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 ( DynFlags, DynFlag(..), opt_LiberateCaseThreshold )
12 import CoreLint ( showPass, 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)
52 Better code, because 'a' is free inside the inner letrec, rather
53 than needing projection from v.
55 Other examples we'd like to catch with this kind of transformation
61 We'd like to avoid the redundant pattern match, transforming to
65 last (x:(y:ys)) = last' y ys
68 last' _ (y:ys) = last' y ys
70 (is this necessarily an improvement)
77 drop n (x:xs) = drop (n-1) xs
79 Would like to pass n along unboxed.
82 To think about (Apr 94)
85 Main worry: duplicating code excessively. At the moment we duplicate
86 the entire binding group once at each recursive call. But there may
87 be a group of recursive calls which share a common set of evaluated
88 free variables, in which case the duplication is a plain waste.
90 Another thing we could consider adding is some unfold-threshold thing,
91 so that we'll only duplicate if the size of the group rhss isn't too
97 The ``level'' of a binder tells how many
98 recursive defns lexically enclose the binding
99 A recursive defn "encloses" its RHS, not its
102 letrec f = let g = ... in ...
107 Here, the level of @f@ is zero, the level of @g@ is one,
108 and the level of @h@ is zero (NB not one).
111 type LibCaseLevel = Int
113 topLevel :: LibCaseLevel
120 Int -- Bomb-out size for deciding if
121 -- potential liberatees are too big.
122 -- (passed in from cmd-line args)
124 LibCaseLevel -- Current level
126 (IdEnv LibCaseLevel) -- Binds all non-top-level in-scope Ids
127 -- (top-level and imported things have
130 (IdEnv CoreBind) -- Binds *only* recursively defined
131 -- Ids, to their own binding group,
132 -- and *only* in their own RHSs
134 [(Id,LibCaseLevel)] -- Each of these Ids was scrutinised by an
135 -- enclosing case expression, with the
136 -- specified number of enclosing
137 -- recursive bindings; furthermore,
138 -- the Id is bound at a lower level
139 -- than the case expression. The
140 -- order is insignificant; it's a bag
143 initEnv :: Int -> LibCaseEnv
144 initEnv bomb_size = LibCaseEnv bomb_size 0 emptyVarEnv emptyVarEnv []
146 bombOutSize (LibCaseEnv bomb_size _ _ _ _) = bomb_size
153 liberateCase :: DynFlags -> [CoreBind] -> IO [CoreBind]
154 liberateCase dflags binds
156 showPass dflags "Liberate case" ;
157 let { binds' = do_prog (initEnv opt_LiberateCaseThreshold) binds } ;
158 endPass dflags "Liberate case" Opt_D_verbose_core2core binds'
159 {- no specific flag for dumping -}
163 do_prog env (bind:binds) = bind' : do_prog env' binds
165 (env', bind') = libCaseBind env bind
172 libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
174 libCaseBind env (NonRec binder rhs)
175 = (addBinders env [binder], NonRec binder (libCase env rhs))
177 libCaseBind env (Rec pairs)
178 = (env_body, Rec pairs')
180 (binders, rhss) = unzip pairs
182 env_body = addBinders env binders
184 pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
186 env_rhs = if all rhs_small_enough rhss then extended_env else env
188 -- We extend the rec-env by binding each Id to its rhs, first
189 -- processing the rhs with an *un-extended* environment, so
190 -- that the same process doesn't occur for ever!
192 extended_env = addRecBinds env [ (binder, libCase env_body rhs)
193 | (binder, rhs) <- pairs ]
195 rhs_small_enough rhs = couldBeSmallEnoughToInline lIBERATE_BOMB_SIZE rhs
196 lIBERATE_BOMB_SIZE = bombOutSize env
204 libCase :: LibCaseEnv
208 libCase env (Var v) = libCaseId env v
209 libCase env (Lit lit) = Lit lit
210 libCase env (Type ty) = Type ty
211 libCase env (App fun arg) = App (libCase env fun) (libCase env arg)
212 libCase env (Note note body) = Note note (libCase env body)
214 libCase env (Lam binder body)
215 = Lam binder (libCase (addBinders env [binder]) body)
217 libCase env (Let bind body)
218 = Let bind' (libCase env_body body)
220 (env_body, bind') = libCaseBind env bind
222 libCase env (Case scrut bndr alts)
223 = Case (libCase env scrut) bndr (map (libCaseAlt env_alts) alts)
225 env_alts = addBinders env_with_scrut [bndr]
226 env_with_scrut = case scrut of
227 Var scrut_var -> addScrutedVar env scrut_var
230 libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
236 libCaseId :: LibCaseEnv -> Id -> CoreExpr
238 | Just the_bind <- lookupRecId env v -- It's a use of a recursive thing
239 , not (null free_scruts) -- with free vars scrutinised in RHS
240 = Let the_bind (Var v)
246 rec_id_level = lookupLevel env v
247 free_scruts = freeScruts env rec_id_level
255 addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
256 addBinders (LibCaseEnv bomb lvl lvl_env rec_env scruts) binders
257 = LibCaseEnv bomb lvl lvl_env' rec_env scruts
259 lvl_env' = extendVarEnvList lvl_env (binders `zip` repeat lvl)
261 addRecBinds :: LibCaseEnv -> [(Id,CoreExpr)] -> LibCaseEnv
262 addRecBinds (LibCaseEnv bomb lvl lvl_env rec_env scruts) pairs
263 = LibCaseEnv bomb lvl' lvl_env' rec_env' scruts
266 lvl_env' = extendVarEnvList lvl_env [(binder,lvl) | (binder,_) <- pairs]
267 rec_env' = extendVarEnvList rec_env [(binder, Rec pairs) | (binder,_) <- pairs]
269 addScrutedVar :: LibCaseEnv
270 -> Id -- This Id is being scrutinised by a case expression
273 addScrutedVar env@(LibCaseEnv bomb lvl lvl_env rec_env scruts) scrut_var
275 = LibCaseEnv bomb lvl lvl_env rec_env scruts'
276 -- Add to scruts iff the scrut_var is being scrutinised at
277 -- a deeper level than its defn
281 scruts' = (scrut_var, lvl) : scruts
282 bind_lvl = case lookupVarEnv lvl_env scrut_var of
286 lookupRecId :: LibCaseEnv -> Id -> Maybe CoreBind
287 lookupRecId (LibCaseEnv bomb lvl lvl_env rec_env scruts) id
288 = lookupVarEnv rec_env id
290 lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
291 lookupLevel (LibCaseEnv bomb lvl lvl_env rec_env scruts) id
292 = case lookupVarEnv lvl_env id of
296 freeScruts :: LibCaseEnv
297 -> LibCaseLevel -- Level of the recursive Id
298 -> [Id] -- Ids that are scrutinised between the binding
299 -- of the recursive Id and here
300 freeScruts (LibCaseEnv bomb lvl lvl_env rec_env scruts) rec_bind_lvl
301 = [v | (v,scrut_lvl) <- scruts, scrut_lvl > rec_bind_lvl]