2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[Simplify]{The main module of the simplifier}
7 module Simplify ( simplTopBinds, simplExpr ) where
9 #include "HsVersions.h"
11 import DynFlags ( dopt, DynFlag(Opt_D_dump_inlinings),
16 import SimplUtils ( mkCase, mkLam,
17 SimplCont(..), DupFlag(..), LetRhsFlag(..),
18 mkRhsStop, mkBoringStop, mkLazyArgStop, pushContArgs,
19 contResultType, countArgs, contIsDupable, contIsRhsOrArg,
20 getContArgs, interestingCallContext, interestingArg, isStrictType,
21 preInlineUnconditionally, postInlineUnconditionally,
22 interestingArgContext, inlineMode, activeInline, activeRule
24 import Id ( Id, idType, idInfo, idArity, isDataConWorkId,
25 idUnfolding, setIdUnfolding, isDeadBinder,
26 idNewDemandInfo, setIdInfo,
27 setIdOccInfo, zapLamIdInfo, setOneShotLambda
29 import MkId ( eRROR_ID )
30 import Literal ( mkStringLit )
31 import IdInfo ( OccInfo(..), isLoopBreaker,
32 setArityInfo, zapDemandInfo,
36 import NewDemand ( isStrictDmd )
37 import Unify ( coreRefineTys, dataConCanMatch )
38 import DataCon ( DataCon, dataConTyCon, dataConRepStrictness, isVanillaDataCon,
39 dataConInstArgTys, dataConTyVars )
40 import TyCon ( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
42 import PprCore ( pprParendExpr, pprCoreExpr )
43 import CoreUnfold ( mkUnfolding, callSiteInline )
44 import CoreUtils ( exprIsDupable, exprIsTrivial, needsCaseBinding,
45 exprIsConApp_maybe, mkPiTypes, findAlt,
46 exprType, exprIsHNF, findDefault, mergeAlts,
47 exprOkForSpeculation, exprArity,
48 mkCoerce, mkCoerce2, mkSCC, mkInlineMe, applyTypeToArg
50 import Rules ( lookupRule )
51 import BasicTypes ( isMarkedStrict )
52 import CostCentre ( currentCCS )
53 import Type ( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
54 splitFunTy_maybe, splitFunTy, coreEqType, splitTyConApp_maybe,
57 import Var ( tyVarKind, mkTyVar )
58 import VarEnv ( elemVarEnv, emptyVarEnv )
59 import TysPrim ( realWorldStatePrimTy )
60 import PrelInfo ( realWorldPrimId )
61 import BasicTypes ( TopLevelFlag(..), isTopLevel,
64 import Name ( mkSysTvName )
65 import StaticFlags ( opt_PprStyle_Debug )
68 import Maybes ( orElse )
70 import Util ( notNull, filterOut )
74 The guts of the simplifier is in this module, but the driver loop for
75 the simplifier is in SimplCore.lhs.
78 -----------------------------------------
79 *** IMPORTANT NOTE ***
80 -----------------------------------------
81 The simplifier used to guarantee that the output had no shadowing, but
82 it does not do so any more. (Actually, it never did!) The reason is
83 documented with simplifyArgs.
86 -----------------------------------------
87 *** IMPORTANT NOTE ***
88 -----------------------------------------
89 Many parts of the simplifier return a bunch of "floats" as well as an
90 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
92 All "floats" are let-binds, not case-binds, but some non-rec lets may
93 be unlifted (with RHS ok-for-speculation).
97 -----------------------------------------
98 ORGANISATION OF FUNCTIONS
99 -----------------------------------------
101 - simplify all top-level binders
102 - for NonRec, call simplRecOrTopPair
103 - for Rec, call simplRecBind
106 ------------------------------
107 simplExpr (applied lambda) ==> simplNonRecBind
108 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
109 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
111 ------------------------------
112 simplRecBind [binders already simplfied]
113 - use simplRecOrTopPair on each pair in turn
115 simplRecOrTopPair [binder already simplified]
116 Used for: recursive bindings (top level and nested)
117 top-level non-recursive bindings
119 - check for PreInlineUnconditionally
123 Used for: non-top-level non-recursive bindings
124 beta reductions (which amount to the same thing)
125 Because it can deal with strict arts, it takes a
126 "thing-inside" and returns an expression
128 - check for PreInlineUnconditionally
129 - simplify binder, including its IdInfo
138 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
139 Used for: binding case-binder and constr args in a known-constructor case
140 - check for PreInLineUnconditionally
144 ------------------------------
145 simplLazyBind: [binder already simplified, RHS not]
146 Used for: recursive bindings (top level and nested)
147 top-level non-recursive bindings
148 non-top-level, but *lazy* non-recursive bindings
149 [must not be strict or unboxed]
150 Returns floats + an augmented environment, not an expression
151 - substituteIdInfo and add result to in-scope
152 [so that rules are available in rec rhs]
155 - float if exposes constructor or PAP
159 completeNonRecX: [binder and rhs both simplified]
160 - if the the thing needs case binding (unlifted and not ok-for-spec)
166 completeLazyBind: [given a simplified RHS]
167 [used for both rec and non-rec bindings, top level and not]
168 - try PostInlineUnconditionally
169 - add unfolding [this is the only place we add an unfolding]
174 Right hand sides and arguments
175 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
176 In many ways we want to treat
177 (a) the right hand side of a let(rec), and
178 (b) a function argument
179 in the same way. But not always! In particular, we would
180 like to leave these arguments exactly as they are, so they
181 will match a RULE more easily.
186 It's harder to make the rule match if we ANF-ise the constructor,
187 or eta-expand the PAP:
189 f (let { a = g x; b = h x } in (a,b))
192 On the other hand if we see the let-defns
197 then we *do* want to ANF-ise and eta-expand, so that p and q
198 can be safely inlined.
200 Even floating lets out is a bit dubious. For let RHS's we float lets
201 out if that exposes a value, so that the value can be inlined more vigorously.
204 r = let x = e in (x,x)
206 Here, if we float the let out we'll expose a nice constructor. We did experiments
207 that showed this to be a generally good thing. But it was a bad thing to float
208 lets out unconditionally, because that meant they got allocated more often.
210 For function arguments, there's less reason to expose a constructor (it won't
211 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
212 So for the moment we don't float lets out of function arguments either.
217 For eta expansion, we want to catch things like
219 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
221 If the \x was on the RHS of a let, we'd eta expand to bring the two
222 lambdas together. And in general that's a good thing to do. Perhaps
223 we should eta expand wherever we find a (value) lambda? Then the eta
224 expansion at a let RHS can concentrate solely on the PAP case.
227 %************************************************************************
229 \subsection{Bindings}
231 %************************************************************************
234 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
236 simplTopBinds env binds
237 = -- Put all the top-level binders into scope at the start
238 -- so that if a transformation rule has unexpectedly brought
239 -- anything into scope, then we don't get a complaint about that.
240 -- It's rather as if the top-level binders were imported.
241 simplRecBndrs env (bindersOfBinds binds) `thenSmpl` \ (env, bndrs') ->
242 simpl_binds env binds bndrs' `thenSmpl` \ (floats, _) ->
243 freeTick SimplifierDone `thenSmpl_`
244 returnSmpl (floatBinds floats)
246 -- We need to track the zapped top-level binders, because
247 -- they should have their fragile IdInfo zapped (notably occurrence info)
248 -- That's why we run down binds and bndrs' simultaneously.
