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 Unify ( coreRefineTys, dataConCanMatch )
38 import DataCon ( DataCon, dataConTyCon, dataConRepStrictness, isVanillaDataCon,
39 dataConInstArgTys, dataConTyVars )
40 import TyCon ( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
42 import PprCore ( pprParendExpr, pprCoreExpr )
43 import CoreUnfold ( mkUnfolding, callSiteInline )
44 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
45 exprIsConApp_maybe, mkPiTypes, findAlt,
46 exprType, exprIsHNF, findDefault, mergeAlts,
47 exprOkForSpeculation, exprArity,
48 mkCoerce, mkCoerce2, mkSCC, mkInlineMe, applyTypeToArg
50 import Rules ( lookupRule )
51 import BasicTypes ( isMarkedStrict )
52 import CostCentre ( currentCCS )
53 import Type ( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
54 splitFunTy_maybe, splitFunTy, coreEqType, splitTyConApp_maybe,
57 import Var ( tyVarKind, mkTyVar )
58 import VarEnv ( elemVarEnv, emptyVarEnv )
59 import TysPrim ( realWorldStatePrimTy )
60 import PrelInfo ( realWorldPrimId )
61 import BasicTypes ( TopLevelFlag(..), isTopLevel,
64 import Name ( mkSysTvName )
65 import StaticFlags ( opt_PprStyle_Debug )
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 ASSERT2( 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
615 -- If the unfolding is a value, the demand info may
616 -- go pear-shaped, so we nuke it. Example:
618 -- case x of (p,q) -> h p q x
619 -- Here x is certainly demanded. But after we've nuked
620 -- the case, we'll get just
621 -- let x = (a,b) in h a b x
622 -- and now x is not demanded (I'm assuming h is lazy)
623 -- This really happens. Similarly
624 -- let f = \x -> e in ...f..f...
625 -- After inling f at some of its call sites the original binding may
626 -- (for example) be no longer strictly demanded.
627 -- The solution here is a bit ad hoc...
628 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
629 final_info | loop_breaker = new_bndr_info
630 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
631 | otherwise = info_w_unf
633 final_id = new_bndr `setIdInfo` final_info
635 -- These seqs forces the Id, and hence its IdInfo,
636 -- and hence any inner substitutions
638 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
639 returnSmpl (unitFloat env final_id new_rhs, env)
642 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
643 loop_breaker = isLoopBreaker occ_info
644 old_info = idInfo old_bndr
645 occ_info = occInfo old_info
650 %************************************************************************
652 \subsection[Simplify-simplExpr]{The main function: simplExpr}
654 %************************************************************************
656 The reason for this OutExprStuff stuff is that we want to float *after*
657 simplifying a RHS, not before. If we do so naively we get quadratic
658 behaviour as things float out.
660 To see why it's important to do it after, consider this (real) example:
674 a -- Can't inline a this round, cos it appears twice
678 Each of the ==> steps is a round of simplification. We'd save a
679 whole round if we float first. This can cascade. Consider
684 let f = let d1 = ..d.. in \y -> e
688 in \x -> ...(\y ->e)...
690 Only in this second round can the \y be applied, and it
691 might do the same again.
695 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
696 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
698 expr_ty' = substTy env (exprType expr)
699 -- The type in the Stop continuation, expr_ty', is usually not used
700 -- It's only needed when discarding continuations after finding
701 -- a function that returns bottom.
702 -- Hence the lazy substitution
705 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
706 -- Simplify an expression, given a continuation
707 simplExprC env expr cont
708 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
709 returnSmpl (wrapFloats floats expr)
711 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
712 -- Simplify an expression, returning floated binds
714 simplExprF env (Var v) cont = simplVar env v cont
715 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
716 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
717 simplExprF env (Note note expr) cont = simplNote env note expr cont
718 simplExprF env (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg (Just env) cont)
720 simplExprF env (Type ty) cont
721 = ASSERT( contIsRhsOrArg cont )
722 simplType env ty `thenSmpl` \ ty' ->
723 rebuild env (Type ty') cont
725 simplExprF env (Case scrut bndr case_ty alts) cont
726 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
727 = -- Simplify the scrutinee with a Select continuation
728 simplExprF env scrut (Select NoDup bndr alts env cont)
731 = -- If case-of-case is off, simply simplify the case expression
732 -- in a vanilla Stop context, and rebuild the result around it
733 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
734 rebuild env case_expr' cont
736 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
737 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
739 simplExprF env (Let (Rec pairs) body) cont
740 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
741 -- NB: bndrs' don't have unfoldings or rules
742 -- We add them as we go down
744 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
745 addFloats env floats $ \ env ->
746 simplExprF env body cont
748 -- A non-recursive let is dealt with by simplNonRecBind
749 simplExprF env (Let (NonRec bndr rhs) body) cont
750 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
751 simplExprF env body cont
754 ---------------------------------
755 simplType :: SimplEnv -> InType -> SimplM OutType
756 -- Kept monadic just so we can do the seqType
758 = seqType new_ty `seq` returnSmpl new_ty
760 new_ty = substTy env ty
764 %************************************************************************
768 %************************************************************************
771 simplLam env fun cont
774 zap_it = mkLamBndrZapper fun (countArgs cont)
775 cont_ty = contResultType cont
777 -- Type-beta reduction
778 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) mb_arg_se body_cont)
779 = ASSERT( isTyVar bndr )
780 do { tick (BetaReduction bndr)
781 ; ty_arg' <- case mb_arg_se of
782 Just arg_se -> simplType (setInScope arg_se env) ty_arg
