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
27 import CmdLineOpts ( SimplifierSwitch(..) )
29 import CoreUtils ( manifestlyWHNF )
30 import Id ( idType, isBottomingId, idWantsToBeINLINEd, dataConArgTys,
31 getIdArity, GenId{-instance Eq-}
33 import IdInfo ( arityMaybe )
34 import Maybes ( maybeToBool )
35 import PrelInfo ( augmentId, buildId, realWorldStateTy )
36 import PrimOp ( primOpIsCheap )
39 import Type ( eqTy, isPrimType, maybeAppDataTyCon, getTyVar_maybe )
40 import TyVar ( GenTyVar{-instance Eq-} )
41 import Util ( isIn, panic )
48 The function @floatExposesHNF@ tells whether let/case floating will
49 expose a head normal form. It is passed booleans indicating the
54 :: Bool -- Float let(rec)s out of rhs
55 -> Bool -- Float cheap primops out of rhs
56 -> Bool -- OK to duplicate code
57 -> GenCoreExpr bdr Id tyvar uvar
60 floatExposesHNF float_lets float_primops ok_to_dup rhs
63 try (Case (Prim _ _) (PrimAlts alts deflt) )
64 | float_primops && (null alts || ok_to_dup)
65 = or (try_deflt deflt : map try_alt alts)
67 try (Let bind body) | float_lets = try body
71 -- because it *will* become one.
72 -- likewise for `augment g h'
74 try (App (App (Var bld) _) _) | bld == buildId = True
75 try (App (App (App (Var aug) _) _) _) | aug == augmentId = True
77 try other = manifestlyWHNF other
78 {- but *not* necessarily "manifestlyBottom other"...
80 We may want to float a let out of a let to expose WHNFs,
81 but to do that to expose a "bottom" is a Bad Idea:
83 in ...error ...y... -- manifestly bottom using y
87 in let x = ...error ...y...
90 as y is only used in case of an error, we do not want
91 to allocate it eagerly as that's a waste.
94 try_alt (lit,rhs) = try rhs
96 try_deflt NoDefault = False
97 try_deflt (BindDefault _ rhs) = try rhs
101 Eta reduction on ordinary lambdas
102 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
103 We have a go at doing
105 \ x y -> f x y ===> f
107 But we only do this if it gets rid of a whole lambda, not part.
108 The idea is that lambdas are often quite helpful: they indicate
109 head normal forms, so we don't want to chuck them away lightly.
110 But if they expose a simple variable then we definitely win. Even
111 if they expose a type application we win. So we check for this special
116 f xs = [y | (y,_) <- xs]
118 gives rise to a recursive function for the list comprehension, and
119 f turns out to be just a single call to this recursive function.
122 mkValLamTryingEta :: [Id] -- Args to the lambda
123 -> CoreExpr -- Lambda body
126 mkValLamTryingEta [] body = body
128 mkValLamTryingEta orig_ids body
129 = reduce_it (reverse orig_ids) body
131 bale_out = mkValLam orig_ids body
133 reduce_it [] residual
134 | residual_ok residual = residual
135 | otherwise = bale_out
137 reduce_it (id:ids) (App fun (VarArg arg))
139 && not (idType id `eqTy` realWorldStateTy)
140 -- *never* eta-reduce away a PrimIO state token! (WDP 94/11)
143 reduce_it ids other = bale_out
145 is_elem = isIn "mkValLamTryingEta"
148 residual_ok :: CoreExpr -> Bool -- Checks for type application
149 -- and function not one of the
152 residual_ok (Var v) = not (v `is_elem` orig_ids)
153 -- Fun mustn't be one of the bound ids
154 residual_ok (App fun arg)
155 | notValArg arg = residual_ok fun
156 residual_ok other = False
161 @etaExpandCount@ takes an expression, E, and returns an integer n,
164 E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn)
166 is a safe transformation. In particular, the transformation should
167 not cause work to be duplicated, unless it is ``cheap'' (see
168 @manifestlyCheap@ below).
170 @etaExpandCount@ errs on the conservative side. It is always safe to
173 An application of @error@ is special, because it can absorb as many
174 arguments as you care to give it. For this special case we return
175 100, to represent "infinity", which is a bit of a hack.
178 etaExpandCount :: GenCoreExpr bdr Id tyvar uvar
179 -> Int -- Number of extra args you can safely abstract
181 etaExpandCount (Lam (ValBinder _) body)
182 = 1 + etaExpandCount body
184 etaExpandCount (Let bind body)
185 | all manifestlyCheap (rhssOfBind bind)
186 = etaExpandCount body
188 etaExpandCount (Case scrut alts)
189 | manifestlyCheap scrut
190 = minimum [etaExpandCount rhs | rhs <- rhssOfAlts alts]
192 etaExpandCount fun@(Var _) = eta_fun fun
193 etaExpandCount (App fun arg)
194 | notValArg arg = eta_fun fun
195 | otherwise = case etaExpandCount fun of
197 n -> n-1 -- Knock off one
199 etaExpandCount other = 0 -- Give up
202 -- Scc (pessimistic; ToDo),
203 -- Let with non-whnf rhs(s),
204 -- Case with non-whnf scrutinee
206 -----------------------------
207 eta_fun :: GenCoreExpr bdr Id tv uv -- The function
208 -> Int -- How many args it can safely be applied to
210 eta_fun (App fun arg) | notValArg arg = eta_fun fun
213 | isBottomingId v -- Bottoming ids have "infinite arity"
214 = 10000 -- Blargh. Infinite enough!
