2 % (c) The AQUA Project, Glasgow University, 1994-1998
4 \section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop}
7 module LiberateCase ( liberateCase ) where
9 #include "HsVersions.h"
11 import CmdLineOpts ( opt_D_verbose_core2core, opt_LiberateCaseThreshold )
12 import CoreLint ( beginPass, endPass )
14 import CoreUnfold ( couldBeSmallEnoughToInline )
20 This module walks over @Core@, and looks for @case@ on free variables.
22 if there is case on a free on the route to the recursive call,
23 then the recursive call is replaced with an unfolding.
32 => the inner f is replaced.
41 (note the NEED for shadowing)
43 => Run Andr\'e's wonder pass ...
50 Better code, because 'a' is free inside the inner letrec, rather
51 than needing projection from v.
53 Other examples we'd like to catch with this kind of transformation
59 We'd like to avoid the redundant pattern match, transforming to
63 last (x:(y:ys)) = last' y ys
66 last' _ (y:ys) = last' y ys
68 (is this necessarily an improvement)
75 drop n (x:xs) = drop (n-1) xs
77 Would like to pass n along unboxed.
80 To think about (Apr 94)
83 Main worry: duplicating code excessively. At the moment we duplicate
84 the entire binding group once at each recursive call. But there may
85 be a group of recursive calls which share a common set of evaluated
86 free variables, in which case the duplication is a plain waste.
88 Another thing we could consider adding is some unfold-threshold thing,
89 so that we'll only duplicate if the size of the group rhss isn't too
95 The ``level'' of a binder tells how many
96 recursive defns lexically enclose the binding
97 A recursive defn "encloses" its RHS, not its
100 letrec f = let g = ... in ...
105 Here, the level of @f@ is zero, the level of @g@ is one,
106 and the level of @h@ is zero (NB not one).
109 type LibCaseLevel = Int
111 topLevel :: LibCaseLevel
118 Int -- Bomb-out size for deciding if
119 -- potential liberatees are too big.
120 -- (passed in from cmd-line args)
122 LibCaseLevel -- Current level
124 (IdEnv LibCaseLevel) -- Binds all non-top-level in-scope Ids
125 -- (top-level and imported things have
128 (IdEnv CoreBind)-- Binds *only* recursively defined
129 -- Ids, to their own binding group,
130 -- and *only* in their own RHSs
132 [(Id,LibCaseLevel)] -- Each of these Ids was scrutinised by an
133 -- enclosing case expression, with the
134 -- specified number of enclosing
135 -- recursive bindings; furthermore,
136 -- the Id is bound at a lower level
137 -- than the case expression. The
138 -- order is insignificant; it's a bag
141 initEnv :: Int -> LibCaseEnv
142 initEnv bomb_size = LibCaseEnv bomb_size 0 emptyVarEnv emptyVarEnv []
144 bombOutSize (LibCaseEnv bomb_size _ _ _ _) = bomb_size
151 liberateCase :: [CoreBind] -> IO [CoreBind]
154 beginPass "Liberate case" ;
155 let { binds' = do_prog (initEnv opt_LiberateCaseThreshold) binds } ;
156 endPass "Liberate case"
157 opt_D_verbose_core2core {- no specific flag for dumping -}
162 do_prog env (bind:binds) = bind' : do_prog env' binds
164 (env', bind') = libCaseBind env bind
171 libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
173 libCaseBind env (NonRec binder rhs)
174 = (addBinders env [binder], NonRec binder (libCase env rhs))
176 libCaseBind env (Rec pairs)
177 = (env_body, Rec pairs')
179 (binders, rhss) = unzip pairs
181 env_body = addBinders env binders
183 pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
185 env_rhs = if all rhs_small_enough rhss then extended_env else env
187 -- We extend the rec-env by binding each Id to its rhs, first
188 -- processing the rhs with an *un-extended* environment, so
189 -- that the same process doesn't occur for ever!
192 = addRecBinds env [ (binder, libCase env_body rhs)
193 | (binder, rhs) <- pairs ]
195 -- Why "localiseId" above? Because we're creating a new local
196 -- copy of the original binding. In particular, the original
197 -- binding might have been for a top-level, and this copy clearly
198 -- will not be top-level!
