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 IdInfo ( OccInfo(..), isLoopBreaker,
30 setArityInfo, zapDemandInfo,
34 import NewDemand ( isStrictDmd )
35 import TcGadt ( dataConCanMatch )
36 import DataCon ( dataConTyCon, dataConRepStrictness )
37 import TyCon ( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
39 import PprCore ( pprParendExpr, pprCoreExpr )
40 import CoreUnfold ( mkUnfolding, callSiteInline )
41 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
42 exprIsConApp_maybe, mkPiTypes, findAlt,
43 exprType, exprIsHNF, findDefault, mergeAlts,
44 exprOkForSpeculation, exprArity,
45 mkCoerce, mkSCC, mkInlineMe, applyTypeToArg,
48 import Rules ( lookupRule )
49 import BasicTypes ( isMarkedStrict )
50 import CostCentre ( currentCCS )
51 import Type ( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
52 coreEqType, splitTyConApp_maybe,
53 isTyVarTy, isFunTy, tcEqType
55 import Coercion ( Coercion, coercionKind,
56 mkTransCoercion, mkSymCoercion, splitCoercionKind_maybe, decomposeCo )
57 import VarEnv ( elemVarEnv, emptyVarEnv )
58 import TysPrim ( realWorldStatePrimTy )
59 import PrelInfo ( realWorldPrimId )
60 import BasicTypes ( TopLevelFlag(..), isTopLevel,
65 import Maybes ( orElse )
67 import Util ( notNull, filterOut )
71 The guts of the simplifier is in this module, but the driver loop for
72 the simplifier is in SimplCore.lhs.
75 -----------------------------------------
76 *** IMPORTANT NOTE ***
77 -----------------------------------------
78 The simplifier used to guarantee that the output had no shadowing, but
79 it does not do so any more. (Actually, it never did!) The reason is
80 documented with simplifyArgs.
83 -----------------------------------------
84 *** IMPORTANT NOTE ***
85 -----------------------------------------
86 Many parts of the simplifier return a bunch of "floats" as well as an
87 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
89 All "floats" are let-binds, not case-binds, but some non-rec lets may
90 be unlifted (with RHS ok-for-speculation).
94 -----------------------------------------
95 ORGANISATION OF FUNCTIONS
96 -----------------------------------------
98 - simplify all top-level binders
99 - for NonRec, call simplRecOrTopPair
100 - for Rec, call simplRecBind
103 ------------------------------
104 simplExpr (applied lambda) ==> simplNonRecBind
105 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
106 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
108 ------------------------------
109 simplRecBind [binders already simplfied]
110 - use simplRecOrTopPair on each pair in turn
112 simplRecOrTopPair [binder already simplified]
113 Used for: recursive bindings (top level and nested)
114 top-level non-recursive bindings
116 - check for PreInlineUnconditionally
120 Used for: non-top-level non-recursive bindings
121 beta reductions (which amount to the same thing)
122 Because it can deal with strict arts, it takes a
123 "thing-inside" and returns an expression
125 - check for PreInlineUnconditionally
126 - simplify binder, including its IdInfo
135 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
136 Used for: binding case-binder and constr args in a known-constructor case
137 - check for PreInLineUnconditionally
141 ------------------------------
142 simplLazyBind: [binder already simplified, RHS not]
143 Used for: recursive bindings (top level and nested)
144 top-level non-recursive bindings
145 non-top-level, but *lazy* non-recursive bindings
146 [must not be strict or unboxed]
147 Returns floats + an augmented environment, not an expression
148 - substituteIdInfo and add result to in-scope
149 [so that rules are available in rec rhs]
152 - float if exposes constructor or PAP
156 completeNonRecX: [binder and rhs both simplified]
157 - if the the thing needs case binding (unlifted and not ok-for-spec)
163 completeLazyBind: [given a simplified RHS]
164 [used for both rec and non-rec bindings, top level and not]
165 - try PostInlineUnconditionally
166 - add unfolding [this is the only place we add an unfolding]
171 Right hand sides and arguments
172 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
173 In many ways we want to treat
174 (a) the right hand side of a let(rec), and
175 (b) a function argument
176 in the same way. But not always! In particular, we would
177 like to leave these arguments exactly as they are, so they
178 will match a RULE more easily.
183 It's harder to make the rule match if we ANF-ise the constructor,
184 or eta-expand the PAP:
186 f (let { a = g x; b = h x } in (a,b))
189 On the other hand if we see the let-defns
194 then we *do* want to ANF-ise and eta-expand, so that p and q
195 can be safely inlined.
197 Even floating lets out is a bit dubious. For let RHS's we float lets
198 out if that exposes a value, so that the value can be inlined more vigorously.
201 r = let x = e in (x,x)
203 Here, if we float the let out we'll expose a nice constructor. We did experiments
204 that showed this to be a generally good thing. But it was a bad thing to float
205 lets out unconditionally, because that meant they got allocated more often.
207 For function arguments, there's less reason to expose a constructor (it won't
208 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
209 So for the moment we don't float lets out of function arguments either.
214 For eta expansion, we want to catch things like
216 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
218 If the \x was on the RHS of a let, we'd eta expand to bring the two
219 lambdas together. And in general that's a good thing to do. Perhaps
220 we should eta expand wherever we find a (value) lambda? Then the eta
221 expansion at a let RHS can concentrate solely on the PAP case.
224 %************************************************************************
226 \subsection{Bindings}
228 %************************************************************************
231 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
233 simplTopBinds env binds
234 = -- Put all the top-level binders into scope at the start
235 -- so that if a transformation rule has unexpectedly brought
236 -- anything into scope, then we don't get a complaint about that.
237 -- It's rather as if the top-level binders were imported.
238 simplRecBndrs env (bindersOfBinds binds) `thenSmpl` \ (env, bndrs') ->
239 simpl_binds env binds bndrs' `thenSmpl` \ (floats, _) ->
240 freeTick SimplifierDone `thenSmpl_`
241 returnSmpl (floatBinds floats)
243 -- We need to track the zapped top-level binders, because
244 -- they should have their fragile IdInfo zapped (notably occurrence info)
245 -- That's why we run down binds and bndrs' simultaneously.
246 simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
247 simpl_binds env [] bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
248 simpl_binds env (bind:binds) bs = simpl_bind env bind bs `thenSmpl` \ (floats,env) ->
249 addFloats env floats $ \env ->
250 simpl_binds env binds (drop_bs bind bs)
252 drop_bs (NonRec _ _) (_ : bs) = bs
253 drop_bs (Rec prs) bs = drop (length prs) bs
255 simpl_bind env bind bs
256 = getDOptsSmpl `thenSmpl` \ dflags ->
257 if dopt Opt_D_dump_inlinings dflags then
258 pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
260 simpl_bind1 env bind bs
262 simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
263 simpl_bind1 env (Rec pairs) bs' = simplRecBind env TopLevel pairs bs'
267 %************************************************************************
269 \subsection{simplNonRec}
271 %************************************************************************
273 simplNonRecBind is used for
274 * non-top-level non-recursive lets in expressions
278 * An unsimplified (binder, rhs) pair
279 * The env for the RHS. It may not be the same as the
280 current env because the bind might occur via (\x.E) arg
282 It uses the CPS form because the binding might be strict, in which
283 case we might discard the continuation:
284 let x* = error "foo" in (...x...)
