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 | preInlineUnconditionally env NotTopLevel bndr new_rhs
369 -- This happens; for example, the case_bndr during case of
370 -- known constructor: case (a,b) of x { (p,q) -> ... }
371 -- Here x isn't mentioned in the RHS, so we don't want to
372 -- create the (dead) let-binding let x = (a,b) in ...
374 -- Similarly, single occurrences can be inlined vigourously
375 -- e.g. case (f x, g y) of (a,b) -> ....
376 -- If a,b occur once we can avoid constructing the let binding for them.
377 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
381 = simplBinder env bndr `thenSmpl` \ (env, bndr') ->
382 completeNonRecX env False {- Non-strict; pessimistic -}
383 bndr bndr' new_rhs thing_inside
385 completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
386 = mkAtomicArgs is_strict
387 True {- OK to float unlifted -}
388 new_rhs `thenSmpl` \ (aux_binds, rhs2) ->
390 -- Make the arguments atomic if necessary,
391 -- adding suitable bindings
392 addAtomicBindsE env (fromOL aux_binds) $ \ env ->
393 completeLazyBind env NotTopLevel
394 old_bndr new_bndr rhs2 `thenSmpl` \ (floats, env) ->
395 addFloats env floats thing_inside
399 %************************************************************************
401 \subsection{Lazy bindings}
403 %************************************************************************
405 simplRecBind is used for
406 * recursive bindings only
409 simplRecBind :: SimplEnv -> TopLevelFlag
410 -> [(InId, InExpr)] -> [OutId]
411 -> SimplM (FloatsWith SimplEnv)
412 simplRecBind env top_lvl pairs bndrs'
413 = go env pairs bndrs' `thenSmpl` \ (floats, env) ->
414 returnSmpl (flattenFloats floats, env)
416 go env [] _ = returnSmpl (emptyFloats env, env)
418 go env ((bndr, rhs) : pairs) (bndr' : bndrs')
419 = simplRecOrTopPair env top_lvl bndr bndr' rhs `thenSmpl` \ (floats, env) ->
420 addFloats env floats (\env -> go env pairs bndrs')
424 simplRecOrTopPair is used for
425 * recursive bindings (whether top level or not)
426 * top-level non-recursive bindings
428 It assumes the binder has already been simplified, but not its IdInfo.
431 simplRecOrTopPair :: SimplEnv
433 -> InId -> OutId -- Binder, both pre-and post simpl
434 -> InExpr -- The RHS and its environment
435 -> SimplM (FloatsWith SimplEnv)
437 simplRecOrTopPair env top_lvl bndr bndr' rhs
438 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
439 = tick (PreInlineUnconditionally bndr) `thenSmpl_`
440 returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
443 = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
444 -- May not actually be recursive, but it doesn't matter
448 simplLazyBind is used for
449 * recursive bindings (whether top level or not)
450 * top-level non-recursive bindings
451 * non-top-level *lazy* non-recursive bindings
453 [Thus it deals with the lazy cases from simplNonRecBind, and all cases
454 from SimplRecOrTopBind]
457 1. It assumes that the binder is *already* simplified,
458 and is in scope, but not its IdInfo
460 2. It assumes that the binder type is lifted.
462 3. It does not check for pre-inline-unconditionallly;
463 that should have been done already.
466 simplLazyBind :: SimplEnv
467 -> TopLevelFlag -> RecFlag
468 -> InId -> OutId -- Binder, both pre-and post simpl
469 -> InExpr -> SimplEnv -- The RHS and its environment
470 -> SimplM (FloatsWith SimplEnv)
472 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
474 (env1,bndr2) = addLetIdInfo env bndr bndr1
475 rhs_env = setInScope rhs_se env1
476 is_top_level = isTopLevel top_lvl
477 ok_float_unlifted = not is_top_level && isNonRec is_rec
478 rhs_cont = mkRhsStop (idType bndr2)
480 -- Simplify the RHS; note the mkRhsStop, which tells
481 -- the simplifier that this is the RHS of a let.
482 simplExprF rhs_env rhs rhs_cont `thenSmpl` \ (floats, rhs1) ->
484 -- If any of the floats can't be floated, give up now
485 -- (The allLifted predicate says True for empty floats.)
486 if (not ok_float_unlifted && not (allLifted floats)) then
487 completeLazyBind env1 top_lvl bndr bndr2
488 (wrapFloats floats rhs1)
491 -- ANF-ise a constructor or PAP rhs
492 mkAtomicArgs False {- Not strict -}
493 ok_float_unlifted rhs1 `thenSmpl` \ (aux_binds, rhs2) ->
495 -- If the result is a PAP, float the floats out, else wrap them
496 -- By this time it's already been ANF-ised (if necessary)
497 if isEmptyFloats floats && isNilOL aux_binds then -- Shortcut a common case
498 completeLazyBind env1 top_lvl bndr bndr2 rhs2
500 else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
501 -- WARNING: long dodgy argument coming up
502 -- WANTED: a better way to do this
504 -- We can't use "exprIsCheap" instead of exprIsHNF,
505 -- because that causes a strictness bug.
506 -- x = let y* = E in case (scc y) of { T -> F; F -> T}
507 -- The case expression is 'cheap', but it's wrong to transform to
508 -- y* = E; x = case (scc y) of {...}
509 -- Either we must be careful not to float demanded non-values, or
510 -- we must use exprIsHNF for the test, which ensures that the
511 -- thing is non-strict. So exprIsHNF => bindings are non-strict
512 -- I think. The WARN below tests for this.
514 -- We use exprIsTrivial here because we want to reveal lone variables.
515 -- E.g. let { x = letrec { y = E } in y } in ...
516 -- Here we definitely want to float the y=E defn.
517 -- exprIsHNF definitely isn't right for that.
519 -- Again, the floated binding can't be strict; if it's recursive it'll
520 -- be non-strict; if it's non-recursive it'd be inlined.
522 -- Note [SCC-and-exprIsTrivial]
524 -- y = let { x* = E } in scc "foo" x
525 -- then we do *not* want to float out the x binding, because
526 -- it's strict! Fortunately, exprIsTrivial replies False to
529 -- There's a subtlety here. There may be a binding (x* = e) in the
530 -- floats, where the '*' means 'will be demanded'. So is it safe
531 -- to float it out? Answer no, but it won't matter because
532 -- we only float if (a) arg' is a WHNF, or (b) it's going to top level
533 -- and so there can't be any 'will be demanded' bindings in the floats.
535 ASSERT2( is_top_level || not (any demanded_float (floatBinds floats)),
536 ppr (filter demanded_float (floatBinds floats)) )
538 tick LetFloatFromLet `thenSmpl_` (
539 addFloats env1 floats $ \ env2 ->
540 addAtomicBinds env2 (fromOL aux_binds) $ \ env3 ->
541 completeLazyBind env3 top_lvl bndr bndr2 rhs2)
544 completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
547 demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
548 -- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
549 demanded_float (Rec _) = False
554 %************************************************************************
556 \subsection{Completing a lazy binding}
558 %************************************************************************
561 * deals only with Ids, not TyVars
562 * takes an already-simplified binder and RHS
563 * is used for both recursive and non-recursive bindings
564 * is used for both top-level and non-top-level bindings
566 It does the following:
567 - tries discarding a dead binding
568 - tries PostInlineUnconditionally
569 - add unfolding [this is the only place we add an unfolding]
572 It does *not* attempt to do let-to-case. Why? Because it is used for
573 - top-level bindings (when let-to-case is impossible)
574 - many situations where the "rhs" is known to be a WHNF
575 (so let-to-case is inappropriate).
