructing the let binding for them.
396 | preInlineUnconditionally env NotTopLevel bndr new_rhs
397 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
399 -- NB: completeLazyBind uses postInlineUnconditionally; no need to do that here
404 %************************************************************************
406 \subsection{Lazy bindings}
408 %************************************************************************
410 simplRecBind is used for
411 * recursive bindings only
414 simplRecBind :: SimplEnv -> TopLevelFlag
415 -> [(InId, InExpr)] -> [OutId]
416 -> SimplM (FloatsWith SimplEnv)
417 simplRecBind env top_lvl pairs bndrs'
418 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
419 returnSmpl (flattenFloats floats, env)
421 go env [] _ = returnSmpl (emptyFloats env, env)
423 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
424 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
425 addFloats env floats (\env -> go env pairs bndrs')
429 simplRecOrTopPair is used for
430 * recursive bindings (whether top level or not)
431 * top-level non-recursive bindings
433 It assumes the binder has already been simplified, but not its IdInfo.
436 simplRecOrTopPair :: SimplEnv
438 -> InId -> OutId -- Binder, both pre-and post simpl
439 -> InExpr -- The RHS and its environment
440 -> SimplM (FloatsWith SimplEnv)
442 simplRecOrTopPair env top_lvl bndr bndr' rhs
443 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
444 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
445 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
448 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
449 -- May not actually be recursive, but it doesn't matter
453 simplLazyBind is used for
454 * recursive bindings (whether top level or not)
455 * top-level non-recursive bindings
456 * non-top-level *lazy* non-recursive bindings
458 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
459 from SimplRecOrTopBind]
462 1. It assumes that the binder is *already* simplified,
463 and is in scope, but not its IdInfo
465 2. It assumes that the binder type is lifted.
467 3. It does not check for pre-inline-unconditionallly;
468 that should have been done already.
471 simplLazyBind :: SimplEnv
472 -> TopLevelFlag -> RecFlag
473 -> InId -> OutId -- Binder, both pre-and post simpl
474 -> InExpr -> SimplEnv -- The RHS and its environment
475 -> SimplM (FloatsWith SimplEnv)
477 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
479 (env1,bndr2) = addLetIdInfo env bndr bndr1
480 rhs_env = setInScope rhs_se env1
481 is_top_level = isTopLevel top_lvl
482 ok_float_unlifted = not is_top_level && isNonRec is_rec
483 rhs_cont = mkRhsStop (idType bndr2)
485 -- Simplify the RHS; note the mkRhsStop, which tells
486 -- the simplifier that this is the RHS of a let.
487 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
489 -- If any of the floats can't be floated, give up now
490 -- (The allLifted predicate says True for empty floats.)
491 if (not ok_float_unlifted && not (allLifted floats)) then
492 completeLazyBind env1 top_lvl bndr bndr2
493 (wrapFloats floats rhs1)
496 -- ANF-ise a constructor or PAP rhs
497 mkAtomicArgs False {- Not strict -}
498 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
500 -- If the result is a PAP, float the floats out, else wrap them
501 -- By this time it's already been ANF-ised (if necessary)
502 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
503 completeLazyBind env1 top_lvl bndr bndr2 rhs2
505 else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
506 -- WARNING: long dodgy argument coming up
507 -- WANTED: a better way to do this
509 -- We can't use "exprIsCheap" instead of exprIsHNF,
510 -- because that causes a strictness bug.
511 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
512 -- The case expression is 'cheap', but it's wrong to transform to
513 -- y* = E; x = case (scc y) of {...}
514 -- Either we must be careful not to float demanded non-values, or
515 -- we must use exprIsHNF for the test, which ensures that the
516 -- thing is non-strict. So exprIsHNF => bindings are non-strict
517 -- I think. The WARN below tests for this.
519 -- We use exprIsTrivial here because we want to reveal lone variables.
520 -- E.g. let { x = letrec { y = E } in y } in ...
521 -- Here we definitely want to float the y=E defn.
522 -- exprIsHNF definitely isn't right for that.
524 -- Again, the floated binding can't be strict; if it's recursive it'll
525 -- be non-strict; if it's non-recursive it'd be inlined.
527 -- Note [SCC-and-exprIsTrivial]
529 -- y = let { x* = E } in scc "foo" x
530 -- then we do *not* want to float out the x binding, because
531 -- it's strict! Fortunately, exprIsTrivial replies False to
534 -- There's a subtlety here. There may be a binding (x* = e) in the
535 -- floats, where the '*' means 'will be demanded'. So is it safe
536 -- to float it out? Answer no, but it won't matter because
537 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
538 -- and so there can't be any 'will be demanded' bindings in the floats.
540 WARN( not (is_top_level || not (any demanded_float (floatBinds floats))),
541 ppr (filter demanded_float (floatBinds floats)) )
543 tick LetFloatFromLet `thenSmpl_` (
544 addFloats env1 floats $ \ env2 ->
545 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
546 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
549 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
552 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
553 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
554 demanded_float (Rec _) = False
559 %************************************************************************
561 \subsection{Completing a lazy binding}
563 %************************************************************************
566 * deals only with Ids, not TyVars
567 * takes an already-simplified binder and RHS
568 * is used for both recursive and non-recursive bindings
569 * is used for both top-level and non-top-level bindings
571 It does the following:
572 - tries discarding a dead binding
573 - tries PostInlineUnconditionally
574 - add unfolding [this is the only place we add an unfolding]
577 It does *not* attempt to do let-to-case. Why? Because it is used for
578 - top-level bindings (when let-to-case is impossible)
579 - many situations where the "rhs" is known to be a WHNF
580 (so let-to-case is inappropriate).
583 completeLazyBind :: SimplEnv
584 -> TopLevelFlag -- Flag stuck into unfolding
585 -> InId -- Old binder
586 -> OutId -- New binder
587 -> OutExpr -- Simplified RHS
588 -> SimplM (FloatsWith SimplEnv)
589 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
590 -- by extending the substitution (e.g. let x = y in ...)
591 -- The new binding (if any) is returned as part of the floats.
592 -- NB: the returned SimplEnv has the right SubstEnv, but you should
593 -- (as usual) use the in-scope-env from the floats
595 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
596 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
597 = -- Drop the binding
598 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
599 -- pprTrace "Inline unconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
600 returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
601 -- Use the substitution to make quite, quite sure that the substitution
602 -- will happen, since we are going to discard the binding
607 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
609 -- Add the unfolding *only* for non-loop-breakers
610 -- Making loop breakers not have an unfolding at all
611 -- means that we can avoid tests in exprIsConApp, for example.