249 simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
250 simpl_binds env [] bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
251 simpl_binds env (bind:binds) bs = simpl_bind env bind bs `thenSmpl` \ (floats,env) ->
252 addFloats env floats $ \env ->
253 simpl_binds env binds (drop_bs bind bs)
255 drop_bs (NonRec _ _) (_ : bs) = bs
256 drop_bs (Rec prs) bs = drop (length prs) bs
258 simpl_bind env bind bs
259 = getDOptsSmpl `thenSmpl` \ dflags ->
260 if dopt Opt_D_dump_inlinings dflags then
261 pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
263 simpl_bind1 env bind bs
265 simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
266 simpl_bind1 env (Rec pairs) bs' = simplRecBind env TopLevel pairs bs'
270 %************************************************************************
272 \subsection{simplNonRec}
274 %************************************************************************
276 simplNonRecBind is used for
277 * non-top-level non-recursive lets in expressions
281 * An unsimplified (binder, rhs) pair
282 * The env for the RHS. It may not be the same as the
283 current env because the bind might occur via (\x.E) arg
285 It uses the CPS form because the binding might be strict, in which
286 case we might discard the continuation:
287 let x* = error "foo" in (...x...)
289 It needs to turn unlifted bindings into a @case@. They can arise
290 from, say: (\x -> e) (4# + 3#)
293 simplNonRecBind :: SimplEnv
295 -> InExpr -> SimplEnv -- Arg, with its subst-env
296 -> OutType -- Type of thing computed by the context
297 -> (SimplEnv -> SimplM FloatsWithExpr) -- The body
298 -> SimplM FloatsWithExpr
300 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
302 = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
305 simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
306 = simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
308 simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
309 | preInlineUnconditionally env NotTopLevel bndr rhs
310 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
311 thing_inside (extendIdSubst env bndr (mkContEx rhs_se rhs))
313 | isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty -- A strict let
314 = -- Don't use simplBinder because that doesn't keep
315 -- fragile occurrence info in the substitution
316 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr1) ->
317 simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty $ \ env1 rhs1 ->
319 -- Now complete the binding and simplify the body
321 (env2,bndr2) = addLetIdInfo env1 bndr bndr1
323 if needsCaseBinding bndr_ty rhs1
325 thing_inside env2 `thenSmpl` \ (floats, body) ->
326 returnSmpl (emptyFloats env2, Case rhs1 bndr2 (exprType body)
327 [(DEFAULT, [], wrapFloats floats body)])
329 completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
331 | otherwise -- Normal, lazy case
332 = -- Don't use simplBinder because that doesn't keep
333 -- fragile occurrence info in the substitution
334 simplNonRecBndr env bndr `thenSmpl` \ (env, bndr') ->
335 simplLazyBind env NotTopLevel NonRecursive
336 bndr bndr' rhs rhs_se `thenSmpl` \ (floats, env) ->
337 addFloats env floats thing_inside
340 bndr_ty = idType bndr
343 A specialised variant of simplNonRec used when the RHS is already simplified, notably
344 in knownCon. It uses case-binding where necessary.
347 simplNonRecX :: SimplEnv
348 -> InId -- Old binder
349 -> OutExpr -- Simplified RHS
350 -> (SimplEnv -> SimplM FloatsWithExpr)
351 -> SimplM FloatsWithExpr
353 simplNonRecX env bndr new_rhs thing_inside
354 | needsCaseBinding (idType bndr) new_rhs
355 -- Make this test *before* the preInlineUnconditionally
356 -- Consider case I# (quotInt# x y) of
357 -- I# v -> let w = J# v in ...
358 -- If we gaily inline (quotInt# x y) for v, we end up building an
360 -- let w = J# (quotInt# x y) in ...
361 -- because quotInt# can fail.
362 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
363 thing_inside env `thenSmpl` \ (floats, body) ->
364 let body' = wrapFloats floats body in
365 returnSmpl (emptyFloats env, Case new_rhs bndr' (exprType body') [(DEFAULT, [], body')])
367 {- No, no, no! Do not try preInlineUnconditionally
368 Doing so risks exponential behaviour, because new_rhs has been simplified once already
369 In the cases described by the folowing commment, postInlineUnconditionally will
370 catch many of the relevant cases.
371 -- This happens; for example, the case_bndr during case of
372 -- known constructor: case (a,b) of x { (p,q) -> ... }
373 -- Here x isn't mentioned in the RHS, so we don't want to
374 -- create the (dead) let-binding let x = (a,b) in ...
376 -- Similarly, single occurrences can be inlined vigourously
377 -- e.g. case (f x, g y) of (a,b) -> ....
378 -- If a,b occur once we can avoid constructing the let binding for them.
379 | preInlineUnconditionally env NotTopLevel bndr new_rhs
380 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
384 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
385 completeNonRecX env False {- Non-strict; pessimistic -}
386 bndr bndr' new_rhs thing_inside
388 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
389 = mkAtomicArgs is_strict
390 True {- OK to float unlifted -}
391 new_rhs `thenSmpl` \ (aux_binds, rhs2) ->
393 -- Make the arguments atomic if necessary,
394 -- adding suitable bindings
395 addAtomicBindsE env (fromOL aux_binds) $ \ env ->
396 completeLazyBind env NotTopLevel
397 old_bndr new_bndr rhs2 `thenSmpl` \ (floats, env) ->
398 addFloats env floats thing_inside
402 %************************************************************************
404 \subsection{Lazy bindings}
406 %************************************************************************
408 simplRecBind is used for
409 * recursive bindings only
412 simplRecBind :: SimplEnv -> TopLevelFlag
413 -> [(InId, InExpr)] -> [OutId]
414 -> SimplM (FloatsWith SimplEnv)
415 simplRecBind env top_lvl pairs bndrs'
416 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
417 returnSmpl (flattenFloats floats, env)
419 go env [] _ = returnSmpl (emptyFloats env, env)
421 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
422 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
423 addFloats env floats (\env -> go env pairs bndrs')
427 simplRecOrTopPair is used for
428 * recursive bindings (whether top level or not)
429 * top-level non-recursive bindings
431 It assumes the binder has already been simplified, but not its IdInfo.
434 simplRecOrTopPair :: SimplEnv
436 -> InId -> OutId -- Binder, both pre-and post simpl
437 -> InExpr -- The RHS and its environment
438 -> SimplM (FloatsWith SimplEnv)
440 simplRecOrTopPair env top_lvl bndr bndr' rhs
441 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
442 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
443 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
446 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
447 -- May not actually be recursive, but it doesn't matter
451 simplLazyBind is used for
452 * recursive bindings (whether top level or not)
453 * top-level non-recursive bindings
454 * non-top-level *lazy* non-recursive bindings
456 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
457 from SimplRecOrTopBind]
460 1. It assumes that the binder is *already* simplified,
461 and is in scope, but not its IdInfo
463 2. It assumes that the binder type is lifted.
465 3. It does not check for pre-inline-unconditionallly;
466 that should have been done already.
469 simplLazyBind :: SimplEnv
470 -> TopLevelFlag -> RecFlag
471 -> InId -> OutId -- Binder, both pre-and post simpl
472 -> InExpr -> SimplEnv -- The RHS and its environment
473 -> SimplM (FloatsWith SimplEnv)
475 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
477 (env1,bndr2) = addLetIdInfo env bndr bndr1
478 rhs_env = setInScope rhs_se env1
479 is_top_level = isTopLevel top_lvl
480 ok_float_unlifted = not is_top_level && isNonRec is_rec
481 rhs_cont = mkRhsStop (idType bndr2)
483 -- Simplify the RHS; note the mkRhsStop, which tells
484 -- the simplifier that this is the RHS of a let.
485 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
487 -- If any of the floats can't be floated, give up now
488 -- (The allLifted predicate says True for empty floats.)
489 if (not ok_float_unlifted && not (allLifted floats)) then
490 completeLazyBind env1 top_lvl bndr bndr2
491 (wrapFloats floats rhs1)
494 -- ANF-ise a constructor or PAP rhs
495 mkAtomicArgs False {- Not strict -}
496 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
498 -- If the result is a PAP, float the floats out, else wrap them
499 -- By this time it's already been ANF-ised (if necessary)
500 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
501 completeLazyBind env1 top_lvl bndr bndr2 rhs2
503 else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
504 -- WARNING: long dodgy argument coming up
505 -- WANTED: a better way to do this
507 -- We can't use "exprIsCheap" instead of exprIsHNF,
508 -- because that causes a strictness bug.
