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 if needsCaseBinding bndr_ty rhs1
325 thing_inside env2 `thenSmpl` \ (floats, body) ->
326 returnSmpl (emptyFloats env2, Case rhs1 bndr2 (exprType body)
327 [(DEFAULT, [], wrapFloats floats body)])
329 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
331 | otherwise -- Normal, lazy case
332 = -- Don't use simplBinder because that doesn't keep
333 -- fragile occurrence info in the substitution
334 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr') ->
335 simplLazyBind env NotTopLevel NonRecursive
336 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
337 addFloats env floats thing_inside
340 bndr_ty = idType bndr
343 A specialised variant of simplNonRec used when the RHS is already simplified, notably
344 in knownCon. It uses case-binding where necessary.
347 simplNonRecX :: SimplEnv
348 -> InId -- Old binder
349 -> OutExpr -- Simplified RHS
350 -> (SimplEnv -> SimplM FloatsWithExpr)
351 -> SimplM FloatsWithExpr
353 simplNonRecX env bndr new_rhs thing_inside
354 | needsCaseBinding (idType bndr) new_rhs
355 -- Make this test *before* the preInlineUnconditionally
356 -- Consider case I# (quotInt# x y) of
357 -- I# v -> let w = J# v in ...
358 -- If we gaily inline (quotInt# x y) for v, we end up building an
360 -- let w = J# (quotInt# x y) in ...
361 -- because quotInt# can fail.
362 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
363 thing_inside env `thenSmpl` \ (floats, body) ->
364 let body' = wrapFloats floats body in
365 returnSmpl (emptyFloats env, Case new_rhs bndr' (exprType body') [(DEFAULT, [], body')])
367 | preInlineUnconditionally env NotTopLevel bndr new_rhs
368 -- This happens; for example, the case_bndr during case of
369 -- known constructor: case (a,b) of x { (p,q) -> ... }
370 -- Here x isn't mentioned in the RHS, so we don't want to
371 -- create the (dead) let-binding let x = (a,b) in ...
373 -- Similarly, single occurrences can be inlined vigourously
374 -- e.g. case (f x, g y) of (a,b) -> ....
375 -- If a,b occur once we can avoid constructing the let binding for them.
376 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
379 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
380 completeNonRecX env False {- Non-strict; pessimistic -}
381 bndr bndr' new_rhs thing_inside
383 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
384 = mkAtomicArgs is_strict
385 True {- OK to float unlifted -}
386 new_rhs `thenSmpl` \ (aux_binds, rhs2) ->
388 -- Make the arguments atomic if necessary,
389 -- adding suitable bindings
390 addAtomicBindsE env (fromOL aux_binds) $ \ env ->
391 completeLazyBind env NotTopLevel
392 old_bndr new_bndr rhs2 `thenSmpl` \ (floats, env) ->
393 addFloats env floats thing_inside
397 %************************************************************************
399 \subsection{Lazy bindings}
401 %************************************************************************
403 simplRecBind is used for
404 * recursive bindings only
407 simplRecBind :: SimplEnv -> TopLevelFlag
408 -> [(InId, InExpr)] -> [OutId]
409 -> SimplM (FloatsWith SimplEnv)
410 simplRecBind env top_lvl pairs bndrs'
411 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
412 returnSmpl (flattenFloats floats, env)
414 go env [] _ = returnSmpl (emptyFloats env, env)
416 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
417 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
418 addFloats env floats (\env -> go env pairs bndrs')
422 simplRecOrTopPair is used for
423 * recursive bindings (whether top level or not)
424 * top-level non-recursive bindings
426 It assumes the binder has already been simplified, but not its IdInfo.
429 simplRecOrTopPair :: SimplEnv
431 -> InId -> OutId -- Binder, both pre-and post simpl
432 -> InExpr -- The RHS and its environment
433 -> SimplM (FloatsWith SimplEnv)
435 simplRecOrTopPair env top_lvl bndr bndr' rhs
436 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
437 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
438 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
441 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
442 -- May not actually be recursive, but it doesn't matter
446 simplLazyBind is used for
447 * recursive bindings (whether top level or not)
448 * top-level non-recursive bindings
449 * non-top-level *lazy* non-recursive bindings
451 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
452 from SimplRecOrTopBind]
455 1. It assumes that the binder is *already* simplified,
456 and is in scope, but not its IdInfo
458 2. It assumes that the binder type is lifted.
460 3. It does not check for pre-inline-unconditionallly;
461 that should have been done already.
464 simplLazyBind :: SimplEnv
465 -> TopLevelFlag -> RecFlag
466 -> InId -> OutId -- Binder, both pre-and post simpl
467 -> InExpr -> SimplEnv -- The RHS and its environment
468 -> SimplM (FloatsWith SimplEnv)
470 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
472 (env1,bndr2) = addLetIdInfo env bndr bndr1
473 rhs_env = setInScope rhs_se env1
474 is_top_level = isTopLevel top_lvl
475 ok_float_unlifted = not is_top_level && isNonRec is_rec
476 rhs_cont = mkRhsStop (idType bndr2)
478 -- Simplify the RHS; note the mkRhsStop, which tells
479 -- the simplifier that this is the RHS of a let.
480 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
482 -- If any of the floats can't be floated, give up now
483 -- (The allLifted predicate says True for empty floats.)
484 if (not ok_float_unlifted && not (allLifted floats)) then
485 completeLazyBind env1 top_lvl bndr bndr2
486 (wrapFloats floats rhs1)
489 -- ANF-ise a constructor or PAP rhs
490 mkAtomicArgs False {- Not strict -}
491 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
493 -- If the result is a PAP, float the floats out, else wrap them
494 -- By this time it's already been ANF-ised (if necessary)
495 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
496 completeLazyBind env1 top_lvl bndr bndr2 rhs2
498 else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
499 -- WARNING: long dodgy argument coming up
500 -- WANTED: a better way to do this
502 -- We can't use "exprIsCheap" instead of exprIsHNF,
503 -- because that causes a strictness bug.
504 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
505 -- The case expression is 'cheap', but it's wrong to transform to
506 -- y* = E; x = case (scc y) of {...}
507 -- Either we must be careful not to float demanded non-values, or
508 -- we must use exprIsHNF for the test, which ensures that the
509 -- thing is non-strict. So exprIsHNF => bindings are non-strict
510 -- I think. The WARN below tests for this.
512 -- We use exprIsTrivial here because we want to reveal lone variables.
513 -- E.g. let { x = letrec { y = E } in y } in ...
514 -- Here we definitely want to float the y=E defn.
515 -- exprIsHNF definitely isn't right for that.
517 -- Again, the floated binding can't be strict; if it's recursive it'll
518 -- be non-strict; if it's non-recursive it'd be inlined.