612 -- This is important: if exprIsConApp says 'yes' for a recursive
613 -- thing, then we can get into an infinite loop
614 -- If the unfolding is a value, the demand info may
615 -- go pear-shaped, so we nuke it. Example:
617 -- case x of (p,q) -> h p q x
618 -- Here x is certainly demanded. But after we've nuked
619 -- the case, we'll get just
620 -- let x = (a,b) in h a b x
621 -- and now x is not demanded (I'm assuming h is lazy)
622 -- This really happens. Similarly
623 -- let f = \x -> e in ...f..f...
624 -- After inling f at some of its call sites the original binding may
625 -- (for example) be no longer strictly demanded.
626 -- The solution here is a bit ad hoc...
627 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
628 final_info | loop_breaker = new_bndr_info
629 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
630 | otherwise = info_w_unf
632 final_id = new_bndr `setIdInfo` final_info
634 -- These seqs forces the Id, and hence its IdInfo,
635 -- and hence any inner substitutions
637 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
638 returnSmpl (unitFloat env final_id new_rhs, env)
641 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
642 loop_breaker = isLoopBreaker occ_info
643 old_info = idInfo old_bndr
644 occ_info = occInfo old_info
649 %************************************************************************
651 \subsection[Simplify-simplExpr]{The main function: simplExpr}
653 %************************************************************************
655 The reason for this OutExprStuff stuff is that we want to float *after*
656 simplifying a RHS, not before. If we do so naively we get quadratic
657 behaviour as things float out.
659 To see why it's important to do it after, consider this (real) example:
673 a -- Can't inline a this round, cos it appears twice
677 Each of the ==> steps is a round of simplification. We'd save a
678 whole round if we float first. This can cascade. Consider
683 let f = let d1 = ..d.. in \y -> e
687 in \x -> ...(\y ->e)...
689 Only in this second round can the \y be applied, and it
690 might do the same again.
694 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
695 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
697 expr_ty' = substTy env (exprType expr)
698 -- The type in the Stop continuation, expr_ty', is usually not used
699 -- It's only needed when discarding continuations after finding
700 -- a function that returns bottom.
701 -- Hence the lazy substitution
704 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
705 -- Simplify an expression, given a continuation
706 simplExprC env expr cont
707 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
708 returnSmpl (wrapFloats floats expr)
710 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
711 -- Simplify an expression, returning floated binds
713 simplExprF env (Var v) cont = simplVar env v cont
714 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
715 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
716 simplExprF env (Note note expr) cont = simplNote env note expr cont
717 simplExprF env (Cast body co) cont = simplCast env body co cont
718 simplExprF env (App fun arg) cont = simplExprF env fun
719 (ApplyTo NoDup arg (Just env) cont)
721 simplExprF env (Type ty) cont
722 = ASSERT( contIsRhsOrArg cont )
723 simplType env ty `thenSmpl` \ ty' ->
724 rebuild env (Type ty') cont
726 simplExprF env (Case scrut bndr case_ty alts) cont
727 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
728 = -- Simplify the scrutinee with a Select continuation
729 simplExprF env scrut (Select NoDup bndr alts env cont)
732 = -- If case-of-case is off, simply simplify the case expression
733 -- in a vanilla Stop context, and rebuild the result around it
734 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
735 rebuild env case_expr' cont
737 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
738 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
740 simplExprF env (Let (Rec pairs) body) cont
741 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
742 -- NB: bndrs' don't have unfoldings or rules
743 -- We add them as we go down
745 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
746 addFloats env floats $ \ env ->
747 simplExprF env body cont
749 -- A non-recursive let is dealt with by simplNonRecBind
750 simplExprF env (Let (NonRec bndr rhs) body) cont
751 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
752 simplExprF env body cont
755 ---------------------------------
756 simplType :: SimplEnv -> InType -> SimplM OutType
757 -- Kept monadic just so we can do the seqType
759 = seqType new_ty `seq` returnSmpl new_ty
761 new_ty = substTy env ty
765 %************************************************************************
769 %************************************************************************
772 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont -> SimplM FloatsWithExpr
773 simplCast env body co cont
776 | (s1, k1) <- coercionKind co
777 , s1 `tcEqType` k1 = cont
778 addCoerce co1 (CoerceIt co2 cont)
779 | (s1, k1) <- coercionKind co1
780 , (l1, t1) <- coercionKind co2
781 -- coerce T1 S1 (coerce S1 K1 e)
784 -- coerce T1 K1 e, otherwise
786 -- For example, in the initial form of a worker
787 -- we may find (coerce T (coerce S (\x.e))) y
788 -- and we'd like it to simplify to e[y/x] in one round
790 , s1 `coreEqType` t1 = cont -- The coerces cancel out
791 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
793 addCoerce co (ApplyTo dup arg arg_se cont)
794 | not (isTypeArg arg) -- This whole case only works for value args
795 -- Could upgrade to have equiv thing for type apps too
796 , Just (s1s2, t1t2) <- splitCoercionKind_maybe co
798 -- co : s1s2 :=: t1t2
799 -- (coerce (T1->T2) (S1->S2) F) E
801 -- coerce T2 S2 (F (coerce S1 T1 E))
803 -- t1t2 must be a function type, T1->T2, because it's applied
804 -- to something but s1s2 might conceivably not be
806 -- When we build the ApplyTo we can't mix the out-types
807 -- with the InExpr in the argument, so we simply substitute
808 -- to make it all consistent. It's a bit messy.
809 -- But it isn't a common case.