509 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
510 -- The case expression is 'cheap', but it's wrong to transform to
511 -- y* = E; x = case (scc y) of {...}
512 -- Either we must be careful not to float demanded non-values, or
513 -- we must use exprIsHNF for the test, which ensures that the
514 -- thing is non-strict. So exprIsHNF => bindings are non-strict
515 -- I think. The WARN below tests for this.
517 -- We use exprIsTrivial here because we want to reveal lone variables.
518 -- E.g. let { x = letrec { y = E } in y } in ...
519 -- Here we definitely want to float the y=E defn.
520 -- exprIsHNF definitely isn't right for that.
522 -- Again, the floated binding can't be strict; if it's recursive it'll
523 -- be non-strict; if it's non-recursive it'd be inlined.
525 -- Note [SCC-and-exprIsTrivial]
527 -- y = let { x* = E } in scc "foo" x
528 -- then we do *not* want to float out the x binding, because
529 -- it's strict! Fortunately, exprIsTrivial replies False to
532 -- There's a subtlety here. There may be a binding (x* = e) in the
533 -- floats, where the '*' means 'will be demanded'. So is it safe
534 -- to float it out? Answer no, but it won't matter because
535 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
536 -- and so there can't be any 'will be demanded' bindings in the floats.
538 ASSERT2( is_top_level || not (any demanded_float (floatBinds floats)),
539 ppr (filter demanded_float (floatBinds floats)) )
541 tick LetFloatFromLet `thenSmpl_` (
542 addFloats env1 floats $ \ env2 ->
543 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
544 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
547 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
550 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
551 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
552 demanded_float (Rec _) = False
557 %************************************************************************
559 \subsection{Completing a lazy binding}
561 %************************************************************************
564 * deals only with Ids, not TyVars
565 * takes an already-simplified binder and RHS
566 * is used for both recursive and non-recursive bindings
567 * is used for both top-level and non-top-level bindings
569 It does the following:
570 - tries discarding a dead binding
571 - tries PostInlineUnconditionally
572 - add unfolding [this is the only place we add an unfolding]
575 It does *not* attempt to do let-to-case. Why? Because it is used for
576 - top-level bindings (when let-to-case is impossible)
577 - many situations where the "rhs" is known to be a WHNF
578 (so let-to-case is inappropriate).
581 completeLazyBind :: SimplEnv
582 -> TopLevelFlag -- Flag stuck into unfolding
583 -> InId -- Old binder
584 -> OutId -- New binder
585 -> OutExpr -- Simplified RHS
586 -> SimplM (FloatsWith SimplEnv)
587 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
588 -- by extending the substitution (e.g. let x = y in ...)
589 -- The new binding (if any) is returned as part of the floats.
590 -- NB: the returned SimplEnv has the right SubstEnv, but you should
591 -- (as usual) use the in-scope-env from the floats
593 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
594 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
595 = -- Drop the binding
596 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
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
612 -- If the unfolding is a value, the demand info may
613 -- go pear-shaped, so we nuke it. Example:
615 -- case x of (p,q) -> h p q x
616 -- Here x is certainly demanded. But after we've nuked
617 -- the case, we'll get just
618 -- let x = (a,b) in h a b x
619 -- and now x is not demanded (I'm assuming h is lazy)
620 -- This really happens. Similarly
621 -- let f = \x -> e in ...f..f...
622 -- After inling f at some of its call sites the original binding may
623 -- (for example) be no longer strictly demanded.
624 -- The solution here is a bit ad hoc...
625 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
626 final_info | loop_breaker = new_bndr_info
627 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
628 | otherwise = info_w_unf
630 final_id = new_bndr `setIdInfo` final_info
632 -- These seqs forces the Id, and hence its IdInfo,
633 -- and hence any inner substitutions
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 (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg (Just env) cont)
716 simplExprF env (Type ty) cont
717 = ASSERT( contIsRhsOrArg cont )
718 simplType env ty `thenSmpl` \ ty' ->
719 rebuild env (Type ty') cont
721 simplExprF env (Case scrut bndr case_ty alts) cont
722 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
723 = -- Simplify the scrutinee with a Select continuation
724 simplExprF env scrut (Select NoDup bndr alts env cont)
727 = -- If case-of-case is off, simply simplify the case expression
728 -- in a vanilla Stop context, and rebuild the result around it
729 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
730 rebuild env case_expr' cont
732 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
733 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
735 simplExprF env (Let (Rec pairs) body) cont
736 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
737 -- NB: bndrs' don't have unfoldings or rules
738 -- We add them as we go down
740 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
741 addFloats env floats $ \ env ->
742 simplExprF env body cont
744 -- A non-recursive let is dealt with by simplNonRecBind
745 simplExprF env (Let (NonRec bndr rhs) body) cont
746 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
747 simplExprF env body cont
750 ---------------------------------
751 simplType :: SimplEnv -> InType -> SimplM OutType
752 -- Kept monadic just so we can do the seqType
754 = seqType new_ty `seq` returnSmpl new_ty
756 new_ty = substTy env ty
760 %************************************************************************
764 %************************************************************************
767 simplLam env fun cont
770 zap_it = mkLamBndrZapper fun (countArgs cont)
771 cont_ty = contResultType cont
773 -- Type-beta reduction
774 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) mb_arg_se body_cont)
775 = ASSERT( isTyVar bndr )
776 do { tick (BetaReduction bndr)
777 ; ty_arg' <- case mb_arg_se of
778 Just arg_se -> simplType (setInScope arg_se env) ty_arg
779 Nothing -> return ty_arg
780 ; go (extendTvSubst env bndr ty_arg') body body_cont }
782 -- Ordinary beta reduction
783 go env (Lam bndr body) cont@(ApplyTo _ arg (Just arg_se) body_cont)
784 = do { tick (BetaReduction bndr)
785 ; simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
786 go env body body_cont }
788 go env (Lam bndr body) cont@(ApplyTo _ arg Nothing body_cont)
789 = do { tick (BetaReduction bndr)
790 ; simplNonRecX env (zap_it bndr) arg $ \ env ->
791 go env body body_cont }
793 -- Not enough args, so there are real lambdas left to put in the result
794 go env lam@(Lam _ _) cont
795 = do { (env, bndrs') <- simplLamBndrs env bndrs
796 ; body' <- simplExpr env body
797 ; (floats, new_lam) <- mkLam env bndrs' body' cont
798 ; addFloats env floats $ \ env ->
799 rebuild env new_lam cont }
801 (bndrs,body) = collectBinders lam
803 -- Exactly enough args
804 go env expr cont = simplExprF env expr cont
806 mkLamBndrZapper :: CoreExpr -- Function
807 -> Int -- Number of args supplied, *including* type args
808 -> Id -> Id -- Use this to zap the binders
809 mkLamBndrZapper fun n_args
810 | n_args >= n_params fun = \b -> b -- Enough args
811 | otherwise = \b -> zapLamIdInfo b
813 -- NB: we count all the args incl type args
814 -- so we must count all the binders (incl type lambdas)
815 n_params (Note _ e) = n_params e
816 n_params (Lam b e) = 1 + n_params e
817 n_params other = 0::Int
821 %************************************************************************
825 %************************************************************************
828 simplNote env (Coerce to from) body cont
830 addCoerce s1 k1 cont -- Drop redundant coerces. This can happen if a polymoprhic
831 -- (coerce a b e) is instantiated with a=ty1 b=ty2 and the
832 -- two are the same. This happens a lot in Happy-generated parsers
833 | s1 `coreEqType` k1 = cont
835 addCoerce s1 k1 (CoerceIt t1 cont)
836 -- coerce T1 S1 (coerce S1 K1 e)
839 -- coerce T1 K1 e, otherwise
841 -- For example, in the initial form of a worker
842 -- we may find (coerce T (coerce S (\x.e))) y
843 -- and we'd like it to simplify to e[y/x] in one round
845 | t1 `coreEqType` k1 = cont -- The coerces cancel out
846 | otherwise = CoerceIt t1 cont -- They don't cancel, but
847 -- the inner one is redundant
849 addCoerce t1t2 s1s2 (ApplyTo dup arg mb_arg_se cont)
850 | not (isTypeArg arg), -- This whole case only works for value args
851 -- Could upgrade to have equiv thing for type apps too
852 Just (s1, s2) <- splitFunTy_maybe s1s2
853 -- (coerce (T1->T2) (S1->S2) F) E
855 -- coerce T2 S2 (F (coerce S1 T1 E))
857 -- t1t2 must be a function type, T1->T2, because it's applied to something
858 -- but s1s2 might conceivably not be
860 -- When we build the ApplyTo we can't mix the out-types
861 -- with the InExpr in the argument, so we simply substitute
862 -- to make it all consistent. It's a bit messy.