578 completeLazyBind :: SimplEnv
579 -> TopLevelFlag -- Flag stuck into unfolding
580 -> InId -- Old binder
581 -> OutId -- New binder
582 -> OutExpr -- Simplified RHS
583 -> SimplM (FloatsWith SimplEnv)
584 -- We return a new SimplEnv, because completeLazyBind may choose to do its work
585 -- by extending the substitution (e.g. let x = y in ...)
586 -- The new binding (if any) is returned as part of the floats.
587 -- NB: the returned SimplEnv has the right SubstEnv, but you should
588 -- (as usual) use the in-scope-env from the floats
590 completeLazyBind env top_lvl old_bndr new_bndr new_rhs
591 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
592 = -- Drop the binding
593 tick (PostInlineUnconditionally old_bndr) `thenSmpl_`
594 returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
595 -- Use the substitution to make quite, quite sure that the substitution
596 -- will happen, since we are going to discard the binding
601 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
603 -- Add the unfolding *only* for non-loop-breakers
604 -- Making loop breakers not have an unfolding at all
605 -- means that we can avoid tests in exprIsConApp, for example.
606 -- This is important: if exprIsConApp says 'yes' for a recursive
607 -- thing, then we can get into an infinite loop
609 -- If the unfolding is a value, the demand info may
610 -- go pear-shaped, so we nuke it. Example:
612 -- case x of (p,q) -> h p q x
613 -- Here x is certainly demanded. But after we've nuked
614 -- the case, we'll get just
615 -- let x = (a,b) in h a b x
616 -- and now x is not demanded (I'm assuming h is lazy)
617 -- This really happens. Similarly
618 -- let f = \x -> e in ...f..f...
619 -- After inling f at some of its call sites the original binding may
620 -- (for example) be no longer strictly demanded.
621 -- The solution here is a bit ad hoc...
622 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
623 final_info | loop_breaker = new_bndr_info
624 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
625 | otherwise = info_w_unf
627 final_id = new_bndr `setIdInfo` final_info
629 -- These seqs forces the Id, and hence its IdInfo,
630 -- and hence any inner substitutions
632 returnSmpl (unitFloat env final_id new_rhs, env)
635 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
636 loop_breaker = isLoopBreaker occ_info
637 old_info = idInfo old_bndr
638 occ_info = occInfo old_info
643 %************************************************************************
645 \subsection[Simplify-simplExpr]{The main function: simplExpr}
647 %************************************************************************
649 The reason for this OutExprStuff stuff is that we want to float *after*
650 simplifying a RHS, not before. If we do so naively we get quadratic
651 behaviour as things float out.
653 To see why it's important to do it after, consider this (real) example:
667 a -- Can't inline a this round, cos it appears twice
671 Each of the ==> steps is a round of simplification. We'd save a
672 whole round if we float first. This can cascade. Consider
677 let f = let d1 = ..d.. in \y -> e
681 in \x -> ...(\y ->e)...
683 Only in this second round can the \y be applied, and it
684 might do the same again.
688 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
689 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
691 expr_ty' = substTy env (exprType expr)
692 -- The type in the Stop continuation, expr_ty', is usually not used
693 -- It's only needed when discarding continuations after finding
694 -- a function that returns bottom.
695 -- Hence the lazy substitution
698 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
699 -- Simplify an expression, given a continuation
700 simplExprC env expr cont
701 = simplExprF env expr cont `thenSmpl` \ (floats, expr) ->
702 returnSmpl (wrapFloats floats expr)
704 simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
705 -- Simplify an expression, returning floated binds
707 simplExprF env (Var v) cont = simplVar env v cont
708 simplExprF env (Lit lit) cont = rebuild env (Lit lit) cont
709 simplExprF env expr@(Lam _ _) cont = simplLam env expr cont
710 simplExprF env (Note note expr) cont = simplNote env note expr cont
711 simplExprF env (App fun arg) cont = simplExprF env fun (ApplyTo NoDup arg env cont)
713 simplExprF env (Type ty) cont
714 = ASSERT( contIsRhsOrArg cont )
715 simplType env ty `thenSmpl` \ ty' ->
716 rebuild env (Type ty') cont
718 simplExprF env (Case scrut bndr case_ty alts) cont
719 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
720 = -- Simplify the scrutinee with a Select continuation
721 simplExprF env scrut (Select NoDup bndr alts env cont)
724 = -- If case-of-case is off, simply simplify the case expression
725 -- in a vanilla Stop context, and rebuild the result around it
726 simplExprC env scrut case_cont `thenSmpl` \ case_expr' ->
727 rebuild env case_expr' cont
729 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
730 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
732 simplExprF env (Let (Rec pairs) body) cont
733 = simplRecBndrs env (map fst pairs) `thenSmpl` \ (env, bndrs') ->
734 -- NB: bndrs' don't have unfoldings or rules
735 -- We add them as we go down
737 simplRecBind env NotTopLevel pairs bndrs' `thenSmpl` \ (floats, env) ->
738 addFloats env floats $ \ env ->
739 simplExprF env body cont
741 -- A non-recursive let is dealt with by simplNonRecBind
742 simplExprF env (Let (NonRec bndr rhs) body) cont
743 = simplNonRecBind env bndr rhs env (contResultType cont) $ \ env ->
744 simplExprF env body cont
747 ---------------------------------
748 simplType :: SimplEnv -> InType -> SimplM OutType
749 -- Kept monadic just so we can do the seqType
751 = seqType new_ty `seq` returnSmpl new_ty
753 new_ty = substTy env ty
757 %************************************************************************
761 %************************************************************************
764 simplLam env fun cont
767 zap_it = mkLamBndrZapper fun (countArgs cont)
768 cont_ty = contResultType cont
770 -- Type-beta reduction
771 go env (Lam bndr body) (ApplyTo _ (Type ty_arg) arg_se body_cont)
772 = ASSERT( isTyVar bndr )
773 tick (BetaReduction bndr) `thenSmpl_`
774 simplType (setInScope arg_se env) ty_arg `thenSmpl` \ ty_arg' ->
775 go (extendTvSubst env bndr ty_arg') body body_cont
777 -- Ordinary beta reduction
778 go env (Lam bndr body) cont@(ApplyTo _ arg arg_se body_cont)
779 = tick (BetaReduction bndr) `thenSmpl_`
780 simplNonRecBind env (zap_it bndr) arg arg_se cont_ty $ \ env ->
781 go env body body_cont
783 -- Not enough args, so there are real lambdas left to put in the result
784 go env lam@(Lam _ _) cont
785 = simplLamBndrs env bndrs `thenSmpl` \ (env, bndrs') ->
786 simplExpr env body `thenSmpl` \ body' ->
787 mkLam env bndrs' body' cont `thenSmpl` \ (floats, new_lam) ->
788 addFloats env floats $ \ env ->
789 rebuild env new_lam cont
791 (bndrs,body) = collectBinders lam
793 -- Exactly enough args
794 go env expr cont = simplExprF env expr cont
796 mkLamBndrZapper :: CoreExpr -- Function
797 -> Int -- Number of args supplied, *including* type args
798 -> Id -> Id -- Use this to zap the binders
799 mkLamBndrZapper fun n_args
800 | n_args >= n_params fun = \b -> b -- Enough args
801 | otherwise = \b -> zapLamIdInfo b
803 -- NB: we count all the args incl type args
804 -- so we must count all the binders (incl type lambdas)
805 n_params (Note _ e) = n_params e
806 n_params (Lam b e) = 1 + n_params e
807 n_params other = 0::Int
811 %************************************************************************
815 %************************************************************************
818 simplNote env (Coerce to from) body cont
820 addCoerce s1 k1 cont -- Drop redundant coerces. This can happen if a polymoprhic
821 -- (coerce a b e) is instantiated with a=ty1 b=ty2 and the
822 -- two are the same. This happens a lot in Happy-generated parsers
823 | s1 `coreEqType` k1 = cont
825 addCoerce s1 k1 (CoerceIt t1 cont)
826 -- coerce T1 S1 (coerce S1 K1 e)
829 -- coerce T1 K1 e, otherwise
831 -- For example, in the initial form of a worker
832 -- we may find (coerce T (coerce S (\x.e))) y
833 -- and we'd like it to simplify to e[y/x] in one round
835 | t1 `coreEqType` k1 = cont -- The coerces cancel out
836 | otherwise = CoerceIt t1 cont -- They don't cancel, but
837 -- the inner one is redundant
839 addCoerce t1t2 s1s2 (ApplyTo dup arg arg_se cont)
840 | not (isTypeArg arg), -- This whole case only works for value args
841 -- Could upgrade to have equiv thing for type apps too
842 Just (s1, s2) <- splitFunTy_maybe s1s2
843 -- (coerce (T1->T2) (S1->S2) F) E
845 -- coerce T2 S2 (F (coerce S1 T1 E))
847 -- t1t2 must be a function type, T1->T2, because it's applied to something
848 -- but s1s2 might conceivably not be
850 -- When we build the ApplyTo we can't mix the out-types
851 -- with the InExpr in the argument, so we simply substitute
852 -- to make it all consistent. It's a bit messy.