812 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
813 -- t2 :=: s2 with left and right on the curried form:
814 -- (->) t1 t2 :=: (->) s1 s2
815 [co1, co2] = decomposeCo 2 co
816 new_arg = mkCoerce (mkSymCoercion co1) arg'
817 arg' = case arg_se of
819 Just arg_se -> substExpr (setInScope arg_se env) arg
820 result = ApplyTo dup new_arg (Just $ zapSubstEnv env)
822 addCoerce co cont = CoerceIt co cont
824 simplType env co `thenSmpl` \ co' ->
825 simplExprF env body (addCoerce co' cont)
828 %************************************************************************
832 %************************************************************************
835 simplLam env fun cont
838 zap_it = mkLamBndrZapper fun (countArgs cont)
839 cont_ty = contResultType cont
841 -- Type-beta reduction
842 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) mb_arg_se body_cont)
843 = ASSERT( isTyVar bndr )
844 do { tick (BetaReduction bndr)
845 ; ty_arg' <- case mb_arg_se of
846 Just arg_se -> simplType (setInScope arg_se env) ty_arg
847 Nothing -> return ty_arg
848 ; go (extendTvSubst env bndr ty_arg') body body_cont }
850 -- Ordinary beta reduction
851 go env (Lam bndr body) cont@(ApplyTo _ arg (Just arg_se) body_cont)
852 = do { tick (BetaReduction bndr)
853 ; simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
854 go env body body_cont }
856 go env (Lam bndr body) cont@(ApplyTo _ arg Nothing body_cont)
857 = do { tick (BetaReduction bndr)
858 ; simplNonRecX env (zap_it bndr) arg $ \ env ->
859 go env body body_cont }
861 -- Not enough args, so there are real lambdas left to put in the result
862 go env lam@(Lam _ _) cont
863 = do { (env, bndrs') <- simplLamBndrs env bndrs
864 ; body' <- simplExpr env body
865 ; (floats, new_lam) <- mkLam env bndrs' body' cont
866 ; addFloats env floats $ \ env ->
867 rebuild env new_lam cont }
869 (bndrs,body) = collectBinders lam
871 -- Exactly enough args
872 go env expr cont = simplExprF env expr cont
874 mkLamBndrZapper :: CoreExpr -- Function
875 -> Int -- Number of args supplied, *including* type args
876 -> Id -> Id -- Use this to zap the binders
877 mkLamBndrZapper fun n_args
878 | n_args >= n_params fun = \b -> b -- Enough args
879 | otherwise = \b -> zapLamIdInfo b
881 -- NB: we count all the args incl type args
882 -- so we must count all the binders (incl type lambdas)
883 n_params (Note _ e) = n_params e
884 n_params (Lam b e) = 1 + n_params e
885 n_params other = 0::Int
889 %************************************************************************
893 %************************************************************************
898 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
899 -- inlining. All other CCCSs are mapped to currentCCS.
900 simplNote env (SCC cc) e cont
901 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
902 rebuild env (mkSCC cc e') cont
904 -- See notes with SimplMonad.inlineMode
905 simplNote env InlineMe e cont
906 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
907 = -- Don't inline inside an INLINE expression
908 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
909 rebuild env (mkInlineMe e') cont
911 | otherwise -- Dissolve the InlineMe note if there's
912 -- an interesting context of any kind to combine with
913 -- (even a type application -- anything except Stop)
914 = simplExprF env e cont
916 simplNote env (CoreNote s) e cont
917 = simplExpr env e `thenSmpl` \ e' ->
918 rebuild env (Note (CoreNote s) e') cont
922 %************************************************************************
924 \subsection{Dealing with calls}
926 %************************************************************************
929 simplVar env var cont
930 = case substId env var of
931 DoneEx e -> simplExprF (zapSubstEnv env) e cont
932 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
933 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
934 -- Note [zapSubstEnv]
935 -- The template is already simplified, so don't re-substitute.
936 -- This is VITAL. Consider
938 -- let y = \z -> ...x... in
940 -- We'll clone the inner \x, adding x->x' in the id_subst
941 -- Then when we inline y, we must *not* replace x by x' in
942 -- the inlined copy!!
944 ---------------------------------------------------------
945 -- Dealing with a call site
947 completeCall env var occ_info cont
948 = -- Simplify the arguments
949 getDOptsSmpl `thenSmpl` \ dflags ->
951 chkr = getSwitchChecker env
952 (args, call_cont) = getContArgs chkr var cont
955 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
956 (contResultType call_cont) $ \ env args ->
958 -- Next, look for rules or specialisations that match
960 -- It's important to simplify the args first, because the rule-matcher
961 -- doesn't do substitution as it goes. We don't want to use subst_args
962 -- (defined in the 'where') because that throws away useful occurrence info,
963 -- and perhaps-very-important specialisations.
965 -- Some functions have specialisations *and* are strict; in this case,
966 -- we don't want to inline the wrapper of the non-specialised thing; better
967 -- to call the specialised thing instead.
968 -- We used to use the black-listing mechanism to ensure that inlining of
969 -- the wrapper didn't occur for things that have specialisations till a
970 -- later phase, so but now we just try RULES first
972 -- You might think that we shouldn't apply rules for a loop breaker:
973 -- doing so might give rise to an infinite loop, because a RULE is
974 -- rather like an extra equation for the function:
975 -- RULE: f (g x) y = x+y
978 -- But it's too drastic to disable rules for loop breakers.
979 -- Even the foldr/build rule would be disabled, because foldr
980 -- is recursive, and hence a loop breaker:
981 -- foldr k z (build g) = g k z
982 -- So it's up to the programmer: rules can cause divergence
985 in_scope = getInScope env
987 maybe_rule = case activeRule env of
988 Nothing -> Nothing -- No rules apply
989 Just act_fn -> lookupRule act_fn in_scope rules var args
992 Just (rule_name, rule_rhs) ->
993 tick (RuleFired rule_name) `thenSmpl_`
994 (if dopt Opt_D_dump_inlinings dflags then
995 pprTrace "Rule fired" (vcat [
996 text "Rule:" <+> ftext rule_name,
997 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
998 text "After: " <+> pprCoreExpr rule_rhs,
999 text "Cont: " <+> ppr call_cont])
1002 simplExprF env rule_rhs call_cont ;
1004 Nothing -> -- No rules
1006 -- Next, look for an inlining
1008 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
1009 interesting_cont = interestingCallContext (notNull args)
1012 active_inline = activeInline env var occ_info
1013 maybe_inline = callSiteInline dflags active_inline occ_info
1014 var arg_infos interesting_cont
1016 case maybe_inline of {
1017 Just unfolding -- There is an inlining!
1018 -> tick (UnfoldingDone var) `thenSmpl_`
1019 (if dopt Opt_D_dump_inlinings dflags then
1020 pprTrace "Inlining done" (vcat [
1021 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1022 text "Inlined fn: " <+> ppr unfolding,
1023 text "Cont: " <+> ppr call_cont])
1026 simplExprF env unfolding (pushContArgs args call_cont)
1029 Nothing -> -- No inlining!
1032 rebuild env (mkApps (Var var) args) call_cont
1036 %************************************************************************
1038 \subsection{Arguments}
1040 %************************************************************************
1043 ---------------------------------------------------------
1044 -- Simplifying the arguments of a call
1046 simplifyArgs :: SimplEnv
1047 -> OutType -- Type of the function
1048 -> Bool -- True if the fn has RULES
1049 -> [(InExpr, Maybe SimplEnv, Bool)] -- Details of the arguments
1050 -> OutType -- Type of the continuation
1051 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1052 -> SimplM FloatsWithExpr
1054 -- [CPS-like because of strict arguments]
1056 -- Simplify the arguments to a call.