520 -- Note [SCC-and-exprIsTrivial]
522 -- y = let { x* = E } in scc "foo" x
523 -- then we do *not* want to float out the x binding, because
524 -- it's strict! Fortunately, exprIsTrivial replies False to
527 -- There's a subtlety here. There may be a binding (x* = e) in the
528 -- floats, where the '*' means 'will be demanded'. So is it safe
529 -- to float it out? Answer no, but it won't matter because
530 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
531 -- and so there can't be any 'will be demanded' bindings in the floats.
533 ASSERT2( is_top_level || not (any demanded_float (floatBinds floats)),
534 ppr (filter demanded_float (floatBinds floats)) )
536 tick LetFloatFromLet `thenSmpl_` (
537 addFloats env1 floats $ \ env2 ->
538 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
539 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
542 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
545 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
546 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
547 demanded_float (Rec _) = False
552 %************************************************************************
554 \subsection{Completing a lazy binding}
556 %************************************************************************
559 * deals only with Ids, not TyVars
560 * takes an already-simplified binder and RHS
561 * is used for both recursive and non-recursive bindings
562 * is used for both top-level and non-top-level bindings
564 It does the following:
565 - tries discarding a dead binding
566 - tries PostInlineUnconditionally
567 - add unfolding [this is the only place we add an unfolding]
570 It does *not* attempt to do let-to-case. Why? Because it is used for
571 - top-level bindings (when let-to-case is impossible)
572 - many situations where the "rhs" is known to be a WHNF
573 (so let-to-case is inappropriate).
576 completeLazyBind :: SimplEnv
577 -> TopLevelFlag -- Flag stuck into unfolding
578 -> InId -- Old binder
579 -> OutId -- New binder
580 -> OutExpr -- Simplified RHS
581 -> SimplM (FloatsWith SimplEnv)
582 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
583 -- by extending the substitution (e.g. let x = y in ...)
584 -- The new binding (if any) is returned as part of the floats.
585 -- NB: the returned SimplEnv has the right SubstEnv, but you should
586 -- (as usual) use the in-scope-env from the floats
588 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
589 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
590 = -- Drop the binding
591 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
592 returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
593 -- Use the substitution to make quite, quite sure that the substitution
594 -- will happen, since we are going to discard the binding
599 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
601 -- Add the unfolding *only* for non-loop-breakers
602 -- Making loop breakers not have an unfolding at all
603 -- means that we can avoid tests in exprIsConApp, for example.
604 -- This is important: if exprIsConApp says 'yes' for a recursive
605 -- thing, then we can get into an infinite loop
607 -- If the unfolding is a value, the demand info may
608 -- go pear-shaped, so we nuke it. Example:
610 -- case x of (p,q) -> h p q x
611 -- Here x is certainly demanded. But after we've nuked
612 -- the case, we'll get just
613 -- let x = (a,b) in h a b x
614 -- and now x is not demanded (I'm assuming h is lazy)
615 -- This really happens. Similarly
616 -- let f = \x -> e in ...f..f...
617 -- After inling f at some of its call sites the original binding may
618 -- (for example) be no longer strictly demanded.
619 -- The solution here is a bit ad hoc...
620 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
621 final_info | loop_breaker = new_bndr_info
622 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
623 | otherwise = info_w_unf
625 final_id = new_bndr `setIdInfo` final_info
627 -- These seqs forces the Id, and hence its IdInfo,
628 -- and hence any inner substitutions
630 returnSmpl (unitFloat env final_id new_rhs, env)
633 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
634 loop_breaker = isLoopBreaker occ_info
635 old_info = idInfo old_bndr
636 occ_info = occInfo old_info
641 %************************************************************************
643 \subsection[Simplify-simplExpr]{The main function: simplExpr}
645 %************************************************************************
647 The reason for this OutExprStuff stuff is that we want to float *after*
648 simplifying a RHS, not before. If we do so naively we get quadratic
649 behaviour as things float out.
651 To see why it's important to do it after, consider this (real) example:
665 a -- Can't inline a this round, cos it appears twice
669 Each of the ==> steps is a round of simplification. We'd save a
670 whole round if we float first. This can cascade. Consider
675 let f = let d1 = ..d.. in \y -> e
679 in \x -> ...(\y ->e)...
681 Only in this second round can the \y be applied, and it
682 might do the same again.
686 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
687 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
689 expr_ty' = substTy env (exprType expr)
690 -- The type in the Stop continuation, expr_ty', is usually not used
691 -- It's only needed when discarding continuations after finding
692 -- a function that returns bottom.
693 -- Hence the lazy substitution
696 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
697 -- Simplify an expression, given a continuation
698 simplExprC env expr cont
699 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
700 returnSmpl (wrapFloats floats expr)
702 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
703 -- Simplify an expression, returning floated binds
705 simplExprF env (Var v) cont = simplVar env v cont
706 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
707 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
708 simplExprF env (Note note expr) cont = simplNote env note expr cont
709 simplExprF env (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg env cont)
711 simplExprF env (Type ty) cont
712 = ASSERT( contIsRhsOrArg cont )
713 simplType env ty `thenSmpl` \ ty' ->
714 rebuild env (Type ty') cont
716 simplExprF env (Case scrut bndr case_ty alts) cont
717 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
718 = -- Simplify the scrutinee with a Select continuation
719 simplExprF env scrut (Select NoDup bndr alts env cont)
722 = -- If case-of-case is off, simply simplify the case expression
723 -- in a vanilla Stop context, and rebuild the result around it
724 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
725 rebuild env case_expr' cont
727 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
728 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
730 simplExprF env (Let (Rec pairs) body) cont
731 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
732 -- NB: bndrs' don't have unfoldings or rules
733 -- We add them as we go down
735 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
