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, mkDataConAlt,
17 SimplCont(..), DupFlag(..), LetRhsFlag(..),
18 mkRhsStop, mkBoringStop, mkLazyArgStop, pushContArgs,
19 contResultType, countArgs, contIsDupable, contIsRhsOrArg,
20 getContArgs, interestingCallContext, interestingArg, isStrictType,
21 preInlineUnconditionally, postInlineUnconditionally,
22 interestingArgContext, inlineMode, activeInline, activeRule
24 import Id ( Id, idType, idInfo, idArity, isDataConWorkId,
25 idUnfolding, setIdUnfolding, isDeadBinder,
26 idNewDemandInfo, setIdInfo,
27 setIdOccInfo, zapLamIdInfo, setOneShotLambda
29 import MkId ( eRROR_ID )
30 import Literal ( mkStringLit )
31 import IdInfo ( OccInfo(..), isLoopBreaker,
32 setArityInfo, zapDemandInfo,
36 import NewDemand ( isStrictDmd )
37 import TcGadt ( dataConCanMatch )
38 import DataCon ( DataCon, dataConTyCon, dataConRepStrictness )
39 import TyCon ( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
41 import PprCore ( pprParendExpr, pprCoreExpr )
42 import CoreUnfold ( mkUnfolding, callSiteInline )
43 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
44 exprIsConApp_maybe, mkPiTypes, findAlt,
45 exprType, exprIsHNF, findDefault, mergeAlts,
46 exprOkForSpeculation, exprArity,
47 mkCoerce, mkSCC, mkInlineMe, applyTypeToArg
49 import Rules ( lookupRule )
50 import BasicTypes ( isMarkedStrict )
51 import CostCentre ( currentCCS )
52 import Type ( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
53 splitFunTy_maybe, splitFunTy, coreEqType, splitTyConApp_maybe,
54 isTyVarTy, mkTyVarTys, isFunTy, tcEqType
56 import Coercion ( Coercion, coercionKind,
57 mkTransCoercion, mkLeftCoercion, mkRightCoercion,
58 mkSymCoercion, splitCoercionKind_maybe, decomposeCo )
59 import Var ( tyVarKind, mkTyVar )
60 import VarEnv ( elemVarEnv, emptyVarEnv )
61 import TysPrim ( realWorldStatePrimTy )
62 import PrelInfo ( realWorldPrimId )
63 import BasicTypes ( TopLevelFlag(..), isTopLevel,
68 import Maybes ( orElse )
70 import Util ( notNull, filterOut )
74 The guts of the simplifier is in this module, but the driver loop for
75 the simplifier is in SimplCore.lhs.
78 -----------------------------------------
79 *** IMPORTANT NOTE ***
80 -----------------------------------------
81 The simplifier used to guarantee that the output had no shadowing, but
82 it does not do so any more. (Actually, it never did!) The reason is
83 documented with simplifyArgs.
86 -----------------------------------------
87 *** IMPORTANT NOTE ***
88 -----------------------------------------
89 Many parts of the simplifier return a bunch of "floats" as well as an
90 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
92 All "floats" are let-binds, not case-binds, but some non-rec lets may
93 be unlifted (with RHS ok-for-speculation).
97 -----------------------------------------
98 ORGANISATION OF FUNCTIONS
99 -----------------------------------------
101 - simplify all top-level binders
102 - for NonRec, call simplRecOrTopPair
103 - for Rec, call simplRecBind
106 ------------------------------
107 simplExpr (applied lambda) ==> simplNonRecBind
108 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
109 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
111 ------------------------------
112 simplRecBind [binders already simplfied]
113 - use simplRecOrTopPair on each pair in turn
115 simplRecOrTopPair [binder already simplified]
116 Used for: recursive bindings (top level and nested)
117 top-level non-recursive bindings
119 - check for PreInlineUnconditionally
123 Used for: non-top-level non-recursive bindings
124 beta reductions (which amount to the same thing)
125 Because it can deal with strict arts, it takes a
126 "thing-inside" and returns an expression
128 - check for PreInlineUnconditionally
129 - simplify binder, including its IdInfo
138 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
139 Used for: binding case-binder and constr args in a known-constructor case
140 - check for PreInLineUnconditionally
144 ------------------------------
145 simplLazyBind: [binder already simplified, RHS not]
146 Used for: recursive bindings (top level and nested)
147 top-level non-recursive bindings
148 non-top-level, but *lazy* non-recursive bindings
149 [must not be strict or unboxed]
150 Returns floats + an augmented environment, not an expression
151 - substituteIdInfo and add result to in-scope
152 [so that rules are available in rec rhs]
155 - float if exposes constructor or PAP
159 completeNonRecX: [binder and rhs both simplified]
160 - if the the thing needs case binding (unlifted and not ok-for-spec)
166 completeLazyBind: [given a simplified RHS]
167 [used for both rec and non-rec bindings, top level and not]
168 - try PostInlineUnconditionally
169 - add unfolding [this is the only place we add an unfolding]
174 Right hand sides and arguments
175 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
176 In many ways we want to treat
177 (a) the right hand side of a let(rec), and
178 (b) a function argument
179 in the same way. But not always! In particular, we would
180 like to leave these arguments exactly as they are, so they
181 will match a RULE more easily.
186 It's harder to make the rule match if we ANF-ise the constructor,
187 or eta-expand the PAP:
189 f (let { a = g x; b = h x } in (a,b))
192 On the other hand if we see the let-defns
197 then we *do* want to ANF-ise and eta-expand, so that p and q
198 can be safely inlined.
200 Even floating lets out is a bit dubious. For let RHS's we float lets
201 out if that exposes a value, so that the value can be inlined more vigorously.
204 r = let x = e in (x,x)
206 Here, if we float the let out we'll expose a nice constructor. We did experiments
207 that showed this to be a generally good thing. But it was a bad thing to float
208 lets out unconditionally, because that meant they got allocated more often.
210 For function arguments, there's less reason to expose a constructor (it won't
211 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
212 So for the moment we don't float lets out of function arguments either.
217 For eta expansion, we want to catch things like
219 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
221 If the \x was on the RHS of a let, we'd eta expand to bring the two
222 lambdas together. And in general that's a good thing to do. Perhaps
223 we should eta expand wherever we find a (value) lambda? Then the eta
224 expansion at a let RHS can concentrate solely on the PAP case.
227 %************************************************************************
229 \subsection{Bindings}
231 %************************************************************************
234 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
236 simplTopBinds env binds
237 = -- Put all the top-level binders into scope at the start
238 -- so that if a transformation rule has unexpectedly brought
239 -- anything into scope, then we don't get a complaint about that.
240 -- It's rather as if the top-level binders were imported.
241 simplRecBndrs env (bindersOfBinds binds) `thenSmpl` \ (env, bndrs') ->
242 simpl_binds env binds bndrs' `thenSmpl` \ (floats, _) ->
243 freeTick SimplifierDone `thenSmpl_`
244 returnSmpl (floatBinds floats)
246 -- We need to track the zapped top-level binders, because
247 -- they should have their fragile IdInfo zapped (notably occurrence info)
248 -- That's why we run down binds and bndrs' simultaneously.
249 simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
250 simpl_binds env [] bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
251 simpl_binds env (bind:binds) bs = simpl_bind env bind bs `thenSmpl` \ (floats,env) ->
252 addFloats env floats $ \env ->
253 simpl_binds env binds (drop_bs bind bs)
255 drop_bs (NonRec _ _) (_ : bs) = bs
256 drop_bs (Rec prs) bs = drop (length prs) bs
258 simpl_bind env bind bs
259 = getDOptsSmpl `thenSmpl` \ dflags ->
260 if dopt Opt_D_dump_inlinings dflags then
261 pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
263 simpl_bind1 env bind bs
265 simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
266 simpl_bind1 env (Rec pairs) bs' = simplRecBind env TopLevel pairs bs'
270 %************************************************************************
272 \subsection{simplNonRec}
274 %************************************************************************
276 simplNonRecBind is used for
277 * non-top-level non-recursive lets in expressions
281 * An unsimplified (binder, rhs) pair
282 * The env for the RHS. It may not be the same as the
283 current env because the bind might occur via (\x.E) arg
285 It uses the CPS form because the binding might be strict, in which
286 case we might discard the continuation:
287 let x* = error "foo" in (...x...)