1057 -- This part of the simplifier may break the no-shadowing invariant
1059 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1060 -- where f is strict in its second arg
1061 -- If we simplify the innermost one first we get (...(\a -> e)...)
1062 -- Simplifying the second arg makes us float the case out, so we end up with
1063 -- case y of (a,b) -> f (...(\a -> e)...) e'
1064 -- So the output does not have the no-shadowing invariant. However, there is
1065 -- no danger of getting name-capture, because when the first arg was simplified
1066 -- we used an in-scope set that at least mentioned all the variables free in its
1067 -- static environment, and that is enough.
1069 -- We can't just do innermost first, or we'd end up with a dual problem:
1070 -- case x of (a,b) -> f e (...(\a -> e')...)
1072 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1073 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1074 -- continuations. We have to keep the right in-scope set around; AND we have
1075 -- to get the effect that finding (error "foo") in a strict arg position will
1076 -- discard the entire application and replace it with (error "foo"). Getting
1077 -- all this at once is TOO HARD!
1079 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1080 = go env fn_ty args thing_inside
1082 go env fn_ty [] thing_inside = thing_inside env []
1083 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1084 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1085 thing_inside env (arg':args')
1087 simplifyArg env fn_ty has_rules (arg, Nothing, _) cont_ty thing_inside
1088 = thing_inside env arg -- Already simplified
1090 simplifyArg env fn_ty has_rules (Type ty_arg, Just se, _) cont_ty thing_inside
1091 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1092 thing_inside env (Type new_ty_arg)
1094 simplifyArg env fn_ty has_rules (val_arg, Just arg_se, is_strict) cont_ty thing_inside
1096 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1098 | otherwise -- Lazy argument
1099 -- DO NOT float anything outside, hence simplExprC
1100 -- There is no benefit (unlike in a let-binding), and we'd
1101 -- have to be very careful about bogus strictness through
1102 -- floating a demanded let.
1103 = simplExprC (setInScope arg_se env) val_arg
1104 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1105 thing_inside env arg1
1107 arg_ty = funArgTy fn_ty
1110 simplStrictArg :: LetRhsFlag
1111 -> SimplEnv -- The env of the call
1112 -> InExpr -> SimplEnv -- The arg plus its env
1113 -> OutType -- arg_ty: type of the argument
1114 -> OutType -- cont_ty: Type of thing computed by the context
1115 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1116 -- Takes an expression of type rhs_ty,
1117 -- returns an expression of type cont_ty
1118 -- The env passed to this continuation is the
1119 -- env of the call, plus any new in-scope variables
1120 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1122 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1123 = simplExprF (setInScope arg_env call_env) arg
1124 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1125 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1126 -- to simplify the argument
1127 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1131 %************************************************************************
1133 \subsection{mkAtomicArgs}
1135 %************************************************************************
1137 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1138 constructor application and, if so, converts it to ANF, so that the
1139 resulting thing can be inlined more easily. Thus
1146 There are three sorts of binding context, specified by the two
1152 N N Top-level or recursive Only bind args of lifted type
1154 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1155 but lazy unlifted-and-ok-for-speculation
1157 Y Y Non-top-level, non-recursive, Bind all args
1158 and strict (demanded)
1165 there is no point in transforming to
1167 x = case (y div# z) of r -> MkC r
1169 because the (y div# z) can't float out of the let. But if it was
1170 a *strict* let, then it would be a good thing to do. Hence the
1171 context information.
1174 mkAtomicArgsE :: SimplEnv
1175 -> Bool -- A strict binding
1176 -> OutExpr -- The rhs
1177 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1178 -> SimplM FloatsWithExpr
1180 mkAtomicArgsE env is_strict rhs thing_inside
1181 | (Var fun, args) <- collectArgs rhs, -- It's an application
1182 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1183 = go env (Var fun) args
1185 | otherwise = thing_inside env rhs
1188 go env fun [] = thing_inside env fun
1190 go env fun (arg : args)
1191 | exprIsTrivial arg -- Easy case
1192 || no_float_arg -- Can't make it atomic
1193 = go env (App fun arg) args
1196 = do { arg_id <- newId FSLIT("a") arg_ty
1197 ; completeNonRecX env False {- pessimistic -} arg_id arg_id arg $ \env ->
1198 go env (App fun (Var arg_id)) args }
1200 arg_ty = exprType arg
1201 no_float_arg = not is_strict && (isUnLiftedType arg_ty) && not (exprOkForSpeculation arg)
1204 -- Old code: consider rewriting to be more like mkAtomicArgsE
1206 mkAtomicArgs :: Bool -- A strict binding
1207 -> Bool -- OK to float unlifted args
1209 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1210 OutExpr) -- things that need case-binding,
1211 -- if the strict-binding flag is on
1213 mkAtomicArgs is_strict ok_float_unlifted rhs
1214 | (Var fun, args) <- collectArgs rhs, -- It's an application
1215 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1216 = go fun nilOL [] args -- Have a go
1218 | otherwise = bale_out -- Give up
1221 bale_out = returnSmpl (nilOL, rhs)
1223 go fun binds rev_args []
1224 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1226 go fun binds rev_args (arg : args)
1227 | exprIsTrivial arg -- Easy case
1228 = go fun binds (arg:rev_args) args
1230 | not can_float_arg -- Can't make this arg atomic
1231 = bale_out -- ... so give up
1233 | otherwise -- Don't forget to do it recursively
1234 -- E.g. x = a:b:c:[]
1235 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1236 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1237 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1238 (Var arg_id : rev_args) args
1240 arg_ty = exprType arg
1241 can_float_arg = is_strict
1242 || not (isUnLiftedType arg_ty)
1243 || (ok_float_unlifted && exprOkForSpeculation arg)
1246 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1247 -> (SimplEnv -> SimplM (FloatsWith a))
1248 -> SimplM (FloatsWith a)
1249 addAtomicBinds env [] thing_inside = thing_inside env
1250 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1251 addAtomicBinds env bs thing_inside
1255 %************************************************************************
1257 \subsection{The main rebuilder}
1259 %************************************************************************
1262 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1264 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1265 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1266 rebuild env expr (CoerceIt co cont) = rebuild env (mkCoerce co expr) cont
1267 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1268 rebuild env expr (ApplyTo _ arg mb_se cont) = rebuildApp env expr arg mb_se cont
1270 rebuildApp env fun arg mb_se cont
1271 = do { arg' <- simplArg env arg mb_se
1272 ; rebuild env (App fun arg') cont }
1274 simplArg :: SimplEnv -> CoreExpr -> Maybe SimplEnv -> SimplM CoreExpr
1275 simplArg env arg Nothing = return arg -- The arg is already simplified
1276 simplArg env arg (Just arg_env) = simplExpr (setInScope arg_env env) arg
1278 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1282 %************************************************************************