853 -- But it isn't a common case.
855 (t1,t2) = splitFunTy t1t2
856 new_arg = mkCoerce2 s1 t1 (substExpr arg_env arg)
857 arg_env = setInScope arg_se env
859 ApplyTo dup new_arg (zapSubstEnv env) (addCoerce t2 s2 cont)
861 addCoerce to' _ cont = CoerceIt to' cont
863 simplType env to `thenSmpl` \ to' ->
864 simplType env from `thenSmpl` \ from' ->
865 simplExprF env body (addCoerce to' from' cont)
868 -- Hack: we only distinguish subsumed cost centre stacks for the purposes of
869 -- inlining. All other CCCSs are mapped to currentCCS.
870 simplNote env (SCC cc) e cont
871 = simplExpr (setEnclosingCC env currentCCS) e `thenSmpl` \ e' ->
872 rebuild env (mkSCC cc e') cont
874 -- See notes with SimplMonad.inlineMode
875 simplNote env InlineMe e cont
876 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
877 = -- Don't inline inside an INLINE expression
878 simplExpr (setMode inlineMode env ) e `thenSmpl` \ e' ->
879 rebuild env (mkInlineMe e') cont
881 | otherwise -- Dissolve the InlineMe note if there's
882 -- an interesting context of any kind to combine with
883 -- (even a type application -- anything except Stop)
884 = simplExprF env e cont
886 simplNote env (CoreNote s) e cont
887 = simplExpr env e `thenSmpl` \ e' ->
888 rebuild env (Note (CoreNote s) e') cont
892 %************************************************************************
894 \subsection{Dealing with calls}
896 %************************************************************************
899 simplVar env var cont
900 = case substId env var of
901 DoneEx e -> simplExprF (zapSubstEnv env) e cont
902 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
903 DoneId var1 occ -> completeCall (zapSubstEnv env) var1 occ cont
904 -- Note [zapSubstEnv]
905 -- The template is already simplified, so don't re-substitute.
906 -- This is VITAL. Consider
908 -- let y = \z -> ...x... in
910 -- We'll clone the inner \x, adding x->x' in the id_subst
911 -- Then when we inline y, we must *not* replace x by x' in
912 -- the inlined copy!!
914 ---------------------------------------------------------
915 -- Dealing with a call site
917 completeCall env var occ_info cont
918 = -- Simplify the arguments
919 getDOptsSmpl `thenSmpl` \ dflags ->
921 chkr = getSwitchChecker env
922 (args, call_cont) = getContArgs chkr var cont
925 simplifyArgs env fn_ty (interestingArgContext var call_cont) args
926 (contResultType call_cont) $ \ env args ->
928 -- Next, look for rules or specialisations that match
930 -- It's important to simplify the args first, because the rule-matcher
931 -- doesn't do substitution as it goes. We don't want to use subst_args
932 -- (defined in the 'where') because that throws away useful occurrence info,
933 -- and perhaps-very-important specialisations.
935 -- Some functions have specialisations *and* are strict; in this case,
936 -- we don't want to inline the wrapper of the non-specialised thing; better
937 -- to call the specialised thing instead.
938 -- We used to use the black-listing mechanism to ensure that inlining of
939 -- the wrapper didn't occur for things that have specialisations till a
940 -- later phase, so but now we just try RULES first
942 -- You might think that we shouldn't apply rules for a loop breaker:
943 -- doing so might give rise to an infinite loop, because a RULE is
944 -- rather like an extra equation for the function:
945 -- RULE: f (g x) y = x+y
948 -- But it's too drastic to disable rules for loop breakers.
949 -- Even the foldr/build rule would be disabled, because foldr
950 -- is recursive, and hence a loop breaker:
951 -- foldr k z (build g) = g k z
952 -- So it's up to the programmer: rules can cause divergence
955 in_scope = getInScope env
957 maybe_rule = case activeRule env of
958 Nothing -> Nothing -- No rules apply
959 Just act_fn -> lookupRule act_fn in_scope rules var args
962 Just (rule_name, rule_rhs) ->
963 tick (RuleFired rule_name) `thenSmpl_`
964 (if dopt Opt_D_dump_inlinings dflags then
965 pprTrace "Rule fired" (vcat [
966 text "Rule:" <+> ftext rule_name,
967 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
968 text "After: " <+> pprCoreExpr rule_rhs,
969 text "Cont: " <+> ppr call_cont])
972 simplExprF env rule_rhs call_cont ;
974 Nothing -> -- No rules
976 -- Next, look for an inlining
978 arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
979 interesting_cont = interestingCallContext (notNull args)
982 active_inline = activeInline env var occ_info
983 maybe_inline = callSiteInline dflags active_inline occ_info
984 var arg_infos interesting_cont
986 case maybe_inline of {
987 Just unfolding -- There is an inlining!
988 -> tick (UnfoldingDone var) `thenSmpl_`
989 (if dopt Opt_D_dump_inlinings dflags then
990 pprTrace "Inlining done" (vcat [
991 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
992 text "Inlined fn: " <+> ppr unfolding,
993 text "Cont: " <+> ppr call_cont])
996 makeThatCall env var unfolding args call_cont
999 Nothing -> -- No inlining!
1002 rebuild env (mkApps (Var var) args) call_cont
1005 makeThatCall :: SimplEnv
1007 -> InExpr -- Inlined function rhs
1008 -> [OutExpr] -- Arguments, already simplified
1009 -> SimplCont -- After the call
1010 -> SimplM FloatsWithExpr
1011 -- Similar to simplLam, but this time
1012 -- the arguments are already simplified
1013 makeThatCall orig_env var fun@(Lam _ _) args cont
1014 = go orig_env fun args
1016 zap_it = mkLamBndrZapper fun (length args)
1018 -- Type-beta reduction
1019 go env (Lam bndr body) (Type ty_arg : args)
1020 = ASSERT( isTyVar bndr )
1021 tick (BetaReduction bndr) `thenSmpl_`
1022 go (extendTvSubst env bndr ty_arg) body args
1024 -- Ordinary beta reduction
1025 go env (Lam bndr body) (arg : args)
1026 = tick (BetaReduction bndr) `thenSmpl_`
1027 simplNonRecX env (zap_it bndr) arg $ \ env ->
1030 -- Not enough args, so there are real lambdas left to put in the result
1032 = simplExprF env fun (pushContArgs orig_env args cont)
1033 -- NB: orig_env; the correct environment to capture with
1034 -- the arguments.... env has been augmented with substitutions
1035 -- from the beta reductions.