286 It needs to turn unlifted bindings into a @case@. They can arise
287 from, say: (\x -> e) (4# + 3#)
290 simplNonRecBind :: SimplEnv
292 -> InExpr -> SimplEnv -- Arg, with its subst-env
293 -> OutType -- Type of thing computed by the context
294 -> (SimplEnv -> SimplM FloatsWithExpr) -- The body
295 -> SimplM FloatsWithExpr
297 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
299 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
302 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
303 = simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
305 simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
306 | preInlineUnconditionally env NotTopLevel bndr rhs
307 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
308 thing_inside (extendIdSubst env bndr (mkContEx rhs_se rhs))
310 | isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty -- A strict let
311 = -- Don't use simplBinder because that doesn't keep
312 -- fragile occurrence info in the substitution
313 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr1) ->
314 simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty $ \ env1 rhs1 ->
316 -- Now complete the binding and simplify the body
318 (env2,bndr2) = addLetIdInfo env1 bndr bndr1
320 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
322 | otherwise -- Normal, lazy case
323 = -- Don't use simplBinder because that doesn't keep
324 -- fragile occurrence info in the substitution
325 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr') ->
326 simplLazyBind env NotTopLevel NonRecursive
327 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
328 addFloats env floats thing_inside
331 bndr_ty = idType bndr
334 A specialised variant of simplNonRec used when the RHS is already simplified, notably
335 in knownCon. It uses case-binding where necessary.
338 simplNonRecX :: SimplEnv
339 -> InId -- Old binder
340 -> OutExpr -- Simplified RHS
341 -> (SimplEnv -> SimplM FloatsWithExpr)
342 -> SimplM FloatsWithExpr
344 simplNonRecX env bndr new_rhs thing_inside
345 = do { (env, bndr') <- simplBinder env bndr
346 ; completeNonRecX env False {- Non-strict; pessimistic -}
347 bndr bndr' new_rhs thing_inside }
350 completeNonRecX :: SimplEnv
351 -> Bool -- Strict binding
352 -> InId -- Old binder
353 -> OutId -- New binder
354 -> OutExpr -- Simplified RHS
355 -> (SimplEnv -> SimplM FloatsWithExpr)
356 -> SimplM FloatsWithExpr
358 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
359 | needsCaseBinding (idType new_bndr) new_rhs
360 -- Make this test *before* the preInlineUnconditionally
361 -- Consider case I# (quotInt# x y) of
362 -- I# v -> let w = J# v in ...
363 -- If we gaily inline (quotInt# x y) for v, we end up building an
365 -- let w = J# (quotInt# x y) in ...
366 -- because quotInt# can fail.
367 = do { (floats, body) <- thing_inside env
368 ; let body' = wrapFloats floats body
369 ; return (emptyFloats env, Case new_rhs new_bndr (exprType body)
370 [(DEFAULT, [], body')]) }
373 = -- Make the arguments atomic if necessary,
374 -- adding suitable bindings
375 -- pprTrace "completeNonRecX" (ppr new_bndr <+> ppr new_rhs) $
376 mkAtomicArgsE env is_strict new_rhs $ \ env new_rhs ->
377 completeLazyBind env NotTopLevel
378 old_bndr new_bndr new_rhs `thenSmpl` \ (floats, env) ->
379 addFloats env floats thing_inside
381 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
382 Doing so risks exponential behaviour, because new_rhs has been simplified once already
383 In the cases described by the folowing commment, postInlineUnconditionally will
384 catch many of the relevant cases.
385 -- This happens; for example, the case_bndr during case of
386 -- known constructor: case (a,b) of x { (p,q) -> ... }
387 -- Here x isn't mentioned in the RHS, so we don't want to
388 -- create the (dead) let-binding let x = (a,b) in ...
390 -- Similarly, single occurrences can be inlined vigourously
391 -- e.g. case (f x, g y) of (a,b) -> ....
392 -- If a,b occur once we can avoid constructing the let binding for them.
393 | preInlineUnconditionally env NotTopLevel bndr new_rhs
394 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
396 -- NB: completeLazyBind uses postInlineUnconditionally; no need to do that here
401 %************************************************************************
403 \subsection{Lazy bindings}
405 %************************************************************************
407 simplRecBind is used for
408 * recursive bindings only
411 simplRecBind :: SimplEnv -> TopLevelFlag
412 -> [(InId, InExpr)] -> [OutId]
413 -> SimplM (FloatsWith SimplEnv)
414 simplRecBind env top_lvl pairs bndrs'
415 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
416 returnSmpl (flattenFloats floats, env)
418 go env [] _ = returnSmpl (emptyFloats env, env)
420 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
421 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
422 addFloats env floats (\env -> go env pairs bndrs')
426 simplRecOrTopPair is used for
427 * recursive bindings (whether top level or not)
428 * top-level non-recursive bindings
430 It assumes the binder has already been simplified, but not its IdInfo.
433 simplRecOrTopPair :: SimplEnv
435 -> InId -> OutId -- Binder, both pre-and post simpl
436 -> InExpr -- The RHS and its environment
437 -> SimplM (FloatsWith SimplEnv)
439 simplRecOrTopPair env top_lvl bndr bndr' rhs
440 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
441 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
442 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
445 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
446 -- May not actually be recursive, but it doesn't matter
450 simplLazyBind is used for
451 * recursive bindings (whether top level or not)
452 * top-level non-recursive bindings
453 * non-top-level *lazy* non-recursive bindings
455 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
456 from SimplRecOrTopBind]
459 1. It assumes that the binder is *already* simplified,
460 and is in scope, but not its IdInfo
462 2. It assumes that the binder type is lifted.
464 3. It does not check for pre-inline-unconditionallly;
465 that should have been done already.
468 simplLazyBind :: SimplEnv
469 -> TopLevelFlag -> RecFlag
470 -> InId -> OutId -- Binder, both pre-and post simpl
471 -> InExpr -> SimplEnv -- The RHS and its environment
472 -> SimplM (FloatsWith SimplEnv)
474 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
476 (env1,bndr2) = addLetIdInfo env bndr bndr1
477 rhs_env = setInScope rhs_se env1
478 is_top_level = isTopLevel top_lvl
479 ok_float_unlifted = not is_top_level && isNonRec is_rec
480 rhs_cont = mkRhsStop (idType bndr2)
482 -- Simplify the RHS; note the mkRhsStop, which tells
483 -- the simplifier that this is the RHS of a let.
484 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
486 -- If any of the floats can't be floated, give up now
487 -- (The allLifted predicate says True for empty floats.)
488 if (not ok_float_unlifted && not (allLifted floats)) then
489 completeLazyBind env1 top_lvl bndr bndr2
490 (wrapFloats floats rhs1)
493 -- ANF-ise a constructor or PAP rhs
494 mkAtomicArgs False {- Not strict -}
495 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
497 -- If the result is a PAP, float the floats out, else wrap them
498 -- By this time it's already been ANF-ised (if necessary)
499 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
500 completeLazyBind env1 top_lvl bndr bndr2 rhs2
502 else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
503 -- WARNING: long dodgy argument coming up
504 -- WANTED: a better way to do this
506 -- We can't use "exprIsCheap" instead of exprIsHNF,
507 -- because that causes a strictness bug.
508 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
509 -- The case expression is 'cheap', but it's wrong to transform to
510 -- y* = E; x = case (scc y) of {...}
511 -- Either we must be careful not to float demanded non-values, or
512 -- we must use exprIsHNF for the test, which ensures that the
513 -- thing is non-strict. So exprIsHNF => bindings are non-strict
514 -- I think. The WARN below tests for this.
516 -- We use exprIsTrivial here because we want to reveal lone variables.
517 -- E.g. let { x = letrec { y = E } in y } in ...
518 -- Here we definitely want to float the y=E defn.
519 -- exprIsHNF definitely isn't right for that.
521 -- Again, the floated binding can't be strict; if it's recursive it'll
522 -- be non-strict; if it's non-recursive it'd be inlined.
524 -- Note [SCC-and-exprIsTrivial]
526 -- y = let { x* = E } in scc "foo" x
527 -- then we do *not* want to float out the x binding, because
528 -- it's strict! Fortunately, exprIsTrivial replies False to
531 -- There's a subtlety here. There may be a binding (x* = e) in the
532 -- floats, where the '*' means 'will be demanded'. So is it safe
533 -- to float it out? Answer no, but it won't matter because
534 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
535 -- and so there can't be any 'will be demanded' bindings in the floats.