783 Nothing -> return ty_arg
784 ; go (extendTvSubst env bndr ty_arg') body body_cont }
786 -- Ordinary beta reduction
787 go env (Lam bndr body) cont@(ApplyTo _ arg (Just arg_se) body_cont)
788 = do { tick (BetaReduction bndr)
789 ; simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
790 go env body body_cont }
792 go env (Lam bndr body) cont@(ApplyTo _ arg Nothing body_cont)
793 = do { tick (BetaReduction bndr)
794 ; simplNonRecX env (zap_it bndr) arg $ \ env ->
795 go env body body_cont }
797 -- Not enough args, so there are real lambdas left to put in the result
798 go env lam@(Lam _ _) cont
799 = do { (env, bndrs') <- simplLamBndrs env bndrs
800 ; body' <- simplExpr env body
801 ; (floats, new_lam) <- mkLam env bndrs' body' cont
802 ; addFloats env floats $ \ env ->
803 rebuild env new_lam cont }
805 (bndrs,body) = collectBinders lam
807 -- Exactly enough args
808 go env expr cont = simplExprF env expr cont
810 mkLamBndrZapper :: CoreExpr -- Function
811 -> Int -- Number of args supplied, *including* type args
812 -> Id -> Id -- Use this to zap the binders
813 mkLamBndrZapper fun n_args
814 | n_args >= n_params fun = \b -> b -- Enough args
815 | otherwise = \b -> zapLamIdInfo b
817 -- NB: we count all the args incl type args
818 -- so we must count all the binders (incl type lambdas)
819 n_params (Note _ e) = n_params e
820 n_params (Lam b e) = 1 + n_params e
821 n_params other = 0::Int
825 %************************************************************************
829 %************************************************************************
832 simplNote env (Coerce to from) body cont
834 addCoerce s1 k1 cont -- Drop redundant coerces. This can happen if a polymoprhic
835 -- (coerce a b e) is instantiated with a=ty1 b=ty2 and the
836 -- two are the same. This happens a lot in Happy-generated parsers
837 | s1 `coreEqType` k1 = cont
839 addCoerce s1 k1 (CoerceIt t1 cont)
840 -- coerce T1 S1 (coerce S1 K1 e)
843 -- coerce T1 K1 e, otherwise
845 -- For example, in the initial form of a worker
846 -- we may find (coerce T (coerce S (\x.e))) y
847 -- and we'd like it to simplify to e[y/x] in one round
849 | t1 `coreEqType` k1 = cont -- The coerces cancel out
850 | otherwise = CoerceIt t1 cont -- They don't cancel, but
851 -- the inner one is redundant
853 addCoerce t1t2 s1s2 (ApplyTo dup arg mb_arg_se cont)
854 | not (isTypeArg arg), -- This whole case only works for value args
855 -- Could upgrade to have equiv thing for type apps too
856 Just (s1, s2) <- splitFunTy_maybe s1s2
857 -- (coerce (T1->T2) (S1->S2) F) E
859 -- coerce T2 S2 (F (coerce S1 T1 E))
861 -- t1t2 must be a function type, T1->T2, because it's applied to something
862 -- but s1s2 might conceivably not be
864 -- When we build the ApplyTo we can't mix the out-types
865 -- with the InExpr in the argument, so we simply substitute
866 -- to make it all consistent. It's a bit messy.
867 -- But it isn't a common case.
869 (t1,t2) = splitFunTy t1t2
870 new_arg = mkCoerce2 s1 t1 arg'
871 arg' = case mb_arg_se of
873 Just arg_se -> substExpr (setInScope arg_se env) arg
875 ApplyTo dup new_arg Nothing (addCoerce t2 s2 cont)
877 addCoerce to' _ cont = CoerceIt to' cont
879 simplType env to `thenSmpl` \ to' ->
880 simplType env from `thenSmpl` \ from' ->
881 simplExprF env body (addCoerce to' from' cont)
884 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
885 -- inlining. All other CCCSs are mapped to currentCCS.
886 simplNote env (SCC cc) e cont
887 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
888 rebuild env (mkSCC cc e') cont
890 -- See notes with SimplMonad.inlineMode
891 simplNote env InlineMe e cont
892 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
893 = -- Don't inline inside an INLINE expression
894 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
895 rebuild env (mkInlineMe e') cont
897 | otherwise -- Dissolve the InlineMe note if there's
898 -- an interesting context of any kind to combine with
899 -- (even a type application -- anything except Stop)
900 = simplExprF env e cont
902 simplNote env (CoreNote s) e cont
903 = simplExpr env e `thenSmpl` \ e' ->
904 rebuild env (Note (CoreNote s) e') cont
908 %************************************************************************
910 \subsection{Dealing with calls}
912 %************************************************************************
915 simplVar env var cont
916 = case substId env var of
917 DoneEx e -> simplExprF (zapSubstEnv env) e cont
918 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
919 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
920 -- Note [zapSubstEnv]
921 -- The template is already simplified, so don't re-substitute.
922 -- This is VITAL. Consider
924 -- let y = \z -> ...x... in
926 -- We'll clone the inner \x, adding x->x' in the id_subst
927 -- Then when we inline y, we must *not* replace x by x' in
928 -- the inlined copy!!
930 ---------------------------------------------------------
931 -- Dealing with a call site
933 completeCall env var occ_info cont
934 = -- Simplify the arguments
935 getDOptsSmpl `thenSmpl` \ dflags ->
937 chkr = getSwitchChecker env
938 (args, call_cont) = getContArgs chkr var cont
941 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
942 (contResultType call_cont) $ \ env args ->
944 -- Next, look for rules or specialisations that match
946 -- It's important to simplify the args first, because the rule-matcher
947 -- doesn't do substitution as it goes. We don't want to use subst_args
948 -- (defined in the 'where') because that throws away useful occurrence info,
949 -- and perhaps-very-important specialisations.
951 -- Some functions have specialisations *and* are strict; in this case,
952 -- we don't want to inline the wrapper of the non-specialised thing; better
953 -- to call the specialised thing instead.
954 -- We used to use the black-listing mechanism to ensure that inlining of
955 -- the wrapper didn't occur for things that have specialisations till a
956 -- later phase, so but now we just try RULES first
958 -- You might think that we shouldn't apply rules for a loop breaker:
959 -- doing so might give rise to an infinite loop, because a RULE is
960 -- rather like an extra equation for the function:
961 -- RULE: f (g x) y = x+y
964 -- But it's too drastic to disable rules for loop breakers.