1284 \subsection{Functions dealing with a case}
1286 %************************************************************************
1288 Blob of helper functions for the "case-of-something-else" situation.
1291 ---------------------------------------------------------
1292 -- Eliminate the case if possible
1294 rebuildCase :: SimplEnv
1295 -> OutExpr -- Scrutinee
1296 -> InId -- Case binder
1297 -> [InAlt] -- Alternatives (inceasing order)
1299 -> SimplM FloatsWithExpr
1301 rebuildCase env scrut case_bndr alts cont
1302 | Just (con,args) <- exprIsConApp_maybe scrut
1303 -- Works when the scrutinee is a variable with a known unfolding
1304 -- as well as when it's an explicit constructor application
1305 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1307 | Lit lit <- scrut -- No need for same treatment as constructors
1308 -- because literals are inlined more vigorously
1309 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1312 = -- Prepare the continuation;
1313 -- The new subst_env is in place
1314 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1315 addFloats env floats $ \ env ->
1318 -- The case expression is annotated with the result type of the continuation
1319 -- This may differ from the type originally on the case. For example
1320 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1323 -- let j a# = <blob>
1324 -- in case(T) a of { True -> j 1#; False -> j 0# }
1325 -- Note that the case that scrutinises a now returns a T not an Int#
1326 res_ty' = contResultType dup_cont
1329 -- Deal with case binder
1330 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1332 -- Deal with the case alternatives
1333 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1335 -- Put the case back together
1336 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1338 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1339 -- The case binder *not* scope over the whole returned case-expression
1340 rebuild env case_expr nondup_cont
1343 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1344 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1345 way, there's a chance that v will now only be used once, and hence
1350 There is a time we *don't* want to do that, namely when
1351 -fno-case-of-case is on. This happens in the first simplifier pass,
1352 and enhances full laziness. Here's the bad case:
1353 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1354 If we eliminate the inner case, we trap it inside the I# v -> arm,
1355 which might prevent some full laziness happening. I've seen this
1356 in action in spectral/cichelli/Prog.hs:
1357 [(m,n) | m <- [1..max], n <- [1..max]]
1358 Hence the check for NoCaseOfCase.
1362 There is another situation when we don't want to do it. If we have
1364 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1365 ...other cases .... }
1367 We'll perform the binder-swap for the outer case, giving
1369 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1370 ...other cases .... }
1372 But there is no point in doing it for the inner case, because w1 can't
1373 be inlined anyway. Furthermore, doing the case-swapping involves
1374 zapping w2's occurrence info (see paragraphs that follow), and that
1375 forces us to bind w2 when doing case merging. So we get
1377 case x of w1 { A -> let w2 = w1 in e1
1378 B -> let w2 = w1 in e2
1379 ...other cases .... }
1381 This is plain silly in the common case where w2 is dead.
1383 Even so, I can't see a good way to implement this idea. I tried
1384 not doing the binder-swap if the scrutinee was already evaluated
1385 but that failed big-time:
1389 case v of w { MkT x ->
1390 case x of x1 { I# y1 ->
1391 case x of x2 { I# y2 -> ...
1393 Notice that because MkT is strict, x is marked "evaluated". But to
1394 eliminate the last case, we must either make sure that x (as well as
1395 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1396 the binder-swap. So this whole note is a no-op.
1400 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1401 any occurrence info (eg IAmDead) in the case binder, because the
1402 case-binder now effectively occurs whenever v does. AND we have to do
1403 the same for the pattern-bound variables! Example:
1405 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1407 Here, b and p are dead. But when we move the argment inside the first
1408 case RHS, and eliminate the second case, we get
1410 case x of { (a,b) -> a b }
1412 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1415 Indeed, this can happen anytime the case binder isn't dead:
1416 case <any> of x { (a,b) ->
1417 case x of { (p,q) -> p } }
1418 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1419 The point is that we bring into the envt a binding
1421 after the outer case, and that makes (a,b) alive. At least we do unless
1422 the case binder is guaranteed dead.
1425 simplCaseBinder env (Var v) case_bndr
1426 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1428 -- Failed try [see Note 2 above]
1429 -- not (isEvaldUnfolding (idUnfolding v))
1431 = simplBinder env (zapOccInfo case_bndr) `thenSmpl` \ (env, case_bndr') ->
1432 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1433 -- We could extend the substitution instead, but it would be
1434 -- a hack because then the substitution wouldn't be idempotent
1435 -- any more (v is an OutId). And this does just as well.
1437 simplCaseBinder env other_scrut case_bndr
1438 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1439 returnSmpl (env, case_bndr')
1441 zapOccInfo :: InId -> InId
1442 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1446 simplAlts does two things:
1448 1. Eliminate alternatives that cannot match, including the
1449 DEFAULT alternative.
1451 2. If the DEFAULT alternative can match only one possible constructor,
1452 then make that constructor explicit.
1454 case e of x { DEFAULT -> rhs }
1456 case e of x { (a,b) -> rhs }
1457 where the type is a single constructor type. This gives better code
1458 when rhs also scrutinises x or e.
1460 Here "cannot match" includes knowledge from GADTs
1462 It's a good idea do do this stuff before simplifying the alternatives, to
1463 avoid simplifying alternatives we know can't happen, and to come up with
1464 the list of constructors that are handled, to put into the IdInfo of the
1465 case binder, for use when simplifying the alternatives.
1467 Eliminating the default alternative in (1) isn't so obvious, but it can
1470 data Colour = Red | Green | Blue
1479 DEFAULT -> [ case y of ... ]
1481 If we inline h into f, the default case of the inlined h can't happen.
1482 If we don't notice this, we may end up filtering out *all* the cases
1483 of the inner case y, which give us nowhere to go!