537 WARN( not (is_top_level || not (any demanded_float (floatBinds floats))),
538 ppr (filter demanded_float (floatBinds floats)) )
540 tick LetFloatFromLet `thenSmpl_` (
541 addFloats env1 floats $ \ env2 ->
542 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
543 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
546 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
549 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
550 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
551 demanded_float (Rec _) = False
556 %************************************************************************
558 \subsection{Completing a lazy binding}
560 %************************************************************************
563 * deals only with Ids, not TyVars
564 * takes an already-simplified binder and RHS
565 * is used for both recursive and non-recursive bindings
566 * is used for both top-level and non-top-level bindings
568 It does the following:
569 - tries discarding a dead binding
570 - tries PostInlineUnconditionally
571 - add unfolding [this is the only place we add an unfolding]
574 It does *not* attempt to do let-to-case. Why? Because it is used for
575 - top-level bindings (when let-to-case is impossible)
576 - many situations where the "rhs" is known to be a WHNF
577 (so let-to-case is inappropriate).
580 completeLazyBind :: SimplEnv
581 -> TopLevelFlag -- Flag stuck into unfolding
582 -> InId -- Old binder
583 -> OutId -- New binder
584 -> OutExpr -- Simplified RHS
585 -> SimplM (FloatsWith SimplEnv)
586 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
587 -- by extending the substitution (e.g. let x = y in ...)
588 -- The new binding (if any) is returned as part of the floats.
589 -- NB: the returned SimplEnv has the right SubstEnv, but you should
590 -- (as usual) use the in-scope-env from the floats
592 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
593 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
594 = -- Drop the binding
595 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
596 -- pprTrace "Inline unconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
597 returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
598 -- Use the substitution to make quite, quite sure that the substitution
599 -- will happen, since we are going to discard the binding
604 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
606 -- Add the unfolding *only* for non-loop-breakers
607 -- Making loop breakers not have an unfolding at all
608 -- means that we can avoid tests in exprIsConApp, for example.
609 -- This is important: if exprIsConApp says 'yes' for a recursive
610 -- thing, then we can get into an infinite loop
611 -- If the unfolding is a value, the demand info may
612 -- go pear-shaped, so we nuke it. Example:
614 -- case x of (p,q) -> h p q x
615 -- Here x is certainly demanded. But after we've nuked
616 -- the case, we'll get just
617 -- let x = (a,b) in h a b x
618 -- and now x is not demanded (I'm assuming h is lazy)
619 -- This really happens. Similarly
620 -- let f = \x -> e in ...f..f...
621 -- After inling f at some of its call sites the original binding may
622 -- (for example) be no longer strictly demanded.
623 -- The solution here is a bit ad hoc...
624 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
625 final_info | loop_breaker = new_bndr_info
626 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
627 | otherwise = info_w_unf
629 final_id = new_bndr `setIdInfo` final_info
631 -- These seqs forces the Id, and hence its IdInfo,
632 -- and hence any inner substitutions
634 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
635 returnSmpl (unitFloat env final_id new_rhs, env)
638 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
639 loop_breaker = isLoopBreaker occ_info
640 old_info = idInfo old_bndr
641 occ_info = occInfo old_info
646 %************************************************************************
648 \subsection[Simplify-simplExpr]{The main function: simplExpr}
650 %************************************************************************
652 The reason for this OutExprStuff stuff is that we want to float *after*
653 simplifying a RHS, not before. If we do so naively we get quadratic
654 behaviour as things float out.
656 To see why it's important to do it after, consider this (real) example:
670 a -- Can't inline a this round, cos it appears twice
674 Each of the ==> steps is a round of simplification. We'd save a
675 whole round if we float first. This can cascade. Consider
680 let f = let d1 = ..d.. in \y -> e
684 in \x -> ...(\y ->e)...
686 Only in this second round can the \y be applied, and it
687 might do the same again.
691 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
692 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
694 expr_ty' = substTy env (exprType expr)
695 -- The type in the Stop continuation, expr_ty', is usually not used
696 -- It's only needed when discarding continuations after finding
697 -- a function that returns bottom.
698 -- Hence the lazy substitution
701 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
702 -- Simplify an expression, given a continuation
703 simplExprC env expr cont
704 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
705 returnSmpl (wrapFloats floats expr)
707 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
708 -- Simplify an expression, returning floated binds
710 simplExprF env (Var v) cont = simplVar env v cont
711 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
712 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
713 simplExprF env (Note note expr) cont = simplNote env note expr cont
714 simplExprF env (Cast body co) cont = simplCast env body co cont
715 simplExprF env (App fun arg) cont = simplExprF env fun
716 (ApplyTo NoDup arg (Just env) cont)
718 simplExprF env (Type ty) cont
719 = ASSERT( contIsRhsOrArg cont )
720 simplType env ty `thenSmpl` \ ty' ->
721 rebuild env (Type ty') cont
723 simplExprF env (Case scrut bndr case_ty alts) cont
724 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
725 = -- Simplify the scrutinee with a Select continuation
726 simplExprF env scrut (Select NoDup bndr alts env cont)
729 = -- If case-of-case is off, simply simplify the case expression
730 -- in a vanilla Stop context, and rebuild the result around it
731 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
732 rebuild env case_expr' cont
734 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
735 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
737 simplExprF env (Let (Rec pairs) body) cont
738 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
739 -- NB: bndrs' don't have unfoldings or rules
740 -- We add them as we go down
742 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
743 addFloats env floats $ \ env ->
744 simplExprF env body cont
746 -- A non-recursive let is dealt with by simplNonRecBind
747 simplExprF env (Let (NonRec bndr rhs) body) cont
748 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
749 simplExprF env body cont
752 ---------------------------------
753 simplType :: SimplEnv -> InType -> SimplM OutType
754 -- Kept monadic just so we can do the seqType
756 = seqType new_ty `seq` returnSmpl new_ty
758 new_ty = substTy env ty
762 %************************************************************************
766 %************************************************************************
769 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont -> SimplM FloatsWithExpr
770 simplCast env body co cont
773 | (s1, k1) <- coercionKind co
774 , s1 `tcEqType` k1 = cont
775 addCoerce co1 (CoerceIt co2 cont)
776 | (s1, k1) <- coercionKind co1
777 , (l1, t1) <- coercionKind co2
778 -- coerce T1 S1 (coerce S1 K1 e)
781 -- coerce T1 K1 e, otherwise
783 -- For example, in the initial form of a worker
784 -- we may find (coerce T (coerce S (\x.e))) y
785 -- and we'd like it to simplify to e[y/x] in one round
787 , s1 `coreEqType` t1 = cont -- The coerces cancel out
788 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
790 addCoerce co (ApplyTo dup arg arg_se cont)
791 | not (isTypeArg arg) -- This whole case only works for value args
792 -- Could upgrade to have equiv thing for type apps too
793 , Just (s1s2, t1t2) <- splitCoercionKind_maybe co
795 -- co : s1s2 :=: t1t2
796 -- (coerce (T1->T2) (S1->S2) F) E
798 -- coerce T2 S2 (F (coerce S1 T1 E))
800 -- t1t2 must be a function type, T1->T2, because it's applied
801 -- to something but s1s2 might conceivably not be
803 -- When we build the ApplyTo we can't mix the out-types
804 -- with the InExpr in the argument, so we simply substitute
805 -- to make it all consistent. It's a bit messy.
806 -- But it isn't a common case.