965 -- Even the foldr/build rule would be disabled, because foldr
966 -- is recursive, and hence a loop breaker:
967 -- foldr k z (build g) = g k z
968 -- So it's up to the programmer: rules can cause divergence
971 in_scope = getInScope env
973 maybe_rule = case activeRule env of
974 Nothing -> Nothing -- No rules apply
975 Just act_fn -> lookupRule act_fn in_scope rules var args
978 Just (rule_name, rule_rhs) ->
979 tick (RuleFired rule_name) `thenSmpl_`
980 (if dopt Opt_D_dump_inlinings dflags then
981 pprTrace "Rule fired" (vcat [
982 text "Rule:" <+> ftext rule_name,
983 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
984 text "After: " <+> pprCoreExpr rule_rhs,
985 text "Cont: " <+> ppr call_cont])
988 simplExprF env rule_rhs call_cont ;
990 Nothing -> -- No rules
992 -- Next, look for an inlining
994 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
995 interesting_cont = interestingCallContext (notNull args)
998 active_inline = activeInline env var occ_info
999 maybe_inline = callSiteInline dflags active_inline occ_info
1000 var arg_infos interesting_cont
1002 case maybe_inline of {
1003 Just unfolding -- There is an inlining!
1004 -> tick (UnfoldingDone var) `thenSmpl_`
1005 (if dopt Opt_D_dump_inlinings dflags then
1006 pprTrace "Inlining done" (vcat [
1007 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1008 text "Inlined fn: " <+> ppr unfolding,
1009 text "Cont: " <+> ppr call_cont])
1012 simplExprF env unfolding (pushContArgs args call_cont)
1015 Nothing -> -- No inlining!
1018 rebuild env (mkApps (Var var) args) call_cont
1022 %************************************************************************
1024 \subsection{Arguments}
1026 %************************************************************************
1029 ---------------------------------------------------------
1030 -- Simplifying the arguments of a call
1032 simplifyArgs :: SimplEnv
1033 -> OutType -- Type of the function
1034 -> Bool -- True if the fn has RULES
1035 -> [(InExpr, Maybe SimplEnv, Bool)] -- Details of the arguments
1036 -> OutType -- Type of the continuation
1037 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1038 -> SimplM FloatsWithExpr
1040 -- [CPS-like because of strict arguments]
1042 -- Simplify the arguments to a call.
1043 -- This part of the simplifier may break the no-shadowing invariant
1045 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1046 -- where f is strict in its second arg
1047 -- If we simplify the innermost one first we get (...(\a -> e)...)
1048 -- Simplifying the second arg makes us float the case out, so we end up with
1049 -- case y of (a,b) -> f (...(\a -> e)...) e'
1050 -- So the output does not have the no-shadowing invariant. However, there is
1051 -- no danger of getting name-capture, because when the first arg was simplified
1052 -- we used an in-scope set that at least mentioned all the variables free in its
1053 -- static environment, and that is enough.
1055 -- We can't just do innermost first, or we'd end up with a dual problem:
1056 -- case x of (a,b) -> f e (...(\a -> e')...)
1058 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1059 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1060 -- continuations. We have to keep the right in-scope set around; AND we have
1061 -- to get the effect that finding (error "foo") in a strict arg position will
1062 -- discard the entire application and replace it with (error "foo"). Getting
1063 -- all this at once is TOO HARD!
1065 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1066 = go env fn_ty args thing_inside
1068 go env fn_ty [] thing_inside = thing_inside env []
1069 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1070 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1071 thing_inside env (arg':args')
1073 simplifyArg env fn_ty has_rules (arg, Nothing, _) cont_ty thing_inside
1074 = thing_inside env arg -- Already simplified
1076 simplifyArg env fn_ty has_rules (Type ty_arg, Just se, _) cont_ty thing_inside
1077 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1078 thing_inside env (Type new_ty_arg)
1080 simplifyArg env fn_ty has_rules (val_arg, Just arg_se, is_strict) cont_ty thing_inside
1082 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1084 | otherwise -- Lazy argument
1085 -- DO NOT float anything outside, hence simplExprC
1086 -- There is no benefit (unlike in a let-binding), and we'd
1087 -- have to be very careful about bogus strictness through
1088 -- floating a demanded let.
1089 = simplExprC (setInScope arg_se env) val_arg
1090 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1091 thing_inside env arg1
1093 arg_ty = funArgTy fn_ty
1096 simplStrictArg :: LetRhsFlag
1097 -> SimplEnv -- The env of the call
1098 -> InExpr -> SimplEnv -- The arg plus its env
1099 -> OutType -- arg_ty: type of the argument
1100 -> OutType -- cont_ty: Type of thing computed by the context
1101 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1102 -- Takes an expression of type rhs_ty,
1103 -- returns an expression of type cont_ty
1104 -- The env passed to this continuation is the
1105 -- env of the call, plus any new in-scope variables
1106 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1108 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1109 = simplExprF (setInScope arg_env call_env) arg
1110 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1111 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1112 -- to simplify the argument
1113 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1117 %************************************************************************
1119 \subsection{mkAtomicArgs}
1121 %************************************************************************
1123 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1124 constructor application and, if so, converts it to ANF, so that the
1125 resulting thing can be inlined more easily. Thus
1132 There are three sorts of binding context, specified by the two
1138 N N Top-level or recursive Only bind args of lifted type
1140 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1141 but lazy unlifted-and-ok-for-speculation
1143 Y Y Non-top-level, non-recursive, Bind all args
1144 and strict (demanded)
1151 there is no point in transforming to
1153 x = case (y div# z) of r -> MkC r
1155 because the (y div# z) can't float out of the let. But if it was
1156 a *strict* let, then it would be a good thing to do. Hence the
1157 context information.