289 It needs to turn unlifted bindings into a @case@. They can arise
290 from, say: (\x -> e) (4# + 3#)
293 simplNonRecBind :: SimplEnv
295 -> InExpr -> SimplEnv -- Arg, with its subst-env
296 -> OutType -- Type of thing computed by the context
297 -> (SimplEnv -> SimplM FloatsWithExpr) -- The body
298 -> SimplM FloatsWithExpr
300 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
302 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
305 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
306 = simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
308 simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
309 | preInlineUnconditionally env NotTopLevel bndr rhs
310 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
311 thing_inside (extendIdSubst env bndr (mkContEx rhs_se rhs))
313 | isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty -- A strict let
314 = -- Don't use simplBinder because that doesn't keep
315 -- fragile occurrence info in the substitution
316 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr1) ->
317 simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty $ \ env1 rhs1 ->
319 -- Now complete the binding and simplify the body
321 (env2,bndr2) = addLetIdInfo env1 bndr bndr1
323 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
325 | otherwise -- Normal, lazy case
326 = -- Don't use simplBinder because that doesn't keep
327 -- fragile occurrence info in the substitution
328 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr') ->
329 simplLazyBind env NotTopLevel NonRecursive
330 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
331 addFloats env floats thing_inside
334 bndr_ty = idType bndr
337 A specialised variant of simplNonRec used when the RHS is already simplified, notably
338 in knownCon. It uses case-binding where necessary.
341 simplNonRecX :: SimplEnv
342 -> InId -- Old binder
343 -> OutExpr -- Simplified RHS
344 -> (SimplEnv -> SimplM FloatsWithExpr)
345 -> SimplM FloatsWithExpr
347 simplNonRecX env bndr new_rhs thing_inside
348 = do { (env, bndr') <- simplBinder env bndr
349 ; completeNonRecX env False {- Non-strict; pessimistic -}
350 bndr bndr' new_rhs thing_inside }
353 completeNonRecX :: SimplEnv
354 -> Bool -- Strict binding
355 -> InId -- Old binder
356 -> OutId -- New binder
357 -> OutExpr -- Simplified RHS
358 -> (SimplEnv -> SimplM FloatsWithExpr)
359 -> SimplM FloatsWithExpr
361 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
362 | needsCaseBinding (idType new_bndr) new_rhs
363 -- Make this test *before* the preInlineUnconditionally
364 -- Consider case I# (quotInt# x y) of
365 -- I# v -> let w = J# v in ...
366 -- If we gaily inline (quotInt# x y) for v, we end up building an
368 -- let w = J# (quotInt# x y) in ...
369 -- because quotInt# can fail.
370 = do { (floats, body) <- thing_inside env
371 ; let body' = wrapFloats floats body
372 ; return (emptyFloats env, Case new_rhs new_bndr (exprType body)
373 [(DEFAULT, [], body')]) }
376 = -- Make the arguments atomic if necessary,
377 -- adding suitable bindings
378 -- pprTrace "completeNonRecX" (ppr new_bndr <+> ppr new_rhs) $
379 mkAtomicArgsE env is_strict new_rhs $ \ env new_rhs ->
380 completeLazyBind env NotTopLevel
381 old_bndr new_bndr new_rhs `thenSmpl` \ (floats, env) ->
382 addFloats env floats thing_inside
384 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
385 Doing so risks exponential behaviour, because new_rhs has been simplified once already
386 In the cases described by the folowing commment, postInlineUnconditionally will
387 catch many of the relevant cases.
388 -- This happens; for example, the case_bndr during case of
389 -- known constructor: case (a,b) of x { (p,q) -> ... }
390 -- Here x isn't mentioned in the RHS, so we don't want to
391 -- create the (dead) let-binding let x = (a,b) in ...
393 -- Similarly, single occurrences can be inlined vigourously
394 -- e.g. case (f x, g y) of (a,b) -> ....
395 -- If a,b occur once we can avoid constructing the let binding for them.
396 | preInlineUnconditionally env NotTopLevel bndr new_rhs
397 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
399 -- NB: completeLazyBind uses postInlineUnconditionally; no need to do that here
404 %************************************************************************
406 \subsection{Lazy bindings}
408 %************************************************************************
410 simplRecBind is used for
411 * recursive bindings only
414 simplRecBind :: SimplEnv -> TopLevelFlag
415 -> [(InId, InExpr)] -> [OutId]
416 -> SimplM (FloatsWith SimplEnv)
417 simplRecBind env top_lvl pairs bndrs'
418 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
419 returnSmpl (flattenFloats floats, env)
421 go env [] _ = returnSmpl (emptyFloats env, env)
423 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
424 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
425 addFloats env floats (\env -> go env pairs bndrs')
429 simplRecOrTopPair is used for
430 * recursive bindings (whether top level or not)
431 * top-level non-recursive bindings
433 It assumes the binder has already been simplified, but not its IdInfo.
436 simplRecOrTopPair :: SimplEnv
438 -> InId -> OutId -- Binder, both pre-and post simpl
439 -> InExpr -- The RHS and its environment
440 -> SimplM (FloatsWith SimplEnv)
442 simplRecOrTopPair env top_lvl bndr bndr' rhs
443 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
444 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
445 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
448 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
449 -- May not actually be recursive, but it doesn't matter
453 simplLazyBind is used for
454 * recursive bindings (whether top level or not)
455 * top-level non-recursive bindings
456 * non-top-level *lazy* non-recursive bindings
458 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
459 from SimplRecOrTopBind]
462 1. It assumes that the binder is *already* simplified,
463 and is in scope, but not its IdInfo
465 2. It assumes that the binder type is lifted.
467 3. It does not check for pre-inline-unconditionallly;
468 that should have been done already.
471 simplLazyBind :: SimplEnv
472 -> TopLevelFlag -> RecFlag
473 -> InId -> OutId -- Binder, both pre-and post simpl
474 -> InExpr -> SimplEnv -- The RHS and its environment
475 -> SimplM (FloatsWith SimplEnv)
477 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
479 (env1,bndr2) = addLetIdInfo env bndr bndr1
480 rhs_env = setInScope rhs_se env1
481 is_top_level = isTopLevel top_lvl
482 ok_float_unlifted = not is_top_level && isNonRec is_rec
483 rhs_cont = mkRhsStop (idType bndr2)
485 -- Simplify the RHS; note the mkRhsStop, which tells
486 -- the simplifier that this is the RHS of a let.
487 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
489 -- If any of the floats can't be floated, give up now
490 -- (The allLifted predicate says True for empty floats.)
491 if (not ok_float_unlifted && not (allLifted floats)) then
492 completeLazyBind env1 top_lvl bndr bndr2
493 (wrapFloats floats rhs1)
496 -- ANF-ise a constructor or PAP rhs
497 mkAtomicArgs False {- Not strict -}
498 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
500 -- If the result is a PAP, float the floats out, else wrap them
501 -- By this time it's already been ANF-ised (if necessary)
502 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
503 completeLazyBind env1 top_lvl bndr bndr2 rhs2
505 else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
506 -- WARNING: long dodgy argument coming up
507 -- WANTED: a better way to do this
509 -- We can't use "exprIsCheap" instead of exprIsHNF,
510 -- because that causes a strictness bug.