809 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
810 -- t2 :=: s2 with left and right on the curried form:
811 -- (->) t1 t2 :=: (->) s1 s2
812 [co1, co2] = decomposeCo 2 co
813 new_arg = mkCoerce (mkSymCoercion co1) arg'
814 arg' = case arg_se of
816 Just arg_se -> substExpr (setInScope arg_se env) arg
817 result = ApplyTo dup new_arg (Just $ zapSubstEnv env)
819 addCoerce co cont = CoerceIt co cont
821 simplType env co `thenSmpl` \ co' ->
822 simplExprF env body (addCoerce co' cont)
825 %************************************************************************
829 %************************************************************************
832 simplLam env fun cont
835 zap_it = mkLamBndrZapper fun (countArgs cont)
836 cont_ty = contResultType cont
838 -- Type-beta reduction
839 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) mb_arg_se body_cont)
840 = ASSERT( isTyVar bndr )
841 do { tick (BetaReduction bndr)
842 ; ty_arg' <- case mb_arg_se of
843 Just arg_se -> simplType (setInScope arg_se env) ty_arg
844 Nothing -> return ty_arg
845 ; go (extendTvSubst env bndr ty_arg') body body_cont }
847 -- Ordinary beta reduction
848 go env (Lam bndr body) cont@(ApplyTo _ arg (Just arg_se) body_cont)
849 = do { tick (BetaReduction bndr)
850 ; simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
851 go env body body_cont }
853 go env (Lam bndr body) cont@(ApplyTo _ arg Nothing body_cont)
854 = do { tick (BetaReduction bndr)
855 ; simplNonRecX env (zap_it bndr) arg $ \ env ->
856 go env body body_cont }
858 -- Not enough args, so there are real lambdas left to put in the result
859 go env lam@(Lam _ _) cont
860 = do { (env, bndrs') <- simplLamBndrs env bndrs
861 ; body' <- simplExpr env body
862 ; (floats, new_lam) <- mkLam env bndrs' body' cont
863 ; addFloats env floats $ \ env ->
864 rebuild env new_lam cont }
866 (bndrs,body) = collectBinders lam
868 -- Exactly enough args
869 go env expr cont = simplExprF env expr cont
871 mkLamBndrZapper :: CoreExpr -- Function
872 -> Int -- Number of args supplied, *including* type args
873 -> Id -> Id -- Use this to zap the binders
874 mkLamBndrZapper fun n_args
875 | n_args >= n_params fun = \b -> b -- Enough args
876 | otherwise = \b -> zapLamIdInfo b
878 -- NB: we count all the args incl type args
879 -- so we must count all the binders (incl type lambdas)
880 n_params (Note _ e) = n_params e
881 n_params (Lam b e) = 1 + n_params e
882 n_params other = 0::Int
886 %************************************************************************
890 %************************************************************************
895 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
896 -- inlining. All other CCCSs are mapped to currentCCS.
897 simplNote env (SCC cc) e cont
898 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
899 rebuild env (mkSCC cc e') cont
901 -- See notes with SimplMonad.inlineMode
902 simplNote env InlineMe e cont
903 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
904 = -- Don't inline inside an INLINE expression
905 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
906 rebuild env (mkInlineMe e') cont
908 | otherwise -- Dissolve the InlineMe note if there's
909 -- an interesting context of any kind to combine with
910 -- (even a type application -- anything except Stop)
911 = simplExprF env e cont
913 simplNote env (CoreNote s) e cont
914 = simplExpr env e `thenSmpl` \ e' ->
915 rebuild env (Note (CoreNote s) e') cont
919 %************************************************************************
921 \subsection{Dealing with calls}
923 %************************************************************************
926 simplVar env var cont
927 = case substId env var of
928 DoneEx e -> simplExprF (zapSubstEnv env) e cont
929 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
930 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
931 -- Note [zapSubstEnv]
932 -- The template is already simplified, so don't re-substitute.
933 -- This is VITAL. Consider
935 -- let y = \z -> ...x... in
937 -- We'll clone the inner \x, adding x->x' in the id_subst
938 -- Then when we inline y, we must *not* replace x by x' in
939 -- the inlined copy!!
941 ---------------------------------------------------------
942 -- Dealing with a call site
944 completeCall env var occ_info cont
945 = -- Simplify the arguments
946 getDOptsSmpl `thenSmpl` \ dflags ->
948 chkr = getSwitchChecker env
949 (args, call_cont) = getContArgs chkr var cont
952 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
953 (contResultType call_cont) $ \ env args ->
955 -- Next, look for rules or specialisations that match
957 -- It's important to simplify the args first, because the rule-matcher
958 -- doesn't do substitution as it goes. We don't want to use subst_args
959 -- (defined in the 'where') because that throws away useful occurrence info,
960 -- and perhaps-very-important specialisations.
962 -- Some functions have specialisations *and* are strict; in this case,
963 -- we don't want to inline the wrapper of the non-specialised thing; better
964 -- to call the specialised thing instead.
965 -- We used to use the black-listing mechanism to ensure that inlining of
966 -- the wrapper didn't occur for things that have specialisations till a
967 -- later phase, so but now we just try RULES first
969 -- You might think that we shouldn't apply rules for a loop breaker:
970 -- doing so might give rise to an infinite loop, because a RULE is
971 -- rather like an extra equation for the function:
972 -- RULE: f (g x) y = x+y
975 -- But it's too drastic to disable rules for loop breakers.
976 -- Even the foldr/build rule would be disabled, because foldr
977 -- is recursive, and hence a loop breaker:
978 -- foldr k z (build g) = g k z
979 -- So it's up to the programmer: rules can cause divergence
982 in_scope = getInScope env
984 maybe_rule = case activeRule env of
985 Nothing -> Nothing -- No rules apply
986 Just act_fn -> lookupRule act_fn in_scope rules var args
989 Just (rule_name, rule_rhs) ->
990 tick (RuleFired rule_name) `thenSmpl_`
991 (if dopt Opt_D_dump_inlinings dflags then
992 pprTrace "Rule fired" (vcat [
993 text "Rule:" <+> ftext rule_name,
994 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
995 text "After: " <+> pprCoreExpr rule_rhs,
996 text "Cont: " <+> ppr call_cont])
999 simplExprF env rule_rhs call_cont ;
1001 Nothing -> -- No rules
1003 -- Next, look for an inlining
1005 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
1006 interesting_cont = interestingCallContext (notNull args)
1009 active_inline = activeInline env var occ_info
1010 maybe_inline = callSiteInline dflags active_inline occ_info
1011 var arg_infos interesting_cont
1013 case maybe_inline of {
1014 Just unfolding -- There is an inlining!
1015 -> tick (UnfoldingDone var) `thenSmpl_`
1016 (if dopt Opt_D_dump_inlinings dflags then
1017 pprTrace "Inlining done" (vcat [
1018 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1019 text "Inlined fn: " <+> ppr unfolding,
1020 text "Cont: " <+> ppr call_cont])
1023 simplExprF env unfolding (pushContArgs args call_cont)
1026 Nothing -> -- No inlining!
1029 rebuild env (mkApps (Var var) args) call_cont
1033 %************************************************************************
1035 \subsection{Arguments}
1037 %************************************************************************
1040 ---------------------------------------------------------
1041 -- Simplifying the arguments of a call
1043 simplifyArgs :: SimplEnv
1044 -> OutType -- Type of the function
1045 -> Bool -- True if the fn has RULES
1046 -> [(InExpr, Maybe SimplEnv, Bool)] -- Details of the arguments
1047 -> OutType -- Type of the continuation
1048 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1049 -> SimplM FloatsWithExpr
1051 -- [CPS-like because of strict arguments]
1053 -- Simplify the arguments to a call.
1054 -- This part of the simplifier may break the no-shadowing invariant
1056 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1057 -- where f is strict in its second arg
1058 -- If we simplify the innermost one first we get (...(\a -> e)...)
1059 -- Simplifying the second arg makes us float the case out, so we end up with
1060 -- case y of (a,b) -> f (...(\a -> e)...) e'
1061 -- So the output does not have the no-shadowing invariant. However, there is
1062 -- no danger of getting name-capture, because when the first arg was simplified
1063 -- we used an in-scope set that at least mentioned all the variables free in its
1064 -- static environment, and that is enough.