1160 mkAtomicArgsE :: SimplEnv
1161 -> Bool -- A strict binding
1162 -> OutExpr -- The rhs
1163 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1164 -> SimplM FloatsWithExpr
1166 mkAtomicArgsE env is_strict rhs thing_inside
1167 | (Var fun, args) <- collectArgs rhs, -- It's an application
1168 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1169 = go env (Var fun) args
1171 | otherwise = thing_inside env rhs
1174 go env fun [] = thing_inside env fun
1176 go env fun (arg : args)
1177 | exprIsTrivial arg -- Easy case
1178 || no_float_arg -- Can't make it atomic
1179 = go env (App fun arg) args
1182 = do { arg_id <- newId FSLIT("a") arg_ty
1183 ; completeNonRecX env False {- pessimistic -} arg_id arg_id arg $ \env ->
1184 go env (App fun (Var arg_id)) args }
1186 arg_ty = exprType arg
1187 no_float_arg = not is_strict && (isUnLiftedType arg_ty) && not (exprOkForSpeculation arg)
1190 -- Old code: consider rewriting to be more like mkAtomicArgsE
1192 mkAtomicArgs :: Bool -- A strict binding
1193 -> Bool -- OK to float unlifted args
1195 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1196 OutExpr) -- things that need case-binding,
1197 -- if the strict-binding flag is on
1199 mkAtomicArgs is_strict ok_float_unlifted rhs
1200 | (Var fun, args) <- collectArgs rhs, -- It's an application
1201 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1202 = go fun nilOL [] args -- Have a go
1204 | otherwise = bale_out -- Give up
1207 bale_out = returnSmpl (nilOL, rhs)
1209 go fun binds rev_args []
1210 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1212 go fun binds rev_args (arg : args)
1213 | exprIsTrivial arg -- Easy case
1214 = go fun binds (arg:rev_args) args
1216 | not can_float_arg -- Can't make this arg atomic
1217 = bale_out -- ... so give up
1219 | otherwise -- Don't forget to do it recursively
1220 -- E.g. x = a:b:c:[]
1221 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1222 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1223 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1224 (Var arg_id : rev_args) args
1226 arg_ty = exprType arg
1227 can_float_arg = is_strict
1228 || not (isUnLiftedType arg_ty)
1229 || (ok_float_unlifted && exprOkForSpeculation arg)
1232 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1233 -> (SimplEnv -> SimplM (FloatsWith a))
1234 -> SimplM (FloatsWith a)
1235 addAtomicBinds env [] thing_inside = thing_inside env
1236 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1237 addAtomicBinds env bs thing_inside
1241 %************************************************************************
1243 \subsection{The main rebuilder}
1245 %************************************************************************
1248 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1250 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1251 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1252 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1253 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1254 rebuild env expr (ApplyTo _ arg mb_se cont) = rebuildApp env expr arg mb_se cont
1256 rebuildApp env fun arg mb_se cont
1257 = do { arg' <- simplArg env arg mb_se
1258 ; rebuild env (App fun arg') cont }
1260 simplArg :: SimplEnv -> CoreExpr -> Maybe SimplEnv -> SimplM CoreExpr
1261 simplArg env arg Nothing = return arg -- The arg is already simplified
1262 simplArg env arg (Just arg_env) = simplExpr (setInScope arg_env env) arg
1264 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1268 %************************************************************************
1270 \subsection{Functions dealing with a case}
1272 %************************************************************************
1274 Blob of helper functions for the "case-of-something-else" situation.
1277 ---------------------------------------------------------
1278 -- Eliminate the case if possible
1280 rebuildCase :: SimplEnv
1281 -> OutExpr -- Scrutinee
1282 -> InId -- Case binder
1283 -> [InAlt] -- Alternatives (inceasing order)
1285 -> SimplM FloatsWithExpr
1287 rebuildCase env scrut case_bndr alts cont
1288 | Just (con,args) <- exprIsConApp_maybe scrut
1289 -- Works when the scrutinee is a variable with a known unfolding
1290 -- as well as when it's an explicit constructor application
1291 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1293 | Lit lit <- scrut -- No need for same treatment as constructors
1294 -- because literals are inlined more vigorously
1295 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1298 = -- Prepare the continuation;
1299 -- The new subst_env is in place
1300 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1301 addFloats env floats $ \ env ->
1304 -- The case expression is annotated with the result type of the continuation
1305 -- This may differ from the type originally on the case. For example
1306 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1309 -- let j a# = <blob>
1310 -- in case(T) a of { True -> j 1#; False -> j 0# }
1311 -- Note that the case that scrutinises a now returns a T not an Int#
1312 res_ty' = contResultType dup_cont
1315 -- Deal with case binder
1316 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1318 -- Deal with the case alternatives
1319 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1321 -- Put the case back together
1322 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1324 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1325 -- The case binder *not* scope over the whole returned case-expression
1326 rebuild env case_expr nondup_cont
1329 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1330 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1331 way, there's a chance that v will now only be used once, and hence
1336 There is a time we *don't* want to do that, namely when
1337 -fno-case-of-case is on. This happens in the first simplifier pass,
1338 and enhances full laziness. Here's the bad case:
1339 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1340 If we eliminate the inner case, we trap it inside the I# v -> arm,
1341 which might prevent some full laziness happening. I've seen this
1342 in action in spectral/cichelli/Prog.hs:
1343 [(m,n) | m <- [1..max], n <- [1..max]]
1344 Hence the check for NoCaseOfCase.
1348 There is another situation when we don't want to do it. If we have
1350 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1351 ...other cases .... }
1353 We'll perform the binder-swap for the outer case, giving
1355 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1356 ...other cases .... }
1358 But there is no point in doing it for the inner case, because w1 can't
1359 be inlined anyway. Furthermore, doing the case-swapping involves
1360 zapping w2's occurrence info (see paragraphs that follow), and that
1361 forces us to bind w2 when doing case merging. So we get
1363 case x of w1 { A -> let w2 = w1 in e1
1364 B -> let w2 = w1 in e2
1365 ...other cases .... }
1367 This is plain silly in the common case where w2 is dead.
1369 Even so, I can't see a good way to implement this idea. I tried
1370 not doing the binder-swap if the scrutinee was already evaluated
1371 but that failed big-time:
1375 case v of w { MkT x ->
1376 case x of x1 { I# y1 ->
1377 case x of x2 { I# y2 -> ...
1379 Notice that because MkT is strict, x is marked "evaluated". But to
1380 eliminate the last case, we must either make sure that x (as well as
1381 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1382 the binder-swap. So this whole note is a no-op.
1386 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1387 any occurrence info (eg IAmDead) in the case binder, because the
1388 case-binder now effectively occurs whenever v does. AND we have to do
1389 the same for the pattern-bound variables! Example:
1391 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1393 Here, b and p are dead. But when we move the argment inside the first
1394 case RHS, and eliminate the second case, we get
1396 case x of { (a,b) -> a b }
1398 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1401 Indeed, this can happen anytime the case binder isn't dead:
1402 case <any> of x { (a,b) ->
1403 case x of { (p,q) -> p } }
1404 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1405 The point is that we bring into the envt a binding
1407 after the outer case, and that makes (a,b) alive. At least we do unless
1408 the case binder is guaranteed dead.