511 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
512 -- The case expression is 'cheap', but it's wrong to transform to
513 -- y* = E; x = case (scc y) of {...}
514 -- Either we must be careful not to float demanded non-values, or
515 -- we must use exprIsHNF for the test, which ensures that the
516 -- thing is non-strict. So exprIsHNF => bindings are non-strict
517 -- I think. The WARN below tests for this.
519 -- We use exprIsTrivial here because we want to reveal lone variables.
520 -- E.g. let { x = letrec { y = E } in y } in ...
521 -- Here we definitely want to float the y=E defn.
522 -- exprIsHNF definitely isn't right for that.
524 -- Again, the floated binding can't be strict; if it's recursive it'll
525 -- be non-strict; if it's non-recursive it'd be inlined.
527 -- Note [SCC-and-exprIsTrivial]
529 -- y = let { x* = E } in scc "foo" x
530 -- then we do *not* want to float out the x binding, because
531 -- it's strict! Fortunately, exprIsTrivial replies False to
534 -- There's a subtlety here. There may be a binding (x* = e) in the
535 -- floats, where the '*' means 'will be demanded'. So is it safe
536 -- to float it out? Answer no, but it won't matter because
537 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
538 -- and so there can't be any 'will be demanded' bindings in the floats.
540 WARN( not (is_top_level || not (any demanded_float (floatBinds floats))),
541 ppr (filter demanded_float (floatBinds floats)) )
543 tick LetFloatFromLet `thenSmpl_` (
544 addFloats env1 floats $ \ env2 ->
545 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
546 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
549 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
552 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
553 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
554 demanded_float (Rec _) = False
559 %************************************************************************
561 \subsection{Completing a lazy binding}
563 %************************************************************************
566 * deals only with Ids, not TyVars
567 * takes an already-simplified binder and RHS
568 * is used for both recursive and non-recursive bindings
569 * is used for both top-level and non-top-level bindings
571 It does the following:
572 - tries discarding a dead binding
573 - tries PostInlineUnconditionally
574 - add unfolding [this is the only place we add an unfolding]
577 It does *not* attempt to do let-to-case. Why? Because it is used for
578 - top-level bindings (when let-to-case is impossible)
579 - many situations where the "rhs" is known to be a WHNF
580 (so let-to-case is inappropriate).
583 completeLazyBind :: SimplEnv
584 -> TopLevelFlag -- Flag stuck into unfolding
585 -> InId -- Old binder
586 -> OutId -- New binder
587 -> OutExpr -- Simplified RHS
588 -> SimplM (FloatsWith SimplEnv)
589 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
590 -- by extending the substitution (e.g. let x = y in ...)
591 -- The new binding (if any) is returned as part of the floats.
592 -- NB: the returned SimplEnv has the right SubstEnv, but you should
593 -- (as usual) use the in-scope-env from the floats
595 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
596 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
597 = -- Drop the binding
598 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
599 -- pprTrace "Inline unconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
600 returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
601 -- Use the substitution to make quite, quite sure that the substitution
602 -- will happen, since we are going to discard the binding
607 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
609 -- Add the unfolding *only* for non-loop-breakers
610 -- Making loop breakers not have an unfolding at all
611 -- means that we can avoid tests in exprIsConApp, for example.
612 -- This is important: if exprIsConApp says 'yes' for a recursive
613 -- thing, then we can get into an infinite loop
615 -- If the unfolding is a value, the demand info may
616 -- go pear-shaped, so we nuke it. Example:
618 -- case x of (p,q) -> h p q x
619 -- Here x is certainly demanded. But after we've nuked
620 -- the case, we'll get just
621 -- let x = (a,b) in h a b x
622 -- and now x is not demanded (I'm assuming h is lazy)
623 -- This really happens. Similarly
624 -- let f = \x -> e in ...f..f...
625 -- After inling f at some of its call sites the original binding may
626 -- (for example) be no longer strictly demanded.
627 -- The solution here is a bit ad hoc...
628 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
629 final_info | loop_breaker = new_bndr_info
630 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
631 | otherwise = info_w_unf
633 final_id = new_bndr `setIdInfo` final_info
635 -- These seqs forces the Id, and hence its IdInfo,
636 -- and hence any inner substitutions
638 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
639 returnSmpl (unitFloat env final_id new_rhs, env)
642 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
643 loop_breaker = isLoopBreaker occ_info
644 old_info = idInfo old_bndr
645 occ_info = occInfo old_info
650 %************************************************************************
652 \subsection[Simplify-simplExpr]{The main function: simplExpr}
654 %************************************************************************
656 The reason for this OutExprStuff stuff is that we want to float *after*
657 simplifying a RHS, not before. If we do so naively we get quadratic
658 behaviour as things float out.
660 To see why it's important to do it after, consider this (real) example:
674 a -- Can't inline a this round, cos it appears twice
678 Each of the ==> steps is a round of simplification. We'd save a
679 whole round if we float first. This can cascade. Consider
684 let f = let d1 = ..d.. in \y -> e
688 in \x -> ...(\y ->e)...
690 Only in this second round can the \y be applied, and it
691 might do the same again.
695 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
696 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
698 expr_ty' = substTy env (exprType expr)
699 -- The type in the Stop continuation, expr_ty', is usually not used
700 -- It's only needed when discarding continuations after finding
701 -- a function that returns bottom.
702 -- Hence the lazy substitution
705 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
706 -- Simplify an expression, given a continuation
707 simplExprC env expr cont
708 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
709 returnSmpl (wrapFloats floats expr)
711 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
712 -- Simplify an expression, returning floated binds
714 simplExprF env (Var v) cont = simplVar env v cont
715 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
716 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
717 simplExprF env (Note note expr) cont = simplNote env note expr cont
718 simplExprF env (Cast body co) cont = simplCast env body co cont
719 simplExprF env (App fun arg) cont = simplExprF env fun
720 (ApplyTo NoDup arg (Just env) cont)
722 simplExprF env (Type ty) cont
723 = ASSERT( contIsRhsOrArg cont )
724 simplType env ty `thenSmpl` \ ty' ->
725 rebuild env (Type ty') cont
727 simplExprF env (Case scrut bndr case_ty alts) cont
728 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
729 = -- Simplify the scrutinee with a Select continuation
730 simplExprF env scrut (Select NoDup bndr alts env cont)
733 = -- If case-of-case is off, simply simplify the case expression
734 -- in a vanilla Stop context, and rebuild the result around it
735 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
736 rebuild env case_expr' cont
738 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
739 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
741 simplExprF env (Let (Rec pairs) body) cont
742 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
743 -- NB: bndrs' don't have unfoldings or rules
744 -- We add them as we go down
746 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
747 addFloats env floats $ \ env ->
748 simplExprF env body cont
750 -- A non-recursive let is dealt with by simplNonRecBind
751 simplExprF env (Let (NonRec bndr rhs) body) cont
752 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
753 simplExprF env body cont
756 ---------------------------------
757 simplType :: SimplEnv -> InType -> SimplM OutType
758 -- Kept monadic just so we can do the seqType
760 = seqType new_ty `seq` returnSmpl new_ty
762 new_ty = substTy env ty
766 %************************************************************************
770 %************************************************************************
773 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont -> SimplM FloatsWithExpr
774 simplCast env body co cont
777 | (s1, k1) <- coercionKind co
778 , s1 `tcEqType` k1 = cont
779 addCoerce co1 (CoerceIt co2 cont)
780 | (s1, k1) <- coercionKind co1
781 , (l1, t1) <- coercionKind co2
782 -- coerce T1 S1 (coerce S1 K1 e)
785 -- coerce T1 K1 e, otherwise
787 -- For example, in the initial form of a worker
788 -- we may find (coerce T (coerce S (\x.e))) y
789 -- and we'd like it to simplify to e[y/x] in one round
791 , s1 `coreEqType` t1 = cont -- The coerces cancel out
792 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
794 addCoerce co (ApplyTo dup arg arg_se cont)
795 | not (isTypeArg arg) -- This whole case only works for value args
796 -- Could upgrade to have equiv thing for type apps too
797 , Just (s1s2, t1t2) <- splitCoercionKind_maybe co
799 -- co : s1s2 :=: t1t2
800 -- (coerce (T1->T2) (S1->S2) F) E
802 -- coerce T2 S2 (F (coerce S1 T1 E))
804 -- t1t2 must be a function type, T1->T2, because it's applied
805 -- to something but s1s2 might conceivably not be
807 -- When we build the ApplyTo we can't mix the out-types
808 -- with the InExpr in the argument, so we simply substitute
809 -- to make it all consistent. It's a bit messy.