1487 simplAlts :: SimplEnv
1489 -> OutId -- Case binder
1490 -> [InAlt] -> SimplCont
1491 -> SimplM [OutAlt] -- Includes the continuation
1493 simplAlts env scrut case_bndr' alts cont'
1494 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1495 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1496 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1497 -- We need the mergeAlts in case the new default_alt
1498 -- has turned into a constructor alternative.
1500 (alts_wo_default, maybe_deflt) = findDefault alts
1501 imposs_cons = case scrut of
1502 Var v -> otherCons (idUnfolding v)
1505 -- "imposs_deflt_cons" are handled either by the context,
1506 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1507 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1509 simplDefault :: SimplEnv
1510 -> OutId -- Case binder; need just for its type. Note that as an
1511 -- OutId, it has maximum information; this is important.
1512 -- Test simpl013 is an example
1513 -> [AltCon] -- These cons can't happen when matching the default
1516 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1517 -- mergeAlts expects
1520 simplDefault env case_bndr' imposs_cons cont Nothing
1521 = return [] -- No default branch
1523 simplDefault env case_bndr' imposs_cons cont (Just rhs)
1524 | -- This branch handles the case where we are
1525 -- scrutinisng an algebraic data type
1526 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1527 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1528 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1529 -- case x of { DEFAULT -> e }
1530 -- and we don't want to fill in a default for them!
1531 Just all_cons <- tyConDataCons_maybe tycon,
1532 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1533 -- which GHC allows, then the case expression will have at most a default
1534 -- alternative. We don't want to eliminate that alternative, because the
1535 -- invariant is that there's always one alternative. It's more convenient
1537 -- case x of { DEFAULT -> e }
1538 -- as it is, rather than transform it to
1539 -- error "case cant match"
1540 -- which would be quite legitmate. But it's a really obscure corner, and
1541 -- not worth wasting code on.
1543 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1544 poss_data_cons = filterOut (`elem` imposs_data_cons) all_cons
1545 gadt_imposs | all isTyVarTy inst_tys = []
1546 | otherwise = filter (cant_match inst_tys) poss_data_cons
1547 final_poss = filterOut (`elem` gadt_imposs) poss_data_cons
1549 = case final_poss of
1550 [] -> returnSmpl [] -- Eliminate the default alternative
1551 -- altogether if it can't match
1553 [con] -> -- It matches exactly one constructor, so fill it in
1554 do { tick (FillInCaseDefault case_bndr')
1555 ; con_alt <- mkDataConAlt con inst_tys rhs
1556 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1557 -- The simplAlt must succeed with Just because we have
1558 -- already filtered out construtors that can't match
1561 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1564 = simplify_default imposs_cons
1566 cant_match tys data_con = not (dataConCanMatch data_con tys)
1568 simplify_default imposs_cons
1569 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1570 -- Record the constructors that the case-binder *can't* be.
1571 ; rhs' <- simplExprC env' rhs cont
1572 ; return [(DEFAULT, [], rhs')] }
1574 simplAlt :: SimplEnv
1575 -> [AltCon] -- These constructors can't be present when
1576 -- matching this alternative
1577 -> OutId -- The case binder
1580 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1582 -- Simplify an alternative, returning the type refinement for the
1583 -- alternative, if the alternative does any refinement at all
1584 -- Nothing => the alternative is inaccessible
1586 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1587 | con `elem` imposs_cons -- This case can't match
1590 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1591 -- TURGID DUPLICATION, needed only for the simplAlt call
1592 -- in mkDupableAlt. Clean this up when moving to FC
1593 = ASSERT( null bndrs )
1594 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1595 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1597 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1598 -- Record the constructors that the case-binder *can't* be.
1600 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1601 = ASSERT( null bndrs )
1602 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1603 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1605 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1607 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1608 = -- Deal with the pattern-bound variables
1609 -- Mark the ones that are in ! positions in the data constructor
1610 -- as certainly-evaluated.
1611 -- NB: it happens that simplBinders does *not* erase the OtherCon
1612 -- form of unfolding, so it's ok to add this info before
1613 -- doing simplBinders
1614 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1616 -- Bind the case-binder to (con args)
1617 let unf = mkUnfolding False (mkConApp con con_args)
1618 inst_tys' = tyConAppArgs (idType case_bndr')
1619 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1620 env' = mk_rhs_env env case_bndr' unf
1622 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1623 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1625 -- add_evals records the evaluated-ness of the bound variables of
1626 -- a case pattern. This is *important*. Consider
1627 -- data T = T !Int !Int
1629 -- case x of { T a b -> T (a+1) b }
1631 -- We really must record that b is already evaluated so that we don't
1632 -- go and re-evaluate it when constructing the result.
1633 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1635 cat_evals dc vs strs
1639 go (v:vs) strs | isTyVar v = v : go vs strs
1640 go (v:vs) (str:strs)
1641 | isMarkedStrict str = evald_v : go vs strs
1642 | otherwise = zapped_v : go vs strs
1644 zapped_v = zap_occ_info v
1645 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1646 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1648 -- If the case binder is alive, then we add the unfolding
1650 -- to the envt; so vs are now very much alive
1651 -- Note [Aug06] I can't see why this actually matters
1652 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1653 | otherwise = zapOccInfo
1655 mk_rhs_env env case_bndr' case_bndr_unf
1656 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1660 %************************************************************************
1662 \subsection{Known constructor}
1664 %************************************************************************
1666 We are a bit careful with occurrence info. Here's an example
1668 (\x* -> case x of (a*, b) -> f a) (h v, e)
1670 where the * means "occurs once". This effectively becomes
1671 case (h v, e) of (a*, b) -> f a)