1037 makeThatCall env var fun args cont
1038 = simplExprF env fun (pushContArgs env args cont)
1042 %************************************************************************
1044 \subsection{Arguments}
1046 %************************************************************************
1049 ---------------------------------------------------------
1050 -- Simplifying the arguments of a call
1052 simplifyArgs :: SimplEnv
1053 -> OutType -- Type of the function
1054 -> Bool -- True if the fn has RULES
1055 -> [(InExpr, SimplEnv, Bool)] -- Details of the arguments
1056 -> OutType -- Type of the continuation
1057 -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
1058 -> SimplM FloatsWithExpr
1060 -- [CPS-like because of strict arguments]
1062 -- Simplify the arguments to a call.
1063 -- This part of the simplifier may break the no-shadowing invariant
1065 -- f (...(\a -> e)...) (case y of (a,b) -> e')
1066 -- where f is strict in its second arg
1067 -- If we simplify the innermost one first we get (...(\a -> e)...)
1068 -- Simplifying the second arg makes us float the case out, so we end up with
1069 -- case y of (a,b) -> f (...(\a -> e)...) e'
1070 -- So the output does not have the no-shadowing invariant. However, there is
1071 -- no danger of getting name-capture, because when the first arg was simplified
1072 -- we used an in-scope set that at least mentioned all the variables free in its
1073 -- static environment, and that is enough.
1075 -- We can't just do innermost first, or we'd end up with a dual problem:
1076 -- case x of (a,b) -> f e (...(\a -> e')...)
1078 -- I spent hours trying to recover the no-shadowing invariant, but I just could
1079 -- not think of an elegant way to do it. The simplifier is already knee-deep in
1080 -- continuations. We have to keep the right in-scope set around; AND we have
1081 -- to get the effect that finding (error "foo") in a strict arg position will
1082 -- discard the entire application and replace it with (error "foo"). Getting
1083 -- all this at once is TOO HARD!
1085 simplifyArgs env fn_ty has_rules args cont_ty thing_inside
1086 = go env fn_ty args thing_inside
1088 go env fn_ty [] thing_inside = thing_inside env []
1089 go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty $ \ env arg' ->
1090 go env (applyTypeToArg fn_ty arg') args $ \ env args' ->
1091 thing_inside env (arg':args')
1093 simplifyArg env fn_ty has_rules (Type ty_arg, se, _) cont_ty thing_inside
1094 = simplType (setInScope se env) ty_arg `thenSmpl` \ new_ty_arg ->
1095 thing_inside env (Type new_ty_arg)
1097 simplifyArg env fn_ty has_rules (val_arg, arg_se, is_strict) cont_ty thing_inside
1099 = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
1101 | otherwise -- Lazy argument
1102 -- DO NOT float anything outside, hence simplExprC
1103 -- There is no benefit (unlike in a let-binding), and we'd
1104 -- have to be very careful about bogus strictness through
1105 -- floating a demanded let.
1106 = simplExprC (setInScope arg_se env) val_arg
1107 (mkLazyArgStop arg_ty has_rules) `thenSmpl` \ arg1 ->
1108 thing_inside env arg1
1110 arg_ty = funArgTy fn_ty
1113 simplStrictArg :: LetRhsFlag
1114 -> SimplEnv -- The env of the call
1115 -> InExpr -> SimplEnv -- The arg plus its env
1116 -> OutType -- arg_ty: type of the argument
1117 -> OutType -- cont_ty: Type of thing computed by the context
1118 -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)
1119 -- Takes an expression of type rhs_ty,
1120 -- returns an expression of type cont_ty
1121 -- The env passed to this continuation is the
1122 -- env of the call, plus any new in-scope variables
1123 -> SimplM FloatsWithExpr -- An expression of type cont_ty
1125 simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
1126 = simplExprF (setInScope arg_env call_env) arg
1127 (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
1128 -- Notice the way we use arg_env (augmented with in-scope vars from call_env)
1129 -- to simplify the argument
1130 -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
1134 %************************************************************************
1136 \subsection{mkAtomicArgs}
1138 %************************************************************************
1140 mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
1141 constructor application and, if so, converts it to ANF, so that the
1142 resulting thing can be inlined more easily. Thus
1149 There are three sorts of binding context, specified by the two
1155 N N Top-level or recursive Only bind args of lifted type
1157 N Y Non-top-level and non-recursive, Bind args of lifted type, or
1158 but lazy unlifted-and-ok-for-speculation
1160 Y Y Non-top-level, non-recursive, Bind all args
1161 and strict (demanded)
1168 there is no point in transforming to
1170 x = case (y div# z) of r -> MkC r
1172 because the (y div# z) can't float out of the let. But if it was
1173 a *strict* let, then it would be a good thing to do. Hence the
1174 context information.
1177 mkAtomicArgs :: Bool -- A strict binding
1178 -> Bool -- OK to float unlifted args
1180 -> SimplM (OrdList (OutId,OutExpr), -- The floats (unusually) may include
1181 OutExpr) -- things that need case-binding,
1182 -- if the strict-binding flag is on
1184 mkAtomicArgs is_strict ok_float_unlifted rhs
1185 | (Var fun, args) <- collectArgs rhs, -- It's an application
1186 isDataConWorkId fun || valArgCount args < idArity fun -- And it's a constructor or PAP
1187 = go fun nilOL [] args -- Have a go
1189 | otherwise = bale_out -- Give up
1192 bale_out = returnSmpl (nilOL, rhs)
1194 go fun binds rev_args []
1195 = returnSmpl (binds, mkApps (Var fun) (reverse rev_args))
1197 go fun binds rev_args (arg : args)
1198 | exprIsTrivial arg -- Easy case
1199 = go fun binds (arg:rev_args) args
1201 | not can_float_arg -- Can't make this arg atomic
1202 = bale_out -- ... so give up
1204 | otherwise -- Don't forget to do it recursively
1205 -- E.g. x = a:b:c:[]
1206 = mkAtomicArgs is_strict ok_float_unlifted arg `thenSmpl` \ (arg_binds, arg') ->
1207 newId FSLIT("a") arg_ty `thenSmpl` \ arg_id ->
1208 go fun ((arg_binds `snocOL` (arg_id,arg')) `appOL` binds)
1209 (Var arg_id : rev_args) args
1211 arg_ty = exprType arg
1212 can_float_arg = is_strict
1213 || not (isUnLiftedType arg_ty)
1214 || (ok_float_unlifted && exprOkForSpeculation arg)
1217 addAtomicBinds :: SimplEnv -> [(OutId,OutExpr)]
1218 -> (SimplEnv -> SimplM (FloatsWith a))
1219 -> SimplM (FloatsWith a)
1220 addAtomicBinds env [] thing_inside = thing_inside env
1221 addAtomicBinds env ((v,r):bs) thing_inside = addAuxiliaryBind env (NonRec v r) $ \ env ->
1222 addAtomicBinds env bs thing_inside
1224 addAtomicBindsE :: SimplEnv -> [(OutId,OutExpr)]
1225 -> (SimplEnv -> SimplM FloatsWithExpr)
1226 -> SimplM FloatsWithExpr
1227 -- Same again, but this time we're in an expression context,
1228 -- and may need to do some case bindings
1230 addAtomicBindsE env [] thing_inside
1232 addAtomicBindsE env ((v,r):bs) thing_inside
1233 | needsCaseBinding (idType v) r
1234 = addAtomicBindsE (addNewInScopeIds env [v]) bs thing_inside `thenSmpl` \ (floats, expr) ->
1235 WARN( exprIsTrivial expr, ppr v <+> pprCoreExpr expr )
1236 (let body = wrapFloats floats expr in
1237 returnSmpl (emptyFloats env, Case r v (exprType body) [(DEFAULT,[],body)]))
1240 = addAuxiliaryBind env (NonRec v r) $ \ env ->
1241 addAtomicBindsE env bs thing_inside
1245 %************************************************************************
1247 \subsection{The main rebuilder}
1249 %************************************************************************
1252 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
1254 rebuild env expr (Stop _ _ _) = rebuildDone env expr
1255 rebuild env expr (ArgOf _ _ _ cont_fn) = cont_fn env expr
1256 rebuild env expr (CoerceIt to_ty cont) = rebuild env (mkCoerce to_ty expr) cont
1257 rebuild env expr (Select _ bndr alts se cont) = rebuildCase (setInScope se env) expr bndr alts cont
1258 rebuild env expr (ApplyTo _ arg se cont) = rebuildApp (setInScope se env) expr arg cont
1260 rebuildApp env fun arg cont
1261 = simplExpr env arg `thenSmpl` \ arg' ->
1262 rebuild env (App fun arg') cont
1264 rebuildDone env expr = returnSmpl (emptyFloats env, expr)