810 -- But it isn't a common case.
813 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
814 -- t2 :=: s2 with left and right on the curried form:
815 -- (->) t1 t2 :=: (->) s1 s2
816 [co1, co2] = decomposeCo 2 co
817 new_arg = mkCoerce (mkSymCoercion co1) (substExpr arg_env arg)
818 arg_env = setInScope arg_se env
819 result = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
820 addCoerce co cont = CoerceIt co cont
822 simplType env co `thenSmpl` \ co' ->
823 simplExprF env body (addCoerce co' cont)
826 %************************************************************************
830 %************************************************************************
833 simplLam env fun cont
836 zap_it = mkLamBndrZapper fun (countArgs cont)
837 cont_ty = contResultType cont
839 -- Type-beta reduction
840 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) mb_arg_se body_cont)
841 = ASSERT( isTyVar bndr )
842 do { tick (BetaReduction bndr)
843 ; ty_arg' <- case mb_arg_se of
844 Just arg_se -> simplType (setInScope arg_se env) ty_arg
845 Nothing -> return ty_arg
846 ; go (extendTvSubst env bndr ty_arg') body body_cont }
848 -- Ordinary beta reduction
849 go env (Lam bndr body) cont@(ApplyTo _ arg (Just arg_se) body_cont)
850 = do { tick (BetaReduction bndr)
851 ; simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
852 go env body body_cont }
854 go env (Lam bndr body) cont@(ApplyTo _ arg Nothing body_cont)
855 = do { tick (BetaReduction bndr)
856 ; simplNonRecX env (zap_it bndr) arg $ \ env ->
857 go env body body_cont }
859 -- Not enough args, so there are real lambdas left to put in the result
860 go env lam@(Lam _ _) cont
861 = do { (env, bndrs') <- simplLamBndrs env bndrs
862 ; body' <- simplExpr env body
863 ; (floats, new_lam) <- mkLam env bndrs' body' cont
864 ; addFloats env floats $ \ env ->
865 rebuild env new_lam cont }
867 (bndrs,body) = collectBinders lam
869 -- Exactly enough args
870 go env expr cont = simplExprF env expr cont
872 mkLamBndrZapper :: CoreExpr -- Function
873 -> Int -- Number of args supplied, *including* type args
874 -> Id -> Id -- Use this to zap the binders
875 mkLamBndrZapper fun n_args
876 | n_args >= n_params fun = \b -> b -- Enough args
877 | otherwise = \b -> zapLamIdInfo b
879 -- NB: we count all the args incl type args
880 -- so we must count all the binders (incl type lambdas)
881 n_params (Note _ e) = n_params e
882 n_params (Lam b e) = 1 + n_params e
883 n_params other = 0::Int
887 %************************************************************************
891 %************************************************************************
896 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
897 -- inlining. All other CCCSs are mapped to currentCCS.
898 simplNote env (SCC cc) e cont
899 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
900 rebuild env (mkSCC cc e') cont
902 -- See notes with SimplMonad.inlineMode
903 simplNote env InlineMe e cont
904 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
905 = -- Don't inline inside an INLINE expression
906 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
907 rebuild env (mkInlineMe e') cont
909 | otherwise -- Dissolve the InlineMe note if there's
910 -- an interesting context of any kind to combine with
911 -- (even a type application -- anything except Stop)
912 = simplExprF env e cont
914 simplNote env (CoreNote s) e cont
915 = simplExpr env e `thenSmpl` \ e' ->
916 rebuild env (Note (CoreNote s) e') cont
920 %************************************************************************
922 \subsection{Dealing with calls}
924 %************************************************************************
927 simplVar env var cont
928 = case substId env var of
929 DoneEx e -> simplExprF (zapSubstEnv env) e cont
930 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
931 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
932 -- Note [zapSubstEnv]
933 -- The template is already simplified, so don't re-substitute.
934 -- This is VITAL. Consider
936 -- let y = \z -> ...x... in
938 -- We'll clone the inner \x, adding x->x' in the id_subst
939 -- Then when we inline y, we must *not* replace x by x' in
940 -- the inlined copy!!
942 ---------------------------------------------------------
943 -- Dealing with a call site
945 completeCall env var occ_info cont
946 = -- Simplify the arguments
947 getDOptsSmpl `thenSmpl` \ dflags ->
949 chkr = getSwitchChecker env
950 (args, call_cont) = getContArgs chkr var cont
953 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
954 (contResultType call_cont) $ \ env args ->
956 -- Next, look for rules or specialisations that match
958 -- It's important to simplify the args first, because the rule-matcher
959 -- doesn't do substitution as it goes. We don't want to use subst_args
960 -- (defined in the 'where') because that throws away useful occurrence info,
961 -- and perhaps-very-important specialisations.
963 -- Some functions have specialisations *and* are strict; in this case,
964 -- we don't want to inline the wrapper of the non-specialised thing; better
965 -- to call the specialised thing instead.
966 -- We used to use the black-listing mechanism to ensure that inlining of
967 -- the wrapper didn't occur for things that have specialisations till a
968 -- later phase, so but now we just try RULES first
970 -- You might think that we shouldn't apply rules for a loop breaker:
971 -- doing so might give rise to an infinite loop, because a RULE is
972 -- rather like an extra equation for the function:
973 -- RULE: f (g x) y = x+y
976 -- But it's too drastic to disable rules for loop breakers.
977 -- Even the foldr/build rule would be disabled, because foldr
978 -- is recursive, and hence a loop breaker:
979 -- foldr k z (build g) = g k z
980 -- So it's up to the programmer: rules can cause divergence
983 in_scope = getInScope env
985 maybe_rule = case activeRule env of
986 Nothing -> Nothing -- No rules apply
987 Just act_fn -> lookupRule act_fn in_scope rules var args
990 Just (rule_name, rule_rhs) ->
991 tick (RuleFired rule_name) `thenSmpl_`
992 (if dopt Opt_D_dump_inlinings dflags then
993 pprTrace "Rule fired" (vcat [
994 text "Rule:" <+> ftext rule_name,
995 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
996 text "After: " <+> pprCoreExpr rule_rhs,
997 text "Cont: " <+> ppr call_cont])
1000 simplExprF env rule_rhs call_cont ;
1002 Nothing -> -- No rules
1004 -- Next, look for an inlining
1006 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
1007 interesting_cont = interestingCallContext (notNull args)
1010 active_inline = activeInline env var occ_info
1011 maybe_inline = callSiteInline dflags active_inline occ_info
1012 var arg_infos interesting_cont
1014 case maybe_inline of {
1015 Just unfolding -- There is an inlining!
1016 -> tick (UnfoldingDone var) `thenSmpl_`
1017 (if dopt Opt_D_dump_inlinings dflags then
1018 pprTrace "Inlining done" (vcat [
1019 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1020 text "Inlined fn: " <+> ppr unfolding,
1021 text "Cont: " <+> ppr call_cont])
1024 simplExprF env unfolding (pushContArgs args call_cont)
1027 Nothing -> -- No inlining!
1030 rebuild env (mkApps (Var var) args) call_cont
1034 %************************************************************************
1036 \subsection{Arguments}
1038 %************************************************************************
1041 ---------------------------------------------------------
1042 -- Simplifying the arguments of a call
1044 simplifyArgs :: SimplEnv
1045 -> OutType -- Type of the function
1046 -> Bool -- True if the fn has RULES
1047 -> [(InExpr, Maybe SimplEnv, Bool)] -- Details of the arguments
1048 -> OutType -- Type of the continuation
1049 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1050 -> SimplM FloatsWithExpr
1052 -- [CPS-like because of strict arguments]
1054 -- Simplify the arguments to a call.