1673 let a* = h v; b = e in f a
1677 All this should happen in one sweep.
1680 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1681 -> InId -> [InAlt] -> SimplCont
1682 -> SimplM FloatsWithExpr
1684 knownCon env scrut con args bndr alts cont
1685 = tick (KnownBranch bndr) `thenSmpl_`
1686 case findAlt con alts of
1687 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1688 simplNonRecX env bndr scrut $ \ env ->
1689 -- This might give rise to a binding with non-atomic args
1690 -- like x = Node (f x) (g x)
1691 -- but simplNonRecX will atomic-ify it
1692 simplExprF env rhs cont
1694 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1695 simplNonRecX env bndr scrut $ \ env ->
1696 simplExprF env rhs cont
1698 (DataAlt dc, bs, rhs)
1699 -> ASSERT( n_drop_tys + length bs == length args )
1700 bind_args env dead_bndr bs (drop n_drop_tys args) $ \ env ->
1702 -- It's useful to bind bndr to scrut, rather than to a fresh
1703 -- binding x = Con arg1 .. argn
1704 -- because very often the scrut is a variable, so we avoid
1705 -- creating, and then subsequently eliminating, a let-binding
1706 -- BUT, if scrut is a not a variable, we must be careful
1707 -- about duplicating the arg redexes; in that case, make
1708 -- a new con-app from the args
1709 bndr_rhs = case scrut of
1712 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1713 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1714 -- args are aready OutExprs, but bs are InIds
1716 simplNonRecX env bndr bndr_rhs $ \ env ->
1717 simplExprF env rhs cont
1719 dead_bndr = isDeadBinder bndr
1720 n_drop_tys = tyConArity (dataConTyCon dc)
1723 bind_args env dead_bndr [] _ thing_inside = thing_inside env
1725 bind_args env dead_bndr (b:bs) (Type ty : args) thing_inside
1726 = ASSERT( isTyVar b )
1727 bind_args (extendTvSubst env b ty) dead_bndr bs args thing_inside
1729 bind_args env dead_bndr (b:bs) (arg : args) thing_inside
1732 b' = if dead_bndr then b else zapOccInfo b
1733 -- Note that the binder might be "dead", because it doesn't occur
1734 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1735 -- Nevertheless we must keep it if the case-binder is alive, because it may
1736 -- be used in teh con_app
1738 simplNonRecX env b' arg $ \ env ->
1739 bind_args env dead_bndr bs args thing_inside
1743 %************************************************************************
1745 \subsection{Duplicating continuations}
1747 %************************************************************************
1750 prepareCaseCont :: SimplEnv
1751 -> [InAlt] -> SimplCont
1752 -> SimplM (FloatsWith (SimplCont,SimplCont))
1753 -- Return a duplicatable continuation, a non-duplicable part
1754 -- plus some extra bindings (that scope over the entire
1757 -- No need to make it duplicatable if there's only one alternative
1758 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1759 prepareCaseCont env alts cont = mkDupableCont env cont
1763 mkDupableCont :: SimplEnv -> SimplCont
1764 -> SimplM (FloatsWith (SimplCont, SimplCont))
1766 mkDupableCont env cont
1767 | contIsDupable cont
1768 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1770 mkDupableCont env (CoerceIt ty cont)
1771 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1772 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1774 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1775 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1776 -- Do *not* duplicate an ArgOf continuation
1777 -- Because ArgOf continuations are opaque, we gain nothing by
1778 -- propagating them into the expressions, and we do lose a lot.
1779 -- Here's an example:
1780 -- && (case x of { T -> F; F -> T }) E
1781 -- Now, && is strict so we end up simplifying the case with
1782 -- an ArgOf continuation. If we let-bind it, we get
1784 -- let $j = \v -> && v E
1785 -- in simplExpr (case x of { T -> F; F -> T })
1786 -- (ArgOf (\r -> $j r)
1787 -- And after simplifying more we get
1789 -- let $j = \v -> && v E
1790 -- in case of { T -> $j F; F -> $j T }
1791 -- Which is a Very Bad Thing
1793 -- The desire not to duplicate is the entire reason that
1794 -- mkDupableCont returns a pair of continuations.
1796 -- The original plan had:
1797 -- e.g. (...strict-fn...) [...hole...]
1799 -- let $j = \a -> ...strict-fn...
1800 -- in $j [...hole...]
1802 mkDupableCont env (ApplyTo _ arg mb_se cont)
1803 = -- e.g. [...hole...] (...arg...)
1805 -- let a = ...arg...
1806 -- in [...hole...] a
1807 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1808 ; addFloats env floats $ \ env -> do
1809 { arg1 <- simplArg env arg mb_se
1810 ; (floats2, arg2) <- mkDupableArg env arg1
1811 ; return (floats2, (ApplyTo OkToDup arg2 Nothing dup_cont, nondup_cont)) }}
1813 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1814 -- | not (exprIsDupable rhs && contIsDupable case_cont) -- See notes below
1815 -- | not (isDeadBinder case_bndr)
1816 | all isDeadBinder bs
1817 = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
1819 scrut_ty = substTy se (idType case_bndr)
1821 {- Note [Single-alternative cases]
1822 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1823 This case is just like the ArgOf case. Here's an example:
1827 case (case x of I# x' ->
1829 True -> I# (negate# x')
1830 False -> I# x') of y {
1832 Because the (case x) has only one alternative, we'll transform to
1834 case (case x' <# 0# of
1835 True -> I# (negate# x')
1836 False -> I# x') of y {
1838 But now we do *NOT* want to make a join point etc, giving
1840 let $j = \y -> MkT y
1842 True -> $j (I# (negate# x'))
1844 In this case the $j will inline again, but suppose there was a big
1845 strict computation enclosing the orginal call to MkT. Then, it won't
1846 "see" the MkT any more, because it's big and won't get duplicated.
1847 And, what is worse, nothing was gained by the case-of-case transform.
1849 When should use this case of mkDupableCont?
1850 However, matching on *any* single-alternative case is a *disaster*;
1851 e.g. case (case ....) of (a,b) -> (# a,b #)
1852 We must push the outer case into the inner one!
1855 * Match [(DEFAULT,_,_)], but in the common case of Int,
1856 the alternative-filling-in code turned the outer case into
1857 case (...) of y { I# _ -> MkT y }
1859 * Match on single alternative plus (not (isDeadBinder case_bndr))
1860 Rationale: pushing the case inwards won't eliminate the construction.
1861 But there's a risk of
1862 case (...) of y { (a,b) -> let z=(a,b) in ... }
1863 Now y looks dead, but it'll come alive again. Still, this
1864 seems like the best option at the moment.
1866 * Match on single alternative plus (all (isDeadBinder bndrs))
1867 Rationale: this is essentially seq.
1869 * Match when the rhs is *not* duplicable, and hence would lead to a
1870 join point. This catches the disaster-case above. We can test
1871 the *un-simplified* rhs, which is fine. It might get bigger or
1872 smaller after simplification; if it gets smaller, this case might
1873 fire next time round. NB also that we must test contIsDupable
1874 case_cont *btoo, because case_cont might be big!
1876 HOWEVER: I found that this version doesn't work well, because
1877 we can get let x = case (...) of { small } in ...case x...
1878 When x is inlined into its full context, we find that it was a bad
1879 idea to have pushed the outer case inside the (...) case.