1066 -- We can't just do innermost first, or we'd end up with a dual problem:
1067 -- case x of (a,b) -> f e (...(\a -> e')...)
1069 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1070 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1071 -- continuations. We have to keep the right in-scope set around; AND we have
1072 -- to get the effect that finding (error "foo") in a strict arg position will
1073 -- discard the entire application and replace it with (error "foo"). Getting
1074 -- all this at once is TOO HARD!
1076 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1077 = go env fn_ty args thing_inside
1079 go env fn_ty [] thing_inside = thing_inside env []
1080 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1081 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1082 thing_inside env (arg':args')
1084 simplifyArg env fn_ty has_rules (arg, Nothing, _) cont_ty thing_inside
1085 = thing_inside env arg -- Already simplified
1087 simplifyArg env fn_ty has_rules (Type ty_arg, Just se, _) cont_ty thing_inside
1088 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1089 thing_inside env (Type new_ty_arg)
1091 simplifyArg env fn_ty has_rules (val_arg, Just arg_se, is_strict) cont_ty thing_inside
1093 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1095 | otherwise -- Lazy argument
1096 -- DO NOT float anything outside, hence simplExprC
1097 -- There is no benefit (unlike in a let-binding), and we'd
1098 -- have to be very careful about bogus strictness through
1099 -- floating a demanded let.
1100 = simplExprC (setInScope arg_se env) val_arg
1101 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1102 thing_inside env arg1
1104 arg_ty = funArgTy fn_ty
1107 simplStrictArg :: LetRhsFlag
1108 -> SimplEnv -- The env of the call
1109 -> InExpr -> SimplEnv -- The arg plus its env
1110 -> OutType -- arg_ty: type of the argument
1111 -> OutType -- cont_ty: Type of thing computed by the context
1112 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1113 -- Takes an expression of type rhs_ty,
1114 -- returns an expression of type cont_ty
1115 -- The env passed to this continuation is the
1116 -- env of the call, plus any new in-scope variables
1117 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1119 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1120 = simplExprF (setInScope arg_env call_env) arg
1121 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1122 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1123 -- to simplify the argument
1124 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1128 %************************************************************************
1130 \subsection{mkAtomicArgs}
1132 %************************************************************************
1134 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1135 constructor application and, if so, converts it to ANF, so that the
1136 resulting thing can be inlined more easily. Thus
1143 There are three sorts of binding context, specified by the two
1149 N N Top-level or recursive Only bind args of lifted type
1151 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1152 but lazy unlifted-and-ok-for-speculation
1154 Y Y Non-top-level, non-recursive, Bind all args
1155 and strict (demanded)
1162 there is no point in transforming to
1164 x = case (y div# z) of r -> MkC r
1166 because the (y div# z) can't float out of the let. But if it was
1167 a *strict* let, then it would be a good thing to do. Hence the
1168 context information.
1171 mkAtomicArgsE :: SimplEnv
1172 -> Bool -- A strict binding
1173 -> OutExpr -- The rhs
1174 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1175 -> SimplM FloatsWithExpr
1177 mkAtomicArgsE env is_strict rhs thing_inside
1178 | (Var fun, args) <- collectArgs rhs, -- It's an application
1179 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1180 = go env (Var fun) args
1182 | otherwise = thing_inside env rhs
1185 go env fun [] = thing_inside env fun
1187 go env fun (arg : args)
1188 | exprIsTrivial arg -- Easy case
1189 || no_float_arg -- Can't make it atomic
1190 = go env (App fun arg) args
1193 = do { arg_id <- newId FSLIT("a") arg_ty
1194 ; completeNonRecX env False {- pessimistic -} arg_id arg_id arg $ \env ->
1195 go env (App fun (Var arg_id)) args }
1197 arg_ty = exprType arg
1198 no_float_arg = not is_strict && (isUnLiftedType arg_ty) && not (exprOkForSpeculation arg)
1201 -- Old code: consider rewriting to be more like mkAtomicArgsE
1203 mkAtomicArgs :: Bool -- A strict binding
1204 -> Bool -- OK to float unlifted args
1206 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1207 OutExpr) -- things that need case-binding,
1208 -- if the strict-binding flag is on
1210 mkAtomicArgs is_strict ok_float_unlifted rhs
1211 | (Var fun, args) <- collectArgs rhs, -- It's an application
1212 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1213 = go fun nilOL [] args -- Have a go
1215 | otherwise = bale_out -- Give up
1218 bale_out = returnSmpl (nilOL, rhs)
1220 go fun binds rev_args []
1221 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1223 go fun binds rev_args (arg : args)
1224 | exprIsTrivial arg -- Easy case
1225 = go fun binds (arg:rev_args) args
1227 | not can_float_arg -- Can't make this arg atomic
1228 = bale_out -- ... so give up
1230 | otherwise -- Don't forget to do it recursively
1231 -- E.g. x = a:b:c:[]
1232 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1233 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1234 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1235 (Var arg_id : rev_args) args
1237 arg_ty = exprType arg
1238 can_float_arg = is_strict
1239 || not (isUnLiftedType arg_ty)
1240 || (ok_float_unlifted && exprOkForSpeculation arg)
1243 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1244 -> (SimplEnv -> SimplM (FloatsWith a))
1245 -> SimplM (FloatsWith a)
1246 addAtomicBinds env [] thing_inside = thing_inside env
1247 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1248 addAtomicBinds env bs thing_inside
1252 %************************************************************************
1254 \subsection{The main rebuilder}
1256 %************************************************************************
1259 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1261 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1262 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1263 rebuild env expr (CoerceIt co cont) = rebuild env (mkCoerce co expr) cont
1264 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1265 rebuild env expr (ApplyTo _ arg mb_se cont) = rebuildApp env expr arg mb_se cont
1267 rebuildApp env fun arg mb_se cont
1268 = do { arg' <- simplArg env arg mb_se
1269 ; rebuild env (App fun arg') cont }
1271 simplArg :: SimplEnv -> CoreExpr -> Maybe SimplEnv -> SimplM CoreExpr
1272 simplArg env arg Nothing = return arg -- The arg is already simplified
1273 simplArg env arg (Just arg_env) = simplExpr (setInScope arg_env env) arg
1275 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1279 %************************************************************************
1281 \subsection{Functions dealing with a case}
1283 %************************************************************************
1285 Blob of helper functions for the "case-of-something-else" situation.
1288 ---------------------------------------------------------
1289 -- Eliminate the case if possible
1291 rebuildCase :: SimplEnv
1292 -> OutExpr -- Scrutinee
1293 -> InId -- Case binder
1294 -> [InAlt] -- Alternatives (inceasing order)
1296 -> SimplM FloatsWithExpr
1298 rebuildCase env scrut case_bndr alts cont
1299 | Just (con,args) <- exprIsConApp_maybe scrut
1300 -- Works when the scrutinee is a variable with a known unfolding
1301 -- as well as when it's an explicit constructor application
1302 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1304 | Lit lit <- scrut -- No need for same treatment as constructors
1305 -- because literals are inlined more vigorously
1306 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1309 = -- Prepare the continuation;
1310 -- The new subst_env is in place
1311 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1312 addFloats env floats $ \ env ->
1315 -- The case expression is annotated with the result type of the continuation
1316 -- This may differ from the type originally on the case. For example
1317 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1320 -- let j a# = <blob>
1321 -- in case(T) a of { True -> j 1#; False -> j 0# }
1322 -- Note that the case that scrutinises a now returns a T not an Int#
1323 res_ty' = contResultType dup_cont
1326 -- Deal with case binder
1327 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1329 -- Deal with the case alternatives
1330 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1332 -- Put the case back together
1333 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1335 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1336 -- The case binder *not* scope over the whole returned case-expression
1337 rebuild env case_expr nondup_cont
1340 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1341 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1342 way, there's a chance that v will now only be used once, and hence
1347 There is a time we *don't* want to do that, namely when
1348 -fno-case-of-case is on. This happens in the first simplifier pass,
1349 and enhances full laziness. Here's the bad case:
1350 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1351 If we eliminate the inner case, we trap it inside the I# v -> arm,
1352 which might prevent some full laziness happening. I've seen this
1353 in action in spectral/cichelli/Prog.hs:
1354 [(m,n) | m <- [1..max], n <- [1..max]]