1055 -- This part of the simplifier may break the no-shadowing invariant
1057 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1058 -- where f is strict in its second arg
1059 -- If we simplify the innermost one first we get (...(\a -> e)...)
1060 -- Simplifying the second arg makes us float the case out, so we end up with
1061 -- case y of (a,b) -> f (...(\a -> e)...) e'
1062 -- So the output does not have the no-shadowing invariant. However, there is
1063 -- no danger of getting name-capture, because when the first arg was simplified
1064 -- we used an in-scope set that at least mentioned all the variables free in its
1065 -- static environment, and that is enough.
1067 -- We can't just do innermost first, or we'd end up with a dual problem:
1068 -- case x of (a,b) -> f e (...(\a -> e')...)
1070 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1071 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1072 -- continuations. We have to keep the right in-scope set around; AND we have
1073 -- to get the effect that finding (error "foo") in a strict arg position will
1074 -- discard the entire application and replace it with (error "foo"). Getting
1075 -- all this at once is TOO HARD!
1077 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1078 = go env fn_ty args thing_inside
1080 go env fn_ty [] thing_inside = thing_inside env []
1081 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1082 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1083 thing_inside env (arg':args')
1085 simplifyArg env fn_ty has_rules (arg, Nothing, _) cont_ty thing_inside
1086 = thing_inside env arg -- Already simplified
1088 simplifyArg env fn_ty has_rules (Type ty_arg, Just se, _) cont_ty thing_inside
1089 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1090 thing_inside env (Type new_ty_arg)
1092 simplifyArg env fn_ty has_rules (val_arg, Just arg_se, is_strict) cont_ty thing_inside
1094 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1096 | otherwise -- Lazy argument
1097 -- DO NOT float anything outside, hence simplExprC
1098 -- There is no benefit (unlike in a let-binding), and we'd
1099 -- have to be very careful about bogus strictness through
1100 -- floating a demanded let.
1101 = simplExprC (setInScope arg_se env) val_arg
1102 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1103 thing_inside env arg1
1105 arg_ty = funArgTy fn_ty
1108 simplStrictArg :: LetRhsFlag
1109 -> SimplEnv -- The env of the call
1110 -> InExpr -> SimplEnv -- The arg plus its env
1111 -> OutType -- arg_ty: type of the argument
1112 -> OutType -- cont_ty: Type of thing computed by the context
1113 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1114 -- Takes an expression of type rhs_ty,
1115 -- returns an expression of type cont_ty
1116 -- The env passed to this continuation is the
1117 -- env of the call, plus any new in-scope variables
1118 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1120 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1121 = simplExprF (setInScope arg_env call_env) arg
1122 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1123 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1124 -- to simplify the argument
1125 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1129 %************************************************************************
1131 \subsection{mkAtomicArgs}
1133 %************************************************************************
1135 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1136 constructor application and, if so, converts it to ANF, so that the
1137 resulting thing can be inlined more easily. Thus
1144 There are three sorts of binding context, specified by the two
1150 N N Top-level or recursive Only bind args of lifted type
1152 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1153 but lazy unlifted-and-ok-for-speculation
1155 Y Y Non-top-level, non-recursive, Bind all args
1156 and strict (demanded)
1163 there is no point in transforming to
1165 x = case (y div# z) of r -> MkC r
1167 because the (y div# z) can't float out of the let. But if it was
1168 a *strict* let, then it would be a good thing to do. Hence the
1169 context information.
1172 mkAtomicArgsE :: SimplEnv
1173 -> Bool -- A strict binding
1174 -> OutExpr -- The rhs
1175 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1176 -> SimplM FloatsWithExpr
1178 mkAtomicArgsE env is_strict rhs thing_inside
1179 | (Var fun, args) <- collectArgs rhs, -- It's an application
1180 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1181 = go env (Var fun) args
1183 | otherwise = thing_inside env rhs
1186 go env fun [] = thing_inside env fun
1188 go env fun (arg : args)
1189 | exprIsTrivial arg -- Easy case
1190 || no_float_arg -- Can't make it atomic
1191 = go env (App fun arg) args
1194 = do { arg_id <- newId FSLIT("a") arg_ty
1195 ; completeNonRecX env False {- pessimistic -} arg_id arg_id arg $ \env ->
1196 go env (App fun (Var arg_id)) args }
1198 arg_ty = exprType arg
1199 no_float_arg = not is_strict && (isUnLiftedType arg_ty) && not (exprOkForSpeculation arg)
1202 -- Old code: consider rewriting to be more like mkAtomicArgsE
1204 mkAtomicArgs :: Bool -- A strict binding
1205 -> Bool -- OK to float unlifted args
1207 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1208 OutExpr) -- things that need case-binding,
1209 -- if the strict-binding flag is on
1211 mkAtomicArgs is_strict ok_float_unlifted rhs
1212 | (Var fun, args) <- collectArgs rhs, -- It's an application
1213 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1214 = go fun nilOL [] args -- Have a go
1216 | otherwise = bale_out -- Give up
1219 bale_out = returnSmpl (nilOL, rhs)
1221 go fun binds rev_args []
1222 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1224 go fun binds rev_args (arg : args)
1225 | exprIsTrivial arg -- Easy case
1226 = go fun binds (arg:rev_args) args
1228 | not can_float_arg -- Can't make this arg atomic
1229 = bale_out -- ... so give up
1231 | otherwise -- Don't forget to do it recursively
1232 -- E.g. x = a:b:c:[]
1233 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1234 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1235 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1236 (Var arg_id : rev_args) args
1238 arg_ty = exprType arg
1239 can_float_arg = is_strict
1240 || not (isUnLiftedType arg_ty)
1241 || (ok_float_unlifted && exprOkForSpeculation arg)
1244 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1245 -> (SimplEnv -> SimplM (FloatsWith a))
1246 -> SimplM (FloatsWith a)
1247 addAtomicBinds env [] thing_inside = thing_inside env
1248 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1249 addAtomicBinds env bs thing_inside
1253 %************************************************************************
1255 \subsection{The main rebuilder}
1257 %************************************************************************
1260 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1262 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1263 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1264 rebuild env expr (CoerceIt co cont) = rebuild env (mkCoerce co expr) cont
1265 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1266 rebuild env expr (ApplyTo _ arg mb_se cont) = rebuildApp env expr arg mb_se cont
1268 rebuildApp env fun arg mb_se cont
1269 = do { arg' <- simplArg env arg mb_se
1270 ; rebuild env (App fun arg') cont }
1272 simplArg :: SimplEnv -> CoreExpr -> Maybe SimplEnv -> SimplM CoreExpr
1273 simplArg env arg Nothing = return arg -- The arg is already simplified
1274 simplArg env arg (Just arg_env) = simplExpr (setInScope arg_env env) arg
1276 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1280 %************************************************************************