736 addFloats env floats $ \ env ->
737 simplExprF env body cont
739 -- A non-recursive let is dealt with by simplNonRecBind
740 simplExprF env (Let (NonRec bndr rhs) body) cont
741 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
742 simplExprF env body cont
745 ---------------------------------
746 simplType :: SimplEnv -> InType -> SimplM OutType
747 -- Kept monadic just so we can do the seqType
749 = seqType new_ty `seq` returnSmpl new_ty
751 new_ty = substTy env ty
755 %************************************************************************
759 %************************************************************************
762 simplLam env fun cont
765 zap_it = mkLamBndrZapper fun (countArgs cont)
766 cont_ty = contResultType cont
768 -- Type-beta reduction
769 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
770 = ASSERT( isTyVar bndr )
771 tick (BetaReduction bndr) `thenSmpl_`
772 simplType (setInScope arg_se env) ty_arg `thenSmpl` \ ty_arg' ->
773 go (extendTvSubst env bndr ty_arg') body body_cont
775 -- Ordinary beta reduction
776 go env (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
777 = tick (BetaReduction bndr) `thenSmpl_`
778 simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
779 go env body body_cont
781 -- Not enough args, so there are real lambdas left to put in the result
782 go env lam@(Lam _ _) cont
783 = simplLamBndrs env bndrs `thenSmpl` \ (env, bndrs') ->
784 simplExpr env body `thenSmpl` \ body' ->
785 mkLam env bndrs' body' cont `thenSmpl` \ (floats, new_lam) ->
786 addFloats env floats $ \ env ->
787 rebuild env new_lam cont
789 (bndrs,body) = collectBinders lam
791 -- Exactly enough args
792 go env expr cont = simplExprF env expr cont
794 mkLamBndrZapper :: CoreExpr -- Function
795 -> Int -- Number of args supplied, *including* type args
796 -> Id -> Id -- Use this to zap the binders
797 mkLamBndrZapper fun n_args
798 | n_args >= n_params fun = \b -> b -- Enough args
799 | otherwise = \b -> zapLamIdInfo b
801 -- NB: we count all the args incl type args
802 -- so we must count all the binders (incl type lambdas)
803 n_params (Note _ e) = n_params e
804 n_params (Lam b e) = 1 + n_params e
805 n_params other = 0::Int
809 %************************************************************************
813 %************************************************************************
816 simplNote env (Coerce to from) body cont
818 addCoerce s1 k1 cont -- Drop redundant coerces. This can happen if a polymoprhic
819 -- (coerce a b e) is instantiated with a=ty1 b=ty2 and the
820 -- two are the same. This happens a lot in Happy-generated parsers
821 | s1 `coreEqType` k1 = cont
823 addCoerce s1 k1 (CoerceIt t1 cont)
824 -- coerce T1 S1 (coerce S1 K1 e)
827 -- coerce T1 K1 e, otherwise
829 -- For example, in the initial form of a worker
830 -- we may find (coerce T (coerce S (\x.e))) y
831 -- and we'd like it to simplify to e[y/x] in one round
833 | t1 `coreEqType` k1 = cont -- The coerces cancel out
834 | otherwise = CoerceIt t1 cont -- They don't cancel, but
835 -- the inner one is redundant
837 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
838 | not (isTypeArg arg), -- This whole case only works for value args
839 -- Could upgrade to have equiv thing for type apps too
840 Just (s1, s2) <- splitFunTy_maybe s1s2
841 -- (coerce (T1->T2) (S1->S2) F) E
843 -- coerce T2 S2 (F (coerce S1 T1 E))
845 -- t1t2 must be a function type, T1->T2, because it's applied to something
846 -- but s1s2 might conceivably not be
848 -- When we build the ApplyTo we can't mix the out-types
849 -- with the InExpr in the argument, so we simply substitute
850 -- to make it all consistent. It's a bit messy.
851 -- But it isn't a common case.
853 (t1,t2) = splitFunTy t1t2
854 new_arg = mkCoerce2 s1 t1 (substExpr arg_env arg)
855 arg_env = setInScope arg_se env
857 ApplyTo dup new_arg (zapSubstEnv env) (addCoerce t2 s2 cont)
859 addCoerce to' _ cont = CoerceIt to' cont
861 simplType env to `thenSmpl` \ to' ->
862 simplType env from `thenSmpl` \ from' ->
863 simplExprF env body (addCoerce to' from' cont)
866 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
867 -- inlining. All other CCCSs are mapped to currentCCS.
868 simplNote env (SCC cc) e cont
869 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
870 rebuild env (mkSCC cc e') cont
872 -- See notes with SimplMonad.inlineMode
873 simplNote env InlineMe e cont
874 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
875 = -- Don't inline inside an INLINE expression
876 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
877 rebuild env (mkInlineMe e') cont
879 | otherwise -- Dissolve the InlineMe note if there's
880 -- an interesting context of any kind to combine with
881 -- (even a type application -- anything except Stop)
882 = simplExprF env e cont
884 simplNote env (CoreNote s) e cont
885 = simplExpr env e `thenSmpl` \ e' ->
886 rebuild env (Note (CoreNote s) e') cont
890 %************************************************************************
892 \subsection{Dealing with calls}
894 %************************************************************************
897 simplVar env var cont
898 = case substId env var of
899 DoneEx e -> simplExprF (zapSubstEnv env) e cont
900 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
901 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
902 -- Note [zapSubstEnv]
903 -- The template is already simplified, so don't re-substitute.
904 -- This is VITAL. Consider
906 -- let y = \z -> ...x... in
908 -- We'll clone the inner \x, adding x->x' in the id_subst
909 -- Then when we inline y, we must *not* replace x by x' in
910 -- the inlined copy!!
912 ---------------------------------------------------------
913 -- Dealing with a call site
915 completeCall env var occ_info cont
916 = -- Simplify the arguments
917 getDOptsSmpl `thenSmpl` \ dflags ->
919 chkr = getSwitchChecker env
920 (args, call_cont) = getContArgs chkr var cont
923 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
924 (contResultType call_cont) $ \ env args ->
926 -- Next, look for rules or specialisations that match
928 -- It's important to simplify the args first, because the rule-matcher
929 -- doesn't do substitution as it goes. We don't want to use subst_args
930 -- (defined in the 'where') because that throws away useful occurrence info,
931 -- and perhaps-very-important specialisations.
933 -- Some functions have specialisations *and* are strict; in this case,
934 -- we don't want to inline the wrapper of the non-specialised thing; better
935 -- to call the specialised thing instead.
936 -- We used to use the black-listing mechanism to ensure that inlining of
937 -- the wrapper didn't occur for things that have specialisations till a
938 -- later phase, so but now we just try RULES first
940 -- You might think that we shouldn't apply rules for a loop breaker:
941 -- doing so might give rise to an infinite loop, because a RULE is
942 -- rather like an extra equation for the function:
943 -- RULE: f (g x) y = x+y
946 -- But it's too drastic to disable rules for loop breakers.