1355 Hence the check for NoCaseOfCase.
1359 There is another situation when we don't want to do it. If we have
1361 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1362 ...other cases .... }
1364 We'll perform the binder-swap for the outer case, giving
1366 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1367 ...other cases .... }
1369 But there is no point in doing it for the inner case, because w1 can't
1370 be inlined anyway. Furthermore, doing the case-swapping involves
1371 zapping w2's occurrence info (see paragraphs that follow), and that
1372 forces us to bind w2 when doing case merging. So we get
1374 case x of w1 { A -> let w2 = w1 in e1
1375 B -> let w2 = w1 in e2
1376 ...other cases .... }
1378 This is plain silly in the common case where w2 is dead.
1380 Even so, I can't see a good way to implement this idea. I tried
1381 not doing the binder-swap if the scrutinee was already evaluated
1382 but that failed big-time:
1386 case v of w { MkT x ->
1387 case x of x1 { I# y1 ->
1388 case x of x2 { I# y2 -> ...
1390 Notice that because MkT is strict, x is marked "evaluated". But to
1391 eliminate the last case, we must either make sure that x (as well as
1392 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1393 the binder-swap. So this whole note is a no-op.
1397 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1398 any occurrence info (eg IAmDead) in the case binder, because the
1399 case-binder now effectively occurs whenever v does. AND we have to do
1400 the same for the pattern-bound variables! Example:
1402 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1404 Here, b and p are dead. But when we move the argment inside the first
1405 case RHS, and eliminate the second case, we get
1407 case x of { (a,b) -> a b }
1409 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1412 Indeed, this can happen anytime the case binder isn't dead:
1413 case <any> of x { (a,b) ->
1414 case x of { (p,q) -> p } }
1415 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1416 The point is that we bring into the envt a binding
1418 after the outer case, and that makes (a,b) alive. At least we do unless
1419 the case binder is guaranteed dead.
1422 simplCaseBinder env (Var v) case_bndr
1423 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1425 -- Failed try [see Note 2 above]
1426 -- not (isEvaldUnfolding (idUnfolding v))
1428 = simplBinder env (zapOccInfo case_bndr) `thenSmpl` \ (env, case_bndr') ->
1429 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1430 -- We could extend the substitution instead, but it would be
1431 -- a hack because then the substitution wouldn't be idempotent
1432 -- any more (v is an OutId). And this does just as well.
1434 simplCaseBinder env other_scrut case_bndr
1435 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1436 returnSmpl (env, case_bndr')
1438 zapOccInfo :: InId -> InId
1439 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1443 simplAlts does two things:
1445 1. Eliminate alternatives that cannot match, including the
1446 DEFAULT alternative.
1448 2. If the DEFAULT alternative can match only one possible constructor,
1449 then make that constructor explicit.
1451 case e of x { DEFAULT -> rhs }
1453 case e of x { (a,b) -> rhs }
1454 where the type is a single constructor type. This gives better code
1455 when rhs also scrutinises x or e.
1457 Here "cannot match" includes knowledge from GADTs
1459 It's a good idea do do this stuff before simplifying the alternatives, to
1460 avoid simplifying alternatives we know can't happen, and to come up with
1461 the list of constructors that are handled, to put into the IdInfo of the
1462 case binder, for use when simplifying the alternatives.
1464 Eliminating the default alternative in (1) isn't so obvious, but it can
1467 data Colour = Red | Green | Blue
1476 DEFAULT -> [ case y of ... ]
1478 If we inline h into f, the default case of the inlined h can't happen.
1479 If we don't notice this, we may end up filtering out *all* the cases
1480 of the inner case y, which give us nowhere to go!
1484 simplAlts :: SimplEnv
1486 -> OutId -- Case binder
1487 -> [InAlt] -> SimplCont
1488 -> SimplM [OutAlt] -- Includes the continuation
1490 simplAlts env scrut case_bndr' alts cont'
1491 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1492 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1493 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1494 -- We need the mergeAlts in case the new default_alt
1495 -- has turned into a constructor alternative.
1497 (alts_wo_default, maybe_deflt) = findDefault alts
1498 imposs_cons = case scrut of
1499 Var v -> otherCons (idUnfolding v)
1502 -- "imposs_deflt_cons" are handled either by the context,
1503 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1504 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1506 simplDefault :: SimplEnv
1507 -> OutId -- Case binder; need just for its type. Note that as an
1508 -- OutId, it has maximum information; this is important.
1509 -- Test simpl013 is an example
1510 -> [AltCon] -- These cons can't happen when matching the default
1513 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1514 -- mergeAlts expects
1517 simplDefault env case_bndr' imposs_cons cont Nothing
1518 = return [] -- No default branch
1520 simplDefault env case_bndr' imposs_cons cont (Just rhs)
1521 | -- This branch handles the case where we are
1522 -- scrutinisng an algebraic data type
1523 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1524 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1525 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1526 -- case x of { DEFAULT -> e }
1527 -- and we don't want to fill in a default for them!
1528 Just all_cons <- tyConDataCons_maybe tycon,
1529 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1530 -- which GHC allows, then the case expression will have at most a default
1531 -- alternative. We don't want to eliminate that alternative, because the
1532 -- invariant is that there's always one alternative. It's more convenient
1534 -- case x of { DEFAULT -> e }
1535 -- as it is, rather than transform it to
1536 -- error "case cant match"
1537 -- which would be quite legitmate. But it's a really obscure corner, and
1538 -- not worth wasting code on.
1540 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1541 poss_data_cons = filterOut (`elem` imposs_data_cons) all_cons
1542 gadt_imposs | all isTyVarTy inst_tys = []
1543 | otherwise = filter (cant_match inst_tys) poss_data_cons
1544 final_poss = filterOut (`elem` gadt_imposs) poss_data_cons
1546 = case final_poss of
1547 [] -> returnSmpl [] -- Eliminate the default alternative
1548 -- altogether if it can't match
1550 [con] -> -- It matches exactly one constructor, so fill it in
1551 do { tick (FillInCaseDefault case_bndr')
1552 ; us <- getUniquesSmpl
1553 ; let (ex_tvs, co_tvs, arg_ids) =
1554 dataConRepInstPat us con inst_tys
1555 ; let con_alt = (DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, rhs)
1556 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1557 -- The simplAlt must succeed with Just because we have
1558 -- already filtered out construtors that can't match
1561 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1564 = simplify_default imposs_cons
1566 cant_match tys data_con = not (dataConCanMatch data_con tys)
1568 simplify_default imposs_cons
1569 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1570 -- Record the constructors that the case-binder *can't* be.
1571 ; rhs' <- simplExprC env' rhs cont
1572 ; return [(DEFAULT, [], rhs')] }
1574 simplAlt :: SimplEnv
1575 -> [AltCon] -- These constructors can't be present when
1576 -- matching this alternative
1577 -> OutId -- The case binder
1580 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1582 -- Simplify an alternative, returning the type refinement for the
1583 -- alternative, if the alternative does any refinement at all
1584 -- Nothing => the alternative is inaccessible
1586 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1587 | con `elem` imposs_cons -- This case can't match
1590 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1591 -- TURGID DUPLICATION, needed only for the simplAlt call
1592 -- in mkDupableAlt. Clean this up when moving to FC
1593 = ASSERT( null bndrs )
1594 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1595 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1597 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1598 -- Record the constructors that the case-binder *can't* be.
1600 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1601 = ASSERT( null bndrs )
1602 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1603 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1605 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1607 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1608 = -- Deal with the pattern-bound variables
1609 -- Mark the ones that are in ! positions in the data constructor
1610 -- as certainly-evaluated.