1282 \subsection{Functions dealing with a case}
1284 %************************************************************************
1286 Blob of helper functions for the "case-of-something-else" situation.
1289 ---------------------------------------------------------
1290 -- Eliminate the case if possible
1292 rebuildCase :: SimplEnv
1293 -> OutExpr -- Scrutinee
1294 -> InId -- Case binder
1295 -> [InAlt] -- Alternatives (inceasing order)
1297 -> SimplM FloatsWithExpr
1299 rebuildCase env scrut case_bndr alts cont
1300 | Just (con,args) <- exprIsConApp_maybe scrut
1301 -- Works when the scrutinee is a variable with a known unfolding
1302 -- as well as when it's an explicit constructor application
1303 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1305 | Lit lit <- scrut -- No need for same treatment as constructors
1306 -- because literals are inlined more vigorously
1307 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1310 = -- Prepare the continuation;
1311 -- The new subst_env is in place
1312 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1313 addFloats env floats $ \ env ->
1316 -- The case expression is annotated with the result type of the continuation
1317 -- This may differ from the type originally on the case. For example
1318 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1321 -- let j a# = <blob>
1322 -- in case(T) a of { True -> j 1#; False -> j 0# }
1323 -- Note that the case that scrutinises a now returns a T not an Int#
1324 res_ty' = contResultType dup_cont
1327 -- Deal with case binder
1328 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1330 -- Deal with the case alternatives
1331 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1333 -- Put the case back together
1334 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1336 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1337 -- The case binder *not* scope over the whole returned case-expression
1338 rebuild env case_expr nondup_cont
1341 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1342 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1343 way, there's a chance that v will now only be used once, and hence
1348 There is a time we *don't* want to do that, namely when
1349 -fno-case-of-case is on. This happens in the first simplifier pass,
1350 and enhances full laziness. Here's the bad case:
1351 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1352 If we eliminate the inner case, we trap it inside the I# v -> arm,
1353 which might prevent some full laziness happening. I've seen this
1354 in action in spectral/cichelli/Prog.hs:
1355 [(m,n) | m <- [1..max], n <- [1..max]]
1356 Hence the check for NoCaseOfCase.
1360 There is another situation when we don't want to do it. If we have
1362 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1363 ...other cases .... }
1365 We'll perform the binder-swap for the outer case, giving
1367 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1368 ...other cases .... }
1370 But there is no point in doing it for the inner case, because w1 can't
1371 be inlined anyway. Furthermore, doing the case-swapping involves
1372 zapping w2's occurrence info (see paragraphs that follow), and that
1373 forces us to bind w2 when doing case merging. So we get
1375 case x of w1 { A -> let w2 = w1 in e1
1376 B -> let w2 = w1 in e2
1377 ...other cases .... }
1379 This is plain silly in the common case where w2 is dead.
1381 Even so, I can't see a good way to implement this idea. I tried
1382 not doing the binder-swap if the scrutinee was already evaluated
1383 but that failed big-time:
1387 case v of w { MkT x ->
1388 case x of x1 { I# y1 ->
1389 case x of x2 { I# y2 -> ...
1391 Notice that because MkT is strict, x is marked "evaluated". But to
1392 eliminate the last case, we must either make sure that x (as well as
1393 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1394 the binder-swap. So this whole note is a no-op.
1398 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1399 any occurrence info (eg IAmDead) in the case binder, because the
1400 case-binder now effectively occurs whenever v does. AND we have to do
1401 the same for the pattern-bound variables! Example:
1403 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1405 Here, b and p are dead. But when we move the argment inside the first
1406 case RHS, and eliminate the second case, we get
1408 case x of { (a,b) -> a b }
1410 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1413 Indeed, this can happen anytime the case binder isn't dead:
1414 case <any> of x { (a,b) ->
1415 case x of { (p,q) -> p } }
1416 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1417 The point is that we bring into the envt a binding
1419 after the outer case, and that makes (a,b) alive. At least we do unless
1420 the case binder is guaranteed dead.
1423 simplCaseBinder env (Var v) case_bndr
1424 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1426 -- Failed try [see Note 2 above]
1427 -- not (isEvaldUnfolding (idUnfolding v))
1429 = simplBinder env (zapOccInfo case_bndr) `thenSmpl` \ (env, case_bndr') ->
1430 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1431 -- We could extend the substitution instead, but it would be
1432 -- a hack because then the substitution wouldn't be idempotent
1433 -- any more (v is an OutId). And this does just as well.
1435 simplCaseBinder env other_scrut case_bndr
1436 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1437 returnSmpl (env, case_bndr')
1439 zapOccInfo :: InId -> InId
1440 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1444 simplAlts does two things:
1446 1. Eliminate alternatives that cannot match, including the
1447 DEFAULT alternative.
1449 2. If the DEFAULT alternative can match only one possible constructor,
1450 then make that constructor explicit.
1452 case e of x { DEFAULT -> rhs }
1454 case e of x { (a,b) -> rhs }
1455 where the type is a single constructor type. This gives better code
1456 when rhs also scrutinises x or e.
1458 Here "cannot match" includes knowledge from GADTs
1460 It's a good idea do do this stuff before simplifying the alternatives, to
1461 avoid simplifying alternatives we know can't happen, and to come up with
1462 the list of constructors that are handled, to put into the IdInfo of the
1463 case binder, for use when simplifying the alternatives.
1465 Eliminating the default alternative in (1) isn't so obvious, but it can
1468 data Colour = Red | Green | Blue
1477 DEFAULT -> [ case y of ... ]
1479 If we inline h into f, the default case of the inlined h can't happen.
1480 If we don't notice this, we may end up filtering out *all* the cases
1481 of the inner case y, which give us nowhere to go!
1485 simplAlts :: SimplEnv
1487 -> OutId -- Case binder
1488 -> [InAlt] -> SimplCont
1489 -> SimplM [OutAlt] -- Includes the continuation
1491 simplAlts env scrut case_bndr' alts cont'
1492 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1493 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1494 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1495 -- We need the mergeAlts in case the new default_alt
1496 -- has turned into a constructor alternative.
1498 (alts_wo_default, maybe_deflt) = findDefault alts
1499 imposs_cons = case scrut of
1500 Var v -> otherCons (idUnfolding v)
1503 -- "imposs_deflt_cons" are handled either by the context,
1504 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1505 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1507 simplDefault :: SimplEnv
1508 -> OutId -- Case binder; need just for its type. Note that as an
1509 -- OutId, it has maximum information; this is important.
1510 -- Test simpl013 is an example
1511 -> [AltCon] -- These cons can't happen when matching the default
1514 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1515 -- mergeAlts expects
1518 simplDefault env case_bndr' imposs_cons cont Nothing
1519 = 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 ; con_alt <- mkDataConAlt con inst_tys rhs
1553 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1554 -- The simplAlt must succeed with Just because we have
1555 -- already filtered out construtors that can't match
1558 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1561 = simplify_default imposs_cons
1563 cant_match tys data_con = not (dataConCanMatch data_con tys)
1565 simplify_default imposs_cons
1566 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1567 -- Record the constructors that the case-binder *can't* be.
1568 ; rhs' <- simplExprC env' rhs cont
1569 ; return [(DEFAULT, [], rhs')] }
1571 simplAlt :: SimplEnv
1572 -> [AltCon] -- These constructors can't be present when
1573 -- matching this alternative
1574 -> OutId -- The case binder
1577 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1579 -- Simplify an alternative, returning the type refinement for the
1580 -- alternative, if the alternative does any refinement at all
1581 -- Nothing => the alternative is inaccessible
1583 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1584 | con `elem` imposs_cons -- This case can't match
1587 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1588 -- TURGID DUPLICATION, needed only for the simplAlt call
1589 -- in mkDupableAlt. Clean this up when moving to FC
1590 = ASSERT( null bndrs )
1591 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1592 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1594 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1595 -- Record the constructors that the case-binder *can't* be.
1597 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1598 = ASSERT( null bndrs )
1599 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1600 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1602 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1604 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1605 = -- Deal with the pattern-bound variables
1606 -- Mark the ones that are in ! positions in the data constructor
1607 -- as certainly-evaluated.