947 -- Even the foldr/build rule would be disabled, because foldr
948 -- is recursive, and hence a loop breaker:
949 -- foldr k z (build g) = g k z
950 -- So it's up to the programmer: rules can cause divergence
953 in_scope = getInScope env
955 maybe_rule = case activeRule env of
956 Nothing -> Nothing -- No rules apply
957 Just act_fn -> lookupRule act_fn in_scope rules var args
960 Just (rule_name, rule_rhs) ->
961 tick (RuleFired rule_name) `thenSmpl_`
962 (if dopt Opt_D_dump_inlinings dflags then
963 pprTrace "Rule fired" (vcat [
964 text "Rule:" <+> ftext rule_name,
965 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
966 text "After: " <+> pprCoreExpr rule_rhs,
967 text "Cont: " <+> ppr call_cont])
970 simplExprF env rule_rhs call_cont ;
972 Nothing -> -- No rules
974 -- Next, look for an inlining
976 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
977 interesting_cont = interestingCallContext (notNull args)
980 active_inline = activeInline env var occ_info
981 maybe_inline = callSiteInline dflags active_inline occ_info
982 var arg_infos interesting_cont
984 case maybe_inline of {
985 Just unfolding -- There is an inlining!
986 -> tick (UnfoldingDone var) `thenSmpl_`
987 (if dopt Opt_D_dump_inlinings dflags then
988 pprTrace "Inlining done" (vcat [
989 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
990 text "Inlined fn: " <+> ppr unfolding,
991 text "Cont: " <+> ppr call_cont])
994 makeThatCall env var unfolding args call_cont
997 Nothing -> -- No inlining!
1000 rebuild env (mkApps (Var var) args) call_cont
1003 makeThatCall :: SimplEnv
1005 -> InExpr -- Inlined function rhs
1006 -> [OutExpr] -- Arguments, already simplified
1007 -> SimplCont -- After the call
1008 -> SimplM FloatsWithExpr
1009 -- Similar to simplLam, but this time
1010 -- the arguments are already simplified
1011 makeThatCall orig_env var fun@(Lam _ _) args cont
1012 = go orig_env fun args
1014 zap_it = mkLamBndrZapper fun (length args)
1016 -- Type-beta reduction
1017 go env (Lam bndr body) (Type ty_arg : args)
1018 = ASSERT( isTyVar bndr )
1019 tick (BetaReduction bndr) `thenSmpl_`
1020 go (extendTvSubst env bndr ty_arg) body args
1022 -- Ordinary beta reduction
1023 go env (Lam bndr body) (arg : args)
1024 = tick (BetaReduction bndr) `thenSmpl_`
1025 simplNonRecX env (zap_it bndr) arg $ \ env ->
1028 -- Not enough args, so there are real lambdas left to put in the result
1030 = simplExprF env fun (pushContArgs orig_env args cont)
1031 -- NB: orig_env; the correct environment to capture with
1032 -- the arguments.... env has been augmented with substitutions
1033 -- from the beta reductions.
1035 makeThatCall env var fun args cont
1036 = simplExprF env fun (pushContArgs env args cont)
1040 %************************************************************************
1042 \subsection{Arguments}
1044 %************************************************************************
1047 ---------------------------------------------------------
1048 -- Simplifying the arguments of a call
1050 simplifyArgs :: SimplEnv
1051 -> OutType -- Type of the function
1052 -> Bool -- True if the fn has RULES
1053 -> [(InExpr, SimplEnv, Bool)] -- Details of the arguments
1054 -> OutType -- Type of the continuation
1055 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1056 -> SimplM FloatsWithExpr
1058 -- [CPS-like because of strict arguments]
1060 -- Simplify the arguments to a call.
1061 -- This part of the simplifier may break the no-shadowing invariant
1063 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1064 -- where f is strict in its second arg
1065 -- If we simplify the innermost one first we get (...(\a -> e)...)
1066 -- Simplifying the second arg makes us float the case out, so we end up with
1067 -- case y of (a,b) -> f (...(\a -> e)...) e'
1068 -- So the output does not have the no-shadowing invariant. However, there is
1069 -- no danger of getting name-capture, because when the first arg was simplified
1070 -- we used an in-scope set that at least mentioned all the variables free in its
1071 -- static environment, and that is enough.
1073 -- We can't just do innermost first, or we'd end up with a dual problem:
1074 -- case x of (a,b) -> f e (...(\a -> e')...)
1076 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1077 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1078 -- continuations. We have to keep the right in-scope set around; AND we have
1079 -- to get the effect that finding (error "foo") in a strict arg position will
1080 -- discard the entire application and replace it with (error "foo"). Getting
1081 -- all this at once is TOO HARD!
1083 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1084 = go env fn_ty args thing_inside
1086 go env fn_ty [] thing_inside = thing_inside env []
1087 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1088 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1089 thing_inside env (arg':args')
1091 simplifyArg env fn_ty has_rules (Type ty_arg, se, _) cont_ty thing_inside
1092 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1093 thing_inside env (Type new_ty_arg)
1095 simplifyArg env fn_ty has_rules (val_arg, arg_se, is_strict) cont_ty thing_inside
1097 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1099 | otherwise -- Lazy argument
1100 -- DO NOT float anything outside, hence simplExprC
1101 -- There is no benefit (unlike in a let-binding), and we'd
1102 -- have to be very careful about bogus strictness through
1103 -- floating a demanded let.
1104 = simplExprC (setInScope arg_se env) val_arg
1105 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1106 thing_inside env arg1
1108 arg_ty = funArgTy fn_ty
1111 simplStrictArg :: LetRhsFlag
1112 -> SimplEnv -- The env of the call
1113 -> InExpr -> SimplEnv -- The arg plus its env
1114 -> OutType -- arg_ty: type of the argument
1115 -> OutType -- cont_ty: Type of thing computed by the context
1116 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1117 -- Takes an expression of type rhs_ty,
1118 -- returns an expression of type cont_ty
1119 -- The env passed to this continuation is the
1120 -- env of the call, plus any new in-scope variables
1121 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1123 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1124 = simplExprF (setInScope arg_env call_env) arg
1125 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1126 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1127 -- to simplify the argument
1128 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1132 %************************************************************************
1134 \subsection{mkAtomicArgs}
1136 %************************************************************************
1138 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1139 constructor application and, if so, converts it to ANF, so that the
1140 resulting thing can be inlined more easily. Thus
1147 There are three sorts of binding context, specified by the two
1153 N N Top-level or recursive Only bind args of lifted type
1155 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1156 but lazy unlifted-and-ok-for-speculation
1158 Y Y Non-top-level, non-recursive, Bind all args
1159 and strict (demanded)
1166 there is no point in transforming to
1168 x = case (y div# z) of r -> MkC r
1170 because the (y div# z) can't float out of the let. But if it was
1171 a *strict* let, then it would be a good thing to do. Hence the
1172 context information.