1611 -- NB: it happens that simplBinders does *not* erase the OtherCon
1612 -- form of unfolding, so it's ok to add this info before
1613 -- doing simplBinders
1614 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1616 -- Bind the case-binder to (con args)
1617 let unf = mkUnfolding False (mkConApp con con_args)
1618 inst_tys' = tyConAppArgs (idType case_bndr')
1619 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1620 env' = mk_rhs_env env case_bndr' unf
1622 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1623 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1625 -- add_evals records the evaluated-ness of the bound variables of
1626 -- a case pattern. This is *important*. Consider
1627 -- data T = T !Int !Int
1629 -- case x of { T a b -> T (a+1) b }
1631 -- We really must record that b is already evaluated so that we don't
1632 -- go and re-evaluate it when constructing the result.
1633 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1635 cat_evals dc vs strs
1639 go (v:vs) strs | isTyVar v = v : go vs strs
1640 go (v:vs) (str:strs)
1641 | isMarkedStrict str = evald_v : go vs strs
1642 | otherwise = zapped_v : go vs strs
1644 zapped_v = zap_occ_info v
1645 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1646 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1648 -- If the case binder is alive, then we add the unfolding
1650 -- to the envt; so vs are now very much alive
1651 -- Note [Aug06] I can't see why this actually matters
1652 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1653 | otherwise = zapOccInfo
1655 mk_rhs_env env case_bndr' case_bndr_unf
1656 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1660 %************************************************************************
1662 \subsection{Known constructor}
1664 %************************************************************************
1666 We are a bit careful with occurrence info. Here's an example
1668 (\x* -> case x of (a*, b) -> f a) (h v, e)
1670 where the * means "occurs once". This effectively becomes
1671 case (h v, e) of (a*, b) -> f a)
1673 let a* = h v; b = e in f a
1677 All this should happen in one sweep.
1680 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1681 -> InId -> [InAlt] -> SimplCont
1682 -> SimplM FloatsWithExpr
1684 knownCon env scrut con args bndr alts cont
1685 = tick (KnownBranch bndr) `thenSmpl_`
1686 case findAlt con alts of
1687 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1688 simplNonRecX env bndr scrut $ \ env ->
1689 -- This might give rise to a binding with non-atomic args
1690 -- like x = Node (f x) (g x)
1691 -- but simplNonRecX will atomic-ify it
1692 simplExprF env rhs cont
1694 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1695 simplNonRecX env bndr scrut $ \ env ->
1696 simplExprF env rhs cont
1698 (DataAlt dc, bs, rhs)
1699 -> -- ASSERT( n_drop_tys + length bs == length args )
1700 bind_args env dead_bndr bs (drop n_drop_tys args) $ \ env ->
1702 -- It's useful to bind bndr to scrut, rather than to a fresh
1703 -- binding x = Con arg1 .. argn
1704 -- because very often the scrut is a variable, so we avoid
1705 -- creating, and then subsequently eliminating, a let-binding
1706 -- BUT, if scrut is a not a variable, we must be careful
1707 -- about duplicating the arg redexes; in that case, make
1708 -- a new con-app from the args
1709 bndr_rhs = case scrut of
1712 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1713 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1714 -- args are aready OutExprs, but bs are InIds
1716 simplNonRecX env bndr bndr_rhs $ \ env ->
1717 simplExprF env rhs cont
1719 dead_bndr = isDeadBinder bndr
1720 n_drop_tys = tyConArity (dataConTyCon dc)
1723 bind_args env dead_bndr [] _ thing_inside = thing_inside env
1725 bind_args env dead_bndr (b:bs) (Type ty : args) thing_inside
1726 = ASSERT( isTyVar b )
1727 bind_args (extendTvSubst env b ty) dead_bndr bs args thing_inside
1729 bind_args env dead_bndr (b:bs) (arg : args) thing_inside
1732 b' = if dead_bndr then b else zapOccInfo b
1733 -- Note that the binder might be "dead", because it doesn't occur
1734 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1735 -- Nevertheless we must keep it if the case-binder is alive, because it may
1736 -- be used in teh con_app
1738 simplNonRecX env b' arg $ \ env ->
1739 bind_args env dead_bndr bs args thing_inside
1743 %************************************************************************
1745 \subsection{Duplicating continuations}
1747 %************************************************************************
1750 prepareCaseCont :: SimplEnv
1751 -> [InAlt] -> SimplCont
1752 -> SimplM (FloatsWith (SimplCont,SimplCont))
1753 -- Return a duplicatable continuation, a non-duplicable part
1754 -- plus some extra bindings (that scope over the entire
1757 -- No need to make it duplicatable if there's only one alternative
1758 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1759 prepareCaseCont env alts cont = mkDupableCont env cont
1763 mkDupableCont :: SimplEnv -> SimplCont
1764 -> SimplM (FloatsWith (SimplCont, SimplCont))
1766 mkDupableCont env cont
1767 | contIsDupable cont
1768 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1770 mkDupableCont env (CoerceIt ty cont)
1771 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1772 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1774 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1775 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1776 -- Do *not* duplicate an ArgOf continuation
1777 -- Because ArgOf continuations are opaque, we gain nothing by
1778 -- propagating them into the expressions, and we do lose a lot.
1779 -- Here's an example:
1780 -- && (case x of { T -> F; F -> T }) E
1781 -- Now, && is strict so we end up simplifying the case with
1782 -- an ArgOf continuation. If we let-bind it, we get
1784 -- let $j = \v -> && v E
1785 -- in simplExpr (case x of { T -> F; F -> T })
1786 -- (ArgOf (\r -> $j r)
1787 -- And after simplifying more we get
1789 -- let $j = \v -> && v E
1790 -- in case of { T -> $j F; F -> $j T }
1791 -- Which is a Very Bad Thing
1793 -- The desire not to duplicate is the entire reason that
1794 -- mkDupableCont returns a pair of continuations.
1796 -- The original plan had:
1797 -- e.g. (...strict-fn...) [...hole...]
1799 -- let $j = \a -> ...strict-fn...
1800 -- in $j [...hole...]
1802 mkDupableCont env (ApplyTo _ arg mb_se cont)
1803 = -- e.g. [...hole...] (...arg...)
1805 -- let a = ...arg...
1806 -- in [...hole...] a
1807 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1808 ; addFloats env floats $ \ env -> do
1809 { arg1 <- simplArg env arg mb_se
1810 ; (floats2, arg2) <- mkDupableArg env arg1
1811 ; return (floats2, (ApplyTo OkToDup arg2 Nothing dup_cont, nondup_cont)) }}
1813 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1814 -- | not (exprIsDupable rhs && contIsDupable case_cont) -- See notes below
1815 -- | not (isDeadBinder case_bndr)
1816 | all isDeadBinder bs
1817 = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
1819 scrut_ty = substTy se (idType case_bndr)
1821 {- Note [Single-alternative cases]
1822 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1823 This case is just like the ArgOf case. Here's an example:
1827 case (case x of I# x' ->
1829 True -> I# (negate# x')
1830 False -> I# x') of y {
1832 Because the (case x) has only one alternative, we'll transform to
1834 case (case x' <# 0# of
1835 True -> I# (negate# x')
1836 False -> I# x') of y {
1838 But now we do *NOT* want to make a join point etc, giving
1840 let $j = \y -> MkT y
1842 True -> $j (I# (negate# x'))
1844 In this case the $j will inline again, but suppose there was a big
1845 strict computation enclosing the orginal call to MkT. Then, it won't
1846 "see" the MkT any more, because it's big and won't get duplicated.