863 -- But it isn't a common case.
865 (t1,t2) = splitFunTy t1t2
866 new_arg = mkCoerce2 s1 t1 arg'
867 arg' = case mb_arg_se of
869 Just arg_se -> substExpr (setInScope arg_se env) arg
871 ApplyTo dup new_arg Nothing (addCoerce t2 s2 cont)
873 addCoerce to' _ cont = CoerceIt to' cont
875 simplType env to `thenSmpl` \ to' ->
876 simplType env from `thenSmpl` \ from' ->
877 simplExprF env body (addCoerce to' from' cont)
880 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
881 -- inlining. All other CCCSs are mapped to currentCCS.
882 simplNote env (SCC cc) e cont
883 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
884 rebuild env (mkSCC cc e') cont
886 -- See notes with SimplMonad.inlineMode
887 simplNote env InlineMe e cont
888 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
889 = -- Don't inline inside an INLINE expression
890 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
891 rebuild env (mkInlineMe e') cont
893 | otherwise -- Dissolve the InlineMe note if there's
894 -- an interesting context of any kind to combine with
895 -- (even a type application -- anything except Stop)
896 = simplExprF env e cont
898 simplNote env (CoreNote s) e cont
899 = simplExpr env e `thenSmpl` \ e' ->
900 rebuild env (Note (CoreNote s) e') cont
904 %************************************************************************
906 \subsection{Dealing with calls}
908 %************************************************************************
911 simplVar env var cont
912 = case substId env var of
913 DoneEx e -> simplExprF (zapSubstEnv env) e cont
914 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
915 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
916 -- Note [zapSubstEnv]
917 -- The template is already simplified, so don't re-substitute.
918 -- This is VITAL. Consider
920 -- let y = \z -> ...x... in
922 -- We'll clone the inner \x, adding x->x' in the id_subst
923 -- Then when we inline y, we must *not* replace x by x' in
924 -- the inlined copy!!
926 ---------------------------------------------------------
927 -- Dealing with a call site
929 completeCall env var occ_info cont
930 = -- Simplify the arguments
931 getDOptsSmpl `thenSmpl` \ dflags ->
933 chkr = getSwitchChecker env
934 (args, call_cont) = getContArgs chkr var cont
937 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
938 (contResultType call_cont) $ \ env args ->
940 -- Next, look for rules or specialisations that match
942 -- It's important to simplify the args first, because the rule-matcher
943 -- doesn't do substitution as it goes. We don't want to use subst_args
944 -- (defined in the 'where') because that throws away useful occurrence info,
945 -- and perhaps-very-important specialisations.
947 -- Some functions have specialisations *and* are strict; in this case,
948 -- we don't want to inline the wrapper of the non-specialised thing; better
949 -- to call the specialised thing instead.
950 -- We used to use the black-listing mechanism to ensure that inlining of
951 -- the wrapper didn't occur for things that have specialisations till a
952 -- later phase, so but now we just try RULES first
954 -- You might think that we shouldn't apply rules for a loop breaker:
955 -- doing so might give rise to an infinite loop, because a RULE is
956 -- rather like an extra equation for the function:
957 -- RULE: f (g x) y = x+y
960 -- But it's too drastic to disable rules for loop breakers.
961 -- Even the foldr/build rule would be disabled, because foldr
962 -- is recursive, and hence a loop breaker:
963 -- foldr k z (build g) = g k z
964 -- So it's up to the programmer: rules can cause divergence
967 in_scope = getInScope env
969 maybe_rule = case activeRule env of
970 Nothing -> Nothing -- No rules apply
971 Just act_fn -> lookupRule act_fn in_scope rules var args
974 Just (rule_name, rule_rhs) ->
975 tick (RuleFired rule_name) `thenSmpl_`
976 (if dopt Opt_D_dump_inlinings dflags then
977 pprTrace "Rule fired" (vcat [
978 text "Rule:" <+> ftext rule_name,
979 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
980 text "After: " <+> pprCoreExpr rule_rhs,
981 text "Cont: " <+> ppr call_cont])
984 simplExprF env rule_rhs call_cont ;
986 Nothing -> -- No rules
988 -- Next, look for an inlining
990 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
991 interesting_cont = interestingCallContext (notNull args)
994 active_inline = activeInline env var occ_info
995 maybe_inline = callSiteInline dflags active_inline occ_info
996 var arg_infos interesting_cont
998 case maybe_inline of {
999 Just unfolding -- There is an inlining!
1000 -> tick (UnfoldingDone var) `thenSmpl_`
1001 (if dopt Opt_D_dump_inlinings dflags then
1002 pprTrace "Inlining done" (vcat [
1003 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1004 text "Inlined fn: " <+> ppr unfolding,
1005 text "Cont: " <+> ppr call_cont])
1008 simplExprF env unfolding (pushContArgs args call_cont)
1011 Nothing -> -- No inlining!
1014 rebuild env (mkApps (Var var) args) call_cont
1018 %************************************************************************
1020 \subsection{Arguments}
1022 %************************************************************************
1025 ---------------------------------------------------------
1026 -- Simplifying the arguments of a call
1028 simplifyArgs :: SimplEnv
1029 -> OutType -- Type of the function
1030 -> Bool -- True if the fn has RULES
1031 -> [(InExpr, Maybe SimplEnv, Bool)] -- Details of the arguments
1032 -> OutType -- Type of the continuation
1033 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1034 -> SimplM FloatsWithExpr
1036 -- [CPS-like because of strict arguments]
1038 -- Simplify the arguments to a call.
1039 -- This part of the simplifier may break the no-shadowing invariant
1041 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1042 -- where f is strict in its second arg
1043 -- If we simplify the innermost one first we get (...(\a -> e)...)
1044 -- Simplifying the second arg makes us float the case out, so we end up with
1045 -- case y of (a,b) -> f (...(\a -> e)...) e'
1046 -- So the output does not have the no-shadowing invariant. However, there is
1047 -- no danger of getting name-capture, because when the first arg was simplified
1048 -- we used an in-scope set that at least mentioned all the variables free in its
1049 -- static environment, and that is enough.
1051 -- We can't just do innermost first, or we'd end up with a dual problem:
1052 -- case x of (a,b) -> f e (...(\a -> e')...)
1054 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1055 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1056 -- continuations. We have to keep the right in-scope set around; AND we have
1057 -- to get the effect that finding (error "foo") in a strict arg position will
1058 -- discard the entire application and replace it with (error "foo"). Getting
1059 -- all this at once is TOO HARD!
1061 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1062 = go env fn_ty args thing_inside
1064 go env fn_ty [] thing_inside = thing_inside env []
1065 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1066 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1067 thing_inside env (arg':args')
1069 simplifyArg env fn_ty has_rules (arg, Nothing, _) cont_ty thing_inside
1070 = thing_inside env arg -- Already simplified
1072 simplifyArg env fn_ty has_rules (Type ty_arg, Just se, _) cont_ty thing_inside
1073 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1074 thing_inside env (Type new_ty_arg)
1076 simplifyArg env fn_ty has_rules (val_arg, Just arg_se, is_strict) cont_ty thing_inside
1078 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1080 | otherwise -- Lazy argument
1081 -- DO NOT float anything outside, hence simplExprC
1082 -- There is no benefit (unlike in a let-binding), and we'd
1083 -- have to be very careful about bogus strictness through
1084 -- floating a demanded let.