1175 mkAtomicArgs :: Bool -- A strict binding
1176 -> Bool -- OK to float unlifted args
1178 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1179 OutExpr) -- things that need case-binding,
1180 -- if the strict-binding flag is on
1182 mkAtomicArgs is_strict ok_float_unlifted rhs
1183 | (Var fun, args) <- collectArgs rhs, -- It's an application
1184 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1185 = go fun nilOL [] args -- Have a go
1187 | otherwise = bale_out -- Give up
1190 bale_out = returnSmpl (nilOL, rhs)
1192 go fun binds rev_args []
1193 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1195 go fun binds rev_args (arg : args)
1196 | exprIsTrivial arg -- Easy case
1197 = go fun binds (arg:rev_args) args
1199 | not can_float_arg -- Can't make this arg atomic
1200 = bale_out -- ... so give up
1202 | otherwise -- Don't forget to do it recursively
1203 -- E.g. x = a:b:c:[]
1204 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1205 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1206 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1207 (Var arg_id : rev_args) args
1209 arg_ty = exprType arg
1210 can_float_arg = is_strict
1211 || not (isUnLiftedType arg_ty)
1212 || (ok_float_unlifted && exprOkForSpeculation arg)
1215 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1216 -> (SimplEnv -> SimplM (FloatsWith a))
1217 -> SimplM (FloatsWith a)
1218 addAtomicBinds env [] thing_inside = thing_inside env
1219 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1220 addAtomicBinds env bs thing_inside
1222 addAtomicBindsE :: SimplEnv -> [(OutId,OutExpr)]
1223 -> (SimplEnv -> SimplM FloatsWithExpr)
1224 -> SimplM FloatsWithExpr
1225 -- Same again, but this time we're in an expression context,
1226 -- and may need to do some case bindings
1228 addAtomicBindsE env [] thing_inside
1230 addAtomicBindsE env ((v,r):bs) thing_inside
1231 | needsCaseBinding (idType v) r
1232 = addAtomicBindsE (addNewInScopeIds env [v]) bs thing_inside `thenSmpl` \ (floats, expr) ->
1233 WARN( exprIsTrivial expr, ppr v <+> pprCoreExpr expr )
1234 (let body = wrapFloats floats expr in
1235 returnSmpl (emptyFloats env, Case r v (exprType body) [(DEFAULT,[],body)]))
1238 = addAuxiliaryBind env (NonRec v r) $ \ env ->
1239 addAtomicBindsE env bs thing_inside
1243 %************************************************************************
1245 \subsection{The main rebuilder}
1247 %************************************************************************
1250 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1252 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1253 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1254 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1255 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1256 rebuild env expr (ApplyTo _ arg se cont) = rebuildApp (setInScope se env) expr arg cont
1258 rebuildApp env fun arg cont
1259 = simplExpr env arg `thenSmpl` \ arg' ->
1260 rebuild env (App fun arg') cont
1262 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1266 %************************************************************************
1268 \subsection{Functions dealing with a case}
1270 %************************************************************************
1272 Blob of helper functions for the "case-of-something-else" situation.
1275 ---------------------------------------------------------
1276 -- Eliminate the case if possible
1278 rebuildCase :: SimplEnv
1279 -> OutExpr -- Scrutinee
1280 -> InId -- Case binder
1281 -> [InAlt] -- Alternatives (inceasing order)
1283 -> SimplM FloatsWithExpr
1285 rebuildCase env scrut case_bndr alts cont
1286 | Just (con,args) <- exprIsConApp_maybe scrut
1287 -- Works when the scrutinee is a variable with a known unfolding
1288 -- as well as when it's an explicit constructor application
1289 = knownCon env (DataAlt con) args case_bndr alts cont
1291 | Lit lit <- scrut -- No need for same treatment as constructors
1292 -- because literals are inlined more vigorously
1293 = knownCon env (LitAlt lit) [] case_bndr alts cont
1296 = -- Prepare the continuation;
1297 -- The new subst_env is in place
1298 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1299 addFloats env floats $ \ env ->
1302 -- The case expression is annotated with the result type of the continuation
1303 -- This may differ from the type originally on the case. For example
1304 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1307 -- let j a# = <blob>
1308 -- in case(T) a of { True -> j 1#; False -> j 0# }
1309 -- Note that the case that scrutinises a now returns a T not an Int#
1310 res_ty' = contResultType dup_cont
1313 -- Deal with case binder
1314 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1316 -- Deal with the case alternatives
1317 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1319 -- Put the case back together
1320 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1322 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1323 -- The case binder *not* scope over the whole returned case-expression
1324 rebuild env case_expr nondup_cont
1327 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1328 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1329 way, there's a chance that v will now only be used once, and hence
1334 There is a time we *don't* want to do that, namely when
1335 -fno-case-of-case is on. This happens in the first simplifier pass,
1336 and enhances full laziness. Here's the bad case:
1337 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1338 If we eliminate the inner case, we trap it inside the I# v -> arm,
1339 which might prevent some full laziness happening. I've seen this
1340 in action in spectral/cichelli/Prog.hs:
1341 [(m,n) | m <- [1..max], n <- [1..max]]
1342 Hence the check for NoCaseOfCase.
1346 There is another situation when we don't want to do it. If we have
1348 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1349 ...other cases .... }
1351 We'll perform the binder-swap for the outer case, giving
1353 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1354 ...other cases .... }
1356 But there is no point in doing it for the inner case, because w1 can't
1357 be inlined anyway. Furthermore, doing the case-swapping involves
1358 zapping w2's occurrence info (see paragraphs that follow), and that
1359 forces us to bind w2 when doing case merging. So we get
1361 case x of w1 { A -> let w2 = w1 in e1
1362 B -> let w2 = w1 in e2
1363 ...other cases .... }
1365 This is plain silly in the common case where w2 is dead.
1367 Even so, I can't see a good way to implement this idea. I tried
1368 not doing the binder-swap if the scrutinee was already evaluated
1369 but that failed big-time:
1373 case v of w { MkT x ->
1374 case x of x1 { I# y1 ->
1375 case x of x2 { I# y2 -> ...
1377 Notice that because MkT is strict, x is marked "evaluated". But to
1378 eliminate the last case, we must either make sure that x (as well as
1379 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1380 the binder-swap. So this whole note is a no-op.
1384 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1385 any occurrence info (eg IAmDead) in the case binder, because the
1386 case-binder now effectively occurs whenever v does. AND we have to do
1387 the same for the pattern-bound variables! Example:
1389 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1391 Here, b and p are dead. But when we move the argment inside the first
1392 case RHS, and eliminate the second case, we get
1394 case x of { (a,b) -> a b }
1396 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1399 Indeed, this can happen anytime the case binder isn't dead:
1400 case <any> of x { (a,b) ->
1401 case x of { (p,q) -> p } }
1402 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1403 The point is that we bring into the envt a binding
1405 after the outer case, and that makes (a,b) alive. At least we do unless
1406 the case binder is guaranteed dead.
1409 simplCaseBinder env (Var v) case_bndr
1410 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1412 -- Failed try [see Note 2 above]
1413 -- not (isEvaldUnfolding (idUnfolding v))
1415 = simplBinder env (zap case_bndr) `thenSmpl` \ (env, case_bndr') ->
1416 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1417 -- We could extend the substitution instead, but it would be
1418 -- a hack because then the substitution wouldn't be idempotent
1419 -- any more (v is an OutId). And this does just as well.
1421 zap b = b `setIdOccInfo` NoOccInfo
1423 simplCaseBinder env other_scrut case_bndr
1424 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1425 returnSmpl (env, case_bndr')
1429 simplAlts does two things:
1431 1. Eliminate alternatives that cannot match, including the
1432 DEFAULT alternative.
1434 2. If the DEFAULT alternative can match only one possible constructor,
1435 then make that constructor explicit.