1411 simplCaseBinder env (Var v) case_bndr
1412 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1414 -- Failed try [see Note 2 above]
1415 -- not (isEvaldUnfolding (idUnfolding v))
1417 = simplBinder env (zapOccInfo case_bndr) `thenSmpl` \ (env, case_bndr') ->
1418 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1419 -- We could extend the substitution instead, but it would be
1420 -- a hack because then the substitution wouldn't be idempotent
1421 -- any more (v is an OutId). And this does just as well.
1423 simplCaseBinder env other_scrut case_bndr
1424 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1425 returnSmpl (env, case_bndr')
1427 zapOccInfo :: InId -> InId
1428 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1432 simplAlts does two things:
1434 1. Eliminate alternatives that cannot match, including the
1435 DEFAULT alternative.
1437 2. If the DEFAULT alternative can match only one possible constructor,
1438 then make that constructor explicit.
1440 case e of x { DEFAULT -> rhs }
1442 case e of x { (a,b) -> rhs }
1443 where the type is a single constructor type. This gives better code
1444 when rhs also scrutinises x or e.
1446 Here "cannot match" includes knowledge from GADTs
1448 It's a good idea do do this stuff before simplifying the alternatives, to
1449 avoid simplifying alternatives we know can't happen, and to come up with
1450 the list of constructors that are handled, to put into the IdInfo of the
1451 case binder, for use when simplifying the alternatives.
1453 Eliminating the default alternative in (1) isn't so obvious, but it can
1456 data Colour = Red | Green | Blue
1465 DEFAULT -> [ case y of ... ]
1467 If we inline h into f, the default case of the inlined h can't happen.
1468 If we don't notice this, we may end up filtering out *all* the cases
1469 of the inner case y, which give us nowhere to go!
1473 simplAlts :: SimplEnv
1475 -> OutId -- Case binder
1476 -> [InAlt] -> SimplCont
1477 -> SimplM [OutAlt] -- Includes the continuation
1479 simplAlts env scrut case_bndr' alts cont'
1480 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1481 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1482 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1483 -- We need the mergeAlts in case the new default_alt
1484 -- has turned into a constructor alternative.
1486 (alts_wo_default, maybe_deflt) = findDefault alts
1487 imposs_cons = case scrut of
1488 Var v -> otherCons (idUnfolding v)
1491 -- "imposs_deflt_cons" are handled either by the context,
1492 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1493 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1495 simplDefault :: SimplEnv
1496 -> OutId -- Case binder; need just for its type. Note that as an
1497 -- OutId, it has maximum information; this is important.
1498 -- Test simpl013 is an example
1499 -> [AltCon] -- These cons can't happen when matching the default
1502 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1503 -- mergeAlts expects
1506 simplDefault env case_bndr' imposs_cons cont Nothing
1507 = return [] -- No default branch
1508 simplDefault env case_bndr' imposs_cons cont (Just rhs)
1509 | -- This branch handles the case where we are
1510 -- scrutinisng an algebraic data type
1511 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1512 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1513 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1514 -- case x of { DEFAULT -> e }
1515 -- and we don't want to fill in a default for them!
1516 Just all_cons <- tyConDataCons_maybe tycon,
1517 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1518 -- which GHC allows, then the case expression will have at most a default
1519 -- alternative. We don't want to eliminate that alternative, because the
1520 -- invariant is that there's always one alternative. It's more convenient
1522 -- case x of { DEFAULT -> e }
1523 -- as it is, rather than transform it to
1524 -- error "case cant match"
1525 -- which would be quite legitmate. But it's a really obscure corner, and
1526 -- not worth wasting code on.
1528 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1529 poss_data_cons = filterOut (`elem` imposs_data_cons) all_cons
1530 gadt_imposs | all isTyVarTy inst_tys = []
1531 | otherwise = filter (cant_match inst_tys) poss_data_cons
1532 final_poss = filterOut (`elem` gadt_imposs) poss_data_cons
1534 = case final_poss of
1535 [] -> returnSmpl [] -- Eliminate the default alternative
1536 -- altogether if it can't match
1538 [con] -> -- It matches exactly one constructor, so fill it in
1539 do { con_alt <- mkDataConAlt case_bndr' con inst_tys rhs
1540 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1541 -- The simplAlt must succeed with Just because we have
1542 -- already filtered out construtors that can't match
1545 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1548 = simplify_default imposs_cons
1550 cant_match tys data_con = not (dataConCanMatch data_con tys)
1552 simplify_default imposs_cons
1553 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1554 -- Record the constructors that the case-binder *can't* be.
1555 ; rhs' <- simplExprC env' rhs cont
1556 ; return [(DEFAULT, [], rhs')] }
1558 mkDataConAlt :: Id -> DataCon -> [OutType] -> InExpr -> SimplM InAlt
1559 -- Make a data-constructor alternative to replace the DEFAULT case
1560 -- NB: there's something a bit bogus here, because we put OutTypes into an InAlt
1561 mkDataConAlt case_bndr con tys rhs
1562 = do { tick (FillInCaseDefault case_bndr)
1563 ; args <- mk_args con tys
1564 ; return (DataAlt con, args, rhs) }
1566 mk_args con inst_tys
1567 = do { (tv_bndrs, inst_tys') <- mk_tv_bndrs con inst_tys
1568 ; let arg_tys = dataConInstArgTys con inst_tys'
1569 ; arg_ids <- mapM (newId FSLIT("a")) arg_tys
1570 ; returnSmpl (tv_bndrs ++ arg_ids) }
1572 mk_tv_bndrs con inst_tys
1573 | isVanillaDataCon con
1574 = return ([], inst_tys)
1576 = do { tv_uniqs <- getUniquesSmpl
1577 ; let new_tvs = zipWith mk tv_uniqs (dataConTyVars con)
1578 mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv)
1579 ; return (new_tvs, mkTyVarTys new_tvs) }
1581 simplAlt :: SimplEnv
1582 -> [AltCon] -- These constructors can't be present when
1583 -- matching this alternative
1584 -> OutId -- The case binder
1587 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1589 -- Simplify an alternative, returning the type refinement for the
1590 -- alternative, if the alternative does any refinement at all
1591 -- Nothing => the alternative is inaccessible
1593 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1594 | con `elem` imposs_cons -- This case can't match
1597 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1598 -- TURGID DUPLICATION, needed only for the simplAlt call
1599 -- in mkDupableAlt. Clean this up when moving to FC
1600 = ASSERT( null bndrs )
1601 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1602 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1604 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1605 -- Record the constructors that the case-binder *can't* be.