1085 = simplExprC (setInScope arg_se env) val_arg
1086 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1087 thing_inside env arg1
1089 arg_ty = funArgTy fn_ty
1092 simplStrictArg :: LetRhsFlag
1093 -> SimplEnv -- The env of the call
1094 -> InExpr -> SimplEnv -- The arg plus its env
1095 -> OutType -- arg_ty: type of the argument
1096 -> OutType -- cont_ty: Type of thing computed by the context
1097 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1098 -- Takes an expression of type rhs_ty,
1099 -- returns an expression of type cont_ty
1100 -- The env passed to this continuation is the
1101 -- env of the call, plus any new in-scope variables
1102 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1104 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1105 = simplExprF (setInScope arg_env call_env) arg
1106 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1107 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1108 -- to simplify the argument
1109 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1113 %************************************************************************
1115 \subsection{mkAtomicArgs}
1117 %************************************************************************
1119 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1120 constructor application and, if so, converts it to ANF, so that the
1121 resulting thing can be inlined more easily. Thus
1128 There are three sorts of binding context, specified by the two
1134 N N Top-level or recursive Only bind args of lifted type
1136 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1137 but lazy unlifted-and-ok-for-speculation
1139 Y Y Non-top-level, non-recursive, Bind all args
1140 and strict (demanded)
1147 there is no point in transforming to
1149 x = case (y div# z) of r -> MkC r
1151 because the (y div# z) can't float out of the let. But if it was
1152 a *strict* let, then it would be a good thing to do. Hence the
1153 context information.
1156 mkAtomicArgs :: Bool -- A strict binding
1157 -> Bool -- OK to float unlifted args
1159 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1160 OutExpr) -- things that need case-binding,
1161 -- if the strict-binding flag is on
1163 mkAtomicArgs is_strict ok_float_unlifted rhs
1164 | (Var fun, args) <- collectArgs rhs, -- It's an application
1165 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1166 = go fun nilOL [] args -- Have a go
1168 | otherwise = bale_out -- Give up
1171 bale_out = returnSmpl (nilOL, rhs)
1173 go fun binds rev_args []
1174 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1176 go fun binds rev_args (arg : args)
1177 | exprIsTrivial arg -- Easy case
1178 = go fun binds (arg:rev_args) args
1180 | not can_float_arg -- Can't make this arg atomic
1181 = bale_out -- ... so give up
1183 | otherwise -- Don't forget to do it recursively
1184 -- E.g. x = a:b:c:[]
1185 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1186 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1187 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1188 (Var arg_id : rev_args) args
1190 arg_ty = exprType arg
1191 can_float_arg = is_strict
1192 || not (isUnLiftedType arg_ty)
1193 || (ok_float_unlifted && exprOkForSpeculation arg)
1196 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1197 -> (SimplEnv -> SimplM (FloatsWith a))
1198 -> SimplM (FloatsWith a)
1199 addAtomicBinds env [] thing_inside = thing_inside env
1200 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1201 addAtomicBinds env bs thing_inside
1203 addAtomicBindsE :: SimplEnv -> [(OutId,OutExpr)]
1204 -> (SimplEnv -> SimplM FloatsWithExpr)
1205 -> SimplM FloatsWithExpr
1206 -- Same again, but this time we're in an expression context,
1207 -- and may need to do some case bindings
1209 addAtomicBindsE env [] thing_inside
1211 addAtomicBindsE env ((v,r):bs) thing_inside
1212 | needsCaseBinding (idType v) r
1213 = addAtomicBindsE (addNewInScopeIds env [v]) bs thing_inside `thenSmpl` \ (floats, expr) ->
1214 WARN( exprIsTrivial expr, ppr v <+> pprCoreExpr expr )
1215 (let body = wrapFloats floats expr in
1216 returnSmpl (emptyFloats env, Case r v (exprType body) [(DEFAULT,[],body)]))
1219 = addAuxiliaryBind env (NonRec v r) $ \ env ->
1220 addAtomicBindsE env bs thing_inside
1224 %************************************************************************
1226 \subsection{The main rebuilder}
1228 %************************************************************************
1231 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1233 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1234 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1235 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1236 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1237 rebuild env expr (ApplyTo _ arg mb_se cont) = rebuildApp env expr arg mb_se cont
1239 rebuildApp env fun arg mb_se cont
1240 = do { arg' <- simplArg env arg mb_se
1241 ; rebuild env (App fun arg') cont }
1243 simplArg :: SimplEnv -> CoreExpr -> Maybe SimplEnv -> SimplM CoreExpr
1244 simplArg env arg Nothing = return arg -- The arg is already simplified
1245 simplArg env arg (Just arg_env) = simplExpr (setInScope arg_env env) arg
1247 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1251 %************************************************************************
1253 \subsection{Functions dealing with a case}
1255 %************************************************************************
1257 Blob of helper functions for the "case-of-something-else" situation.
1260 ---------------------------------------------------------
1261 -- Eliminate the case if possible
1263 rebuildCase :: SimplEnv
1264 -> OutExpr -- Scrutinee
1265 -> InId -- Case binder
1266 -> [InAlt] -- Alternatives (inceasing order)
1268 -> SimplM FloatsWithExpr
1270 rebuildCase env scrut case_bndr alts cont
1271 | Just (con,args) <- exprIsConApp_maybe scrut
1272 -- Works when the scrutinee is a variable with a known unfolding
1273 -- as well as when it's an explicit constructor application
1274 = knownCon env (DataAlt con) args case_bndr alts cont
1276 | Lit lit <- scrut -- No need for same treatment as constructors
1277 -- because literals are inlined more vigorously
1278 = knownCon env (LitAlt lit) [] case_bndr alts cont
1281 = -- Prepare the continuation;
1282 -- The new subst_env is in place
1283 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1284 addFloats env floats $ \ env ->
1287 -- The case expression is annotated with the result type of the continuation
1288 -- This may differ from the type originally on the case. For example
1289 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1292 -- let j a# = <blob>
1293 -- in case(T) a of { True -> j 1#; False -> j 0# }
1294 -- Note that the case that scrutinises a now returns a T not an Int#
1295 res_ty' = contResultType dup_cont
1298 -- Deal with case binder
1299 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1301 -- Deal with the case alternatives
1302 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1304 -- Put the case back together
1305 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1307 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1308 -- The case binder *not* scope over the whole returned case-expression
1309 rebuild env case_expr nondup_cont
1312 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1313 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1314 way, there's a chance that v will now only be used once, and hence
1319 There is a time we *don't* want to do that, namely when
1320 -fno-case-of-case is on. This happens in the first simplifier pass,
1321 and enhances full laziness. Here's the bad case:
1322 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1323 If we eliminate the inner case, we trap it inside the I# v -> arm,
1324 which might prevent some full laziness happening. I've seen this
1325 in action in spectral/cichelli/Prog.hs:
1326 [(m,n) | m <- [1..max], n <- [1..max]]
1327 Hence the check for NoCaseOfCase.
1331 There is another situation when we don't want to do it. If we have
1333 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1334 ...other cases .... }
1336 We'll perform the binder-swap for the outer case, giving
1338 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1339 ...other cases .... }
1341 But there is no point in doing it for the inner case, because w1 can't
1342 be inlined anyway. Furthermore, doing the case-swapping involves
1343 zapping w2's occurrence info (see paragraphs that follow), and that
1344 forces us to bind w2 when doing case merging. So we get
1346 case x of w1 { A -> let w2 = w1 in e1
1347 B -> let w2 = w1 in e2
1348 ...other cases .... }
1350 This is plain silly in the common case where w2 is dead.
1352 Even so, I can't see a good way to implement this idea. I tried
1353 not doing the binder-swap if the scrutinee was already evaluated
1354 but that failed big-time:
1358 case v of w { MkT x ->
1359 case x of x1 { I# y1 ->
1360 case x of x2 { I# y2 -> ...