1437 case e of x { DEFAULT -> rhs }
1439 case e of x { (a,b) -> rhs }
1440 where the type is a single constructor type. This gives better code
1441 when rhs also scrutinises x or e.
1443 Here "cannot match" includes knowledge from GADTs
1445 It's a good idea do do this stuff before simplifying the alternatives, to
1446 avoid simplifying alternatives we know can't happen, and to come up with
1447 the list of constructors that are handled, to put into the IdInfo of the
1448 case binder, for use when simplifying the alternatives.
1450 Eliminating the default alternative in (1) isn't so obvious, but it can
1453 data Colour = Red | Green | Blue
1462 DEFAULT -> [ case y of ... ]
1464 If we inline h into f, the default case of the inlined h can't happen.
1465 If we don't notice this, we may end up filtering out *all* the cases
1466 of the inner case y, which give us nowhere to go!
1470 simplAlts :: SimplEnv
1472 -> OutId -- Case binder
1473 -> [InAlt] -> SimplCont
1474 -> SimplM [OutAlt] -- Includes the continuation
1476 simplAlts env scrut case_bndr' alts cont'
1477 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1478 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1479 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1480 -- We need the mergeAlts in case the new default_alt
1481 -- has turned into a constructor alternative.
1483 (alts_wo_default, maybe_deflt) = findDefault alts
1484 imposs_cons = case scrut of
1485 Var v -> otherCons (idUnfolding v)
1488 -- "imposs_deflt_cons" are handled either by the context,
1489 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1490 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1492 simplDefault :: SimplEnv
1493 -> OutId -- Case binder; need just for its type. Note that as an
1494 -- OutId, it has maximum information; this is important.
1495 -- Test simpl013 is an example
1496 -> [AltCon] -- These cons can't happen when matching the default
1499 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1500 -- mergeAlts expects
1503 simplDefault env case_bndr' imposs_cons cont Nothing
1504 = return [] -- No default branch
1505 simplDefault env case_bndr' imposs_cons cont (Just rhs)
1506 | -- This branch handles the case where we are
1507 -- scrutinisng an algebraic data type
1508 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1509 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1510 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1511 -- case x of { DEFAULT -> e }
1512 -- and we don't want to fill in a default for them!
1513 Just all_cons <- tyConDataCons_maybe tycon,
1514 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1515 -- which GHC allows, then the case expression will have at most a default
1516 -- alternative. We don't want to eliminate that alternative, because the
1517 -- invariant is that there's always one alternative. It's more convenient
1519 -- case x of { DEFAULT -> e }
1520 -- as it is, rather than transform it to
1521 -- error "case cant match"
1522 -- which would be quite legitmate. But it's a really obscure corner, and
1523 -- not worth wasting code on.
1525 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1526 poss_data_cons = filterOut (`elem` imposs_data_cons) all_cons
1527 gadt_imposs | all isTyVarTy inst_tys = []
1528 | otherwise = filter (cant_match inst_tys) poss_data_cons
1529 final_poss = filterOut (`elem` gadt_imposs) poss_data_cons
1531 = case final_poss of
1532 [] -> returnSmpl [] -- Eliminate the default alternative
1533 -- altogether if it can't match
1535 [con] -> -- It matches exactly one constructor, so fill it in
1536 do { con_alt <- mkDataConAlt case_bndr' con inst_tys rhs
1537 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1538 -- The simplAlt must succeed with Just because we have
1539 -- already filtered out construtors that can't match
1542 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1545 = simplify_default imposs_cons
1547 cant_match tys data_con = not (dataConCanMatch data_con tys)
1549 simplify_default imposs_cons
1550 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1551 -- Record the constructors that the case-binder *can't* be.
1552 ; rhs' <- simplExprC env' rhs cont
1553 ; return [(DEFAULT, [], rhs')] }
1555 mkDataConAlt :: Id -> DataCon -> [OutType] -> InExpr -> SimplM InAlt
1556 -- Make a data-constructor alternative to replace the DEFAULT case
1557 -- NB: there's something a bit bogus here, because we put OutTypes into an InAlt
1558 mkDataConAlt case_bndr con tys rhs
1559 = do { tick (FillInCaseDefault case_bndr)
1560 ; args <- mk_args con tys
1561 ; return (DataAlt con, args, rhs) }
1563 mk_args con inst_tys
1564 = do { (tv_bndrs, inst_tys') <- mk_tv_bndrs con inst_tys
1565 ; let arg_tys = dataConInstArgTys con inst_tys'
1566 ; arg_ids <- mapM (newId FSLIT("a")) arg_tys
1567 ; returnSmpl (tv_bndrs ++ arg_ids) }
1569 mk_tv_bndrs con inst_tys
1570 | isVanillaDataCon con
1571 = return ([], inst_tys)
1573 = do { tv_uniqs <- getUniquesSmpl
1574 ; let new_tvs = zipWith mk tv_uniqs (dataConTyVars con)
1575 mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv)
1576 ; return (new_tvs, mkTyVarTys new_tvs) }
1578 simplAlt :: SimplEnv
1579 -> [AltCon] -- These constructors can't be present when
1580 -- matching this alternative
1581 -> OutId -- The case binder
1584 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1586 -- Simplify an alternative, returning the type refinement for the
1587 -- alternative, if the alternative does any refinement at all
1588 -- Nothing => the alternative is inaccessible
1590 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1591 | con `elem` imposs_cons -- This case can't match
1594 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1595 -- TURGID DUPLICATION, needed only for the simplAlt call
1596 -- in mkDupableAlt. Clean this up when moving to FC
1597 = ASSERT( null bndrs )
1598 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1599 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1601 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1602 -- Record the constructors that the case-binder *can't* be.
1604 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1605 = ASSERT( null bndrs )
1606 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1607 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1609 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1611 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1612 | isVanillaDataCon con
1613 = -- Deal with the pattern-bound variables
1614 -- Mark the ones that are in ! positions in the data constructor
1615 -- as certainly-evaluated.