1882 mkDupableCont env (Select _ case_bndr alts se cont)
1883 = -- e.g. (case [...hole...] of { pi -> ei })
1885 -- let ji = \xij -> ei
1886 -- in case [...hole...] of { pi -> ji xij }
1887 do { tick (CaseOfCase case_bndr)
1888 ; let alt_env = setInScope se env
1889 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1890 -- NB: call mkDupableCont here, *not* prepareCaseCont
1891 -- We must make a duplicable continuation, whereas prepareCaseCont
1892 -- doesn't when there is a single case branch
1893 ; addFloats alt_env floats1 $ \ alt_env -> do
1895 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1896 -- NB: simplBinder does not zap deadness occ-info, so
1897 -- a dead case_bndr' will still advertise its deadness
1898 -- This is really important because in
1899 -- case e of b { (# a,b #) -> ... }
1900 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1901 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1902 -- In the new alts we build, we have the new case binder, so it must retain
1905 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1906 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1907 (mkBoringStop (contResultType dup_cont)),
1911 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1912 -- Let-bind the thing if necessary
1913 mkDupableArg env arg
1915 = return (emptyFloats env, arg)
1917 = do { arg_id <- newId FSLIT("a") (exprType arg)
1918 ; tick (CaseOfCase arg_id)
1919 -- Want to tick here so that we go round again,
1920 -- and maybe copy or inline the code.
1921 -- Not strictly CaseOfCase, but never mind
1922 ; return (unitFloat env arg_id arg, Var arg_id) }
1923 -- What if the arg should be case-bound?
1924 -- This has been this way for a long time, so I'll leave it,
1925 -- but I can't convince myself that it's right.
1927 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1928 -> SimplM (FloatsWith [InAlt])
1929 -- Absorbs the continuation into the new alternatives
1931 mkDupableAlts env case_bndr' alts dupable_cont
1934 go env [] = returnSmpl (emptyFloats env, [])
1936 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1937 ; addFloats env floats1 $ \ env -> do
1938 { (floats2, alts') <- go env alts
1939 ; returnSmpl (floats2, case mb_alt' of
1940 Just alt' -> alt' : alts'
1944 mkDupableAlt env case_bndr' cont alt
1945 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1947 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1949 Just (reft, (con, bndrs', rhs')) ->
1950 -- Safe to say that there are no handled-cons for the DEFAULT case
1952 if exprIsDupable rhs' then
1953 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
1954 -- It is worth checking for a small RHS because otherwise we
1955 -- get extra let bindings that may cause an extra iteration of the simplifier to
1956 -- inline back in place. Quite often the rhs is just a variable or constructor.
1957 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1958 -- iterations because the version with the let bindings looked big, and so wasn't
1959 -- inlined, but after the join points had been inlined it looked smaller, and so
1962 -- NB: we have to check the size of rhs', not rhs.
1963 -- Duplicating a small InAlt might invalidate occurrence information
1964 -- However, if it *is* dupable, we return the *un* simplified alternative,
1965 -- because otherwise we'd need to pair it up with an empty subst-env....
1966 -- but we only have one env shared between all the alts.
1967 -- (Remember we must zap the subst-env before re-simplifying something).
1968 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1972 rhs_ty' = exprType rhs'
1973 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1975 | isTyVar bndr = not (bndr `elemVarEnv` reft)
1976 -- Don't abstract over tyvar binders which are refined away
1977 -- See Note [Refinement] below
1978 | otherwise = not (isDeadBinder bndr)
1979 -- The deadness info on the new Ids is preserved by simplBinders
1981 -- If we try to lift a primitive-typed something out
1982 -- for let-binding-purposes, we will *caseify* it (!),
1983 -- with potentially-disastrous strictness results. So
1984 -- instead we turn it into a function: \v -> e
1985 -- where v::State# RealWorld#. The value passed to this function
1986 -- is realworld#, which generates (almost) no code.
1988 -- There's a slight infelicity here: we pass the overall
1989 -- case_bndr to all the join points if it's used in *any* RHS,
1990 -- because we don't know its usage in each RHS separately
1992 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1993 -- we make the join point into a function whenever used_bndrs'
1994 -- is empty. This makes the join-point more CPR friendly.
1995 -- Consider: let j = if .. then I# 3 else I# 4
1996 -- in case .. of { A -> j; B -> j; C -> ... }
1998 -- Now CPR doesn't w/w j because it's a thunk, so
1999 -- that means that the enclosing function can't w/w either,
2000 -- which is a lose. Here's the example that happened in practice:
2001 -- kgmod :: Int -> Int -> Int
2002 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2006 -- I have seen a case alternative like this:
2007 -- True -> \v -> ...
2008 -- It's a bit silly to add the realWorld dummy arg in this case, making
2011 -- (the \v alone is enough to make CPR happy) but I think it's rare
2013 ( if not (any isId used_bndrs')
2014 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
2015 returnSmpl ([rw_id], [Var realWorldPrimId])
2017 returnSmpl (used_bndrs', varsToCoreExprs used_bndrs')
2018 ) `thenSmpl` \ (final_bndrs', final_args) ->
2020 -- See comment about "$j" name above
2021 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
2022 -- Notice the funky mkPiTypes. If the contructor has existentials
2023 -- it's possible that the join point will be abstracted over
2024 -- type varaibles as well as term variables.
2025 -- Example: Suppose we have
2026 -- data T = forall t. C [t]
2028 -- case (case e of ...) of
2029 -- C t xs::[t] -> rhs
2030 -- We get the join point
2031 -- let j :: forall t. [t] -> ...
2032 -- j = /\t \xs::[t] -> rhs
2034 -- case (case e of ...) of
2035 -- C t xs::[t] -> j t xs
2037 -- We make the lambdas into one-shot-lambdas. The
2038 -- join point is sure to be applied at most once, and doing so
2039 -- prevents the body of the join point being floated out by
2040 -- the full laziness pass
2041 really_final_bndrs = map one_shot final_bndrs'
2042 one_shot v | isId v = setOneShotLambda v
2044 join_rhs = mkLams really_final_bndrs rhs'
2045 join_call = mkApps (Var join_bndr) final_args
2047 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2054 MkT :: a -> b -> T a
2058 MkT a' b (p::a') (q::b) -> [p,w]
2060 The danger is that we'll make a join point
2064 and that's ill-typed, because (p::a') but (w::a).
2066 Solution so far: don't abstract over a', because the type refinement
2067 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2069 Then we must abstract over any refined free variables. Hmm. Maybe we
2070 could just abstract over *all* free variables, thereby lambda-lifting
2071 the join point? We should try this.