1362 Notice that because MkT is strict, x is marked "evaluated". But to
1363 eliminate the last case, we must either make sure that x (as well as
1364 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1365 the binder-swap. So this whole note is a no-op.
1369 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1370 any occurrence info (eg IAmDead) in the case binder, because the
1371 case-binder now effectively occurs whenever v does. AND we have to do
1372 the same for the pattern-bound variables! Example:
1374 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1376 Here, b and p are dead. But when we move the argment inside the first
1377 case RHS, and eliminate the second case, we get
1379 case x of { (a,b) -> a b }
1381 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1384 Indeed, this can happen anytime the case binder isn't dead:
1385 case <any> of x { (a,b) ->
1386 case x of { (p,q) -> p } }
1387 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1388 The point is that we bring into the envt a binding
1390 after the outer case, and that makes (a,b) alive. At least we do unless
1391 the case binder is guaranteed dead.
1394 simplCaseBinder env (Var v) case_bndr
1395 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1397 -- Failed try [see Note 2 above]
1398 -- not (isEvaldUnfolding (idUnfolding v))
1400 = simplBinder env (zap case_bndr) `thenSmpl` \ (env, case_bndr') ->
1401 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1402 -- We could extend the substitution instead, but it would be
1403 -- a hack because then the substitution wouldn't be idempotent
1404 -- any more (v is an OutId). And this does just as well.
1406 zap b = b `setIdOccInfo` NoOccInfo
1408 simplCaseBinder env other_scrut case_bndr
1409 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1410 returnSmpl (env, case_bndr')
1414 simplAlts does two things:
1416 1. Eliminate alternatives that cannot match, including the
1417 DEFAULT alternative.
1419 2. If the DEFAULT alternative can match only one possible constructor,
1420 then make that constructor explicit.
1422 case e of x { DEFAULT -> rhs }
1424 case e of x { (a,b) -> rhs }
1425 where the type is a single constructor type. This gives better code
1426 when rhs also scrutinises x or e.
1428 Here "cannot match" includes knowledge from GADTs
1430 It's a good idea do do this stuff before simplifying the alternatives, to
1431 avoid simplifying alternatives we know can't happen, and to come up with
1432 the list of constructors that are handled, to put into the IdInfo of the
1433 case binder, for use when simplifying the alternatives.
1435 Eliminating the default alternative in (1) isn't so obvious, but it can
1438 data Colour = Red | Green | Blue
1447 DEFAULT -> [ case y of ... ]
1449 If we inline h into f, the default case of the inlined h can't happen.
1450 If we don't notice this, we may end up filtering out *all* the cases
1451 of the inner case y, which give us nowhere to go!
1455 simplAlts :: SimplEnv
1457 -> OutId -- Case binder
1458 -> [InAlt] -> SimplCont
1459 -> SimplM [OutAlt] -- Includes the continuation
1461 simplAlts env scrut case_bndr' alts cont'
1462 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1463 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1464 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1465 -- We need the mergeAlts in case the new default_alt
1466 -- has turned into a constructor alternative.
1468 (alts_wo_default, maybe_deflt) = findDefault alts
1469 imposs_cons = case scrut of
1470 Var v -> otherCons (idUnfolding v)
1473 -- "imposs_deflt_cons" are handled either by the context,
1474 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1475 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1477 simplDefault :: SimplEnv
1478 -> OutId -- Case binder; need just for its type. Note that as an
1479 -- OutId, it has maximum information; this is important.
1480 -- Test simpl013 is an example
1481 -> [AltCon] -- These cons can't happen when matching the default
1484 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1485 -- mergeAlts expects
1488 simplDefault env case_bndr' imposs_cons cont Nothing
1489 = return [] -- No default branch
1490 simplDefault env case_bndr' imposs_cons cont (Just rhs)
1491 | -- This branch handles the case where we are
1492 -- scrutinisng an algebraic data type
1493 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1494 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1495 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1496 -- case x of { DEFAULT -> e }
1497 -- and we don't want to fill in a default for them!
1498 Just all_cons <- tyConDataCons_maybe tycon,
1499 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1500 -- which GHC allows, then the case expression will have at most a default
1501 -- alternative. We don't want to eliminate that alternative, because the
1502 -- invariant is that there's always one alternative. It's more convenient
1504 -- case x of { DEFAULT -> e }
1505 -- as it is, rather than transform it to
1506 -- error "case cant match"
1507 -- which would be quite legitmate. But it's a really obscure corner, and
1508 -- not worth wasting code on.
1510 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1511 poss_data_cons = filterOut (`elem` imposs_data_cons) all_cons
1512 gadt_imposs | all isTyVarTy inst_tys = []
1513 | otherwise = filter (cant_match inst_tys) poss_data_cons
1514 final_poss = filterOut (`elem` gadt_imposs) poss_data_cons
1516 = case final_poss of
1517 [] -> returnSmpl [] -- Eliminate the default alternative
1518 -- altogether if it can't match
1520 [con] -> -- It matches exactly one constructor, so fill it in
1521 do { con_alt <- mkDataConAlt case_bndr' con inst_tys rhs
1522 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1523 -- The simplAlt must succeed with Just because we have
1524 -- already filtered out construtors that can't match
1527 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1530 = simplify_default imposs_cons
1532 cant_match tys data_con = not (dataConCanMatch data_con tys)
1534 simplify_default imposs_cons
1535 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1536 -- Record the constructors that the case-binder *can't* be.
1537 ; rhs' <- simplExprC env' rhs cont
1538 ; return [(DEFAULT, [], rhs')] }
1540 mkDataConAlt :: Id -> DataCon -> [OutType] -> InExpr -> SimplM InAlt
1541 -- Make a data-constructor alternative to replace the DEFAULT case
1542 -- NB: there's something a bit bogus here, because we put OutTypes into an InAlt
1543 mkDataConAlt case_bndr con tys rhs
1544 = do { tick (FillInCaseDefault case_bndr)
1545 ; args <- mk_args con tys
1546 ; return (DataAlt con, args, rhs) }
1548 mk_args con inst_tys
1549 = do { (tv_bndrs, inst_tys') <- mk_tv_bndrs con inst_tys
1550 ; let arg_tys = dataConInstArgTys con inst_tys'
1551 ; arg_ids <- mapM (newId FSLIT("a")) arg_tys
1552 ; returnSmpl (tv_bndrs ++ arg_ids) }
1554 mk_tv_bndrs con inst_tys
1555 | isVanillaDataCon con
1556 = return ([], inst_tys)
1558 = do { tv_uniqs <- getUniquesSmpl
1559 ; let new_tvs = zipWith mk tv_uniqs (dataConTyVars con)
1560 mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv)
1561 ; return (new_tvs, mkTyVarTys new_tvs) }
1563 simplAlt :: SimplEnv
1564 -> [AltCon] -- These constructors can't be present when
1565 -- matching this alternative
1566 -> OutId -- The case binder
1569 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1571 -- Simplify an alternative, returning the type refinement for the
1572 -- alternative, if the alternative does any refinement at all
1573 -- Nothing => the alternative is inaccessible
1575 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1576 | con `elem` imposs_cons -- This case can't match
1579 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1580 -- TURGID DUPLICATION, needed only for the simplAlt call
1581 -- in mkDupableAlt. Clean this up when moving to FC
1582 = ASSERT( null bndrs )
1583 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1584 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1586 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1587 -- Record the constructors that the case-binder *can't* be.
1589 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1590 = ASSERT( null bndrs )
1591 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1592 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1594 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1596 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1597 | isVanillaDataCon con
1598 = -- Deal with the pattern-bound variables
1599 -- Mark the ones that are in ! positions in the data constructor
1600 -- as certainly-evaluated.