1616 -- NB: it happens that simplBinders does *not* erase the OtherCon
1617 -- form of unfolding, so it's ok to add this info before
1618 -- doing simplBinders
1619 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1621 -- Bind the case-binder to (con args)
1622 let unf = mkUnfolding False (mkConApp con con_args)
1623 inst_tys' = tyConAppArgs (idType case_bndr')
1624 con_args = map Type inst_tys' ++ map varToCoreExpr vs'
1625 env' = mk_rhs_env env case_bndr' unf
1627 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1628 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1630 | otherwise -- GADT case
1632 (tvs,ids) = span isTyVar vs
1634 simplBinders env tvs `thenSmpl` \ (env1, tvs') ->
1635 case coreRefineTys con tvs' (idType case_bndr') of {
1636 Nothing -- Inaccessible
1637 | opt_PprStyle_Debug -- Hack: if debugging is on, generate an error case
1639 -> let rhs' = mkApps (Var eRROR_ID)
1640 [Type (substTy env (exprType rhs)),
1641 Lit (mkStringLit "Impossible alternative (GADT)")]
1643 simplBinders env1 ids `thenSmpl` \ (env2, ids') ->
1644 returnSmpl (Just (emptyVarEnv, (DataAlt con, tvs' ++ ids', rhs')))
1646 | otherwise -- Filter out the inaccessible branch
1649 Just refine@(tv_subst_env, _) -> -- The normal case
1652 env2 = refineSimplEnv env1 refine
1653 -- Simplify the Ids in the refined environment, so their types
1654 -- reflect the refinement. Usually this doesn't matter, but it helps
1655 -- in mkDupableAlt, when we want to float a lambda that uses these binders
1656 -- Furthermore, it means the binders contain maximal type information
1658 simplBinders env2 (add_evals con ids) `thenSmpl` \ (env3, ids') ->
1659 let unf = mkUnfolding False con_app
1660 con_app = mkConApp con con_args
1661 con_args = map varToCoreExpr vs' -- NB: no inst_tys'
1662 env_w_unf = mk_rhs_env env3 case_bndr' unf
1665 simplExprC env_w_unf rhs cont' `thenSmpl` \ rhs' ->
1666 returnSmpl (Just (tv_subst_env, (DataAlt con, vs', rhs'))) }
1669 -- add_evals records the evaluated-ness of the bound variables of
1670 -- a case pattern. This is *important*. Consider
1671 -- data T = T !Int !Int
1673 -- case x of { T a b -> T (a+1) b }
1675 -- We really must record that b is already evaluated so that we don't
1676 -- go and re-evaluate it when constructing the result.
1677 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1679 cat_evals dc vs strs
1683 go (v:vs) strs | isTyVar v = v : go vs strs
1684 go (v:vs) (str:strs)
1685 | isMarkedStrict str = evald_v : go vs strs
1686 | otherwise = zapped_v : go vs strs
1688 zapped_v = zap_occ_info v
1689 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1690 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1692 -- If the case binder is alive, then we add the unfolding
1694 -- to the envt; so vs are now very much alive
1695 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1696 | otherwise = \id -> id `setIdOccInfo` NoOccInfo
1698 mk_rhs_env env case_bndr' case_bndr_unf
1699 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1703 %************************************************************************
1705 \subsection{Known constructor}
1707 %************************************************************************
1709 We are a bit careful with occurrence info. Here's an example
1711 (\x* -> case x of (a*, b) -> f a) (h v, e)
1713 where the * means "occurs once". This effectively becomes
1714 case (h v, e) of (a*, b) -> f a)
1716 let a* = h v; b = e in f a
1720 All this should happen in one sweep.
1723 knownCon :: SimplEnv -> AltCon -> [OutExpr]
1724 -> InId -> [InAlt] -> SimplCont
1725 -> SimplM FloatsWithExpr
1727 knownCon env con args bndr alts cont
1728 = tick (KnownBranch bndr) `thenSmpl_`
1729 case findAlt con alts of
1730 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1731 simplNonRecX env bndr scrut $ \ env ->
1732 -- This might give rise to a binding with non-atomic args
1733 -- like x = Node (f x) (g x)
1734 -- but no harm will be done
1735 simplExprF env rhs cont
1738 LitAlt lit -> Lit lit
1739 DataAlt dc -> mkConApp dc args
1741 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1742 simplNonRecX env bndr (Lit lit) $ \ env ->
1743 simplExprF env rhs cont
1745 (DataAlt dc, bs, rhs)
1746 -> ASSERT( n_drop_tys + length bs == length args )
1747 bind_args env bs (drop n_drop_tys args) $ \ env ->
1749 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1750 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1751 -- args are aready OutExprs, but bs are InIds
1753 simplNonRecX env bndr con_app $ \ env ->
1754 simplExprF env rhs cont
1756 n_drop_tys | isVanillaDataCon dc = tyConArity (dataConTyCon dc)
1758 -- Vanilla data constructors lack type arguments in the pattern
1761 bind_args env [] _ thing_inside = thing_inside env
1763 bind_args env (b:bs) (Type ty : args) thing_inside
1764 = ASSERT( isTyVar b )
1765 bind_args (extendTvSubst env b ty) bs args thing_inside
1767 bind_args env (b:bs) (arg : args) thing_inside
1769 simplNonRecX env b arg $ \ env ->
1770 bind_args env bs args thing_inside
1774 %************************************************************************
1776 \subsection{Duplicating continuations}
1778 %************************************************************************
1781 prepareCaseCont :: SimplEnv
1782 -> [InAlt] -> SimplCont
1783 -> SimplM (FloatsWith (SimplCont,SimplCont))
1784 -- Return a duplicatable continuation, a non-duplicable part
1785 -- plus some extra bindings (that scope over the entire
1788 -- No need to make it duplicatable if there's only one alternative
1789 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1790 prepareCaseCont env alts cont = mkDupableCont env cont
1794 mkDupableCont :: SimplEnv -> SimplCont
1795 -> SimplM (FloatsWith (SimplCont, SimplCont))
1797 mkDupableCont env cont
1798 | contIsDupable cont
1799 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1801 mkDupableCont env (CoerceIt ty cont)
1802 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1803 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1805 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1806 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1807 -- Do *not* duplicate an ArgOf continuation
1808 -- Because ArgOf continuations are opaque, we gain nothing by
1809 -- propagating them into the expressions, and we do lose a lot.
1810 -- Here's an example:
1811 -- && (case x of { T -> F; F -> T }) E
1812 -- Now, && is strict so we end up simplifying the case with
1813 -- an ArgOf continuation. If we let-bind it, we get
1815 -- let $j = \v -> && v E
1816 -- in simplExpr (case x of { T -> F; F -> T })
1817 -- (ArgOf (\r -> $j r)
1818 -- And after simplifying more we get
1820 -- let $j = \v -> && v E
1821 -- in case of { T -> $j F; F -> $j T }
1822 -- Which is a Very Bad Thing
1824 -- The desire not to duplicate is the entire reason that
1825 -- mkDupableCont returns a pair of continuations.
1827 -- The original plan had:
1828 -- e.g. (...strict-fn...) [...hole...]
1830 -- let $j = \a -> ...strict-fn...
1831 -- in $j [...hole...]
1833 mkDupableCont env (ApplyTo _ arg se cont)
1834 = -- e.g. [...hole...] (...arg...)
1836 -- let a = ...arg...