1608 -- NB: it happens that simplBinders does *not* erase the OtherCon
1609 -- form of unfolding, so it's ok to add this info before
1610 -- doing simplBinders
1611 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1613 -- Bind the case-binder to (con args)
1614 let unf = mkUnfolding False (mkConApp con con_args)
1615 inst_tys' = tyConAppArgs (idType case_bndr')
1616 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1617 env' = mk_rhs_env env case_bndr' unf
1619 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1620 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1622 -- add_evals records the evaluated-ness of the bound variables of
1623 -- a case pattern. This is *important*. Consider
1624 -- data T = T !Int !Int
1626 -- case x of { T a b -> T (a+1) b }
1628 -- We really must record that b is already evaluated so that we don't
1629 -- go and re-evaluate it when constructing the result.
1630 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1632 cat_evals dc vs strs
1636 go (v:vs) strs | isTyVar v = v : go vs strs
1637 go (v:vs) (str:strs)
1638 | isMarkedStrict str = evald_v : go vs strs
1639 | otherwise = zapped_v : go vs strs
1641 zapped_v = zap_occ_info v
1642 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1643 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1645 -- If the case binder is alive, then we add the unfolding
1647 -- to the envt; so vs are now very much alive
1648 -- Note [Aug06] I can't see why this actually matters
1649 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1650 | otherwise = zapOccInfo
1652 mk_rhs_env env case_bndr' case_bndr_unf
1653 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1657 %************************************************************************
1659 \subsection{Known constructor}
1661 %************************************************************************
1663 We are a bit careful with occurrence info. Here's an example
1665 (\x* -> case x of (a*, b) -> f a) (h v, e)
1667 where the * means "occurs once". This effectively becomes
1668 case (h v, e) of (a*, b) -> f a)
1670 let a* = h v; b = e in f a
1674 All this should happen in one sweep.
1677 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1678 -> InId -> [InAlt] -> SimplCont
1679 -> SimplM FloatsWithExpr
1681 knownCon env scrut con args bndr alts cont
1682 = tick (KnownBranch bndr) `thenSmpl_`
1683 case findAlt con alts of
1684 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1685 simplNonRecX env bndr scrut $ \ env ->
1686 -- This might give rise to a binding with non-atomic args
1687 -- like x = Node (f x) (g x)
1688 -- but simplNonRecX will atomic-ify it
1689 simplExprF env rhs cont
1691 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1692 simplNonRecX env bndr scrut $ \ env ->
1693 simplExprF env rhs cont
1695 (DataAlt dc, bs, rhs)
1696 -> ASSERT( n_drop_tys + length bs == length args )
1697 bind_args env dead_bndr bs (drop n_drop_tys args) $ \ env ->
1699 -- It's useful to bind bndr to scrut, rather than to a fresh
1700 -- binding x = Con arg1 .. argn
1701 -- because very often the scrut is a variable, so we avoid
1702 -- creating, and then subsequently eliminating, a let-binding
1703 -- BUT, if scrut is a not a variable, we must be careful
1704 -- about duplicating the arg redexes; in that case, make
1705 -- a new con-app from the args
1706 bndr_rhs = case scrut of
1709 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1710 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1711 -- args are aready OutExprs, but bs are InIds
1713 simplNonRecX env bndr bndr_rhs $ \ env ->
1714 simplExprF env rhs cont
1716 n_drop_tys = tyConArity (dataConTyCon dc)
1719 bind_args env dead_bndr [] _ thing_inside = thing_inside env
1721 bind_args env dead_bndr (b:bs) (Type ty : args) thing_inside
1722 = ASSERT( isTyVar b )
1723 bind_args (extendTvSubst env b ty) dead_bndr bs args thing_inside
1725 bind_args env dead_bndr (b:bs) (arg : args) thing_inside
1728 b' = if dead_bndr then b else zapOccInfo b
1729 -- Note that the binder might be "dead", because it doesn't occur
1730 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1731 -- Nevertheless we must keep it if the case-binder is alive, because it may
1732 -- be used in teh con_app
1734 simplNonRecX env b' arg $ \ env ->
1735 bind_args env dead_bndr bs args thing_inside
1739 %************************************************************************
1741 \subsection{Duplicating continuations}
1743 %************************************************************************
1746 prepareCaseCont :: SimplEnv
1747 -> [InAlt] -> SimplCont
1748 -> SimplM (FloatsWith (SimplCont,SimplCont))
1749 -- Return a duplicatable continuation, a non-duplicable part
1750 -- plus some extra bindings (that scope over the entire
1753 -- No need to make it duplicatable if there's only one alternative
1754 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1755 prepareCaseCont env alts cont = mkDupableCont env cont
1759 mkDupableCont :: SimplEnv -> SimplCont
1760 -> SimplM (FloatsWith (SimplCont, SimplCont))
1762 mkDupableCont env cont
1763 | contIsDupable cont
1764 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1766 mkDupableCont env (CoerceIt ty cont)
1767 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1768 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1770 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1771 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1772 -- Do *not* duplicate an ArgOf continuation
1773 -- Because ArgOf continuations are opaque, we gain nothing by
1774 -- propagating them into the expressions, and we do lose a lot.
1775 -- Here's an example:
1776 -- && (case x of { T -> F; F -> T }) E
1777 -- Now, && is strict so we end up simplifying the case with
1778 -- an ArgOf continuation. If we let-bind it, we get
1780 -- let $j = \v -> && v E
1781 -- in simplExpr (case x of { T -> F; F -> T })
1782 -- (ArgOf (\r -> $j r)
1783 -- And after simplifying more we get
1785 -- let $j = \v -> && v E
1786 -- in case of { T -> $j F; F -> $j T }
1787 -- Which is a Very Bad Thing
1789 -- The desire not to duplicate is the entire reason that
1790 -- mkDupableCont returns a pair of continuations.
1792 -- The original plan had:
1793 -- e.g. (...strict-fn...) [...hole...]
1795 -- let $j = \a -> ...strict-fn...
1796 -- in $j [...hole...]
1798 mkDupableCont env (ApplyTo _ arg mb_se cont)
1799 = -- e.g. [...hole...] (...arg...)
1801 -- let a = ...arg...
1802 -- in [...hole...] a
1803 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1804 ; addFloats env floats $ \ env -> do
1805 { arg1 <- simplArg env arg mb_se
1806 ; (floats2, arg2) <- mkDupableArg env arg1
1807 ; return (floats2, (ApplyTo OkToDup arg2 Nothing dup_cont, nondup_cont)) }}
1809 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1810 -- | not (exprIsDupable rhs && contIsDupable case_cont) -- See notes below
1811 -- | not (isDeadBinder case_bndr)
1812 | all isDeadBinder bs
1813 = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
1815 scrut_ty = substTy se (idType case_bndr)
1817 {- Note [Single-alternative cases]
1818 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1819 This case is just like the ArgOf case. Here's an example:
1823 case (case x of I# x' ->
1825 True -> I# (negate# x')
1826 False -> I# x') of y {
1828 Because the (case x) has only one alternative, we'll transform to
1830 case (case x' <# 0# of
1831 True -> I# (negate# x')
1832 False -> I# x') of y {
1834 But now we do *NOT* want to make a join point etc, giving
1836 let $j = \y -> MkT y
1838 True -> $j (I# (negate# x'))
1840 In this case the $j will inline again, but suppose there was a big
1841 strict computation enclosing the orginal call to MkT. Then, it won't
1842 "see" the MkT any more, because it's big and won't get duplicated.