1268 %************************************************************************
1270 \subsection{Functions dealing with a case}
1272 %************************************************************************
1274 Blob of helper functions for the "case-of-something-else" situation.
1277 ---------------------------------------------------------
1278 -- Eliminate the case if possible
1280 rebuildCase :: SimplEnv
1281 -> OutExpr -- Scrutinee
1282 -> InId -- Case binder
1283 -> [InAlt] -- Alternatives (inceasing order)
1285 -> SimplM FloatsWithExpr
1287 rebuildCase env scrut case_bndr alts cont
1288 | Just (con,args) <- exprIsConApp_maybe scrut
1289 -- Works when the scrutinee is a variable with a known unfolding
1290 -- as well as when it's an explicit constructor application
1291 = knownCon env (DataAlt con) args case_bndr alts cont
1293 | Lit lit <- scrut -- No need for same treatment as constructors
1294 -- because literals are inlined more vigorously
1295 = knownCon env (LitAlt lit) [] case_bndr alts cont
1298 = -- Prepare the continuation;
1299 -- The new subst_env is in place
1300 prepareCaseCont env alts cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1301 addFloats env floats $ \ env ->
1304 -- The case expression is annotated with the result type of the continuation
1305 -- This may differ from the type originally on the case. For example
1306 -- case(T) (case(Int#) a of { True -> 1#; False -> 0# }) of
1309 -- let j a# = <blob>
1310 -- in case(T) a of { True -> j 1#; False -> j 0# }
1311 -- Note that the case that scrutinises a now returns a T not an Int#
1312 res_ty' = contResultType dup_cont
1315 -- Deal with case binder
1316 simplCaseBinder env scrut case_bndr `thenSmpl` \ (alt_env, case_bndr') ->
1318 -- Deal with the case alternatives
1319 simplAlts alt_env scrut case_bndr' alts dup_cont `thenSmpl` \ alts' ->
1321 -- Put the case back together
1322 mkCase scrut case_bndr' res_ty' alts' `thenSmpl` \ case_expr ->
1324 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1325 -- The case binder *not* scope over the whole returned case-expression
1326 rebuild env case_expr nondup_cont
1329 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1330 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1331 way, there's a chance that v will now only be used once, and hence
1336 There is a time we *don't* want to do that, namely when
1337 -fno-case-of-case is on. This happens in the first simplifier pass,
1338 and enhances full laziness. Here's the bad case:
1339 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1340 If we eliminate the inner case, we trap it inside the I# v -> arm,
1341 which might prevent some full laziness happening. I've seen this
1342 in action in spectral/cichelli/Prog.hs:
1343 [(m,n) | m <- [1..max], n <- [1..max]]
1344 Hence the check for NoCaseOfCase.
1348 There is another situation when we don't want to do it. If we have
1350 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1351 ...other cases .... }
1353 We'll perform the binder-swap for the outer case, giving
1355 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1356 ...other cases .... }
1358 But there is no point in doing it for the inner case, because w1 can't
1359 be inlined anyway. Furthermore, doing the case-swapping involves
1360 zapping w2's occurrence info (see paragraphs that follow), and that
1361 forces us to bind w2 when doing case merging. So we get
1363 case x of w1 { A -> let w2 = w1 in e1
1364 B -> let w2 = w1 in e2
1365 ...other cases .... }
1367 This is plain silly in the common case where w2 is dead.
1369 Even so, I can't see a good way to implement this idea. I tried
1370 not doing the binder-swap if the scrutinee was already evaluated
1371 but that failed big-time:
1375 case v of w { MkT x ->
1376 case x of x1 { I# y1 ->
1377 case x of x2 { I# y2 -> ...
1379 Notice that because MkT is strict, x is marked "evaluated". But to
1380 eliminate the last case, we must either make sure that x (as well as
1381 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1382 the binder-swap. So this whole note is a no-op.
1386 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1387 any occurrence info (eg IAmDead) in the case binder, because the
1388 case-binder now effectively occurs whenever v does. AND we have to do
1389 the same for the pattern-bound variables! Example:
1391 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1393 Here, b and p are dead. But when we move the argment inside the first
1394 case RHS, and eliminate the second case, we get
1396 case x of { (a,b) -> a b }
1398 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1401 Indeed, this can happen anytime the case binder isn't dead:
1402 case <any> of x { (a,b) ->
1403 case x of { (p,q) -> p } }
1404 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1405 The point is that we bring into the envt a binding
1407 after the outer case, and that makes (a,b) alive. At least we do unless
1408 the case binder is guaranteed dead.
1411 simplCaseBinder env (Var v) case_bndr
1412 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
1414 -- Failed try [see Note 2 above]
1415 -- not (isEvaldUnfolding (idUnfolding v))
1417 = simplBinder env (zap case_bndr) `thenSmpl` \ (env, case_bndr') ->
1418 returnSmpl (modifyInScope env v case_bndr', case_bndr')
1419 -- We could extend the substitution instead, but it would be
1420 -- a hack because then the substitution wouldn't be idempotent
1421 -- any more (v is an OutId). And this does just as well.
1423 zap b = b `setIdOccInfo` NoOccInfo
1425 simplCaseBinder env other_scrut case_bndr
1426 = simplBinder env case_bndr `thenSmpl` \ (env, case_bndr') ->
1427 returnSmpl (env, case_bndr')
1431 simplAlts does two things:
1433 1. Eliminate alternatives that cannot match, including the
1434 DEFAULT alternative.
1436 2. If the DEFAULT alternative can match only one possible constructor,
1437 then make that constructor explicit.
1439 case e of x { DEFAULT -> rhs }
1441 case e of x { (a,b) -> rhs }
1442 where the type is a single constructor type. This gives better code
1443 when rhs also scrutinises x or e.
1445 Here "cannot match" includes knowledge from GADTs
1447 It's a good idea do do this stuff before simplifying the alternatives, to
1448 avoid simplifying alternatives we know can't happen, and to come up with
1449 the list of constructors that are handled, to put into the IdInfo of the
1450 case binder, for use when simplifying the alternatives.
1452 Eliminating the default alternative in (1) isn't so obvious, but it can
1455 data Colour = Red | Green | Blue
1464 DEFAULT -> [ case y of ... ]
1466 If we inline h into f, the default case of the inlined h can't happen.
1467 If we don't notice this, we may end up filtering out *all* the cases
1468 of the inner case y, which give us nowhere to go!