1607 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1608 = ASSERT( null bndrs )
1609 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1610 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1612 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1614 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1615 | isVanillaDataCon con
1616 = -- Deal with the pattern-bound variables
1617 -- Mark the ones that are in ! positions in the data constructor
1618 -- as certainly-evaluated.
1619 -- NB: it happens that simplBinders does *not* erase the OtherCon
1620 -- form of unfolding, so it's ok to add this info before
1621 -- doing simplBinders
1622 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1624 -- Bind the case-binder to (con args)
1625 let unf = mkUnfolding False (mkConApp con con_args)
1626 inst_tys' = tyConAppArgs (idType case_bndr')
1627 con_args = map Type inst_tys' ++ map varToCoreExpr vs'
1628 env' = mk_rhs_env env case_bndr' unf
1630 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1631 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1633 | otherwise -- GADT case
1635 (tvs,ids) = span isTyVar vs
1637 simplBinders env tvs `thenSmpl` \ (env1, tvs') ->
1638 case coreRefineTys con tvs' (idType case_bndr') of {
1639 Nothing -- Inaccessible
1640 | opt_PprStyle_Debug -- Hack: if debugging is on, generate an error case
1642 -> let rhs' = mkApps (Var eRROR_ID)
1643 [Type (substTy env (exprType rhs)),
1644 Lit (mkStringLit "Impossible alternative (GADT)")]
1646 simplBinders env1 ids `thenSmpl` \ (env2, ids') ->
1647 returnSmpl (Just (emptyVarEnv, (DataAlt con, tvs' ++ ids', rhs')))
1649 | otherwise -- Filter out the inaccessible branch
1652 Just refine@(tv_subst_env, _) -> -- The normal case
1655 env2 = refineSimplEnv env1 refine
1656 -- Simplify the Ids in the refined environment, so their types
1657 -- reflect the refinement. Usually this doesn't matter, but it helps
1658 -- in mkDupableAlt, when we want to float a lambda that uses these binders
1659 -- Furthermore, it means the binders contain maximal type information
1661 simplBinders env2 (add_evals con ids) `thenSmpl` \ (env3, ids') ->
1662 let unf = mkUnfolding False con_app
1663 con_app = mkConApp con con_args
1664 con_args = map varToCoreExpr vs' -- NB: no inst_tys'
1665 env_w_unf = mk_rhs_env env3 case_bndr' unf
1668 simplExprC env_w_unf rhs cont' `thenSmpl` \ rhs' ->
1669 returnSmpl (Just (tv_subst_env, (DataAlt con, vs', rhs'))) }
1672 -- add_evals records the evaluated-ness of the bound variables of
1673 -- a case pattern. This is *important*. Consider
1674 -- data T = T !Int !Int
1676 -- case x of { T a b -> T (a+1) b }
1678 -- We really must record that b is already evaluated so that we don't
1679 -- go and re-evaluate it when constructing the result.
1680 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1682 cat_evals dc vs strs
1686 go (v:vs) strs | isTyVar v = v : go vs strs
1687 go (v:vs) (str:strs)
1688 | isMarkedStrict str = evald_v : go vs strs
1689 | otherwise = zapped_v : go vs strs
1691 zapped_v = zap_occ_info v
1692 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1693 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1695 -- If the case binder is alive, then we add the unfolding
1697 -- to the envt; so vs are now very much alive
1698 -- Note [Aug06] I can't see why this actually matters
1699 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1700 | otherwise = zapOccInfo
1702 mk_rhs_env env case_bndr' case_bndr_unf
1703 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1707 %************************************************************************
1709 \subsection{Known constructor}
1711 %************************************************************************
1713 We are a bit careful with occurrence info. Here's an example
1715 (\x* -> case x of (a*, b) -> f a) (h v, e)
1717 where the * means "occurs once". This effectively becomes
1718 case (h v, e) of (a*, b) -> f a)