1843 And, what is worse, nothing was gained by the case-of-case transform.
1845 When should use this case of mkDupableCont?
1846 However, matching on *any* single-alternative case is a *disaster*;
1847 e.g. case (case ....) of (a,b) -> (# a,b #)
1848 We must push the outer case into the inner one!
1851 * Match [(DEFAULT,_,_)], but in the common case of Int,
1852 the alternative-filling-in code turned the outer case into
1853 case (...) of y { I# _ -> MkT y }
1855 * Match on single alternative plus (not (isDeadBinder case_bndr))
1856 Rationale: pushing the case inwards won't eliminate the construction.
1857 But there's a risk of
1858 case (...) of y { (a,b) -> let z=(a,b) in ... }
1859 Now y looks dead, but it'll come alive again. Still, this
1860 seems like the best option at the moment.
1862 * Match on single alternative plus (all (isDeadBinder bndrs))
1863 Rationale: this is essentially seq.
1865 * Match when the rhs is *not* duplicable, and hence would lead to a
1866 join point. This catches the disaster-case above. We can test
1867 the *un-simplified* rhs, which is fine. It might get bigger or
1868 smaller after simplification; if it gets smaller, this case might
1869 fire next time round. NB also that we must test contIsDupable
1870 case_cont *btoo, because case_cont might be big!
1872 HOWEVER: I found that this version doesn't work well, because
1873 we can get let x = case (...) of { small } in ...case x...
1874 When x is inlined into its full context, we find that it was a bad
1875 idea to have pushed the outer case inside the (...) case.
1878 mkDupableCont env (Select _ case_bndr alts se cont)
1879 = -- e.g. (case [...hole...] of { pi -> ei })
1881 -- let ji = \xij -> ei
1882 -- in case [...hole...] of { pi -> ji xij }
1883 do { tick (CaseOfCase case_bndr)
1884 ; let alt_env = setInScope se env
1885 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1886 -- NB: call mkDupableCont here, *not* prepareCaseCont
1887 -- We must make a duplicable continuation, whereas prepareCaseCont
1888 -- doesn't when there is a single case branch
1889 ; addFloats alt_env floats1 $ \ alt_env -> do
1891 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1892 -- NB: simplBinder does not zap deadness occ-info, so
1893 -- a dead case_bndr' will still advertise its deadness
1894 -- This is really important because in
1895 -- case e of b { (# a,b #) -> ... }
1896 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1897 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1898 -- In the new alts we build, we have the new case binder, so it must retain
1901 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1902 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1903 (mkBoringStop (contResultType dup_cont)),
1907 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1908 -- Let-bind the thing if necessary
1909 mkDupableArg env arg
1911 = return (emptyFloats env, arg)
1913 = do { arg_id <- newId FSLIT("a") (exprType arg)
1914 ; tick (CaseOfCase arg_id)
1915 -- Want to tick here so that we go round again,
1916 -- and maybe copy or inline the code.
1917 -- Not strictly CaseOfCase, but never mind
1918 ; return (unitFloat env arg_id arg, Var arg_id) }
1919 -- What if the arg should be case-bound?
1920 -- This has been this way for a long time, so I'll leave it,
1921 -- but I can't convince myself that it's right.
1923 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1924 -> SimplM (FloatsWith [InAlt])
1925 -- Absorbs the continuation into the new alternatives
1927 mkDupableAlts env case_bndr' alts dupable_cont
1930 go env [] = returnSmpl (emptyFloats env, [])
1932 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1933 ; addFloats env floats1 $ \ env -> do
1934 { (floats2, alts') <- go env alts
1935 ; returnSmpl (floats2, case mb_alt' of
1936 Just alt' -> alt' : alts'
1940 mkDupableAlt env case_bndr' cont alt
1941 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1943 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1945 Just (reft, (con, bndrs', rhs')) ->
1946 -- Safe to say that there are no handled-cons for the DEFAULT case
1948 if exprIsDupable rhs' then
1949 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
1950 -- It is worth checking for a small RHS because otherwise we
1951 -- get extra let bindings that may cause an extra iteration of the simplifier to
1952 -- inline back in place. Quite often the rhs is just a variable or constructor.
1953 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1954 -- iterations because the version with the let bindings looked big, and so wasn't
1955 -- inlined, but after the join points had been inlined it looked smaller, and so
1958 -- NB: we have to check the size of rhs', not rhs.
1959 -- Duplicating a small InAlt might invalidate occurrence information
1960 -- However, if it *is* dupable, we return the *un* simplified alternative,
1961 -- because otherwise we'd need to pair it up with an empty subst-env....
1962 -- but we only have one env shared between all the alts.
1963 -- (Remember we must zap the subst-env before re-simplifying something).
1964 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1968 rhs_ty' = exprType rhs'
1969 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1971 | isTyVar bndr = not (bndr `elemVarEnv` reft)
1972 -- Don't abstract over tyvar binders which are refined away
1973 -- See Note [Refinement] below
1974 | otherwise = not (isDeadBinder bndr)
1975 -- The deadness info on the new Ids is preserved by simplBinders
1977 -- If we try to lift a primitive-typed something out
1978 -- for let-binding-purposes, we will *caseify* it (!),
1979 -- with potentially-disastrous strictness results. So
1980 -- instead we turn it into a function: \v -> e
1981 -- where v::State# RealWorld#. The value passed to this function
1982 -- is realworld#, which generates (almost) no code.
1984 -- There's a slight infelicity here: we pass the overall
1985 -- case_bndr to all the join points if it's used in *any* RHS,
1986 -- because we don't know its usage in each RHS separately
1988 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1989 -- we make the join point into a function whenever used_bndrs'
1990 -- is empty. This makes the join-point more CPR friendly.
1991 -- Consider: let j = if .. then I# 3 else I# 4
1992 -- in case .. of { A -> j; B -> j; C -> ... }
1994 -- Now CPR doesn't w/w j because it's a thunk, so
1995 -- that means that the enclosing function can't w/w either,
1996 -- which is a lose. Here's the example that happened in practice:
1997 -- kgmod :: Int -> Int -> Int
1998 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2002 -- I have seen a case alternative like this:
2003 -- True -> \v -> ...
2004 -- It's a bit silly to add the realWorld dummy arg in this case, making
2007 -- (the \v alone is enough to make CPR happy) but I think it's rare
2009 ( if not (any isId used_bndrs')
2010 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
2011 returnSmpl ([rw_id], [Var realWorldPrimId])
2013 returnSmpl (used_bndrs', varsToCoreExprs used_bndrs')
2014 ) `thenSmpl` \ (final_bndrs', final_args) ->
2016 -- See comment about "$j" name above
2017 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
2018 -- Notice the funky mkPiTypes. If the contructor has existentials
2019 -- it's possible that the join point will be abstracted over
2020 -- type varaibles as well as term variables.
2021 -- Example: Suppose we have
2022 -- data T = forall t. C [t]
2024 -- case (case e of ...) of
2025 -- C t xs::[t] -> rhs
2026 -- We get the join point
2027 -- let j :: forall t. [t] -> ...
2028 -- j = /\t \xs::[t] -> rhs
2030 -- case (case e of ...) of
2031 -- C t xs::[t] -> j t xs
2033 -- We make the lambdas into one-shot-lambdas. The
2034 -- join point is sure to be applied at most once, and doing so
2035 -- prevents the body of the join point being floated out by
2036 -- the full laziness pass
2037 really_final_bndrs = map one_shot final_bndrs'
2038 one_shot v | isId v = setOneShotLambda v
2040 join_rhs = mkLams really_final_bndrs rhs'
2041 join_call = mkApps (Var join_bndr) final_args
2043 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2050 MkT :: a -> b -> T a
2054 MkT a' b (p::a') (q::b) -> [p,w]
2056 The danger is that we'll make a join point
2060 and that's ill-typed, because (p::a') but (w::a).
2062 Solution so far: don't abstract over a', because the type refinement
2063 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2065 Then we must abstract over any refined free variables. Hmm. Maybe we
2066 could just abstract over *all* free variables, thereby lambda-lifting
2067 the join point? We should try this.