2 % (c) The AQUA Project, Glasgow University, 1993-1996
4 \section[SimplUtils]{The simplifier utilities}
7 #include "HsVersions.h"
13 mkTyLamTryingEta, mkValLamTryingEta,
19 simplIdWantsToBeINLINEd,
21 type_ok_for_let_to_case
25 IMPORT_DELOOPER(SmplLoop) -- paranoia checking
28 import CmdLineOpts ( SimplifierSwitch(..) )
30 import CoreUtils ( manifestlyWHNF )
31 import Id ( idType, isBottomingId, idWantsToBeINLINEd, dataConArgTys,
32 getIdArity, GenId{-instance Eq-}
34 import IdInfo ( arityMaybe )
35 import Maybes ( maybeToBool )
36 import PrelVals ( augmentId, buildId )
37 import PrimOp ( primOpIsCheap )
40 import Type ( eqTy, isPrimType, maybeAppDataTyConExpandingDicts, getTyVar_maybe )
41 import TysWiredIn ( realWorldStateTy )
42 import TyVar ( GenTyVar{-instance Eq-} )
43 import Util ( isIn, panic )
50 The function @floatExposesHNF@ tells whether let/case floating will
51 expose a head normal form. It is passed booleans indicating the
56 :: Bool -- Float let(rec)s out of rhs
57 -> Bool -- Float cheap primops out of rhs
58 -> Bool -- OK to duplicate code
59 -> GenCoreExpr bdr Id tyvar uvar
62 floatExposesHNF float_lets float_primops ok_to_dup rhs
65 try (Case (Prim _ _) (PrimAlts alts deflt) )
66 | float_primops && (null alts || ok_to_dup)
67 = or (try_deflt deflt : map try_alt alts)
69 try (Let bind body) | float_lets = try body
73 -- because it *will* become one.
74 -- likewise for `augment g h'
76 try (App (App (Var bld) _) _) | bld == buildId = True
77 try (App (App (App (Var aug) _) _) _) | aug == augmentId = True
79 try other = manifestlyWHNF other
80 {- but *not* necessarily "manifestlyBottom other"...
82 We may want to float a let out of a let to expose WHNFs,
83 but to do that to expose a "bottom" is a Bad Idea:
85 in ...error ...y... -- manifestly bottom using y
89 in let x = ...error ...y...
92 as y is only used in case of an error, we do not want
93 to allocate it eagerly as that's a waste.
96 try_alt (lit,rhs) = try rhs
98 try_deflt NoDefault = False
99 try_deflt (BindDefault _ rhs) = try rhs
103 Eta reduction on ordinary lambdas
104 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
105 We have a go at doing
107 \ x y -> f x y ===> f
109 But we only do this if it gets rid of a whole lambda, not part.
110 The idea is that lambdas are often quite helpful: they indicate
111 head normal forms, so we don't want to chuck them away lightly.
112 But if they expose a simple variable then we definitely win. Even
113 if they expose a type application we win. So we check for this special
118 f xs = [y | (y,_) <- xs]
120 gives rise to a recursive function for the list comprehension, and
121 f turns out to be just a single call to this recursive function.
124 mkValLamTryingEta :: [Id] -- Args to the lambda
125 -> CoreExpr -- Lambda body
128 mkValLamTryingEta [] body = body
130 mkValLamTryingEta orig_ids body
131 = reduce_it (reverse orig_ids) body
133 bale_out = mkValLam orig_ids body
135 reduce_it [] residual
136 | residual_ok residual = residual
137 | otherwise = bale_out
139 reduce_it (id:ids) (App fun (VarArg arg))
141 && not (idType id `eqTy` realWorldStateTy)
142 -- *never* eta-reduce away a PrimIO state token! (WDP 94/11)
145 reduce_it ids other = bale_out
147 is_elem = isIn "mkValLamTryingEta"
150 residual_ok :: CoreExpr -> Bool -- Checks for type application
151 -- and function not one of the
154 residual_ok (Var v) = not (v `is_elem` orig_ids)
155 -- Fun mustn't be one of the bound ids
156 residual_ok (App fun arg)
157 | notValArg arg = residual_ok fun
158 residual_ok other = False
163 @etaExpandCount@ takes an expression, E, and returns an integer n,
166 E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn)
168 is a safe transformation. In particular, the transformation should
169 not cause work to be duplicated, unless it is ``cheap'' (see
170 @manifestlyCheap@ below).
172 @etaExpandCount@ errs on the conservative side. It is always safe to
175 An application of @error@ is special, because it can absorb as many
176 arguments as you care to give it. For this special case we return
177 100, to represent "infinity", which is a bit of a hack.
180 etaExpandCount :: GenCoreExpr bdr Id tyvar uvar
181 -> Int -- Number of extra args you can safely abstract
183 etaExpandCount (Lam (ValBinder _) body)
184 = 1 + etaExpandCount body
186 etaExpandCount (Let bind body)
187 | all manifestlyCheap (rhssOfBind bind)
188 = etaExpandCount body
190 etaExpandCount (Case scrut alts)
191 | manifestlyCheap scrut
192 = minimum [etaExpandCount rhs | rhs <- rhssOfAlts alts]
194 etaExpandCount fun@(Var _) = eta_fun fun
195 etaExpandCount (App fun arg)
196 | notValArg arg = eta_fun fun
197 | otherwise = case etaExpandCount fun of
199 n -> n-1 -- Knock off one
201 etaExpandCount other = 0 -- Give up
204 -- Scc (pessimistic; ToDo),
205 -- Let with non-whnf rhs(s),
206 -- Case with non-whnf scrutinee
208 -----------------------------
209 eta_fun :: GenCoreExpr bdr Id tv uv -- The function
210 -> Int -- How many args it can safely be applied to
212 eta_fun (App fun arg) | notValArg arg = eta_fun fun
215 | isBottomingId v -- Bottoming ids have "infinite arity"
216 = 10000 -- Blargh. Infinite enough!