1720 let a* = h v; b = e in f a
1724 All this should happen in one sweep.
1727 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1728 -> InId -> [InAlt] -> SimplCont
1729 -> SimplM FloatsWithExpr
1731 knownCon env scrut con args bndr alts cont
1732 = tick (KnownBranch bndr) `thenSmpl_`
1733 case findAlt con alts of
1734 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1735 simplNonRecX env bndr scrut $ \ env ->
1736 -- This might give rise to a binding with non-atomic args
1737 -- like x = Node (f x) (g x)
1738 -- but simplNonRecX will atomic-ify it
1739 simplExprF env rhs cont
1741 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1742 simplNonRecX env bndr scrut $ \ env ->
1743 simplExprF env rhs cont
1745 (DataAlt dc, bs, rhs)
1746 -> ASSERT( n_drop_tys + length bs == length args )
1747 bind_args env dead_bndr bs (drop n_drop_tys args) $ \ env ->
1749 -- It's useful to bind bndr to scrut, rather than to a fresh
1750 -- binding x = Con arg1 .. argn
1751 -- because very often the scrut is a variable, so we avoid
1752 -- creating, and then subsequently eliminating, a let-binding
1753 -- BUT, if scrut is a not a variable, we must be careful
1754 -- about duplicating the arg redexes; in that case, make
1755 -- a new con-app from the args
1756 bndr_rhs = case scrut of
1759 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1760 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1761 -- args are aready OutExprs, but bs are InIds
1763 simplNonRecX env bndr bndr_rhs $ \ env ->
1764 simplExprF env rhs cont
1766 dead_bndr = isDeadBinder bndr
1767 n_drop_tys | isVanillaDataCon dc = tyConArity (dataConTyCon dc)
1769 -- Vanilla data constructors lack type arguments in the pattern
1772 bind_args env dead_bndr [] _ thing_inside = thing_inside env
1774 bind_args env dead_bndr (b:bs) (Type ty : args) thing_inside
1775 = ASSERT( isTyVar b )
1776 bind_args (extendTvSubst env b ty) dead_bndr bs args thing_inside
1778 bind_args env dead_bndr (b:bs) (arg : args) thing_inside
1781 b' = if dead_bndr then b else zapOccInfo b
1782 -- Note that the binder might be "dead", because it doesn't occur
1783 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1784 -- Nevertheless we must keep it if the case-binder is alive, because it may
1785 -- be used in teh con_app
1787 simplNonRecX env b' arg $ \ env ->
1788 bind_args env dead_bndr bs args thing_inside
1792 %************************************************************************
1794 \subsection{Duplicating continuations}
1796 %************************************************************************
1799 prepareCaseCont :: SimplEnv
1800 -> [InAlt] -> SimplCont
1801 -> SimplM (FloatsWith (SimplCont,SimplCont))
1802 -- Return a duplicatable continuation, a non-duplicable part
1803 -- plus some extra bindings (that scope over the entire
1806 -- No need to make it duplicatable if there's only one alternative
1807 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1808 prepareCaseCont env alts cont = mkDupableCont env cont
1812 mkDupableCont :: SimplEnv -> SimplCont
1813 -> SimplM (FloatsWith (SimplCont, SimplCont))
1815 mkDupableCont env cont
1816 | contIsDupable cont
1817 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1819 mkDupableCont env (CoerceIt ty cont)
1820 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1821 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1823 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1824 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1825 -- Do *not* duplicate an ArgOf continuation
1826 -- Because ArgOf continuations are opaque, we gain nothing by
1827 -- propagating them into the expressions, and we do lose a lot.
1828 -- Here's an example:
1829 -- && (case x of { T -> F; F -> T }) E
1830 -- Now, && is strict so we end up simplifying the case with
1831 -- an ArgOf continuation. If we let-bind it, we get
1833 -- let $j = \v -> && v E
1834 -- in simplExpr (case x of { T -> F; F -> T })
1835 -- (ArgOf (\r -> $j r)
1836 -- And after simplifying more we get
1838 -- let $j = \v -> && v E
1839 -- in case of { T -> $j F; F -> $j T }
1840 -- Which is a Very Bad Thing
1842 -- The desire not to duplicate is the entire reason that
1843 -- mkDupableCont returns a pair of continuations.
1845 -- The original plan had:
1846 -- e.g. (...strict-fn...) [...hole...]
1848 -- let $j = \a -> ...strict-fn...
1849 -- in $j [...hole...]
1851 mkDupableCont env (ApplyTo _ arg mb_se cont)
1852 = -- e.g. [...hole...] (...arg...)
1854 -- let a = ...arg...
1855 -- in [...hole...] a
1856 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1857 ; addFloats env floats $ \ env -> do
1858 { arg1 <- simplArg env arg mb_se
1859 ; (floats2, arg2) <- mkDupableArg env arg1
1860 ; return (floats2, (ApplyTo OkToDup arg2 Nothing dup_cont, nondup_cont)) }}
1862 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1863 -- | not (exprIsDupable rhs && contIsDupable case_cont) -- See notes below
1864 -- | not (isDeadBinder case_bndr)
1865 | all isDeadBinder bs
1866 = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
1868 scrut_ty = substTy se (idType case_bndr)
1870 {- Note [Single-alternative cases]
1871 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1872 This case is just like the ArgOf case. Here's an example:
1876 case (case x of I# x' ->
1878 True -> I# (negate# x')
1879 False -> I# x') of y {
1881 Because the (case x) has only one alternative, we'll transform to
1883 case (case x' <# 0# of
1884 True -> I# (negate# x')
1885 False -> I# x') of y {
1887 But now we do *NOT* want to make a join point etc, giving
1889 let $j = \y -> MkT y
1891 True -> $j (I# (negate# x'))
1893 In this case the $j will inline again, but suppose there was a big
1894 strict computation enclosing the orginal call to MkT. Then, it won't
1895 "see" the MkT any more, because it's big and won't get duplicated.
1896 And, what is worse, nothing was gained by the case-of-case transform.
1898 When should use this case of mkDupableCont?
1899 However, matching on *any* single-alternative case is a *disaster*;
1900 e.g. case (case ....) of (a,b) -> (# a,b #)
1901 We must push the outer case into the inner one!
1904 * Match [(DEFAULT,_,_)], but in the common case of Int,
1905 the alternative-filling-in code turned the outer case into
1906 case (...) of y { I# _ -> MkT y }
1908 * Match on single alternative plus (not (isDeadBinder case_bndr))
1909 Rationale: pushing the case inwards won't eliminate the construction.
1910 But there's a risk of
1911 case (...) of y { (a,b) -> let z=(a,b) in ... }
1912 Now y looks dead, but it'll come alive again. Still, this
1913 seems like the best option at the moment.
1915 * Match on single alternative plus (all (isDeadBinder bndrs))
1916 Rationale: this is essentially seq.
1918 * Match when the rhs is *not* duplicable, and hence would lead to a
1919 join point. This catches the disaster-case above. We can test
1920 the *un-simplified* rhs, which is fine. It might get bigger or
1921 smaller after simplification; if it gets smaller, this case might
1922 fire next time round. NB also that we must test contIsDupable
1923 case_cont *btoo, because case_cont might be big!
1925 HOWEVER: I found that this version doesn't work well, because
1926 we can get let x = case (...) of { small } in ...case x...
1927 When x is inlined into its full context, we find that it was a bad
1928 idea to have pushed the outer case inside the (...) case.