217 | maybeToBool arity_maybe -- We know the arity
220 arity_maybe = arityMaybe (getIdArity v)
221 arity = case arity_maybe of { Just arity -> arity }
223 eta_fun other = 0 -- Give up
226 @manifestlyCheap@ looks at a Core expression and returns \tr{True} if
227 it is obviously in weak head normal form, or is cheap to get to WHNF.
228 By ``cheap'' we mean a computation we're willing to duplicate in order
229 to bring a couple of lambdas together. The main examples of things
230 which aren't WHNF but are ``cheap'' are:
235 where e, and all the ei are cheap; and
240 where e and b are cheap; and
244 where op is a cheap primitive operator
247 manifestlyCheap :: GenCoreExpr bndr Id tv uv -> Bool
249 manifestlyCheap (Var _) = True
250 manifestlyCheap (Lit _) = True
251 manifestlyCheap (Con _ _) = True
252 manifestlyCheap (SCC _ e) = manifestlyCheap e
253 manifestlyCheap (Lam x e) = if isValBinder x then True else manifestlyCheap e
254 manifestlyCheap (Prim op _) = primOpIsCheap op
256 manifestlyCheap (Let bind body)
257 = manifestlyCheap body && all manifestlyCheap (rhssOfBind bind)
259 manifestlyCheap (Case scrut alts)
260 = manifestlyCheap scrut && all manifestlyCheap (rhssOfAlts alts)
262 manifestlyCheap other_expr -- look for manifest partial application
263 = case (collectArgs other_expr) of { (fun, _, _, vargs) ->
266 Var f | isBottomingId f -> True -- Application of a function which
267 -- always gives bottom; we treat this as
268 -- a WHNF, because it certainly doesn't
269 -- need to be shared!
272 num_val_args = length vargs
274 num_val_args == 0 || -- Just a type application of
275 -- a variable (f t1 t2 t3)
277 case (arityMaybe (getIdArity f)) of
279 Just arity -> num_val_args < arity
285 Eta reduction on type lambdas
286 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
287 We have a go at doing
289 /\a -> <expr> a ===> <expr>
291 where <expr> doesn't mention a.
292 This is sometimes quite useful, because we can get the sequence:
294 f ab d = let d1 = ...d... in
295 letrec f' b x = ...d...(f' b)... in
299 f.Int b = letrec f' b x = ...dInt...(f' b)... in
304 f' b x = ...dInt...(f' b)...
307 Now we really want to simplify to
311 and then replace all the f's with f.Ints.
313 N.B. We are careful not to partially eta-reduce a sequence of type
314 applications since this breaks the specialiser:
316 /\ a -> f Char# a =NO=> f Char#
319 mkTyLamTryingEta :: [TyVar] -> CoreExpr -> CoreExpr
321 mkTyLamTryingEta tyvars tylam_body
323 tyvars == tyvar_args && -- Same args in same order
324 check_fun fun -- Function left is ok
326 -- Eta reduction worked
329 -- The vastly common case
330 mkTyLam tyvars tylam_body
332 (tyvar_args, fun) = strip_tyvar_args [] tylam_body
334 strip_tyvar_args args_so_far tyapp@(App fun (TyArg ty))
335 = case getTyVar_maybe ty of
336 Just tyvar_arg -> strip_tyvar_args (tyvar_arg:args_so_far) fun
337 Nothing -> (args_so_far, tyapp)
339 strip_tyvar_args args_so_far (App _ (UsageArg _))
340 = panic "SimplUtils.mkTyLamTryingEta: strip_tyvar_args UsageArg"
342 strip_tyvar_args args_so_far fun
345 check_fun (Var f) = True -- Claim: tyvars not mentioned by type of f
346 check_fun other = False
352 Given a type generate the case alternatives
356 if there's one constructor, or
360 if there's many, or if it's a primitive type.
365 :: Type -- type of RHS
366 -> SmplM InAlts -- result
368 mkIdentityAlts rhs_ty
370 = newId rhs_ty `thenSmpl` \ binder ->
371 returnSmpl (PrimAlts [] (BindDefault (binder, bad_occ_info) (Var binder)))
374 = case (maybeAppDataTyCon rhs_ty) of
375 Just (tycon, ty_args, [data_con]) -> -- algebraic type suitable for unpacking
377 inst_con_arg_tys = dataConArgTys data_con ty_args
379 newIds inst_con_arg_tys `thenSmpl` \ new_bindees ->
381 new_binders = [ (b, bad_occ_info) | b <- new_bindees ]
385 [(data_con, new_binders, mkCon data_con [] ty_args (map VarArg new_bindees))]
389 _ -> -- Multi-constructor or abstract algebraic type
390 newId rhs_ty `thenSmpl` \ binder ->
391 returnSmpl (AlgAlts [] (BindDefault (binder,bad_occ_info) (Var binder)))
393 bad_occ_info = ManyOcc 0 -- Non-committal!
397 simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool
399 simplIdWantsToBeINLINEd id env
400 = if switchIsSet env IgnoreINLINEPragma
402 else idWantsToBeINLINEd id
404 type_ok_for_let_to_case :: Type -> Bool
406 type_ok_for_let_to_case ty
407 = case (maybeAppDataTyCon ty) of
409 Just (tycon, ty_args, []) -> False
410 Just (tycon, ty_args, non_null_data_cons) -> True
411 -- Null data cons => type is abstract