1601 -- NB: it happens that simplBinders does *not* erase the OtherCon
1602 -- form of unfolding, so it's ok to add this info before
1603 -- doing simplBinders
1604 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1606 -- Bind the case-binder to (con args)
1607 let unf = mkUnfolding False (mkConApp con con_args)
1608 inst_tys' = tyConAppArgs (idType case_bndr')
1609 con_args = map Type inst_tys' ++ map varToCoreExpr vs'
1610 env' = mk_rhs_env env case_bndr' unf
1612 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1613 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1615 | otherwise -- GADT case
1617 (tvs,ids) = span isTyVar vs
1619 simplBinders env tvs `thenSmpl` \ (env1, tvs') ->
1620 case coreRefineTys con tvs' (idType case_bndr') of {
1621 Nothing -- Inaccessible
1622 | opt_PprStyle_Debug -- Hack: if debugging is on, generate an error case
1624 -> let rhs' = mkApps (Var eRROR_ID)
1625 [Type (substTy env (exprType rhs)),
1626 Lit (mkStringLit "Impossible alternative (GADT)")]
1628 simplBinders env1 ids `thenSmpl` \ (env2, ids') ->
1629 returnSmpl (Just (emptyVarEnv, (DataAlt con, tvs' ++ ids', rhs')))
1631 | otherwise -- Filter out the inaccessible branch
1634 Just refine@(tv_subst_env, _) -> -- The normal case
1637 env2 = refineSimplEnv env1 refine
1638 -- Simplify the Ids in the refined environment, so their types
1639 -- reflect the refinement. Usually this doesn't matter, but it helps
1640 -- in mkDupableAlt, when we want to float a lambda that uses these binders
1641 -- Furthermore, it means the binders contain maximal type information
1643 simplBinders env2 (add_evals con ids) `thenSmpl` \ (env3, ids') ->
1644 let unf = mkUnfolding False con_app
1645 con_app = mkConApp con con_args
1646 con_args = map varToCoreExpr vs' -- NB: no inst_tys'
1647 env_w_unf = mk_rhs_env env3 case_bndr' unf
1650 simplExprC env_w_unf rhs cont' `thenSmpl` \ rhs' ->
1651 returnSmpl (Just (tv_subst_env, (DataAlt con, vs', rhs'))) }
1654 -- add_evals records the evaluated-ness of the bound variables of
1655 -- a case pattern. This is *important*. Consider
1656 -- data T = T !Int !Int
1658 -- case x of { T a b -> T (a+1) b }
1660 -- We really must record that b is already evaluated so that we don't
1661 -- go and re-evaluate it when constructing the result.
1662 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1664 cat_evals dc vs strs
1668 go (v:vs) strs | isTyVar v = v : go vs strs
1669 go (v:vs) (str:strs)
1670 | isMarkedStrict str = evald_v : go vs strs
1671 | otherwise = zapped_v : go vs strs
1673 zapped_v = zap_occ_info v
1674 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1675 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1677 -- If the case binder is alive, then we add the unfolding
1679 -- to the envt; so vs are now very much alive
1680 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1681 | otherwise = \id -> id `setIdOccInfo` NoOccInfo
1683 mk_rhs_env env case_bndr' case_bndr_unf
1684 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1688 %************************************************************************
1690 \subsection{Known constructor}
1692 %************************************************************************
1694 We are a bit careful with occurrence info. Here's an example
1696 (\x* -> case x of (a*, b) -> f a) (h v, e)
1698 where the * means "occurs once". This effectively becomes
1699 case (h v, e) of (a*, b) -> f a)
1701 let a* = h v; b = e in f a
1705 All this should happen in one sweep.
1708 knownCon :: SimplEnv -> AltCon -> [OutExpr]
1709 -> InId -> [InAlt] -> SimplCont
1710 -> SimplM FloatsWithExpr
1712 knownCon env con args bndr alts cont
1713 = tick (KnownBranch bndr) `thenSmpl_`
1714 case findAlt con alts of
1715 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1716 simplNonRecX env bndr scrut $ \ env ->
1717 -- This might give rise to a binding with non-atomic args
1718 -- like x = Node (f x) (g x)
1719 -- but no harm will be done
1720 simplExprF env rhs cont
1723 LitAlt lit -> Lit lit
1724 DataAlt dc -> mkConApp dc args
1726 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1727 simplNonRecX env bndr (Lit lit) $ \ env ->
1728 simplExprF env rhs cont
1730 (DataAlt dc, bs, rhs)
1731 -> ASSERT( n_drop_tys + length bs == length args )
1732 bind_args env bs (drop n_drop_tys args) $ \ env ->
1734 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1735 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1736 -- args are aready OutExprs, but bs are InIds
1738 simplNonRecX env bndr con_app $ \ env ->
1739 simplExprF env rhs cont
1741 n_drop_tys | isVanillaDataCon dc = tyConArity (dataConTyCon dc)
1743 -- Vanilla data constructors lack type arguments in the pattern
1746 bind_args env [] _ thing_inside = thing_inside env
1748 bind_args env (b:bs) (Type ty : args) thing_inside
1749 = ASSERT( isTyVar b )
1750 bind_args (extendTvSubst env b ty) bs args thing_inside
1752 bind_args env (b:bs) (arg : args) thing_inside
1754 simplNonRecX env b arg $ \ env ->
1755 bind_args env bs args thing_inside
1759 %************************************************************************
1761 \subsection{Duplicating continuations}
1763 %************************************************************************
1766 prepareCaseCont :: SimplEnv
1767 -> [InAlt] -> SimplCont
1768 -> SimplM (FloatsWith (SimplCont,SimplCont))
1769 -- Return a duplicatable continuation, a non-duplicable part
1770 -- plus some extra bindings (that scope over the entire
1773 -- No need to make it duplicatable if there's only one alternative
1774 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1775 prepareCaseCont env alts cont = mkDupableCont env cont
1779 mkDupableCont :: SimplEnv -> SimplCont
1780 -> SimplM (FloatsWith (SimplCont, SimplCont))
1782 mkDupableCont env cont
1783 | contIsDupable cont
1784 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1786 mkDupableCont env (CoerceIt ty cont)
1787 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1788 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1790 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1791 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1792 -- Do *not* duplicate an ArgOf continuation
1793 -- Because ArgOf continuations are opaque, we gain nothing by
1794 -- propagating them into the expressions, and we do lose a lot.
1795 -- Here's an example:
1796 -- && (case x of { T -> F; F -> T }) E
1797 -- Now, && is strict so we end up simplifying the case with
1798 -- an ArgOf continuation. If we let-bind it, we get
1800 -- let $j = \v -> && v E
1801 -- in simplExpr (case x of { T -> F; F -> T })
1802 -- (ArgOf (\r -> $j r)
1803 -- And after simplifying more we get
1805 -- let $j = \v -> && v E
1806 -- in case of { T -> $j F; F -> $j T }
1807 -- Which is a Very Bad Thing
1809 -- The desire not to duplicate is the entire reason that
1810 -- mkDupableCont returns a pair of continuations.
1812 -- The original plan had:
1813 -- e.g. (...strict-fn...) [...hole...]
1815 -- let $j = \a -> ...strict-fn...
1816 -- in $j [...hole...]
1818 mkDupableCont env (ApplyTo _ arg mb_se cont)
1819 = -- e.g. [...hole...] (...arg...)
1821 -- let a = ...arg...
1822 -- in [...hole...] a
1823 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1824 ; addFloats env floats $ \ env -> do
1825 { arg1 <- simplArg env arg mb_se
1826 ; (floats2, arg2) <- mkDupableArg env arg1
1827 ; return (floats2, (ApplyTo OkToDup arg2 Nothing dup_cont, nondup_cont)) }}
1829 mkDupableCont env cont@(Select _ case_bndr [_] se _)
1830 = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
1832 scrut_ty = substTy se (idType case_bndr)
1833 -- This case is just like the previous one. Here's an example:
1834 -- data T a = MkT !a
1835 -- ...(MkT (abs x))...