1931 mkDupableCont env (Select _ case_bndr alts se cont)
1932 = -- e.g. (case [...hole...] of { pi -> ei })
1934 -- let ji = \xij -> ei
1935 -- in case [...hole...] of { pi -> ji xij }
1936 do { tick (CaseOfCase case_bndr)
1937 ; let alt_env = setInScope se env
1938 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1939 -- NB: call mkDupableCont here, *not* prepareCaseCont
1940 -- We must make a duplicable continuation, whereas prepareCaseCont
1941 -- doesn't when there is a single case branch
1942 ; addFloats alt_env floats1 $ \ alt_env -> do
1944 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1945 -- NB: simplBinder does not zap deadness occ-info, so
1946 -- a dead case_bndr' will still advertise its deadness
1947 -- This is really important because in
1948 -- case e of b { (# a,b #) -> ... }
1949 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1950 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1951 -- In the new alts we build, we have the new case binder, so it must retain
1954 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1955 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1956 (mkBoringStop (contResultType dup_cont)),
1960 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1961 -- Let-bind the thing if necessary
1962 mkDupableArg env arg
1964 = return (emptyFloats env, arg)
1966 = do { arg_id <- newId FSLIT("a") (exprType arg)
1967 ; tick (CaseOfCase arg_id)
1968 -- Want to tick here so that we go round again,
1969 -- and maybe copy or inline the code.
1970 -- Not strictly CaseOfCase, but never mind
1971 ; return (unitFloat env arg_id arg, Var arg_id) }
1972 -- What if the arg should be case-bound?
1973 -- This has been this way for a long time, so I'll leave it,
1974 -- but I can't convince myself that it's right.
1976 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1977 -> SimplM (FloatsWith [InAlt])
1978 -- Absorbs the continuation into the new alternatives
1980 mkDupableAlts env case_bndr' alts dupable_cont
1983 go env [] = returnSmpl (emptyFloats env, [])
1985 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1986 ; addFloats env floats1 $ \ env -> do
1987 { (floats2, alts') <- go env alts
1988 ; returnSmpl (floats2, case mb_alt' of
1989 Just alt' -> alt' : alts'
1993 mkDupableAlt env case_bndr' cont alt
1994 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1996 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1998 Just (reft, (con, bndrs', rhs')) ->
1999 -- Safe to say that there are no handled-cons for the DEFAULT case
2001 if exprIsDupable rhs' then
2002 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
2003 -- It is worth checking for a small RHS because otherwise we
2004 -- get extra let bindings that may cause an extra iteration of the simplifier to
2005 -- inline back in place. Quite often the rhs is just a variable or constructor.
2006 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
2007 -- iterations because the version with the let bindings looked big, and so wasn't
2008 -- inlined, but after the join points had been inlined it looked smaller, and so
2011 -- NB: we have to check the size of rhs', not rhs.
2012 -- Duplicating a small InAlt might invalidate occurrence information
2013 -- However, if it *is* dupable, we return the *un* simplified alternative,
2014 -- because otherwise we'd need to pair it up with an empty subst-env....
2015 -- but we only have one env shared between all the alts.
2016 -- (Remember we must zap the subst-env before re-simplifying something).
2017 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
2021 rhs_ty' = exprType rhs'
2022 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
2024 | isTyVar bndr = not (bndr `elemVarEnv` reft)
2025 -- Don't abstract over tyvar binders which are refined away
2026 -- See Note [Refinement] below
2027 | otherwise = not (isDeadBinder bndr)
2028 -- The deadness info on the new Ids is preserved by simplBinders
2030 -- If we try to lift a primitive-typed something out
2031 -- for let-binding-purposes, we will *caseify* it (!),
2032 -- with potentially-disastrous strictness results. So
2033 -- instead we turn it into a function: \v -> e
2034 -- where v::State# RealWorld#. The value passed to this function
2035 -- is realworld#, which generates (almost) no code.
2037 -- There's a slight infelicity here: we pass the overall
2038 -- case_bndr to all the join points if it's used in *any* RHS,
2039 -- because we don't know its usage in each RHS separately
2041 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
2042 -- we make the join point into a function whenever used_bndrs'
2043 -- is empty. This makes the join-point more CPR friendly.
2044 -- Consider: let j = if .. then I# 3 else I# 4
2045 -- in case .. of { A -> j; B -> j; C -> ... }
2047 -- Now CPR doesn't w/w j because it's a thunk, so
2048 -- that means that the enclosing function can't w/w either,
2049 -- which is a lose. Here's the example that happened in practice:
2050 -- kgmod :: Int -> Int -> Int
2051 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2055 -- I have seen a case alternative like this:
2056 -- True -> \v -> ...
2057 -- It's a bit silly to add the realWorld dummy arg in this case, making
2060 -- (the \v alone is enough to make CPR happy) but I think it's rare
2062 ( if not (any isId used_bndrs')
2063 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
2064 returnSmpl ([rw_id], [Var realWorldPrimId])
2066 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
2067 ) `thenSmpl` \ (final_bndrs', final_args) ->
2069 -- See comment about "$j" name above
2070 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
2071 -- Notice the funky mkPiTypes. If the contructor has existentials
2072 -- it's possible that the join point will be abstracted over
2073 -- type varaibles as well as term variables.
2074 -- Example: Suppose we have
2075 -- data T = forall t. C [t]
2077 -- case (case e of ...) of
2078 -- C t xs::[t] -> rhs
2079 -- We get the join point
2080 -- let j :: forall t. [t] -> ...
2081 -- j = /\t \xs::[t] -> rhs
2083 -- case (case e of ...) of
2084 -- C t xs::[t] -> j t xs
2086 -- We make the lambdas into one-shot-lambdas. The
2087 -- join point is sure to be applied at most once, and doing so
2088 -- prevents the body of the join point being floated out by
2089 -- the full laziness pass
2090 really_final_bndrs = map one_shot final_bndrs'
2091 one_shot v | isId v = setOneShotLambda v
2093 join_rhs = mkLams really_final_bndrs rhs'
2094 join_call = mkApps (Var join_bndr) final_args
2096 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2103 MkT :: a -> b -> T a
2107 MkT a' b (p::a') (q::b) -> [p,w]
2109 The danger is that we'll make a join point
2113 and that's ill-typed, because (p::a') but (w::a).
2115 Solution so far: don't abstract over a', because the type refinement
2116 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2118 Then we must abstract over any refined free variables. Hmm. Maybe we
2119 could just abstract over *all* free variables, thereby lambda-lifting
2120 the join point? We should try this.