1847 And, what is worse, nothing was gained by the case-of-case transform.
1849 When should use this case of mkDupableCont?
1850 However, matching on *any* single-alternative case is a *disaster*;
1851 e.g. case (case ....) of (a,b) -> (# a,b #)
1852 We must push the outer case into the inner one!
1855 * Match [(DEFAULT,_,_)], but in the common case of Int,
1856 the alternative-filling-in code turned the outer case into
1857 case (...) of y { I# _ -> MkT y }
1859 * Match on single alternative plus (not (isDeadBinder case_bndr))
1860 Rationale: pushing the case inwards won't eliminate the construction.
1861 But there's a risk of
1862 case (...) of y { (a,b) -> let z=(a,b) in ... }
1863 Now y looks dead, but it'll come alive again. Still, this
1864 seems like the best option at the moment.
1866 * Match on single alternative plus (all (isDeadBinder bndrs))
1867 Rationale: this is essentially seq.
1869 * Match when the rhs is *not* duplicable, and hence would lead to a
1870 join point. This catches the disaster-case above. We can test
1871 the *un-simplified* rhs, which is fine. It might get bigger or
1872 smaller after simplification; if it gets smaller, this case might
1873 fire next time round. NB also that we must test contIsDupable
1874 case_cont *btoo, because case_cont might be big!
1876 HOWEVER: I found that this version doesn't work well, because
1877 we can get let x = case (...) of { small } in ...case x...
1878 When x is inlined into its full context, we find that it was a bad
1879 idea to have pushed the outer case inside the (...) case.
1882 mkDupableCont env (Select _ case_bndr alts se cont)
1883 = -- e.g. (case [...hole...] of { pi -> ei })
1885 -- let ji = \xij -> ei
1886 -- in case [...hole...] of { pi -> ji xij }
1887 do { tick (CaseOfCase case_bndr)
1888 ; let alt_env = setInScope se env
1889 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1890 -- NB: call mkDupableCont here, *not* prepareCaseCont
1891 -- We must make a duplicable continuation, whereas prepareCaseCont
1892 -- doesn't when there is a single case branch
1893 ; addFloats alt_env floats1 $ \ alt_env -> do
1895 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1896 -- NB: simplBinder does not zap deadness occ-info, so
1897 -- a dead case_bndr' will still advertise its deadness
1898 -- This is really important because in
1899 -- case e of b { (# a,b #) -> ... }
1900 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1901 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1902 -- In the new alts we build, we have the new case binder, so it must retain
1905 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1906 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1907 (mkBoringStop (contResultType dup_cont)),
1911 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1912 -- Let-bind the thing if necessary
1913 mkDupableArg env arg
1915 = return (emptyFloats env, arg)
1917 = do { arg_id <- newId FSLIT("a") (exprType arg)
1918 ; tick (CaseOfCase arg_id)
1919 -- Want to tick here so that we go round again,
1920 -- and maybe copy or inline the code.
1921 -- Not strictly CaseOfCase, but never mind
1922 ; return (unitFloat env arg_id arg, Var arg_id) }
1923 -- What if the arg should be case-bound?
1924 -- This has been this way for a long time, so I'll leave it,
1925 -- but I can't convince myself that it's right.
1927 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1928 -> SimplM (FloatsWith [InAlt])
1929 -- Absorbs the continuation into the new alternatives
1931 mkDupableAlts env case_bndr' alts dupable_cont
1934 go env [] = returnSmpl (emptyFloats env, [])
1936 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1937 ; addFloats env floats1 $ \ env -> do
1938 { (floats2, alts') <- go env alts
1939 ; returnSmpl (floats2, case mb_alt' of
1940 Just alt' -> alt' : alts'
1944 mkDupableAlt env case_bndr' cont alt
1945 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1947 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1949 Just (reft, (con, bndrs', rhs')) ->
1950 -- Safe to say that there are no handled-cons for the DEFAULT case
1952 if exprIsDupable rhs' then
1953 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
1954 -- It is worth checking for a small RHS because otherwise we
1955 -- get extra let bindings that may cause an extra iteration of the simplifier to
1956 -- inline back in place. Quite often the rhs is just a variable or constructor.
1957 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1958 -- iterations because the version with the let bindings looked big, and so wasn't
1959 -- inlined, but after the join points had been inlined it looked smaller, and so
1962 -- NB: we have to check the size of rhs', not rhs.
1963 -- Duplicating a small InAlt might invalidate occurrence information
1964 -- However, if it *is* dupable, we return the *un* simplified alternative,
1965 -- because otherwise we'd need to pair it up with an empty subst-env....
1966 -- but we only have one env shared between all the alts.
1967 -- (Remember we must zap the subst-env before re-simplifying something).
1968 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1972 rhs_ty' = exprType rhs'
1973 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1975 | isTyVar bndr = not (bndr `elemVarEnv` reft)
1976 -- Don't abstract over tyvar binders which are refined away
1977 -- See Note [Refinement] below
1978 | otherwise = not (isDeadBinder bndr)
1979 -- The deadness info on the new Ids is preserved by simplBinders
1981 -- If we try to lift a primitive-typed something out
1982 -- for let-binding-purposes, we will *caseify* it (!),
1983 -- with potentially-disastrous strictness results. So
1984 -- instead we turn it into a function: \v -> e
1985 -- where v::State# RealWorld#. The value passed to this function
1986 -- is realworld#, which generates (almost) no code.
1988 -- There's a slight infelicity here: we pass the overall
1989 -- case_bndr to all the join points if it's used in *any* RHS,
1990 -- because we don't know its usage in each RHS separately
1992 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1993 -- we make the join point into a function whenever used_bndrs'
1994 -- is empty. This makes the join-point more CPR friendly.
1995 -- Consider: let j = if .. then I# 3 else I# 4
1996 -- in case .. of { A -> j; B -> j; C -> ... }
1998 -- Now CPR doesn't w/w j because it's a thunk, so
1999 -- that means that the enclosing function can't w/w either,
2000 -- which is a lose. Here's the example that happened in practice:
2001 -- kgmod :: Int -> Int -> Int
2002 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2006 -- I have seen a case alternative like this:
2007 -- True -> \v -> ...
2008 -- It's a bit silly to add the realWorld dummy arg in this case, making
2011 -- (the \v alone is enough to make CPR happy) but I think it's rare
2013 ( if not (any isId used_bndrs')
2014 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
2015 returnSmpl ([rw_id], [Var realWorldPrimId])
2017 returnSmpl (used_bndrs', varsToCoreExprs used_bndrs')
2018 ) `thenSmpl` \ (final_bndrs', final_args) ->
2020 -- See comment about "$j" name above
2021 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
2022 -- Notice the funky mkPiTypes. If the contructor has existentials
2023 -- it's possible that the join point will be abstracted over
2024 -- type varaibles as well as term variables.
2025 -- Example: Suppose we have
2026 -- data T = forall t. C [t]
2028 -- case (case e of ...) of
2029 -- C t xs::[t] -> rhs
2030 -- We get the join point
2031 -- let j :: forall t. [t] -> ...
2032 -- j = /\t \xs::[t] -> rhs
2034 -- case (case e of ...) of
2035 -- C t xs::[t] -> j t xs
2037 -- We make the lambdas into one-shot-lambdas. The
2038 -- join point is sure to be applied at most once, and doing so
2039 -- prevents the body of the join point being floated out by
2040 -- the full laziness pass
2041 really_final_bndrs = map one_shot final_bndrs'
2042 one_shot v | isId v = setOneShotLambda v
2044 join_rhs = mkLams really_final_bndrs rhs'
2045 join_call = mkApps (Var join_bndr) final_args
2047 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2054 MkT :: a -> b -> T a
2058 MkT a' b (p::a') (q::b) -> [p,w]
2060 The danger is that we'll make a join point
2064 and that's ill-typed, because (p::a') but (w::a).
2066 Solution so far: don't abstract over a', because the type refinement
2067 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2069 Then we must abstract over any refined free variables. Hmm. Maybe we
2070 could just abstract over *all* free variables, thereby lambda-lifting
2071 the join point? We should try this.