1837 -- case (case x of I# x' ->
1839 -- True -> I# (negate# x')
1840 -- False -> I# x') of y {
1842 -- Because the (case x) has only one alternative, we'll transform to
1843 -- case x of I# x' ->
1844 -- case (case x' <# 0# of
1845 -- True -> I# (negate# x')
1846 -- False -> I# x') of y {
1848 -- But now we do *NOT* want to make a join point etc, giving
1849 -- case x of I# x' ->
1850 -- let $j = \y -> MkT y
1851 -- in case x' <# 0# of
1852 -- True -> $j (I# (negate# x'))
1853 -- False -> $j (I# x')
1854 -- In this case the $j will inline again, but suppose there was a big
1855 -- strict computation enclosing the orginal call to MkT. Then, it won't
1856 -- "see" the MkT any more, because it's big and won't get duplicated.
1857 -- And, what is worse, nothing was gained by the case-of-case transform.
1859 -- NB: Originally I matched [(DEFAULT,_,_)], but in the common
1860 -- case of Int, the alternative-filling-in code turned the outer case into
1861 -- case (...) of y { I# _ -> MkT y }
1862 -- and that doesn't match the DEFAULT!
1863 -- Now I match on any single-alternative case.
1864 -- I hope that is the right thing to do!
1866 mkDupableCont env (Select _ case_bndr alts se cont)
1867 = -- e.g. (case [...hole...] of { pi -> ei })
1869 -- let ji = \xij -> ei
1870 -- in case [...hole...] of { pi -> ji xij }
1871 do { tick (CaseOfCase case_bndr)
1872 ; let alt_env = setInScope se env
1873 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1874 -- NB: call mkDupableCont here, *not* prepareCaseCont
1875 -- We must make a duplicable continuation, whereas prepareCaseCont
1876 -- doesn't when there is a single case branch
1877 ; addFloats alt_env floats1 $ \ alt_env -> do
1879 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1880 -- NB: simplBinder does not zap deadness occ-info, so
1881 -- a dead case_bndr' will still advertise its deadness
1882 -- This is really important because in
1883 -- case e of b { (# a,b #) -> ... }
1884 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1885 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1886 -- In the new alts we build, we have the new case binder, so it must retain
1889 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1890 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1891 (mkBoringStop (contResultType dup_cont)),
1895 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1896 -- Let-bind the thing if necessary
1897 mkDupableArg env arg
1899 = return (emptyFloats env, arg)
1901 = do { arg_id <- newId FSLIT("a") (exprType arg)
1902 ; tick (CaseOfCase arg_id)
1903 -- Want to tick here so that we go round again,
1904 -- and maybe copy or inline the code.
1905 -- Not strictly CaseOfCase, but never mind
1906 ; return (unitFloat env arg_id arg, Var arg_id) }
1907 -- What if the arg should be case-bound?
1908 -- This has been this way for a long time, so I'll leave it,
1909 -- but I can't convince myself that it's right.
1911 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1912 -> SimplM (FloatsWith [InAlt])
1913 -- Absorbs the continuation into the new alternatives
1915 mkDupableAlts env case_bndr' alts dupable_cont
1918 go env [] = returnSmpl (emptyFloats env, [])
1920 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1921 ; addFloats env floats1 $ \ env -> do
1922 { (floats2, alts') <- go env alts
1923 ; returnSmpl (floats2, case mb_alt' of
1924 Just alt' -> alt' : alts'
1928 mkDupableAlt env case_bndr' cont alt
1929 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1931 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1933 Just (reft, (con, bndrs', rhs')) ->
1934 -- Safe to say that there are no handled-cons for the DEFAULT case
1936 if exprIsDupable rhs' then
1937 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
1938 -- It is worth checking for a small RHS because otherwise we
1939 -- get extra let bindings that may cause an extra iteration of the simplifier to
1940 -- inline back in place. Quite often the rhs is just a variable or constructor.
1941 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1942 -- iterations because the version with the let bindings looked big, and so wasn't
1943 -- inlined, but after the join points had been inlined it looked smaller, and so
1946 -- NB: we have to check the size of rhs', not rhs.
1947 -- Duplicating a small InAlt might invalidate occurrence information
1948 -- However, if it *is* dupable, we return the *un* simplified alternative,
1949 -- because otherwise we'd need to pair it up with an empty subst-env....
1950 -- but we only have one env shared between all the alts.
1951 -- (Remember we must zap the subst-env before re-simplifying something).
1952 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1956 rhs_ty' = exprType rhs'
1957 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1959 | isTyVar bndr = not (bndr `elemVarEnv` reft)
1960 -- Don't abstract over tyvar binders which are refined away
1961 -- See Note [Refinement] below
1962 | otherwise = not (isDeadBinder bndr)
1963 -- The deadness info on the new Ids is preserved by simplBinders
1965 -- If we try to lift a primitive-typed something out
1966 -- for let-binding-purposes, we will *caseify* it (!),
1967 -- with potentially-disastrous strictness results. So
1968 -- instead we turn it into a function: \v -> e
1969 -- where v::State# RealWorld#. The value passed to this function
1970 -- is realworld#, which generates (almost) no code.
1972 -- There's a slight infelicity here: we pass the overall
1973 -- case_bndr to all the join points if it's used in *any* RHS,
1974 -- because we don't know its usage in each RHS separately
1976 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1977 -- we make the join point into a function whenever used_bndrs'
1978 -- is empty. This makes the join-point more CPR friendly.
1979 -- Consider: let j = if .. then I# 3 else I# 4
1980 -- in case .. of { A -> j; B -> j; C -> ... }
1982 -- Now CPR doesn't w/w j because it's a thunk, so
1983 -- that means that the enclosing function can't w/w either,
1984 -- which is a lose. Here's the example that happened in practice:
1985 -- kgmod :: Int -> Int -> Int
1986 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1990 -- I have seen a case alternative like this:
1991 -- True -> \v -> ...
1992 -- It's a bit silly to add the realWorld dummy arg in this case, making
1995 -- (the \v alone is enough to make CPR happy) but I think it's rare
1997 ( if not (any isId used_bndrs')
1998 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
1999 returnSmpl ([rw_id], [Var realWorldPrimId])
2001 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
2002 ) `thenSmpl` \ (final_bndrs', final_args) ->
2004 -- See comment about "$j" name above
2005 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
2006 -- Notice the funky mkPiTypes. If the contructor has existentials
2007 -- it's possible that the join point will be abstracted over
2008 -- type varaibles as well as term variables.
2009 -- Example: Suppose we have
2010 -- data T = forall t. C [t]
2012 -- case (case e of ...) of
2013 -- C t xs::[t] -> rhs
2014 -- We get the join point
2015 -- let j :: forall t. [t] -> ...
2016 -- j = /\t \xs::[t] -> rhs
2018 -- case (case e of ...) of
2019 -- C t xs::[t] -> j t xs
2021 -- We make the lambdas into one-shot-lambdas. The
2022 -- join point is sure to be applied at most once, and doing so
2023 -- prevents the body of the join point being floated out by
2024 -- the full laziness pass
2025 really_final_bndrs = map one_shot final_bndrs'
2026 one_shot v | isId v = setOneShotLambda v
2028 join_rhs = mkLams really_final_bndrs rhs'
2029 join_call = mkApps (Var join_bndr) final_args
2031 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2038 MkT :: a -> b -> T a
2042 MkT a' b (p::a') (q::b) -> [p,w]
2044 The danger is that we'll make a join point
2048 and that's ill-typed, because (p::a') but (w::a).
2050 Solution so far: don't abstract over a', because the type refinement
2051 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2053 Then we must abstract over any refined free variables. Hmm. Maybe we
2054 could just abstract over *all* free variables, thereby lambda-lifting
2055 the join point? We should try this.