219 | maybeToBool arity_maybe -- We know the arity
222 arity_maybe = arityMaybe (getIdArity v)
223 arity = case arity_maybe of { Just arity -> arity }
225 eta_fun other = 0 -- Give up
228 @manifestlyCheap@ looks at a Core expression and returns \tr{True} if
229 it is obviously in weak head normal form, or is cheap to get to WHNF.
230 By ``cheap'' we mean a computation we're willing to duplicate in order
231 to bring a couple of lambdas together. The main examples of things
232 which aren't WHNF but are ``cheap'' are:
237 where e, and all the ei are cheap; and
242 where e and b are cheap; and
246 where op is a cheap primitive operator
249 manifestlyCheap :: GenCoreExpr bndr Id tv uv -> Bool
251 manifestlyCheap (Var _) = True
252 manifestlyCheap (Lit _) = True
253 manifestlyCheap (Con _ _) = True
254 manifestlyCheap (SCC _ e) = manifestlyCheap e
255 manifestlyCheap (Coerce _ _ e) = manifestlyCheap e
256 manifestlyCheap (Lam x e) = if isValBinder x then True else manifestlyCheap e
257 manifestlyCheap (Prim op _) = primOpIsCheap op
259 manifestlyCheap (Let bind body)
260 = manifestlyCheap body && all manifestlyCheap (rhssOfBind bind)
262 manifestlyCheap (Case scrut alts)
263 = manifestlyCheap scrut && all manifestlyCheap (rhssOfAlts alts)
265 manifestlyCheap other_expr -- look for manifest partial application
266 = case (collectArgs other_expr) of { (fun, _, _, vargs) ->
269 Var f | isBottomingId f -> True -- Application of a function which
270 -- always gives bottom; we treat this as
271 -- a WHNF, because it certainly doesn't
272 -- need to be shared!
275 num_val_args = length vargs
277 num_val_args == 0 || -- Just a type application of
278 -- a variable (f t1 t2 t3)
280 case (arityMaybe (getIdArity f)) of
282 Just arity -> num_val_args < arity
288 Eta reduction on type lambdas
289 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
290 We have a go at doing
292 /\a -> <expr> a ===> <expr>
294 where <expr> doesn't mention a.
295 This is sometimes quite useful, because we can get the sequence:
297 f ab d = let d1 = ...d... in
298 letrec f' b x = ...d...(f' b)... in
302 f.Int b = letrec f' b x = ...dInt...(f' b)... in
307 f' b x = ...dInt...(f' b)...
310 Now we really want to simplify to
314 and then replace all the f's with f.Ints.
316 N.B. We are careful not to partially eta-reduce a sequence of type
317 applications since this breaks the specialiser:
319 /\ a -> f Char# a =NO=> f Char#
322 mkTyLamTryingEta :: [TyVar] -> CoreExpr -> CoreExpr
324 mkTyLamTryingEta tyvars tylam_body
326 tyvars == tyvar_args && -- Same args in same order
327 check_fun fun -- Function left is ok
329 -- Eta reduction worked
332 -- The vastly common case
333 mkTyLam tyvars tylam_body
335 (tyvar_args, fun) = strip_tyvar_args [] tylam_body
337 strip_tyvar_args args_so_far tyapp@(App fun (TyArg ty))
338 = case getTyVar_maybe ty of
339 Just tyvar_arg -> strip_tyvar_args (tyvar_arg:args_so_far) fun
340 Nothing -> (args_so_far, tyapp)
342 strip_tyvar_args args_so_far (App _ (UsageArg _))
343 = panic "SimplUtils.mkTyLamTryingEta: strip_tyvar_args UsageArg"
345 strip_tyvar_args args_so_far fun
348 check_fun (Var f) = True -- Claim: tyvars not mentioned by type of f
349 check_fun other = False
355 Given a type generate the case alternatives
359 if there's one constructor, or
363 if there's many, or if it's a primitive type.
368 :: Type -- type of RHS
369 -> SmplM InAlts -- result
371 mkIdentityAlts rhs_ty
373 = newId rhs_ty `thenSmpl` \ binder ->
374 returnSmpl (PrimAlts [] (BindDefault (binder, bad_occ_info) (Var binder)))
377 = case (maybeAppDataTyConExpandingDicts rhs_ty) of
378 Just (tycon, ty_args, [data_con]) -> -- algebraic type suitable for unpacking
380 inst_con_arg_tys = dataConArgTys data_con ty_args
382 newIds inst_con_arg_tys `thenSmpl` \ new_bindees ->
384 new_binders = [ (b, bad_occ_info) | b <- new_bindees ]
388 [(data_con, new_binders, mkCon data_con [] ty_args (map VarArg new_bindees))]
392 _ -> -- Multi-constructor or abstract algebraic type
393 newId rhs_ty `thenSmpl` \ binder ->
394 returnSmpl (AlgAlts [] (BindDefault (binder,bad_occ_info) (Var binder)))
396 bad_occ_info = ManyOcc 0 -- Non-committal!
400 simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool
402 simplIdWantsToBeINLINEd id env
403 = if switchIsSet env IgnoreINLINEPragma
405 else idWantsToBeINLINEd id
407 type_ok_for_let_to_case :: Type -> Bool
409 type_ok_for_let_to_case ty
410 = case (maybeAppDataTyConExpandingDicts ty) of
412 Just (tycon, ty_args, []) -> False
413 Just (tycon, ty_args, non_null_data_cons) -> True
414 -- Null data cons => type is abstract