1472 simplAlts :: SimplEnv
1474 -> OutId -- Case binder
1475 -> [InAlt] -> SimplCont
1476 -> SimplM [OutAlt] -- Includes the continuation
1478 simplAlts env scrut case_bndr' alts cont'
1479 = do { mb_alts <- mapSmpl (simplAlt env imposs_cons case_bndr' cont') alts_wo_default
1480 ; default_alts <- simplDefault env case_bndr' imposs_deflt_cons cont' maybe_deflt
1481 ; return (mergeAlts default_alts [alt' | Just (_, alt') <- mb_alts]) }
1482 -- We need the mergeAlts in case the new default_alt
1483 -- has turned into a constructor alternative.
1485 (alts_wo_default, maybe_deflt) = findDefault alts
1486 imposs_cons = case scrut of
1487 Var v -> otherCons (idUnfolding v)
1490 -- "imposs_deflt_cons" are handled either by the context,
1491 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1492 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1494 simplDefault :: SimplEnv
1495 -> OutId -- Case binder; need just for its type. Note that as an
1496 -- OutId, it has maximum information; this is important.
1497 -- Test simpl013 is an example
1498 -> [AltCon] -- These cons can't happen when matching the default
1501 -> SimplM [OutAlt] -- One branch or none; we use a list because it's what
1502 -- mergeAlts expects
1505 simplDefault env case_bndr' imposs_cons cont Nothing
1506 = return [] -- No default branch
1507 simplDefault env case_bndr' imposs_cons cont (Just rhs)
1508 | -- This branch handles the case where we are
1509 -- scrutinisng an algebraic data type
1510 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1511 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1512 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1513 -- case x of { DEFAULT -> e }
1514 -- and we don't want to fill in a default for them!
1515 Just all_cons <- tyConDataCons_maybe tycon,
1516 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1517 -- which GHC allows, then the case expression will have at most a default
1518 -- alternative. We don't want to eliminate that alternative, because the
1519 -- invariant is that there's always one alternative. It's more convenient
1521 -- case x of { DEFAULT -> e }
1522 -- as it is, rather than transform it to
1523 -- error "case cant match"
1524 -- which would be quite legitmate. But it's a really obscure corner, and
1525 -- not worth wasting code on.
1527 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1528 poss_data_cons = filterOut (`elem` imposs_data_cons) all_cons
1529 gadt_imposs | all isTyVarTy inst_tys = []
1530 | otherwise = filter (cant_match inst_tys) poss_data_cons
1531 final_poss = filterOut (`elem` gadt_imposs) poss_data_cons
1533 = case final_poss of
1534 [] -> returnSmpl [] -- Eliminate the default alternative
1535 -- altogether if it can't match
1537 [con] -> -- It matches exactly one constructor, so fill it in
1538 do { con_alt <- mkDataConAlt case_bndr' con inst_tys rhs
1539 ; Just (_, alt') <- simplAlt env [] case_bndr' cont con_alt
1540 -- The simplAlt must succeed with Just because we have
1541 -- already filtered out construtors that can't match
1544 two_or_more -> simplify_default (map DataAlt gadt_imposs ++ imposs_cons)
1547 = simplify_default imposs_cons
1549 cant_match tys data_con = not (dataConCanMatch data_con tys)
1551 simplify_default imposs_cons
1552 = do { let env' = mk_rhs_env env case_bndr' (mkOtherCon imposs_cons)
1553 -- Record the constructors that the case-binder *can't* be.
1554 ; rhs' <- simplExprC env' rhs cont
1555 ; return [(DEFAULT, [], rhs')] }
1557 mkDataConAlt :: Id -> DataCon -> [OutType] -> InExpr -> SimplM InAlt
1558 -- Make a data-constructor alternative to replace the DEFAULT case
1559 -- NB: there's something a bit bogus here, because we put OutTypes into an InAlt
1560 mkDataConAlt case_bndr con tys rhs
1561 = do { tick (FillInCaseDefault case_bndr)
1562 ; args <- mk_args con tys
1563 ; return (DataAlt con, args, rhs) }
1565 mk_args con inst_tys
1566 = do { (tv_bndrs, inst_tys') <- mk_tv_bndrs con inst_tys
1567 ; let arg_tys = dataConInstArgTys con inst_tys'
1568 ; arg_ids <- mapM (newId FSLIT("a")) arg_tys
1569 ; returnSmpl (tv_bndrs ++ arg_ids) }
1571 mk_tv_bndrs con inst_tys
1572 | isVanillaDataCon con
1573 = return ([], inst_tys)
1575 = do { tv_uniqs <- getUniquesSmpl
1576 ; let new_tvs = zipWith mk tv_uniqs (dataConTyVars con)
1577 mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv)
1578 ; return (new_tvs, mkTyVarTys new_tvs) }
1580 simplAlt :: SimplEnv
1581 -> [AltCon] -- These constructors can't be present when
1582 -- matching this alternative
1583 -> OutId -- The case binder
1586 -> SimplM (Maybe (TvSubstEnv, OutAlt))
1588 -- Simplify an alternative, returning the type refinement for the
1589 -- alternative, if the alternative does any refinement at all
1590 -- Nothing => the alternative is inaccessible
1592 simplAlt env imposs_cons case_bndr' cont' (con, bndrs, rhs)
1593 | con `elem` imposs_cons -- This case can't match
1596 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1597 -- TURGID DUPLICATION, needed only for the simplAlt call
1598 -- in mkDupableAlt. Clean this up when moving to FC
1599 = ASSERT( null bndrs )
1600 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1601 returnSmpl (Just (emptyVarEnv, (DEFAULT, [], rhs')))
1603 env' = mk_rhs_env env case_bndr' (mkOtherCon handled_cons)
1604 -- Record the constructors that the case-binder *can't* be.
1606 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1607 = ASSERT( null bndrs )
1608 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1609 returnSmpl (Just (emptyVarEnv, (LitAlt lit, [], rhs')))
1611 env' = mk_rhs_env env case_bndr' (mkUnfolding False (Lit lit))
1613 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1614 | isVanillaDataCon con
1615 = -- Deal with the pattern-bound variables
1616 -- Mark the ones that are in ! positions in the data constructor
1617 -- as certainly-evaluated.