200 -- It is enough to change just the binder, because subsequent
201 -- simplification will propagate the right info from the binder.
203 -- Why does it matter? Because the codeGen keeps a separate
204 -- environment for top-level Ids, and it is disastrous for it
205 -- to think that something is top-level when it isn't.
207 -- [May 98: all this is now handled by SimplCore.tidyCore]
209 rhs_small_enough rhs = couldBeSmallEnoughToInline lIBERATE_BOMB_SIZE rhs
211 lIBERATE_BOMB_SIZE = bombOutSize env
219 libCase :: LibCaseEnv
223 libCase env (Var v) = libCaseId env v
224 libCase env (Lit lit) = Lit lit
225 libCase env (Type ty) = Type ty
226 libCase env (App fun arg) = App (libCase env fun) (libCase env arg)
227 libCase env (Note note body) = Note note (libCase env body)
229 libCase env (Lam binder body)
230 = Lam binder (libCase (addBinders env [binder]) body)
232 libCase env (Let bind body)
233 = Let bind' (libCase env_body body)
235 (env_body, bind') = libCaseBind env bind
237 libCase env (Case scrut bndr alts)
238 = Case (libCase env scrut) bndr (map (libCaseAlt env_alts) alts)
240 env_alts = addBinders env_with_scrut [bndr]
241 env_with_scrut = case scrut of
242 Var scrut_var -> addScrutedVar env scrut_var
245 libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
251 libCaseId :: LibCaseEnv -> Id -> CoreExpr
253 | maybeToBool maybe_rec_bind && -- It's a use of a recursive thing
254 there_are_free_scruts -- with free vars scrutinised in RHS
255 = Let the_bind (Var v)
261 maybe_rec_bind :: Maybe CoreBind -- The binding of the recursive thingy
262 maybe_rec_bind = lookupRecId env v
263 Just the_bind = maybe_rec_bind
265 rec_id_level = lookupLevel env v
267 there_are_free_scruts = freeScruts env rec_id_level
275 addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
276 addBinders (LibCaseEnv bomb lvl lvl_env rec_env scruts) binders
277 = LibCaseEnv bomb lvl lvl_env' rec_env scruts
279 lvl_env' = extendVarEnvList lvl_env (binders `zip` repeat lvl)
281 addRecBinds :: LibCaseEnv -> [(Id,CoreExpr)] -> LibCaseEnv
282 addRecBinds (LibCaseEnv bomb lvl lvl_env rec_env scruts) pairs
283 = LibCaseEnv bomb lvl' lvl_env' rec_env' scruts
286 lvl_env' = extendVarEnvList lvl_env [(binder,lvl) | (binder,_) <- pairs]
287 rec_env' = extendVarEnvList rec_env [(binder, Rec pairs) | (binder,_) <- pairs]
289 addScrutedVar :: LibCaseEnv
290 -> Id -- This Id is being scrutinised by a case expression
293 addScrutedVar env@(LibCaseEnv bomb lvl lvl_env rec_env scruts) scrut_var
295 = LibCaseEnv bomb lvl lvl_env rec_env scruts'
296 -- Add to scruts iff the scrut_var is being scrutinised at
297 -- a deeper level than its defn
301 scruts' = (scrut_var, lvl) : scruts
302 bind_lvl = case lookupVarEnv lvl_env scrut_var of
306 lookupRecId :: LibCaseEnv -> Id -> Maybe CoreBind
307 lookupRecId (LibCaseEnv bomb lvl lvl_env rec_env scruts) id
309 = lookupVarEnv rec_env id
311 = case (lookupVarEnv rec_env id) of
316 lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
317 lookupLevel (LibCaseEnv bomb lvl lvl_env rec_env scruts) id
318 = case lookupVarEnv lvl_env id of
322 freeScruts :: LibCaseEnv
323 -> LibCaseLevel -- Level of the recursive Id
324 -> Bool -- True <=> there is an enclosing case of a variable
325 -- bound outside (ie level <=) the recursive Id.
326 freeScruts (LibCaseEnv bomb lvl lvl_env rec_env scruts) rec_bind_lvl
327 = not (null free_scruts)
329 free_scruts = [v | (v,lvl) <- scruts, lvl > rec_bind_lvl]