1837 -- in [...hole...] a
1838 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1839 ; addFloats env floats $ \ env -> do
1840 { arg1 <- simplExpr (setInScope se env) arg
1841 ; (floats2, arg2) <- mkDupableArg env arg1
1842 ; return (floats2, (ApplyTo OkToDup arg2 (zapSubstEnv se) dup_cont, nondup_cont)) }}
1844 mkDupableCont env (Select _ case_bndr alts se cont)
1845 = -- e.g. (case [...hole...] of { pi -> ei })
1847 -- let ji = \xij -> ei
1848 -- in case [...hole...] of { pi -> ji xij }
1849 do { tick (CaseOfCase case_bndr)
1850 ; let alt_env = setInScope se env
1851 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1852 -- NB: call mkDupableCont here, *not* prepareCaseCont
1853 -- We must make a duplicable continuation, whereas prepareCaseCont
1854 -- doesn't when there is a single case branch
1855 ; addFloats alt_env floats1 $ \ alt_env -> do
1857 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1858 -- NB: simplBinder does not zap deadness occ-info, so
1859 -- a dead case_bndr' will still advertise its deadness
1860 -- This is really important because in
1861 -- case e of b { (# a,b #) -> ... }
1862 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1863 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1864 -- In the new alts we build, we have the new case binder, so it must retain
1867 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1868 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1869 (mkBoringStop (contResultType dup_cont)),
1873 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1874 -- Let-bind the thing if necessary
1875 mkDupableArg env arg
1877 = return (emptyFloats env, arg)
1879 = do { arg_id <- newId FSLIT("a") (exprType arg)
1880 ; tick (CaseOfCase arg_id)
1881 -- Want to tick here so that we go round again,
1882 -- and maybe copy or inline the code.
1883 -- Not strictly CaseOfCase, but never mind
1884 ; return (unitFloat env arg_id arg, Var arg_id) }
1885 -- What if the arg should be case-bound?
1886 -- This has been this way for a long time, so I'll leave it,
1887 -- but I can't convince myself that it's right.
1889 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1890 -> SimplM (FloatsWith [InAlt])
1891 -- Absorbs the continuation into the new alternatives
1893 mkDupableAlts env case_bndr' alts dupable_cont
1896 go env [] = returnSmpl (emptyFloats env, [])
1898 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1899 ; addFloats env floats1 $ \ env -> do
1900 { (floats2, alts') <- go env alts
1901 ; returnSmpl (floats2, case mb_alt' of
1902 Just alt' -> alt' : alts'
1906 mkDupableAlt env case_bndr' cont alt
1907 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1909 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1911 Just (reft, (con, bndrs', rhs')) ->
1912 -- Safe to say that there are no handled-cons for the DEFAULT case
1914 if exprIsDupable rhs' then
1915 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
1916 -- It is worth checking for a small RHS because otherwise we
1917 -- get extra let bindings that may cause an extra iteration of the simplifier to
1918 -- inline back in place. Quite often the rhs is just a variable or constructor.
1919 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1920 -- iterations because the version with the let bindings looked big, and so wasn't
1921 -- inlined, but after the join points had been inlined it looked smaller, and so
1924 -- NB: we have to check the size of rhs', not rhs.
1925 -- Duplicating a small InAlt might invalidate occurrence information
1926 -- However, if it *is* dupable, we return the *un* simplified alternative,
1927 -- because otherwise we'd need to pair it up with an empty subst-env....
1928 -- but we only have one env shared between all the alts.
1929 -- (Remember we must zap the subst-env before re-simplifying something).
1930 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1934 rhs_ty' = exprType rhs'
1935 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1937 | isTyVar bndr = not (bndr `elemVarEnv` reft)
1938 -- Don't abstract over tyvar binders which are refined away
1939 -- See Note [Refinement] below
1940 | otherwise = not (isDeadBinder bndr)
1941 -- The deadness info on the new Ids is preserved by simplBinders
1943 -- If we try to lift a primitive-typed something out
1944 -- for let-binding-purposes, we will *caseify* it (!),
1945 -- with potentially-disastrous strictness results. So
1946 -- instead we turn it into a function: \v -> e
1947 -- where v::State# RealWorld#. The value passed to this function
1948 -- is realworld#, which generates (almost) no code.
1950 -- There's a slight infelicity here: we pass the overall
1951 -- case_bndr to all the join points if it's used in *any* RHS,
1952 -- because we don't know its usage in each RHS separately
1954 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1955 -- we make the join point into a function whenever used_bndrs'
1956 -- is empty. This makes the join-point more CPR friendly.
1957 -- Consider: let j = if .. then I# 3 else I# 4
1958 -- in case .. of { A -> j; B -> j; C -> ... }
1960 -- Now CPR doesn't w/w j because it's a thunk, so
1961 -- that means that the enclosing function can't w/w either,
1962 -- which is a lose. Here's the example that happened in practice:
1963 -- kgmod :: Int -> Int -> Int
1964 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1968 -- I have seen a case alternative like this:
1969 -- True -> \v -> ...
1970 -- It's a bit silly to add the realWorld dummy arg in this case, making
1973 -- (the \v alone is enough to make CPR happy) but I think it's rare
1975 ( if not (any isId used_bndrs')
1976 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
1977 returnSmpl ([rw_id], [Var realWorldPrimId])
1979 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
1980 ) `thenSmpl` \ (final_bndrs', final_args) ->
1982 -- See comment about "$j" name above
1983 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
1984 -- Notice the funky mkPiTypes. If the contructor has existentials
1985 -- it's possible that the join point will be abstracted over
1986 -- type varaibles as well as term variables.
1987 -- Example: Suppose we have
1988 -- data T = forall t. C [t]
1990 -- case (case e of ...) of
1991 -- C t xs::[t] -> rhs
1992 -- We get the join point
1993 -- let j :: forall t. [t] -> ...
1994 -- j = /\t \xs::[t] -> rhs
1996 -- case (case e of ...) of
1997 -- C t xs::[t] -> j t xs
1999 -- We make the lambdas into one-shot-lambdas. The
2000 -- join point is sure to be applied at most once, and doing so
2001 -- prevents the body of the join point being floated out by
2002 -- the full laziness pass
2003 really_final_bndrs = map one_shot final_bndrs'
2004 one_shot v | isId v = setOneShotLambda v
2006 join_rhs = mkLams really_final_bndrs rhs'
2007 join_call = mkApps (Var join_bndr) final_args
2009 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2016 MkT :: a -> b -> T a
2020 MkT a' b (p::a') (q::b) -> [p,w]
2022 The danger is that we'll make a join point
2026 and that's ill-typed, because (p::a') but (w::a).
2028 Solution so far: don't abstract over a', because the type refinement
2029 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2031 Then we must abstract over any refined free variables. Hmm. Maybe we
2032 could just abstract over *all* free variables, thereby lambda-lifting
2033 the join point? We should try this.