1618 -- NB: it happens that simplBinders does *not* erase the OtherCon
1619 -- form of unfolding, so it's ok to add this info before
1620 -- doing simplBinders
1621 simplBinders env (add_evals con vs) `thenSmpl` \ (env, vs') ->
1623 -- Bind the case-binder to (con args)
1624 let unf = mkUnfolding False (mkConApp con con_args)
1625 inst_tys' = tyConAppArgs (idType case_bndr')
1626 con_args = map Type inst_tys' ++ map varToCoreExpr vs'
1627 env' = mk_rhs_env env case_bndr' unf
1629 simplExprC env' rhs cont' `thenSmpl` \ rhs' ->
1630 returnSmpl (Just (emptyVarEnv, (DataAlt con, vs', rhs')))
1632 | otherwise -- GADT case
1634 (tvs,ids) = span isTyVar vs
1636 simplBinders env tvs `thenSmpl` \ (env1, tvs') ->
1637 case coreRefineTys con tvs' (idType case_bndr') of {
1638 Nothing -- Inaccessible
1639 | opt_PprStyle_Debug -- Hack: if debugging is on, generate an error case
1641 -> let rhs' = mkApps (Var eRROR_ID)
1642 [Type (substTy env (exprType rhs)),
1643 Lit (mkStringLit "Impossible alternative (GADT)")]
1645 simplBinders env1 ids `thenSmpl` \ (env2, ids') ->
1646 returnSmpl (Just (emptyVarEnv, (DataAlt con, tvs' ++ ids', rhs')))
1648 | otherwise -- Filter out the inaccessible branch
1651 Just refine@(tv_subst_env, _) -> -- The normal case
1654 env2 = refineSimplEnv env1 refine
1655 -- Simplify the Ids in the refined environment, so their types
1656 -- reflect the refinement. Usually this doesn't matter, but it helps
1657 -- in mkDupableAlt, when we want to float a lambda that uses these binders
1658 -- Furthermore, it means the binders contain maximal type information
1660 simplBinders env2 (add_evals con ids) `thenSmpl` \ (env3, ids') ->
1661 let unf = mkUnfolding False con_app
1662 con_app = mkConApp con con_args
1663 con_args = map varToCoreExpr vs' -- NB: no inst_tys'
1664 env_w_unf = mk_rhs_env env3 case_bndr' unf
1667 simplExprC env_w_unf rhs cont' `thenSmpl` \ rhs' ->
1668 returnSmpl (Just (tv_subst_env, (DataAlt con, vs', rhs'))) }
1671 -- add_evals records the evaluated-ness of the bound variables of
1672 -- a case pattern. This is *important*. Consider
1673 -- data T = T !Int !Int
1675 -- case x of { T a b -> T (a+1) b }
1677 -- We really must record that b is already evaluated so that we don't
1678 -- go and re-evaluate it when constructing the result.
1679 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1681 cat_evals dc vs strs
1685 go (v:vs) strs | isTyVar v = v : go vs strs
1686 go (v:vs) (str:strs)
1687 | isMarkedStrict str = evald_v : go vs strs
1688 | otherwise = zapped_v : go vs strs
1690 zapped_v = zap_occ_info v
1691 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1692 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1694 -- If the case binder is alive, then we add the unfolding
1696 -- to the envt; so vs are now very much alive
1697 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1698 | otherwise = \id -> id `setIdOccInfo` NoOccInfo
1700 mk_rhs_env env case_bndr' case_bndr_unf
1701 = modifyInScope env case_bndr' (case_bndr' `setIdUnfolding` case_bndr_unf)
1705 %************************************************************************
1707 \subsection{Known constructor}
1709 %************************************************************************
1711 We are a bit careful with occurrence info. Here's an example
1713 (\x* -> case x of (a*, b) -> f a) (h v, e)
1715 where the * means "occurs once". This effectively becomes
1716 case (h v, e) of (a*, b) -> f a)
1718 let a* = h v; b = e in f a
1722 All this should happen in one sweep.
1725 knownCon :: SimplEnv -> AltCon -> [OutExpr]
1726 -> InId -> [InAlt] -> SimplCont
1727 -> SimplM FloatsWithExpr
1729 knownCon env con args bndr alts cont
1730 = tick (KnownBranch bndr) `thenSmpl_`
1731 case findAlt con alts of
1732 (DEFAULT, bs, rhs) -> ASSERT( null bs )
1733 simplNonRecX env bndr scrut $ \ env ->
1734 -- This might give rise to a binding with non-atomic args
1735 -- like x = Node (f x) (g x)
1736 -- but no harm will be done
1737 simplExprF env rhs cont
1740 LitAlt lit -> Lit lit
1741 DataAlt dc -> mkConApp dc args
1743 (LitAlt lit, bs, rhs) -> ASSERT( null bs )
1744 simplNonRecX env bndr (Lit lit) $ \ env ->
1745 simplExprF env rhs cont
1747 (DataAlt dc, bs, rhs)
1748 -> ASSERT( n_drop_tys + length bs == length args )
1749 bind_args env bs (drop n_drop_tys args) $ \ env ->
1751 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1752 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1753 -- args are aready OutExprs, but bs are InIds
1755 simplNonRecX env bndr con_app $ \ env ->
1756 simplExprF env rhs cont
1758 n_drop_tys | isVanillaDataCon dc = tyConArity (dataConTyCon dc)
1760 -- Vanilla data constructors lack type arguments in the pattern
1763 bind_args env [] _ thing_inside = thing_inside env
1765 bind_args env (b:bs) (Type ty : args) thing_inside
1766 = ASSERT( isTyVar b )
1767 bind_args (extendTvSubst env b ty) bs args thing_inside
1769 bind_args env (b:bs) (arg : args) thing_inside
1771 simplNonRecX env b arg $ \ env ->
1772 bind_args env bs args thing_inside
1776 %************************************************************************
1778 \subsection{Duplicating continuations}
1780 %************************************************************************
1783 prepareCaseCont :: SimplEnv
1784 -> [InAlt] -> SimplCont
1785 -> SimplM (FloatsWith (SimplCont,SimplCont))
1786 -- Return a duplicatable continuation, a non-duplicable part
1787 -- plus some extra bindings (that scope over the entire
1790 -- No need to make it duplicatable if there's only one alternative
1791 prepareCaseCont env [alt] cont = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1792 prepareCaseCont env alts cont = mkDupableCont env cont
1796 mkDupableCont :: SimplEnv -> SimplCont
1797 -> SimplM (FloatsWith (SimplCont, SimplCont))
1799 mkDupableCont env cont
1800 | contIsDupable cont
1801 = returnSmpl (emptyFloats env, (cont, mkBoringStop (contResultType cont)))
1803 mkDupableCont env (CoerceIt ty cont)
1804 = mkDupableCont env cont `thenSmpl` \ (floats, (dup_cont, nondup_cont)) ->
1805 returnSmpl (floats, (CoerceIt ty dup_cont, nondup_cont))
1807 mkDupableCont env cont@(ArgOf _ arg_ty _ _)
1808 = returnSmpl (emptyFloats env, (mkBoringStop arg_ty, cont))
1809 -- Do *not* duplicate an ArgOf continuation
1810 -- Because ArgOf continuations are opaque, we gain nothing by
1811 -- propagating them into the expressions, and we do lose a lot.
1812 -- Here's an example:
1813 -- && (case x of { T -> F; F -> T }) E
1814 -- Now, && is strict so we end up simplifying the case with
1815 -- an ArgOf continuation. If we let-bind it, we get
1817 -- let $j = \v -> && v E
1818 -- in simplExpr (case x of { T -> F; F -> T })
1819 -- (ArgOf (\r -> $j r)
1820 -- And after simplifying more we get
1822 -- let $j = \v -> && v E
1823 -- in case of { T -> $j F; F -> $j T }
1824 -- Which is a Very Bad Thing
1826 -- The desire not to duplicate is the entire reason that
1827 -- mkDupableCont returns a pair of continuations.
1829 -- The original plan had:
1830 -- e.g. (...strict-fn...) [...hole...]
1832 -- let $j = \a -> ...strict-fn...
1833 -- in $j [...hole...]
1835 mkDupableCont env (ApplyTo _ arg se cont)
1836 = -- e.g. [...hole...] (...arg...)
1838 -- let a = ...arg...
1839 -- in [...hole...] a
1840 do { (floats, (dup_cont, nondup_cont)) <- mkDupableCont env cont
1841 ; addFloats env floats $ \ env -> do
1842 { arg1 <- simplExpr (setInScope se env) arg
1843 ; (floats2, arg2) <- mkDupableArg env arg1
1844 ; return (floats2, (ApplyTo OkToDup arg2 (zapSubstEnv se) dup_cont, nondup_cont)) }}
1846 mkDupableCont env (Select _ case_bndr alts se cont)
1847 = -- e.g. (case [...hole...] of { pi -> ei })
1849 -- let ji = \xij -> ei
1850 -- in case [...hole...] of { pi -> ji xij }
1851 do { tick (CaseOfCase case_bndr)
1852 ; let alt_env = setInScope se env
1853 ; (floats1, (dup_cont, nondup_cont)) <- mkDupableCont alt_env cont
1854 -- NB: call mkDupableCont here, *not* prepareCaseCont
1855 -- We must make a duplicable continuation, whereas prepareCaseCont
1856 -- doesn't when there is a single case branch
1857 ; addFloats alt_env floats1 $ \ alt_env -> do
1859 { (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1860 -- NB: simplBinder does not zap deadness occ-info, so
1861 -- a dead case_bndr' will still advertise its deadness
1862 -- This is really important because in
1863 -- case e of b { (# a,b #) -> ... }
1864 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1865 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1866 -- In the new alts we build, we have the new case binder, so it must retain
1869 ; (floats2, alts') <- mkDupableAlts alt_env case_bndr' alts dup_cont
1870 ; return (floats2, (Select OkToDup case_bndr' alts' (zapSubstEnv se)
1871 (mkBoringStop (contResultType dup_cont)),
1875 mkDupableArg :: SimplEnv -> OutExpr -> SimplM (FloatsWith OutExpr)
1876 -- Let-bind the thing if necessary
1877 mkDupableArg env arg
1879 = return (emptyFloats env, arg)
1881 = do { arg_id <- newId FSLIT("a") (exprType arg)
1882 ; tick (CaseOfCase arg_id)
1883 -- Want to tick here so that we go round again,
1884 -- and maybe copy or inline the code.
1885 -- Not strictly CaseOfCase, but never mind
1886 ; return (unitFloat env arg_id arg, Var arg_id) }
1887 -- What if the arg should be case-bound?
1888 -- This has been this way for a long time, so I'll leave it,
1889 -- but I can't convince myself that it's right.
1891 mkDupableAlts :: SimplEnv -> OutId -> [InAlt] -> SimplCont
1892 -> SimplM (FloatsWith [InAlt])
1893 -- Absorbs the continuation into the new alternatives
1895 mkDupableAlts env case_bndr' alts dupable_cont
1898 go env [] = returnSmpl (emptyFloats env, [])
1900 = do { (floats1, mb_alt') <- mkDupableAlt env case_bndr' dupable_cont alt
1901 ; addFloats env floats1 $ \ env -> do
1902 { (floats2, alts') <- go env alts
1903 ; returnSmpl (floats2, case mb_alt' of
1904 Just alt' -> alt' : alts'
1908 mkDupableAlt env case_bndr' cont alt
1909 = simplAlt env [] case_bndr' cont alt `thenSmpl` \ mb_stuff ->
1911 Nothing -> returnSmpl (emptyFloats env, Nothing) ;
1913 Just (reft, (con, bndrs', rhs')) ->
1914 -- Safe to say that there are no handled-cons for the DEFAULT case
1916 if exprIsDupable rhs' then
1917 returnSmpl (emptyFloats env, Just (con, bndrs', rhs'))
1918 -- It is worth checking for a small RHS because otherwise we
1919 -- get extra let bindings that may cause an extra iteration of the simplifier to
1920 -- inline back in place. Quite often the rhs is just a variable or constructor.
1921 -- The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1922 -- iterations because the version with the let bindings looked big, and so wasn't
1923 -- inlined, but after the join points had been inlined it looked smaller, and so
1926 -- NB: we have to check the size of rhs', not rhs.
1927 -- Duplicating a small InAlt might invalidate occurrence information
1928 -- However, if it *is* dupable, we return the *un* simplified alternative,
1929 -- because otherwise we'd need to pair it up with an empty subst-env....
1930 -- but we only have one env shared between all the alts.
1931 -- (Remember we must zap the subst-env before re-simplifying something).
1932 -- Rather than do this we simply agree to re-simplify the original (small) thing later.
1936 rhs_ty' = exprType rhs'
1937 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1939 | isTyVar bndr = not (bndr `elemVarEnv` reft)
1940 -- Don't abstract over tyvar binders which are refined away
1941 -- See Note [Refinement] below
1942 | otherwise = not (isDeadBinder bndr)
1943 -- The deadness info on the new Ids is preserved by simplBinders
1945 -- If we try to lift a primitive-typed something out
1946 -- for let-binding-purposes, we will *caseify* it (!),
1947 -- with potentially-disastrous strictness results. So
1948 -- instead we turn it into a function: \v -> e
1949 -- where v::State# RealWorld#. The value passed to this function
1950 -- is realworld#, which generates (almost) no code.
1952 -- There's a slight infelicity here: we pass the overall
1953 -- case_bndr to all the join points if it's used in *any* RHS,
1954 -- because we don't know its usage in each RHS separately
1956 -- We used to say "&& isUnLiftedType rhs_ty'" here, but now
1957 -- we make the join point into a function whenever used_bndrs'
1958 -- is empty. This makes the join-point more CPR friendly.
1959 -- Consider: let j = if .. then I# 3 else I# 4
1960 -- in case .. of { A -> j; B -> j; C -> ... }
1962 -- Now CPR doesn't w/w j because it's a thunk, so
1963 -- that means that the enclosing function can't w/w either,
1964 -- which is a lose. Here's the example that happened in practice:
1965 -- kgmod :: Int -> Int -> Int
1966 -- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1970 -- I have seen a case alternative like this:
1971 -- True -> \v -> ...
1972 -- It's a bit silly to add the realWorld dummy arg in this case, making
1975 -- (the \v alone is enough to make CPR happy) but I think it's rare
1977 ( if not (any isId used_bndrs')
1978 then newId FSLIT("w") realWorldStatePrimTy `thenSmpl` \ rw_id ->
1979 returnSmpl ([rw_id], [Var realWorldPrimId])
1981 returnSmpl (used_bndrs', map varToCoreExpr used_bndrs')
1982 ) `thenSmpl` \ (final_bndrs', final_args) ->
1984 -- See comment about "$j" name above
1985 newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty') `thenSmpl` \ join_bndr ->
1986 -- Notice the funky mkPiTypes. If the contructor has existentials
1987 -- it's possible that the join point will be abstracted over
1988 -- type varaibles as well as term variables.
1989 -- Example: Suppose we have
1990 -- data T = forall t. C [t]
1992 -- case (case e of ...) of
1993 -- C t xs::[t] -> rhs
1994 -- We get the join point
1995 -- let j :: forall t. [t] -> ...
1996 -- j = /\t \xs::[t] -> rhs
1998 -- case (case e of ...) of
1999 -- C t xs::[t] -> j t xs
2001 -- We make the lambdas into one-shot-lambdas. The
2002 -- join point is sure to be applied at most once, and doing so
2003 -- prevents the body of the join point being floated out by
2004 -- the full laziness pass
2005 really_final_bndrs = map one_shot final_bndrs'
2006 one_shot v | isId v = setOneShotLambda v
2008 join_rhs = mkLams really_final_bndrs rhs'
2009 join_call = mkApps (Var join_bndr) final_args
2011 returnSmpl (unitFloat env join_bndr join_rhs, Just (con, bndrs', join_call)) }
2018 MkT :: a -> b -> T a
2022 MkT a' b (p::a') (q::b) -> [p,w]
2024 The danger is that we'll make a join point
2028 and that's ill-typed, because (p::a') but (w::a).
2030 Solution so far: don't abstract over a', because the type refinement
2031 maps [a' -> a] . Ultimately that won't work when real refinement goes on.
2033 Then we must abstract over any refined free variables. Hmm. Maybe we
2034 could just abstract over *all* free variables, thereby lambda-lifting
2035 the join point? We should try this.